Gender differences in two forms of cold-water swim analgesia.

Female rats display lower shock thresholds and less morphine analgesia than male rats, differences which are sensitive to gonadal manipulations. Baseline pain thresholds and morphine analgesia also vary across the estrous cycle of female rats. The first experiment demonstrated that 'nonopioid' continuous cold-water swim (CCWS: 2 degrees C for 3.5 min) analgesia and 'opioid' intermittent cold-water swim (ICWS: 2 degrees C, 18 10-sec swims and 10-sec rests over 3 min) analgesia are subject to gender-specific effects. The magnitudes of CCWS and ICWS analgesia were significantly lower in female rats as compared to either age-matched or weight-matched male rats. The jump test displayed a more consistent pattern than the tail-flick test. The second experiment failed to show differences in the magnitude of CCWS analgesia across the female estrous cycle. These data demonstrate that both opioid (ICWS) and nonopioid (CCWS) forms of stress-induced analgesia are sensitive to gender differences, and suggest that male gonadal hormones may enhance analgesic responsivity.