Volume 19, Number 2/May 1981
Published by the MIT Press
ISSN 0028-3967

NEUROSCIENCES

Research Program Bulletin:

 

The F.O. Schmitt Lecture in Neuroscience 1980

THE RELATIONSHIP BETWEEN FUNCTION
AND ENERGY METABOLISM:
ITS USE IN THE LOCALIZATION
OF FUNCTIONAL ACTIVITY
IN THE NERVOUS SYSTEM

Louis Sokoloff
NEUROSCIENCES RESEARCH PROGRAM

165 Allandale Street, Jamaica Plain Station, Boston, MA 02130
Telephone: (617) 522-6700 Cable: NEUROCENT

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© 1981 by the Massachusetts Institute of Technology
THE F.O. SCHMITT LECTURE IN NEUROSCIENCE 1980

THE RELATIONSHIP BETWEEN FUNCTION AND
ENERGY METABOLISM: ITS USE IN THE
LOCALIZATION OF FUNCTIONAL ACTIVITY

IN THE NERVOUS SYSTEM

Louis Sokoloff
National Institute of Mental Health
Bethesda, Maryland
Louis Sokoloff is Chief of the Laboratory of Cerebral Metabolism of
the National Institute of Mental Health, Bethesda, Maryland.

This text is based in part on the author’s F.O. Schmitt Lecture in
Neuroscience, delivered at Kresge Auditorium, Massachusetts Institute
of Technology, March 18, 1980. It was the seventh in this series of

lectures.
FOREWORD

The F.O. Schmitt Lecture and Prize in Neuroscience was established
by the Associates of the Neurosciences Research Program in 1973 to
mark the seventienth birthday of Francis O. Schmitt, founder of the
organization. The purpose of the award is to recognize, encourage,
and advance the achievement of excellence in neuroscience.

John Z. Young was the first recipient of the award and subsequent
awardees have been Solomon H. Snyder and Leslie Iversen, Vernon B.
Mountcastle, Victor Hamburger, Roger Guillemin, and Stephen W.
Kuffler.

Louis Sokoloff received the F.O. Schmitt Prize for 1980 in recog-
nition of his contributions to neurochemistry and cerebral physiology.
The Prize was presented with the following citation:

“For his synthesis of neurochemistry, enzyme kinetics, and circula-
tory physiology in the development of techniques to quantitate
cerebral metabolism and cerebral blood flow; for his use of these
techniques to demonstrate the relations between glucose utilization
and the functional activity of the brain and, in particular, to elucidate
brain mechanisms of visual information processing, of psychotropic
drug action, and of circadian rhythmicity; for his catalytic contribu-
tions to the work of other neuroscientists in their applications of his
discoveries in clinical determinations of cerebral blood flow and
metabolism by positron emission tomography; and for his pioneering
work, which helped develop the field of modern neurochemistry.”
INTRODUCTION

The brain is a complex, heterogeneous organ composed of many
anatomical and functional components with markedly different levels
of activity that vary independently with time and function. Other
tissues are generally far more homogeneous, with most of their cells
acting similarly and synchronously in response to a common stimulus
or regulatory influence. The central nervous system, however, consists
of innumerable subunits, each integrated into its own set of functional
pathways and networks and subserving only one or a few of the many
activities in which the nervous system participates. Understanding
how the nervous system functions requires knowledge not only of the
mechanisms of excitation and inhibition but even more so of their
precise localization in the nervous system and the relationships of
neural subunits to specific functions.

Historically, studies of the central nervous system have concentrated
heavily on localization of function and mapping of pathways related
to specific functions. These have been carried out neuroanatomically
and histologically with staining and degeneration techniques, behav-
iorally with ablation and stimulation techniques, electrophysiologicaily
with electrical recording and evoked electrical responses, and histo-
chemically with a variety of techniques, including fluorescent and
immunofluorescent methods and autoradiography of orthograde and
retrograde axoplasmic flow. Many of these conventional methods
suffer from a sampling problem. They generally permit examination
of only one potential pathway at a time, and only positive results are
interpretable. Furthermore, the demonstration of a pathway reveals
only a potential for function; it does not reveal its significance in
normal function.

Tissues that do physical and/or chemical work, such as heart, kidney,
and skeletal muscle, exhibit a close relationship between energy metab-
olism and functional activity. From measurement of energy metabolism
it is then possible to estimate the level of functional activity. The ex-
istence of a similar relationship in the tissues of the central nervous
system has been more difficult to prove, partly because of uncertainty
about the nature of the work associated with nervous functional
activity, but mainly because of the difficulty in assessing the levels of
functional and metabolic activities in the same functional component
of the brain at the same time. Much of our present knowledge of
cerebral energy metabolism in vivo has been obtained by means of
the nitrous oxide technique of Kety and Schmidt (1948a) and its
modifications (Scheinberg and Stead, 1949; Lassen and Munck, 1955;
Ekl6df et al., 1973; Gjedde et al., 1975), which measure the average

Neurosciences Res. Prog. Bull., Vol. 19, No. 2 159
160 Energy Metabolism for Functional Localization in the Nervous System

rates of energy metabolism in the brain as a whole. These methods
have demonstrated changes in cerebral metabolic rate in association
with gross or diffuse alterations of cerebral function and/or structure,
as, for example, those that occur during postnatal development, aging,
senility, anesthesia, disorders of consciousness, and convulsive states
(Kety, 1950, 1957; Lassen, 1959; Sokoloff, 1960, 1976). They have
not detected changes in cerebral metabolic rate in a number of con-
ditions with, perhaps, more subtle alterations in cerebral functional
activity; for example, deep slow-wave sleep, performance of mental
arithmetic, sedation and tranquilization, schizophrenia, and LSD-
induced psychosis (Kety, 1950; Lassen, 1959; Sokoloff, 1969). It is
possible that there are no changes in cerebral energy metabolism in
these conditions. The apparent lack of change could also be explained
by either a redistribution of local levels of functional and metabolic
activity without significant change in the average of the brain as a
whole or the restriction of altered metabolic activity to regions too
small to be detected in measurements of the brain as a whole. What
has clearly been needed is a method that measures the rates of energy
metabolism in specific discrete regions of the brain in normal and
altered states of functional activity.

Kety and his associates (Landau et al., 1955; Freygang and Sokoloff,
1958; Kety, 1960; Reivich et al., 1969) developed a quantitative auto-
radiographic technique to measure the local tissue concentrations of
chemically inert, diffusible, radioactive tracers that they used to deter-
mine the rates of blood flow simultaneously in all the structural com-
ponents visible and identifiable in autoradiographs of serial sections of
the brain. The application of this quantitative autoradiographic tech-
nique to the determination of local cerebral metabolic rate has proved
to be more difficult because of the inherently greater complexity of
the problem and the unsuitability of the labeled species of the normal
substrates of cerebral energy metabolism, oxygen and glucose. The
radioisotopes of oxygen have too short a physical half-life. Both
oxygen and glucose are too rapidly converted to carbon dioxide, and
CO, is too rapidly cleared from the cerebral tissues. Sacks (1957), for
example, has found in man significant losses of '*CO, from the brain
within two minutes after the onset of an intravenous infusion of
['*C] glucose, labeled either uniformly, in the C-1, C-2, or C-6 posi-
tions. These limitations of [14C] glucose have been avoided by the use
of 2-deoxy-D-['4C] glucose, a labeled analogue of glucose with special
properties that make it particularly appropriate for this application
(Sokoloff et al., 1977). It is metabolized through part of the pathway
of glucose metabolism at a definable rate relative to that of glucose.
Unlike glucose, however, its product, ['4C] deoxyglucose-6-phosphate,
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 161

is essentially trapped in the tissues, allowing the application of the
quantitative autoradiographic technique. The use of radioactive 2-
deoxyglucose (DG) to trace glucose utilization and the autoradiographic
technique to achieve regional localization has recently led to the
development of amethod that measures the rates of glucose utilization
simultaneously in all components of the central nervous system in
the normal conscious state and during experimental physiological,
pharmacological, and pathological conditions (Sokoloff et al., 1977).
Because the procedure is so designed that the concentrations of radio-
activity in the tissues during autoradiography are more or less propor-
tional to the rates of glucose utilization, the autoradiographs provide
pictorial representations of the relative rates of glucose utilization in
all the cerebral structures visualized. Numerous studies with this
method have established that there is a close relationship between
functional activity and energy metabolism in the central nervous sys-
tem (Sokoloff, 1977; Plum et al., 1976), and the method has become
a potent new tool for mapping functional neural pathways on the
basis of evoked metabolic responses.

THEORY

The method is derived from a model based on the biochemical proper-
ties of 2-deoxyglucose (Figure 1) (Sokoloff et al., 1977). DG is trans-
ported bidirectionally between blood and brain by the same carrier
that transports glucose across the blood-brain barrier (Bidder, 1968;
Bachelard, 1971; Oldendorf, 1971). In the cerebral tissues it is phos-
phorylated by hexokinase to 2-deoxyglucose-6-phosphate (DG-
6-P) (Sols and Crane, 1954). Deoxyglucose and glucose are, therefore,
competitive substrates for both blood-brain transport and hexokinase-
catalyzed phosphorylation. Unlike glucose-6-phosphate (G-6-P), how-
ever, which is metabolized further eventually to CO, and water and to
a lesser degree via the hexosemonophosphate shunt, DG-6-P cannot be
converted to fructose-6-phosphate and is not a substrate for G-6-P
dehydrogenase (Sols and Crane, 1954). There is very little glucose-6-
phosphatase activity in brain (Hers, 1957) and even less deoxyglucose-
6-phosphatase activity (Sokoloff et al., 1977). Deoxyglucose-6-
phosphate, once formed, is, therefore, essentially trapped in the cere-
bral tissues, at least long enough for the duration of the measurement.
The half-lives of ['*C] deoxyglucose-6-phosphate in the various cere-
bral tissues have been experimentally estimated; the average half-lives
are 7.7 (S.D. = + 1.6) and 9.7 (S.D. = + 2.6) hours in gray and white
matter, respectively (Sokoloff et al., 1977). The shortest half-life is
6.1 hours in the inferior colliculus (Sokoloff et al., 1977).
162 Energy Metabolism for Functional Localization in the Nervous System

 

 

 

 

 

- PLASMA BRAIN TISSUE
Precursor Poot I Metabolic Products
. 1.
[4C] Deoxyglucose = (l4C]Deoxyglucase © fuc}Deoxyglucose-6-Phosphate
(Cp) (Cz) (Ca)
TOTALS COMETATN = Cj = Ce + Cy
Glucose =—4 Glucose — —+ Glucose-6-Phosphate
(Cp) () (Cy)

| t
i
| CO2 + H20
I

 

 

Figure 1.

Diagrammatic representation of the theoretical model. ce represents the total '*C concen-
tration in a single homogeneous tissue of the brain. C ‘3 and Cp represent the concentrations
of ['*C]}deoxyglucose and glucose, respectively, in the arterial plasma; Cf and Cp represent
their respective concentrations in the tissue pools that serve as substrates for hexokinase.
C*, represents the concentration of [?*C] deoxyglucose-6-phosphate in the tissue. The con-
stants k*, k*, and k¥, represent the rate constants for carrier-mediated transport of ['*C]-
deoxyglucose from plasma to tissue, for carrier-mediated transport back from tissue to
plasma, and for phosphorylation by hexokinase, respectively. The constants k,,k,,andk,
are the equivalent rate constants for glucose. ['*C]Deoxyglucose and glucose share and
compete for the carrier that transports both between plasma and tissue and for hexokinase,
which phosphorylates them to their respective hexose-6-phosphates. The dashed arrow rep-
resents the possibility of glucose-6-phosphate hydrolysis by glucose-6-phosphatase activity, if
any. [Sokoloff et al., 1977]

If the interval of time is kept short enough, for example less than
one hour, to allow the assumption of negligible loss of [!4C] DG-6-P
from the tissues, then the quantity of ['*C] DG-6-P accumulated in
any cerebral tissue at any given time following the introduction of
(44C] DG into the circulation is equal to the integral of the rate of
[4C] DG phosphorylation by hexokinase in that tissue during that
interval of time. This integral is in turn related to the amount of
glucose that has been phosphorylated over the same interval, depend-
ing on the time courses of the relative concentrations of [!'*C] DG and
glucose in the precursor pools and the Michaelis-Menten kinetic con-
stants for hexokinase with respect to both ['*C]DG and glucose.
With cerebral glucose consumption in a steady state, the amount of
glucose phosphorylated during the interval of time equals the steady
state flux of glucose through the hexokinase-catalyzed step times
the duration of the interval, and the net rate of flux of glucose through
this step equals the rate of glucose utilization.

