What is Topic 6 in INI CET (AIIMS PG)?

Topic 6 is a topic in the Botany syllabus of INI CET (AIIMS PG). It carries a weight of 3% in the exam. Our notes cover the key concepts, formulas, and problem-solving approaches you need to master this topic.

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Topic 6 — Botany | INI CET (AIIMS PG) Study Notes

Autacoids: Histamine and Antihistamines covers autacoids — histamine and antihistamines for INI CET (AIIMS PG).

Autacoids: Locally produced, short-acting chemical messengers (histamine, serotonin, prostaglandins, leukotrienes, kinins).

Histamine:

Synthesis: Histidine (via histidine decarboxylase) → Histamine
Storage: Mast cells and basophils (in granules — preformed, released immediately)
Release triggers: Type I hypersensitivity (IgE-mediated), complement fragments (C3a, C5a — anaphylatoxins), physical agents (cold, trauma), drugs (morphine, tubocurarine), food allergens
Receptors:
- H₁ receptor (GPCR — Gq): Smooth muscle (bronchoconstriction, GI contraction), vascular endothelium (↑NO → vasodilation, ↑vascular permeability → edema), nasal mucosa (sneezing, rhinorrhea) — ALLERGIC reactions
- H₂ receptor (GPCR — Gs): Gastric parietal cells (↑HCl secretion) — PEPTIC ULCER disease; heart (↑HR), uterus (relaxation)

Antihistamines (H₁ blockers):

First-generation (sedating) — cross BBB:
- Diphenhydramine: Allergy, motion sickness (antiemetic), insomnia; side effects: sedation, anticholinergic (dry mouth, urinary retention, constipation)
- Dimenhydrinate: Motion sickness (Dramamine)
- Promethazine: Allergy, sedation, antiemetic
- Chlorpheniramine: Common cold (less effective than for true allergy)
Second-generation (non-sedating) — do NOT cross BBB significantly:
- Cetirizine: Active metabolite of hydroxyzine; some anticholinergic effect
- Loratadine: Non-sedating even at high doses
- Desloratadine: Active metabolite of loratadine
- Fexofenadine: Active metabolite of terfenadine (terfenadine withdrawn for QT prolongation)
- Levocetirizine: Active isomer of cetirizine
- Azelastine: Nasal spray for allergic rhinitis

Clinical Uses of H₁ Antihistamines:

Allergic rhinitis (seasonal/perennial) — nasal congestion, sneezing, rhinorrhea, itching
Allergic conjunctivitis
Urticaria (hives) and angioedema
Anaphylaxis (adjunct to epinephrine — epinephrine is first-line for anaphylaxis)
Motion sickness (first-generation — antiemetic effect via vestibular pathways)
Insomnia (first-generation — CNS depression)
Common cold (limited value — rhinovirus not histamine-mediated)

⚡ Exam Tip for INI CET (AIIMS PG): Second-generation antihistamines (cetirizine, loratadine, fexofenadine) are non-sedating because they do NOT cross the blood-brain barrier. First-generation (diphenhydramine) cause sedation because they DO cross the BBB and block H₁ receptors in the CNS.