Involvement of spinal kappa opioid receptors in a type of footshock induced analgesia in mice.

We have studied the effects of several opioid antagonists on a type of footshock stress-induced analgesia (FSIA) measured by the tail-flick test in male mice. Naloxone injected either subcutaneously (0.1-10 mg/kg) or intrathecally (1-20 micrograms) antagonized FSIA at higher doses than those that blocked a similar degree of analgesia induced by morphine. Intracerebroventricular (i.c.v.) naloxone (1-20 micrograms) did not modify the FSIA while antagonizing the i.c.v. morphine-induced analgesia. As a consequence, the antagonism of the FSIA by naloxone probably occurs at the level of the spinal cord and through receptors different than mu. The delta selective antagonist naltrindole (0.1-3 mg/kg s.c.) did not antagonize the analgesic effects of the stress. Nor-binaltorphimine, a kappa selective antagonist, blocked the FSIA when administered systemically (1-4 mg/kg i.p.) or locally (0.1-1 microgram i.t.). These results strongly suggest that spinal kappa opioid receptors are responsible for this type of endogenous analgesia.