Neurohormones And Behavior - Chemical Bases Of Behavior
serotonin norepinephrine brain epinephrine oxidase monoamine effects nervous activity central
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True hormones are substances secreted by endocrine glands and carried to other parts of the body by the blood where they have profound metabolic effects. The secretions of the posterior pituitary are instances of true hormones secreted by nervous tissue (neurohormones). Recently, the concept of neurohormones, or neurohumors, has been extended to include substances which are chemical mediators of nerve impulses (e.g., transmission at the synapse) or which regulate nervous transmission in some way. These substances are of interest to psychiatry because they probably play a vital role in the activity of the central nervous system and, hence, in behavior. In addition, evidence is accumulating that they may participate in some way in the etiology, pathogenesis, or manifestations of some forms of mental disorder. Behavioral aspects of four of these substances will be briefly discussed: serotonin, norepinephrine, epinephrine, and acetylcholine.
SEROTONIN Serotonin (5-hydroxytryptamine) is a potent vasoconstrictorsuggests it plays a role in brain function. Also, 5-hydroxytrypNEUROHORMONES-CHEMICAL-BASES Fig. 2. Metabolism of serotonin. Reserpine acts at A to free serotonin. Monoamine oxidase inhibitors, such as iproniazid, act at B to impede the destruction of serotonin. (The aldehyde intermediate between serotonin and 5-hydroxyindoleacetic acid is not shown.) tophan decarboxylase and monoamine oxidase, the enzymes which make and destroy it (Fig. 2), have a distribution in brain similar to that of serotonin (9, 28). More direct evidence for its role in behavior is provided by the observation that the intravenous administration of 5-hydroxytryptophan causes marked excitement and autonomic disturbances in dogs and other experimental animals (69).
Some evidence for the action of serotonin at the cellular level is given by Woolley and Shaw (75), who studied the effects of serotonin on human and rat brain tissue cultures in vitro. Oligodendroglial cells, one of the connective tissue elements of nervous tissue, are normally seen to pulsate rhythmically. Serotonin causes these cells to contract strongly; the addition of an antimetabolite of serotonin (e.g., medmain) counteracts these tetanic contractions and normal pulsations of the cells return. Woolley and Shaw speculate that the pulsating oligodendroglia might serve as stirring devices, facilitating the circulation of extravascular fluids and thus insuring adequate supply of oxygen and nutrients to the nervous elements and removal of waste products. If the pulsations are brought about normally by the periodic release of serotonin and its enzymatic destruction, an excess of serotonin would cause prolonged contraction. A deficiency of serotonin, or an excess of an antimetabolite of serotonin, would be associated with prolonged relaxation. In either case, the slowed pulsation of oligodendroglial cells might be followed by an anoxia with resulting changes in mental functions.
Additional light is thrown on the role of serotonin in behavior by considering its relationship to the enzyme, monoamine oxidase. This enzyme oxidizes serotonin to 5-hydroxyindole acetic acid which appears in the urine (Fig. 2). Recently, a class of drugs have come into use in medicine for the treatment of depression which, among their other pharmacologic properties, are inhibitors of monoamine oxidase. This suggests the possibility that these monoamine oxidase inhibitors acquire their anti-depressant properties by bringing about an increase in brain serotonin. Indeed, a marked increase in brain serotonin follows 5-hydroxytryptophan administration in animals pretreated with a mono-amine oxidase inhibitor (10).
Additional evidence for the role of the serotonin-monoamine oxidase system is provided by recent investigations of Aprison and Ferster (1, 2). They studied the behavioral effects of 5- hydroxytryptophan by operant conditioning technics (see Section III). The behavioral effects of 5-hydroxytryptophan were found to be enhanced if the animals were given iproniazid, a monoamine oxidase inhibitor, beforehand. Furthermore, the return of the behavior under study to baseline levels of performance had the same time course as the return of brain monoamine oxidase activity to normal (1). This strongly suggests that the marked behavioral changes which follow the administration of 5-hydroxytryptophan in combination with iproniazid are due to an increase of free serotonin in the brain. Most recently Aprison and his associates have given 5-hydroxytryptophan alone and shown that the amount of free serotonin in the diencephalon and telencephalon correlates with the behavioral effects of the drug (3).
