NEUROTRANSMITTERS SCIENTIFIC AMERICAN 230: 58-71, 1974 These chemicals released from nerve-fiber endings are the messengers by means of which nerve cells communicate. Neurotransmitters mediate functions ranging from muscle contraction to the control of behavior n 1901 the noted English physiologist J. N. Langley observed that the in- jection of an extract of the adrenal gland into an animal stimulated tissues innervated by the sympathetic nerves:” the nerves of the autonomic nervous sys- tem that increase the heart rate, raise the blood pressure and cause smooth mus- cles to contract. Just three years before that John J. Abel of Johns Hopkins Uni- versity had isolated the hormone adrena- line from the adrenal gland, and so Langley’s observation prompted T. R. Elliott, his student at the University of Cambridge, to inject adrenaline into ex- perimental animals. Elliott saw that the hormone, like the crude extract, pro- duced a response in a number of organs that was similar to the response evoked by the electrical stimulation of sympa- thetic nerves. He thereupon made the brilliant and germinal suggestion that adrenaline might be released from sym- pathetic nerves and then cause a re- sponse in muscle cells with which the nerves form junctions. Elliott thus first enunciated the concept of neural com- munication by means of chemical trans- mitters. A neurotransmitter is a chemical that is discharged from a nerve-fiber ending. It reaches and is recognized by a receptor on the surface of a postsynap- tic nerve cell or other excitable postjunc- tional cell and either stimulates or inhib- its the second cell. Today it is clear that many different neurotransmitters influ- ence a variety of tissues and physiologi- cal processes. Neurotransmitters make the heart beat faster or slower and make muscles contract or relax. They cause glands to synthesize hormone-producing enzymes or to secrete hormones. And they are the agents through which the brain regulates movement and changes mood and behavior. Elliott’s concept of chemical neuro- transmission was accepted slowly. Lang- by Julius Axelrod ley, who disliked theories of any kind, discouraged further speculation by El- liott until more facts were available. That took time. The first definite evidence for neurochemical transmission was ob- tained in 1921 by Otto Loewi, who was then working at the University of Graz in Austria, through an elegant and cru- cial experiment. Loewi put the heart of a frog in a bath in which the heart could be kept beating. The fluid bathing the heart was allowed to perfuse a second heart. When Loewi stimulated the first heart’s vagus nerve (a nerve of the para- sympathetic system that reduces the heart rate), the beat of the second heart was slowed, showing that some sub- stance was liberated from the stimulated vagus nerve, was transported by the fluid and influenced the perfused heart. The substance was later identified by Sir Henry Dale as acetylcholine, one of the first neurotransmitters to be recognized. In a similar experiment the stimulation of the accelerans nerve (the sympathetic nerve that increases the heart rate) of a frog heart speeded up the beat of an un- stimulated perfused heart. In 1946 the Swedish physiologist Ulf von Euler iso- lated the neurotransmitter of the sym- pathetic system and identified it as noradrenaline. The Transmitters To be classed as a neurotransmitter a chemical should fulfill a certain set of criteria. Nerves should have the enzymes required to produce the chemical; when nerves are stimulated, they should liber- ate the chemical, which should then react with a specific receptor on the post- junctional cell and produce a biological response; mechanisms should be avail- able to terminate the actions of the chemical rapidly. On the hasis of these criteria two compounds are now estab- Reprinted by the ” lished as neurotransmitters: acetylcho- line and noradrenaline. Nerves that con- tain them are respectively called cholin- ergic and noradrenergic nerves. There are a number of other nerve chemicals that meet many of the listed criteria but have not yet been shown to meet them all. These “putative” transmitters are dopamine, adrenaline, serotonin, octop- amine, histamine, gamma aminobutyric acid, glutamic acid, aspartic acid and glycine. This article will deal mainly with one class of neurotransmitters, the catechol- amines, since more is known about these compounds than about some other trans- mitters and since many of the principles governing their disposition appear to govern those of transmitters in general. The catecholamines include noradren- aline (also known as norepinephrine), dopamine and adrenaline (or epineph- rine). They have in common a chemical structure that consists of a benzene ring on which there are two adjacent hydrox- yl