Inhibition of [H-3]forskolin binding by Gpp[NH]p may reflect adenylate cyclase coupling to G(i)

THE effect of the GTP analogue guanylyl-5'-imidodiphosphate (Gpp[NH]p) on [ H-3]forskolin binding was studied in rat brain using autoradiography. In th e striatum the presence of 100 mu M Gpp[NH]p produced a 40% increase in bin ding, whereas a decrease of about 30% was observed with low Gpp[NH]p concen trations (0.1 mu M). In the molecular layer of the cerebellum all concentra tions of Gpp[NH]p decreased [H-3]forskolin binding. The decrease in binding disappeared in both striatum and the molecular layer of cerebellum in sect ions pretreated with 100 mu M N-ethylmaleimide (NEM) for 10 min. NEM pretre atment did not significantly affect the stimulation of [H-3]forskolin bindi ng by micromolar concentrations of Gpp[NH]p in the striatum, but reversed t he decrease observed in the molecular layer of the cerebellum, to an increa se. Based on these data we suggest that the effects of Gpp[NH]p on [H-3]for skolin binding may involve both G(s) and G(i), where stimulation produces a n increase and decrease in binding, respectively. The regional effects of G pp[NH]p may reflect differences in the responsiveness of adenylate cyclase to G(s)- and G(i)-mediated effects. NeuroReport 10:2423-2427 (C) 1999 Lippi ncott Williams & Wilkins.