RESIDUAL CA2-ACETATE VIA A G-PROTEIN ALPHA(S)-SUBUNIT IN RAT HYPOTHALAMIC NEURONS( CHANNEL CURRENT MODULATION BY MEGESTROL)

1. The inhibition of voltage-activated Ca2+ channel currents by the or ally active progesterone derivative, megestrol acetate (MA), was exami ned in freshly dissociated rat ventromedial hypothalamic nucleus (VMN) neurones using the whole-cell voltage-clamp technique with 10 mn;I Ba 2+ as the charge carrier. 2. The steady-state inhibition of the peak h igh-threshold Ca2+ channel current evoked by depolarization from -80 t o -10 mV by MA increased in a concentration-dependent fashion. MA inhi bited a fraction of the whole-cell Ca2+ channel current while progeste rone had no effect on the peak Ca2+ channel current (7 % at 10 mu M). The low-threshold Ca2+ (T-type) current, evoked from -100 to -30 mV, w as unaffected by NIB. 3. Intracellular dialysis with MA had no effect on the Ca2+ channel current. Concomitant extracellular perfusion of MA showed normal inhibitory activity, suggesting that the MA binding sit e can only be accessed extracellularly. 4. The high-threshold Ca2+ cha nnel current in VMN neurones was found to consist of four pharmacologi cally distinguishable components: an N-type current, an L-type current , a P-type current, and a residual current. MA had no effect on the N- , L- and P-type Ca2+ channel currents, but inhibited the residual curr ent. 5. In neurones isolated from cholera toxin-treated animals, the M A-induced inhibition of the Ca2+ channel current tvas significantly di minished, suggesting a G-protein alpha(s)-subunit involvement. 6. Trea tment with antisense phosphothio-oligodeoxynucleotides to the G alpha( s)-subunit (antisense-G alpha(s)) significantly reduced the MA-induced inhibition of the Ca2+ channel current. Treatment with either sense-G alpha(s) or antisense-G alpha(11) had no effect, confirming a G alpha (s)-subunit involvement. 7. These results suggest that appetite enhanc ement induced by MA in cachectic patients may in part be due to a nove l central nervous system action, that is, inhibition of a fraction of the whole-cell Ca2+ channel current to attenuate the firing of VMN neu rones that may be involved in satiety mechanisms.