Nociceptive sensitivity and opioid antinociception and antihyperalgesia in Freund's adjuvant-induced arthritic male and female rats.

The present study was designed to examine sex differences in complete Freund's adjuvant (CFA)-induced mechanical hyperalgesia and sex differences in opioid antinociception and anti-hyperalgesia. Female rats developed inflammation and hyperalgesia faster and exhibited greater peak hyperalgesia than male rats. In arthritic (CFA-treated) rats, lower thresholds were observed during estrus and proestrus, and in nonarthritic (vehicle-treated) rats, lower thresholds were observed during proestrus. Morphine and oxycodone were more potent in male than female arthritic rats, and butorphanol was more potent and effective in male than female arthritic rats. The potency of morphine was increased in arthritic rats, although to a greater magnitude in males. The potency of oxycodone was increased in male but not female arthritic rats. The potency of butorphanol was increased in arthritic male rats and the maximal antinociceptive effect of butorphanol was increased in arthritic female rats, but it did not result in greater than 20% antinociception. Morphine, oxycodone, and butorphanol all produced antihyperalgesic effects (returning thresholds of arthritic rats to the thresholds of nonarthritic rats) with greater potency in males than females. The peripherally acting opioid agonist loperamide produced intermediate levels of antinociception in male and female arthritic rats and no antinociception in nonarthritic rats. Loperamide was more potent in male than female arthritic rats at producing antihyperalgesia. These data demonstrate sex differences in arthritis-induced hyperalgesia and responsiveness to opioid analgesics. In arthritic rats, the antinociceptive effects of opioid agonists are most probably mediated by both central and peripheral opioid receptors, whereas their antihyperalgesic effects are mediated primarily by actions at peripheral opioid receptors.