PHARMACOLOGY OF 5-HYDROXYTRYPTAMINE

(5-HT)/ SEROTONIN

Sunilchandra.U

Ist yr PhD,VPT

5-HT

• Amine autacoid • serotonin- in serum, on blood clotting• Enteramine- in enterochromaffin cells

of GIT- smooth muscle contraction• 1950’s- 5-HT: 5- HYDROXY TRYPTAMINE

5-HYDROXYTRYPTAMINE (SEROTONIN)

• 3-[beta-aminoethyl]-5-hydroxyindole) • widely distributed in the animal and plant kingdoms• In vertebrates, tunicates, mollusks, arthropods,

coelenterates, fruits-banana, pear, pineapple, tomato,,nuts. • Venoms (e.g stinging nettle, wasps, and

scorpions) • In enterochromaffin cells throughout the

gastrointestinal (GI) tract (90%)• In storage granules in platelets• Throughout the central nervous system

(CNS)

5-HT: SYNTHESIS AND METABOLISM

• Tryptophan, is actively transported into the brain by a carrier protein that also transports other large neutral and branched chain amino acids

• oxidase that requires O2 and a reduced pteridine cofactor, is the rate-limiting enzyme in the synthetic pathway.

Tryptophan PCPA T .hydoxylase

5-hydroxytryptophan

5-HT .L –decarboxylase

5-HT (hydroxytryptamine) / Serotonin

5-HT…contd

• The enzyme that converts L-5-hydroxytryptophan to 5-HT, aromatic L-amino acid decarboxylase, is the same enzyme that decarboxylates L-dopa in catecholamine synthesis

• Storage: in secretory granules by a vesicular transporter and released by exocytosis.

Action terminated by• Neuronal uptake mediated by a specific 5-HT transporter

localized in the membrane of serotonergic axon terminals and in the membrane of platelets (where it takes up 5-HT from the blood).

• This uptake system is the only way that platelets acquire 5-HT (they lack the enzymes required to synthesize 5-HT).

5-HT…contd

Metabolism • Oxidative deamination by MAO-A, with subsequent

conversion of the aldehyde to 5-hydroxyindole acetic acid (5-HIAA) by an aldehyde Dehydrogenase

• Reduction of the acetaldehyde to an alcohol, 5-hydroxytryptophol, is normally insignificant

5-HIAA • LEVELS: Marker for carcinoid syndrome, providing a

reliable diagnostic test for the disease• actively transported out of the brain by a process that

is sensitive to probenecid. • Metabolite excreted in the urine

• Another close relative of 5-HT, melatonin (5-methoxy-N-acetyltryptamine), is formed by sequential N-acetylation and O-methylation

• Melatonin is the principal indoleamine in the pineal gland, where it may be said to constitute a pigment of the imagination

• MAO-A preferentially metabolizes 5-HT and norepinephrine; clorgyline is a specific inhibitor of this enzyme.

• MAO-B prefers b-phenylethylamine and benzylamine as substrates; low dose selegiline is a relatively selective inhibitor of MAO-B.

• Dopamine and tryptamine are metabolized equally well by both isoforms.

• Neurons contain both isoforms of MAO, localized primarily in the outer membrane of mitochondria.

• MAO-B - isoform in platelets

5-HT RECEPTOR TYPES

• 7 TYPES• 5-HT1, 5-HT2, 5-HT3, 5-HT4. 5-HT5, 5-HT6 AND 5-HT7

• GPCR: 5-HT1, 5-HT2, 5-HT4 -- 5-HT7. • Ligand gated ion channel: 5-HT3

• SUBTYPES: Total: 14 Reecptor subtypes• 5-HT1- A,B,D,E,F• 5-HT2--- A,B,C • 5-HT5------A,B

IMPORTANT ones

• 5-HT1A, B, D: autoreceptors- inhibit serotonergic activity in brain0 e g: Buspirone. Sumatriptan (agonist)

• 5-HT2A: platelets, vasc smth M, : Ketanserin(antagonist)

• 5-HT3 : Myenetric plexuses: emesis, peristalsis: Ondansetron (antagonist);

• 5-HT4 :prokinetic: intestinal peristalsis, secretion: Metoclopramide, cisapride( agonist)

5-HT1 RECEPTORS • All 5 members of the 5-HT1 receptor subfamily inhibit

adenylyl cyclase. 5-HT1A

• found in the raphe nuclei of the brainstem, where it functions as an inhibitory, somatodendritic autoreceptor on cell bodies of serotonergic neurons

• activates a receptor-operated K+ channel and inhibits a voltage-gated Ca2+ channel, a common property of receptors coupled to the pertussis toxin–sensitive Gi/Go family of G proteins.

