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

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Abstract
1 Conductance increases to γ-aminobutyric acid (GABA) were recorded in the gm6b and opener muscle of the spiny lobsters, Panulirus interruptus and P. argus. 2 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. 3 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. 4 Muscimol was able to mimic the action of GABA while baclofen was without effect. Bicuculline was a weak blocker. 5 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. 6 GABA-induced responses of the gm6b muscle of Homarus americanus were blocked almost completely by picrotoxin 10−6 m. 7 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.