groups and an ethylamine side chain. Noradrenaline is present in peripheral nerves, the brain and the spinal cord and in the medulla, or inner core, of the adrenal gland. In peripheral tissues and in the brain noradrenaline acts as a neu- rotransmitter, that is, it exerts most of its effect locally on postjunctional cells. In the adrenal medulla it functions as a hormone, that is, it is released into the bloodstream and acts on distant target organs. Dopamine, once thought to be simply an intermediate in the synthesis of noradrenaline and adrenaline, is also a neurotransmitter in its own right in the brain, where it functions in nerves that influence movement and behavior. The third catecholamine, adrenaline, is large- ly concentrated in the adrenal medulla. It is discharged into the bloodstream in fear, anger or other stress and acts as a hormone on a number of organs, includ- 59 U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE National Institutes of Health ing the heart, the liver and the intestines. Just in the past year it has developed that adrenaline is probably also a neuro- transmitter, since it is found in nerves in the brain. Techniques developed a decade ago in Sweden made it possible to visualize catecholamines in neurons directly, by the fluorescent glow they emit after treatment with formaldehyde vapor. Fluorescence photomicrography, TERMINALS “A A fi \ a \ f Thee § f \ \Vanicosiry py SYMPATHETIC NEURON, or nerve cell (a), consists of a cell body, 5 long axon and nu- merous nerve terminals studded with varicosities. inzymes involved in the synthesis of the transmitters are made in the nucleus and transporied down the axen to the varicosities, where the transmitters are manufactured and stored. A varivosity sad its junction with an- other cell are enlarged (6). The transmitter (colored dots) is stered in vesicles. When the nerve is excited and depolarized, vesicles move to the cell membrane and fuse with it, re- leasing transmitter into the junctional gap. The transminer reaches receptors on the effector eell that recognize only it, net any other chemicals (black shapes) that are present in the gap. &0 gates as a wave of depolarization that moves along the nerve axon to the end- ings. As Bernhard Katz of University College London first showed tor acetyl- choline, the depolarization causes a quantum—a packet or spurt, as it were— of the transmitter to be discharged from the nerve ending into the synaptic cleft. Bicchemical evidence recently ob- tained in our laboratory and others shows that noradrenaline is released from aerves in much the same way. The vesicles in the endings contain not only noradrenaiine but also the enzyme, DBH, that converts dopamine into nor- adrenaline. When the sympathetic nerve is stimulated electrically, noradrenaline and the enzyme are released in about the same proportions in which they are pres- ent in the vesicles, The only way that could happen would be through the fu- sion of the vesicle with the outer mem- brane of the nerve, followed by the for- mation of an opening large enough to allow molecules of noradrenaline to be extruded along with the much larger molecules of the enzyme. Such a release mechanism is called exocytosis, The de- tailed events whereby the vesicle fuses with the neural membrane and makes an opening to discharge its soluble contents are uncertain, as is the subsequent fate of the vesicle. We do know that certain conditicns prevent the release of norad- renaline and DBH. One is the presence of vinblastin, a compound that breaks down the protein structures in nerve cells called neurotubules. Another is the presence of cytocholasin-beta, a sub- stance that disrupts the function of the contractile filament system in cells, A third is the absence of calcium, These findings suggest that the long, tubelike protein structures may orient the vesicles to @ site on the neuronal membrane from which the release occurs. It is well lnown that microfilaments in cells other than nerves, such as muscle cells, can be activated by calcium so that they con- tract. It is therefore possible that de- polarization causes calcium to activate a contractile filament on the neural mem- brane, which thereupon contracts to make 2n opening large enough so that the soluble contents of the vesicle can be discharged. The observation that DBH is released from nerves suggested to Richard Wein- shilboum, a research associate in my lab- oratory, that the enzyme might find its way into the bloodstream. We devised a sensitive assay for the enzyme and found it is indeed present in the blood, and we and others went on to measure the amount of ihe enzyme (which is found specifically in sympathetic nerves) in a SYMPATHETIC-NERVE TERMINALS from the iris of a rat’s eve minals, studded with varicositios where the