GI Pharmacology — 5-HT & Ondansetron

Current · V5 (2025) → H2.vii Historical · V4 (2023) → O2.ii 1 exam appearance

2012B Q17

Exam question

Classify the 5HT receptors and give examples of pharmacological agents that affect them (60% of marks). Outline the pharmacology of ondansetron (40% of marks).

CICMWrecks answer

Master answer

5HT Receptors

5-HT receptor subtypes comprise the largest known neurotransmitter-receptor family.

4 5-HT receptor families recognized
All members of GPCR superfamily except 5HT3
5HT3 is a ligand gated ion channel that conducts Na and K
Overall effects
- CNS
  - Vasoconstriction
  - Increased inotropy and chronotropy
- Resp
  - Bronchial smooth muscle constriction
- CNS
  - Sleep-wake cycle
  - Temperature regulation
  - Appetite
  - Mood
  - Impulsivity
  - Anxiety and Depression
  - Sexual behaviour

	Mechanism	Sites of Action	Effect	Drugs
5HT1	– GiPCR – Some subtypes also act as ligand gated ion channels	Inhibitory autoreceptor on serotonergic cells Endothelium	– Inhibition of serotonin secretion – Endothelial No release	Agonist – Sumitriptan (migraine) – Ergotamine – LSD
5HT2	Gq or G11PCR	CNS – Pre-frontal – Parietal – Somatosensory – Choroid plexus GI Tract Platelets	– Platelet aggregation and vasoconstriction	Agonist – LSD Antagonist – Clozapine – Risperidone
5HT3	Ligand gated Na and K channel	CNS – Area Postrema (CTZ) – NTS (Vomiting centre) GI Tract – Parasympathetic nerve terminals	Emesis	Antagonist Ondansetron
5HT4	GsPCR	CNS GI Tract – Myenteric plexus – Secretory cells	GI peristalsis and secretion

Additional receptors cloned
- 5HT5~7 (GsPCR) Unknown clinical relenvance
Other drugs with effects on serotonergic system
- Increase serotonin release
  - MDMA
- Decrease serotonin reuptake
  - Tramadol
    - 1R2R(+) Tramadol
  - SSRI
  - Tricyclic Antidepressants
- Decrease serotonin metabolism
  - MAO Inhibitors

Mooney 2016

Pharmacology of Ondansetron

Answer by Mooney

Past papers

Exam appearances

1 appearance

Exam	Exact exam wording	Candidate success
2012B Q17	Classify the 5HT receptors and give examples of pharmacological agents that affect them (60% of marks). Outline the pharmacology of ondansetron (40% of marks).	45.4%