serotonin agonist &antagonist

serotonin(5-hydroxytryptamine)

By

Dr. Shipra Jain

Asst. Professor,

Dept. of Pharmacology

MGMC&H, Jaipur

HISTORY In 1935, Vittorio Erspamer showed that an extract

from enterochromaffin cells made intestines contract. Two years later, Erspamer was able to show that it was a

previously unknown amine, which he named enteramine. In 1948, Maurice M. Rapport, Arda Green, and Irvine Page of the

Cleveland Clinic discovered a vasoconstrictor substance in blood serum and named it SEROTONIN.

5-HT was synthesised in 1951 .In 1957,Gaddum & Picarelli proposed 2 distinct 5-HT receptor

subtypes-M & D receptors.In 1979, Peroutka & Snyder provided evidence for 2 distinct

recognition sites for 5-HT.

ANATOMY• The neurons of the raphe nuclei are the principal source of 5-HT

release in the brain.• The raphe nuclei are neurons grouped into 9 pairs and

distributed along the entire length of the brainstem, centered around the reticular formation.

• Axons from the neurons of the raphe nuclei form a neurotransmitter system, reaching almost every part of the CNS.

• Axons of neurons in the lower raphe nuclei terminate in the cerebellum and spinal cord, while the axons of the higher nuclei spread out in the entire brain.

DOPAMENERGIC and SEROTONERGIC pathway in Brain

THE CHEMICAL STRUCTURE

SEROTONIN SYNTHESIS

Pathway for serotonin synthesis from tryptophan. Abbreviations: THP = tryptophan hydroxylase, DHPR = dihydropteridine reductase, H2B = dihydrobiopterin, H4B = tetrahyrobiopterin, 5-HT = 5-hydroxytryptophan, AADC = aromatic L-amino acid decarboxylase.

Hydroxylation at C5 is rate limiting step ,can be blocked by p-chlorophenylalanine & p-chloroamphetamine.

5-HT is concentrated in synaptic vesicles by a vesicle associated transporter (VAT) that is blocked by reserpine.

Serotonin or 5-Hydroxytryptamine (5-HT) is a monoamine

neurotransmitter.

Biochemically derived from tryptophan, serotonin is primarily found in the

• Enterochromaffin cells in Gastrointestinal tract (˃ 90 %)• Platelets• Central nervous system (CNS)

TERMINATION• Serotonergic action is terminated primarily via uptake of 5-HT

from the synapse.

• This is accomplished through the specific monoamine transporter for 5-HT, SERT, on the presynaptic neuron.

• Another monoamine transporter known as PMAT has been regarded to be important in the clearance of Serotonin.

5-ht RECEPTORS

• The 5-HT receptors are the receptors for serotonin. • They are located on the cell membrane of nerve cells and

mediate the effects of serotonin as the endogenous ligand .• All 5-HT receptors (except 5-HT3) are G protein-coupled, seven

transmembrane (or heptahelical) receptors that activate an intracellular second messenger cascade.

SEROTONIN RECEPTORSFamily Type Mechanism Potential

5-HT1 G-protein coupled. Decreasing cellular levels of cAMP. Inhibitory

5-HT2 G-protein coupled. Increasing cellular levels of IP3 and DAG. Excitatory

5-HT3Ligand-gated Na+ and K+ cation channel.

Depolarizing plasma membrane. Excitatory

5-HT4 G-protein coupled. Increasing cellular levels of cAMP. Excitatory

5-HT5 G-protein coupled. Decreasing cellular levels of cAMP. Inhibitory

5-HT6 G-protein coupled. Increasing cellular levels of cAMP. Excitatory

5-HT7 G-protein coupled. Increasing cellular levels of cAMP. Excitatory

5- HT 1 RECEPTOR• G protein coupled receptor acts by ↓ cAMP• It is of 6 types• 5-HT 1A & 5-HT 1B are autoreceptors• 5-HT 1A –Localized in raphe nuclei & hippocampus

Selective agonist- 8-OH DPATSelective antagonist- WAY100635

• 5- HT 1B - Localized in Substantia niagra, globus pallidus ,basal gangliaSelective agonist- Sumatriptan, CP93129

• 5-HT 1D – Localized in brain5-HT 1B/1D cause constriction of cranial blood vesssels. Selective agonist- SumatriptanIt also cause inhibition of NA release from sympathetic nerve endings & inflammatory neuropeptides from nerve endings in cranial blood vessels

