Department of Molecular Biology

Unit Eukaryotic Gene Expression and Signal Transduction

get a look at the movie

 get a free slideshow about                  

Introduction 

Serotonin (5-hydroxytryptamine, 5-HT) is widely distributed in animals and plants, occurring in vertebrates, fruits, nuts, and venoms. A number of congeners of serotonin are also found in nature and have been shown to possess a variety of peripheral and central nervous system activities. Of particular interest over the years is the psychotomimetic activity displayed by several serotonin-related compounds such as N,N-dimethyl tryptamine, 5-hydroxy-N,N-dimethyltryptamine (bufotenine), and 4-phosyphoryloxy-N, N-dimethyl-tryptamine (psilocybin). 

Although serotonin may be obtained from a variety of dietary sources, endogenous 5-HT is synthesized in situ from
tryptophan through the actions of the enzymes tryptophan hydroxylase and aromatic L-amino acid decarboxylase. Both dietary and endogenous 5-HT are rapidly metabolized and inactivated by monoamine oxidase and aldehyde dehydrogenase to the major metabolite, 5-hydroxy-indoleacetic acid (5-HIAA). 

The biosynthesis of serotonin from the amino acid tryptophan is similar to that found for the catecholamines, and 5-hydroxytryptophan can cross the BBB to increase central levels of 5-HT. p-Chlorophenylalanine can competitively inhibit tryptophan hydroxylase to prevent serotonin synthesis.

Although serotonin is metabolized by monoamine oxidase to 5-hydroxyindoleacetic acid, most of the serotonin released into the post-synaptic space is removed by the neuron through a reuptake mechanism inhibited by the tricyclic antidepressants (see lecture notes for selective serotonin reuptake inhibitors.

Of the chemical neurotransmitter substances, serotonin is perhaps the most implicated in the etiology or treatment of various disorders, particularly those of the central nervous system, including anxiety, depression, obsessive-compulsive disorder, schizophrenia, stroke, obesity, pain, hypertension, vascular disorders, migraine, and nausea. A major factor in our understanding of the role of 5-HT in these disorders is the recent rapid advance made in understanding the physiological role of various serotonin receptor subtypes. This review will summarize the physiological functions of serotonin--those drugs currently available that act by mimicking or antagonizing the actions of serotonin--and the future development of serotonergic agents. 

SEROTONIN (red spheres) secreted by a presynaptic cell binds to receptors (shades of green) on a postsynaptic cell and directs the postsynaptic cell to fire or stop firing. The cell's response is influenced by the amount of serotonin in the cleft and by the types of receptors; serotonin receptors come in at least 13 "flavors."
Serotonin levels in synapses are reduced by two kinds of presynaptic molecules: autoreceptors (orange), which direct the cells to inhibit serotonin production, and reuptake transporters (yellow), which absorb the neurotransmitter. Several antidepressants, including Prozac and Paxil, increase synaptic serotonin by inhibiting its
reuptake.

Serotonin was first isolated from blood in 1948 by Page and coworkers and was later identified in the central nervous system. As is the case for most neurotransmitters, it has a relatively simple chemical structure but displays complex pharmacological properties. Based on the similarity of this structure to the structures of norepinephrine and dopamine, it is not surprising that serotonin, like its catecholamine counterparts, possesses a diversity of pharmacological effects, both centrally and peripherally. It is found in three main areas of the body: the intestinal wall (where it causes increased gastrointestinal motility); blood vessels (where large vessels are constricted); and the central nervous system (CNS). 

The most widely studied effects have been those on the CNS. The functions of serotonin are numerous and appear to involve control of appetite, sleep, memory and learning, temperature regulation, mood, behavior (including sexual and hallucinogenic behavior), cardiovascular function, muscle contraction, endocrine regulation, and depression. Peripherally, serotonin appears to play a major role in platelet homeostasis, motility of the GI tract, and carcinoid tumor secretion. This represents quite a broad spectrum of pharmacological and psychological effects, considering the fact that the average human adult possesses only about 10 mg of 5-HT. 

Subsequent to his discovery of serotonin, Page commented that no physiological substance known possesses such diverse actions in the body as does 5-HT. 

Chemical neurotransmitters (CNTs) produce their effects as a consequence of interactions with appropriate receptors. As is the case with all the CNTs, serotonin is synthesized in brain neurons and stored in vesicles. Upon a nerve impulse, it is released into the synaptic cleft, where it interacts with various postsynaptic receptors. 

The actions of 5-HT are terminated by three major mechanisms: diffusion; metabolism; and uptake back into the synaptic cleft through the actions of specific amine membrane transported systems. These events are summarized in Figure 1.
[missing from this edition]. 