These relationships can be mathematically defined and an opera-
tional equation derived if the following assumptions are made: (1) a
steady state for glucose (i.c., constant plasma glucose concentration
and constant rate of glucose consumption) throughout the period of
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 163

the procedure; (2) homogeneous tissue compartment within which
the concentrations of [!*C] DG and glucose are uniform and exchange
directly with the plasma; and (3) tracer concentrations of [*C]DG
(i.e., molecular concentrations of free {!4C] DG essentially equal to
zero). The operational equation that defines R;, the rate of glucose
consumption per unit mass of tissue, i, in terms of measurable vari-
ables is presented in Figure 2.

The rate constants are determined in a separate group of animals
by a nonlinear, iterative process. This process provides the least
squares best-fit of an equation that defines the time course of total
tissue '4C concentration in terms of the time, the history of the
plasma concentration, and the rate constants to the experimentally
determined time courses of tissue and plasma concentrations of !*C

General Equation for Measurement of Reaction Rates with Tracers:

Labeled Product Formed in Interval of Time, O to T

Rate of Reaction =
e oF eae Isotope Effect Integrated Specific Activity
Correction Factor of Precursor

 

Operational Equation of ['*c] Deoxyglucose Method:

Labeled Product Formed in Interval of Time, O to T

 

Total “C in Tissue '4C in Precursor Remaining in Tissue ot Time, T
at Time, T

_—_—

t
cHT) - kt euroannt/ ch elke+ks)t dt
0

Ri = T T
Vin'Km || f (8 (ke 4) T [ (SB) ole +t
ieee | (ae) at e7 2 7K3 I ()e 27Kgll dt

tsotope Effect Integrated Plasma Correction for Lag in Tissue
Correction Specific Activity Equilibration with Plasma
Factor

 

 

 

 

 

Integrated Precursor Specific Activity in Tissue

Figure 2.

Operational equation of radioactive deoxyglucose method and its functional anatomy. 7 rep-
resents the time at the termination of the experimental period; A equals the ratio of the
distribution space of deoxyglucose in the tissue to that of glucose; ® equals the fraction of
glucose that, once phosphorylated, continues down the glycolytic pathway; and Kn and
Vy, and K,, and V,, represent the familiar Michaelis-Menten kinetic constants of hexo-
kinase for deoxyglucose and glucose, respectively. The other symbols are the same as those
defined in Figure 1. [Sokoloff, 1978]
Table 1

Values of Rate Constants in the Normal Conscious Albino Rat [Sokoloff et al., 1977]

 

 

 

Distribution
Rate constants volume
Structure (min7!) (ml/g)
kt kt kt kY/(kE+k2)

Gray matter

Visual cortex 0.189 + 0.048 0.279 + 0.176 0.063 + 0.040 0.553
Auditory cortex 0.226 + 0.068 0.241 + 0.198 0.067 + 0.057 0.734
Parietal cortex 0.194 + 0.051 0.257 + 0.175 0.062 + 0.045 0.608
Sensory-motor cortex 0.193 + 0.037 0.208 + 0.112 0.049 + 0.035 0.751
Thalamus 0.188 + 0.045 0.218 + 0.144 0.053 + 0.043 0.694
Medial geniculate body 0.219 + 0.055 0.259 + 0.164 0.055 + 0.040 0.697
Lateral geniculate body 0.172 + 0.038 0.220 + 0.134 0.055 + 0.040 0.625
Hypothalamus 0.158 + 0.032 0.266 + 0.119 0.043 + 0.032 0.587
Hippocampus 0.169 + 0.043 0.260 + 0.166 0.056 + 0.040 0.535
Amygdala 0.149 + 0.028 0.235 + 0.109 0.032 + 0.026 0.558
Caudate-putamen 0.176 + 0.041 0.200 + 0.140 0.061 + 0.050 0.674
Superior colliculus 0.198 + 0.054 0.240 + 0.166 0.046 + 0.042 0.692
Pontine gray matter 0.170 + 0.040 0.246 + 0.142 0.037 + 0.033 0.601
Cerebellar cortex 0.225 + 0.066 0.392 + 0.229 0.059 + 0.031 0.499
Cerebellar nucleus 0.207 + 0.042 0.194 40.111 0.038 + 0.035 0.892

Mean ¢ S.E.M. 0.189 + 0.012 0.245 + 0.040 0.052 + 0.010 0.647 + 0.073
White matter

Corpus callosum 0.085 + 0.015 0.135 + 0.075 0.019 + 0.033 0.552

Genu of corpus callosum 0.076 + 0.013 0.131 + 0.075 0.019 + 0.034 0.507
Internal capsule 0.077 + 0.015 0.134 + 0.085 0.023 + 0.039 0.490

Mean + S.E.M. 0.079 + 0.008 0.133 + 0.046 0.020 + 0.020 0.516 + 0.171

Half-life of
Precursor pool
(min)

Log ,2/(k* +k*)

2.03
2.25
2.17
2.70
2.56
2.21
2.52
2.58
2.19
2.60
2.66
2.42
2.45
1.54
2.99

2.39 + 0.40

4.50
4.62
4.41

4.51 + 0.90

 

ol

WI9ISAG SNOAION 9Y] Ul UOTJEZTeIO'T [euooUN 10} wWsToqejap AsIoug
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 165

(Sokoloff et al., 1977). The rate constants have thus far been com-
pletely determined only in normal, conscious albino rats (Table 1).
Partial analyses indicate that the values are quite similar in the con-
scious monkey (Kennedy et al., 1978).

The A, ®, and the enzyme kinetic constants are grouped together to
constitute a single, lumped constant (Figure 2). It can be shown
mathematically that this lumped constant is equal to the asymptotic
value of the product of the ratio of the cerebral extraction ratios of
[4C] DG and glucose and the ratio of the arterial blood to plasma
specific activities when the arterial plasma [1*C]DG concentration
is maintained constant (Sokoloff et al., 1977). The lumped constant
is also determined in a separate group of animals from arterial and
venous cerebral blood samples drawn during a programmed intravenous
infusion that produces and maintains a constant arterial plasma
[4C] DG concentration (Sokoloff et al., 1977). An example of such a
determination in a conscious monkey is illustrated in Figure 3. Thus
far the lumped constant has been determined only in the albino rat,
monkey, cat, and dog (Table 2). The lumped constant appears to be
characteristic of the species and does not appear to change significantly
in a wide range of physiological conditions (Table 2) (Sokoloff et al.,
1977).

Despite its complex appearance, the operational equation is really
nothing more than a general statement of the standard relationship by
which rates of enzyme-catalyzed reactions are determined from
measurements made with radioactive tracers (Figure 2). The numerator
of the equation represents the amount of radioactive product formed
in a given interval of time; it is equal to C# the combined concentra-
tions of [14*C] DG and [!4C] DG-6-P in the tissue at time, 7, measured
by the quantitative autoradiographic technique, less a term that rep-
resents the free unmetabolized [!*C]DG still remaining in the tissue.
The denominator represents the integrated specific activity of the pre-
cursor pool times a factor, the lumped constant, which is equivalent
to a correction factor for an isotope effect. The term with the ex-
ponential factor in the denominator takes into the account the lag in
the equilibration of the tissue precursor pool with the plasma.

EXPERIMENTAL PROCEDURE FOR MEASUREMENT OF LOCAL
CEREBRAL GLUCOSE UTILIZATION

Theoretical Considerations in the Design of the Procedure

The operational equation of the method specifies the variables to be
measured in order to determine R;, the local rate of glucose con-
166 Energy Metabolism for Functional Localization in the Nervous System

2007 {¢c]DEOxYGLUCOSE A 2 4
Og ne P
Ny ct

(50- 3
St
r 2

GLUCOSE

a
3
(4/6)
NOLLVHLNSONOD 3S00T19

 

[4doG CONCENTRATION
fumCi/ml}
3
°o

(cR-c¥ sox
P| (Ca- Cy Cy,
wo

A/Cp
B/C;

 

E

i minus LUMPED CONSTANT

Ch/Cp (Cy Cyc

°

 

CA Calfick- ch cK

 

 

° 5 to 5 20 25 30 35
TIME (min)

Figure 3.

Data obtained and their use in determination of the lumped constant and the combination
of rate constants, (K* + &¥), in a representative experiment. A. Time courses of arterial blood
and plasma concentrations of ['*C] DG and glucose and cerebral venous blood concentrations
of ['*C] DG and glucose during programmed intravenous infusion of ['*C] DG. B. Arithmetic
plot of the function derived from the variables in A and combined as indicated in the formula
on the ordinate against time. This function declines exponentially, with a rate constant equal
to (k*¥ +k), until it reaches an asymptotic value equal to the lumped constant, 0.35, in
this experiment (dashed line). C. Semilogarithmic plot of the curve in B less the lumped con-
stant, i.e., its asymptotic value. Solid circles represent actual values. This curve is analyzed
into two components by a standard curve-peeling technique to yield the two straight lines
representing the separate components. Open circles are points for the fast component, ob-
tained by subtracting the values for the slow component from the solid circles. The rate
constants for these two components represent the values of (k¥ + K¥) for two compartments;
the fast and slow compartments are assumed to represent gray and white matter, respectively.
In this experiment the values for (k¥ + k¥) were found to equal 0.462 (half-time = 1.5 min)
and 0.154 (half-time = 4.5 min) in gray and white matter, respectively. [Kennedy et al.,
1978]
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 167

Table 2
Values of the Lumped Constant in the Albino Rat, Rhesus Monkey, Cat, and Dog
[Sokoloff, 1979]

 

 

Animal No. of animals Mean ¢ S.D. S.E.M.
Albino rat:

Conscious 15 0.464 + 0.099* + 0.026
Anesthetized 9 0.512 + 0.118* + 0.039
Conscious (5% CO, ) 2 0.463 + 0.122* + 0.086
Combined 26 0.481 + 0.119 + 0.023
Rhesus monkey:

Conscious 7 0.344 + 0.095 + 0.036
Cat:

Anesthetized 6 0.411 + 0.013 + 0.005
Dog (beagle puppy):

Conscious 7 0.558 + 0.082 + 0.031

 

*No statistically significant difference between normal conscious and anesthetized rats

(0.3 < p < 0.4) and conscious rats breathing 5% CO, (p > 0.9).

The values were obtained as follows: rat, Sokoloff et al., 1977; monkey, Kennedy et al., 1978;
cat, (M. Miyaoka, J. Magnes, C. Kennedy, M. Shinohara, and L. Sokoloff, unpuplished data);
dog, Duffy et al., 1979.

sumption in the brain. The following variables are measured in each
experiment: (1) the entire history of the arterial plasma [!4C] deoxy-
glucose concentration, C*, from zero time to the time of killing, 7;
(2) the steady-state arterial plasma glucose level, G , over the same
interval; and (3) the local concentration of !*C in the tissue at the
time of killing, C}(T). The rate constants, k¥, kz, and k*, and the
lumped constant, y Ve K,,/® Vin KX, are not "measured in each ex-
periment; the values for these constants that are used are those deter-
mined separately in other groups of animals, as described above and
presented in Tables 1} and 2.

The operational equation is generally applicable with all types of
arterial plasma ['*C]DG concentration curves. Its configuration,
however, suggests that a declining curve, approaching zero by the time
of killing, is the choice to minimize certain potential errors. The
quantitative autoradiographic technique measures only total “C
concentration in the tissue and does not distinguish between [!4C] DG-
6-P and ['4C]DG. It is, however, [!4C]DG-6-P concentration that
must be known to determine glucose consumption. ['*C]DG-6-P
concentration is calculated in the numerator of the operational
equation, which equals the total tissue C content, C*(7), minus
168 Energy Metabolism for Functional Localization in the Nervous System

the [}4C] DG concentration present in the tissue, estimated by the term
containing the exponential factor and rate constants. In the denom-
inator of the operational equation there is also a term containing an
exponential factor and rate constants. Both these terms have the use-
ful property of approaching zero with increasing time if Cr is also
allowed to approach zero. The rate constants, k*, k}, and k}, are not
measured in the same animals in which local glucose consumption is
being measured. It is conceivable that the rate constants in Table 1
are not equally applicable in all physiological, pharmacological, and
pathological states. One possible solution is to determine the rate
constants for each condition to be studied. An alternative solution,
and the one chosen, is to administer the ['*C] DG as a single intra-
venous pulse at zero time and to allow sufficient time for the clear-
ance of {!*C]DG from the plasma and the terms containing the rate
constants to fall to levels too low to influence the final result. To
wait until these terms reach zero is impractical because of the long
time required and the risk of effects of the small but finite rate of
loss of ['*C]DG-6-P from the tissues. A reasonable time interval is
45 minutes; by this time the plasma level has fallen to very low levels,
and, on the basis of the values of (ki + k}) in Table 1, the expo-
nential factors have declined through at least ten half-lives.

