Contribution of conditioned opioid analgesia to the shock-induced associative US-preexposure deficit

The modulatory effect of conditioned opioid analgesia on learning in the US-preexposure paradigm was examined in three experiments using water-deprived rats. In Experiment 1, it was found that tailflick latencies increased immediately after the rats were exposed to a context in which footshock had previously been administered. Prolonged nonreinforced exposure to the context attenuated this analgesia. Experiment 2 tested the possibility that the effectiveness of CS-US pairings in an excitatory context might be reduced by a conditioned analgesic response that lessens the perceived intensity of the US. Administration of the opiate antagonist naloxone prior to CS-US pairings in the excitatory context reduced the US-preexposure deficit—that is, the retarded response to the CS—but did not eliminate it, suggesting that part of the observed deficit resulted from conditioned activation of the endogenous opioid system. In Experiment 3, it was found that exposure to the excitatory context immediately prior to a CS-US pairing in an associatively neutral context resulted in a conditioned response deficit, indicating that the analgesia elicited by the excitatory context was sufficient to reduce US effectiveness. In combination with other recent reports, these results suggest that the associative deficit resulting from preexposure to a shock US may, in certain instances, represent the sum of several different associative processes.