It must be remembered, however, that monoamine oxidases are involved in the metabolism of other biologically active amines found in the central nervous system, such as norepinephrine. It is possible that the antidepressant properties of monoamine oxidase inhibitors are the result of interferences with the latter systems. Also, there are potent and effective antidepressants in clinical use that have no monoamine oxidase inhibiting properties at all.
Still another clue to the action of serotonin is provided by investigations with reserpine, a potent tranquilizer and hypotensive agent much used in medicine (Fig. 3). Brain serotonin exists largely in a bound form which renders it biologically inactive and prevents it from being destroyed by the action of the enzyme, monoamine oxidase. Reserpine appears to exert its central action by affecting the serotonin binding sites for a prolonged period (63). The free serotonin released by the action of reserpine is rapidly metabolized (Fig. 2), bringing about an over-all reduction in free serotonin. The low concentration of free serotonin that persists apparently represents a balance between new serotonin being synthesized in the body and the destruction by monoamine oxidase of the serotonin freed from binding sites. The level returns to normal when the binding sites recover or new sites are formed. The transient stimulation or excitement sometimes seen when reserpine is first given may represent a period of free serotonin excess, the hormone having been unbound but not yet metabolized. The tranquilization characteristic of reserpine usually begins several days after the drug is started. The clinical effects of the drug persist, however, several days after the drug is discontinued, presumably corresponding to the time required for serotonin binding sites to return to normal. In this interpretation, as in the case of monoamine oxidase inhibition discussed above, free serotonin is associated with stimulating effect and serotonin deficiency with depressant or tranquilizing effects.
Additional interest in serotonin derives from its possible relationship to certain psychotomimetics. These are substances which produce psychotic-like aberrations of mental function such as hallucinations, feelings of unreality, and paranoid thinking. Many of these substances, such as lysergic acid diethylamide (LSD), contain an indole nucleus and have some structural similarity to serotonin (Fig. 3). Woolley and Shaw (74) made the interesting hypothesis that the psychotomimetic properties of LSD might be due to blockage of serotonin from its normal receptor sites. In other words, the structural similarity of LSD to serotonin might be sufficient for LSD to occupy the normal receptor sites of serotonin but enough dissimilar that LSD does not perform the normal physiologic functions of serotonin. Thus, mental aberrations might be associated with insufficient serotonin at normal receptor sites brought about by other substances, at present unknown, with some structural similarity to serotonin. This hypothesis was suggested by the observation that serotonin and LSD, as well as some other psychotomimetics, do act as anti-metabolites in certain experimental situations. Thus, for example, LSD has been shown to inhibit the accelerator nerve of the heart of certain clams, the Venus mercenceria, for which serotonin is the chemical mediator (71). An antagonism has been shown between LSD and serotonin in isolated tissue preparations such as rat uterus and carotid artery smooth muscle (27).
However, LSD has been found to have effects similar to serotonin in other test systems (75). For example, the action of LSD 7 Fig. 3. Psychotomimetics (mescaline, bufotenine, psilocybin, and LSD) and a tranquilizer (reserpine) with powerful behavioral effects. Two structural nuclei important in biochemical aspects of behavior, catechol and indole, are shown also.
on rat and human brain tissue studied in vitro is similar to that of serotonin: sustained contractions of the oligodendroglia (75). Such observations suggest that mental aberrations might be related to excess of serotonin rather than a deficiency. In fact, there are proponents of both the excess and deficiency theories of the role of serotonin in mental abnormalities. One further observation might be mentioned here. Costa (17) reported that LSD can either increase or decrease the effects of serotonin, depending on its concentration. It may well be, as Himwich has suggested (36), that an excess might be associated with some forms of mental aberration and a deficiency with others.
Norepinephrine, like epinephrine, is found peripherally in the adrenal medulla and in adrenergic nerves. It mediates postganglionic sympathetic nerve transmission to effector organs. Its role in the central nervous system is less clear, but it is present in large concentrations in the hypothalamus (70), a structure much concerned with autonomic effects (see Chapter 3). This suggests the possibility that norepinephrine is a chemical mediator of central nervous activity (70).
