Sex Differences in Periaqueductal Gray Kinase Activation After Morphine Tolerance

Abstract

Morphine is a potent opioid analgesic with very high propensity for tolerance development. The analgesic and tolerance properties of opioids are partially regulated by the midbrain ventrolateral periaqueductal gray (vlPAG). There is a growing body of literature that highlights the sex differences in pain processing and opioid effects. However, the nature of sex differences at the level of kinase effectors of the μ‐receptor is unknown. Here, we investigate the intracellular kinase activation and localization patterns in the vlPAG of male and female mice following acute and chronic morphine treatment. Wild‐type mice were treated with morphine (10 mg/kg, i.p.) or saline in a chronic or acute administration paradigm. We performed fluorescence immunohistochemistry to map the localization and activation of extracellular signal‐regulated kinase 1/2 (ERK 1/2), protein kinase‐C (PKC), and protein kinase‐A (PKA). We observed elevated baseline levels of phosphorylated forms of the kinases ERK 1/2, PKC and PKA in females. We observed significantly greater activated ERK 1/2 in the vlPAG chronic morphine‐treated animals which co‐localized with the endosomal marker Eea1. We note that phosphorylated PKC tends to localize to the plasma membrane near the μ‐opioid receptor in the vlPAG following chronic morphine treatment. Further, we observed that although PKA activation is increased following chronic morphine treatment, there is a significant reduction in nuclear translocation of this kinase. Across these kinases, the magnitude of increase of kinase activation was more in males after morphine tolerance. Taken together, this study demonstrates a differential activation and localization of ERK 1/2, PKC and PKA across sexes and opens avenues to explore the role of chronic morphine treatment on G‐protein signaling and kinase nuclear transport.Support or Funding InformationBrain and Behavior Research Foundation to ENB. Department of Biology, Utah State University