Shock controllability and the nature of stress-induced analgesia.

Three experiments were conducted to examine the impact of the escapability of shock on the nature of the analgesia produced by shock. In Experiment 1, rats were exposed to 0, 20, 40, or 80 escapable or yoked inescapable shocks. Tail-flick testing revealed a "double-peak" pattern in which analgesia was present after 20 and 80 shocks. This was true for both escapable and inescapable shock. Analgesia after 20 escapable or inescapable shocks was insensitive to naltrexone, as was the analgesia after 80 escapable shocks. However, the analgesia after 80 inescapable shocks was completely blocked by naltrexone. Moreover, in Experiment 2 the analgesia following 80 inescapable shocks persisted for at least 2 hr, whereas it dissipated rapidly following 80 escapable shocks. Further, the analgesia produced by escapable shock even dissipated with the continued occurrence of escapable shock. Finally, shock controllability was shown to alter the analgesia produced by subsequent exposure to shock. Prior experience with controllable shock completely blocked the late-appearing naltrexone reversible analgesia; prior experience with uncontrollable shock led it to appear sooner.