Mechanistic Sex Differences Underlying the Development of Chronic Pain




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Chronic pain impacts approximately 1 out of every 3 Americans every year. Despite extensive preclinical research, very few new pain therapeutics have been approved for use in the past decade. It has become increasingly clear that the mechanisms underlying the development of chronic pain are different in males and females, and understanding these sexually dimorphic mechanisms will aid in development of efficacious chronic pain therapeutics. We hypothesized that male and female rodents have separate mechanisms underlying the transition from an acute to chronic pain state. We first tested this hypothesis through a series of experiments using pharmacological manipulations in the hyperalgesic priming model. In this model animals received an initial injection of interleukin 6 (IL-6) or IL-6 receptor (IL-6r) and mechanical hypersensitivity is measured through von Frey filament testing. Following the resolution of the initial hypersensitivity caused by IL-6r injection, animals received a subthreshold dose of Prostaglandin E2 (PGE2) through an intraplantar (I.Pl.) injection. For animals that received a priming dose of IL-6r or IL-6, the second injection of PGE2 caused mechanical hypersensitivity. A single injection of IL-6r did not increase levels of activated spinal microglia in either male or female mice at 3h, but the PGE2 injection did increase levels of activated microglia in both male and female animals at 3h post injection. Animals then received an intrathecal (I.T.) injection of either TNP-ATP, a P2X Receptor (P2XR) antagonist, or Skepinone, a p38 MAPK inhibitor, at the time of IL-6r injection. TNP-ATP blocked the mechanical hypersensitivity cause by intraplantar (I.Pl.) IL-6r in both male and female mice but blocked precipitation of priming by injection of PGE2 in male mice only. Skepinone had no effect on the response to IL-6r, but blocked priming in male mice. Neither TNP-ATP nor Skepinone could reverse priming one established in either sex. Because chronic pain disproportionately impacts women, we next sought to understand female-specific mechanisms underlying chronic pain in various preclinical animal models. In the hyperalgesic priming model we found that male and female mice had similar levels of mechanical hypersensitivity following I.Pl. injection of IL-6. We found that female mice have a prolonged response to the PGE2 I.Pl. injection. Additionally, using priming precipitated with I.T. PGE2 caused an increased and prolonged mechanical hypersensitivity in female mice when compared with male mice. This enhanced and prolonged response to PGE2 is dependent on estrogen and local mRNA translation in the spinal cord (S.C.). Prolactin (PRL) in sensory neurons is necessary for hyperalgesic priming female mice by not male mice. We established this by using both ∆PRL, a PRL receptor antagonis, and a transgenic mouse line that had ablated PRL in Nav1.8+ neurons. In our final set of experiments, we examined the female-specific role of Calcitonin Gene Related Peptide (CGRP) in mouse models of chronic pain. Mice received an I.T. injection of either Olcegpant or CGRP8-37, both of which are CGRP antagonists. Hyperalgesic priming was blocked in female mice that received I.T. Olcegepant at the time of IL-6r. Olcegepant at the time of PGE2 did not reverse established priming. CGRP8-37 was able to both block and reverse hyperalgesic priming specifically in female mice. A monoclonal CGRP antibody was also able to block hyperalgesic priming specifically in female mice. In the Spared-Nerve Injury model, CGRP8-37 had a transient effect specifically in female mice. CGRP applied intrathecally caused a long-lasting hindpaw sensitivity in female mice which could be reversed by CLP-257, a KCC2 activator. Spinal cord electrophysiology revealed that CGRP shifted GABA-A reversal potentials to more positive values, again in female mice. These findings demonstrate sex-specific mechanisms underlying the transition from acute to chronic pain, and create a strong rationale to explore the development of sex-specific analgesics going forward.



Chronic pain, Sex factors in disease, Calcitonin gene-related peptide