Determinants of the nature of environmentally induced hypoalgesia

Factors that determine whether naltrexone and pentobarbital anesthesia block the hypoalgesia produced by electric shock were investigated. The number of shocks delivered, the time since the last shock, and whether the subject is removed from the shock environment for testing were all found to be critical. One shock was followed by an initial hypoalgesia that was reversed by naltrexone but was not affected by pentobarbital. Moreover, this initial hypoalgesia was not eliminated by removing the subject from the shock apparatus. When testing was conducted in the shock context, this early hypoalgesia was followed by a hypoalgesia that was still reversed by naltrexone but differed from the early response in that it was eliminated by pentobarbital anesthesia. This second hypoalgesia did not occur when testing was conducted after removing the animal from the shock apparatus. Five shocks were also followed by two separable responses. Here the first response was also not reduced by pentobarbital, but it was not blocked by naltrexone. This initial response was also not prevented by removal from the shock context, but, again, the second response did not occur after removal. The pattern after 80 shocks was quite different. Here the hypoalgesic response did not appear to change in character across the 10 min of testing or with removal from the shock apparatus. The response persisted for 10 min in the shock context or after removal and was blocked by both naltrexone and pentobarbital throughout its entire duration. These results reconcile findings from different laboratories and bear on a number of theoretical models of stress-induced hypoalgesia.