Sex differences in effects of opioid blockade on stress-induced freezing behavior

The present experiment examined the effects of naloxone on freezing behavior in male and female rats following stress and no-stress conditions. Twelve male and 12 female Wistar rats were exposed to 10 min of mild, unpredictable footshock stress and to a comparable no-stress condition. Immediately following stress or no-stress conditions, subjects were injected with naloxone or saline, and two independent observers measured 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 conditions. These results reveal a sex difference in effects of naloxone on freezing behavior and suggest that sex differences may exist with respect to the role of endogenous opioids under stress.