Intrathecal co-administration of NMDA antagonist and NK-1 antagonist reduces MAC of isoflurane in rats.

Background: Intravenous administration of N-methyl-D-aspartate (NMDA) receptor antagonists and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonists reportedly reduce the minimum alveolar anaesthetic concentration (MAC) for inhalation anaesthetics. If pain perception can be prevented by the intrathecal administration of antinociceptive receptor antagonists, these agents may reduce the requirements for inhalation anaesthetics. We studied the effect of intrathecal administration of an AMPA/kainate receptor antagonist, a metabotropic glutamate (mGlu) receptor antagonist and co-administration of NMDA and a neurokinin-1(NK-1) receptor antagonist drugs at low doses on the MAC. Methods: After Wistar rats (n=36) were fitted with indwelling intrathecal catheters, the MAC of isoflurane was determined following intrathecal administration of a non-NMDA receptor antagonist (CNQX) at 10 μg, a mGlu receptor antagonist (AP3) at 10 μg, or a combination of NMDA receptor antagonist (APV) at 0.01 μg to 1 μg with NK-1 receptor antagonist (CP96345, CP) at 0.1 μg to 10 μg. Subsequently, a reversal dose of intrathecal NMDA with substance P (SP) was administered, and the MAC of isoflurane was redetermined. Conscious rats (n=15) were also examined for the presence of locomotor dysfunction following the intrathecal co-administration of APV and CP. Results: Neither CNQX nor AP3 reduced the MAC of isoflurane. APV at 0.01 μg plus CP at 1 μg, as well as APV at 0.1 μg plus CP at 10 μg, reduced the MAC of isoflurane, with respective reductions of 7.6% and 14%; (P<0.05). Co-administration of NMDA plus SP reversed the decrease in the MAC of isoflurane. Locomotive activity was not changed. Conclusions: The NMDA receptor and the NK-1 receptor are important determinants of the MAC of isoflurane, exerting this influence by inhibition of pain transmission in the spinal cord, while mGlu and AMPA receptors have no effect on the MAC of isoflurane.