Sex, pain, and opioids: interdependent influences of sex and pain modality on dynorphin-mediated antinociception in rats.

The role of dynorphin A (1-17; Dyn) and its associated kappa opioid receptor (KOR) in nociception represents a longstanding scientific conundrum: Dyn and KOR (Dyn/KOR) have variously been reported to inhibit, facilitate, or have no effect on pain. We investigated whether interactions between sex and pain type (which are usually ignored) influenced Dyn/KOR-mediated antinociception. Blockade of the spinal α(2)-noradrenergic receptor (α(2)-NAR) using yohimbine elicited comparable spinal Dyn release in females and males. Nevertheless, the yohimbine-induced antinociception exhibited sexual dimorphism that depended on the pain test used: in the intraperitoneal acetic acid-induced writhing test, yohimbine produced antinociception only in females, whereas in the intraplantar formalin-induced paw flinch test, antinociception was observed only in males. In females and males, both intrathecal Dyn antibodies and spinal KOR blockade eliminated the yohimbine-induced antinociception, indicating that Dyn/KOR mediated it. However, despite the conditional nature of spinal Dyn/KOR-mediated yohimbine antinociception, both intraplantar formalin and intraperitoneal acetic acid activated spinal Dyn neurons that expressed α(2)-NARs. Moreover, Dyn terminals apposed KOR-expressing spinal nociceptive neurons in both sexes. This similar organization suggests that the sexually dimorphic interdependent effects of sex and pain type may result from the presence of nonfunctional (silent) KORs on nociceptive spinal neurons that are responsive to intraplantar formalin (in females) versus intraperitoneal acetic acid (in males). Our findings that spinal Dyn/KOR-mediated antinociception depends on interactions between sex and pain type underscore the importance of using both sexes and multiple pain models when investigating Dyn/KOR antinociception.