Contribution of different opioid systems to footshock-induced analgesia and motor suppression

Mice, subjected footshock, showed a significant increase of pain threshold and motor suppression immediately after the footshock and both effects were partially antagonized by pretreatment with naloxone at high doses. The magnitude of footshock-induced analgesia, but not motor suppression, was significantly less in morphine-tolerant mice than in non-tolerant mice. On the other hand, the magnitude of motor suppression, but not analgesia, was significantly less in ethylketocyclazocine- and pentazocine-tolerant mice than in non-tolerant mice. Furthermore, the apparent development of tolerance to both phenomena by successive daily footshocks was strikingly different. Tolerance to footshock-induced analgesia was induced on day 4 by the successive footshocks, while tolerance to motor suppression was not observed until day 17 of successive daily footshocks. In addition, the antinociceptive effect of morphine was significantly attenuated in mice tolerant to footshock-induced analgesia. These results suggest that different opioid systems may participate in footshock-induced analgesia and motor suppression. In addition, it is suggested that footshock-induced analgesia may be mediated by mu and/or delta receptors and motor suppression may be mediated by kappa and/or delta receptors.