Unraveling the Role of Histamine, Prostaglandins, 5HT, and Antihistamines in Human Health

Role of Histamine, Prostaglandins, 5HT and Antihistaminics

Introduction

Histamine, Prostaglandins, and 5-Hydroxytryptamine (5HT) are important autacoids.
Play roles in inflammation, allergy, gastric secretion, neurotransmission.
Antihistamines counteract effects of histamine.

Role of Histamine

H1 receptors: Allergy, inflammation, vasodilation, bronchoconstriction.
H2 receptors: Gastric acid secretion.
H3 receptors: Neurotransmission.
H4 receptors: Immune cell chemotaxis.

Role of Prostaglandins

Synthesized from arachidonic acid via COX enzymes.
Roles:

Inflammation & fever (PGE2, PGI2)
Gastric mucosal protection (PGE2)
Uterine contraction (PGF2α)
Vasodilation/constriction

Visual: Pathway of prostaglandin synthesis.

Role of 5HT (Serotonin)

Found in GI tract, CNS, and platelets.
Functions:

Mood regulation
GI motility
Platelet aggregation
Vasoconstriction/vasodilation

Distribution of serotonin in the body.

Histaminergic Agonists

List:

Betahistine
Histamine phosphate
Impromidine (H2 selective)
2-Methylhistamine (H1 selective)
4-Methylhistamine (H2 selective)

Histamine - Mechanism of Action

Binds to H1, H2, H3, H4 receptors.
GPCR mediated responses:

H1: ↑ IP3/DAG → Smooth muscle contraction, capillary permeability.
H2: ↑ cAMP → Gastric acid secretion.

Histamine - Indications & Adverse Effects

Indications:

Diagnostic agent (asthma, gastric acid secretion test)

Adverse Effects:

Hypotension
Bronchospasm
Flushing
Headache

Antihistamines - Classification

H1 Antihistamines:

First-generation: Diphenhydramine, Dimenhydrinate, Hydroxyzine, etc.
Second-generation: Loratadine, Cetirizine, Fexofenadine, etc.

H2 Antihistamines:

Ranitidine, Famotidine (not focus here)

H1 Antihistamines - Mechanism of Action

Block H1 receptors competitively.
Inhibit allergic responses.
First-gen cross BBB → sedation.
Second-gen do not cross BBB → non-sedating.

First-Generation H1 Antihistamines

Drugs: Diphenhydramine, Dimenhydrinate, Hydroxyzine, Promethazine, Pheniramine, Cyproheptadine, Cinnarizine, Chlorpheniramine, Triprolidine.
Uses: Allergy, motion sickness, insomnia, nausea.
Adverse Effects: Sedation, dry mouth, dizziness.
Drug Interactions: CNS depressants, alcohol, MAOIs.

Second-Generation H1 Antihistamines

Drugs: Fexofenadine, Loratadine, Cetirizine, Levocetirizine, Ebastine.
Uses: Allergic rhinitis, urticaria.
Adverse Effects: Less sedation, headache.
Drug Interactions: Ketoconazole, erythromycin (CYP450 interaction).

Presentation for Role of Histamine, Prostaglandins, 5HT, and Antihistamines in Human Health

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