• Hiroyuki Sumikura and Lars Arendt-Nielsen.
• Department of Anesthesiology, St. Marianna University School of Medicine, 2-16-1 Sugao, Miyamae-ku, Kawasaki 216-0015, Japan.
• J Anesth. 2003 Jan 1; 17 (2): 92-7.

PurposeIt has been reported that intrathecal coadministration of a GABA(A) agonist and a glutamate antagonist induces synergistic antinociceptive effects in rats. We hypothesized that this synergistic antinociceptive effect might induce a synergistic reduction in the minimum alveolar anesthetic concentration (MAC).MethodsThe MAC for sevoflurane was determined before and after intrathecal administration of muscimol (GABA(A) agonist, 0.1-10 microg), AP-5 (N-methyl-D-aspartate [NMDA] antagonist, 0.1-10 microg), and YM872 (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA] antagonist, 0.1-10 microg) in rats. The effects of coadministration of muscimol and AP-5 or YM872 on MAC reduction were also tested.ResultsIntrathecal administration of muscimol at doses of 1 and 10 microg significantly reduced the MAC by 34.5% +/- 3.3% and 46.9% +/- 4.4%, respectively (P < 0.01). Intrathecal administration of AP-5 at a dose of 10 microg and YM872 at a dose of 10 microg significantly reduced the MAC by 25.8% +/- 2.4% and 31.4% +/- 6.3%, respectively (P < 0.01). No additional reductions in MAC values were observed when a GABA(A) agonist was combined with either an NMDA or an AMPA antagonist.ConclusionIntrathecal coadministration of a GABA(A) agonist and an NMDA or AMPA antagonist, which has been reported to produce a synergistic antinociceptive effect, did not induce a synergistic reduction in the MAC. The antinociceptive effect may not be the predominant factor in the reduction of the MAC induced by these drugs.

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