Vol. 289, Issue 1, 321-328, April 1999

A-Current Down-Modulated by  Receptor in Frog Pituitary Melanotrope Cells Through a G Protein-Dependent Pathway¹

Olivier Soriani², Frank Le Foll, François Roman², François P. Monnet³, Hubert Vaudry and Lionel Cazin

European Institute for Peptide Research (IFRMP No. 23), Laboratory of Cellular and Molecular Neuroendocrinology, Institut National de la Santé et de la Recherche Médicale U413, Unité Associée Centre National de la Recherche Scientifique, University of Rouen, Mont-Saint-Aignan, France

Gramicidin perforated patch-clamp recordings were used to study the effects of two  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  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 · ml¹; 12 h) suppressed the inhibition of I_A by (+)-pentazocine (20 µM). It is concluded that in frog melanotrope cells, a cholera toxin-sensitive, G protein-dependent inhibition of I_A through a  1 receptor activation, at least partially, underlies the excitatory effect of  ligands.