The time courses of the concentrations of ['*C]DG and [!*C] DG-
6-P in arterial plasma and representative gray and white matter fol-
lowing an intravenous pulse of [!4C]DG are illustrated in Figure 4.
As the plasma concentration falls from its peak following the pulse,
the tissue concentrations of ['4C]DG first rise until the tissues and
plasma reach equilibrium. As the plasma concentration continues to
fall below its equilibrium levels, there is a net loss of [1*C]DG back
to the plasma, as well as continued conversion of tissue ['*C]DG to
['4C] DG-6-P, and the concentrations of free [!4C]DG in the tissues
then decline (Figure 4A). The higher the blood flow of the tissue, the
more rapidly it initially takes up [!'4C] DG, but it reaches equilibrium
with plasma sooner and loses [!*C] DG more rapidly after the point of
equilibrium. These opposing effects of blood flow before and after
equilibrium tend to cancel out the effects of blood flow. By 45
minutes the tissue and plasma levels of free ['*C]DG have reached
very low levels. On the other hand, the ['*C] DG-6-P concentrations in
the tissues rise continuously and by 45 minutes are responsible for
most of the '4C in the tissues, particularly in gray matter (Figure 4A).
The numerator of the operational equation represents the final total
tissue 4C concentration, measured autoradiographically, minus the
final point on the tissue [‘4C] DG curve and is equal, therefore, to the
final ['4C] DG-6-P concentration in the tissue (Figure 4A).
 

 

  

 

 

 

 

 

 

T T T T T T T T T T T F T T T T T t
A J
c 1600 |¢ ['*C] Deoxyglucose ('*C] Deoxyglucose- 41600 — 1000 B
8 6-Phosphate 2 g 900 Integrals
= 1400 o—+ Plasma 1400 9° 3 300 —_ Plasma a |
8 5 1200 1200 3 8 oO init
2 a 4 % = > 70H  ----- White 8698 4
ya —
% = 1000 41000 8 = 3 E 600 4
8 5 3 8 8 2 500 4
3 2 goof 4800 = > 3a
eo 23 = 00 4
ka 600 4600 o 5 3 :
get > 8 O 300} 4
a 4400 2 3 Qo
= i e = ™ : |
200}, : +200 100k J
LT = rn 1 1 1 1 1 1. 1 1 1 1 1 L 1
5 10 15 20 25 30 35 40 45 5 10 15 20 25 30 35 40 45
Time (minutes) Time (minutes)

Figure 4.

Graphical representation of the significant variables in the operational equation used to calculate local cerebral glucose utilization. A. Time courses of ['*C] DG
concentrations in arterial plasma and in average gray and white matter and {'4C] DG-6-P concentrations in average gray and white matter following an intra-
venous pulse of 50 uCi of ['*C]deoxyglucose. The plasma curve is derived from measurements of plasma [!*C] DG concentration. The tissue concentrations
were calculated from the plasma curve and the mean values of k¥*, KE, and k¥ for gray and white matter in Table 1 according to the second term in the
numerator of the operational equation. The [*C] DG-6-P concentrations in the tissues were calculated from the same variables by integration of the product
of k# and the tissue concentration of [**C] DG. The arrows point to the concentrations of (?*C] DG and [!*C] DG-6-P in the tissues at the time of killing; the
autoradiographic technique measures the total 44C content (i.e., the sum of these concentrations) at that time, which is equal to C*(T), the first term in the
numerator of the operational equation. Note that at the time of killing, the total “C content represents mainly [?*C] deoxyglucose-6-phosphate concentra-
tion, especially in gray matter. B. Time courses of ratios of [14C] deoxyglucose to glucose concentrations (i.e., specific activities) in plasma and average gray
and white matter. The curve for plasma was determined by division of the plasma curve in (A) by the plasma glucose concentrations. The curves for the
tissues were calculated by differentation of the function in brackets in the denominator of the operational equation. The integrals in (B) are the integrals
of the specific activities with respect to time and represent the areas under the curves. The integrals under the tissue curves are equivalent to all of the denom-
inator of the operational equation, except for the lumped constant. Note that by the time of killing, the integrals of the tissue curves approach equality with
each other and with that of the plasma curve. [Sokoloff et al., 1977]

Z ‘ON ‘61 ‘IOA “‘TIng ‘301g ‘soy soousTosOINEN,

691
170 Energy Metabolism for Functional Localization in the Nervous System

The physical significance of the denominator of the operational
equation is illustrated in Figure 4B. The curves in Figure 4B are
derived from the curves for ['4C]DG concentration in plasma and
average gray and white matter in Figure 4A by dividing them by the
glucose concentrations in those tissues. They represent, in effect,
the time courses of the specific activities in those tissues. The integrals
in Figure 4B are the integrated specific activities, i.e., the areas under
each of the curves between zero and 45 minutes. The denominator
of the operational equation is equal to the product of the lumped
constant and the integral appropriate to the tissue. It should be noted
that the integrals for gray and white matter are almost equal to the
integral for plasma (Figure 4B). As can be seen from the operational
equation, this phenomenon merely reflects the diminished contribu-
tions of the terms containing the exponential factors at 45 minutes
after the pulse of ['*C] DG; at infinite time all the integrals would be
equal to the integral of the plasma curve. It may be recalled that the
model assumes only a single compartment for free [!*C] DG in each
tissue. It can be shown that, at infinite time following a pulse, the
integrals of the specific activities of all compartments, either in series
or parallel, that derive their (!4C] DG ultimately from the plasma com-
partment become equal to each other and to the integral of the plasma
specific activity.* It would then be immaterial if there were, indeed,
more than one compartment, and 45 minutes is sufficiently close to
infinity (i.e., at least 10 half-lives) to minimize possible errors due to
that assumption.

Experimental Protocol

The animals are prepared for the experiment by the insertion of
polyethylene catheters in an artery and vein. Any convenient artery
or vein can be used. In the rat the femoral or the tail arteries and
veins have been found satisfactory. In the monkey and cat the femoral
vessels are probably most convenient. The catheters are inserted under
anesthesia, and anesthetic agents without long-lasting aftereffects
should be used. Light halothane anesthesia, with or without supple-
mentation with nitrous oxide, has been found to be quite satisfactory.
At least two hours are allowed for recovery from the surgery and
anesthesia before initiation of the experiment.

The design of the experimental procedure for the measurement of
local cerebral glucose utilization was based on the theoretical con-
siderations discussed above. At zero time a pulse of 125 wCi (no more
than 2.5 pmoles) of [!4C]deoxyglucose per kg of body weight is

*C, Patlak, unpublished.
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 171

administered to the animal via the venous catheter. Arterial sampling
is initiated with the onset of the pulse, and timed 50- to 100-yl samples
of arterial blood are collected consecutively as rapidly as possible
during the early period so as not to miss the peak of the arterial curve.
Arterial sampling is continued at less frequent intervals later in the
experimental period but at sufficient frequency to define fully the
arterial curve. The arterial blood samples are immediately centrifuged
to separate the plasma, which is stored on ice until assayed for ['*C] DG
by liquid scintillation counting and glucose concentrations by standard
enzymatic methods. At approximately 45 minutes the animal is de-
capitated, and the brain is removed and frozen in Freon XII or iso-
pentane maintained between -50° and -75°C with liquid nitrogen.
When fully frozen, the brain is stored at -70°C until sectioned and
autoradiographed. The experimental period may be limited to 30
minutes. This is theoretically permissible and may sometimes be
necessary for reasons of experimental expediency, but greater errors
due to possible inaccuracies in the rate constants may result.

Autoradiographic Measurement of Tissue '*C Concentration

The !*C concentrations in localized regions of the brain are measured
by a modification of the quantitative autoradiographic technique
previously decribed (Reivich et al., 1969). The frozen brain is coated
with chilled embedding medium (Lipshaw Manufacturing Co., Detroit
MI) and fixed to object-holders appropriate to the microtome to be
used.

Brain sections, precisely 20 wm in thickness, are prepared in a cryo-
stat maintained at -21°C to -22°C. The brain sections are picked up
on glass cover slips, dried on a hot plate at 60°C for at least 5 minutes,
and placed sequentially in an X-ray cassette. A set of [!4C] methyl-
methacrylate standards (Amersham Corp., Arlington Heights, IL),
which include a blank and a series of progressively increasing '*C con-
centration, is also placed in the cassette. These standards must pre-
viously have been calibrated for their autoradiographic equivalence to
the !*C concentrations in brain sections, 20 wm in thickness, prepared
as described above. The method of calibration has been described
previously (Reivich et al., 1969).

Autoradiographs are prepared from these sections directly in the X-
ray cassette with Kodak single-coated, blue-sensitive Medical X-ray
Film, Type SB-5 (Eastman Kodak Co., Rochester, NY). The exposure
time is generally 5 to 6 days with the doses used as described above,
and the exposed films are developed according to the instructions
supplied with the film. The SB-5 X-ray film is rapid but coarse-grained.
For finer-grained autoradiographs and, therefore, better-defined
172 Energy Metabolism for Functional Localization in the Nervous System

images with higher resolution, it is possible to use mammographic
films, such as DuPont LoDose or Kodak MR-1 films, or fine-grain
panchromatic film, such as Kodak Plus-X, but the exposure times
are 2 to 3 times longer. The autoradiographs provide a pictorial
representation of the relative ‘*C concentrations in the various cere-
bral structures and the plastic standards. A calibration curve of the
relationship between optical density and tissue '*C concentrations
for each film is obtained by densitometric measurements of the
portions of the film representing the various standards. The local
tissue concentrations are then determined from the calibration curve
and the optical densities of the film in the regions representing the
cerebral structures of interest. Local cerebral glucose utilization is
calculated from the local tissue concentrations of *C and the plasma
[14C] DG and glucose concentrations according to the operationai
equation (Figure 2).

Computerized Color-Coded Image-Processing

The autoradiographs provide pictorial representations of only the rela-
tive and not the actual rates of glucose utilization in all the structures of
the nervous system. Furthermore, the resolution of differences in rela-
tive rates is limited by the ability of the human eye to recognize the dif-
ferences in shades of gray. Manual densitometric analysis permits the
computation of actual rates of glucose utilization with a fair degree of re-
solution, but it generates enormous tables of data that fail to convey the
tremendous heterogeneity of metabolic rates, even within anatomic
structures, or the full information contained within the autoradio-
graphs. Goochee and coworkers (1980) have developed a compu-
terized image-processing system to analyze and transform the audio-
radiographs into color-coded maps of the distribution of the actual
rates of glucose utilization exactly where they are located throughout
the central nervous system. The autoradiographs are scanned auto-
matically by a computer-controlled scanning microdensitometer. The
optical density of each spot in the autoradiograph, from 25 to 100 um
as selected, is stored in a computer, converted to 4C concentration
on the basis of the optical densities of the calibrated 14C plastic
standards, and then converted to local rates of glucose utilization by
solution of the operational equation of the method. Colors are assigned
to narrow ranges of the rates of glucose utilization, and the auto-
radiographs are then displayed in a color TV monitor in color, along
with a calibrated color scale for identifying the rate of glucose utiliza-
tion in each spot of the autoradiograph from its color. These color
maps add a third dimension, the rate of glucose utilization on a color
scale, to the spatial dimensions already present on the autoradiographs.
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 173

RATES OF LOCAL CEREBRAL GLUCOSE UTILIZATION
IN THE NORMAL CONSCIOUS STATE

Thus far quantitative measurements of local cerebral glucose utilization
have been reported only for the albino rat (Sokoloff et al., 1977) and
monkey (Kennedy et al., 1978). These values are presented in Table 3.
The rates of local cerebral glucose utilization in the normal conscious
rat vary widely throughout the brain. The values in white structures
tend to group together and are always considerably below those of
gray structures. The average value in gray matter is approximately
three times that of white matter, but the individual values vary from
approximately 50 to 200 umoles of glucose/100 g/min. The highest
values are in the structures involved in auditory functions, with the
inferior colliculus clearly the most metabolically active structure in
the brain.

The rates of local cerebral glucose utilization in the conscious mon-
key exhibit similar heterogeneity, but they are generally one-third to
one-half the values in corresponding structures of the rat brain (Table
3). The differences in rates in the rat and monkey brain are consistent
with the different cellular packing densities in the brains of these two
species.

