SEROTONIN

SEROTONIN


SEROTONIN

  • Chemically 5-hydroxytryptamine.
  • Synthesized from tryptophan.
  • Produced by hydroxylation –> Followed by tryptophan decarboxylation.
  • Stored in vesicles.
  • Exerts effects due to serotonin receptors activation (5-HT1-HT7 receptors).
    • Action is terminated by reuptake.

Actions:

  • On i.v. injection: 
    • Produces triphasic response to blood pressure.
    • Early sharp fall (due to Bezold Jarisch reflex) —> brief rise (due to vasoconstriction) –> prolonged fall (due to arteriolar dilation) in blood pressure.
  • Increases platelet aggregation.
  • Powerful smooth muscle stimulator.
    • Increases peristalsis & constricts bronchi.
  • Gastroprotective action – 
    • By decreasing acid secretion & increasing mucus production.
  • Involved in regulation of sleep, cognition, behaviour & mood.

Serotonin receptors for selective serotonin drugs & actions:

5 HT1 – Subtypes: 5 HT1A & 5 HT1B 5 HT 2A/2C 5 HT 3 5HT 4
  • 5 HT1A
    • Presynaptic autoreceptor.
    • Modulates serotonin release.
  • Partial agonists:
    • Buspirone, isapirone, gepirone).
  • Useful as anti-anxiety drug.
  • Responsible for direct serotonin actions.
  • Inotropic receptor.
    • Note: All other 5 HT receptors are G-protein coupled receptors).
  • Mediates most reflex &  indirect serotonin actions
  • In GIT.
  • Responsible increased peristalsis.
  • 5 HT1B –
    • Cause constriction of cranial vessels.
  • Agonists:
    • Sumatriptan, naratriptan.
    • Frovatriptan: Longest acting.
    • Rizatriptan: Fastest acting 5HT 1B/1D  agonist.
  • Useful for acute migraine attacks.

  • Antagonists:
    • Ketanserin & ritanserin.
  • Useful as antihypertensive agents.
  • Clozapine & risperidone 
    • Atypical antipsychotic agents.
    • Acts via antagonistic 5 HT 2A/2C activity.
  • Antagonists:
    • Ondansetron, grainsetron & tropisetron.
  • Doc for chemotherapy-induced vomiting.
  • Agonists:
    • Cisapride, mosapride, renzapride & tegaserod.
  • Useful in GERD treatment.

Non-selective drugs:

1. Cyproheptadine:

  • Blocks 5 HT2a, H1 & muscarinic receptors.
  • Increases appetite.
  • Used in children for weight gain.

2. Methysergide:

  • 5 HT2A/2C antagonist.
  • 5 HT1 agonist.
  • Indicated for migraine prophylaxis.
  • Causes pulmonary, endocardial & retroperitoneal fibrosis (on prolonged use).

3. LSD:

  • Ergot derivative.
  • Powerful hallucinogen.
  • Acts as an agonist for 5 HT 1a, 5 HT 2A/2C  & 5 HT 5-7 receptors.

Exam Important

  • Serotonin is chemically 5-hydroxytryptamine, synthesized from tryptophan
  • On i.v. injection of serotonin produces triphasic response on blood pressure.
  • 5 HT-3 is inotropic receptor; All other 5-HT receptors are G-protein coupled receptors.
  • Frovatriptan is longest acting & Rizatriptan is fastest acting 5HT 1B/1D agonist.
  • Ondansetron is Doc for chemotherapy-induced vomiting.
  • Cyproheptadine blocks 5 HT2a, H1 & muscarinic receptors, used in children for weight gain.
  • Methysergide is a 5 HT2A/2C antagonist5 HT1 agonist.
  • Methysergide is indicated for migraine prophylaxis.
  • LSD is ergot derivative & powerful hallucinogen
  • LSD acts as an agonist for 5 HT 1a, 5 HT 2A/2C  & 5 HT 5-7 receptors.
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