noradrenaline is stored, emit a green glow after treatment with formaldehyde, showing that are enlarged 2.440 diameters in this fluerescence micrograph made they contain noradrenaline, one of the neurotransmitters. The ter- hs David Facubowite of the National Insiitute of Mental Health. VARICOSITIES on noradrenaline-containing nerve endings from crograph made by Floyd Bloom of the National Institute of Mental a rat pineal gland are enlarged 90,000 diameters in an electron mi- Health. The varicosities contain vesicles, many with dense cores. RADIOACTIVE NORADRENALINE is shown by radioautogra- developed silver grains (black blots) in a photographic film laid on phy to be localized in the vesicles. A pineal-tissue sample was tak- the sample. The developed grains were strikingly localized over the en from rats injected with labeled noradrenaline. The radioactivity vesicles, which were thus identified as sites of transmitter storage. él TYROSINE e CW OL e ® TYROSINE > HYDROXYLASE DOPA Q DOPA DECARBOXYLASE ——> | DOPAMINE DOPAMINE BETA-HYDROXYLASE 9 --—> (D8H; NORADRENALINE PHENYLETHANOL AMINE N-METHYLTRANSFERASE ——> (PNMT} ADRENALINE @ HYDAOGEN © OXYGEN @ nitRocen @ cAnBon 62 variety? of disease states. It is low in the hereditary disorder of the autonomic sys- ter called familial dysautonomia and in Down's syndrome (mongolism), and ib is high in torsion dystonia (a neurological disease involving muscle spasticity), in neuroblsstoma (a cancer of nervous tis- sue) and in certain fomns of hyperten- sion. The findings suggest that in each of these diseases there are abnormalities in the functioning of the sympathetic nervous system. Action and Inactivation Once the neurortansmitter is liberated it diffuses across the cleft between the nerve terminal and adjacent cells. The capacity of a neighboring effector cell to respond to the transmitter then depends on the ability of a receptor on the post- junctional cell’s surface to selectively recognize and combine with the aeuro- transmitter. Wheu the receptor and transmitter interact, a series of everts fs triggered that causes the elector cell ta carry out its special fimction. Some of these responses occur rapidly (in a frac- tion of a second), as in the propagation of verve transmission across a synapse; others occur slowly, in minutes or some- times hours, as in the synthesis of intra- cellular enzymes. There are two recep- tors that recognize noradrenaline, alpha and beta adrenergic receptors, and there is one for dopamine. These receptors can be distinguished from one another by the specific response each elicits and by the ability of specific drugs to block those responses. The beta adrenergic receptors turn on an effector cell, and ‘they do so by means of adenosine 3’5’ monophosphate, or cy- clic AMP, the universal “second mes- senger” that mediates between hormones and many cellular activities elicited by those hormones [see “Cyclic AMP,” by Tra Pastan; ScrenTiFIC AMERICAN, Au- gust, 1972]. Investigators have traced several of the steps in the activation of the receptor by noradrenaline by study- ing the interaction of nor adrenaline and fat cells or cells of the tiver or of the PRODUCTION of catecholamine transmit ters is accomplished by four enzymes teol- or) in sympathetic-nerve terminals and in the adrenal gland. Tyrosine, an amino acid, is transformed into the intermediate dopa. Removal of a carboxyl (COOH) group forms dopamine, which is itself a transmit- ter and is also the precursor of the trans- mitter noradrenaline. Ta the adrenal gland the process continnes with the addition of a methyl CCH.) group to form adrenaline. pineal gland. We have iowd the pineal, which makes a horus called melatonin that inhibits the activity of sex glands, perticularly suitaple. fins heavily supplied with nerves conteining norad- renaline [see “The Pineal Gland,” by Richard J, Wurtman and Julius Axelrod; Screnriptc Asenican, fuly, 1965]. Mel- atonin is synthesized in a number of steps, one of which, the conversion of serotonin to N- acetylserctonin, is cata- lyzed by the enzyme serotonin N-acetyl- eg ansferase, it is that eazyme’s synthesis that is controlled by the beta adrenergic receptor. When noradrenaline is released irom « nerve innervating the pineal, it interacts with beta adrenergic receptors on the outside of the membrane of a pineal cell, Once a receptor is occupied hy noradrenaline, the enzyme adenylate vclase, on the inner surface of the cell meinbrane, is activated. The adenylate cyclase thea converts the cellular energy carrier ATP to cyclic AMP, which in hurn stimulates che syathesis of serotonin N-ecetyltransferase [see illustration on vpposite page}. This complex series of eveuts can be turned off by propranolol, a drug that preveuts the noradrenaline from combining with the beta adrener- sic