The paucity of behavior effects on administering norepinephrine to experimental animals is probably related to its poor penetration of the blood-brain barrier. However, the administration of larger doses of this amine to insure that more than a negligible amount enters the brain does produce restless and excited behavior, possibly due to action on central sympathetic centers (14).
Brain norepinephrine, like serotonin, exists chiefly in a biologically inactive bound form. Like serotonin, it is released in active free form by reserpine and rapidly metabolized by mono-amine oxidase. Clinical antidepressants, such as iproniazid, which are monoamine oxidase inhibitors increase the level of brain norepinephrine as they do serotonin (37). Finally, norepinephrine has a distribution in brain similar to that of serotonin. This raises the question as to whether the antidepressant properties of monoamine oxidase inhibitors and tranquilizing properties of reserpine are due to changes in brain levels of free norepinephrine rather than changes in serotonin. This question remains unsettled (67).
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Additional interest in the role of norepinephrine in behavior has been aroused by its possible relationship to psychosis. Hoffer and his associates (39) observed that epinephrine left exposed to -sunlight turned pink and developed psychotomimetic properties. They believed "pink epinephrine" to contain adrenochrome, an unstable oxidative product of epinephrine and norepinephrine, 'believed by the Hoffer group to have psychotomimetic properties. Adrenolutin, a further product of oxidative metabolism of nor-epinephrine, is also psychotomimetic (Fig. 4). Note that these compounds contain the now familiar indole nucleus seen in serotonin and LSD, as well as reserpine. Again we have the suggestion that compounds containing indole are of importance in the chemistry of behavior, and especially the chemistry of abnormal behavior. Although there is not general agreement as to the psychotomimetic properties of these unstable compounds, the observations of Hoffer and others suggest the possible in7
Fig. 4. The structural formulas of epinephrine and norepinephrine, both catechol amines, have been drawn in a manner to emphasize their similarity to indolic compounds. Adrenochrome and adrenolutin, metabolic products of epinephrine and norepinephrine, are believed to be psychotomimetics.
volvement of epinephrine and norepinephrine metabolism in certain abnormal mental states. It may be that abnormal quantities of endogenous metabolites of brain norepinephrine, such as adrenochrome, adrenolutin, or some related substance, are involved in the etiology or pathogenesis of some forms of psychosis. It should be pointed out, however, that not all indolic substances induce abnormal mental states and that some very potent psychotomimetics have quite different structures.
The injection of epinephrine into human subjects or animals. produces motor restlessness, irritability, and evidence of general sympathetic stimulation. Subjects often report unaccountable feelings of anxiety. These changes are probably due to release of large amounts of epinephrine from the adrenal medulla, a small quantity of which may enter the brain. However, these effects are probably mediated peripherally. Epinephrine, like norepinephrine and serotonin, is released from an inactive bound form by reserpine and is rapidly destroyed by monoamine oxidase. Accordingly, some of the same arguments made for the participation of norepinephrine and serotonin in mental changes. might be made for epinephrine as well. However, epinephrine-is not normally present in the brain in significant quantities. and it is likely to be less important than norepinephrine, a metabolite occurring in the degradation of epinephrine.
The proportion of epinephrine and norepinephrine released by the adrenal medulla in response to a physical or psychologic stress appears to vary from species to species, among individuals of the same species, and in the same individual under different circumstances. Furthermore, these differences appear to be correlated with differences in emotional behavior. It has been shown that aggressive, attacking animals secrete excessive norepinephrine and that timid, retreating animals secrete excessive epinephrine (29). Studies with human subjects suggest an analogous correlation between the proportion of the two catechol amines secreted and different emotional states. Persons who tend to react to stress.
or threat with "anger directed outward," i.e., with aggressive behavior, show physiologic reactions indicative of norepinephrine excess. Those who tend to react in a similar situation with "anger directed inward," i.e., with anxiety, show physiologic reactions indicative of epinephrine excess (26, 65).
