LEARNED HELPLESSNESS INDUCING FOOT SHOCK CAN EXACERBATE MORPHINE RESPONSIVENESS

Exposure to inescapable tail shock or foot shock has been shown to pro duce effects on a number of learning tasks. Tail-shock exposure is als o known to influence nociception and morphine reactivity. The present series of experiments investigated the effects of foot shock known to induce learned helplessness effects in our laboratory on the subsequen t reactivity to morphine. A first set of experiments investigated the hypoalgesic response to a 4 mg/kg dose morphine over 4 consecutive day s following exposure to foot shock. Experiment 1A did not reveal an ef fect of foot shock on morphine-induced hypoalgesia when testing was co nducted in a novel context. In Experiment 1B, we observed an increased hypoalgesic response to morphine when testing was conducted in the sh ock context. The findings of Experiment 1B were replicated in Experime nt 2 and extended to assess the contribution of conditioned fear hypoa lgesia to these effects. The possible mechanisms responsible for these findings are discussed.