Exogenous histamine increases the somatostatin receptor/effector system in the rat frontoparietal cortex

The present study examined the effects of histamine on somatostatin-like immunoreactivity levels, binding of 125I-[Tyr11]somatostatin to its specific receptors, somatostatin inhibition of basal and forskolin-stimulated adenylyl cyclase activity and inhibitory guanine-nucleotide binding protein (Gi) function in the rat frontoparietal cortex. An intracerebroventricular (i.c.v.) dose of 10 micrograms or 1 microgram of histamine induced an increase in the number of specific 125I-[Tyr11]somatostatin receptors (590 +/- 22 vs 358 +/- 12 fmol/mg protein, P < 0.001 and 455 +/- 20 vs. 342 +/- 21 fmol/mg protein, P < 0.01, respectively) together with a decrease in their apparent affinity (0.76 +/- 0.04 vs 0.39 +/- 0.02 nM, P < 0.001 and 0.60 +/- 0.03 vs 0.39 +/- 0.05 nM, P < 0.01, respectively) in rat frontoparietal cortex membranes. This increase in tracer binding was not due to a direct effect of histamine on the somatostatin receptors since no change in binding was produced when histamine was added directly to the incubation medium. No significant differences were seen for either the basal or forskolin-stimulated adenylyl cyclase activity in frontoparietal cortex membranes of histamine-treated rats as compared with the control group. In rats treated with 10 micrograms of histamine, however, somatostatin caused a significantly greater inhibition of basal and forskolin-stimulated adenylyl cyclase activity as compared to the control group (33 +/- 4% vs 19 +/- 1% inhibition, P < 0.05 and 31 +/- 1% vs 21 +/- 3% inhibition, P < 0.05, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)