Abstracts

Rationale: Ambient GABA in the brain activates GABA_A receptors to produce tonic inhibition. Membrane potential influences both GABA transport and GABA_A receptors and could thereby regulate tonic inhibition. Methods: We investigated the voltage-dependence of tonic inhibition in cultured hippocampal neurons using patch clamp techniques. Results: Tonic GABA_A conductance increased with depolarization. The capacitance-specific tonic conductance was 15 and 30 pS/pF at -80 mV and -40 mV, respectively. Inhibition of vesicular or nonvesicular GABA release did not prevent the voltage-dependent increase of tonic conductance. Currents evoked with exogenous GABA (1 mM) were outwardly-rectifying similar to tonic inhibition due to endogenous GABA. These results indicate that the voltage-dependence of tonic inhibition resulted from intrinsic GABA_A receptor properties rather than elevated ambient GABA. Following transient depolarization to +40 mV, tonic inhibition measured at -60 mV was increased by 75%. This novel form of modulation of tonic inhibition, termed post-depolarization potentiation (PDP), recovered slowly with a time constant of 63 s. PDP was increased by exogenous GABA and inhibited by GABA_A receptor antagonists applied during depolarization. PDP also occurred following inhibition of vesicular and nonvesicular release of GABA, indicating that PDP was not due to ambient GABA elevation. Measurements of reversal potential showed PDP was due to increased conductance and not Cl- shifts. These findings indicate that voltage-dependent modulation of extrasynaptic GABA_A receptors persist upon repolarization. To assess the functional significance of PDP, we used waveforms that replicated epileptiform activity during voltage-clamp experiments. PDP was produced by this pathophysiologic depolarization. Conclusions: These data show that depolarization produces prolonged potentiation of tonic inhibition due to intrinsic voltage-dependence of GABA_A receptors. These properties are well suited to limit excitability during pathophysiologic depolarization accompanied by rises in ambient GABA, such as occur during seizures and ischemia.