Serotonin: The Neurotransmitter for the '90s

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By Ronald F. Borne, Ph.D., Department of Medicinal Chemistry, School
of Pharmacy, University of Mississippi

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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).

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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-hydroxyindoleacetic acid (5-HIAA).

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 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 transporter systems. These events are summarized in Figure
1. 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 matabolism (step 6).

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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.

5-HT1 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:

 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 subtype 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 gastointestinal and
vascular smooth muscle contraction, platelet aggregation, hypertension,
migraine, and neuronal depolarization. Antagonists have potential use
as antipsychotic 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.

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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 unapprovd
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 inhibitor
of 5-HT uptake, fluoxetine the least. Sertaline is the most selective
for 5-HT versus NE uptake, fluoxetine the lese 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 recemic 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 exreted 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 differnet neurotransmitter receptor systems.

Sumatriptan is administerd 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 agent metoclopramide. 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 l

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