Adenosine trisphosphate-sensitive potassium (K-ATP) channels in the substantia nigra pars reticulata were studied in rat brain slices using whole-cell patch clamp recording. Substantia nigra pars reticula neurons were identified as such by their spontaneous action potential firing at mean rate of 15.3 Hz1 virtual absence of hyperpolarization-activated inward current Ih1 and unresponsiveness to dopamine (30 microM), quinirole (10 microM) and (Met)enkephalin (10 microM). Intracellular dialysis with Mg(2+0-ATP-free pipette solutions caused a slowly developing membrane hyperpolarization (13 +/- 4 mV), accompanied by a cessation of action potential firing, or an outward current (79 +/- 30 pA at around -60 mV), which were reversed b the sulphonylurea K-ATO channel blockers tolbutamide (100 microM) and glibenclamide (3 microM). When Mg(2+0-ATP (2 mM) was included in the recording pipette no membrane hyperpolarization or outward current was observed. Neither the sulphonylureas nor the potassium channel activator lemakalim (200 MicroM) altered membrane potential, firing rate or holding current under these recording conditions. The outward current induced by dialysis with Mg(2+)-ATP-free solutions reversed polarity negative to -94 +/- 9 mV (9 cells), close to the estimated K+ equilibrium potential (-105 mV) for the conditions used, and was associated with a conductance increase that was blocked by Ba2+ (100 microM). The current blocked by the sulphonylureas had a similar reversal potential (-97 +/- 7 MV; 13 cells), and both currents were voltage independent over the range -50 to -100 mV with slope conductance of approximately 2.0 nS. Outward synaptic current were evoked by single shock electrical simulation, in the presence of glutamate receptor antagonists, at a holding potential of -50 mV. These synaptic currents were blocked by bicuculline (10 microM) and reversed polarity at around -65 mV, close to the Cl- equilibrium potential, and were thus mediated by GABAA receptors. They were reversibly depressed by 37 +/- 14% in lemakalim (200 microM) in 6/12 cells tested, an effect that was partially reversed by tolbutamide (200 microM). It is concluded that functional K-ATP channels are present both presynaptically and postsynaptically in the substantia nigra pars reticulata. Postsynaptic K-ATP channels may control excitability in conditions where intracellular ATP is reduced, whereas presynaptic K-ATP channels, sensitive to the potassium channel activator lemakalim, can modulate the release of GABA, which probably arises from fibres of extranigral origin. Pharmacological differences between these two sites could be exploited to treat epilepsies, dyskinesias and akinesia.