Chapter 11: Histamine and Antihistaminics

Study Notes

Histamine Overview

Histamine is a "tissue amine" predominantly found in animal tissues and certain plants, notably stinging nettle.
Initially studied by Dale in the early 20th century, it is linked to allergic reactions and hypersensitivity phenomena.
Plays crucial physiological roles and is largely concentrated in mast cells, particularly in:
- Skin
- Gastric and intestinal mucosa
- Lungs
- Liver
- Placenta
Non-mast cell histamine exists in the brain, epidermis, gastric mucosa, and growing regions.
Turnover rate of mast cell histamine is slow while non-mast cell histamine has a rapid turnover.

Synthesis, Storage, and Destruction

Biochemically known as β imidazolylethylamine, histamine is synthesized from the amino acid histidine and is rapidly degraded via oxidation and methylation.
Within mast cells, histamine is stored in granules, bound to an acidic protein and heparin; exocytosis releases histamine into the extracellular fluid.
Histamine is inactivated when taken orally due to absorption and degradation by the liver.

Histamine Receptors

Four histaminergic receptors have been classified: H₁, H₂, H₃, and H₄.
H₁ and H₂ receptors were first identified, with H₁ being specifically blocked by early antihistamines.
The first H₁ antagonist, burimamide, was developed by Sir James Black in 1972.
H₃ receptor, identified in 1983, acts primarily as an autoreceptor in the brain, modulating the release of histamine, yet lacks significant clinical applicability.
The H₄ receptor, discovered in 2001, shares significant homology with the H₁ receptor and binds a variety of ligands, including 4-methyl histamine.

Description

Explore the role of histamine in allergic reactions and its pharmacological significance as studied in the early 20th century. This quiz covers the basics of histamine, its presence in various tissues, and its implications in hypersensitivity and tissue injury. Test your knowledge on this essential mediator in allergic responses.