EFFECTS OF GENERAL ANESTHESIA

General anesthesia produced by thiopental reduces the rates of glucose
utilization in all structures of the rat brain (Table 4) (Sokoloff et al.,
1977). The effects are not uniform, however. The greatest reductions
occur in the gray structures, particularly those of the primary sensory
pathways. The effects in white matter, though definitely present, are
relatively small compared to those of gray matter. These results are in
agreement with those of previous studies in which anesthesia has been
found to decrease the cerebral metabolic rate of the brain as a whole
(Kety, 1950; Lassen, 1959; Sokoloff, 1976).

RELATION BETWEEN LOCAL FUNCTIONAL ACTIVITY AND
ENERGY METABOLISM

The results of a variety of applications of the method demonstrate a
clear relationship between local cerebral functional activity and glu-
cose consumption. The most striking demonstrations of the close
coupling between function and energy metabolism are seen with
experimentally inducéd local alterations in functional activity that
are restricted to a few specific areas in the brain. The effects on local
glucose consumption are then so pronounced that they are not only
174 Energy Metabolism for Functional Localization in the Nervous System

Table 3
Representative Values for Local Cerebral Glucose Utilization in the Normal
Albino Rat and Monkey (umoles/100g/min)

 

Structure Albino rat* (10) Monkeyf (7)

 

Gray matter

Visual cortex 107+ 6 59+2
Auditory cortex j62+ 5 719+4
Parietal cortex 112+ 5 47+4
Sensory-motor cortex 120+ 5 44+3
Thalamus: lateral nucleus 116+ 5 5442
Thalamus: ventral nucleus 109+ 5 4342
Medial geniculate body 131+ 5 65 +3
Lateral geniculate body 96+ 2 3941
Hypothalamus 54+ 2 2541
Mamillary body 121+ 5 57 +3
Hippocampus 79+ 3 39 +2
Amygdala §2+ 2 25 +2
Caudate-putamen 1102 4 52 +3
Nucleus accumbens 82+ 3 3642
Globus-pallidus 58+ 2 26+2
Substantia nigra 58+ 3 29+2
Vestibular nucleus 128+ 5 66 +3
Cochlear nucleus 113+ 7 5123
Superior olivary nucleus 133+ 7 6344
Inferior colliculus 197 +10 103 +6
Superior colliculus 95+ 5 5544
Pontine gray matter 62+ 3 28 +1
Cerebellar cortex 57+ 2 31+2
Cerebellar nuclei 100+ 4 45+2
White matter

Corpus callosum : 40+ 2 il+1
Internal capsule 334 2 1341
Cerebellar white matter 37+ 2 1221

 

The values are the means + standard errors from measurements made in the number of
animals indicated in parentheses.

*From Sokoloff et al., 1977.
+From Kennedy et al., 1978.
Neurosciences Res. Prog. Bull., Vol. 19, No. 2

Table 4

175

Effects of Thiopental Anesthesia on Local Cerebral Glucose Utilization in the Rat*

[Sokoloff et al., 1977]

 

Local cerebral glucose utilization

 

 

(umoles/100g/min)

Structure Control (6)+ Anesthetized (8)} % Effect
Gray matter

Visual cortex 11145 6423 -42
Auditory cortex 157 +5 81+3 ~ 48
Parietal cortex 107 +3 65122 - 39
Sensory-motor cortex 118 +3 67+2 - 43
Lateral geniculate body 9242 53+3 -42
Medial geniculate body 126 +6 63 +3 ~ 50
Thalamus: lateral nucleus 108 +3 5842 ~ 46
Thalamus: ventral nucleus 98+3 $541 ~44
Hypothalamus 63 +3 43+2 - 32
Caudate-putamen 111+4 72+3 - 35
Hippocampus: Ammon’s horn 7941 5641 -29
Amygdala 5644 41+2 -27
Cochlear nucleus 12447 79+5 - 36
Lateral lemniscus 11447 75+4 - 34
Inferior colliculus 198 +7 131 +8 - 34
Superior olivary nucleus 141+5 10447 - 26
Superior colliculus 99 +3 59 +3 ~40
Vestibular nucleus 13344 81+4 ~ 39
Pontine gray matter 69 +3 46 +3 ~ 33
Cerebellar cortex 6642 44+2 - 33
Cerebellar nucleus 106 +4 75+4 -29
White matter

Corpus callosum 42+2 30+2 -29
Genu of corpus callosum 3545 30+2 -14
Internal capsule 3542 20+2 -17
Cerebellar white matter 38 +2 29+2 -24

 

*Determined at 30 min following pulse of ['*C] deoxyglucose.
{The values are the means + standard errors obtained in the number of animals indicated

in parentheses. All the differences are statistically significant at the p < 0.05 level.
176 Energy Metabolism for Functional Localization in the Nervous System

observed in the quantitative results but can be visualized directly on
the autoradiographs, which are really pictorial representations of the
relative rates of glucose utilization in the various structural com-
ponents of the brain.

Effects of Increased Functional Activity

Effects of Sciatic Nerve Stimulation

Electrical stimulation of one sciatic nerve in the rat under barbiturate
anesthesia causes pronounced increases in glucose consumption (i.e.,
increased optical density’ in the autoradiographs) in the ipsilateral
dorsal horn of the lumbar spinal cord (Kennedy et al., 1975).

Effects of Olfactory Stimulation

The [!4C] deoxyglucose method has been used to map the olfactory
system of the rat (Sharp et al., 1975). Olfactory stimulation with amyl
acetate has been found to produce increased labeling in localized
regions of the olfactory bulb. Preliminary results obtained with other
odors, such as camphor and cheese, suggest different spatial patterns
of increased metabolic activity with different odors.

Effects of Experimental Focal Seizures

The local injection of penicillin into the hand-face area of the motor
cortex of the Rhesus monkey has been shown to induce electrical
discharges in the adjacent cortex and to result in recurrent focal
seizures involving the face, arm, and hand on the contralateral side
(Caveness, 1969). Such seizure activity causes selective increases in
glucose consumption in areas of motor cortex adjacent to the penicillin
locus and in small discrete regions of the putamen, globus pallidus,
caudate nucleus, thalamus, and substantia nigra of the same side
(Figure 5) (Kennedy et al., 1975). Similar studies in the rat have led
to comparable results and have provided evidence on the basis of an
evoked metabolic response of a “mirror” focus in the motor cortex
contralateral to the penicillin-induced epileptogenic focus (Collins
et al., 1976).

Effects of Decreased Functional Activity

Decrements in functional activity result in reduced rates of glucose
utilization. These effects are particularly striking in the auditory and
visual systems of the rat and the visual system of the monkey.
 

 

 

 

 

 

 

Figure 5.

Effects of focal seizures produced by local application of penicillin to motor cortex on local cerebral glucose utilization in the Rhesus monkey. The penicillin
was applied to the hand and face area of the left motor cortex. The left side of the brain is on the left in each of the autoradiographs in the figure. The
numbers are the rates of local cerebral glucose utilization in wmol/100 g tissue/min. Note the following: upper left, motor cortex in region of penicillin
application and corresponding region of contralateral motor cortex; lower left, ipsilateral and contralateral motor cortical regions remote from area of
penicillin applications; upper right, ipsilateral and contralateral putamen and globus pallidus; lower right, ipsilateral and contralateral thalamic nuclei and
substantia nigra. [Sokoloff, 1977]

Z ‘ON ‘61 ‘TOA “Ting ‘301g ‘soy sooustosorneNy

LLI
178 Energy Metabolism for Functional Localization in the Nervous System

Effects of Auditory Deprivation

In the albino rat some of the highest rates of local cerebral glucose
utilization are found in components of the auditory system; i.e.,
auditory cortex, medial geniculate ganglion, inferior colliculus, lateral
lemniscus, superior olive, and cochlear nucleus (Table 3). Bilateral
auditory deprivation by occlusion of both external auditory canals
with wax markedly depresses the metabolic activity in all of these
areas (Sokoloff, 1977). The reductions are symmetrical bilaterally
and range from 35 to 60%. Unilateral auditory deprivation also de-
presses the glucose consumption of these structures, but to a lesser
degree, and some of the structures are asymmetrically affected. For
example, the metabolic activity of the ipsilateral cochlear nucleus
equals 75% of the activity of the contralateral nucleus. The lateral
lemniscus, superior olive, and medial geniculate ganglion are slightly
lower on the contralateral side, while the contralateral inferior col-
liculus is markedly lower in metabolic activity than the ipsilateral
structure. These results demonstrate that there is some degree of
lateralization and crossing of auditory pathways in the rat.

Visual Deprivation in the Rat

In the rat, the visual system is 80 to 85% crossed at the optic chiasma
(Lashley, 1934; Montero and Guillery, 1968), and unilateral enuclea-
tion removes most of the visual input to the central visual structures
of the contralateral side. In the conscious rat studied 2 to 24 hours
after unilateral enucleation, there are marked decrements in glucose
utilization in the contralateral superior colliculus, lateral geniculate
ganglion, and visual cortex as compared to the ipsilateral side (Kennedy
et al., 1975).

Visual Deprivation in the Monkey

In animals with binocular visual systems, such as the Rhesus monkey,
there is only approximately 50% crossing of the visual pathways, and
the structures of the visual system on each side of the brain receive
equal inputs from both retinas. Although each retina projects more or
less equally to both hemispheres, their projections remain segregated
and terminate in six well-defined laminas in the lateral geniculate
ganglia, three each for the ipsilateral and contralateral eyes (Hubel
and Wiesel, 1968, 1972; Wiesel et al., 1974; Rakic, 1976). This segre-
gation is preserved in the optic radiations that project the monocular
representations of the two eyes for any segment of the visual field to
adjacent regions of Layer IV of the striate cortex (Hubel and Wiesel,
1968, 1972). The cells responding to the input of each monocular
terminal zone are distributed transversely through the thickness of the
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 179

striate cortex, resulting in a mosaic of columns, 0.3 to .5 mm in width,
alternately representing the monocular inputs of the two eyes. The
nature and distribution of these ocular dominance columns have pre-
viously been characterized by electrophysiological techniques (Hubel
and Wiesel, 1968), Nauta degeneration methods (Hubel and Wiesel,
1972), and by autoradiographic visualization of axonal and trans-
neuronal transport of [°H]proline- and {?H]fucose-labeled protein
and/or glycoprotein (Wiesel et al., 1974; Rakic, 1976). Bilateral or
unilateral visual deprivation, either by enucleation or by the insertion
of opaque plastic discs, produces consistent changes in the pattern of
distribution of the rates of glucose consumption, all clearly visible in
the autoradiographs, that coincide closely with the changes in func-
tional activity expected from known physiological and anatomical
properties of the binocular visual system (Kennedy et al., 1976).

In animals with intact binocular vision, no bilateral asymmetry is
seen in the autoradiographs of the structures of the visual system
(Figures 6A and 7A). The lateral geniculate ganglia and oculomotor
nuclei appear to be of fairly uniform density and essentially the same
on both sides (Figure 6A). The visual cortex is also the same on both
sides (Figure 7A), but throughout all of Area 17 there is heterogene-
ous density distributed in a characteristic laminar pattern. These
observations indicate that, in animals with binocular visual input, the
rates of glucose consumption in the visual pathways are essentially
equal on both sides of the brain and relatively uniform in the oculo-
motor nuclei and lateral geniculate ganglia but markedly different in
the various layers of the striate cortex.

Autoradiographs from animals with both eyes occluded exhibit
generally decreased labeling of all components of the visual system,
but the bilateral symmetry is fully retained (Figures 6B and 7B), and
the density within each lateral geniculate body is for the most part
fairly uniform (Figure 6B). In the striate cortex, however, the marked
differences in the densities of the various layers seen in the animals
with intact bilateral vision (Figure 7A) are virtually absent so that,
except for a faint delineation of a band within Layer IV, the concen-
tration of the label is essentially homogeneous throughout the striate
cortex (Figure 7B).

Autoradiographs from monkeys with only monocular input, because
of unilateral visual occlusion, exhibit markedly different patterns from
those described above. Both lateral geniculate bodies exhibit exactly
inverse patterns of alternating dark and light bands, corresponding to
the known laminae representing the regions receiving the different
inputs from the retinas of the intact and occluded eyes (Figure 6C).
Bilateral asymmetry is also seen in the oculomotor nuclear complex;
180 Energy Metabolism for Functional Localization in the Nervous System

 

me

5

aa

5.0mm

Figure 6.

Autoradiography of coronal brain sections of monkey at the level of the lateral geniculate
bodies. Large arrows point to the lateral geniculate bodies; small arrows point to oculo-
motor nuclear complex. A. Animal with intact binocular vision. Note the bilateral sym-
metry and relative homogeneity of the lateral geniculate bodies and oculomotor nuclei.
B. Animal with bilateral visual occlusion. Note the reduced relative densities, the relative
homogeneity, and the bilateral symmetry of the lateral geniculate bodies and oculomotor
nuclei. C. Animal with right eye occluded. The left side of the brain is on the left side
of the photograph. Note the laminae and the inverse order of the dark and light bands in
the two lateral geniculate bodies. Note also the lesser density of the oculomotor nuclear
complex on the side contralateral to the occluded eye. [Kennedy et al., 1976]
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 181

 

Figure 7.