Thus, the actions of 5-HT can be theoretically modulated by agents that stimulate or inhibit its biosynthesis (step 1); agents that block its storage (step 2); agents that stimulate or inhibit its release (step 3); agents that mimic or inhibit its actions at its various postsynaptic receptors (step 4); agents that inhibit its uptake back into the nerve terminal (step 5); agents that affect its metabolism (step 6). 

Of all the CNTs, 5-HT presents the most perplexing array of receptor interactions. In 1957, Gaddum suggested that 5-HT interacted on two different receptors in isolated tissues, one on smooth muscle and one on nervous tissue. Since dibenzyline selectively antagonized smooth muscle, and morphine was selective for nervous tissue, these receptors were named "D"; and "M" receptors, respectively. 

Since that time, and especially in the past decade, there has been tremendous progress in 5-HT receptor identification. It now appears that there are at least four populations of receptors for serotonin: 5-HT1, 5-HT2, 5-HT3, and 5-HT4. Recent cloning studies suggest the existence of 5-HT5, 5-HT6, and 5-HT7 subtypes as well. 

To complicate matters further, evidence has been presented that five distinct subtypes of the 5-HT2 (one of which was formerly named the 5-HT1C receptor, a name that still appears in the literature) and three subtypes of the 5-HT3 receptors exist. The physiological function of each receptor subtype has not been established and is currently the subject of intensive investigation. With the exception of the 5-HT3 receptor, which is a ligand-gated ion channel related to NMDA, GABA and nicotinic receptors, all of the 5-HT receptor subtypes belong to the group of G-protein linked receptors. 

The design of specific agonists and antagonists for each receptor system offers much promise for new drug development. The greatest current interest involves the modulation of 5-HT at receptors in the CNS. The following briefly summarizes our understanding of the function of the most widely studied 5-HT receptors. 

These receptors appear to be involved in the processes of smooth muscle relaxation, contraction of some cardiac and vascular smooth muscle, rejunctional inhibition of neurotransmitter release, and effects in the CNS. 

Five subtypes have been proposed, four of which appear to play a major role in humans: 
More information about Serotonin Receptor Subtypes and Ligands can be found at: 
Serotonin Receptor Subtypes and Ligands 

5-HT1A 

This represents perhaps the most widely studied 5-HT receptor subtype. These receptors are located primarily in the CNS. Agonists facilitate male sexual behavior in rats, hypotension, increase food intake, produce hypothermia, and act as anxiolytics. This receptor has also been widely implicated in depression. 

5-HT1B 

These may serve as autoreceptors; thus, activation causes an inhibition of neurotransmitter release. Agonists inhibit
aggressive behavior and food intake in rodents. These receptors, which have been identified only in rodents and are
apparently absent in humans, are thus only of theoretical interest at present. These receptors may be the counterpart of the 5-HT1D receptor found in other species. 

5-HT1C 

These receptors belong to the same receptor subfamily as the 5-HT2 receptor and have been recently renamed as 5-HT2C receptors. This receptor is located in high density in the choroid plexus and may regulate cerebrospinal fluid production and cerebral circulation. This subtype is speculated to be involved in the regulation of analgesia, sleep, and cardiovascular function. 

5-HT1D 

Located primarily in the CNS, this receptor may play a role as a presynaptic heteroreceptor or as a terminal autoreceptor, being thus involved in the inhibition of neurotransmitter release by mediating a negative feedback effect on transmitter release. This subtype is the most abundant 5-HT1 receptor in the CNS but is also found in vascular smooth muscle mediating contraction. While the role of activation of this receptor sub-type is not fully understood, agonists at this site are effective in treating acute migraine headaches. The development of selective antagonists of this receptor should clarify the functional role of 5-HT1D receptors in the CNS. 

5-HT2 receptors

Located primarily in the vascular smooth muscle, platelets, lung, CNS, and the GI tract, these appear to be involved in gastrointestinal and vascular smooth muscle contraction, platelet aggregation, hypertension, migraine, and neuronal
depolarization. Antagonists have potential use as anti-psychotic agents. Because these receptors belong to the same
receptor subfamily as the former 5-HT1C receptors, they have been recently renamed as 5-HT2A receptors. 

5-HT3 receptors

Located primarily in peripheral and central neurons, these receptors appear to be involved in the depolarization of peripheral neurons, pain, and the emesis reflex. Potential use of agents acting at this site include migraine, anxiety, and cognitive and psychotic disorders. 

5-HT4 receptors 

These receptors are found in the CNS, the heart, and the GI tract. Their activation produces an increase in cyclic
andenosine monophosphate (AMP) and appears to involve activation of neurotransmitter release. The gastric prokinetic activity of metoclopramide has been attributed, in part, to its ability to activate 5-HT4 receptors. 

