Serotonin or5-Hydroxytryptamine

Department of Pharmacology

NEIGRIHMS, Shillong

Contents

• Biosynthesis, storage and metabolism of Serotonin

• Serotonergic Receptors and their distribution

• Actions of Serotonin and Physiological Roles 5-HT receptor antagonists

• Migraine and its drug therapy

• Newer drugs used in Migraine

What is Serotonin?

• Serotonin and Enteramine• Serotonin or 5-Hydroxytryptamine (5-HT) is a

monoamine neurotransmitter, biochemically derived from tryptophan

• Feelings of well-being• Present in GI enterochromaffin cells (90%)

– 90% in EC cells and 10% in neurones of different plexuses• 10% in platelets and brain• Stored in granules like catecholamine• Also present in plants and insects• It is also present in plant like tomato, banana &

pineapple lower animals (mollusks, arthropods, snake and bee venom/sting)

Physiologic Distribution of Serotonin(5-Hydroxytryptamine (5-HT)

10% CNS10% CNS

90% GI tract

90% EC s 10% Neurons

90% GI tract

90% EC s 10% Neurons

5-Hydroxytryptamine5-Hydroxytryptamine

NH2

OH

NH

5

Synthesis, Metabolism and Destruction

• Dietary tryptophan– converted to 5–hydroxy– tryptophan by

tryptophan hydroxylase– then to 5-HT by a non–specific decarboxylase

• Degradation – mainly monoamine oxidase (MAO–A > MAO–

B)– 5–hydroxyls Indole acetic acid (5-HIAA) in

urine

N

C

N

C NH2

COOH COOH

NH2

OH

N

C NH2

OH H

Tryptophan 5-Hydroxytryptophan

5-Hydroxytryptamine

N

C COOH

5-OH Indole Acetaldehyde

5-Hydroxy Indole Acetic Acid

Tryptophan hydroxylase

5-OH Tryptophan decarboxylase

MAO

Aldehyde dehydrogenase

(Rate limiting)

In diet. ActiveCNS transport

Dietary

Serotonin Uptake• Non-specific Decarboxylase

produces 5-HT (also CA)• Amine pump (SERT) actively

takes up Serotononin (also CA and also present in platelets) – Na+ dependent carrier

• Stored in storage vesicles by active uptake – Vesicular monoamine transporter (VMAT 2)

– SLC family• 5-HT transporter proteins - outer

membrane of platelets • Deficiency of tryptophan

hydroxylase 5-HT can’t synthesized in platelets – but actively taken up from the serotonin transporter proteins

MAO

Modulation of Serotonin Uptake

• 5-HT Precursor – ↑Tryptophan in diet ↑ 5-HT production

• Inhibit uptake into CNS (other AA’s)• Inhibit synthesis: p-chlorophenylalanine (PCPA) -

irreversible• Inhibit neuronal re-uptake: Cocaine, SSRA (e.g.

fluoxetine, Paroxetin), TCA (e.g. Imipramine)• Inhibit storage-deplete (VMAT-2): Reserpine• Inhibit metabolism: MAO inhibitors (MAO-A)• Promote release: p-chloroamphetamine - then depletes

(e.g. fenfluramine, dexfenfluramine to ↓ appetite)

5-HT Receptors

• These receptors are of seven types (5-HT1, 5-HT2 ,….5-HT7)• 5-HT1 (A to F subtypes) and 5-HT2 have (A - D) subtypes

• 5-HT1A 5-HT1B…. 5-HT1F• 5-HT2A 5-HT2B…. 5-HT2D

• All are GPCR except 5-HT3 – All are cAMP except 5-HT2 (IP3-DAG)– 5-HT3 is a ligand gated Na+ channel

• 5-HT1 inhibits cAMP and 5-HT4, 5-HT5 and 5- HT6 increases cAMP

5-HT1 - Receptors

• All are autoreceptors and inhibit firing of neurones or release of 5-HT

• 5-HT1A – Brainstem and hippocampus (antidepressant – Buspirone)

• 5-HT1D – Basal ganglia and substancia nigra (dopeminergic)

