Effects of naloxone and Mr 1452 on stress-induced changes in nociception of different stimuli in rats.

The effects of naloxone and Mr 1452 on nociceptive responding to heat and pressure following restraint stress were examined. Thresholds for heat and pressure were determined using standard tail immersion and paw pressure tests. Restraint produced significant analgesia to heat but hyperalgesia to pressure. Naloxone reduced this heat analgesia but had no effect on stress-induced hyperalgesia to pressure. Mr 1452 also attenuated the heat analgesia but in contrast to naloxone it potentiated the hyperalgesia to pressure. These results suggest a differential involvement of mu- and k-opioid systems in the mediation of stress-induced changes in nociception.