Effects of morphine and naloxone on restraint-stress ulcers in rats.

Rats were starved for 24 h. Morphine sulphate (1.0, 2.0 or 4.0 mg/kg i.p.) or naloxone-HCl (12.5, 25 or 50 mg/kg i.p.) was administered prior to 3 h of cold-restraint stress. Morphine at 4.0 mg/kg significantly reduced the ulcer severity relative to saline-injected controls and to naloxone-treated rats. Naloxone increased the ulcer incidence and ulcer severity when compared to controls and morphine-treated rats. These results suggest that endogenous opioids released during stress may attenuate the pathological effects of stress.