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Classify 5-HT receptors and outline 5-HT3 antagonist roles
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"5-HT3 antagonist" AND "chemotherapy-induced nausea"
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Serotonergic (5-HT) Receptors: Classification and 5-HT3 Antagonists
Part 1 - Classification of 5-HT Receptors
Key Structural Distinction
All 5-HT receptor families except 5-HT3 are G-protein coupled receptors (GPCRs). The 5-HT3 receptor is unique: it is a ligand-gated Na+/Ca2+ ion channel (ionotropic), explaining why it has a much faster onset of action than the other families.
Table: Serotonin Receptor Subtypes (from your slides + Katzung)
| Receptor | Location | Coupling/Mechanism | Important Actions | Key Drugs |
|---|---|---|---|---|
| 5-HT1 (1A, 1B, 1D, 1E, 1F) | CNS, cranial blood vessels | Gi/o - inhibits AC, opens GIRK channel | Autoreceptors (decrease 5-HT release); constriction of cranial vessels; decreased peptide release from nerve endings | Buspirone (5-HT1A partial agonist); Triptans (5-HT1B/1D agonists); Ergotamine (partial agonist/antagonist at all 5-HT1) |
| 5-HT2 (2A, 2B, 2C) | Platelets, smooth muscle, cerebral cortex (2A), fundus of stomach (2B), choroid (2C) | Gq - activates PLC | Platelet aggregation; smooth muscle contraction; neuronal excitation; CSF production | Ketanserin (2A antagonist); Cyproheptadine (2A antagonist); Methysergide (2A/2C antagonist); Atypical antipsychotics (2A antagonists) |
| 5-HT3 | CTZ, NTS, parasympathetic nerve terminals (GIT), vagal afferents | Ligand-gated Na+/Ca2+ channel (ionotropic) | Vomiting; peristalsis | Ondansetron, Granisetron, Palonosetron, Dolasetron, Tropisetron (antagonists) |
| 5-HT4 | GIT, CNS | Gs - activates AC | Peristalsis; prokinetic effect | Metoclopramide, Prucalopride (agonists) |
| 5-HT5-7 | CNS | Gs/Gi (varies) | Mood, sleep, cognition (largely research targets) | - |
Signalling Summary (as shown in your slide diagram)
- 5-HT1A-1F/5 - Gi/o → inhibits adenylyl cyclase (AC) ↓; opens GIRK K+ channel → hyperpolarisation
- 5-HT4/6/7 - Gs → activates AC ↑
- 5-HT2A-2C - Gq → activates PLC → IP3/DAG pathway
- 5-HT3 - Direct Na+/Ca2+ influx (no G-protein)
Part 2 - 5-HT3 Antagonists: Roles and Pharmacology
Mechanism of Action
5-HT3 receptors are located at two critical sites:
- Centrally - in the vomiting centre and chemoreceptor trigger zone (CTZ/area postrema)
- Peripherally - on vagal and spinal afferent nerves innervating the GIT
Cytotoxic drugs and radiation trigger enterochromaffin cells to release large amounts of 5-HT in the gut wall, which activates peripheral 5-HT3 receptors on vagal afferents and sends emetic signals centrally. 5-HT3 antagonists block both sites, interrupting the emetic reflex at its origin. As Katzung's emphasises, the peripheral (vagal) blockade is actually the predominant mechanism, not simply the central CTZ effect.
Clinical Uses
| Indication | Notes |
|---|---|
| Chemotherapy-induced nausea and vomiting (CINV) | Primary agents for acute CINV (within 24 h). Best given IV 30 min before chemotherapy. Efficacy enhanced by combining with dexamethasone + NK1 antagonist (aprepitant). Palonosetron also covers delayed CINV (>24 h). |
| Postoperative nausea and vomiting (PONV) | Gold standard; replacing older dopamine antagonists (droperidol) due to better safety profile. |
| Radiation-induced nausea and vomiting | Effective prophylaxis for patients undergoing abdominal or total body irradiation. |
| IBS (diarrhoea-predominant) | Alosetron (selective 5-HT3 antagonist) is approved for diarrhoea-predominant IBS in women; reduces colonic transit and visceral hypersensitivity. Use is restricted due to ischaemic colitis risk. |
Drug Comparison: Generations
| Drug | Generation | Half-life | Route | Notes |
|---|---|---|---|---|
| Ondansetron | 1st | 4-9 h | PO, IV | Most widely used; reduce dose in hepatic insufficiency |
| Granisetron | 1st | 4-9 h | PO, IV, transdermal patch | Patch available for extended cover |
| Dolasetron | 1st | 4-9 h | PO, IV | Less commonly used now |
| Tropisetron | 1st | ~8 h | PO, IV | Available outside USA |
| Palonosetron | 2nd | ~40 h | PO, IV | Greater receptor affinity; allosteric binding; positive cooperativity; covers delayed CINV - preferred for highly emetogenic regimens |
| Ramosetron | 2nd | ~9 h | PO, IV | Available in Asia; also used for IBS-D |
Adverse Effects
| Effect | Mechanism/Notes |
|---|---|
| Headache | Most common |
| Constipation | Slowing of colonic transit via 5-HT3 blockade in ENS |
| Dizziness/drowsiness | Central effect |
| QT prolongation | Rare but clinically important; ondansetron at high IV doses carries a specific FDA warning. ECG monitoring warranted in at-risk patients. |
- 5-HT3 antagonists do not block dopamine receptors, so they cause no extrapyramidal side effects - a major advantage over older antiemetics like metoclopramide and prochlorperazine.
- No sedation unlike antihistamines or phenothiazines.
Key Clinical Points
- Motion sickness is not controlled - because the emetic pathway for motion sickness involves the vestibular apparatus and histaminergic/cholinergic pathways, not 5-HT3.
- Combination regimens are standard for highly emetogenic chemotherapy: 5-HT3 antagonist + dexamethasone + NK1 antagonist (aprepitant/fosaprepitant) ± olanzapine.
- Palonosetron is preferred when delayed CINV control is needed, due to its 40-hour half-life and unique allosteric receptor binding.
- Most 5-HT3 antagonists undergo extensive hepatic metabolism; dose reduction is only needed for ondansetron in significant hepatic impairment.
- Among all 5-HT antagonist classes, 5-HT3 blockers are the only ones with broad, widely accepted clinical utility - the other classes (5-HT2, 5-HT1B/D antagonists) have limited use due to side effect profiles or limited availability.
Sources: Your uploaded slides (Table 6.2); Katzung's Basic and Clinical Pharmacology 16e, p. 2809-2826; Yamada's Textbook of Gastroenterology 7e, p. 2496-2501; Morgan & Mikhail's Clinical Anesthesiology 7e.
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