Dose-dependent reductions by naloxone of analgesia induced by cold-water stress.

Animals exposed to cold-water swims, rotation, or inexcapable shocks, display analgesia comparable to that of 10 mg/kg of morphine. The present study investigated whether a narcotic antagonist would eliminate analgesia induced by cold-water swims. In one group of 12 rats, naloxone at 0, 1, 5, 10 and 20 mg/kg was administered at weekly intervals immediately preceding forced cold-water swims (2 degrees C for 3.5 min) and alterations in flinch-jump thresholds were determined 30 min thereafter. In a second group of six rats, the effects of the same dose range of naloxone were determined upon normal flinch-jump thresholds. Naloxone dose-dependently attenuated the cold-water swim-induced analgesia up to a maximal reduction of 50% at 20 mg/kg. In contrast, all doses of naloxone had no effects upon normal flinch-jump thresholds. Since low doses of naloxone completely abolish morphine-induced analgesia, the present data suggest that the analgesia induced by stress is not identical to that of opiates.