receptor. The adenylate cyclase system is in- volved in scores of biological actions. The ability of the pineal cell to carry out its special function, the manufacture of, melatonin, by utilizing the almost uni- versal adenylate-cyclase system depends on the presence of receptors on the cell surface that can specifically recog- nize noradrenaline and of the en- zyme hydroxyindole-O-methyltransfer- ase, uniquely present in the pineal cell, that can convert N-acetylserotonin to melatonin. Once the neurotransmitter has ne acted with the postjunctional cell, actions nuist be rapidly terminated; oth: erwise it would exert its effects for too long and precise contiol would be lost. In the cholinergic nervous system the acetylcholine is rapidly inactivated by the enzyme acetylcholinesterase, which metabolizes the transmitter. In the past 10 years it has become clear that the in- activation of neucotransmitters through wnzymatic transformation is the excep- tion rather than the rule, Catecholamine neurotransmitters are metabolized by two enzymes, catechol-O-methyltrans- ferase (COMT} and monoamine oxidase (MA), the latter is a particularly impor- tant enzyme that removes the amino (NH) group of a wide variety of com- pounds, including serotonin, noradrena- fine, dopamine and adrenaline. There are enzyme-inhibiting chemicals that 4 A EFFECTOR CELL \ CYCLIC AMP 3 | ) N-ACETYL~ [ TRANSFERASE Won | N-ACETYL- SEROTONIN SEROTONIN HIOMT ADENYLATE . CYCLASE ATP MELATONIN MODE OF ACTION of a transmitter is exemplified by the effect of noradrenaline on a pineal cell. Noradrenaline (colored dots) re- leased from a nerve ending binds to a beta-adrenergic receptor on the pineal-cell surface. The receptor thereupon activates the en- zyme adenylate cyclase on the inside of the pineal-cell membrane. The activated adenylate cyclase catalyzes the conversion of adeno- can prevent COMT and MAO from car- rying out their biochemical transforma- tions; when such inhibitors were ad- ministered, the action of noradrenaline was found still to be rapidly terminated. There had to be a method of rapid in- activation other than enzymatic trans- formation. In order to track down such a mecha- nism we injected a cat with radioactive noradrenaline. The labeled transmitter persisted in tissues that were rich in sym- pathetic nerves for many hours, long af- ter its physiological actions were ended, indicating that radioactive noradrenaline was taken up in the sympathetic nerves and held there. My colleagues and I de- signed a simple experiment to prove this. Sympathetic nerves innervating the left salivary gland in rats were destroyed by removing the superior cervical ganglion on the left side of the neck; about seven days after this operation the noradrena- line nerves of the salivary gland on the right side were intact, whereas the nerves on the left side had completely disappeared. When radioactive norad- renaline was injected, the transmitter was found in the right salivary gland but not in the left one. We also found that in cats injected with radioactive noradrenaline the transmitter was re- leased when the sympathetic nerves were stimulated electrically. The experi- ments clearly demonstrated that nor- adrenaline is taken up into, as well as re- leased from, sympathetic nerves. As a re- sult of these experiments we postulated that noradrenaline is rapidly inactivated through its recapture by the sympathetic nerves; once it is back in the nerves, of course, the neurotransmitter cannot exert its effect on postjunctional cells. Leslie L. Iversen of the University of Cam- bridge has since shown that this neuronal recapture by sympathetic nerves is high- ly selective for noradrenaline or com- pounds resembling it in chemical struc- ture. Recent work indicates that uptake by nerves may be the most general mech- anism for the inactivation of neurotrans- mitters. Regulation Chemical transmitters in sympathetic nerves (and presumably in other nerves) are in a state of flux, continually being synthesized, released, metabolized and recaptured, The activity of nerves can also undergo marked fluctuation during periods of stress. In spite of all these dynamic changes the amount of cate- cholamines in tissues remains constant. sine triphosphate (ATP) into cyclic adenosine monophosphate (AMP). The cyclic AMP stimulates synthesis of the enzyme N- acetyltransferase: the enzyme converts serotonin into N.acetyl- serotonin, This is transformed in turn by the pineal cell’s specific enzyme, hydroxyindole-O-methyltransferase (HIQMT), to form melatonin, the pineal-gland hormone that acts on the sex glands. This is owing to a variety of adaptive mechanisms that alter the formation, re- lease and response of catecholamines. There are fasi regulatory changes that require only fractions of a second and