• 5-HT 1E – Localized in cortex, putamen

• 5-HT 1F- Localized in cortex , hippocampusSelective agonist- LY334370

• 5-HT 1P- Localized in enteric nervous systemSelective agonist- 5-HydroxyindalpineSelective antagonist - Renzapride

5- HT 2 RECEPTOR• It is a G protein coupled receptor acts by ↑IP3/DAG• Earlier designated as D type.• It is of 3 types.• 5-HT 2A -Most widely expressed, localized in platelets, vascular &

visceral smooth muscle, cerebral cortex. Mediates most of the direct actions of 5-HT like vasoconstriction,

intestinal, uterine & bronchial contraction, platelet aggregation & activation of cerebral neurons.

Selective agonist – α-methyl-5-HT Selective antagonist – Ketanserin

• 5-HT 2B – Localized in stomach fundusSelective agonist- α-methyl-5-HTSelective antagonist – SB204741

• 5- HT 2C – Localized in choroid plexus, hippocampus, substantia niagra. It elicits vasodilatation through EDRF release.Selective agonist- α-methyl-5-HTSelective antagonist – Mesulergine

• 5- HT 3 receptor- It is a Na+ - K+ ion channel Localized in area postrema, sensory & enteric nerves Mediates indirect & reflex effects of 5-HT at :a) Somatic & autonomic nerve endings- pain, itch ,coronary

chemoreflex (bradycardia, ↓BP due to withdrawal of sympathetic tone, respiratory stimulation or apnea elicited by stimulation of receptors in coronary bed).

b) Nerve endings in myenteric plexus- ↑ peristalsis, emetic reflex

c) Area postrema & NTS in brain stem- nausea, vomiting

Selective agonist – 2-methyl-5-HT, Selective antagonist – Granisetron, Ondansetron ,Tropisetron

• 5- HT 4 receptor- It is a G protein coupled receptor acts by ↑ cAMP Localized in hippocampus & colliculi (↓ K conductance),

smooth muscle of gut (↑ intestinal secretion & peristalsis) Selective agonist- 5-methoxytryptamine, Renzapride

• 5-HT 5 receptor It is a G protein coupled receptor acts by ↓ cAMP Localized in hippocampus It is of 2 types-5-HT 5A & 5-HT 5B

• 5-HT 6 receptor It is a G protein coupled receptor acts by ↑ cAMP Localized in striatum

• 5-HT 7 receptor It is a G protein coupled receptor acts by ↑ cAMP Localized in hypothalamus, intestine.

ACTIONS1. CVS- Arteries are constricted (by action on smooth muscle) &

dilated (through EDRF release) by direct action of 5-HT depending on vascular bed & basal tone.

It releases Adr from adrenal medulla In microcirculation, it dilates arterioles & constricts venules

capillary pressure ↑ & fluid escapes In intact animals, bradycardia seen due to activation of coronary

chemoreflex (Bezold Jarisch refex) through action on vagal afferent nerve endings in coronary bed evoking bradycardia, hypotension & apnoea.

BP – triphasic response is classically seen on iv injectionEarly sharp fall in BP-due to coronary chemo reflexBrief rise in BP- due to vasoconstriction & ↑ COProlonged fall in BP- due to arteriolar vasodilatation & extravasation of fluid.

2.Smooth Muscle-GIT- ↑ peristalsis & diarrhoea (also due to ↑ secretion)Bronchi- It constricts bronchi but is less potent than histamine.

3. Glands – It inhibits gastric secretion (acid & pepsin) but ↑ mucus production ulcer protective property

4.Nerve endings & Adrenal medulla- Activation of afferent nerve endings- tingling & pricking sensation, pain

5.Respiration- Brief stimulation of respiration & hyperventilation. Large doses can cause transient apnea (coronary chemoreflex).

6.Platelets- It causes changes in shape of platelets & is a weak aggregator (5-HT 2A receptor)

7.CNS-Direct injection in brain causes sleepiness, change in body temperature, hunger & behavioural effects.

PATHOPHYSIOLOGICAL ROLES1. Neurotransmitter- It is a neurotransmitter in brain & is

involved in sleep, temperature regulation, thought, cognitive function, behaviour & mood(imbalance causes affective disorders & schizophrenia), vomiting & pain perception

2. Precursor of melatonin in pineal gland- regulates biological clock & maintain circadian rhythm.

3. Neuro-endocrine function- Hypothalamic neurons that control release of anterior pituitary hormones are probably regulated by serotonergic mechanism.