5-HT5 Receptors

A functional mouse 5-HT receptor expressed primarily in the CNS was identified in 1992. The 5-HT5 amino acid sequence is not closely related to other 5-HT receptors, but the pharmacological properties of this receptor reportedly resemble somewhat those of 5-HT1D receptors. A closely related mouse receptor has also been cloned: 5-HT5B, leading to the re-naming of the original 5-HT5 receptor as 5-HT5A. The two 5-HT5 receptors exhibit 77% amino acid sequence homology but less than 50% homology with other cloned 5-HT receptors. The 5-HT5A gene is on mouse chromosome 5, whereas the 5-HT5B gene is on chromosome 1. Rat and human 5-HT5 receptors have also been cloned. To some extent, the 5-HT5 receptors appear to resemble 5-HT1 receptors (e.g., high affinity for 5-HT and 5-CT), however their low homology with other 5-HT1 receptors, together with the failure to demonstrate G-protein coupling, suggested that they represent a distinct family of receptors. There is a report of G-protein coupling for the rat 5-HT5B receptor, but activation of the receptor does not appear to involve cAMP accumulation or phosphoinositide turnover. Some have suggested that 5-HT5 receptors may utilize a novel (perhaps an ion channel) second messenger system. Recently, however, it was shown that, although 5-HT5A receptors are weakly detected on neurons in the cortex, their primary site of expression is non-neuronal. Rat 5-HT5A receptors are expressed in vitro and in vivo by astrocytes and are negatively coupled to adenylate cyclase. A human 5-HT5A, but not 5-HT5B, receptor has been identified; the human 5-HT5A receptor gene, like the rat and mouse counterparts, contain two coding exons separated by a single large intron. Hydropathy analysis indicates seven transmembrane-spanning helical units. The ligand binding characteristics of the human 5-HT5A receptor are generally similar to those of rat and mouse 5-HT5A receptors. Interestingly, however, methiothepin binds with about 60-fold higher affinity (Ki ca. 1 nM) at human than at mouse 5-HT5A receptors. The 5-HT1A ligand 8-OH DPAT reportedly binds with higher affinity at rat 5-HT5B receptors (Ki = 46 nM) than at mouse 5-HT5B receptors (Ki ca. 400 nM); 8-OH DPAT does not bind with significant affinity at mouse or human 5-HT5A receptors (Ki >1,000 nM).

5-HT5A and 5-HT5B receptors are both labeled with [125I]I-LSD; rat 5-HT5B and human 5-HT5A receptors have been labeled with [3H]5-CT. 5-HT binds with modest affinity (Ki = 100–250 nM), whereas 5-CT binds with about 10-fold higher affinity at both receptors. Ergotamine and methiothepin bind with high affinity at human 5-HT5A receptors, whereas agents such as spiperone, sumatriptan, yohimbine, ketanserin, propranolol, zacopride and clozapine bind with much lower affinities (Ki >1,000 nM). The pharmacological function of 5-HT5 receptors is currently unknown; it has been speculated that, on the basis of their localization, they may be involved in motor control, feeding, anxiety, depression, learning, memory consolidation, adaptive behavior, and brain development. 5-HT5A receptors may also be involved in a neuronally-driven mechanism for regulating astrocyte physiology, with relevance to gliosis; disruption of 5-HT neuron-glial interactions may be involved in the development of certain CNS pathologies, including Alzheimer's disease, Down's syndrome, and some drug-induced developmental deficits. Suggestions continue to be made that 5-HT5 receptors may be involved in certain functions previously attributed to 5-HT1D receptors; the possibility exists, however, that because 5-HT5 and 5-HT1D receptors are clearly distinct from one another, earlier studies characterizing 5-HT1D pharmacology may have pooled the three receptors.

5-HT6 Receptors

A cDNA encoding a novel G protein-coupled 5-HT receptor that appeared to be localized exclusively in the CNS was cloned from rat brain. This receptor, termed 5-HT6, exhibits only 36–41% transmembrane homology with 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT2A, and 5-HT2C receptors. Both [125I]I-LSD and [3H]5-HT label 5-HT6 sites and 5-HT displays an affinity of 151 and 56 nM, respectively. With [125I]I-LSD, certain ergots bind with high affinity (e.g., lisuride), whereas others (e.g., mesulergine) bind with only micromolar affinity. 5-Carboxamidotryptamine binds only with modest affinity. A number of typical and atypical neuroleptic agents and tricyclic antidepressants bind with Ki values of <100 nM. In HEK-293 cells stably transfected with this receptor, 5-HT produced a potent, dose-dependent increase in cAMP levels. As such, this was the first cloned 5-HT receptor shown to be coupled to activation of adenylate cyclase. 5-Methoxytryptamine and 5-CT behaved in a similar manner, lisuride acted as a partial agonist, and amoxapine, clozapine and methiothepin act as antagonists. 5-HT6 receptors may be involved in neuropsychiatric disorders. More recently, the human 5-HT6 receptor has been cloned; its gene structure, distribution, and pharmacology are similar to those of the rat receptor. Interestingly, comparison of rat and human 5-HT6 receptors led to the discovery of an apparent frame shift in the latter and led to its resequencing. Rat and human 5-HT6 receptors display 95% homology and differ in structure mostly at their carboxy-terminus end, with the human receptor being two amino acids longer than the rat homolog. Both receptors contain two introns at corresponding positions. Like the rat receptor, the human receptor is positively linked to adenylate cyclase. The pattern of 5-HT6 mRNA expression in human brain indicates highest levels in the caudate nucleus.