5-HT1D • an autoreceptor on axon terminals, inhibiting 5-HT release• Abundantly expressed in substantia nigra and basal gangli• May regulate the firing rate of dopamine (DA)-containing cells

and the release of DA at axonal terminals.

5-HT2 RECEPTORS • Distributed in: CNS, serotonergic terminal areas. • High densities : prefrontal, parietal, and somatosensory

cortex, claustrum, platelets. • Originally were described in stomach fundus. • The 3 subtypes : linked to phospholipase C with the

generation of two second messengers, diacylglycerol (a cofactor in the activation of protein kinase C) and inositol trisphosphate ( mobilizes intracellular stores of Ca2+)

• 5-HT2B: Expression of receptor messenger RNA (mRNA) is highly restricted in the CNS.

• 5-HT2C: High density in the choroid plexus, an epithelial tissue that is the primary site of CSF production.

• 5-HT2C: Implicated in feeding behavior and susceptibility to seizure.

5-HT3 RECEPTORS • Located on parasympathetic terminals in GI

tract : vagal and splanchnic afferents; CNS: solitary tract nucleus and the area postrena

• The only monoamine neurotransmitter receptor• A ligand-operated ion channel receptor. • Activation elicits a rapidly desensitizing

depolarization, mediated by the gating of cations. • In GI tract and the CNS : participate in the emetic

response, providing an anatomical basis for the antiemetic property of 5-HT3 receptor antagonists. In CINV( Chemotherapy induced Nausea& Vomition)

5-HT4 RECEPTORS• Widely distributed throughout the body. • CNS: in neurons of the superior and inferior

colliculi and in the hippocampus.• GI tract: neurons of the myenteric plexus and on• smooth muscle and secretory cells.• Thought to evoke secretion in the alimentary

tract and to facilitate the peristaltic reflex. • Couple to Gs to activate adenylyl cyclase, leading

to a rise in intracellular levels of cyclic AMP, possibly accounting for the utility of prokinetic in GI disorders

OTHER 5-HT RECEPTOR SUBTYPES• 5-HT5A- receptor couples to inhibit adenylyl

cyclase; • 5-HT5B- Functional coupling of the cloned

receptor has not been described.• 5-HT6 and 5-HT7, are linked to activation of

adenylyl cyclase. • 5-HT7 receptors play a role in smooth-muscle

relaxation in the GI tract and the vasculature.• The atypical antipsychotic drug clozapine has

a high affinity for 5-HT6 and 5-HT7 receptors

SITES OF 5-HT ACTION AND PHARMACOLOGICAL EFFECTSENTEROCHROMAFFIN CELLS AND GASTROINTESTINAL TRACT

• Enterochromaffin cells of the GI mucosa (highest density in the duodenum) synthesize and store 5-HT and other autacoids.

• Basal release of enteric 5-HT : Augmented by mechanical stretching and efferent vagal stimulation.

• An additional role in stimulating motility via the myenteric network of neurons

• ECF CELLS- enters the portal vein -metabolized by MAO-A (liver).

• 5-HT that survives hepatic oxidation is rapidly removed by the endothelium of lung capillaries and then inactivated by MAO.

ENTEROCHROMAFFIN CELLS AND GASTROINTESTINAL TRACT….contd

• Enteric 5-HT is released in response to ACh, sympathetic nerve stimulation, increases in intraluminal pressure, and lowered pH, triggering peristaltic contraction.

• 5-HT released by mechanical or vagal stimulation also acts locally to regulate GI function.

• Motility of gastric and intestinal smooth muscle may be either enhanced or inhibited via at least six subtypes of 5-HT receptors

• Abundant 5-HT3 receptors on vagal andother afferent neurons and on enterochromaffin cells play a pivotal role in emesis

PLATELETS• Differ from other formed elements of blood in

expressing mechanisms for uptake, storage, and endocytotic release of 5-HT

• Not synthesized in platelets, but is taken up from the circulation and stored in secretory granules by active transport, similar to the uptake and storage of NE by sympathetic nerve terminals

• A complex local interplay of multiple factors, including 5-HT, regulates thrombosis and hemostasis

• When platelets contact injured endothelium, they release substances that promote platelet aggregation, and secondarily, they release 5-HT

• 5-HT binds to platelet 5-HT2A receptors and elicits a weak aggregation response that is markedly augmented by the presence of collagen.

