Opiate vs non-opiate footshock-induced analgesia (FSIA): the body region shocked is a critical factor.

Previous work has demonstrated that footshock can elicit either opiate or non-opiate analgesia. The present study has demonstrated that one critical factor determining the involvement of endogenous opioids is the body region shocked. Using 90 s shock, front paw shock produced an opiate analgesia which was significantly antagonized by as little as 0.1 mg/kg systemic naloxone and morphine tolerance. In the latter experiment, a parallel recovery of the analgesic potencies of both front paw shock and morphine was observed following 2 weeks of opiate abstinence. In contrast, hind paw shock produced a non-opiate analgesia which failed to be attenuated by 20 mg/kg systemic naloxone and showed no cross-tolerance to morphine. Since identical shock parameters were used for front paw and hind paw shock in the systemic naloxone experiments, stress per se clearly cannot be the crucial factor determining the involvement of endogenous opioids in footshock-induced analgesia. These results were discussed with respect to clinical treatments of pain which utilize somatosensory stimulation.