Rationale: The selective N-methyl-D-aspartate (NMDA) channel blocker MK-801 is known to induce no loss of the righting reflex (LORR) and to stimulate catecholaminergic (CAergic) neurons in rodents, playing a crucial role in arousal. Objectives: We examined whether MK-801 in combination with CA receptor ligands, which inhibit CAergic neuronal activities, could induce anesthesia including LORR. Methods: All drugs were administered systemically to mice. To assess anesthesia, three different behaviors were used: loss of nociceptive response (analgesia in the free-moving state without LORR), LORR, and loss of movement in response to noxious stimulation (immobility under LORR). Results: A very large dose of MK-801 (50 mg/kg) induced neither analgesia nor LORR. In contrast, MK-801 in combination with a small dose of the dopamine (DA) receptor antagonist haloperidol (0.2 mg/kg) dose-dependently produced LORR with a 50 % effective dose (ED50) of 1.6 (0.9-3.0; 95 % confidence limit) mg/kg, but not immobility. The α2-adrenoceptor agonist dexmedetomidine induced not only analgesia, but also immobility in animals treated with MK-801 (5 mg/kg) plus haloperidol (0.2 mg/kg), which then lost their righting reflex. The ED50 value of 0.26 (0.10-0.66) mg/kg (various doses of dexmedetomidine plus a fixed dose of MK-801 and haloperidol) for immobility was approximately three-fold larger than that of 0.09 (0.03-0.23) mg/kg (dexmedetomidine plus vehicle saline) for analgesia. This may occur, as LORR induced by MK-801 plus haloperidol inhibits the pain suppression system. The other ligands had little or no effect. Conclusions: The DAergic stimulant actions of MK-801 may mask its LORR effects by NMDA channel blockade.
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