NOREPINEPHRINE AND SEROTONIN-INDUCED ANTINOCICEPTION ARE BLOCKED BY NALOXONE WITH DIFFERENT DOSAGES

The effects of intrathecally (IT) administered naloxone (Nal) on the a ntinociception produced by IT norepinephrine (NE), serotonin (5-HT), o r morphine (Mor) were observed and compared in rats using the tail-hic k (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 increase s in amplitude and time in TF latency (TFL); b) Nal treatment of 240 a nd 360 nmol has no effects on TFL; c) the antinociception produced by NE (1 nmol) can be blocked by Nal (240 nmol); d) antinociception produ ced by Mor (0.5 nmol) can also be blocked by Nal (240 nmol); e) 240 nm ol 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 antinocic eption at the spinal level, and these effects may be mediated through different types of opiate receptors.