Comparison of the antinociceptive and motor effects of intrathecal opioid agonists in the rat.

This study compared the antinociceptive and motor effects produced by intrathecal administration of selective mu-, delta-, and kappa-opioid receptor agonists in the rat. Changes in nociceptive threshold were measured using the Randall-Selitto paw-withdrawal test and changes in motor coordination were evaluated using the rotarod treadmill test. Each opioid agonist produced statistically significant, dose-dependent increases in mechanical nociceptive thresholds compared to vehicle controls. In the motor coordination studies, DAMGO and DPDPE, but not U50,488H, produced statistically significant decreases in rotarod performance scores compared to vehicle controls. The results of these studies suggest that motor side-effects produced by opioid agonists need to be considered when interpreting the results of antinociceptive tests that are dependent on a normally functioning motor system.