Serotonin may set the tone of brain activity in relationship to the state of behavioral arousal/activity

Serotonin has been implicated in practically every type of behavior, such as appetitive, emotional, motor, cognitive and autonomic. However, from a physiological perspective, it is not clear whether 5-HT affects such behaviors specifically or more generally by coordinating the activity of the nervous system, particularly to set the tone of activity in conjunction with the amount of arousal.

The primary body of data that has contributed to the view that 5-HT has a general effect on behavior by modulating the tone of nervous system activity comes from studies of the firing rate of serotonergic soma in raphe nuclei [29]. Under quiet waking conditions, serotonergic neurons display a slow, clock-like activity of about 1 to 5 spikes/sec, which shows a gradual decline as the animal becomes drowsy and enters slow-wave sleep. A decrease in the regularity of firing accompanies this overall slowing of neuronal activity. During rapid eye movement (REM) sleep, the activity of these neurons becomes silent. In response to certain types of arousing stimuli, the firing rate of these serotonergic neurons increases. Not surprisingly, such data led to the idea that the activity of serotonergic neurons is related to the level of behavioral arousal/activity. Such data also have contributed to the idea that the activity of serotonergic neurons is associated with motor output since atonia of the major skeletal muscle groups occurs during REM sleep. Also oral—buccal motor activity, such as chewing, biting, licking or grooming, causes a marked increase in the firing rate of a subgroup of serotonergic soma that are also activated by somatosensory stimuli applied to the head, neck and face. However, exposing a cat to environmental stressors, such as a loud noise or seeing a dog, although producing strong sympathetic activation and typical behavioral responses, does not alter the firing rate of serotonergic neurons. Thus, the type of motor activity that activates serotonergic soma seems to be repetitive, like that mediated by central pattern generators. Furthermore, activation of serotonergic transmission inhibits information processing in afferent systems. From all such data, it has been suggested that the serotonergic neuronal system functions at the organismic level to integrate functions needed for behavioral output, that is, facilitation of motor output with suppression of activity in sensory systems irrelevant to the ongoing behavior.

Serotonin appears to be involved in a wide variety of physiological functions and behaviors, such as eating, sleep, circadian rhythmicity and neuroendocrine function

Perturbation of the 5-HT system by different types of drugs can elicit alterations in behaviors. Drugs affecting serotonergic neurons and their receptors are used to treat diseases such as depression, anxiety disorders and schizophrenia. In part because of this, 5-HT also has been speculated to be involved specifically in the regulation of all types of behaviors and physiological processes. The possible involvement of 5-HT in three areas, neuroendocrine function, circadian rhythms and feeding behavior, will be highlighted for illustrative purposes.

The hypothalamus secretes several releasing factors and release-inhibiting factors to control the secretion of hormones from the anterior pituitary gland. Serotonin is among the many neurotransmitters that participate in the hypothalamic control of pituitary secretion, particularly in the regulation of adrenocorticotropin (ACTH), prolactin and growth hormone secretion. A direct synaptic connection between serotonergic terminals and corticotropin-releasing hormone (CRH)-containing neurons in the paraventricular nucleus of the hypothalamus has been described. Precursors of 5-HT or drugs that enhance the effect of 5-HT increase CRH in portal blood and ACTH in plasma. In addition to effects at the hypothalamus, 5-HT may have direct effects on the anterior pituitary to stimulate the release of ACTH and at the level of the adrenal cortex to regulate release of corticosterone or cortisol. Actions of serotonin on 5-HT_1A, 5-HT₂, 5-HT₃ and 5-HT₄ receptors seem to be involved in these effects on the hypothalamic—pituitary—adrenal axis [30]. However, what role, if any, is played by serotonin in regulating stress-induced elevations of CRH or the circadian periodicity of the hypothalamic—pituitary—adrenal axis is unclear.

Measurement of these endocrine responses after administration of drugs that increase brain serotonin function provides one of the few methods currently available for assessing such function in humans. Precursors of 5-HT, releasing agents, reuptake inhibitors and receptor agonists and antagonists have been used to probe serotonergic function. For example, intravenous administration of the serotonin precursor l-tryptophan consistently increases plasma concentrations of prolactin and growth hormone but not of ACTH or cortisol. Fenfluramine causes a dose-dependent increase in plasma prolactin. When administered to humans, serotonin agonists that stimulate 5-HT_1A and 5-HT₂ receptors also increase plasma concentrations of ACTH, cortisol, prolactin and perhaps growth hormone. The neuroendocrine response in humans to such agents has been used clinically to assess the functioning of the central serotonergic system in patients with psychiatric disorders.