slower changes that take place after min- utés or even hours. When sympathetic nerves are stimu-’ lated, the conversion of tyrosine to nor- adrenaline in them is rapidly increased. The increased nervous activity specifi- cally affects tyrosine hydroxylase, the enzyme that converts tyrosine to dopa, because its activity is inhibited by nor- adrenaline and dopamine. Any increase in nerve-firing brought ou by stress, cold and certain drugs lowers the level of catecholamines in the nerve terminals. This reduces the negative-feedback ef- fect of noradrenaline and dopamine on tyrosine hydroxylase, so that more tyro- sine is converted to dopa, which in turn is converted to make more catechol- amines. Conversely, when nerve activity is decreased, the catecholamine level rises, slowing down the conversion of tyrosine to dopa by once again inhibiting the tyrosine hydroxylase. Another rapid regulation is accom- plished at the nerve terminal itself, where the alpha adrenergic receptors are situated. When the alpha receptors are 63 activated, they diminish the release of noradrenaline from nerve terminals into the synaptic cleft. When too much nor- adrenaline is released, it accumulates in the synaptic cleft; when the catechol- amine level is high enough, it activates the alpha receptors on the presynaptic nerve terminals and shuts off further release of the neurotransmitter, A slower regulatory process is brought on by prolonged firing of sympathetic nerves, which can step up the manu- facture of the catecholamine-synthesiz- ing enzymes tyrosine hydroxylase, DBH and (to a lesser extent) PNMT; the rise in the enzyme level enables the nerves to make more neurotransmitter. We dis- covered this phenomenon of increased enzyme synthesis when we gave animals reserpine, a versatile drug that lowers the blood pressure and incidentally in- creases sympathetic-nerve firing (which tends to raise the pressure) by a reflex action. The reserpine brought about a gradual increase in tyrosine hydroxylase and DBH in sympathetic nerves and the adrenal gland and of PNMT in the ad- renal gland. Increases in these enzymes were also found in animals exposed to stress, cold, physical restraint, psycho- social stimulation or insulin injection. When the synthesis of proteins was pre- vented by drugs, on the other hand, there was no elevation in enzyme activity after reserpine was given. This indicated that increased nerve activity stimulates the synthesis of new molecules of tyro- sine hydroxylase, DBH and PNMT; with a - greater need for neurotransmitters there is a compensatory increase in the synthesis of enzymes that catalyze the making of these transmitters. In order to learn whether the com- mand for increased synthesis of new tyrosine hydroxylase and DBH mole- cules can be transmitted from one nerve to another we cut the nerve innervating certain noradrenaline cell bodies—the superior cervical ganglia—on one side. When nerves were then stimulated re- flexly by reserpine, there was an eleva- tion of tyrosine hydroxylase and DBH levels in the innervated ganglia but not in the denervated ones. The experiment showed that one nerve can transmit in- formation to another nerve (presumably by means of a chemical signal) that causes the postsynaptic nerve to make new enzyme molecules, Sensitivity In 1855 the German physiologist J. L. Budge observed that when the nerves leading to a rabbit’s right eye were de- stroyed, the pupil of that eye became more dilated than the left pupil. The phenomenon was later explained by the American physiologist Walter B, Can- non, who postulated that as a result of denervation the effector cells somehow become more responsive. He called this effect the “law of denervation supersen- sitivity.” Subsequent work showed that denervation supersensitivity is caused by two separate mechanisms, one pre- synaptic and the other postsynaptic. When nerves are destroyed, presynaptic inactivation by recapture is abolished, thereby leaving the neurotransmitters to react with the postsynaptic site longer. Denervation also causes a profound change in the degree of activity of the postjunctional cell. Recent work with the pineal gland in our laboratory has sug- gested a hypothesis for supersensitivity, and also for subsensitivity, in postjunc- tional cells. As we have seen, noradrena- line stimulates the synthesis of the enzyme serotonin N-acetyltransferase through a beta adrenergic receptor in the postjunctional pineal cell. When the nerves to pineal cells are destroyed (or depleted of noradrenaline by the admin- istration of reserpine), the pineal cells become 10 times as responsive to nor- adrenaline; that is, when the postjunc- tional cell is deprived of its neurotrans- mitter for a period of time, it takes just RELEASE AND INACTIVATION of the neurotransmitter