Nitrous oxide increases serotonin release in the rat spinal cord.

The mechanism of the antinociceptive action of nitrous oxide (N(2)O) is not fully understood. It was reported that N(2)O induces opioid peptide release in the rat midbrain, which can activate the descending inhibitory system in the spinal cord. Although effects of N(2)O on the noradrenergic descending inhibitory system have been established, effects of N(2)O on the serotonergic descending inhibitory system have not been extensively investigated. We measured the extracellular level of serotonin by using in vivo microdialysis in the dorsal horn of the spinal cord in rats. The serotonin release increased to 213.01 +/- 24.87% (mean +/- SEM) of the baseline level from 20 to 40 min after applying N(2)O, which was followed by a gradual decrease. It is suggested that the serotonergic descending pathway is activated by N(2)O.