Autoradiographs of coronal brain sections from Rhesus monkeys at the level of the striate
cortex. A. Animal with normal binocular vision. Note the laminar distribution of the density ;
the dark band corresponds to Layer IV. B. Animal with bilateral visual deprivation. Note
the almost uniform and reduced relative density, especially the virtual disappearance of the
dark band corresponding to Layer IV. C. Animal with right eye occluded. The half-brain
on the left side of the photograph represents the left hemisphere contralateral to the oc-
cluded eye. Note the alternate dark and light striations, each approximately 0.3 to 0.4 mm
in width, that represent the ocular dominance columns. These columns are most apparent
in the dark band corresponding to Layer IV but extend through the entire thickness of the
cortex. The arrows point to regions of bilateral asymmetry where the ocular dominance
columns are absent. These are presumably areas with normally only monocular input. The
one on the left, contralateral to occluded eye, has a continuous dark lamina corresponding
to Layer IV, which is completely absent on the side ipsilateral to the occluded eye. These
regions are believed to be the loci of the cortical representations of the blind spots. [Kennedy
et al., 1976]
182 Energy Metabolism for Functional Localization in the Nervous System

a lower density is apparent in the nuclear complex contralateral to
the occluded eye (Figure 6C). In the striate cortex, the pattern of
distribution of the ['*C]DG-6-P appears to be a composite of the
patterns seen in the animals with intact and bilaterally occluded visual
input. The pattern found in the former regularly alternates with that
of the latter in columns oriented perpendicularly to the cortical
surface (Figure 7C). The dimensions, arrangement, and distribution of
these columns are identical to those of the ocular dominance columns
described by Hubel and Wiesel (Hubel and Wiesel, 1968, 1972; Wiesel
et al., 1974). These columns reflect the interdigitation of the represen-
tations of the two retinas in the visual cortex. Each element in the
visual fields is represented by a pair of contiguous bands in the visual
cortex, one for each of the two retinas or their portions that cor-
respond to agiven point in the visual fields. With symmetrical visual in-
put bilaterally, the columns representing the two eyes are equally active
and, therefore, not visualized in the autoradiographs (Figure 7A).
When one eye is blocked, however, only those columns representing
the blocked eye become metabolically less active, and the autoradio-
graphs then display the alternate bands of normal and depressed ac-
tivities corresponding to the regions of visual cortical representation of
the two eyes (Figure 7C).

There can be seen in the autoradiographs from the animals with uni-
lateral visual deprivation a pair of regions in the folded calcarine
cortex that exhibit bilateral asymmetry (Figure 7C). The ocular domi-
nance columns are absent on both sides, but on the side contralateral
to the occluded eye this region has the appearance of visual cortex
from an animal with normal bilateral vision, and on the ipsilateral side
this region looks like cortex from an animal with both eyes occluded
(Figure 7). These regions are the loci of the cortical representation of
the blind spots of the visual fields and normally have only monocular
input (Kennedy et al., 1975, 1976). The area of the optic disc in the
nasal half of each retina cannot transmit to this region of the con-
tralateral striate cortex which, therefore, receives its sole input from
an area in the temporal half of the ipsilateral retina. Occlusion of one
eye deprives this region of the ipsilateral striate cortex of all input,
while the corresponding region of the contralateral striate cortex re-
tains uninterrupted input from the intact eye. The metabolic re-
flection of this ipsilateral monocular input is seen in the autoradio-
graph in Figure 7C.

The results of these studies with the ['* C] deoxyglucose method in
the binocular visual system of the monkey represent the most dra-
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 183

matic demonstration of the close relationship between physiological
changes in functional activity and the rate of energy metabolism in
specific components of the central nervous system.

MECHANISM OF COUPLING OF LOCAL FUNCTIONAL
ACTIVITY AND ENERGY METABOLISM

In tissues like heart muscle, skeletal muscle, and kidney, which do
readily recognizable physical work, there is a clear quantitative re-
lationship between the work performed and the rate of energy
metabolism. Presumably, at least part of the energy derived from
metabolism is equivalent to the energy expenditure associated with
the physical work and serves to resynthesize high-energy phosphate
bonds consumed in the process. It is less clear what physical work is
performed by nervous tissue. The finding of a close coupling between
local functional activity and glucose utilization suggests, however, that
neural functional activity is associated with some energy-consuming
physical and/or chemical processes.

Electrical activity appears to be the physical process most intimately
involved with functional activity in nervous tissue. Action potentials
are generated by the movement of ions, mainly Na‘ and K*, across cell
membranes down ionic gradients, and energy must be consumed to
restore the ionic gradients to their resting levels. Increased electrical
activity, i.e., increased frequency of action potentials, might be ex-
pected to lead to greater ionic fluxes and require, therefore, more
energy to restore the ionic gradients. Indeed, Yarowsky and coworkers
(1979) have recently found in the superior cervical ganglion in vivo a
direct linear relationship between the frequency of the electrical spike
input and the rate of glucose utilization (see below). The energy re-
quired to transport the ions back across the cell membrane to restore
the ionic gradients is presumably derived from the splitting of ATP by
Na’, K*-ATPase (Albers, 1967; Caldwell, 1968). Once ATP is split,
there are adequate biochemical mechanisms to explain the increased
glucose utilization and energy metabolism. It has been estimated that
more than 40% of the energy consumption of the brain is used for the
maintenance and restoration of ionic gradients and membrane poten-
tials (Whittam, 1962).

This hypothesis implies that the Nat, K*ATPase is a key link in the
coupling of glucose utilization to functional activity. To test this
hypothesis, Mata and coworkers, (1980) have used the [!4C] deoxy-
glucose technique in vitro with electrically stimulated preparations of
184 Energy Metabolism for Functional Localization in the Nervous System

SUPERIOR COLLICULUS

 

 

 

 

 

 

160
z 140 STRATUM
2 GRISEUM
PERFICIAI
s 120 SUPERFICIALE
=
2¢
BE 100
8 a
38 STRATUM
aS 80 LEMNISCI
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BS gy STRATUM
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ws
Ww
C40 F
a
8 aol -- ALBINO
a 2 — PIGMENTED
oO te 7 700 7000
Osi yp, 1 1 fu 1 li ha
0 0.1 1 10 100 1000 10000
LIGHT INTENSITY (lux)
POSTEROLATERAL NUCLEUS OF THE THALAMUS
160
140 +

 

 

 

 

Zz
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8
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Og
os
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S -- ALBINO
QS 20+
g — PIGMENTED
0.3 14 7 700 7000
0 J pol | at lu 1 bi ja
0 0.1 1 10 100 1000 10000

LIGHT INTENSITY (lux)

Figure 8.
Effects of intensity of retinal illumination with randomly spaced light flashes on local cerebral
glucose utilization in components of the visual system of the albino and Norway brown rat.

rat posterior pituitary. Electrical stimulation led to increased glucose
utilization that was blocked by ouabain, an inhibitor of the Na’,
Kt-ATPase but not of the spike activity or the release of vasopressin
by the gland. It is noteworthy that veratridine, an alkaloid that opens
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 185

LATERAL GENICULATE NUCLEUS

 

NUCLEUS

VENTRAL
NUCLEUS

(umoles/100g/min)

 

-- ALBINO

LOCAL CEREBRAL GLUCOSE UTILIZATION

 

 

 

 

 

207 — PIGMENTED
0 0.3 14 7 70 7000
Ly, 1 1 L 1 ‘ 1
0 0.1 1 10 100. 1000 10000
LIGHT INTENSITY (lux)
VISUAL CORTEX
160
z 140+
2
e
a
=
—
°e
Be
og
eo
ae
° >
a
So
ge
ws
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a 207 — PIGMENTED
o3 1.4 7 700 7000
Ole ypz | 14 ba 1. m 4 1
0 0.1 1 10 100. 1000 10000

 

 

 

LIGHT INTENSITY (lux)

Note that the local glucose utilization is proportional to the logarithm of the intensity of il-
lumination, at least at lower levels of intensity, in the primary projection areas of the retina.
[Miyaoka et al., 1979a]

Nat channels, depolarizes the cell membranes, and, therefore, activates
Na‘, K*-ATPase activity, also stimulated glucose utilization in the
posterior pituitary, and this effect was also blocked by ouabain or
tetrodotoxin. These results strongly support the hypothesis that
186 Energy Metabolism for Functional Localization in the Nervous System

COLOR PLATES

Figure 9.

Metabolic activation of ‘whisker barrel” in right sensory cortex of rat by stroking of single
vibrissa on left side of face. The upper left panel includes the entire section of the rat brain at
the level of the “whisker: barrel”. Successive panels illustrate the results of rescanning at
higher resolution and/or zooming. The lower right panel includes a scan at highest resolution;
each pixel is equivalent to 25 um. The experiment was carried out by Hand and coworkers
(1978), and the color-coded image-processing was done by the method of Goochee and co-
workers (1980). {Hand, 1981]

Figure 10A. :
Metabolic activation of the motor and sensory systems of conscious monkey conditioned
to move its left hand and arm 10 to 20 times per min throughout the experimental period.
Sections through brain and cervical spinal cord. Upper right: note increased metabolic activity
in the contralateral motor and sensory cortex and the thalamus. Lower right: note the in-
creased glucose utilization in the contralateral motor and sensory cortex and globus pallidus.
Lower left: note the increased metabolism in the ipsilateral cerebellum and cuneate nucleus.
Upper left: note the increased metabolic rate in the ventral and dorsal horns of the cervical
spinal cord on the side ipsilateral to the arm movement. [Kennedy et al., 1980]

Figure 10B.

Metabolic activation of the motor and sensory systems of conscious monkey conditioned to
move its left hand and arm 10 to 20 times/min throughout the experimental period. Sections
through spinal cord between first cervical and first thoracic segments. Note increased metabolic
activities in left ventral and dorsal horns of segments of cervical cord involved in movement
of left hand and arm. [Kennedy et al., 1980]

Figure 13.

Retina-dependent activation of metabolic activity by apomorphine in superficial layer of
superior colliculus of the rat. Left, control rat, with right eye enucleated, administered saline
intrvenously. Right, experimental rat, with right eye unucleated, administered 1.5 mg/kg
of apomorphine intravenously. Note the bilateral metabolic activation of the deep layers of
the superior colliculus but metabolic activation in the superficial layer only in the ipsilateral
superior colliculus and no response in the superficial layer contralateral to the enucleated eye.
[McCulloch et al., 1980b]

Figure 15.

Influence of visual input on glucose utilization of human cerebral cortex. Left column:
three horizontal sections of brain of normal conscious man studied with ['*F] fluorodeoxy-
glucose technique while eyes were open. Right column: same three sections studied with eyes
closed. Note the decreased glucose utilization in the occipital cortex and the increased meta-
bolic activity in the frontal cortex when the eyes are closed. [Phelps et al., 1980]

Figure 16.

Changes in local cerebral utilization in a human patient with partial complex epilepsy during
seizures. The three images were obtained from the same section of brain studied three times
with the {!®F] fluorodeoxyglucose technique. Left: patient in interictal state. Note the area
with the low rate of glucose utilization in the left temporal cortex compared to the compar-
able area on the right. Middle: patient suffering from severe seizure. Note the marked increase
in glucose utilization in the left temporal cortex and the generalized decrease in metabolic
activity throughout the remainder of the cerebral cortex. Right: patient suffering from more
moderate seizure. Note the increased glucose utilization in the left temporal cortex and the
decreased metabolic activity in the remainder of the cerebral cortex of the left side only.
{Kuhl et al., in press]
 

de Tyh oa
|
| LOCAL CEREBRAL GLUCOSE UTILIZATION
|

 

Te ID

 

SALINE
UNILATERAL ENUCLEATION

 

MONKEY USING LEFT ARM

| as cet
as a}
a

 

 

APOMORPHINE
Us Mes) tee ye

Ne St OTe a2, es eee e/ sd

UMOLES
71886
“MIN

mee TY Pee ESSE PE Re PRE Poe

   
GLUCOSE UTILIZATION IN SPINAL CORD
A eo ds ee er

rad
74

Pees E ee eae ea Re

 

oe t/a) hoes Pea eS

Figure 10B
EYES OPEN

 

EYES CLOSED

 

 

ECAT IMAGES BY PHELPS AND KUHL / COLOR-CODING BY LOM:NIMH

Figure 15

 

INTERICTAL

Figure 16

ICTAL I

ICTAL II

 

 

 
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 187

energy metabolism is coupled to functional activity through the
activity of the Na‘, K* -ATPase.