Serotonin also appears to be involved in the regulation of circadian rhythms. The suprachiasmatic nuclei (SCN) of the hypothalamus generate electrophysiological and metabolic cycles which repeat approximately every 24 hr. When isolated in vitro, the SCN continue to produce 24-hr rhythms in metabolism, vasopressin secretion and spontaneous electrical activity, indicating that circadian time-keeping functions or pacemaker activity are endogenous characteristics of the SCN. Ordinarily, this rhythm is synchronized or entrained to the environmental photoperiod, also about 24 hr. A serotonergic contribution to circadian rhythm regulation has been postulated because the SCN receive very dense serotonergic innervation from the midbrain raphe nuclei. In addition, there is a serotonergic innervation to the intergeniculate leaflet (IGL), an area of brain through which photic information indirectly accesses the SCN.

Serotonin appears to function as an inhibitory transmitter that modulates the effects of light on circadian rhythmicity. Direct application of 5-HT or receptor agonists to the SCN blocks light-induced phase shifts during the subjective night but causes phase advances during the subjective day. Such agents inhibit the excitatory effect of light, measured electrophysiologically, in either the SCN or the lateral geniculate complex. The nonselective 5-HT agonist quipazine resets or shifts the rhythm of spontaneous electrical activity of single cells recorded extracellularly in SCN isolated in brain slices.

Lesions of serotonergic neurons in laboratory animals have been reported by some investigators to disrupt locomotor rhythms or result in loss of the daily rhythm of corticosterone. In the hamster, the median raphe nucleus projects to the SCN, whereas the dorsal raphe nucleus innervates the IGL; furthermore, serotonergic innervation to the SCN, and not the IGL, is necessary for the photic entrainment of locomotor activity [31]. It appears, then, that the SCN circadian pacemaker, or clock, is modulated by stimulation of serotonergic receptors in the SCN and that serotonergic projections to the SCN may modulate the phase of the SCN in intact animals.

The possible involvement of 5-HT in feeding behavior has been an active area of research for many years. Pharmacological studies have contributed primarily to the idea that 5-HT has an inhibitory effect on feeding behavior. Drugs that either directly or indirectly activate postsynaptic 5-HT receptors decrease food consumption, whereas agents that inhibit serotonergic transmission increase food intake. Precisely how this occurs is controversial, with claims that 5-HT governs the selection of macronutrients in the diet, influences responses to the taste qualities of food or modulates gastric activity to reduce feeding. Perhaps the most comprehensive and enduring view is that enhanced serotonergic activity enhances satiety, particularly by increasing the rate of satiation and prolonging the state of satiety [32].

Fenfluramine, originally the racemate and more recently the d-isomer, has been the prototypical drug for studying serotonergic mechanisms in feeding behavior. As mentioned previously, fenfluramine elicits the release of 5-HT and inhibits its reuptake (Fig. 13-7). d-Fenfluramine has an active de-ethylated metabolite, d-norfenfluramine, that contributes to the appetite-suppressant effects of the parent compound. Fenfluramine decreases meal size, rate of eating and eating between meals. This probably is related to its ability in humans to decrease the sensation of hunger and to increase the feeling of “fullness.” Serotonin-reuptake inhibitors, such as fluoxetine and serotonin precursors, mimic these effects. The effects of fenfluramine on feeding behavior are blocked by the nonselective serotonin receptor antagonist metergoline.

Multiple mechanisms in brain appear to be responsible for the effects of serotonergic drugs on satiety; for example, postsynaptic 5-HT_1B receptors are involved in regulating the size of meals eaten, but 5-HT_2C receptors influence the rate of eating. The sites in brain where these drugs, and presumably 5-HT, cause such effects remain to be identified. The paraventricular nucleus (PVN) of the hypothalamus may be an important site, although there are data indicating that actions on the PVN may be sufficient, but not necessary, to reduce caloric intake. In addition to brain mechanisms, 5-HT may act through peripheral mechanisms to produce satiety.

The pharmacological effects produced by drugs such as fenfluramine on feeding behavior in animals have led to its use in the treatment of obesity in humans. In many double-blind, placebo-controlled trials, chronic administration of fenfluramine causes greater weight loss than placebo. Although not as extensively studied clinically, fluoxetine produces similar effects. Weight gain occurs when fenfluramine is stopped, which indicates that the weight loss was related to its administration [33]. Given all of the medical problems associated with obesity, anorectic agents are valuable tools to be used in association with other modalities, such as diet and exercise, in the treatment of the truly obese individual.

5-Hydroxytryptamine not only has important physiological effects of its own but also is the precursor of the hormone melatonin

The human pineal gland weighs about 150 mg and occupies the depression between the superior colliculi at the posterior border of the corpus callosum. Although there are physical connections between the pineal gland and brain, the pineal gland lies “outside” the blood—brain barrier (see Chap. 32) and is innervated primarily by sympathetic nerves arising from the superior cervical ganglia.

