Forskolin Potentiates the Stimulation of Rat Striatal Adenylate Cyclase Mediated by D‐1 Dopamine Receptors, Guanine Nucleotides, and Sodium Fluoride

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Abstract
We report here that forskolin acts in a synergistic manner with dopaminergic agonists, guanine nucleotides, or sodium fluoride to potentiate the stimulation of rat striatal adenylate cyclase mediated by these reagents. In the presence of 100 .mu.M GTP, 100 .mu.M guanyl-5''-yl imidodiphosphate [Gpp(NH)p], or 10 mM NaF, there is a greater than additive increase in forskolin-stimulated enzyme activity as well as a concomitant decrease (two-to fourfold) in the EC50 value for forskolin stimulation of striatal enzyme activity. In the presence of various concentrations of forskolin (10 nM-100 .mu.M), the stimulation of adenylate cyclase elicited by GTP, Gpp(NH)p, and NaF is potentiated 194-1,825%, 122-1,141%, and 208-938%, respectively, compared with the stimulation by these agents above basal activity in the absence of forskolin. With respect to 3,4-dihydroxyphenylethylamine (dopamine) receptor-mediated stimulation of striatal enzyme activity, the stimulation of enzyme activity by dopaminergic agonists, in the absence or presence of forskolin, was GTP-dependent and could be antagonized by the selective D-1 antagonist SCH23390 (100 nM), indicating that these effects are mediated by D-1 dopamine receptors. In the presence of 100 .mu.M GTP, forskolin at various concentrations markedly potentiates the stimulation elicited by submaximal as well as a maximally effective concentrations of dopamine (100 .mu.M) and SKF38393 (1 .mu.M). At higher concentrations of forskolin (10-100 .mu.M) the stimulation elicited by the partial agonist SKF38393 is comparable to that of the full agonist dopamine. In contrast to the effects observed with GTP, in the presence of 100 .mu.M Gpp(NH)p there was no significant stimulation by dopaminergic agonists in the absence or presence of forskolin. These data demonstrate that in rat striatal homogenates forskolin can stimulate adenylate cyclase by (1) its direct action at the catalytic subunit, (2) by potentiating the stimulation elicited by the guanine nucleotide regulatory protein (Ns), and (3) by markedly potentiating the stimulation elicited by partial and full dopaminergic agonists acting at D-1 dopamine receptors.