OMEGA-TOXINS AFFECT NA+ CURRENTS IN NEUROSECRETORY INSECT NEURONS

omega-toxins specifically block certain Ca2+ channels in mammalian neu rons as well as in dorsal unpaired median neurons isolated from the co ckroach Periplaneta americana. In these cockroach neurons both the P/Q -type blockers omega-agatoxin IVA and omega-conotoxin MVIIC but not th e N-type Ca2+ channel blocker omega-conotoxin GVIA affected fast Naf c urrents sensitive to tetrodotoxin and veratridine. Both omega-toxins e nhanced Na+ current decay and thus decreased the amplitudes of the pea k currents. They also led to a slower recovery from inactivation. Toxi n effects developing within a few min were at removed upon washing. Th ey were not use-dependent. The description of the effect of omega-cono toxin MWC on current kinetics in terms of the Hodgkin-Huxley model rev ealed that steady-state parameters were not affected whereas the time constant of inactivation was considerably reduced. Under control condi tions, the inactivation time constant is similar to the time constant of recovery from inactivation. The toxin-induced increase of the latte r time constant and the decrease of the inactivation time constant ind icate that inactivation can no longer be described by first-order kine tics.