5-HT7 Receptors

The 5-HT7 receptor, expressed mainly in the CNS, has been cloned from several species including rat, mouse, guinea pig and human. A low level of expression of 5-HT7 receptors has been detected in the periphery. Alternative splicing probably accounts for two splice variants in the rat; both forms of the receptor, the genes for which contain at least two introns, are positively coupled to adenylate cyclase. The short form of the rat 5-HT7 receptor contains 435 amino acids whereas the long form, as well as mouse 5-HT7 receptors, contains 448 amino acids; the human 5-HT7 receptor falls in between, with 445 amino acids. There is <50% transmembrane sequence homology between 5-HT7 receptors and other 5-HT receptors. Gene mapping studies have led to localization of the human 5-HT7 receptor gene to chromosome 10.

Drugs acting by serotonergic mechanisms

Drugs affecting serotonin synthesis and metabolism

Although an inviting target for drug design, stimulators or inhibitors of the biosynthesis of 5-HT have not been marketed to date. Administration of tryptophan can increase endogenous levels of serotonin and thus has potential value in the treatment of phenylketonuria. An investigational drug, p-chlorophenylalanine, has been shown to decrease serotonin levels by 90% as a result of inhibition of the rate-limiting step in 5-HT synthesis, tryptophan hydroxylase, but no therapeutic applications have been suggested, because of its inherent toxicity. Other inhibitors such as 6-fluorotryptophan and p-chloroamphetamine have also been investigated, but no clinical applications have been uncovered. Inhibitors of monoamine oxidase, as one would expect, have been shown to increase levels of 5-HT. 

Serotonin depletors or releasing agents

Fenfluramine (Pondimin), marketed as an appetite suppressant, is a fairly selective and long-acting 5-HT depleting agent, singling it out from the other phenethylamine anorectic drugs. An increasingly popular "recreational" drug of abuse, 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy, ADAM) has caused widespread concern as a neurotoxin because of its ability to produce long-term depletion of 5-HT from neurons in rat and primate brains. Similar effects have been noted for several illegal analogs related to MDMA, such as the N-ethyl analog known as EVE. Prior to its classification as a Schedule 1 controlled substance, MDMA and its predecessor MDA (3,4-methylenedioxyamphetamine) were synthesized by "street chemists" as designer drugs and were used for various unapproved purposes and made available to psychotherapists. MDMA was originally synthesized in 1914 as an appetite suppressant but was never marketed because of toxic side effects noted early in testing. MDMA has been used as an adjunct to psychotherapy and was popular among psychotherapists because of its ability to reduce anxiety and facilitate communication by giving the patient a calmer understanding of his or her problems. MDMA promotes a significant release of 5-HT from presynaptic vesicles, producing a profound decrease in brain levels of serotonin. The long-term psychological and physical consequences of this depletion has yet to be fully explored. 

Inhibitors of serotonin uptake 

The major mechanism by which the action of serotonin is terminated is by uptake through presynaptic membranes. After 5-HT acts on its various postsynaptic receptors, it is removed from the synaptic cleft back into the nerve terminal through an uptake mechanism involving a specific membrane transporter in a manner similar to that of other biogenic amines. Agents that selectively inhibit this uptake increase the concentration of 5-HT at the postsynaptic receptors and have been found to be quite useful in treating various psychiatric disorders, particularly depression. Approximately 5% of the U.S. population experience a depressive episode requiring psychopharmacological treatment; in any one year, 10-12 million Americans are affected by depression, with the condition twice as common in females than in males. It has been estimated that 15% of patients hospitalized for depression will commit suicide. However, 80-90% of individuals suffering from depression can be successfully treated. 