• 5-HT1B/1D – Cranial Blood Vessels (for constriction – sumatriptan - agonist)

5-HT2

• 5-HT2: Mainly 4 subtypes (5-HT2A ….... 5-HT2D)– Located in all smooth muscles - vascular, visceral– Platelets– Also in 5-HT2– Mediates direct effects of HT

• Vasoconstriction, intestinal, uterine and bronchial constrictions

• Ketanserin, Cyproheptadine, Methysergide are specific antagonists

5-HT3

• Peripheral– located exclusively on neurons and mediate

neurotransmitter release - parasympathetic, sympathetic, sensory and enteric

– cardiac inhibition/activation, pain, initiation of the vomiting reflex

• Central– facilitate dopamine and 5–HT release, inhibit ACh and

noradrenaline release – anxiety, depression, memory, tolerance and dependence

• Ondansetron in specific antagonist

5-HT4-7

• Mucosa, plexuses and smooth muscles of Gut– Augmentation of Intestinal secretion and

peristalsis

• In Brain: hippocampaus area– Specific role is unknown till now– Cizapride – specific agonist of the receptor

Actions of 5-HT - CVSMultiple direct and indirect effects:• Powerful vasoconstrictor except skeletal muscle and heart vessels 5-HT2

mediated)• Overall, large arteries and veins are constricted but in microcirculation

arterioles dilate• Constriction of veins – escape of fluid• Heart: direct ionotropic and chronotropic effects• Reflex action (5-HT3 mediated) in chemoreceptor nerve endings:

Bradycardia and hypotension due to stimulation of sensory nerve endings in baro-receptors, chemo-receptors and in vagal afferents in coronary circulation (Bezold Jarrisch reflex)

• Triphasic response on BP:– Early sharp fall: Coronary chemoreflex– Brief rise in BP: vasoconstriction and increased cardiac output– Prolonged fall in BP: arteriolar dilatation and extravasation of fluid

(Not involved in Physiological Regulation of BP) – PreeclmpsiaKetanserin – 5-HT antagonist (5-HT2)

Actions of 5-HT - GIT

• Entero-chromaffin cells in the mucosa main 5-HT in body

• ↑ GIT motility – partly through direct effect on the smooth muscle cells (5-HT2

receptor)– partly as a result of indirect excitatory effect on enteric neurons

(5-HT3 & 4 receptors)

• Also stimulates vomiting (5-HT3 receptors on vagal afferents and centrally) on 5-HT receptors

• 5-HT inhibit gastric acid and pepsin, secretion however increase mucus production thus it has ulcer protective property

(Physiologically regulates GI motility and peristalsis)

Actions of 5-HT – contd.

• Respiration:– Bronchoconstriction of smooth muscles of Bronchi and reflex

stimulation (bronchial afferents) of Respiration and hyperventiltion

– Boronchoconstriction is of lesser magnitude than histamine

• Platelets:– Responsible physiologically for Haemostasis by promoting

platelet aggregation and clot formation, thereby preventing leakage

– Works in conjunction with collagen and other mediators– Exogenous 5-HT also changes shape of platelets in injury and

promote aggregation (via 5-HT2A)

Actions of 5-HT – Nervous System

• Neurotransmitter in serotonergic neuron (raphe nuclei, pons & medulla)

• These neurons mainly regulate appetite, mood, sleep body temperature, thought, pain perception, vomiting & behaviors (normal and abnormal)

• 5-HT poorly cross blood brain barrier however direct injection in brain produces sleepiness, change in body temperature, vomition & appetite

• Serotonin stimulates adrenal gland (autonomic ganglia) for release catecholamine but lesser than histamine

• Also activates afferent nerve endings – tingling, pricking and pain sensations

• 5-HT is Melatonin precursor in pineal body – biological clock

Actions of 5-HT - Others

• Migraine: – Responsible for causation of migraine– Both antagonist and agonist of 5-HT re used in

migraine treatment

• Carcinoid tumor: – Produces massive 5-HT– Increased bowel motility and bronchoconstriction– Pellagra – due to diversion of tryptophan for 5-HT

synthesis

5-HT antagonists

Antagonists of serotonin

• Ergot derivatives: Ergotamine, ergonovine and methysergide (Carcinoid) etc.

