Antagonists of GABA responses, studied using internally perfused frog dorsal root ganglion neurons

Responses of frog dorsal root ganglion neurons to GABA were studied under conditions of internal perfusion. Conductances to Na, Ca and K were pharmacologically blocked, C1 concentrations were maintained equal on both sides of the membrane and a small holding potential was used. Under these conditions GABA-induced C1 currents could be studied in isolation without shifts in EC1 occurring after GABA application. GABA currents were blocked by a variety of agents. The blockade by bicuculline and Zn was competitive, while that to penicillin was competitive at low concentrations (6 x 10(-5) M) and non-competitive at high concentrations (3 x 10(-4) M). Picrotoxin was non-competitive at all concentrations studied. The time course of the GABA-induced currents was changed in the presence of antagonists, including those that were competitive. These actions appear to be due to a change in the rates of receptor desensitization rather than shifts in EC1. Pretreatment with antagonists increased the degree of inhibition only for picrotoxin as compared to simultaneous application of GABA plus antagonist. The voltage dependence of the GABA response was altered by penicillin but not by other antagonists. GABA responses on frog dorsal root ganglion cell were also depressed by a variety of other metal ions (Cd, Ni, Cu, Co, Mn) and other drugs (strychnine, curare, 4-acetamide, 4'-isothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt, 4,4'-diisothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt trihydrate, bemegride and folic acid). In this preparation bicuculline and the heavy metal ions appear to block at or very near to the agonist binding site, while penicillin probably blocks the ion channel. The non-competitive action of picrotoxin appears not to be channel blockade, but to be due to a slowly equilibrating action at a site different from either the agonist binding site or the channel.