Sex differences in µ-opioid receptor expression in trigeminal ganglia under a myositis condition in rats.

ABSTRACT

BACKGROUND: Peripheral opioid receptor expression is up-regulated under inflammatory conditions, which leads to the increased efficacy of peripherally administered opioids. Sex differences in the effects of inflammation, cytokines and gonadal hormones on µ-opioid receptor (MOR) expression in trigeminal ganglia (TG) are not well understood. METHODS: MOR mRNA and protein levels in TG from male and female Sprague Dawley rats following complete Freund's adjuvant (CFA)-induced muscle inflammation were assessed. Cytokine-induced changes in MOR mRNA expression from TG cultures prepared from intact and gonadectomized male and female, and gonadectomized male rats with testosterone replacement were examined. Behavioural experiments were then performed to examine the efficacy of a peripherally administered MOR agonist in male, female and gonadectomized male rats under a myositis condition. RESULTS: CFA and cytokine treatments induced significant up-regulation of MOR expression in TG from male, but not from female, rats. The cytokine-induced up-regulation of MOR mRNA expression was prevented in TG from orchidectomized (GDX) male rats, which was restored with testosterone replacement. Peripherally administered DAMGO, a specific MOR agonist, significantly attenuated CFA-induced masseter mechanical hypersensitivity only in intact male rats. CONCLUSIONS: Collectively, these data indicate that testosterone plays a key role in the regulation of MOR in TG under inflammatory conditions, and that sex differences in the anti-hyperalgesic effects of peripherally administered opioids are, in part, mediated by peripheral opioid receptor expression levels.