guanylyl-imidodiphosphate and calmidazolium

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 microg/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]-Tyr11-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 Gialpha(2) levels, without altering the amount of Gialpha(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.

Topics: Adenylyl Cyclases; Animals; Colforsin; Frontal Lobe; Guanylyl Imidodiphosphate; Imidazoles; Melatonin; Parietal Lobe; Rats; Rats, Wistar; Receptors, Somatostatin; Somatostatin

Functional interactions between the inhibitory guanine nucleotide-regulatory component (Ni) and the adenylate cyclase catalytic subunit (C) from cerebral cortex have been investigated. The inhibition of adenylate cyclase activity by guanosine 5'-(beta, gamma-imido)triphosphate [Gpp(NH)p] was used as a functional measurement of Ni-C interactions in membranes and cholate extracts. Calmodulin stimulation of C activity was required for the detection of Gpp(NH)p inhibition in these preparations. A similar calmodulin requirement was observed for adenosine receptor-mediated inhibition in membranes. The requirement for calmodulin was demonstrated directly in membranes from which calmodulin had been removed by washing and centrifugation. Adenylate cyclase activity in these preparations was not stimulated by free Ca2+ (1 microM). However, upon the readdition of calmodulin (1 microM), these preparations were stimulated 4-fold by Ca2+. Under these assay conditions, Gpp(NH)p- and adenosine receptor-mediated inhibition was absolutely dependent on Ca2+-calmodulin stimulation. However, forskolin stimulation of activity also restored Gpp(NH)p-mediated inhibition. The following experiments were used to implicate the role of calmodulin in detergent-solubilized preparations: (i) by demonstrating that free Ca2+ was required to observe Gpp(NH)p-mediated inhibition, and (ii) by demonstrating that the calmodulin antagonist, calmidazolium, abolished Gpp(NH)p-mediated inhibition while concomitantly decreasing basal activity. As observed in membranes, detection of guanine nucleotide-mediated inhibition required calmodulin stimulation of the detergent-solubilized adenylate cyclase. These results suggest that stimulation of cerebral cortical C activity by either calmodulin or forskolin is required for Ni-mediated inhibition.

Topics: Adenylyl Cyclase Inhibitors; Adenylyl Cyclases; Animals; Calcium; Calmodulin; Cell Membrane; Cerebral Cortex; Colforsin; GTP-Binding Proteins; Guanylyl Imidodiphosphate; Imidazoles; In Vitro Techniques; Macromolecular Substances; Rats