5-HT & Histamine & Anti-histaminic Drugs

Anti-histaminic Drugs, Pharmacology,
  1. Histamine – Derived from-histidine -stored in mast cell with Heparin.
  2. Type of Histamine Receptor – 4 types
    • H1
    • H2
    • H3
    • H4Allergic & inflammation
  3. Histamine  Non-selective agonist at H1+ H2+ H3 Receptor
  4. Selective H-Agonist-2-methyl histamine, 2-pyridyl-ethyl amine, 2-Thiazolyl ethylamine
  5. Selective H2 –Receptor agonist – 4-methyl histamine, Dimaprit, Impramidine

Action/Role of Histamine:

  1. Blood Vessel :- Dilation of Small Blood vessel  Flushing & hypotension
  2. Increase capillary permeability  Edema through H1 Receptor
  3. Intradermal Inj. :- Triple Response due to Primarily H1 – Receptor mediated
    1. Red Rxn. = due to Vasodilation
    1. Wheal = Exudation of fluid due to increased permeability
    1. Flare = Spreading redness due to Axon reflex
  4. Visceral smooth muscle:- Powerful contractor of visceral S.M via H2 Receptor  Bronchoconstriction & abdominal cramps via H1 receptor
  5. Gastric :- Increase Acid secretion gastric secretion by stimulation of H2-Histaminic Rec.
  6. Neuron – Stimulates nerve ending – may result in pruritis & pain
  7. Brain – Maintain wakefulness through H1-Receptor – Block LCZ
  8. Histamine (H1) – serve as a mediator of inflammation & immediate type of hypersensitivity reaction.


Betahistine:- Histamine analogue – used to control vertigo in Meniere’s disease

H1 Anti – Histaminic:- These drugs act as competitive antagonist at H1-Receptor.

Classification of Anti-histaminic Drugs

On the basis of CNS penetration & Anticholinergic properties, Anti-histaminic drug are classified into 1st generation & 2nd generation.

Antihistaminic, Pharmacology
Classification of Antihistaminic Drugs

1st Generation – Anti-histaminic (H1):-

  • Can penetrate BBB  Sedation & psychomotor impairment so 1st generation Anti-histaminic contradicted in person requiring constant attention (like driving, machinery operators, swimmers etc)
  • 1st Anti-histaminic also posses Anti-Cholinergic activity
  • Major Adverse effect of 1st Generation = Sedation, Psychomotor impairment & Anti-cholinergic effects (dryness of mouth, blurred vision, urinary retention, constipation etc)

2nd Generation Anti-histaminic (H1)

  • Little CNS penetration (No sedation)
  • No Anti-Cholinergic activity
  • Additionally has Anti-allergic [Cetrizine & Axelastine]

Use of Antihistaminic drugs

Based on H1-Blocking actionBased on Anti-cholinergic propertiesOther uses
Allergic condition – itching, urticaria, hay fever etc.Insect bite, prevention of adverse effect due to histamine releaseCommon cold to control rhinorrhoeaMotion sickness (Prophylactic)Parkinsonism (promethazine)Acute muscular dystoniaIdiopathic pruritis Anti-vertigo (cinnarizine)
Pharmacological use of Antihistaminic drugs


  • Cetrizine – It is active metabolite of 1st generation drug hydroxyzine.
  • Levocetirizine – ‘l’ isomer of cetrizine
  • All second generation Anti-histaminic are metabolized to active product except cetrizine & mizolastine.

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