Halothane enhances tonic neuronal inhibition by elevating intracellular calcium.

Whether the major action of anesthetics is to depress the central nervous system (CNS) by reducing excitation or enhancing inhibition remains unknown. Using whole cell patch-clamp recording in hippocampal slices, halothane and pentobarbital were found to prolong the decay time constant (TAU(D)) of GABAA-mediated spontaneous inhibitory postsynaptic currents (sIPSCs). Intracellular administration of the Ca2+ chelator BAPTA or the Ca2+ release inhibitor dantrolene significantly (ANOVA, P less than 0.005) reduced halothane's effect; in contrast, the pentobarbital effect was unchanged. Halothane induced depression of population spike amplitude was blocked by the GABAA antagonist bicuculline. Together, these findings suggest that a major depressant effect of halothane involves enhancement of GABAA-mediated inhibition through release of intraneuronally stored Ca2+.