OPIOID FOOTSHOCK-INDUCED ANALGESIA IN MICE ACUTELY FALLS BY STRESS PROLONGATION

The application of 80 footshocks (S-80) to mice induces a decrease in nociceptive responses as measured by the tail-flick test, which is opi oid mediated as well as prevented by naloxone (10 mg/kg, SC). When the stress is prolonged up to 240 shocks (S-240) (i.e., from 6 min 40 s t o 20 min), no analgesia can be seen immediately after the stress. We h ave examined the two most obvious possibilities, but they do not seem to be responsible for this fact. When morphine (1-5 mg/kg IP) is injec ted in the S-240 situation, a potentiation of its analgesic effects is seen, so that a desensitization of mu opioid receptors is unlikely. O n the other hand, although cortisol (3-30 mg/kg IP) inhibits the analg esic response to S-80, metyrapone (40 and 80 mg/kg IP) and cortexolone (3-18 mg/kg IP) do not cause S-240 to be analgesic. Thus, an increase of endogenous glucocorticoids released during the long-duration stres s does not seem responsible for the lack of analgesia after S-240.