Depression is an affective disorder, the pathogenesis of which cannot be explained by any single cause or theory. The most widely accepted hypothesis involves abnormal function of the catecholamine (primarily norepinephrine) and/or serotonin transmitter systems. In this hypothesis, most forms of depression are associated with a deficiency of norepinephrine and/or serotonin at functionally important adrenergic or serotonergic receptors. Hence drugs that enhance the concentrations of norepinephrine and/or serotonin at these receptors should alleviate to an extent the symptoms of depression. Approaches to the treatment of depression over the years have involved the use of agents (stimulants) that mimic norepinephrine; agents (MAOIs) that increase the levels of NE and 5-HT by inhibiting their metabolism; and drugs that increase these levels at the receptor by inhibiting the uptake of NE and 5-HT. 

The classical tricyclic antidepressants (TCAs) currently available block primarily the uptake of norepinephrine and also, to varying degrees, the uptake of 5-HT--depending on whether they are secondary or tertiary amines. 

Tertiary amines such as imipramine and amitriptyline are more selective inhibitors of 5-HT than catecholamines, compared with secondary amines such as desipramine. More recently, selective 5-HT reuptake inhibitors (SSRIs) have been investigated as potential antidepressants with the anticipation that these agents, unlike the first-generation TCAs, would possess fewer side effects, such as anticholinergic actions and cardiotoxicity, and would be less likely to cause sedation and weight gain. 

Clomipramine (Anafranil) is structurally related to the classical TCAs and was the first medication approved in the United States for the treatment of obsessive-compulsive disorder. It is a potent inhibitor of 5-HT uptake, but effectively inhibits NE uptake as well. In addition, climipramine has affinity for central dopamine D2, histamine H1, and adrenergic alpha-1 receptors and possesses anticholinergic effects. 

Three selective 5-HT uptake inhibitors, also referred to as second-generation antidepressants, have been introduced on the U.S. market. Fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil) have gained immediate acceptance, each appearing in the most recent listing of the top 200 prescription drugs. Fluoxetine recently was approved also for the treatment of obsessive-compulsive disorder. These agents do not appear to possess greater efficacy than the TCAs, nor do they generally possess a faster onset of action; however, they do have the advantage of a lower side-effect profile. Of these three SSRIs, paroxetine is the most potent inhibit or of 5-HT uptake, fluoxetine the least. Sertaline is the most selective for 5-HT versus NE uptake, fluoxetine the less selective. 

Fluoxetine and sertraline produce active metabolites, while paroxetine is metabolized to inactive metabolites. The SSRIs, in general, affect only the uptake of serotonin and display little or no affinity for various receptor systems including muscarinic, adrenergic, dopamine, histamine, or 5-HT receptors. 

In addition to treating depression, several other potential therapeutic applications for SSRIs have been investigated. They include treatment of Alzheimer's disease; modulation of aggressive behavior; treatment of premenstrual syndrome, diabetic neuropathy, and chronic pain; and suppression of alcohol intake. Of particular significance is the observation that 5-HT reduces food consumption by increasing meal-induced satiety and reducing hunger, without producing the behavioral effects of abuse liability associated with amphetamine-like drugs; thus, there is interest in the possible use of SSRIs in the treatment of obesity. 

Venlafaxine (Effexor) is a recently introduced antidepressant, differing from the classical TCAs and the SSRIs chemically and pharmacologically in that it acts as a potent inhibitor of both 5-HT and norepinephrine uptake, as well as weakly inhibiting dopamine uptake. Its major metabolite, O-desmethylvenlafaxine, shares a similar profile. Neither venlafaxine nor its major metabolite have significant affinity for muscarinic, histaminergic, benzodiazephine, mu opioid, or adrenergic alpha-1 receptors. It is administered as a racemic mixture. Both enantiomers inhibit 5-HT and NE uptake, but the (S)(+)-isomer is more selective for 5-HT uptake. Venlafaxine possesses an efficacy equivalent to that of the TCAs and a benign side effect profile similar to the SSRIs. 

Agents acting at serotonin receptors 

Serotonin agonists 

Despite the large number of serotonin analogs that have been prepared and investigated, few have reached the
marketplace. Trazodone (Desyrel), a second-generation antidepressant, possesses a complex mechanism of action. It may act as a presynaptic alpha-2 norepinephrine receptor antagonist, selectively blocking 5-HT uptake as well as possessing 5-HT receptor antagonist properties. Interestingly, it is metabolized to m-chlorophenylpiperazine, a known agaonist at 5-HT receptors and an inhibitor of 5-HT uptake. The anxiolytic agent buspirone (BuSpar) is a partial agonist at 5-HT1A receptors and interacts with other receptor systems as well. 

The recent introduction of sumatriptan (Imitrex) as the first 5-HT1D receptor agonist represents a major advance in the treatment of acute migraine attacks. Migraine headaches affect nearly one in 11 Americans, occurring usually in adults 35-45 years of age. 