4.Nausea & vomiting- especially evoked by cytotoxic drugs or radiotherapy is mediated by release of 5-HT (5-HT 3 receptors in gut ,area postrema & NTS)

5.Migraine – It initiates vasoconstrictor phase of migraine & participates in neurogenic inflammation of cranial blood vessels.

6.Haemostasis – It causes platelet aggregation & clot formation at site of injury to blood vessel & also promotes retraction of injured vessel.

7.Raynaud’s phenomenon- 5-HT release from platelets may trigger acute vasospastic episodes of larger arteries. Ketanserin (5-HT 2 antagonist) is used as a prophylactic.

8. Hypertension - ↑ responsiveness to 5-HT & ↓ uptake & clearance by platelets seen in hypertensives. Ketanserin has anti-hypertensive property

9.Intestinal motility- Enterochromaffin cells & 5-HT containing neurons regulate peristalsis & local reflexes in gut(activated by intestinal distension).

10.Carcinoid tumours-They produce massive amounts of 5-HT. Bowel hypermotility & bronchoconstriction in carcinoid is due to 5-HT.Pellagra may occur due to diversion of tryptophan for synthesizing 5-HT.

DRUGS AFFECTING 5-HT SYSTEM

1. 5-HT Precursor- Tryptophan increases brain 5-HT& produces behavioural effects.

2. Synthesis lnhibitor- p-Chlorophenylalanine (PCPA)selectively inhibits tryptophan hydroxylase (rate limitingstep) & ↓5-HT level in tissues. It is not used clinically due to high toxicity.

3. Uptake Inhibitor – TCAs inhibit 5-HT & NA uptake. Fluoxetine & Sertraline are selective serotonin reuptake inhibitors(SSRI).

4.Storage Inhibitor –Reserpine blocks 5-HT (as well as NA) uptake in storage granules & cause depletion. Fenfluramine selectively releases 5-HT & has anorectic property.

5. Degradation Inhibitor – Nonselective MAO inhibitor (Tranylcypromine) & Selective MAO inhibitor (Clorgyline) ↑ 5-HT content by inhibiting degradation.

6. Neuronal Degeneration- 5,6 dihydroxytryptamine selectively destroys 5-HT neurons.

5-HT AGONISTS1. D-Lysergic acid diethyl amide (LSD)- It is a potent hallucinogen.

It activates 5-HT 1A on raphe cell bodies, 5-HT 2A/2C (responsible for hallucinogenic effect) & 5-HT 5-7 in brain.

2. Azapirones – They include Buspirone, Gepirone & Ipsapirone. It is a new class of anti-anxiety drugs which do not cause sedation. They act as partial agonist of 5-HT 1A receptors in brain.

3. 8-Hydroxydipropylamino tetraline (8-OH DPAT) – It is a highly selective 5- HT 1A agonist used as an experimental tool.

4. Triptans like Sumatriptan, Zolmitriptan,Naratriptan,Rizatriptan- selective 5-HT 1B/1D agonist, constrict cerebral blood vessels & is the most effective treatment of migraine attacks.

5.Cisapride,Mosapride -5-HT 4 agonist. It increases GI motility & is used in GERD, irritable bowel syndrome. Renzapride & Prucalopride is also 5- HT 4 agonist.

5-HT ANTAGONISTS1. Cyproheptadine – It blocks 5-HT 2A receptors & also has H1

anti-histaminic, anticholinergic & sedative properties.It is used to ↑ appetite in children & poor eaters to promote weight gain.Used to control intestinal manifestations of carcinoid & post-gastrectomy dumping syndromes. Also in antagonizing priapism/orgasmic delay caused by 5-HT uptake inhibitors (Fluoxetine & Trazadone).Used in allergic conditions.Side effects- drowsiness, dry mouth, confusion, ataxia, weight gain.

2.Methysergide – It antagonizes actions of 5-HT on smooth muscles including that of blood vessels. Potent 5-HT 2A/2C antagonistUsed in migraine prophylaxis, carcinoid & post-gastrectomy dumping syndromes.Side effects- abdominal, pulmonary & endocardial fibrosis on prolonged use.

