SEX-DIFFERENCES IN EFFECTS OF OPIOID BLOCKADE ON STRESS-INDUCED FREEZING BEHAVIOR

The present experiment examined the effects of naloxone on freezing be havior in male and female rats following stress and no-stress conditio ns. Twelve male and 12 female Wistar rats were exposed to 10 min of mi ld, unpredictable footshock stress and to a comparable no-stress condi tion. Immediately following stress or no-stress conditions, subjects w ere injected with naloxone or saline, and two independent observers me asured freezing behavior. In male rats, naloxone potentiated freezing following stress but had no effect on freezing following no-stress. In females, naloxone did not affect freezing regardless of stress condit ions. These results reveal a sex difference in effects of naloxone on freezing behavior and suggest that sex differences may exist with resp ect to the role of endogenous opioids under stress. (C) 1998 Elsevier Science Inc.