The posterior pituitary is highly enriched with axon terminals that
account for more than 42% of the gland’s total volume (Nordmann,
1977). The gland contains, therefore, an extraordinarily high content
of elements with large areas of membrane surface relative to their
volumes. Such structures are likely to suffer relatively large changes in
‘jonic concentration gradients for a given amount of electrical spike
activity. The increased glucose utilization observed by Mata and
colleagues (1980) in the electrically stimulated posterior pituitary
in vitro probably reflected mainly the metabolic activity of the axonal
terminals. Schwartz and coworkers (1979) have studied the entire
hypothalamo-hypophysial pathway in vivo by means of the [!4C]-
deoxyglucose method. Stimulation of this pathway physiologically
by salt-loading also led to markedly increased glucose utilization in
the posterior pituitary, but surprisingly, there were no detectable
effects in the supraoptic and paraventricular nuclei, the loci of the cell
bodies with projections to the posterior pituitary. Obviously the path-
way had been activated by the osmotic stimulation. The discrepancy
in the effects in the cell bodies and in the regions of termination of
their projections may well reflect the greater sensitivity of axonal
terminals and/or synaptic elements than that of perikarya to metabolic
activation. Indeed, the results of the studies on the binocular system
of the monkey described above also lend support to this possibility.
In the animals with both eyes open, Layer IVB, the layer with pre-
dominantly neuropil and axodendritic connections, is clearly the most
metabolically active portion of the striate cortex (Kennedy et al.,
1976) (Figure 7A). It is precisely this region that shows the greatest
reduction in glucose utilization when both eyes are patched; the other
layers also exhibit some reductions in metabolism but much less so,
and Layer IVB can then hardly be distinguished from the other layers
in the autoradiographs (Figure 7B). It seems likely, then, that the
changes in local cerebral glucose utilization in response to altered
functional activity revealed by the [!4C]deoxyglucose method repre-
sent mainly alterations in the metabolic activity of synaptic terminals
triggered by changes in Na‘, K*-ATPase activity.

APPLICATIONS OF THE DEOXYGLUCOSE METHOD

The results of studies like those described above on the effects of ex-
perimentally induced focal alterations of functional activity on local
188 Energy Metabolism for Functional Localization in the Nervous System

glucose utilization have demonstrated a close coupling between local
functional activity and energy metabolism in the central nervous
system. The effects are often so pronounced that they can be visual-
ized directly on the autoradiographs, which provide pictorial repre-
sentations of the relative rates of glucose utilization throughout the
brain. This technique of autoradiographic visualization of evoked
metabolic responses offers a powerful tool to map functional neural
pathways simultaneously in all anatomical components of the central
nervous system, and extensive use has been made of it for this purpose
(Plum et al., 1976). The results have clearly demonstrated the effec-
tiveness of metabolic responses, either positive or negative, in identify-
ing regions of the central nervous system involved in specific functions.

The method has been used most extensively in qualitative studies in
which regions of altered functional activity are identified by the
change in their visual appearance relative to other regions in the auto-
radiographs. Such qualitative studies are useful only when the effects
are lateralized to one side or when only a few discrete regions are
affected; other regions serve as the controls. Quantitative comparisons
cannot, however, be made for equivalent regions between two or more
animals. To make quantitative comparisons between animals, the
fully quantitative method must be used, which takes into account the
various factors, particularly the plasma glucose level, that influence
the magnitude of labeling of the tissues. The method must be used
quantitatively when the experimental procedure produces systemic
effects and alters metabolism in many regions of the brain.

A comprehensive review of the many qualitative and quantitative
applications of the method is beyond the scope of this report. Only
some of the many neurophysiological, neuroanatomical, pharmacolog-
ical, and pathophysiological applications of the method will be briefly
noted merely to illustrate the broad extent of its potential usefulness.

Neurophysiological and Neuroanatomical Applications

Many of the physiological applications of the ['*C]deoxyglucose
method were in studies designed to test the method and to examine
the relationship between local cerebral functional and metabolic
activities. These applications have been described above. The most
dramatic results have been obtained in the visual systems of the mon-
key and the rat. The method has, for example, been used to define
the nature, conformation, and distribution of the ocular dominance
columns in the striate cortex of the monkey (Figure 7C) (Kennedy
et al., 1976). It has been used by Hubel and coworkers (1978) to do
the same for the orientation columns in the striate cortex of the mon-
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 189

key. A by-product of the studies of the ocular dominance columns
was the identification of the loci of the visual cortical representation
of the blind spots of the visual fields (Figure 7C) (Kennedy et al.,
1976). Studies are currently in progress to map the pathways of
higher visual functions beyond the striate cortex; the results thus far
demonstrate extensive areas of involvement of the inferior temporal
cortex in visual processing (Jarvis et al., 1978). Des Rosiers and
colleagues (1978) have used the method to demonstrate functional
plasticity in the striate cortex of the infant monkey. The ocular domi-
nance columns are already present on the first day of life, but if one eye
is kept patched for three months, the columns representing the open
eye broaden and completely take over the adjacent regions of cortex
containing the columns for the eye that had been patched. Inasmuch
as there is no longer any cortical representation for the patched eye,
the animal becomes functionally blind in one eye. This phenomenon
is almost certainly the basis for the cortical blindness or amblyopia
that often occurs in children with uncorrected strabismus.

There have also been extensive studies of the visual system of the
rat. This species has little if any binocular vision and, therefore, lacks
the ocular dominance columns. Batipps and collaborators (1981) have
compared the rates of local cerebral glucose utilization in albino and
Norway brown rats. The rates were essentially the same throughout
the brain except in the components of the primary visual system. The
metabolic rates in the superior colliculus, lateral geniculate, and visual
cortex of the albino rat were significantly lower than those in the
pigmented rat. Miyaoka and coworkers (1979a) have studied the in-
fluence of the intensity of retinal stimulation with randomly spaced
light flashes on the metabolic rates in the visual systems of the two
strains. In dark-adapted animals there is relatively little difference be-
tween the two strains. With increasing intensity of light, the rates of
glucose utilization first increase in the primary projection areas of the
retina, e.g., superficial layer of the superior colliculus and lateral
geniculate body, and the slopes of the increase are steeper in the
albino rat (Figure 8). At 7 lux, however, the metabolic rates peak in
the albino rat and then decrease with increasing light intensity. In
contrast, the metabolic rates in the pigmented rat rise until they reach
a plateau at about 700 lux, approximately the ambient light intensity
in the laboratory. At this level, the metabolic rates in the visual struc-
tures of the albino rat are considerably below those of the pigmented
rat. These results are consistent with the greater intensity of light
reaching the visual cells of the retina in the albino rats because of lack
of pigment and the subsequent damage to the rods at higher light
intensities. It is of considerable interest that the rates of glucose
190 Energy Metabolism for Functional Localization in the Nervous System

utilization in these visual structures obey the Weber-Fechner Law, i.e.,
the metabolic rate is directly proportional to the logarithm of the
intensity of stimulation (Miyaoka et al., 1979a). Inasmuch as this law
was first developed from behavioral manifestations, these results
simply imply that there is a quantitative relationship between be-
havioral and metabolic responses.

Although less extensive, there have also been applications of the
method to other sensory systems. Studies of the olfactory system
(Sharp et al., 1975) have been discussed above. In addition to the ex-
periments in the auditory system described above, there have been
studies of tonotopic representation in the auditory system. Webster
and coworkers (1978) have obtained clear evidence of selective regions
of metabolic activation in the cochlear nucleus, superior olivary com-
plex, nuclei of the lateral lemnisci, and the inferior colliculus in cats
in response to different frequencies of auditory stimulation. Similar
results have been obtained by Silverman and coworkers (1977) in the
rat and guinea pig. Studies of the sensory cortex have demonstrated
metabolic activation of the “‘whisker barrels” by stimulation of the
whiskers in the rat (Durham and Woolsey, 1977; Hand et al., 1978).
Each whisker is represented in a discrete region of the sensory cortex;
their precise location and extent have been elegantly mapped by Hand
and coworkers (1978) and Hand (1980) by means of the [!*C] deoxy-
glucose method (Figure 9: see color insert).

Thus far, there has been relatively little application of the method to
the physiology of motor functions. Kennedy and collaborators (1980)
have studied monkeys that were conditioned to perform a task with
one hand in response to visual cues; in the monkeys that were per-
forming, they observed metabolic activation throughout the appro-
priate areas of the motor as well as sensory systems from the cortex to
the spinal cord (Figure 10: see color. insert).

An interesting physiological application of the [!4C]deoxyglucose
method has been to the study of circadian rhythms in the central
nervous system. Schwartz and his coworkers (1977, 1980) found that
the suprachiasmatic nucleus in the rat exhibits circadian rhythmicity
in metabolic activity, high during the day and low during the night
(Figure 11). None of the other structures that they examined in the
brain showed rhythmic activity. The normally low activity present
in the nucleus in the dark could be markedly increased by light, but
darkness did not reduce the glucose utilization during the day. The
rhythm is entrained to light; reversal of the light-dark cycle leads not
only to reversal of the rhythm in running activity but also in the
cycle of metabolic activity in the suprachiasmatic nucleus. These
studies lend support to the idea that the suprachiasmatic nucleus has
 

  

 

 

 

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Figure 11.

Circadian rhythms in glucose utilization in suprachiasmatic nucleus in the rat. Left panel, animals entrained to 12 hours of light during day and 12 hours of
darkness during night. Right panel, animals entrained to opposite light-dark regimen. [Schwartz et al., 1980]

TON ‘6T TOA “T[Ng “301g "Soy SoousTOSOINSN|

161
192 Energy Metabolism for Functional Localization in the Nervous System

a role in the organization of circadian rhythms in the central nervous
system.

Much of our knowledge of neurophysiology has been derived from
studies of the electrical activity of the nervous system. Indeed, from
the heavy emphasis that has been placed on electrophysiology, one
might gather that the brain is really an electric organ rather than a
chemical one that functions mainly by the release of chemical trans-
mitters at synapses. Nevertheless, electrical activity is unquestionably
fundamental to the process of conduction, and it is appropriate to
inquire how the local metabolic activities revealed by the ['*C] deoxy-
glucose method are related to the electrical activity of the nervous
system. This question is currently being examined by Yarowsky and
his coworkers (1979) in the superior cervical ganglion of the rat. The
advantage of this structure is that its preganglionic input and post-
ganglionic output can be isolated and electrically stimulated and/or
monitored in vivo. The results thus far indicate a clear relationship
between electrical input to the ganglion and its metabolic activity.
In normal conscious rats the ganglion’s rate of glucose utilization
equals approximately 35 pmoles/100 g/min. This rate is markedly
depressed by anesthesia or denervation and enhanced by electri-
cal stimulation of the afferent nerves. The metabolic activation is
frequency-dependent in the range of 5 to 15 Hz, increasing linearly in
magnitude with increasing frequency of the stimulation (Figure 12).
Similar effects of electrical stimulation on the oxygen and glucose
consumption of the excised ganglion studied in vitro have been ob-
served (Larrabee, 1958; Horowicz and Larrabee, 1958; Friedli, 1977).
Recent studies have also shown that antidromic stimulation of the
postganglionic efferent pathways from the ganglion has similar effects;
stimulation of the external carotid nerve antidromically activates
glucose utilization in the region of distribution of the cell bodies of
this efferent pathway, indicating that not only the preganglionic
axonal terminals are metabolically activated, but the postganglionic
cell bodies as well (Yarowsky et al., 1980). As in the neurohypophysial
pathway (Mata et al., 1980), the effects of electrical stimulation on
energy metabolism in the superior cervical ganglion are probably due
to the ionic currents associated with the spike activity and the con-
sequent activation of the Na‘, K*-ATPase activity to restore the ionic
gradients. Electrical stimulation of the afferents to sympathetic ganglia
have been shown to increase extracellular K* concentration (Friedli,
1977; Galvan et al., 1979). Each spike is normally associated with a
sharp transient rise in extracellular K* concentration, which then
rapidly falls and transiently undershoots before returning to the nor-
mal level (Galvan et al., 1979); ouabain slows the decline in K* con-
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 193

 

5B + LINEAR REGRESSION EQUATION: Y = 27.8 + 1.8 X T
Ba + CORRELATION COEFFICIENT = 8.77 (P ¢ B.@B1)

J
Ss
1
Ld

(3)

pd
+--+

a)

GLUCOSE UTILIZATION
Cumo les/1@@g/min)

a) Tt

ia + b

a 2 4 5 a a i2 14 16
STIMULATION FREQUENCY (Hz)

44
tt ttt

MERN +/- S.E.M. +

 

 

 

Figure 12.