norad- renaline is shown in more detail. The enzyme DBH is stored in the noradrenergic nerve terminals along with the transmitter and is re- leased with it into the junctional gap. The noradrenaline binds to the receptors on the effector cell, eliciting that cell’s response as shown in the illustration on the preceding page. Then the norad- 64 renaline’s action is terminated either through metabolism (1) by the enzymes catechol-O-methyltransferase (COMT) and/or mono- amine oxidase (MAO), or by recapture and storage (2) in the presynaptic sympathetic-nerve terminal; the latter is the more important process. MAO, stored in the membrane of mitochon- dria, also inactivates noradrenaline that leaks out of vesicles (3). one packet of catecholamines to cause the same increase of N-acetyltransferase in the cell as 10 packeis of transmitters would cause in a nurmally innervated cell. If, on the other hand, the pineal ceil is exposed to an excessive amount of catecholamines for a period of time, it lhecomes less responsive: a larger amount of ihe transmitter is required to produce ihe same increase in N-acetyltransferase. These experiments suggest that changes in the responsiveness of excit- alle cells are the result of an alteration in the “avidity” with which the receptor binds the neurotransmitters, If the re- ceptor is exposed to small amounts of catecholamine for some time, it reacts with the neurotransmitter easily; if too many neurotransmitter molecules bom- bard the receptor, it becomes less re- sponsive. Depending on the tissue, this change in sensitivity can come within hours or days, so that it is an effective adaptive mechanism for excitable cells. It is possible that the tolerance that is often developed to a drug taken in ex- cess may reflect subsensitivity on the part of cells that respond to the drug. Role in the Brain The brain has billions of nerves that talk to one another by means of neuro- transmitters. Neurobiologists are just be- ginning to unravel the complex biochem- istry and physiology of chemical trans- thission in the brain. Many different neu- rotransmitters function in brain neurons, but because there are more precise meth- ods of measuring catecholamines and drugs are available that perturb their for- mation, storage, release and metabolism, we know more about brain catechol- amines than about the other neurotrans- mitters, Fluorescence photomicrography and drugs that selectively destroy cate- cholamine-containing nerves have made it possible to locate the noradrenaline, dopamine and serotonin cell bodies and trace the pathways of their axons and nerve endings [sez illustrations on page 71]. The cell bodies of the dopamine- containing nerves are in the area of the brain. stem called the substantia nigra, whence. the dopaminergic axons course through the brain stem, many of them terminating in the caudate nucleus. The dopamine-containing tracts in the cau- date nucleus play an important role in the integration of movement. The elucidation of the ‘biochemistry and pharmacology of dopamine in the brain has led to the developinent of a powerful treatment of a crippling dis- ease, Parkinsonism. The Swedish phar- macologist Arvid Carlsson noted in 1959 NORADRENALINE TYROSINE HYDROX~ YLASE - RAPID REGULATION of catecholamine synthesis is aceoraplished by a feedback mecha- nism: a buildup of dopamine and. novadrenaline inhibits (colored orrows) the activity of tyrosine hydroxylase, which catalyzes the first step ja the synthesis. Au increase in nerve activily reduces the amount of dopamine and noradrenaline in the terminal, removing the inhibition; tyrosine hydroxylase activity increases and more transmitter is synthesized. that when reserpine was given to rats, it sharply reduced the dopamine content of the caudate nucleus in the brain and also caused a Parkinson-like tremor. The ad- ministration of dopa, a dopamine pre- cursor that can get from the blood into the brain more easily: than dopamine, reversed the tremors, ‘These findings prompted Oleh Hornykiewicz, who was then working at the University of Vien- na, to measure the content of dopamine in the brain of patients who had died of Parkinson’s disease. He found that there was virtually no dopamine in the caudate nucleus. The finding led directly to 2 major therapeutic advance by George C. Cotzias of the Brookhaven National Lab- oratory: when dopa, the dopamine pre- cursor that can cross the blood-brain bar- rier, is administered, it makes up the dopamine deficiency and effectively re- lieves the symptoms of Parkinson's dis- ease. This is a good example of how basic research can sometimes lead rapidly to a new treatment for a disease. There are two main nerve tracts con- taining noradrenaline in the brain, the dorsal and ventral pathways. The cell bodies of the