Abstract
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.