Morphine-induced kinase activation and localization in the periaqueductal gray of male and female mice.

Morphine is a potent opioid analgesic with high propensity for the development of antinociceptive tolerance. Morphine antinociception and tolerance are partially regulated by the midbrain ventrolateral periaqueductal gray (vlPAG). However, the majority of research evaluating mu-opioid receptor signaling has focused on males. Here, we investigate kinase activation and localization patterns in the vlPAG following acute and chronic morphine treatment in both sexes. Male and female mice developed rapid antinociceptive tolerance to morphine (10 mg/kg i.p.) on the hot plate assay, but tolerance did not develop in males on the tail flick assay. Quantitative fluorescence immunohistochemistry was used to map and evaluate the activation of extracellular signal-regulated kinase 1/2 (ERK 1/2), protein kinase-C (PKC), and protein kinase-A (PKA). We observed significantly greater phosphorylated ERK 1/2 in the vlPAG of chronic morphine-treated animals which co-localized with the endosomal marker, Eea1. We note that pPKC is significantly elevated in the vlPAG of both sexes following chronic morphine treatment. We also observed that although PKA activity is elevated following chronic morphine treatment in both sexes, there is a significant reduction in the nuclear translocation of its phosphorylated substrate. Taken together, this study demonstrates increased activation of ERK 1/2, PKC, and PKA in response to repeated morphine treatment. The study opens avenues to explore the impact of chronic morphine treatment on G-protein signaling and kinase nuclear transport.

GPR171 Agonist Reduces Chronic Neuropathic and Inflammatory Pain in Male, But Not Female Mice.

Chronic pain is a growing public health crisis that requires exigent and efficacious therapeutics. GPR171 is a promising therapeutic target that is widely expressed through the brain, including within the descending pain modulatory regions. Here, we explore the therapeutic potential of the GPR171 agonist, MS15203, in its ability to alleviate chronic pain in male and female mice using a once-daily systemic dose (10 mg/kg, i.p.) of MS15203 over the course of 5 days. We found that in our models of Complete Freund's Adjuvant (CFA)-induced inflammatory pain and chemotherapy-induced peripheral neuropathy (CIPN), MS15203 did not alleviate thermal hypersensitivity and allodynia, respectively, in female mice. On the other hand, MS15203 treatment decreased the duration of thermal hypersensitivity in CFA-treated male mice following 3 days of once-daily administration. MS15203 treatment also produced an improvement in allodynia in male mice, but not female mice, in neuropathic pain after 5 days of treatment. Gene expression of GPR171 and that of its endogenous ligand BigLEN, encoded by the gene PCSK1N, were unaltered within the periaqueductal gray (PAG) in both male and female mice following inflammatory and neuropathic pain. However, following neuropathic pain in male mice, the protein levels of GPR171 were decreased in the PAG. Treatment with MS15203 then rescued the protein levels of GPR171 in the PAG of these mice. Taken together, our results identify GPR171 as a GPCR that displays sexual dimorphism in alleviation of chronic pain. Further, our results suggest that GPR171 and MS15203 have demonstrable therapeutic potential in the treatment of chronic pain.