Effects of remifentanil on N-methyl-D-aspartate receptor: an electrophysiologic study in rat spinal cord

Remifentanil hydrochloride contained in Ultiva (GlaxoSmithKline, Genval, Belgium) has been incriminated in difficult postoperative pain management, promotion of hyperalgesia, and direct N-methyl-D-aspartate (NMDA) receptor activation, but the involved mechanisms have remained unclear. In the current study, the authors investigated the effects of remifentanil hydrochloride, with and without its vehicle, glycine, on the activation of NMDA receptors and the modulation of NMDA-induced current on neurons inside the lamina II from the dorsal horn of rat spinal cord.

To test these effects, whole cell patch clamp recordings were conducted on acute rat lumbar spinal cord slices. Considering that both components of Ultiva (remifentanil hydrochloride and glycine) could be involved in NMDA receptor activation, experiments were performed first with remifentanil hydrochloride, second with glycine, and third with the two components within Ultiva.

Remifentanil hydrochloride does not induce any current, whereas 3 mm glycine induced a current that was abolished by the specific NMDA glutamate site antagonist D-2-amino-5-phosphonovalerate. Ultiva (remifentanil hydrochloride with its vehicle, glycine) also evoked an inward current that was abolished by D-2-amino-5-phosphonovalerate and not significantly different from the glycine-induced current. Application of remifentanil hydrochloride potentiated the NMDA-induced inward current, and this potentiation was abolished by the mu-opioid receptor antagonist naloxone.

These results show that remifentanil hydrochloride does not directly activate NMDA receptors. The NMDA current recorded after application of Ultiva is related to the presence of glycine. Induced NMDA current is potentiated by application of remifentanil hydrochloride through a pathway involving the mu-opioid receptor.