• If the damaged blood vessel is injured to a depth where vascular smooth muscle is exposed, 5-HT exerts a direct vasoconstrictor effect, thereby contributing to hemostasis, which is enhanced by locally released autocoids (thromboxane A2, kinins, and vasoactive peptides).

• Conversely, 5-HT may stimulate production of NO and antagonize its own vasoconstrictor action (5-HT1 receptor), as well as the vasoconstriction produced by other locally released agents.

CARDIOVASCULAR SYSTEM

• A variety of cardiac responses that result from activation of multiple 5-HT receptor subtypes, stimulation or inhibition of autonomic nerve activity, and reflex responses to 5-HT.

• On BP- IV: Triphasic response-early fall, brief rise follwoed by a prolonged fall

• Early Fall: due to stimulation of coronary chemoreceptors- Bezold- Jarisch Effect

• Middle Pressor: vasoconstriction and increased card. Output

• Late Depressor: arteriolar dilatation and extravasation of fluids

• Heart : Weak Positive inotropic and chronotropic actions on the heart that may be blunted by simultaneous stimulation of afferent nerves from baroreceptors and chemoreceptors.

CVS…contd• Blood vessels: Varies acc. To size of vessel and prevailing sympathetic

activity

• Large vessels: (arteries and veins)-constriction by direct action of 5-HT( via 5-HT2A)- :Contraction, particularly in the splanchnic, Renal, pulmonary, and cerebral vasculatures.

• Arterioles and VSM: inhibitory(dilatory) , the result of stimulated NO and prostaglandin synthesis and blockade of NE release from sympathetic nerves.

• Constricts venules, increased capillary pressure and escaping of fluid from capillaries

• 5-HT amplifies the local constrictor actions of NE, and Ang II

CENTRAL NERVOUS SYSTEM• A multitude of brain functions : Sleep, cognition, sensory

perception, motor activity, temperature regulation, nociception, mood, appetite, sexual behavior, and hormone secretion.

• The principal cell bodies of 5-HT neurons: in raphe nuclei of the brainstem , project throughout the brain and spinal cord.

• In addition to being released at discrete synapses,5-HT released at nonsynaptic varicosities ( axonal swelling, termed varicosities) act as a neuromodulator as well as a neurotransmitter

• Newly formed 5-HT (Nerve terminals contain all of the proteins needed to synthesize 5-HT from L-tryptophan) is rapidly accumulated in synaptic vesicles, where it is protected from MAO.

• 5-HT released by nerve-impulse flow is reaccumulated into the presynaptic terminal by the 5-HT transporter

Respiratory system

• Small direct stimulant effect on bronchiolar smooth muscle in normal humans.

• Facilitate acetylcholine release from bronchial vagal nerve endings.

• In patients with carcinoid syndrome, episodes of bronchoconstriction occur in response to elevated levels of the amine or peptides released from the tumor.

• May also cause hyperventilation as a result of the chemoreceptor reflex or stimulation of bronchial sensory nerve endings

Smooth muscles:

• GIT: increase in GIT motility (5-HT2A-intestine, 5-HT 2B-gastric fundus, 5-HT 4-enetric fundus, 5-HT3-enetric plexuses)

• Release augmented by: stretching(food), vagal stim., lowered pH

• Contraction of uterine Sm.M- Not much significant

Skeletal Muscle

• 5-HT2 receptors are present on skeletal muscle membranes, but their physiologic role is not understood.

• Malignant hyperthermia : a condition precipitated by certain anesthetic and neuromuscular agents that results in severe hyperthermia due to skeletal muscle overactivity and metabolic and autonomic instability.

• In a genetic porcine model of this condition, administration of serotonin or serotonin agonists can precipitate a typical attack. 5-HT2 blockers such as ketanserin can reduce or prevent many manifestations of the condition when evoked in this way.

• However, malignant hyperthermia provoked by anesthetic agents in these animals is not effectively prevented by 5-HT2 antagonists.

• Serotonin syndrome: similar, ut not identical condition precipitated when MAO inhibitors are given with serotonin agonists, especially antidepressants of the selective

• serotonin reuptake inhibitor class (SSRIs).