Extracts of the pineal gland were reported as early as 1917 to lighten frog skin in vitro; in the late 1950s, the pineal hormone, melatonin, which produces this effect was isolated and its chemical structure, 5-methoxy-N-acetyltryptamine, described (Fig. 13-5). Melatonin is synthesized from serotonin, and the pineal gland contains all of the enzymes necessary to synthesize serotonin from tryptophan as well as two additional enzymes required to convert serotonin to melatonin (Fig. 13-5). The rate-limiting enzyme, serotonin N-acetyltransferase (EC 2.31.87), or arlylalkylamine N-acetyltransferase (AANAT), converts serotonin to N-acetylserotonin; this product is converted to melatonin by the enzyme 5-hydroxyindole-O-methyltransferase (HIOMT), which uses S-adenosylmethionine as the methyl donor. The human AANAT gene has been cloned and has considerable sequence identity to the sheep and rat genes. The human gene is localized on chromosome 17. The gene product is a 23.2-kDa protein that contains putative phosphorylation sites. Such sites are likely to be involved in the cAMP-dependent regulation of enzyme activity.

A unique feature of pineal gland physiology is that the synthesis and secretion of melatonin is influenced markedly by the light—dark cycle, acting through a multisynaptic pathway that relays in the superior cervical ganglia of the sympathetic nervous system. During daylight, the synthesis and secretion of melatonin are reduced, as is impulse flow along the sympathetic nerves innervating the pineal gland. At the onset of darkness, there is activation of these nerves, and the increased release of NE from them activates β adrenoceptors on the pineal gland to increase the formation of cAMP, with activation of α₁ adrenoceptors further amplifying the response. This second messenger causes activation of AANAT so as to increase the synthesis of melatonin. The extent of the nighttime increase in AANAT activity is very species-dependent, being, for example, as much as 150-fold in rats but only 1.5-fold in sheep. What type of rhythm is exhibited by humans is not yet known, although mRNA for AANAT is abundant in the pineal gland of humans during the day, as it is in sheep, whereas the transcript is nearly undetectable during the day in the rat pineal gland.

Thus, the pineal gland functions as a neuroendocrine transducer. In mammals, photosensory information impinging on the retina influences the activity of its neuronal projections, which ultimately inhibits or stimulates the secretion of melatonin. A circadian rhythm of melatonin secretion persists in animals housed in continuous darkness. Thus, melatonin synthesis is turned on by an endogenous “clock,” probably located within the SCN of the hypothalamus, with the daily rhythm normally being entrained to the day—night, light—dark cycle [34].

The exact physiological and behavioral effects of melatonin in humans are unclear. Such effects primarily result from the actions of melatonin on the SCN to influence the timing of circadian rhythms. The effects of melatonin are mediated by its activation of specific receptors. Two mammalian receptors for melatonin have been cloned [35], both of which belong to the G protein-coupled receptor family. A third subtype has been cloned from chickens but not yet in mammals. The melatonin_1A receptor is expressed in the hypophyseal pars tuberalis (PT) and the SCN, presumed sites of the reproductive and circadian effects of melatonin. The human melatonin_1B receptor is 60% identical to, and exhibits similarity in, its pharmacological profile and second-messenger coupling to the melatonin_1A receptor. It is found most abundantly in retina and to a lesser extent in brain. It has not been detected in the PT or the SCN. It seems likely that the ability of melatonin to act in the retina to affect some light-dependent functions, such as photopigment disc shedding and phagocytosis, may be due to its activation of the melatonin_1B receptor.

Perhaps the strongest case can be made for melatonin playing a role in reproduction, particularly in seasonally breeding mammals such as hamsters or sheep, which time their reproductive cycles via changes in the photoperiod. Information on day length may be relayed to the hypothalamic—pituitary—gonadal axis by the pattern of melatonin production. Although the effects of melatonin on reproduction were believed to be solely antigonadotropic, melatonin has been shown to be capable of causing progonadotropic effects. The type of effect caused by melatonin is dependent on the time point in the photoperiod when it is administered, the length of the photoperiod, the species and the dose administered. In general, the effects of melatonin are most robust when given around the time of the light-to-dark transition.

Melatonin has potent sedative and hypnotic activity. This has been demonstrated in double-blind, placebo-controlled studies. The hypnotic effect of melatonin seems to be separable from its effects on circadian rhythms. Although far from being fully understood, the entraining effect of melatonin on biological rhythms has led to its being used by humans for disorders that may be related to disturbances of circadian rhythms. For example, it is used to alleviate symptoms of jet lag as well as alertness-related problems in shift workers. Much more research will be needed to establish its efficacy in such conditions. Unfortunately, claims of its usefulness for these types of problems far exceed any controlled clinical data demonstrating such effects. At a different level, but one receiving comparable attention in the popular press as those mentioned above, is the purported antiaging properties of melatonin due to its antioxidant properties. Based on its demonstrated antioxidant properties, seen both in vitro and in vivo, melatonin has been speculated to be part of the natural defense system of the body against the toxic effects caused by free radicals.