noradrenaline-containing tracts are found in the lower part of the brain in the area called the locus cerule- us, or “blue place.” Noradrenaline-con- taming nerve tracts are highly branched and reach many parts of the brain. Among the areas they innervate are the cerebellum and the cerebral cortex, which are concerned with the fine co- ordination of movement, alertness and motion. Another part of the brain in- nervated by noradrenaline neurons is the hypothalamus, which controls many vis- ceral functions of the body such as hun- ger, thirst, temperature regulation, blood pressure, reproduction and behavior. Manipulation of the noradrenaline levels in the brain can change many of the functions of the hypothalamus, particu- larly the “pleasure” centers. Noradrena- line tracts also appear to be involved in mood elevation and depression. Recent- ly nerves containing adrenaline have also been observed in the brain stem. The next few years should show whether these adrenaline-containing tracts also control emotion, mood and behavior. Drugs have been powerful tools for probing the action of neurotransmitters. As our knowledge concerning neuro- transmitters has broadened, so has our understanding of the action of drugs on behavior and on the cardiovascular and motor systems; the two trends have in- teracted nicely. In the early 1950's phar- macologists recognized that the hallu- cinogenie agent lysergic acid diethyla- mine (LSD) not only resembles serotonin 65 in chemical structure but also counter- acts some of its pharmacologic actions (by occupying sites intended for seroto- nin), Several workers therefore proposed that serotonin must have something to do with insanity. Other hallucinogenic agents such as mescaline and ampheta- mine, on the other hand, are related in structure to noradrenaline, In the rmid- 1950's clinical investigators were learn- ing that chemicals such as chlorproma- zine could mitigate psychotic behavicr, and that monoamine oxidase inhibitors and imipramine and related drugs could relieve depression, At about the same time it was observed that reserpine, which was proving valuable not only for hypertension but also for schizophrenia, markedly reduced the levels of norad- yenaline and serotonin in the brain. The observations combined to suggest that these drugs exerted their actions on the brain by interfering wjth neurotrans- mitters. When my colleagues and I found that radioactive noradrenaline can be taken up and released from nerves, we were in a good position to investigate how a drug influences the disposition of injected radioactive transmitters. Effect of Drugs The first compound we examined was cocaine, a potent stimulant that can pro- duce psychosis and that also intensifies the action of noradrenaline. When radio- active noradrenaline was injected into cats that had been given cocaine, the b RELEASE RECAPTURE METABOLISM uptake of catecholamines by the sym- pathetic nerves was prevented, demon- strating that cocaine magnifies the effect of noradrenaline by preventing its cap- ture and inactivation and leaving larger amounts of the catecholamine to react with the effector cell. Antidepressant drugs such as imipramine had the same effect: they blocked the uptake of noradrenaline into sympathetic nerves. By using radioactive noradrenaline we found that amphetamine, which is both a stimulant and a mind-altering drug, af- fects noradrenergic nerves in two ways: it blocks the uptake of noradrenaline and also promotes the release of the neuro- transmitter from nerves. Many drugs that are effective in the treatment of hypertension affect the RESPONSE RESPONSE RESPONSE RESPONSE CERTAIN ADRENERGIC DRUGS increase or decrease the avail- ability of noradrenaline at the adrenergic receptor. Normal release, recapture and metabolism (colored arrows) are illustrated, with a curve representing the normal response of a postjunctional cell (a). Antidepressant drugs enlarge that response in several ways, all of which increase the availability of noradrenaline at the syn- apse. Amphetamine does so by promoting the release of noradren- &6 aline (b). Amphetamine and imipramine and related drugs block recapture (c); the monoamine-oxidase inhibitors interfere with in- activation through metabolism (d). Conversely, reserpine, which reduces blood pressure and may induce depression, reduces the re- sponse by depleting the noradrenaline in storage (e); alpha- methyldopa and other “false transmitters” are stored in the vesicles with noradrenaline and released with it, diluting its effect (f). storage and release of noradrenergic transmitters. Reserpine and guanethi- dine reduce blood pressure by prevent- ing the nerves that raise the pressure from storing noradrenaline. Antihyper- tensive drugs such as alpha methyl! dopa, on the other hand, are transformed