Nitrous oxide analgesia: partial antagonism by naloxone and total reversal after periaqueductal gray lesions in the rat.

Analgesia induced by nitrous oxide was examined using radiant heat tail flick and electrical evoked foot flick tests in rats. Rats exposed to 80 and 60% nitrous oxide expressed statistically significant elevations of percent analgesia (% MPE) compared to air exposed rats. Rats exposed to 30% nitrous oxide showed no significant difference in percent analgesia. Pretreatment with naloxone (10 mg/kg s.c.) produced a significant decrease in %MPE and an increase in variance of response after exposures to 80% nitrous oxide in a double blind study. Kainic acid lesions of the ventral and caudal periaqueductal grey (PAG) reversed analgesia produced by 80% nitrous oxide in a crossover blink study compared to saline lesions. In conclusion, this evidence suggests that the caudal-PAG-raphe mangus-dorsal horn pain inhibition pathway is in part involved in the analgesia induced by nitrous oxide.