ADENOSINE AND OPIATE-LIKE SUBSTANCES MEDIATES ANTINOCICEPTION AT THE SPINAL-CORD

The effects of intrathecally administered naloxone or aminophylline on the antinociception produced by intrathecal NE, 5-HT, morphine or ade nosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) were ob served in rats using the tail-flick test. The results show that: (1) t he antinociception produced by NE with doses of 0.5 or 1.0 nmol could be completely blocked by both naloxone (240 nmol) and aminophylline (1 20 nmol); (2) neither naloxone (240 nmol) nor aminophylline (120 nmol) could alter the antinociception produced by 5-HT with doses of 60 or 120 nmol; and (3) the antinociception produced by morphine (0.5 nmol) could be blocked by both naloxone (240 nmol) and aminophylline (120 nm ol), while the antinociception by NECA (0.5 nmol) could be blocked onl y by aminophylline (120 nmol), but not by naloxone (240 nmol). The res ults suggest that opiate-like substances (OLS) and adenosine are invol ved in the mediation of the NE-produced antinociception, but not in 5- HT-produced antinociception. Results also suggest that NE, OLS and ade nosine may act in a sequential order in the performance of NE-induced antinociception at the spinal level.