Involvement of spinal opioid systems in footshock-induced analgesia: antagonism by naloxone is possible only before induction of analgesia.

Footshock reliably produces analgesia in rats which is mediated either by opiate or non-opiate systems. It has recently been demonstrated that a critical factor determining the involvement of endogenous opioids is the body region shocked; front paw shock produces a naloxone-reversible analgesia and hind paw shock produces an analgesia which fails to be attenuated by this opiate antagonist. The present study demonstrated that a crucial opiate site for the production of front paw footshock-induced analgesia (FSIA) exists within the spinal cord. One microgram of naloxone delivered directly to the lumbosacral cord immediately prior to shock significantly attenuated this analgesia. However, the efficacy of naloxone antagonism was order-dependent in that naloxone failed to antagonize fron paw FSIA if delivered immediately after shock; naloxone could prevent but could not reverse the analgesic state. The body region shocked was again observed to be a critical factor determining the involvement of endogenous opioids since 1 microgram of spinal naloxone failed to antagonize hind paw FSIA. These results were discussed in light of recent evidence proposing a neuromodulatory role of opioids within the spinal cord.