Interactions between NMDA and AMPA glutamate receptor antagonists during halothane anesthesia in the rat

There is increasing evidence that pharmacologic antagonism of glutamatergic neurotransmission can potentiate the anesthetic effects of drugs such as halothane. The purpose of this study was to examine the anesthetic interaction between glutamate receptor antagonists. A competitive NMDA receptor antagonist (CGS 19755) and an AMPA receptor antagonist (NBQX) were administered either alone or in combination prior to determination of the minimum alveolar concentration (MAC) for halothane in the rat. CGS 19755 caused a dose-dependent maximum reduction in halothane MAC of approximately 80%. Doses of NBQX, which were low enough to cause no change in MAC when administered alone, substantially reduced MAC when administered with subanesthetic doses of CGS 19755. This effect decreased as the dose of CGS 19755 was increased. Finally, halothane MAC was reduced to zero when NBQX, in a dose sufficient to reduce halothane MAC by approximately 35% if given alone, was added to a pharmacodynamically similar dose of CGS 19755. Although MAC is believed to predominantly reflect nocioception at the spinal cord level, the results suggests that pharmacologic blockade of glutamatergic neurotransmission is sufficient to result in deep levels of anesthesia. Further, the effect of combinations of NMDA and AMPA receptor antagonists on halothane MAC is consistent with an in vivo physiologic interaction between the NMDA and AMPA receptors.