Many factors have been implicated as initiators of migraine attacks, including stress, smoking, fatigue, glaring lights,
weather, hormonal fluctuations, various foods (including those which contain nitrate and nitrate preservatives),
caffeine-containing beverages, alcohol (especially red wine), and drugs that cause blood vessels to dilate. Migraine
represents a disorder of cerebral vascular regulation and may be the result of a marked, prolonged phase of cranial
vasodilation. During an attack, extravasation of plasma proteins and development of localized inflammation in intracranial vessels also occur. The trigeminal (fifth cranial) nerve has been implicated. Migraines may be initiated by afferent and/or efferent nerves to affected blood vessels. 

Chemical mediators including serotonin, thromboxane A2, prostaglandins and kinins appear to be involved. 5-HT receptors predominate in cranial blood vessels and are widely distributed in the CNS, where they play an important role in controlling cranial circulation and pain. During the prodromal phase of an attack, 5-HT is spontaneously released from platelets. It enters the vessel wall, causing arterial vasoconstriction and lowering the pain threshold. In the absence of 5-HT, extracranial arteries dilate and distend, resulting in a headache. 

Serotonin is released during migraine attacks, and the major metabolite of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), is excreted in increased amounts. Of all the 5-HT receptors, the 5-HT1 subtype has been most widely implicated, since these receptors are mainly located in certain cranial blood vessels. 

Sumatriptan selectively constricts carotid arterial circulation supplying blood to extracranial and intracranial tissues such as meninges, dilation of which is thought to contribute to migraines; it binds with high affinity to 5-HT1D receptors and, to a lesser extent, to 5-HT1A sites. Sumatriptan also may activate a prejunctional inhibitor receptor, which resembles the 5-HT1D receptor on perivascular fibers, resulting in an inhibition of the release of inflammatory neuropeptides that mediate pain. It is highly selective for 5-HT1 receptors and lacks affinity for 5-HT2, 5-HT3, adrenergic, dopamine, acetylcholine, or benzodiazepine receptor sites. The ergot alkaloids such as dihydroergotamine, on the other hand, bind to a number of different neurotransmitter receptor systems. 

Sumatriptan is administered subcutaneously and has been reported to provide complete relief in 86%-96% of patients within 20-60 minutes of injection. There is considerable interest in developing rapid-acting, orally effective 5-HT1D agonists for the treatment of migraines. 

Recently, cisapride (Propulsid) was introduced as a new-generation prokinetic agent indicated for the symptomatic treatment of patients with nocturnal heartburn from gastroesophageal reflux disease (GERD). An estimated 44% of adult Americans experience heartburn at least once a month. Of these, 7%-10% have heartburn on a weekly or more frequent basis, to such an extent that there is a disruption of lifestyle. Treatment generally involves behavioral modification; nonprescription drug therapy, including antacids and alginic acid-antacid combinations; and prescription drug therapy involving acid release inhibitors (H2-antagonists and proton pump inhibitors) and prokinetic agents that increase motility of the GI tract. 

Cisapride is a substituted piperidyl benzamide derivative somewhat structurally related to the prototype prokinetic agentmetoclopramide. It acts as a 5-HT4 agonist in vitro, resulting in increased GI motility and cardiac rate. 
Cisapride is less potent than metoclopramide as a dopamine receptor antagonist and possesses a specific action on the postganglionic nerves of the myenteric plexas of Gi smooth muscles, enhancing the local release of acetylcholine. It has no effect on muscarinic or nicotinic receptor stimulation, nor does it inhibit acetylcholinesterase. 

Serotonin antagonists 

The ergot alkaloids (ergolines) generally display a high affinity but low selectivity for 5-HT binding sites; many also display a high affinity for dopamine and norepinephrine binding sites as well. methysergide, used to prevent migraine headaches, is a potent 5-HT2 antagonist, while the related alkaloid ergotamine, which is used to treat existing migraines, is a 5-HT1 agonist/partial agonist. 

Many hallucinogenic substances such as LSD and psilocin are thought to block serotonin, but, obviously, not all central 5-HT antagonists are hallucinogens. A clinically investigated 5-HT2 antagonist, ketaserin, has been found to be an effective anti-hypertensive agent but is not selective since it may also block alpha1-andrenoreceptors. 

Two 5-HT3 receptor antagonists, ondansetron (zofran) and granisetron (Kytril), have been marketed to treat nausea
associated with radiation and chemotherapy in cancer patients. Nausea and vomiting have consistently appeared among the severe side effects most frequently reported by patients as a result of cancer chemotherapy and radiotherapy. The intensity of these effects varies with the nature of the chemotherapeutic agent, ranging from those with very high emetogenic potential such as cisplatin, dacarbazine, mechlorethamine, and high doses of cytarabine and melphalan to those with a low potential, such as vincristine, chlorambucil, and the androgens and estrogens. 

