We have investigated the effects of sigma ligands [1,3-di(2-tolyl)guanidine (DTG) and (1)-pentazocine] on the electrical activity of cultured frog pituitary melanotrope cells by using the patch-clamp technique. DTG and (1)-pentazocine (10 mM each) induced a reversible depolarization associated with an increase in membrane resistance and action potential firing. In voltage-clamp experiments, DTG and (1)-pentazocine elicited inward currents whose intensity augmented with membrane depolarization. The currents vanished or reversed between -90 and -100 mV, at values close to the K1 equilibrium potential(EK15 -102 mV). DTG (2–500 mM) and (1)-pentazocine (0.2-200 mM) reduced the outward delayed rectifier K1 current [IK(V)] in a dose-dependent manner with EC50 of 64 and 37 mM,respectively. In contrast, naloxone (50 mM) and pirenzepine (10mM) did not affect the sigma ligand-induced inhibition of IK (V).Addition of guanosine-59-O-(3-thiophosphate) in the pipettesolution irreversibly sustained the DTG-induced current whereas guanosine-59-O-(2-thiodiphosphate) virtually suppressed the response. Cholera toxin-pretreatment (1 mg/ml; 18hr) abolished the inward current and the inhibition of IK (V)induced by sigma ligands. In contrast, pretreatment with pertussis toxin (1 mg/ml; 18 hr) had no effect. Taken together, these data indicate that DTG and (1)-pentazocine activate the electrical activity of cultured frog melanotrope cells by reducing both a tonic K1 current and a voltage-dependent [IK (V)] K1conductance through the activation of a cholera toxin-sensitive G-protein.