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

Abstract

1 Conductance increases to -y-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 x 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 Bla 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 GABAactivated chloride-ion channel does not change other functional characteristics of the GABA-induced response.

Chaoborus and Gasterosteus Anti-Predator Responses in Daphnia pulex Are Mediated by Independent Cholinergic and Gabaergic Neuronal Signals

“…GABA receptor subtypes with a single amino acid replacement makes the Drosophila GABA A receptor picrotoxin insensitive [16]. Furthermore, crustacean GABA A receptors were found insensitive towards picrotoxin [17].…”

GABA‐Mediated Presynaptic Inhibition in Crayfish Primary Afferents by Non‐A, Non‐B GABA Receptors

“…In the stomatogastric ganglion of the crab, picrotoxin had no effect on the GABA-activated CI-response either when high concentrations (1 mM) were used, although picrotoxin produced a blockade of GABA-activated K+ responses and also glutamate responses (Marder and Paupardin-Tritsch, 1978). In neuromuscular preparations of the spiny lobster, GABA-activated CI response was insensitive to picrotoxin (Albert et al, 1986). In the present study picrotoxin bath application completely blocked all depolarizations (GABA, muscimol, baclofen, and 3-APA) that are CI-dependent.…”

Differential effect of zinc on the vertebrate GABAA‐receptor complex

“…The current interest in zinc as a putative and novel GABA antagonist followed our original observations of a zincinduced inhibition of the GABA-evoked Cl-conductance recorded from crustacean muscle (Smart & Constanti, 1982). The invertebrate muscle GABA receptor was blocked by 10-10Mm zinc (Smart & Constanti, 1982;Albert et al, 1986). We suggested that zinc was binding to some putative histidine groups resident near the 'mouth' of the GABA-operated ion channel and thereby stabilizing this channel in a closed conformation.…”

“…The invertebrate muscle GABA receptor was blocked by 10-10Mm zinc (Smart & Constanti, 1982;Albert et al, 1986). We suggested that zinc was binding to some putative histidine groups resident near the 'mouth' of the GABA-operated ion channel and thereby stabilizing this channel in a closed conformation.…”