Relationship between average glucose utilization in the superior cervical ganglion of the rat
and the frequency of electrical stimulation of the cervical sympathetic trunk (i.c., pregang-
lionic input). The animals were under urethane anesthesia. The cervical sympathetic trunk was .
transected, and the distal portion was stimulated with 0.3 to 0.4 msec monopolar pulses
administered via a stimulation isolation unit at a maximum current of 500 ywamps and at the
frequency indicated. [Yarowsky et al., 1979]

centration after the spike and eliminates the undershoot. Continuous
stimulation at a frequency of 6 Hz produces a sustained increase in
cellular K* concentration (Galvan et al., 1979). It is likely that the
increased extracellular K* concentration and, almost certainly, in-
creased intracellular Na* concentration activate the Na‘, K*-ATPase,
which in turn leads to the increased glucose utilization.

Pharmacological Applications

The ability of the deoxyglucose method to map the entire brain for
localized regions of altered functional activity on the basis of changes
in energy metabolism offers a potent tool to identify the neural sites
of action of agents with neuropharmacological and psychopharma-
cological actions. It does not, however, discriminate between the
direct and indirect effects of the drug. An entire pathway may be
activated even though the direct action of the drug may be exerted
only at the origin of the pathway. This is of advantage for relating
behavioral effects to central actions, but it is a disadvantage if the
goal is to identify the primary site of action of the drug. To discrim-
inate between direct and indirect actions of a drug, the ['*C] deoxy-
194 Energy Metabolism for Functional Localization in the Nervous System

glucose method must be combined with selectively placed lesions in
the CNS that interrupt afferent pathways to the structure in question.
If the metabolic effect of the drug then remains, then it is due to
direct action; if lost, the effect is likely to be indirect and mediated
via the interrupted pathway. Nevertheless, the method has proved to
be useful in a number of pharmacological studies.

Effects of y-Butyrolactone

y-Hydroxybutyrate and y-butyrolactone, which is hydrolyzed to
y-hydroxybutyrate in plasma, produce trance-like behavioral states
associated with marked suppression of electroencephalographic
activity (Roth and Giarman, 1966). These effects are reversible, and
these drugs have been used clinically as anesthetic adjuvants. There is
evidence that these agents lower neuronal activity in the nigrostriatal
pathway and may act by inhibition of dopaminergic synapses (Roth,
1976). Studies in rats with the ['*C]deoxyglucose technique have
demonstrated that y-butyrolactone produces profound dose-dependent
reductions of glucose utilization throughout the brain (Wolfson et al.,
1977). At the highest doses studied, 600 mg/kg of body weight, glu-
cose utilization was reduced by approximately 75% in gray matter and
33% in white matter, but there was no obvious further specificity
with respect to the local cerebral structures affected. The reversibility
of the effects and the magnitude and diffuseness of the depression of
cerebral metabolic rate suggest that this drug might be considered as
a chemical substitute for hypothermia in conditions in which pro-
found reversible reduction of cerebral metabolism is desired.

Effects of D-Lysergic Acid Diethylamide

The effects of the potent psychotomimetic agent, D-lysergic acid
diethylamide, have been examined in the rat (Shinohara et al., 1976).
In doses of 12.5 to 125 yg/kg, it caused dose-dependent reductions
in glucose utilization in a number of cerebral structures. With increas-
ing dosage, more structures were affected and to a greater degree.
There was no pattern in the distribution of the effects, at least none
discernible at the present level of resolution, that might contribute
to the understanding of the drug’s psychotomimetic actions.

Effects of Morphine Addiction and Withdrawal

Acute morphine administration depresses glucose utilization in many
areas of the brain, but the specific effects of morphine could not be
distinguished from those of the hypercapnia produced by the associated
respiratory depression (Sakurada et al., 1976). In contrast, morphine
addiction, produced within 24 hours by a single subcutaneous in-
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 195

jection of 150 mg/kg of morphine base in an oil emulsion, reduces
glucose utilization in a large number of gray structures in the absence
of changes in arterial pCO,. White matter appears to be unaffected.
Naloxone (1 mg/kg subcutaneously) reduces glucose utilization in a
number of structures when administered to normal rats, but when
given to the morphine-addicted animals produces an acute withdrawal
syndrome and reverses the reductions of glucose utilization in several
structures, most strikingly in the habenula (Sakurada et al., 1976).

Pharmacological Studies of Dopaminergic Systems

The most extensive applications of the deoxyglucose method to
pharmacology have been in studies of dopaminergic systems. Ascend-
ing dopaminergic pathways appear to have a potent influence on
glucose utilization in the forebrain of rats. Electrolytic lesions placed
unilaterally in the lateral hypothalamus or pars compacta of the sub-
stantia nigra caused marked ipsilateral reductions of glucose metab-
olism in numerous forebrain structures rostral to the lesion, particularly
the frontal cerebral cortex, caudate-putamen, and parts of the thala-
mus (Schwartz et al., 1976; Schwartz, 1978). Similar lesions in the
locus coeruleus had no such effects.

Enhancement of dopaminergic synaptic activity by administration
of the agonist of dopamine, apomorphine (Brown and Wolfson, 1978),
or of amphetamine (Wechsler et al., 1979), which stimulates release
of dopamine at the synapse, produces marked increases in glucose
consumption in some of the components of the extrapyramidal sys-
tem known or suspected to contain dopamine-receptive cells. With
both drugs, the greatest increases noted were in the zona reticulata
of the substantia nigra and the subthalamic nucleus. Surprisingly,
none of the components of the dopaminergic mesolimbic system
appeared to be affected.

The studies with amphetamine (Wechsler et al., 1979) were carried
out with the fully quantitative [1*C]deoxyglucose method. The re-
sults in Table 5 illustrate the comprehensiveness with which this
method surveys the entire brain for sites of altered activity due to
actions of the drug. It also allows for quantitative comparison of the
relative potencies of related drugs. For example, in Table 5, the com-
parative effects of d-amphetamine and the less potent dopaminergic
agent, l-amphetamine, are compared; the quantitative results clearly
reveal that the effects of l-amphetamine on local cerebral glucose
utilization are more limited in distribution and of less magnitude
than those of d-amphetamine. Indeed, in similar quantitative studies
with apomorphine, McCulloch and colleagues (1979, 1980a) have
been able to generate complete dose-response curves for the effects of
196 Energy Metabolism for Functional Localization in the Nervous System

the drug on the rates of glucose utilization in various components of
dopaminergic systems. They have also demonstrated metabolically the
development of supersensitivity to apomorphine in rats maintained
chronically on the dopamine antagonist, haloperidol.* In the course
of these studies with apomorphine, McCulloch and coworkers (1980b)
obtained evidence of a retinal dopaminergic system that projects
specifically to the superficial layer of the superior colliculus in the rat.
Apomorphine administration activated metabolism in the superficial
layer of the superior colliculus, as well as in other structures, but the
effect in the superficial layer was prevented by prior enucleation (Fig-
ure 13: see color insert). Miyaokat subsequently observed that intra-
ocular administration of minute amounts of apomorphine caused
increased glucose utilization only in the superficial layer of the superior
colliculus of the contralateral side.

Effects of o- and B-Adrenergic Blocking Agents

Savaki and coworkers (1978) have studied the effects of the a-adren-
ergic blocking agent, phentolamine, and the B-adrenergic blocking
agent, propranolol. Both drugs produce widespread dose-dependent
depressions of glucose utilization throughout the brain but exhibit
particularly striking and opposite effectsin the complete auditory path-
way from the cochlear nucleus to the auditory cortex. Propranolol
markedly depressed and phentolamine markedly enhanced glucose
utilization in this pathway. The functional significance of these effects
is unknown, but they seem to correlate with corresponding effects on
the electrophysiological responsiveness of this sensory system. Pro-
pranolol depresses and phentolamine enhances the amplitude of all
components of evoked auditory responses.f t

Pathophysiological Applications

The application of the DG method to the study of pathological states
has been limited because of uncertainties about the values for the
lumped and rate constants to be used. There are, however, patho-
physiological states in which there is no structural damage to the
tissue, and the standard values of the constants can be used. Several
of these conditions have been and are continuing to be studied by the
['4C] deoxyglucose technique, both qualitatively and quantitatively.

*J, McCulloch, H.E. Savaki, A. Pert, W. Bunney, and L. Sokoloff, unpublished observations.
+M. Miyaoka, unpublished observations.
++T. Furlow and J. Hallenbeck, personal communication.
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 197

Table 5
Effects of d-Amphetamine and 1-Amphetamine on Local Cerebral Glucose
Utilization in the Conscious Ratt [Wechsler et al., 1979]

 

 

Structure Control d-Amphetamine 1-Amphetamine
Gray matter

Visual cortex 102+ 8 135 + 11* 105+ 8
Auditory cortex 160 +11 162+ 6 141+ 6
Parietal cortex 109+ 9 125 +10 116+ 4
Sensory-motor cortex 118+ 8 139+ 9 1ll+ 4
Olfactory cortex 100+ 6 93+ § 94+ 3
Frontal cortex 109 +10 130+ 8 105+ 4
Prefrontal cortex 146210 166+ 7 154+ 4
Thalamus — lateral nucleus 97+ 5 114+ 8 117+ 6
— ventral nucleus 85+ 7 108+ 6* 96+ 4
— habenula 118+10 Fist 5** 822 2**
— dorsomedial nucleus 92+ 6 1ll+ 8 106+ 6
Medial geniculate 116+ 5 119+ 4 116+ 4
Lateral geniculate 19+ 5§ 882 § 84+ 4
Hypothalamus 542 § 56+ 3 522 3
— suprachiasmatic nucleus 94+ 4 754 4** 67+ 1**
— mamillary body 117+ 8 134+ § 1422+ 5*
Lateral olfactory nucleus 8 92+ 6 95+ 5 99+ 6
Au 71+ 4 91+ 4** 81+ 4
Hippocampus — Ammon’s hom 719+ 5 73+ 2 81+ 6
— dentate gyrus 60+ 4 55+ 3 67+ 7
Amygdala 46+ 3 46+ 3 44+ 2
Septal nucleus 56+ 3 55+ 2 544 3
Caudate nucleus 109+ 5 132+ 8* 127: 3*
Nucleus accumbens 76+ § 80+ 3 78+ 3
Globus pallidus 53+ 3 64+ 2* 65+ 3*
Subthatamic nucleus 89+ 6 149 + 10** 107 + 2
Substantia nigra — zona reticulata 58+ 2 105+ 4** 72+ 4
— zona compacta 65+ 4 88+ 6** 72+ 3
Red nucleus 76+ 5 94+ 5* 86+ 2
Vestibular nucleus 121 +11 137+ 5 1302 4
Cochlear nucleus 139+ 6 126+ 1 141+ 5
Superior olivary nucleus 144+ 4 143+ 4 147+ 6
Lateral lemniscus 107+ 3 96+ 5 98+ 3
Inferior colliculus 193 +10 169+ 5 150+ 8**
Dorsal tegmental nucleus 109+ 5 112+ 7 122+ 6
Superior colliculus 80+ 5 89 3 91+ 3
Pontine gray 58:2 4 65+ 3 60+ 1
Cerebellar flocculus 124+10 146:15 153 +10

Cerebellar hemispheres §5+ 3 68+ 6 64+ 2
198 Energy Metabolism for Functional Localization in the Nervous System

Table 5 (continued)

 

 

Structure Control d-Amphetamine 1-Amphetamine
Cerebellar nuclei 102+ 4 105+ 8 110+ 3
White matter ;

Corpus callosum 23+ 3 24+ 2 23+ 1

Genu of corpus callosum 29+ 2 30+ 2 26+ 2
Internal capsule 2i+ 1 24+ 2 19+ 2
Cerebellar white 28+ 1 31+ 2 31+ 2

 

+ All values are the means + standard error of the mean for five animals.
* Significant difference from the control at the p < 0.05 level.
** Sionificant difference from the control at the p < 0.01 level.
§It was not possible to correlate precisely this area on autoradiographs with a specific struc-
ture in the rat brain. It is, however, most likely the lateral olfactory nucleus.

