Many theories have been proposed concerning the mechanisms of general anesthesia Several of these, including the lipid theory and the critical volume hypothesis, make some useful predictions but fail to illuminate the nature of the events that follow anesthetic entry into the neuronal membrane. Gamma-Aminobutyric acid (GABA) is a major inhibitory transmitter in the mammalian brain, and there is considerable evidence that enhancement of GABA-mediated inhibition is associated with sedation and anesthesia. Much research on soluble anesthetics such as barbiturates has centered on GABA-mediated synaptic inhibition mediated by GABA-A receptors. GABA-A receptors mediate an increase in Cl- conductance that underlies the 'fast' inhibitory postsynaptic potential in the brain. The GABA-A receptors are now known to be a family of heterogeneous multi sub-unit receptor proteins. These GABA-A receptors are modulated by a variety of sedative and anesthetic drugs including benzodiazepines and barbiturates. Recently, there have been preliminary reports of volatile anesthetic interactions with GABA-A receptors. The main aim of this proposal is to study the interactions of the anesthetic gases enflurane, halothane and isoflurane with GABA-A receptors. Whole-cell voltage-clamp recordings will be made from cultured rat hippocampal neurons in order to study the interactions of volatile anesthetics with GABA-A receptors. In addition, paired recordings will be made to study the effect of these agents on monosynaptic inhibitory transmission. Anesthetics will be applied to the neurons via the recording solutions, and concentrations of the anesthetics will be determined directly during the experiment by gas chromatography. The specific objectives of this project are: (1) to investigate, in a quantitative manner, the modulation of GABA-A receptors in CNS neurons by three volatile anesthetics, (2) to study the effects of these anesthetics on inhibitory synapses, (3) to study direct effects of anesthetics on neuronal membranes, and (4) to investigate the mechanism by which the anesthetics modulate GABA-A receptors. In obtaining more detailed information about the pharmacology of the GABA-A receptors, it is hoped to make a realistic assessment of the role played by GABA-A receptor mechanisms in the induction and maintenance of anesthesia by these volatile anesthetics.