Innervation of the chemoreceptor trigger zone in the CNS may occur along a number of pathways, including peripheral receptor stimulation in the GI tract, vestibular stimulation, and anticipatory stimulation involving the cerebral cortex. 

A number of different classes of antiemetic agents have been explored over the years. Among them are anticholinergics; antihistaminics; benzodiazepines and barbiturates; cannabinoids; and dopamine antagonists such as the phenothiazines, butyrophenones, and benzamides. Within the latter class, metoclopramide is perhaps the standard of comparison because of its effectiveness in treating nausea associated with high doses of cisplatin. Until recently, metoclopramide was considered the most effecting single agent in treating cisplatin-induced emesis, but its use has been associated with a high incidence of extrapyramidal effects. 5-HT3 receptors have been proposed to play a major role in the physiology of emesis. These receptors are found in high concentrations peripherally in the gut and centrally in the cortical and limbic regions and in or near the chemoreceptor trigger zone, and have been implicated in the vomiting reflex induced by serotonin as a result of chemotherapy. 

During the course of these therapies, mucosal enterochromaffin cells release 5-HT; this stimulates 5-HT3 receptors, evoking vagal efferent discharge and inducing emesis. Both ondansetron and granisetron are potent and selective inhibitors of 5-HT3 receptors both centrally and peripherally. Ondansetron has 100 times greater affinity for 5-HT3 receptors than does metoclopramide, while metoclopramide has 50 times greater affinity for dopamine D2 receptors than for 5-HT3 receptors. 

Granisetron possesses 4,000 to 40,000 times greater affinity for 5-HT3 receptors in the brain than for other receptors, including 5-HT1, 5-HT2, dopamine D2, histamine H1, opioid, benzodiazepine, or adrenergic alpha-1, alpha-2 or beta. Therefore, these selective 5-HT3 receptor antagonists lack many of the side effects associated with other antiemetic agents, particularly lacking the extrapyramidal side effects associated with the dopamine antagonists. 

Ondansetron is available in both injectable and oral formulations, while granisetron is available only in an injectable
formulation at the present time. 

The future 

The future therapeutic applications of receptor-selective serotonin agents are potentially numerous, thus explaining thetremendous investment the pharmaceutical industry is currently making in serotonin research. Some of these potential applications include the following: 

Neuroleptics 

The mechanism of action of current neuroleptics primarily involves antagonism of dopaminergic receptors, but 5-HT2 sites may also be involved since many currently marked neuroleptics have been also shown to antagonize 5-HT2 receptors. 

For example, clozapine (Clozaril) possesses higher affinity for 5-HT2 sites than for dopamine receptors. the role of 5-HT2 receptors in the mechanism of action of the neuroleptics is unclear but may offer a potential area for the development of newer agents. Of particular interest are behavioral studies in the rodent and primates indicating that 5-HT3 antagonist potently antagonize dopamine activity suggesting an important role in controlling limbic dopamine function. These studies suggest a potentially novel approach to the treatment of schizophrenia. 

Antidepressants 

In addition to the continued search for inhibitors of 5-HT and/or NE uptake, 5-HT2 receptors may be attractive targets for new agents in this area. Chronic antidepressant treatment leads to a decrease in the number of 5-HT2 receptors in cortical brain tissue from animals and suicide victims. The potential use of 5-HT2 antagonists as antidepressants is currently being explored. The use of selective inhibitors of 5-HT uptake in the treatment of depression has been previously discussed. 

Fluvoxamine, a potent and selective 5-HT uptake inhibitor, recently was recommended for approval by an FDA advisory committee for the treatment of obsessive-compulsive disorder and has been shown to be effective in depression. 

Anxiolytics 

"Second-generation" anxiolytics such as buspirone bind with high affinity to 5-HT1A receptors, while benzodiazepines have very low affinity for 5-HT sites (primarily affecting GABAminergic function). The development of 5-HT1A agents as potential "anxioselective" drugs may thus provide anxiolytics that lack some of the side effects of benzodiazepine therapy, such as muscle relaxation, sedation, ataxia, and memory impairment. In addition, 5-HT3 antagonists have shown great promise in animal models. 

Cardiovascular agents 

Antagonists of 5-HT2 sites are being explored as antihypertensive agents and for use in peripheral vascular disease,
thrombic or embolic episodes, and cardiopulmonary emergencies, but they may present clinical problems. Flesinoxan, a 5-HT1A agonist, is currently undergoing clinical trials, and many other agonists at this site are bring explored as antihypertensives. 

Analgesics 

The selective 5-HT uptake inhibitor fluoxetine has been shown to produce analgesia in mice following intrathecal
administration. The role of 5-HT in pain perception is being explored. 

