Antihistamines and Histamine: Overview

Histamine Overview

• Histamine Functions
  • Mediator of allergic reactions
  • Involved in physiological processes:
    • Immune response
    • Gastric acid secretion
    • Sleep-wake cycle
    • Cognitive ability
    • Food intake
• Synthesis
  • Synthesized from amino acid histidine
  • Abundant in skin, lungs, gastrointestinal tract
  • Stored in mast cells as granules
• Role in the Brain
  • Functions as a neurotransmitter
  • Present in histaminergic neurons of the hypothalamus

Histamine Receptors (H-receptors)

• Types and Distribution
  • G-protein-coupled receptors
  • H1 and H4: Allergic inflammation
  • H2: Gastric acid secretion

Antihistamines

• General Overview
  • Counteract the action of histamine
  • Term "antihistamine" typically refers to H1-antihistamines for allergy treatment

Allergic Reactions

• Mechanism
  • Occur upon repeated exposure to allergen
  • Sensitized mast cells release histamine
  • Effects include:
    • Dilation and increased permeability of blood vessels
    • Stimulation of sensory nerves
    • Contraction of smooth muscle
• Symptoms
  • Watery eyes, runny nose, sneezing, itching
  • Swelling, hives, difficulty breathing (bronchospasm)
  • Life-threatening anaphylaxis due to vasodilation and bronchoconstriction

Types of H1-antihistamines

• First-Generation
  • Derive from same class as muscarinic, adrenergic, serotonin antagonists
  • Effects:
    • Anti-cholinergic, anti-adrenergic, anti-serotonin
    • Cross blood-brain barrier causing drowsiness, cognitive impairment
    • Used as sedatives (sleeping aids)
• Second-Generation
  • Less likely to cross blood-brain barrier
  • Minimally or non-sedating
  • Highly selective for H1-receptor
  • No anti-cholinergic effects

Conclusion

• H1-antihistamines are primarily used for allergy treatment.
• Different generations have distinct properties and side effects.