It is unclear whether general anesthetics induce enhancement of neural inhibition and/or attenuation of neural excitation. We studied the effects of pentobarbital (5 x 10(-4) mol/L), propofol (5 x 10(-4) mol/L), ketamine (10(-3) mol/L), halothane (1.5 vol%), and isoflurane (2.0 vol%) on both excitatory and inhibitory synaptic transmission in rat hippocampal slices. Excitatory or inhibitory synaptic pathways were isolated using pharmacological antagonists. Extracellular microelectrodes were used to record electrically evoked CA1 neural population spikes (PSs). In the presence of the gamma-aminobutyric acid type A (GABA(A)) receptor antagonist (bicuculline), the inhibitory actions of pentobarbital and propofol were completely antagonized, whereas those of ketamine, halothane, and isoflurane were only partially blocked. To induce the N-methyl-D-aspartate (NMDA) receptor-mediated PS (NMDA PS), the non-NMDA and GABA(A) receptors were blocked in the absence of Mg2+. Ketamine, halothane, and isoflurane decreased the NMDA PS, and pentobarbital and propofol had no effect on the NMDA PS. The non-NMDA receptor-mediated PS (non-NMDA PS) was examined using the antagonists for the NMDA and GABA(A) receptors. Volatile, but not i.v., anesthetics reduced the non-NMDA PS. These findings indicate that pentobarbital and propofol produce inhibitory actions due to enhancement in the GABA(A) receptor; that ketamine reduces NMDA receptor-mediated responses and enhances GABA(A) receptor-mediated responses; and that halothane and isoflurane modulate GABA(A), NMDA, and non-NMDA receptor-mediated synaptic transmission.
Implications: Volatile anesthetics modulate both excitatory and inhibitory synaptic transmission of in vitro rat hippocampal pathways, whereas i.v. anesthetics produce more specific actions on inhibitory synaptic events. These results provide further support the idea that general anesthetics produce drug-specific and distinctive effects on different pathways in the central nervous system.