A-current down-modulated by a receptor in frog pituitary melanotrope cellsthrough a G protein-dependent pathway

Gramicidin perforated patch-clamp recordings were used to study the effects of two sigma 1 receptor ligands, (+)-N-cyclopropylmethyl-N-methyl-1,4-diph enyl-1-ethyl-but-3-en-1-ylamine hydrochloride (JO 1784) and (+)-pentazocine , on the transient outward potassium current (I-A) in cultured frog melanot rope 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 al receptor antagonist, N,N-dipropyl- 2-[4-methoxy-3-2(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE 10 0). Inactivation rate of I-A was best fitted with a double exponential func tion, yielding time constants of 23.7 and 112.5 ms. (+)-Pentazocine (20 mu M) accelerated the current decay, decreasing the time constants to 10.7 and 59 ms, respectively. Current-voltage experiments revealed that (+)-pentazo cine (20 mu M) did neither modify the open-state I/V curves nor the voltage dependence of I-A. However, (+)-pentazocine (20 mu M) shifted the steady-s tate inactivation curve toward more negative potentials and increased the t ime constant of the time-dependent removal of inactivation. In whole-cell e xperiments, internal dialysis of guanosine-5'-O-(3-thiophosphate) (100 mu M ) irreversibly prolonged the response to (+)-pentazocine. In addition, chol era toxin pretreatment (1 mu g.ml(-1); 12 h) suppressed the inhibition of I -A by (+)-pentazocine (20 mu M). It is concluded that in frog melanotrope c ells, a cholera toxin-sensitive, G protein-dependent inhibition of I-A thro ugh a sigma 1 receptor activation, at least partially, underlies the excita tory effect of sigma ligands.