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

HISTAMINE RECEPTORS

• Three receptors have been identified as responsible for the pharmacological effects of histamine. They are H₁, H₂ and H₃ receptors.
• H₁ receptors are present in the bronchial, intestinal smooth muscles and CNS.
• They are also present in the vascular smooth muscles.
• This receptor mediates endothelial cell and smooth muscle responses to histamine.
• Activation of peripheral H₁ receptors causes bronchoconstriction, vasodilatation, increased vascular permeability and stimulation of sensory nerve endings.
• The effector system for the H₁ receptor appears to be activation of phospholipase C, production of inositol-1,4,5-trisphosphate and increased intracellular Ca²⁺. 
• H₂ receptors are present in the gastrointestinal tract and control gastric acid secretion.
• They are also present in the vascular smooth muscles.
• Activation of H₂ receptors stimulates adenylyl cyclase and increases the intracellular concentration of cAMP.
• This receptor increases gastric acid secretion by the parietal cell.
• H₂ receptors on vascular smooth muscle cells make a minor contribution to the vasodilatation caused by histamine release (slower in onset but more sustained).
• H₃ receptors are considered as auto receptors as they are responsible for the feed back regulation of histamine synthesis and release in many organ systems.
• They are also found on certain human autonomic nerve terminal, in arterial tissue and CNS.