Histamine receptors::
Two types of histamine receptors have been identified :(1) H1 receptors involved in most of the action such as allergy triple response headache and contraction of smooth muscles and (2) H2 receptors which are of great significance since they are involved in gastric acid secretion it is now established that H2 receptors stimulation is the final step in the secretion of gastric acid various histamine receptors agonists are as follows:
Specific H1 receptor agonists - 2- methylhistamine
- 2- (2-pyridy ) ethylamine
- 2-(2- thiazolyl) ethylamine
Specific H2 receptor agonists - dimaprit
- 4- methylhistamine
- impromidine
Histamine betazole and triazole act on both H1 and H2 receptors
Histamine H3 receptors:
These receptors are found on histaminergic neurons they control synthesis and release of histamine thus histamine H3 receptors are autoreceptors Recently they have also been found on noradrenergic and serotonergic neurons methylated histamine derivatives (R) $(S) B dimethylhistamine are specific H3 histamine agonists
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