• R M Craft, A H Tseng, D M McNiel, M S Furness, and K C Rice.
• Department of Psychology, Washington State University, Pullman 99164-4820, USA. craft@wsu.edu
• Behav Pharmacol. 2001 Dec 1;12(8):591-602.

AbstractThis study was conducted to determine whether sex differences in opioid antinociception may be explained by sex differences in opioid receptor activation. The time course, dose-effect and selectivity of antagonists that have been previously shown to be relatively mu (beta-funaltrexamine, beta-FNA), kappa (norbinaltorphimine, norBNI), or delta (naltrindole, NTI) receptor selective in male animals were compared in female and male Sprague-Dawley rats using a 52 degrees C hotplate test. In both sexes, beta-FNA (10 or 20 microg intracerebroventricularly [i.c.v.]) dose-dependently blocked the antinociceptive effects of fentanyl (0.056 mg/kg subcutaneously); antagonism was observed 24 h after beta-FNA, and diminished within 7-14 days. In both sexes, norBNI (1 or 10 microg i.c.v.) dose-dependently blocked the antinociceptive effects of U69,593 (1.0 mg/kg subcutaneously); antagonism was maximal by 1-3 days post-norBNI and lasted longer than 56 days. NTI (1 or 10 microg i.c.v.) dose-dependently blocked the antinociceptive effects of [D-Pen2, D-Pen5]enkephalin (DPDPE, 100 nmol i.c.v.) in both sexes; however, the duration of action of NTI was shorter in females than in males. The antinociceptive effects of the mu receptor-preferring agonists fentanyl, morphine and buprenorphine were significantly and dose-dependently antagonized by beta-FNA, but not by norBNI or NTI, in both sexes. Beta-FNA antagonism was significantly greater in females compared with males given morphine, but not fentanyl or buprenorphine. The antinociceptive effects of the kappa receptor-preferring agonists U69,593 and U50,488 were significantly and dose-dependently antagonized by norBNI; U50,488 but not U69,593 was also antagonized to a lesser extent by NTI and beta-FNA, in both sexes. The antinociceptive effect of the delta receptor-preferring agonist SNC 80 was significantly antagonized by NTI, but not by norBNI or beta-FNA, in both sexes. The sex difference in beta-FNA antagonism of morphine suggests that there may be sex differences in functional mu opioid receptor reserve or signal transduction; however, the lack of consistency across all mu agonists weakens this hypothesis. Overall, the opioids tested had very similar receptor selectivity in male and female subjects.

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