INESCAPABLE SHOCK-INDUCED POTENTIATION OF MORPHINE ANALGESIA

Exposure to various stressors potentiates nociceptive and nonnocicepti ve responses to morphine. These phenomena have received little study d espite their seeming generality and importance for understanding analg esia and opiate action. The present experiments characterize inescapab le shock (IS)-induced potentiation of morphine analgesia. Rats were ex posed to IS, equal escapable shocks (ESs), or restraint (control). Pot entiation of analgesia (tail-flick [TF] test and hotplate test) was ob served only in rats given IS 24 or 48 hr earlier, in agreement with pr eviously reported learned-helplessness effects. Finally, no change in tail temperature or motor function was found that could be inaccuratel y interpreted as analgesia. The relevance of these findings to stresso r-induced enhancement of morphine analgesia and potential substates of IS effects are discussed.