Sex differences in opioid-mediated pain inhibitory mechanisms during the interphase in the formalin test

Abstract Many chronic pain conditions are more prevalent in women than men and both fundamental and clinical research supports the implication of endogenous pain inhibitory mechanisms. The goal of this study was to verify if sex differences on endogenous pain inhibitory mechanisms during the formalin test are opioidergic and modulated by sex hormones. Formalin tests were performed with naloxone hydrochloride, a non-selective opioid antagonist in intact and gonadectomized Sprague–Dawley rats of both sexes. Considering the sexual dimorphisms we found, where naloxone preferentially blocked the interphase in female rats, injections of all the possible combinations of μ- (naltrexone hydrochloride), δ- (naltrindole hydrochloride) and κ-selective antagonists (norbinaltorphimine dihydrochloride) were given to evaluate the contribution of these opioid-receptor subtypes to the inhibitory mechanism during the interphase in intact females. Finally, the systemic administration of naloxone methiodide and intrathecal administration of naloxone hydrochloride in intact females allowed us to verify if the action of endogenous opioids that are liberated during the interphase takes place at the periphery or spinally, respectively. The results show that the interphase was almost completely inhibited by naloxone in females while it produced only a slight blockade in males. These results permitted us to conclude that opioids play a major role in the pain inhibitory mechanism of the interphase in females while a non-opioid mechanism seems to be responsible for this inhibitory pathway in males. Using gonadectomized animals of both sexes, we demonstrated the modulation of the opioidergic system of the interphase by sex hormones. The administration of different combinations of selective antagonists for μ-, κ- and δ-opioid receptors in intact females permitted us to conclude that only the combination of κ- and δ-selective antagonists significantly blocked the interphase. The same result was obtained with the combination of the three antagonists, confirming the results with systemic naloxone hydrochloride. Finally, intrathecal administration permitted us to support that the action of naloxone is primarily at the spinal level, even if a supraspinal action cannot be ruled out. These results are of particular interest in showing sexual dimorphisms in endogenous pain modulation mechanisms during the interphase of the formalin test. A clearer understanding of the difference between male an