3. Ketanserin- It is selective 5-HT 2 antagonist (5-HT 2A> 5-HT2C). It blocks 5-HT induced vasoconstriction, platelet aggregation & contraction of airway smooth muscle. It also has α1, H1 & dopaminergic blocking activity.It is used as an effective anti-hypertensive (α1 blockade)Ritanserin is a more selective 5-HT 2A congener of Ketanserin.

4. Clozapine - It is an atypical antipsychotic which blocks dopamine & 5-HT 2A/2C receptors. It is also an inverse agonist at cerebral 5-HT 2A/2C receptors so used in resistant schizophrenia.

5. Risperidone – This atypical antipsychotic is 5-HT 2A & D2 antagonist like Clozapine.It is used in schizophrenia.

6. Ondansetron – It is selective 5-HT 3 antagonist.It is used in controlling nausea & vomiting due to anticancer drugs & radiotherapy.Granisetron & Tropisetron are other selective 5-HT 3 antagonists.

CLINICAL PHARMACOLOGY OF 5-HT

• Serotonin has no clinical application as a drug.• However, its receptor agonism & antagonism has many

clinical implications.• Serotonin is closely related with behaviour, mood & is

therefore important in many neuropsychiatric diseases and their symptomatology.

• It also has a role in many other clinical conditions.

USES 1. Major Depression– SSRIs like Fluoxetine, Sertraline2. Obsessive compulsive disorder – SSRIs3. Phobic disorders - SSRIs4. Premenstrual symptoms - SSRIs5. Somatoform disorders (Hypochondriasis) - SSRIs6. Post traumatic stress disorder - SSRIs7. Neuropathic pain & Fibromyalgia pain8. Anxiety neurosis – Azapirones like Buspirone, Gepirone

Ipsapirone (5- HT 1A partial agonist)

OTHER CLINICAL USES1. Migraine – Triptans like Sumatriptan (Imitrex) , Zolmitriptan

(Zomig) , Naratriptan (Amerge) ,Rizatriptan (Maxalt) – 5-HT 1B/1D agonist

2. Appetite stimulant – Cyproheptadine (5-HT 2A antagonist)3. Schizophrenia – Clozapine (5-HT 2A/2C antagonist)4. Chemotherapy induced nausea & vomiting (CINV) –

Ondansetron (Emeset 4,8mg tabs ,2mg/ml inj in 2ml & 4ml amps), Granisetron (Granicip 1,2mg tabs ,1mg/ml inj in 1ml & 3ml amps) & Tropisetron (5-HT 3 antagonist).

5. GERD – Mozapride available as Moza 2.5mg ,5mg tabs (5-HT 4 agonist )

6. Irritable Bowel Syndrome – Mosapride, Prucalopride (5-HT 4 agonist)

SEROTONIN SYNDROME

• Precipitiating drugs – SSRIs, second generation antidepressants, MAOIs, Linezolid, Tramadol, Meperidine, Fentanyl, Ondansetron, Sumatriptan, MDMA, LSD, St.John’s wort, Ginseng

• Onset within hours.• S/S – Hypertension, hyper-reflexia, tremor, clonus,

hyperthermia, hyperactive bowel sounds, diarrhea, mydriasis, agitation, coma.

• T/T – Sedation by Benzodiazepines, Intubation & ventilation 5-HT 2 block by Cyproheptadine or Chlorpromazine.

What is Migraine?• A debilitating neurobiological headache disorder • Affects 18% of women & 6% of men • Decreases with age • Two categories – 80% = common migraine– 20% = classic migraine (w/ aura)

MIGRAINE• It is a disorder characterized by pulsating headache, usually

restricted to one side, which comes in attacks lasting 4-48 hrs & is often associated with nausea, vomiting, sensitivity to light & sound, flashes of light, vertigo, loose motions & other symptoms.

• It is of 2 typesa) Migraine with aura (classical migraine) – headache is preceded by visual or neurological symptoms.b) Migraine without aura (common migraine)

• 5-HT appears to play a pivotal role in this disorder.