by enzymes in the nerve into substances that resemble the noradrenaline chem- ically. The “false transmitters” are stored and released along with natural neuro- transmitters, diluting them and thus re- ducing their effect. In the past 10 years many psychia- trists and pharmacologists have been struck by the fact that drugs that relieve mental depression also interfere with the uptake, storage, release or metabolism of noradrenaline. Whereas imipramine blocks the uptake of noradrenaline by nerves and amphetamine both releases noradrenaline and blocks its uptake, monoamine oxidase inhibitors, as their name implies, prevent the metabolism of the catecholamine, In other words, all these antidepressants produce similar re- sults by different mechanisms: they in- crease the amount of catecholamine in the synaptic cleft, with the result that more transmitter is available to stimulate the receptor. Conversely, reserpine, a compound that decreases the amount of the chemical transmitters, sometimes produces depression. These considera- tions led to the proposal of a catechol- amine hypothesis cf depressive states, which holds that mental depression is associated with the decreased availabil- ity of brain catecholamine and is relieved by drugs that increase the amount of these transmitters at the adrenergic re- ceptor, Although the hypothesis is not yet entirely substantiated, it has pro- vided a valuable framework within which new approaches to understanding depression can be sought. The introduction in the 1950’s of antipsychotic drugs such as chlorproma- zine and haloperidol revolutionized the treatment of schizophrenia, dramatically reducing the stay of schizophrenics in mental hospitals and saving many bil- lions of dollars in hospital care. Research in the past decade has shown that anti- psychotic drugs also exert their effect on the catecholamine neurotransmitters. Carlsson had observed that antischizo- phrenic drugs caused an increase in the formation of catecholamines in the rat brain, and he formed the hypothesis that this was owing to the drug’s ability to block dopamine receptors. Work by other investigators has confirmed and extended this hypothesis. Antipsychotic drugs do block dopamine receptors in the brain, and there is a strong associa- m\\\\\ L b CSS = aren CAUDATE NUCLEUS _—> NUCLEUS ACCOMBENS OLFACTORY TUBERCLE — SUBSTANTIA NIGRA MEDIAN EMINENCE DOPAMINE TRACTS, the main bundles of nerves containing dopamine, are shown (black) in a drawing of a longitudinal section along the midline of the rat brain. The cell bodies are concentrated in the substantia nigra, and the axons project primarily to the caudate nucleus. A dopamine deficiency in that region causes Parkinsonism, which can be treated with dopa. CEREBRAL CORTEX CEREBELLUM OLFACTORY BULB HYPOTHALAMUS VENTRAL BUNOLE NORADRENALINE TRACTS arise primarily in the locus ceruleus and reach many brain centers, including the cerebellum, cerebral cortex, and hypothalamus. Illustrations on this page are based on maps made by Urban Ungerstedt of Royal Caroline Institute in Sweden. tion between the blocking ability of various drugs and their capacity to re- lieve schizophrenic symptoms. These findings point clearly to the imvolve- ment of the dopaminergic nerves in schizophrenia. Amphetamine has also helped to clari- fy the nature of schizophrenia. Taken repeatedly in large amounts, ampheta- mine produces a psychosis manifested by repetitive and compulsive behavior and hallucinogenic delusions that are indistinguishable from the symptoms ex- hibited by paranoid schizophrenics, Am- phetamine releases catecholamines from nerves in the brain to stimulate both nor- adrenergic and dopamine receptors. Af- ter doing experiments with two forms of amphetamine Solomon H. Snyder of the Johns Hopkins University Medical School hypothesized that the schizo- phrenia-like psychosis the drug induces is due to excessive release of dopamine. The ability of antischizophrenia drugs, which block dopamine receptors, to re- lieve symptoms of amphetamine psycho- sis is consistent with this hypothesis, Although there have been rapid ad- vances in our knowledge of neurotrans- mitters in the past 20 years, much re- mains to be discovered about these com- pounds. Onlv a few of the chemical transmitters of the brain neurons have even been characterized. The role of neurotransmitters in behavior, mood, re- production and learning and in diseases such as depression, schizophrenia, motor disorders and hypertension is beginning to evolve. If only the present trend toward reducing the funds committed to research support can be reversed, ex- citing new discoveries about neurotrans- mitters should soon be made, many of which will surely contribute directly toward the treatment or cure of some of man’s most tragic afflictions,