Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism

Abstract

Recently we have demonstrated that dopamine inhibits action potentials in cultured frog melanotrophs through D 2 receptor-mediated activation of hyperpolarizing potassium current and reduction of calcium and sodium currents. Herein, the respective roles of G proteins,guanosine-5'-triphosphate and adenosine-3′:5′-cyclic-monophosphate in dopamine-induced electrical responses were investigated using the whole-cell patch-clamp technique. Pretreatment of melanotrophs with pertussis toxin (1 μ/ml) abolished the hyperpolarization and arrest of action potentials evoked by dopamine (1 μM) in 77% of the cells studied. Addition ofguanosine-5′- O-(2-thiodiphosphate) (500 μM) to the intracellular solution did not alter the effects of a first exposure to dopamine, but completely blocked the response of cultured melanotrophs to subsequent pulses of dopamine. In cells which were dialysed with guanosine-5′-0-(3-thiotriphosphate) (100 μM) dopamine caused a sustained hyperpolarization and an irreversible inhibition of spikes. Voltage-clamp recordings with electrodes containing guanosine-5'-O-(3-thiotriphosphate), showed that the increase of potassium current and decrease of calcium and sodium currents caused by dopamine were irreversible. These effects were not modified when the pipette contained, in addition toguanosine-5′- O-(3-thiotriphosphate), a high concentration of adenosine-3′:5′-cyclic-monophosphate (100 μM) together with the inhibitor of phosphodiesterases 3-isobutyl-1-methylxanthine (100μM). It is concluded that, in cultured frog melanotrophs, a pertussis toxin-sensitive G protein is implicated in the coupling of dopamine D 2 receptors to activation of potassium channels and inhibition of calcium and sodium channels. Our results also indicate that the G protein-mediated signal transduction does not involve the adenylate cyclase system.