VPT 321: Veterinary Neuropharmacology (2+1) copy 1

SYNTHESIS, STORAGE AND DESTRUCTION OF HISTAMINE

• Histamine is β-imidazolylethylamine.
• It is synthesised locally from the amino acid histidine and degraded rapidly by oxidation and methylation.
• In the mast cells histamine (positively charged) is held by an acidic protein and heparin (negatively charged) within the granules.
• When the granules are extruded by exocytosis, sodium ions from the extracellular fluid undergoes, an exchange with histamine and releases it free.
• Increase in intracellular cAMP inhibits this release.
• Hence drugs that increase intracellular cAMP inhibit histamine release. Eg: Adrenaline.
• Histamine is inactive orally because liver degrades all histamine that is absorbed from the intestines.