The effects of halothane on pharmacologically isolated GABA_A receptor-mediated IPSCs recorded from pyramidal cells and SL-M–SR interneurons. A, Representative traces of stimulus-evoked monosynaptic IPSCs recorded from pyramidal cells using whole-cell voltage-clamp recording (−60 mV) in response to electrical stimulation of the SL-M–SR border were obtained in the presence of glutamate receptor antagonists APV (100 μm) and CNQX (17.2 μm). Under these conditions, only monosynaptic GABA_A receptor-mediated IPSCs could be evoked, and bicuculline (10 μm), a GABA_A receptor antagonist, completely blocked these currents (data not shown). Halothane (0.35 mm, 20 min) depressed evoked IPSC amplitudes and also prolonged the decay phase. These halothane effects were reversible after washout of the anesthetic. B, Stimulus-evoked IPSCs recorded from an interneuron were depressed in a similar manner by halothane. The whole-cell voltage-clamp recordings in interneurons (−60 mV) were obtained as described in A. Graphs of IPSC amplitudes versus experimental time show the time course of halothane-induced depression of GABA_A IPSC amplitudes in pyramidal cells (C, mean ± SD, n = 5) and SL-M–SR interneurons (D, mean ± SD,n = 6). All amplitudes were expressed as a percentage of the average of control recordings taken during the 5 min preceding anesthetic application. E, The amplitude of evoked IPSCs recorded from pyramidal cells and interneurons is expressed as a percentage of control. Halothane (0.35 mm, 20 min) significantly depressed IPSC amplitudes in both pyramidal cells and interneurons. These effects were completely reversed 30 min after halothane wash (*p < 0.05, **p< 0.01 vs control). Halothane also markedly prolonged the decay time constants of IPSCs in both pyramidal cells and interneurons (F). The decay phase was fitted by a single exponential curve, and the time from peak amplitude to 63% of peak amplitude (τ_decay) was measured in averaged traces from at least 20 recordings. Bars represent SEM. n= 6 for pyramidal cells and n = 5 for interneurons (***p < 0.001, ANOVA).