ENHANCEMENT OF MORPHINE ANALGESIA IN RATS FOLLOWING REMOVAL FROM CONTEXTUAL CONDITIONED FEAR CUES

1. Previous studies have shown that morphine analgesia is enhanced whe n analgesia testing is conducted in an environment that has been previ ously paired with shock, but not in a novel or neutral environment. 2. Two experiments were conducted to assess if enhanced morphine analges ia could be demonstrated in a neutral context if rats were first expos ed to conditioned fear cues. This was done by pre-exposing rats to a c ontext previously paired with shock and testing for enhanced morphine analgesia in a neutral context immediately following removal from the conditioned fear context. To determined if conditioned analgesia contr ibuted to the enhanced morphine analgesia, rats were tested for analge sic responsiveness immediately following removal from conditioned fear cues, prior to morphine administration. 3. In Experiment 1, although conditioned analgesia was not observed, a small enhancement of morphin e analgesia was demonstrated in an neutral context in rats pre-exposed to conditioned fear cues, compared to non-conditioned controls. 4. In Experiment 2, which employed more sensitive test procedures, a strong enhancement of morphine analgesia was observed in a neutral context o nly in those rats that demonstrated conditioned analgesia. (C) 1997 El sevier Science Inc.