A GABA‐activated chloride‐conductance not blocked by picrotoxin on spiny lobster neuromuscular preparations

Conductance increases to γ‐aminobutyric acid (GABA) were recorded in the gm6b and opener muscle of the spiny lobsters, Panulirus interruptus and P. argus. GABA‐evoked responses were insensitive to picrotoxin at concentrations as high as 5 × 10⁻⁵ m. Some blockade by picrotoxin was observed at higher concentrations. In normal physiological saline, the reversal potential of the Panulirus GABA‐induced response was near the resting potential. The reversal potential was unaffected by reductions in sodium and calcium. Reduction of chloride by 50% resulted in a greater than 10 mV shift in the reversal potential of the GABA‐induced response. Muscimol was able to mimic the action of GABA while baclofen was without effect. Bicuculline was a weak blocker. Avermectin B₁a irreversibly increased the chloride permeability of the gm6b membrane. This conductance increase was blocked by picrotoxin over a range of concentrations similar to those required for blockade of the GABA‐induced response. GABA‐induced responses of the gm6b muscle of Homarus americanus were blocked almost completely by picrotoxin 10⁻⁶ m. Sensitivity to picrotoxin is not invariably associated with GABA‐activated chloride‐mediated conductance increases. It is suggested that alteration in the binding‐site for picrotoxin on the GABA‐activated chloride‐ion channel does not change other functional characteristics of the GABA‐induced response. 1986 British Pharmacological Society