Serotonin agonist &antagonist
- 1. serotonin (5-hydroxytryptamine) By Dr. Shipra Jain Asst. Professor, Dept. of Pharmacology MGMC&H, Jaipur
- 2. 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.
- 3. 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.
- 5. DOPAMENERGIC and SEROTONERGIC pathway in Brain
- 10. 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.
- 11. 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)
- 12. 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.
- 13. 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.
- 17. SEROTONIN RECEPTORS Family 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-HT3 Ligand- 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
- 19. 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 DPAT Selective antagonist- WAY100635 • 5- HT 1B - Localized in Substantia niagra, globus pallidus ,basal ganglia Selective agonist- Sumatriptan, CP93129
- 20. • 5-HT 1D – Localized in brain 5-HT 1B/1D cause constriction of cranial blood vesssels. Selective agonist- Sumatriptan It 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 , hippocampus Selective agonist- LY334370 • 5-HT 1P- Localized in enteric nervous system Selective agonist- 5-Hydroxyindalpine Selective antagonist - Renzapride
- 21. 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 fundus Selective agonist- α-methyl-5-HT Selective antagonist – SB204741
- 22. • 5- HT 2C – Localized in choroid plexus, hippocampus, substantia niagra. It elicits vasodilatation through EDRF release. Selective agonist- α-methyl-5-HT Selective 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
- 23. 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
- 24. • 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.
- 25. ACTIONS 1. 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.
- 26. BP – triphasic response is classically seen on iv injection Early sharp fall in BP-due to coronary chemo reflex Brief rise in BP- due to vasoconstriction & ↑ CO Prolonged 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.
- 27. 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.
- 28. PATHOPHYSIOLOGICAL ROLES 1. 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.
- 30. 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.
- 31. 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.
- 32. 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 limiting step) & ↓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).
- 34. 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.
- 36. 5-HT AGONISTS 1. 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.
- 37. 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.
- 38. 5-HT ANTAGONISTS 1. 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.
- 39. 2.Methysergide – It antagonizes actions of 5-HT on smooth muscles including that of blood vessels. Potent 5-HT 2A/2C antagonist Used 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.
- 40. 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.
- 41. 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.
- 42. USES 1. Major Depression– SSRIs like Fluoxetine, Sertraline 2. Obsessive compulsive disorder – SSRIs 3. Phobic disorders - SSRIs 4. Premenstrual symptoms - SSRIs 5. Somatoform disorders (Hypochondriasis) - SSRIs 6. Post traumatic stress disorder - SSRIs 7. Neuropathic pain & Fibromyalgia pain 8. Anxiety neurosis – Azapirones like Buspirone, Gepirone Ipsapirone (5- HT 1A partial agonist)
- 43. OTHER CLINICAL USES 1. 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)
- 44. 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.
- 46. 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)
- 47. 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 types a) 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.
- 48. 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
- 49. MIGRAINE - PATHOPHYSIOLOGY VASCULAR 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
- 51. 4 Stages of Migraine 1. Prodrome 2. Aura 3. Headache 4. Postdrome
- 52. 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
- 53. DRUG THERAPY OF MIGRAINE 1. 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.
- 54. 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 combinations b) 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 sc Rizatriptan is more potent, has higher bioavailability with fast onset of action. Dose – 10mg, repeat once after 2 hr if required. d) Antiemetics
- 55. 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 alkaloids b) Sumatriptan c) Antiemetics Prophylactic regime lasting 6 months is recommended.
- 56. PROPHYLAXIS OF MIGRAINE 1. β adrenergic blockers – Propranolol 40mg BD, may be increased upto 160mg BD 2. 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.
- 57. 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.
- 58. 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.