Causes of Migraine

• Increased excitability of CNS• Meningeal blood vessel dilation• Activation of perivascular sensory trigeminal nerves • Pain impulses • Vasoactive neuropeptides contain:– substance P– calcitonin gene-related peptide (CGRP)– neurokinin A

• combination of increased pain sensitivity, tissue and vessel swelling, and inflammation

MIGRAINE - PATHOPHYSIOLOGYVASCULAR THEORY

• Intracerebral blood vessel vasoconstriction – aura• Intracranial/Extra cranial blood vessel vasodilatation – headache

SEROTONIN THEORY• Decreased serotonin levels linked to migraine• Specific serotonin receptors found in blood vessels of brain

• Neurovascular process, in which neural events result in activation of blood vessels, which in turn results in pain and further nerve activation

4 Stages of Migraine

1. Prodrome

2. Aura

3. Headache

4. Postdrome

History of Treatment

• Herbal brews and folk practices • 1200 BC: Egyptians – clay crocodile & magic herbs• 10th century AD: Arabian physicians – garlic or hot iron to

incision at temple• Mid-1600’s AD: Dr. Thomas Willis – enemas, blood letting,

leeches, and natural products • 1870’s: cold bandage on head, quiet room, and sleep

DRUG THERAPY OF MIGRAINE1. Mild Migraine – Cases having <1 attack/month of throbbing

but tolerable headache lasting upto 8 hrs which does not incapacitate the individual. Drugs used include-

a. Analgesics - Paracetamol (0.5-1 gm) or aspirin (300-600mg) 4-6 hourly.

b. NSAIDs – Drugs like Ibuprofen (400-800 mg 8 hourly) , Naproxen (500mg followed by 250mg 8 hourly) , Diclofenac (50mg 8 hourly), Mephenamic acid (500mg 8 hourly) either alone or combined with Paracetamol/ Codeine/ Diazepam/ Diphenhydramine / Caffeine.

c. Antiemetics – Metoclopramide (10mg oral/im) is commonly used. Domperidone (10-20mg oral) & Prochlorperazine (10-25 mg oral/im) are also effective.

2. Moderate Migraine – It is when throbbing headache is more intense, lasts for 6-24 hours, nausea/vomiting & other features are prominent & patient is functionally impaired. 1 or more attacks /month occur. Treatment includes-

a) NSAIDs or their combinationsb) Ergot alkaloids – Ergotamine (1mg oral at half hr interval till

relief is obtained or a total of 6mg is given). Caffeine 100mg is taken with ergotamine.

c) Sumatriptan – 50-100mg oral at onset of migraine, repeated once within 24 hrs if required. It is the only triptan available for parenteral use. Dose – 6mg scRizatriptan is more potent, has higher bioavailability with fast onset of action. Dose – 10mg, repeat once after 2 hr if required.

d) Antiemetics

3. Severe Migraine – Patients suffer 2-3 or more attacks/month of severe throbbing headache lasts 12-48 hours, accompanied by vertigo, vomiting & other symptoms, the patient is grossly incapacitated during attack. Treatment includes-a) Ergot alkaloidsb) Sumatriptanc) AntiemeticsProphylactic regime lasting 6 months is recommended.

PROPHYLAXIS OF MIGRAINE1. β adrenergic blockers – Propranolol 40mg BD, may be

increased upto 160mg BD2. TCAs – Amitriptylline 25-50mg HS. It is better suited for

patients who also suffer from depression.3. Calcium channel blockers – Verapamil is useful. Flunarizine 10-

20 mg OD is more effective.4. Anticonvulsants – Valproic acid 400-1200mg/day &

Gabapentin 300-1200 mg/day. Topiramate 25 mg OD is also useful.

5. 5-HT antagonists – Methysergide & Cyproheptadine may be helpful but seldom used now.

RECENT ADVANCES• Tandospirone – Also known as Metanopirone. It is a 5-HT

1A partial agonist. It is used in China & Japan as an anxiolytic & antidepressant. Dose – 30mg/day.

• Agomelatine – 5-HT 2C antagonist & agonist at melatonin receptor. Used in depression.

• Nelotanserin (APD-125) – selective 5-HT 2A inverse agonist developed by Arena pharmaceuticals for treatment of insomnia.

• Palonosetron (Aloxi) – approved by FDA in 2003 for iv use in preventing delayed chemotherapy induced nausea & vomiting. Oral formulation approved in 2008 for acute chemotherapy induced nausea & vomiting.

CONTD…• Ramosetron – 5-HT 3 antagonist available in Japan & South

east Asian countries used in chemotherapy induced nausea & vomiting.

• Cilansetron – 5-HT 3 antagonist used in Irritable Bowel Syndrome (IBS) where diarrhoea is the dominant symptom.