• Alpha-blockers like phenoxybenzamine• Antihistaminics: Cyproheptadine, cinnarizine• Phenothiazines like chlorpromazine• Ketanserin: used as antihypertensive• Clozapine: for schizophrenia• Metoclopramide, Ondansetron and Granisetron

are currently available as anti-emetic for chemotherapeutic induced nausea and vomiting (5-HT3 antagonists)

Cyproheptadine

• 5-HT2A blocking property• Also H1 antihistamic, anticholinergic and sedative action• Famous for increasing appetite• Uses:

– Allergic reactions like hay fever– Serotonin syndrome– Carcinoid syndrome– Priapism (fluoxetine)– Appetite stimulation in children

• Adverse effects: weight gain, drowsiness, dry mouth etc.

Ketanserine

• Selective 5-HT2 receptor blocker• Additional H1, α1 and dopaminergic blocking

action• Negligible action on other 5-HT receptors• Antagonizes vasoconstriction, platelet

aggregation and airway constriction actions of 5-HT

• Used as antihypertensive agent• Other drugs like – clozapine, risperidone and

ondansetron will be discussed elsewhere !

Ergot Alkaloids and derivatives

1. Ergot alkaloids ---- toxic effects 2. First isolated by Dale & Barger in 1906 from fungus Claviceps

purpurea ---- on rye and other grains3. Contains – LSD, histamine, acetylcholine and tyramine etc.4. They have long been recognized as abortifacients and poisonings

produced an intense burning sensation & gangrenous condition in the limbs as a result of persistent peripheral vasoconstriction

5. Diverse pharmacological actions – agonist, antagonist and partial agonist of serotonin, alpha-receptor and dopaminergic receptors

Classification of ergot alkaloids

1. Natural – Derivatives of the tetra-cyclic compounds (lysergic acid)Amine alkaloids – Ergometrine (ergonovine)Amino acid alkaloids - Ergotamine, Ergotoxine

2. Semi-synthetic – Bromocriptine, Methysergide, Dihydro-ergotamine (DHE)

3. Synthetic – (non lysergic acid derivative) Metergotine

Ergometrine

• Amine ergot alkaloid• Partial agonist of 5-HT receptor in uterus,

placenta and umbilical blood vessels• No antagonistic effect of alpha-receptor• Moderately potent antagonist of 5-HT2 in

intestine• Less dopaminergic action in CTZ – no vomiting• Uterine myometrium• Will be discussed elsewhere !

Ergotamine

• Amino acid alkaloid• Partial agonist of alpha adrenergic and 5-HT1

and 5-HT2 receptors, but no interaction with 5-HT3 or dopaminergic receptors

• Actions: – Sustained vasoconstriction, visceral smooth muscle

contraction and vasomotor centre depression– Antagonizes action of NA and 5-HT in smooth

muscles– Potent emetic via CTZ and oxytotic– Prolong use – vasoconstriction and damage

endothelium

Dihydroergotamine (DHE)

• Hydrogenated ergotamine

• Less serotonergic action than ergotamine and alpha-adrenergic action

• Better blocker of alpha adrenergic receptor

• Less vasoconstrictor and so less intimal damage

• Lesser oxytotic and emetic

Migraine Therapy

What is migraine• Severe, throbbing, pulsating headache usually unilateral headache (few

hours to a few days in duration) • Associated with nausea, vomiting, sensitivity to light and sound, flashes of

light, loose motion and others• Types:

– Classical with aura– Without aura (common)

• Pathophysiology:– Pulsatile dilatation of temporal or certain cranial vessels– Vascular theory: initial vasoconstriction or shunting of blood through carotid

arterio-venous anastomosis causing cerebral ischaemia– Neurogenic theory: depression of cortical electrical activity followed by

depression• Migraine attack associated with (based on histological studies):

• sterile neurogenic perivascular edema • inflammation (clinically effective antimigraine medication reduce

perivascular inflammation)

