The effect of dopamine (DA) was investigated on acutely dissociated rat substantia nigra pars compacta (SNc) neurones by using patch clamp recording. The SNc neurones could be classified into two groups. About 75% of large neurones (>30 microm in diameter) were tyrosine hydroxylase (TH) positive while almost all small neurones (<20 microm) were TH negative. In the large neurones, DA hyperpolarized the membrane, resulting in a reduction of the frequency of spontaneous action potentials in current-clamp mode and induced an inward rectifier K+ current in voltage-clamp mode. Quinpirole, a D2 receptor agonist, mimicked the DA action. S(-)-sulpiride, a D2 receptor antagonist, inhibited the DA-induced current (I(DA)) more effectively than SKF83566, a D1 receptor antagonist. Intracellular application of either guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS) or pertussis toxin (IAP) suppressed I(DA). Guanosine 5'-O-(3-thiotriphosphate) (GTP-gammaS) sustained the DA response. Modulators for cAMP such as forskolin and isobutylmethylxathine, H-89, a protein kinase A inhibitor, and chelerythrine, a protein kinase C inhibitor, had no effect on I(DA). The frequency of DA-induced single channel currents in the inside-out patch configuration, for which the unitary conductance was 56.6pS, was greatly reduced by the replacement of GTP with GDP perfused at the cytosolic side. These results suggest that DA acts on a D2-like receptor and activates directly an IAP-sensitive G protein coupled with inward rectifier K+ channels, resulting in a decrease in the spontaneous firing activities of rat SNc dopaminergic neurones.