Do N -Methyl-d-Aspartate Receptors Mediate the Capacity of Inhaled Anesthetics to Suppress the Temporal Summation that Contributes to Minimum Alveolar Concentration?

Antagonism of N -methyl-d-aspartate (NMDA) receptors markedly decreases the minimum alveolar concentration (MAC) of inhaled anesthetics. To assess the importance of suppression of the temporal summation NMDA receptor component of MAC, we stimulated the tail of rats with trains of electrical pulses of varying interstimulus intervals (ISIs) and determined the inhaled anesthetic concentrations (crossover concentrations) that suppressed movement at different ISIs. The slopes of crossover concentrations versus ISIs provided a measure of temporal summation for each anesthetic. We studied five anesthetics that differ widely in their in vitro capacity to block NMDA receptors. To block NMDA receptor transmission and reveal the NMDA receptor component, the NMDA receptor antagonist, MK801, was separately added during each anesthetic. Halothane, isoflurane, and hexafluorobenzene did not appreciably suppress the NMDA receptor components of temporal summation, which contributed to 21% to 29% of MAC ( P < 0.05 for each). Xenon and o-difluorobenzene suppressed these components to 8% to 0%, respectively, of MAC (neither significant), consistent with their greater NMDA receptor blocking action in vitro. NMDA receptor blockade may contribute to the MAC produced by inhaled anesthetics that potently inhibit NMDA receptors in vitro but not those that have a limited in vitro effect.