Smooth muscle contraction,

increased vascular permeability;

 prostaglandin production;

pruritus-severe itching;

decreased atrioventricular conduction time;

 tachycardia;

 activation of vagal reflexesThr and Asn on transmembrane (TM) 5 bind the imidazole ring; electrostatic interaction between a-NH3+ and Asp on TM3.

acid-secreting cells mediate:

gastric acid secretion by gastric mucosa;

Asp on TM3 binds a-NH3+ of histamine.

Thr and Asp of TM5 bind to imidazole ring.

H3 receptors

CNS autoreceptors.

 Regulate histamine synthesis and release.

FIRST GENERATION H-1 ANTAGONIST