Effects of subchronic and chronic melatonin treatment on somatostatin binding and its effects on adenylyl cyclase activity in the rat frontoparietal cortex

Melatonin and somatostatin are known to exert similar effects on locomotor activity. We have previously demonstrated that acute melatonin treatment regulates somatostatin receptor function in the rat frontoparietal cortex. However, the effects of subchronic and chronic melatonin treatment on the somatostatin receptor‐G protein–adenylyl cyclase system in the rat frontoparietal cortex are unknown. Melatonin was administered subcutaneously at a daily dose of 25  μ g/kg for 4 days, 1 wk or 2 wk. Twenty‐four hours after the last injection, the animals were sacrificed. Melatonin did not alter the somatostatin‐like immunoreactivity content in the frontoparietal cortex from control and melatonin‐treated rats during any of the previously indicated periods. Four days of melatonin administration induced both an increase in the number of [ 125 I]‐Tyr 11 ‐somatostatin receptors and a decrease in the affinity of somatostatin for its receptors in frontoparietal cortical membranes. The increased number of somatostatin receptors in the melatonin‐treated rats was associated with an increased capacity of somatostatin to inhibit basal and forskolin‐stimulated adenylyl cyclase activity. Melatonin administration for 4 days induced a higher adenylyl cyclase activity both under basal conditions and after direct stimulation of the enzyme with forskolin. No significant differences were observed in the function of Gi proteins in the 4‐day melatonin‐treated rats. Western blot analyses showed that the 4‐day melatonin treatment reduced Gi α 2 levels, without altering the amount of Gi α 1 . These melatonin‐induced changes reverted to control values after 7 or 14 days of treatment. Altogether, the present findings suggest that subchronic melatonin treatment modulates the somatostatin receptor/effector system in the rat frontoparietal cortex.