Changes in extracellular magnesium concentration modulate the actions of volatile anesthetics in rat in vitro hippocampal preparations

Our previous clinical studies demonstrated that there were significant correlations between intravenous post-operative pain due to the NMDA receptor-related mechanisms. Using in vitro administration of Mg-free acetate Ringer's solution and a decrease in extracellular Mg²⁺ concentration [Mg] during general anesthesia. It has been reported that changes in [Mg] can influence at hippocampal preparation, we have studied the relationship between [Mg] and the effects of volatile anesthetics, and the role of the NMDA receptor in the Mg-dependent effects of volatile anesthetics. Extracellular recordings were used to record field potentials in rat CA1 pyramidal neurons of transverse hippocampal slices, stimulated electrically via Schaffer collateral fibers input. The depression of the population spike (PS) amplitude by volatile anesthetics (sevoflurane, isoflurane) was strongly dependent on [Mg]. On the other hand the NMDA receptor antagonists (AP5, MK-801) had no effect on the Mg-dependence of anesthetic-induced depression of PSs. The Mg-dependent anesthetic depression, however, was attenuated in the presence of constant extracellular Mg/Ca ratio. Alteration of [Mg] can modulate the actions of volatile anesthetics on the Schaffer collateral fiber input to CA1 pathway in rat hippocampus. It is suggested that this effect is a result of pre- and/or post-synaptic Ca channel-related mechanisms rather than those associated with the NMDA receptor.