These investigations raise the question as to whether the different affects occur first in time and determine the proportion of catechol amines secreted by the adrenal medulla, or vice versa. Electrical stimulation of different brain areas in the hypothalamus has been found to release epinephrine and norepinephrine selectively (23), suggesting the affect-first chain of events. On the other hand, continuous infusion of human subjects with epinephrine is reported to produce the characteristic physiologic concomitants of anxiety, as well as the subjective experience of anxiety (33). These data suggest the possibility that the epinephrine released in a stressful situation mediates the arousal of anxiety in some way. Although continuous infusion of norepinephrine was reported to produce physiologic changes characteristic of "anger directed outward," no subjective experience of anger or behavior suggestive of anger was seen. The situation here may be like that in so many biologic systems: continuous interaction of biochemical and neural elements in an intricate system of checks and balances.
ACh is the chemical mediator of nerve impulses at the neuromuscular synapse of skeletal muscle and the preganglionic synapse of autonomic ganglia. Its possible role as a chemical mediator in the central nervous system and its importance in nerve conduction per se are less well settled questions, but there is positive evidence on both counts (49).
The metabolism of ACh in nervous tissue is outlined in Figure 5. Choline is acetylated by choline acetylase in the presence of coenzyme A and stored in a bound form which is biologically inactive. On appropriate stimulation, e.g., a nerve impulse, a small quantity of biologically active free ACh is released and reacts at the receptor site to form an acetylcholine-receptor complex which mediates the effector activity. The receptor site is 7 cleared of excessive ACh almost immediately by cholinesterase which hydrolyzes the molecule to acetate and choline.
Normally there is an overabundance of cholinesterase. This is probably a biologic safety factor to insure that some is always present. In its absence, neuronal transmission will essentially stop due to the continuous presence of unhydrolyzed acetylcholine.
The considerable evidence that ACh is a central neurohormone has been summarized by Burgen and MacIntosh (15).
ACh and the enzymes necessary for its synthesis and degradation, choline acetylase and cholinesterase, are present in the central nervous tissue, especially in nuclear areas, i.e., areas of high neuron density.
Experimentally, central neurons are very sensitive to ACh (largely excitatory).
Cholinesterase inhibitors produce marked central effects similar to those seen with ACh, suggesting the accumulation of naturally liberated ACh in the brain.
The ACh content of brain varies with the over-all level of nervous activity, being greatest during anesthesia and sleep and smallest during convulsions and emotional excitement.
Neuronal activity is associated with the release of ACh. For example, elevated ACh in the cerebrospinal fluid after seizures suggests release of ACh at the brain surface.
Since ACh seems important in the activity of the neurons of the central nervous tissue, and hence in behavior generally, efforts have been made to correlate changes in activity of this neurohormohe with changes in behavior. This has usually been done by administering ACh to increase its level in brain or to administer anticholinesterases which inhibit the enzyme, cholinesterase, which normally prevents ACh accumulation. The level of ACh may be reduced by giving cholinesterase (Fig. 5). Behavioral correlates of increased or decreased ACh may then be observed.
Profound behavioral changes - reduced motor activity and unresponsiveness to external stimuli - do follow the intraventricular injections of ACh in the cat (48). Cholinesterase has been injected into the ventricles of human patients suffering from chronic psychotic states such as catatonia with behavioral changes occurring within an hour or two. For example, previously unresponsive patients became communicative (19).
The problem has also been approached from the other direction; that is, specific behaviors have been generated and measured in experimental animals by manipulating their environments and the levels of ACh assayed afterwards. Exemplary of this approach are recent studies of Rosenzweig, Krech, and Bennett (61). Individual differences in problem-solving behavior of rats, as solving a maze to secure a food reward, were correlated with levels of cholinesterase activity at the cortex. Some of these results have been difficult to confirm, however.
It has been suggested that ACh also may play a role in psychosis. Whereas psychotomimetics, such as LSD and mescaline, and the neurohormone, serotonin, have an inhibitory effect on synaptic activity, ACh has an enhancing or stimulatory effect.
Thus, disturbances in ACh activity may affect central neural activity and produce mental disturbances (45). There is some indirect evidence for this hypothesis. For example, Pfeiffer and Jenney (60) report that cholinergic agents given in sufficient quantity sometimes produce remissions in schizophrenia. If psychosis in these individuals is somehow related to ACh deficiency, agents that could enable the body to produce more ACh might counteract the process. At least one agent reported to be useful in the treatment of schizophrenia is, in fact, a possible precursor of ACh (Deanol Acetamido-benzoate).
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Neurohormones And Behavior - Chemical Bases Of Behavior