Pharmacotherapy of Migraine

• Three types: Mild, Moderate and Severe• Mild: NSAIDS and Antiemetics (optional)

• Ibuprofen (400 mg 8 hrly)• Paracetamol (500 mg 8 hrly)• Naproxen (250 mg 8 hrly)• Mefenamic acid (500 mg 8 Hrly)• Diclofenac (50 mg 8 Hrly)

– Antiemetics:• Metoclopramide (10 mg oral or IV)• Domperidone (10 mg oral)

Migraine - Moderate

• Intense throbbing headache lasting for 6 – 24 Hrs, nausea and vomiting and functionally impaired patient– NSAIDs– Antiemetics– Specific drugs like ergots and others

(sumatriptan)

Migraine - Severe

• More than 2-3 attacks per month lasting for 12 – 48 hrs, often vertigo and vomiting and patient is completely incapacitated– NSAIDS cannot relieve symptoms– Specific antimigraine drugs like ergot

alkaloids and triptans– Also prophylactic regimens

Ergotamine• Most effective ergot alkaloid• MOA:

– Partial agonist of 5-HT1B/1D– Constriction of dilated temporal vessels– Constriction of carotid AV shunt channels– Reduction of neurogenic inflammation and leakage of plasma in

duramater• Doses: oral/sublingual (1 mg every half an hourly) till relief – 6 mg/

day• Kinetics: only 1% Bioavailability orally – high first pass metabolism

– Given by SL route and rectal– Metabolized in liver and excreted in bile– Sequestrated to tissues and long lastin effect– Crosses BBB

• ADRs: Nausea, vomiting, abdominal pain, muscle cramps etc. Also chest pain, vascular spasm etc.

DHE

• Almost equally effective as ergotamine• Preferred for parenteral administration• Erratic oral absorption – combined with caffeine

100 mg for enhancing oral absorptionErgo status in Migraine:

– Not popular anymore– Regular use is hazardous– No prophylactic value – precipitate on discontinuation– Dull headache– Combination with caffeine, paracetamol, belladona

etc. are available

Selective 5-HT1B/1D agonistSumatriptan

• Selective agonist of 5-HT1B/1D receptor• No interaction with other 5-HT receptors• No interaction with adrenergic, dopaminergic and cholinergic

receptors, GABAMOA:• Blockade of 5-HT1D/1B mediated constriction dilatation of

extracerebral blood vessel• Constriction of arteriovenous shunt of carotid artery• Inhibition of release of 5-HT and inflammatory neuropeptides around

the affected vessels – supression of neurogenic inflammation• Supression of impulse transmission in trigeminovascular systemKinetics:• Poorly absorbed from GIT, bioavailability – 10 – 15% only• Complete absorption after subcutaneous administration• Metabolized by MAO-A and excreted in urine, t1/2 is 2-3 Hrs

Sumatriptan – contd.

• Administration & Doses:– Onset of acute attack– Better tolerated than ergotamine– 50 to 100 mg as initial dose and repeated after 24 Hrs if required– Should not be given is first dose fails– SC dose – 12 mg (1 ml) stat and repeated if required

• Adverse effects: Dose related - Tightness of chest, feeling of heat, paresthesia of limbs, dizziness and weakness (short lasting) – common with SC route– Risk of MI and seizure and death

• Contraindications: IHD, epilepsy, hypertension, pregnancy, hepatic and renal impairment

(Rizatriptan, zolmitriptan, naratriptan, almotriptan)

Prophylaxis of Migraine

• Necessary when attacks are frequent – 2 (two) or more attacks per month

• Aim is to abolish attack totally• Discontinue every 4 - 6 months and observe• Beta-adrenergic blockers – Propranolol (40 mg BD),

timolol etc. (not metoprolol or atenolol)• TCAs: Amitryptylline (25 – 50 mg BD)• Calcium channel blockers: Verapamil – not used now

(flunnarizine – weak Ca channel blocker is effective)• Anticonvulsants: Valproic acid (400 – 1200 mg/day),

gabapentin (300 – 1200 mg per day) and newer topiramate (25 mg OD) – effective

Thank you