Flurazepam effect on GABAergic currents depends on extracellular pH

Abstract

Benzodiazepines (BDZs) are widely used in clinical practice and are known as positive modulators of GABAergic currents. BDZs increase binding affinity and recently they were found to affect GABA(A) receptor gating, including desensitization. Binding and desensitization are also strongly modulated by extracellular pH, a factor that may be severely altered in a pathological brain. It is thus of interest to examine the combined action of BDZ and protons. Pharmacokinetic analysis was based on patch clamp recordings of miniature IPSCs (mIPSCs) and current responses to GABA applications in rat cultured hippocampal neurons. High temporal resolution of currents evoked by exogenous GABA was achieved by using an ultrafast perfusion system (exchange time ca. 80 micros). At acidic pH, flurazepam produced a stronger enhancement of mIPSC amplitudes than at physiological pH. At low GABA concentrations, flurazepam markedly enhanced current amplitudes both at normal and acidic pH, but at the latter, the relative effect was larger. In contrast, at saturating GABA concentrations, flurazepam reduced current amplitudes at both pH 7.2 and 6.0. The slowing of deactivation kinetics by flurazepam decreased with GABA concentration, but at pH 6.0, this trend was shifted toward a higher GABA concentration. Acidification of extracellular medium may significantly affect the susceptibility of phasic and tonic components of GABAergic currents to modulation by BDZs. Quantitative analysis and model simulations indicate that protons and flurazepam additively affect binding and desensitization of GABA(A) receptors.