5-HT : Pathophysiological ROLE Behavior; Sleep-Wake Cycle

• Control of the sleep-wake cycle. • Depletion of 5-HT elicits insomnia that is reversed by the 5-

HT precursor, 5-hydroxytryptophan; treatment with L-tryptophan or nonselective 5-HT agonists accelerates sleep onset and prolongs total sleep time.

• 5-HT antagonists reportedly can increase and decrease slow-wave sleep, probably reflecting interacting or opposing roles for subtypes of 5-HT receptors.

• One relatively consistent finding in humans and in laboratory animals is an increase in slow-wave sleep following administration of a selective 5-HT2A/2C receptor antagonist such as ritanserin.

Aggression and Impulsivity• Critical role in aggression and impulsivity. • Low 5-HIAA is associated with violent suicidal acts, but not

with suicidal ideation per se. • Knockout mice lacking the 5-HT1B receptor exhibit extreme

aggression, suggesting either a role for 5-HT1B receptors in the development of neuronal pathways important in aggression or a direct role in the mediation of aggressive behavior.

• Abnormalities in 5-HT are related to aggressive behaviors.

Anxiety and Depression• A mechanism for altering synaptic availability of 5-HT is

inhibition of presynaptic reaccumulation of neuronally released 5-HT.

• Selective serotonin reuptake inhibitors (SSRIs; e.g., fluoxetine) potentiate and prolong the action of 5-HT released by neuronal activity.

• Effects of 5-HT-active drugs, like the SSRIs, in anxiety and depressive disorders strongly suggest an effect of 5-HT in the neurochemical mediation of these disorders.

• SSRIs are the most widely used treatment for endogenous depression .

Appetite Control Through Serotonin?

• Related to several general behaviors such as sleep,emotion, sex, and appetite

• Studies in lab animlas: reduce food intake in animals, an anorexigenic effect

• A profound reduction in levels of 5-HT in the brain lasts for weeks and is accompanied by a loss of proteins (5-HT transporter and tryptophan hydroxylae) selectively localized in 5-HT neurons

• Sibutramine , an inhibitor of the reuptake of 5-HT, NE, and DA, is used as an appetite suppressant in the management of obesity

• Whether effects on a single neurotransmitter system are primarily responsible for sibutramine’s effects in obese patients is unclear

• Fenfluramine and dexfenfluramine- a racemic chemical more closely related to amphetamine than to serotonin- Anorexigenic drugs/ antiobesity drugs

DRUGS AFFECTING ACTION OF 5-HT 5-HT RECEPTOR AGONISTS:• Selective- Azapirones, Triptans, Cisparide,Renzapride

• Non selective- LSD, Metoclopramide

5-HT RECEPTOR ANTAGONISTS:• Selective- Ketanserin, ritanserin, Risperidone, Ondansetron

• Non selective- Ergot alkaloids, cyproheptadine

5-HT1 RECEPTOR AGONIST DRUGS• 5-HT1A receptor–selective agonists -Antianxiety

drugs: buspirone, gepirone, and ipsapirone• 5-HT1D receptor selective agonists (TRIPTANS.,

sumatriptan, zolmitriptan ,naratriptan rizatriptan ) cause constriction of intracranial blood vessels - Anti migraine drugs(acute migraine attacks)

• Therapy of migraine headaches is complicated by the variable responses among and within individual patients and by the lack of a firm understanding of the pathophysiology of the syndrome.

• The efficacy of antimigraine drugs varies with the absence or presence of aura, duration of the headache, its severity and intensity, and as yet undefined environmental and genetic factors

Triptans: MOA

Two hypotheses • Constricts intracranial blood vessels, including

arteriovenous anastomoses, and restores blood flow to the brain.

• may block the release of proinflammatory neuropeptides at the level of the nerve terminal in the perivascular space.

D-Lysergic Acid Diethylamide (LSD)• Nonselective 5-HT agonist. • Psychedelics (LSD) and other psychotomimetics: mescaline and

psilocybin can induce states of altered awareness, or induce hallucinations and anxiety, probably mediated by 5-HT2A receptors.

• Profoundly alters human behavior, eliciting perception disturbances such as sensory distortion (especially visual and auditory) and hallucinations at low doses ( 1 mg/kg)

• Overactivity of these receptors: a role in the genesis of negative symptoms in schizophrenia and sleep disturbances

• Abuse by human beings and the fascination of scientists with the mechanism of action of LSD.

