Fig. 7. (A ) Acute administration of NO²-arginine had no effect on the analgesic action of dexmedetomidine (Dex) in drug-naïve rats. NO²-arginine was administered intraperitoneally 15 min before administration of dexmedetomidine (50 μg/kg intraperitoneally), and the tail-flick test was administered 40 min later. All values are mean ± standard error of 6 rats. (B ) NO²-arginine (1 and 20 mg/kg intraperitoneally) did not reverse dexmedetomidine tolerance to the analgesic action of dexmedetomidine (100 μg/kg intraperitoneally) in dexmedetomidine-tolerant rats. * P < 0.05 significantly different from control, N = 7–8. (C ) Coadministration of a dose of NO²-arginine (4 μg · kg⁻¹· h⁻¹) that effectively antagonized hypnotic tolerance (fig. 5C) had no effect on analgesic tolerance produced by dexmedetomidine (10 μg · kg⁻¹· h⁻¹for 14 days). Increasing the dose of NO²-arginine to 8 μg · kg⁻¹· h⁻¹was also ineffective. All values are mean ± standard error of 7–8 rats. * P < 0.05, ** P < 0.01 significantly different from control.