Convulsive States

The local injection of penicillin into the motor cortex produces focal
seizures manifested in specific regions of the body contralaterally. The
[!*C] deoxyglucose method has been used to map the spread of
seizure activity within the brain and to identify the structures with
altered functional activity during the seizure. The partial results of
one such experiment in the monkey are illustrated in Figure 5. Discrete
regions of markedly increased glucose utilization, sometimes as much
as 200%, are observed ipsilaterally in the motor cortex, basal ganglia,
particularly the globus pallidus, thalamic nuclei, and contralaterally
in the cerebellar cortex (Kennedy et al., 1975). Kato and coworkers
(1980), Caveness and coworkers (1980), Hosokawa and collaborators
(1980), and Caveness (1980) have carried out the most extensive
studies of the propagation of the seizure activity in newborn and
pubescent monkeys. The results indicate that the brain of the new-
born monkey exhibits similar increases of glucose utilization in
specific structures, but the pattern of distribution of the effects is
less well defined than in the pubescent monkeys. Collins and colleagues
(1976) have carried out similar studies in the rat with similar results
but also obtained evidence on the basis of a local stimulation of glu-
cose utilization of a “mirror focus” in the motor cortex contralateral
to the side with the penicillin-induced epileptogenic focus.

Engel and coworkers (1978) have used the ['*C]DG method to
study seizures kindled in rats by daily electroconvulsive shocks. After
a period of such treatment, the animals exhibit spontaneous seizures.
Their results show marked increases in the limbic system, particularly
the amygdala. The daily administration of the local anesthetic, lido-
caine, kindles similar seizures in rats; Post and colleagues (1979) have
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 199

obtained similar results in such seizures, with particularly pronounced
increases in glucose utilization in the amygdala, hippocampus, and the
entorhinal cortex.

Spreading Cortical Depression

Shinohara and coworkers (1979) studied the effects of local applica-
tions of KC] on the dura overlying the parietal cortex of conscious
rats or directly on the pial surface of the parietal cortex of anesthetized
rats, in order to determine if K* stimulates cerebral metabolism in
vivo as it is well known to do in vitro. The results demonstrate a
marked increase in cerebral cortical glucose utilization in response to
the application of KCl; NaCl has no such effect (Figure 14). Such
application of KCl, however, also produces the phenomenon of
spreading cortical depression. This condition is characterized by a
spread of transient, intense neuronal activity followed by membrane
depolarization, electrical depression, and a negative shift in the cor-
tical DC potential in all directions from the site of initiation at a rate
of 2 to 5 mm/min. The depressed cortex also exhibits a number of
chemical changes, including an increase in extracellular K*, lost, pre-
sumably, from the cells. At the same time that the cortical glucose
utilization is increased, most subcortical structures that are function-
ally connected to the depressed cortex exhibit decreased rates of glu-
cose utilization. During recovery from the spreading cortical depression,
the glucose utilization in the cortex is still increased, but it is dis-
tributed in columns oriented perpendicularly through the cortex. This
columnar arrangement may reflect the columnar functional and
morphological arrangement of the cerebral cortex. It is likely that the
increased glucose utilization in the cortex during spreading cortical
depression is the consequence of the increased extracellular K* and
activation of the Na", K*-ATPase.

Opening of Blood-Brain Barrier

Unilateral opening of the blood-brain barrier in rats by unilateral
carotid injection with a hyperosmotic mannitol solution leads to
widely distributed discrete regions of intensely increased glucose
utilization in the ipsilateral hemisphere (Pappius et al., 1979). These
focal regions of hypermetabolism may reflect local regions of seizure
activity. The prior administration of diazepam in most cases prevents
the appearance of these areas of increased metabolism (Pappius et al.,
1979), and electroencephalographic recordings under similar experi-
mental conditions reveal evidence of seizure activity.*

*C, Fieschi, personal communication.
200 Energy Metabolism for Functional Localization in the Nervous System

 

A 8.

@)

  

FRONTAL CORTEX CONTROL

  

DURING SPREADING DEPRESSION

  

_PARIETAL” CORTEX DURING RECOVERY FROM
SPREADING. DEPRESSION

 

 

 

 

Figure 14.

Autoradiographs of sections of rat brains during spreading cortical depression and during
recovery. The autoradiographs are pictorial representations of the relative rates of glucose
utilization in various parts of the brain; the greater the density, the greater the rate of glucose
utilization. The left sides of the brain are represented by the left hemispheres in the auto-
radiographs. In all the experiments illustrated, the control hemisphere was treated the same
as the experimental side except that equivalent concentrations of NaCl rather than KCl were
used. The NaCl did not lead to any detectable differences from hemispheres over which the
skull was left intact and no NaCl was applied. A. Autoradiographs of sections of brain at
different levels of cerebral cortex from a conscious rat during spreading cortical depression
induced on the left side by application of 5M KCI to the intact dura overlying the left parietal
cortex. The spreading depression was sustained by repeated applications of KCl at 15- to 20-
minute intervals throughout the experimental period. B. Autoradiographs from sections of
brain at the level of the parietal cortex from three animals under barbiturate anesthesia. The
top section is from a normal anesthetized animal; the middle section is from an animal during
unilateral spreading cortical depression induced and sustained by repeated applications of
80 mM KCl in artificial cerebrospinal fluid directly on the surface of the left parieto-occipital
cortex. At the bottom is a comparable section from an animal studied immediately after the
return of cortical d-c potential to normal after a single wave of spreading depression induced
by a single application of 80 mM KC! to the parieto-occipital cortex of the left side. [Shino-
hara et al., 1979]
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 201

Hypoxemia

Pulsinelli and Duffy (1979) have studied the effects of controlled
hypoxemia on local cerebral glucose utilization by means of the
qualitative [!*C]deoxyglucose method. Hypoxemia was achieved by
artificial ventilation of the animals with a mixture of N,, N,O, and
O,, adjusted to maintain the arterial pO, between 28 and 32 mm Hg.
All the animals had had one common carotid artery ligated to limit
the increase in cerebral blood flow and the amount of O, delivered
to the brain. Their autoradiographs provide striking evidence of
marked and disparate changes in glucose utilization in the various
structural components of the brain. The hemisphere ipsilateral to the
carotid ligation was, not unexpectedly, more severely affected. The
most striking effects were markedly higher increases in glucose util-
ization in white matter than in gray matter, presumably due to the Pas-
teur effect, and the appearance of transverse cortical columns of high
activity alternating with columns of low activity. By studies with
black plastic microspheres, Pulsinelli and Duffy were able to show
that the cortical columns were anatomically related to penetrating
cortical arteries, with the columns of high metabolic activity lying
between the arteries.

Miyaoka and coworkers (1979b) have also studied the effects of
moderate hypoxemia in normal, spontaneously breathing, conscious
rats without carotid ligation. The hypoxemia was produced by lower-
ing the O, in the inspired air to approximately 7%. Although this pro-
cedure reduced arterial pO, to approximately 30 mm Hg, the cerebral
hypoxia was probably less than in the studies of Pulsinelli and Duffy
(1979) because of the intact cerebral circulation. The animals re-
mained fully conscious under these experimental conditions, although
they appeared subdued and less active. The quantitative [!4C] deoxy-
glucose method was employed, and rates of glucose utilization were
determined. The results revealed many similarities to those of Pulsinelli
and Duffy (1979). There was a complete redistribution of the local
rates of glucose utilization from the normal pattern. Metabolism in
white matter was markedly increased. Many areas showed decreased
rates of metabolism. Columns were seen in the cerebral cortex, and
the caudate nucleus exhibited a strange lace-like heterogeneity quite
distinct from its normal homogeneity. Despite the widespread
changes, however, overall average glucose utilization remained un-
changed. These results are of relevance to the studies by Kety and
Schmidt (1948b), who found in man that the breathing of 10% O,
produced a wide variety of mental symptoms without altering the
average O, consumption of the brain as a whole. The mental
symptoms were probably the result of metabolic and functional
202 Energy Metabolism for Functional Localization in the Nervous System

changes in specific regions of the brain, detectable only by methods
like the deoxyglucose method that measure metabolic rate in the
structural components of the brain.

Normal Aging

Although, strictly speaking, aging is not a pathophysiological con-
dition, many of its behavioral consequences are directly attributable
to decrements in functions of the central nervous system (Birren et
al., 1963). Normal human aging has been found to be associated with
a decrease in average glucose utilization of the brain as a whole
(Sokoloff, 1966). Smith and coworkers (1980) have employed the
quantitative [!*C]deoxyglucose method to study normal aging in
Sprague-Dawley rats between 5 to 6 and 36 months of age. Their
results show widespread but not homogeneous reductions of local
cerebral glucose utilization with age. The sensory systems, partic-
ularly auditory and visual, are particularly severely affected. The
caudate nucleus is metabolically depressed, and preliminary experi-
ments indicate that it loses responsivity to dopamine agonists, such as
apomorphine, with age.* A striking effect was the loss of metabol-
ically active neuropil in the cerebral cortex; Layer 4 is markedly de-
creased in metabolic activity and extent. Some of these changes may
be related to specific functional disabilities that develop in old age.

MICROSCOPIC RESOLUTION

The resolution of the present ['*C]deoxyglucose method is at best
approximately 100 um. The use of [*H]deoxyglucose does not
greatly improve the resolution when the standard autoradiographic
procedure is used. The limiting factor is the diffusion and migration of
the water-soluble labeled compound in the tissue during the freezing
of the brain and the cutting of the brain sections. Des Rosiers and
Descarries (1978) have been working to extend the resolution of the
method to the light and electron microscopic levels. They use [3H]
deoxyglucose and dipping emulsion techniques. They have reported
that fixation, postfixation, dehydration, and embedding of the brain
by perfusion in situ results in negligible loss or migration of the label
in the tissue. They can localize grain counts over individual cells or
portions of them. Although the method is at present only qualitative,
it is likely that it can eventually be adopted for quantitative use. An
alternative promising approach to microscopic resolution is the use of
freeze-substitution techniques (Ornberg et al., 1979; Sejnowski et al.,
1980).

*C. Smith and J. McCulloch, unpublished observations.
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 203

THE ['®F] FLUORODEOXYGLUCOSE TECHNIQUE

Because the deoxyglucose method requires the measurement of local
concentrations of radioactivity in the individual components of the
brain, it cannot be applied, as originally designed, to man. Recent
developments in computerized emission tomography, however, have
made it possible to measure local concentrations of labeled com-
pounds in vivo in man. Emission tomography requires the use of
y-tadiation, preferably annihilation y-rays derived from positron
emission. A positron-emitting derivative of deoxyglucose, 2-[1!8F]
fluoro-2-deoxy-D-glucose has been synthesized and found to retain
the necessary biochemical properties of 2-deoxyglucose (Reivich et
al., 1979). The method has, therefore, been adapted for use in man
with ['8F]fluorodeoxyglucose and positron-emission tomography
(Reivich et al., 1979; Phelps et al., 1979). The resolution of the
method is still relatively limited, approximately 1 cm, but it is already
proving to be useful in studies of the human visual system (Figure 15:
see color insert) (Phelps et al., 1980) and of clinical conditions, such
as focal epilepsy (Figure 16: see color insert) (Kuhl et al., 1979, 1980).
This technique is of immense potential usefulness for studies of hu-
man local cerebral energy metabolism in normal states and in neurolog-
ical and psychiatric disorders.
204 Energy Metabolism for Functional Localization in the Nervous System

CONCLUDING REMARKS

The deoxyglucose method provides the means to determine quantita-
tively the rates of glucose utilization simultaneously in all structural
and functional components of the central nervous system and to dis-
play them pictorially superimposed on the anatomical structures in
which they occur. Because of the close relationship between local
functional activity and energy metabolism, the method makes it
possible to identify all structures with increased or decreased func-
tional activity in various physiological, pharmacological, and patho-
physiological states. The images provided by the method do resemble
histological sections of nervous tissue, and the method is, therefore,
sometimes misconstrued to be a neuroanatomical method and con-
trasted with physiological methods, such as electrophysiological re-
cording. This classification obscures the most significant and unique
feature of the method. The images are not of structure but of a
dynamic biochemical process, glucose utilization, which is as physio-
logical as electrical activity. In most situations, changes in functional
activity result in changes in energy metabolism, and the images can be
used to visualize and identify the sites of altered activity. The images
are, therefore, analogous to infra-red maps; they record quantitatively
the rates of a kinetic process and display them pictorially exactly
where they exist. The fact that they depict the anatomical structures
is fortuitous; it indicates that the rates of glucose utilization are dis-
tributed according to structure, and specific functions in the nervous
system are associated with specific anatomical structures. The deoxy-
glucose method represents, therefore, in a real sense, a new type of
encephalography, metabolic encephalography. At the very least, it
should serve as a valuable supplement to more conventional methods,
such as electroencephalography. Because, however, it provides a new
means to examine another aspect of function simultaneously in all
parts of the brain, it is hoped that it and its derivative, the [18F] fluoro-
deoxyglucose technique, will open new roads to the understanding
of how the brain works in health and disease.
Neurosciences Res. Prog. Bull., Vol. 19, No. 2 205

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