Remifentanil directly activates human N-methyl-D-aspartate receptors expressed in Xenopus laevis oocytes

Background: Clinical studies suggest that intraoperative administration of the clinical remifentanil formulation Ultiva(R) (GlaxoWellcome GmbH & Co, Bad Oldesloe, Germany) increases postoperative pain and postoperative analgesic requirements, but mechanisms remain unclear. N-methyl-D-aspartate (NMDA) receptors are thought to play a major role in development of postoperative pain and opiate tolerance. The authors hypothesized that Ultiva(R) directly stimulates human NMDA receptors. Methods: To test this hypothesis, the authors expressed human NR1A/NR2A and NR1A/NR2B NMDA receptors in Xenopus laevis oocytes by injection of messenger RNA prepared in vitro. After protein expression, they used a two-electrode voltage clamp to measure currents induced by NMDA receptor agonists and opioids. Results: Noninjected cells were unresponsive to all compounds tested. Glutamate/glycine (1 mM-1 mm each) or Ultiva(R) (0.01 pM-0.1 mM) stimulated NMDA receptors concentration dependently. NR1A/2A EC50 values were 8.0 muM/12 muM for glutamate/glycine and 3.5 nm for Ultiva(R) and NR1A/2B EC50 values were 3.9 muM/1.9 muM for glutamate/glycine and 0.82 muM for Ultiva(R). Glycine in combination with Ultiva(R) showed no additive effect compared with Ultiva(R) alone. Ultiva(R)-induced currents were inhibited by MK-801 (pore blocker) but not by 7-CK (glycine antagonist). D-AP5 (glutamate antagonist), or naloxone. Fentanyl (10 muM) did not stimulate NMDA receptors. Conchision: These data indicate that Ultiva(R) but not fentanyl stimulates NMDA receptors of different subunit combinations (NR1A/2A, NR1A/2B). The mechanism seems to be allosteric activation of the NMDA receptor.