Migraines 

Two experimental "5-HT1-like" agonists (AH 25086 and GR 43175) are being clinically investigated for the treatment of acute migraines. These agents act by activating 5-HT1 receptors that mediate localized vasoconstriction within the carotid vascular bed and appear to be more selective than ergotamine. In addition, 5-HT3 antagonists have been shown to possess effectiveness in migraines. 

Anorectics 

Serotonin is believe to reduce the consumption of food. Selective 5-HT uptake inhibitors (as discussed earlier) and 5-HT releasing agents have been suggested as antiobesity agents. Partial agonists at 5-HT1A receptors have been shown to increase food uptake, so it is of interest to speculate that selective antagonists at these sites may be potentially useful appetite suppressants. Agents acting selectively on serotonergic mechanisms would have the advantage over those agents acting by noradrenergic mechanisms (e.g., amphetamine), because the potential for abuse would be significantly decreased. 5-HT uptake inhibitors have also been shown to be effective in treating bulimia and such other eating disorders as anorexia nervosa. 

Antiemetics 

Several pharmaceutical firms are testing additional antagonists of 5-HT3 receptors as agents that can reduce the distressing vomiting syndrome associated with cancer chemotherapy. Orally effective agents with a rapid onset of action would be highly desirable. 

Senile dementias 

Alzheimer's disease (AD) is the most common form of senile dementia. While the pathogenesis of AD has not been totally clarified, the role of chemical neurotransmitters has been an active area of investigation. Although the involvement of the cholinergic system has received the greatest attention, serotonin appears to also play a major role in the disease. 5-HT function appears to be significantly reduced in Alzheimer's disease as indicated by post-mortem examination of brains of AD patients, which revealed a reduction in receptors of both the 5-HT1 and 5-HT2 type. The role of 5-HT1A receptors in the onset of senile dementia disorders such as Alzheimer's disease is being explored by several groups. The finding that 5-HT1A binding sites are reduced 50% post-mortem in patients with known Alzheimer's-type dementias, coupled with the observation that 5-HT1A receptors are present in large numbers on cholinergic terminals, suggests that binding to these receptors may offer a potential means for identifying early neuronal changes that lead to Alzheimer's-type dementias. The early diagnosis of the onset of these dementias may allow more effective therapeutic utilization of agents, such as cholinergic drugs, for the treatment of these disorders. 5-HT2 receptors also appear to be decreased in Alzheimer's disease.

Overall, there is much evidence to suggest that 5-HT transmission is impaired in senile dementia of the Alzheimer's type and that enhancement of 5-HT function may offer new treatment approaches. Interestingly, 5-HT3 receptor antagonists posses the ability to modify cognitive events and to improve impaired performance in animal models. Ondansetron, for example, possesses a dose-related effect on learning and memory in age-associated memory impairment, suggesting a role for 5-HT3 antagonists in the treatment of cognitive disorders. 

Prokinetic agents 

As stated earlier, the prokinetic activity of metoclopramide has been attributed to its ability to activate 5-HT4 receptors. The recently introduced prokinetic agent cisapride has also been shown, in vitro, to activate 5-HT4 receptors, suggesting that other agents of this class may be introduced in the near future. 

Substance abuse 

In a number of animal studies, 5-HT uptake inhibitors have been shown to decrease the ingestion of alcohol as well as suppressing the self-administration of other abused substances, such as cocaine. Although the clinical usefulness of 5-HT uptake inhibitors has not been established to date, these agents have also been shown to decrease alcohol intake in humans, suggesting an important potential role in treating substance abuse. While the present data are inconclusive, 5-HT3 receptor antagonists appear to reduce the "reward" effects of systemically administered morphine; however, it is not known if they block self-administration of morphine. The anxiety-related consequences of drug withdrawal appear to be improved by 5-HT3 receptor antagonists. 

Nociception 

5-HT3 receptors have been demonstrated on peripheral sensory neurons, and they appear to induce pain and sensitize nociceptive neurons when they are activated. It is interesting to speculate on the potential role of 5-HT3 receptor antagonists in the treatment of pain. 

Conclusion 

Exciting times are ahead for serotonin research. For example, just within the past six months, isolation of genomic clones has been reported for the 5-HT1A, 5-HT1C, and 5-HT2 receptors. One 5-HT3 receptor subunit from neuroblastoma cells has been cloned. Also, recent cloning studies have identified two human gene products of the 5-HT1D receptor, which have been designated as 5-HT1Da and 5-HT1Db receptors. The tremendous recent interest and advances in serotonin receptors, coupled with the intensive research programs in industry and academia for selective agonists at the various 5-HT receptors and subtypes, offer great promise for the development of important new therapeutic agents. 

Analysts project a greater than $10 billion market for serotonin-related drugs in the this millenium.