Kinetic analysis of evoked IPSCs discloses mechanism of antagonism of synaptic GABAA receptors by picrotoxin

Background and purpose: Although picrotoxin is a well-established antagonist of GABA(A) receptors, detailed studies of its action on inhibitory synaptic transmission have not previously been made. Experimental approach: Electrophysiological techniques were used to study the action of picrotoxin on inhibitory postsynaptic currents (IPSCs) evoked in hippocampal neurones, in culture and slice preparations prepared from Wistar rat embryos and juveniles, respectively. Key results: Picrotoxin gradually reduced the amplitude of GABA(A) receptor-mediated eIPSCs in a concentration-dependent manner. This was accompanied by a marked acceleration of the eIPSC decay kinetics, which, in contrast to the effect on amplitude, developed immediately and was completely reversed on washing. The decaying phase of the IPSC could be resolved into two components; 30 mu M picrotoxin reduced tau(fast) by 34% and increased its relative amplitude, while tau(slow) was reduced by 38%, and its relative amplitude decreased. The area under the decaying phase of the normalized eIPSC showed an immediate reduction by 36% in 30 mu M picrotoxin. With increasing concentrations of picrotoxin, this normalized area converged towards 55% of the control, indicating that the rate of relaxation and block has a finite maximum. This implies that picrotoxin does not act by a pore-occluding mechanism (open-channel blocking), and suggests allosteric stabilization of desensitized receptor states as a more likely alternative. This was corroborated by modelling, based on two established microscopic GABA(A) receptor transition schemes. Conclusions and implications: Although the identity of the stabilized state has not been determined unequivocally, picrotoxin effectively traps synaptic GABA(A) receptors in a desensitized state.