Effect of naloxone on the behaviour of rats exposed to a novel environment

It has recently been suggested that endogenous opiates may play a general role in stress responding. To test this hypothesis, naloxone hydrochloride (0.5–4.0 mg/kg SC) was administered to rats exposed to an open field situation. Naloxone treatment produced a decrease in locomotor activity and rearing, and an increase in defaecation. A simple dose-response relationship was not observed, with the most potent effects exerted by the 1 mg/kg dose. Nevertheless, these results indicate that naloxone increases emotionality in the rat and suggest that opioid peptides may be released under conditions of nonpainful stress.