Abstract

Gramicidin perforated patch-clamp recordings were used to study the effects of two ς 1 receptor ligands, (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethyl-but-3-en-1-ylamine hydrochloride (JO 1784) and (+)-pentazocine, on the transient outward potassium current (I_A) in cultured frog melanotrope cells. (+)-Pentazocine reversibly decreased the current amplitude in a dose-dependent manner. The effects of (+)-pentazocine were mimicked by JO 1784 and were markedly reduced by the ς 1 receptor antagonist,N,N-dipropyl-2-[4-methoxy-3–2(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE 100). Inactivation rate of I_A was best fitted with a double exponential function, yielding time constants of 23.7 and 112.5 ms. (+)-Pentazocine (20 μM) accelerated the current decay, decreasing the time constants to 10.7 and 59 ms, respectively. Current-voltage experiments revealed that (+)-pentazocine (20 μM) did neither modify the open-state I/V curves nor the voltage dependence of I_A. However, (+)-pentazocine (20 μM) shifted the steady-state inactivation curve toward more negative potentials and increased the time constant of the time-dependent removal of inactivation. In whole-cell experiments, internal dialysis of guanosine-5′-O-(3-thiophosphate) (100 μM) irreversibly prolonged the response to (+)-pentazocine. In addition, cholera toxin pretreatment (1 μg · ml⁻¹; 12 h) suppressed the inhibition of I_Aby (+)-pentazocine (20 μM). It is concluded that in frog melanotrope cells, a cholera toxin-sensitive, G protein-dependent inhibition ofI_A through a ς 1 receptor activation, at least partially, underlies the excitatory effect of ς ligands.