Sex differences in relation to conditioned fear-induced enhancement of morphine analgesia

A number of studies have reported that both the immediate and proactive eff ects of exposure to a shock stresser are less pronounced in female than in male rats. A separate area of research has demonstrated that female rats ar e less sensitive to the analgesic effects of morphine than males. Experimen ts from our laboratory, as well as others, have found that exposure to a co ntext associated with shock (i.e., conditioned fear context) at the time of morphine administration, enhances the analgesic effects of morphine. Since previous studies have exclusively employed male rats, the purpose of Exper iment I was to determine if a sex difference exists to this context conditi oned fear-induced enhancement of morphine-induced analgesia. The findings o f Experiment 1 showed that females do not appear to exhibit conditioned fea r-induced enhancement of morphine analgesia as compared to males. Experimen t 2 demonstrated that females exhibited higher levels of conditioned fear-i nduced enhancement of morphine analgesia during diestrus I than estrus. Exp eriment 3 demonstrated that females exhibited lower levels of conditioned a nalgesia compared to males, while both groups exhibited similar freezing le vels. The findings of the present experiments suggest that the sex differen ce observed in Experiment 1 may be due to differences in conditioned analge sia. (C) 2001 Elsevier Science Inc. All rights reserved.