Influence of gonadectomy on the antinociceptive effects of opioids in male and female rats.
BACKGROUND
Sex differences in the antinociceptive effects of opioids have been reported in a variety of nociceptive assays, and it has been postulated that these differences are mediated by gonadal hormones.


OBJECTIVE
The present study examined the influence of gonadectomy on opioid antinociception in male and female rats.


METHODS
In a warm-water, tail-withdrawal procedure, the antinociceptive effects of the high-efficacy micro opioids etorphine and morphine; the low-efficacy micro opioids buprenorphine and dezocine; and the low-efficacy, mixed-action opioids butorphanol and nalbuphine were examined in intact and gonadectomized rats of the F344 and Sprague Dawley (SD) strains.


RESULTS
The opioids examined were generally more potent in producing an antinociceptive effect in intact males than intact females, with larger sex differences observed with the less-effective opioids. In F344 males, gonadectomy produced small decreases in the potency of etorphine and morphine, and large decreases in the potency of buprenorphine, dezocine, butorphanol, and nalbuphine. Similar effects were obtained in SD males, with gonadectomy decreasing the potency of each of the opioids tested. In F344 females, gonadectomy produced small increases in the potency of etorphine and large increases in the potency of buprenorphine, dezocine, butorphanol, and nalbuphine. A similar effect was obtained in SD females, as gonadectomy increased the potency of etorphine, morphine, and buprenorphine. In both sexes, gonadectomy had a greater effect in F344 than SD rats.


CONCLUSIONS
These findings suggest that gonadectomy decreases opioid antinociception in male rats and increases opioid antinociception in female rats. Additionally, the influence of gonadectomy on opioid antinociception appears to be determined by the relative effectiveness of the opioid tested and the rodent strain used.
Terner
Jolan M
JM
Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3270, USA. terner@email.unc.edu
Barrett
Andrew C
AC
Grossell
Emily
E
Picker
Mitchell J
MJ
D000700:Analgesics / Q000494:pharmacology*; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D008297:Male; D009919:Orchiectomy; D010052:Ovariectomy; D010147:Pain Measurement / Q000379:methods; D051381:Rats; D011916:Rats, Inbred F344; D017207:Rats, Sprague-Dawley; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D012727:Sex Characteristics; D013045:Species Specificity
D000700:Analgesics; D000701:Analgesics, Opioid