Notes on Histamine and Antihistamines

Histamine Overview

  • Definition: Physiologically active amine C<em>5H</em>9N3C<em>5H</em>9N_3 found in tissue; released by mast cells during allergic reactions.
  • Functions of Histamine:
    • Stimulates gastric secretion
    • Causes capillary dilation
    • Causes bronchial smooth muscle contraction
    • Decreases blood pressure

Historical Context

  • Identified in 1910; recognized as a key mediator in allergies in the 1920s.
  • Antihistamine Introduction: First receptor antagonists introduced in 1937; over 40 compounds developed between 1942-1981.
  • Synthesis:
    • Derived from the amino acid histidine via decarboxylation (enzyme: L-histidine decarboxylase).
    • Primarily synthesized and stored in mast cells.

Histamine Receptors

  • H₁ Receptor: Located in the smooth muscle and central nervous system; responsible for allergic responses; antagonists include Diphenhydramine, Loratadine.
  • H₂ Receptor: Located on parietal cells; stimulates gastric acid secretion; antagonists include Ranitidine.
  • H₃ Receptor: Found in the central nervous system; decreases neurotransmitter release.

Allergic Reactions

  • Definition: Overreaction of the immune system to allergens (e.g., pollen, pet dander).
  • Common Reactions: Eczema, hives, hay fever, asthma, food allergies.
    • Symptoms: Itching, runny nose, conjunctivitis.

Antihistamines

  • Definition: Drugs that block the effects of histamine.
  • Types:
    • First-Generation: Can cause sedation; examples include Diphenhydramine, Chlorpheniramine.
    • Second-Generation: Less sedating; selective for peripheral H₁ receptors, include Loratadine, Cetirizine.
  • Clinical Uses: Allergic rhinitis, urticaria, anaphylaxis.
    • Sedation: First-generation antihistamines induce more sedation vs. second-generation due to blood-brain barrier permeability.

Drug List

  • H₁ Antagonists:
    • 1st Generation: Chlorpheniramine, Diphenhydramine.
    • 2nd Generation: Loratadine, Fexofenadine, Cetirizine.
  • H₂ Antagonists: Cimetidine, Ranitidine, Famotidine.

Summary

  • Histamine plays a crucial role in allergic reactions through its receptors.
  • Antihistamines are vital for managing such reactions, with differing effects based on their generation and properties.