• LSD interacts with brain 5-HT receptors as an agonist/partial agonist. LSD mimics 5-HT at 5-HT1A autoreceptors on raphe cell bodies, partial or full agonists at 5-HT2A and 5-HT2C receptors.

Ergot and the Ergot Alkaloids

• Ergot is the product of a fungus (Claviceps purpurea) that grows on rye and other grains.

• The contamination of an edible grain by a poisonous, parasitic fungus spread death, causing gangrene of the extremities and limbs, and abortion when ingested during pregnancy.

Pharmacological effects of the ergot alkaloids• Effects result from their actions as partial agonists or antagonists at

adrenergic, dopaminergic, and serotonergic receptors • The spectrum of effects depends on the : agent, dosage, species, tissue,

physiological and endocrinological state, and experimental conditions.• Synthetic ergot derivative, bromocriptine (2-bromo-a-ergocryptine), is

used to control the secretion of prolactin - a property derived from its DA agonist effect.

• Other agents : Lysergic acid diethylamide (LSD), a potent hallucinogenic drug, and methysergide, a 5-HT antagonist.

Use of Ergot Alkaloids in Postpartum Hemorrhage

• All natural ergot alkaloids markedly increase the motor activity of the uterus. -contractions become more forceful and prolonged, resting tone is dramatically increased, and sustained contracture can result.

• This precludes their use for induction or facilitation of labor

• Compatible with the use postpartum or after abortion to control bleeding and maintain uterine contraction.

• primarily to prevent postpartum hemorrhage.Ergonovine (ergometrine) is the most active and also is

less toxic than ergotamine. Methylergonovine ( & methyl ergometrine) - Uterine-

stimulating agents in obstetrics.

Ergot alkaloids: Contraindications: • pregnancy, patients with peripheral vascular

disease, coronary artery disease, hypertension, impaired hepatic or renal function, and sepsis.

• Ergot alkaloids should not be taken within 24 hours of the use of the triptans, and should not be used concurrently with other drugs that can cause vasoconstriction.

Adverse Effects• Nausea and vomiting, Leg weakness is common;

muscle pains, occasionally severe,may occur in the extremities, as well as numbness and tingling of extremities.

Bromocriptine

• is extremely effective in reducing the high levels of prolactin that result from pituitary tumors and has even been associated with regression of the tumor in some cases.

• Cabergoline is similar but more potent.• Pseudopregnancy in bitches

Ketanserin and Ritanserin• Potently blocks 5-HT2A receptors, less potently blocks 5-

HT2C receptors, and has no significant effect on 5-HT3 or 5-HT4 receptors or any members of the 5-HT1 receptor family.

• Also has high affinity for a adrenergic and histamine H1 receptors.

• Lowers blood pressure in patients with hypertension, causing a reduction comparable to that seen with b adrenergic-receptor antagonists or diuretics.

• This effect likely relates to its blockade of alpha 1 adrenergic receptors, not its blockade of 5-HT2A receptors.

• Inhibits 5-HT-induced platelet aggregation but does not greatly reduce the capacity of other agents to cause aggregation.

• treatment of bloat in ruminants ……

Methysergide • A congener of methylergonovine.• Blocks 5-HT2A and 5-HT2C receptors but has partial

agonist activity in some preparations. • Inhibits the vasoconstrictor and pressor effects of 5-HT,

as well as the actions of 5-HT on various types of extravascular smooth muscle.

• It can both block and mimic the central effects of 5-HT.• Not selective: it also interacts with 5-HT1 receptors • Therapeutic effects appear primarily to reflect blockade

of 5-HT2 receptors.• Weak vasoconstrictor and oxytocic activity.• Used for the prophylactic treatment of migraine

Cyproheptadine • The structure resembles that of the phenothiazine histamine

H1 receptor antagonists• An effective H1 receptor antagonist. • Also Prominent 5-HT blocking activity on smooth muscle via

its binding to 5-HT2A receptors. I• In addition: Weak anticholinergic activity, mild CNS

depressant• Effective in controlling skin allergies, the accompanying

pruritus. • Appetite stimulant: • The 5-HT blocking actions of cyproheptadine: value in the

postgastrectomy dumping syndrome, GI hypermotility of carcinoid syndrome, and migraine prophylaxis.

• Not, however, a preferred treatment for these conditions. • Side effects : drowsiness. weight gain and increased growth

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