Norepinephrine and serotonin-induced antinociception are blocked by naloxone with different dosages.

The effects of intrathecally (IT) administered naloxone (Nal) on the antinociception produced by IT norepinephrine (NE), serotonin (5-HT), or morphine (Mor) were observed and compared in rats using the tail-flick (TF) assay. The results show that: a) NE, 5-HT, and Mor in doses of 1 nmol, 240 nmol, and 0.5 nmol, respectively, produce similar increases in amplitude and time in TF latency (TFL); b) Nal treatment of 240 and 360 nmol has no effects on TFL; c) the antinociception produced by NE (1 nmol) can be blocked by Nal (240 nmol); d) antinociception produced by Mor (0.5 nmol) can also be blocked by Nal (240 nmol); e) 240 nmol of Nal does not affect the 5-HT (120 nmol)-produced antinociception, while 360 nmol of Nal show a delayed blockade to the 5-HT (120 nmol)-produced antinociception. The results suggest that endogenous opiate-like substances may be involved in both NE- or 5-HT-produced antinociception at the spinal level, and these effects may be mediated through different types of opiate receptors.