Involvement of presynaptic histamine H3 receptors in the modulation of somatostatin binding and its effects on adenylyl cyclase activity in the rat frontoparietal cortex.

Thioperamide (2 mg/kg, l.p.), a histamine H3-receptor antagonist, increased the number of somatostatin (SS) receptors, with no change in the affinity constant, in the rat frontoparietal cortex. This effect was prevented by treatment with (R)-alpha-methylhistamine (3.2 mg/kg, l.p.), a histamine H3-receptor agonist. Thioperamide also induced an increase in SS binding in rats pretreated with mepyramine, a histamine H1-receptor antagonist, or cimetidine, a histamine H2-receptor antagonist. Pretreatment with mepyramine plus cimetidine administered simultaneously antagonized the thioperamide effect on SS binding. The increase in the number of SS receptors was accompanied by a greater SS-mediated inhibition of basal and forskolin-stimulated adenylyl cyclase (AC) activity in frontoparietal cortical membranes in the thioperamide group. Furthermore, the functional activity of the guanine nucleotide-binding inhibitory protein (G1 protein) was not altered by thioperamide or (R)-alpha-methylhistamine administration in frontoparietal cortical membranes. In rats treated with mepyramine plus thioperamide or cimetidine plus thioperamide, the increase in the number of SS receptors was also accompanied by an increased SS inhibition of AC activity. Thioperamide induced a significant increase in SS-like immunoreactivity content in the frontoparietal cortex. Altogether, these results suggest that frontoparietal cortical histamine may play, at least in part, a role in the regulation of the somatostatinergic system.