Shock-induced analgesia on the formalin test: effects of shock severity, naloxone, hypophysectomy, and associative...

Rats were exposed to three shocks, spaced 20 s apart, at two different levels of severity, low (.75 s, 1 mA) and high (3 s, 4 mA). Both shock levels produced a similar suppression of the recuperative behavior elicited by an injection of formalin into a rat's hind paw. Naloxone fully reversed the analgesia produced by the low-severity shock but only partially reversed the analgesia produced by the high-severity shock (Experiment 1). Hypophysectomy did not alter the level of analgesia (Experiment 2). When the rats were tested in a chamber different from the one they were shocked in, both analgesias were totally reversed (Experiment 3). However, imposing a delay between shock and analgesia testing did not reduce analgesia (Experiment 4). These results suggest that analgesia is not directly elicited by the shock but by apparatus stimuli associated with shock. Further support for this position was obtained when it was found that a Pavlovian extinction procedure could completely eliminate analgesia (Experiment 5). In all of the experiments, the freezing response, one of the rat's species-specific defense reactions, was monitored simultaneously with recuperative behavior. A parallel was found between analgesia and this defensive response, a result suggesting that an animal's endogenous analgesic systems may be activated along with the animal's defensive motivational system. The results point to the critical nature of associative variables in the control of endogenous analgesic systems. They also suggest that shock severity is a determinant of analgesia's sensitivity to naloxone.