The Effects of Morphine Tolerance on the PI3K/AKT Intracellular Signaling Pathway During Acute and Chronic Pain in the Central and Peripheral Nervous System of Mice

Abstract

The management of chronic pain with opioids can cause opioid-induced analgesic tolerance and hyperalgesia, complicating clinical pain-management treatments. Mu opioid receptors (MOPs) are inhibitory G-protein coupled receptors. MOPs have been studied for years; however, the intracellular signaling pathways triggered by their activation are not well known. Research presented here sought to determine if opioid induced tolerance is linked to a decreased activity in the PI3K/AKT intracellular signaling pathway. To assess gene expression within this pathway and cGMP nucleotide levels, C57Bl/6 wild type male mice were divided into saline , morphine tolerant (MT), and morphine tolerant with spinal nerve ligation (MT+SNL) groups. Injections were given subcutaneously twice a day for a total of five days. MT mice without SNL developed opioid induced tolerance by day 3 and opioid induced hyperalgesia by day 5. MT+SNL mice had lower TPWL responses for the ipsi (injured) side compared to their contra (uninjured) side. Brainstem, spinal cord, dorsal root ganglia, and sciatic nerves were harvested from mice on day 6 of the behavior testing and were used for qPCR gene expression analysis. Genes chosen for qPCR analysis were Akt1, Akt2, Akt3, Pik3cg (splice variants v1-v3), Pten, Abcc8, Abcc9, Kcnj11 (splice variants v1 and v2), Kcnj8, Oprm1, Jnk3, and nNos1. There were few significant gene expression changes of the PI3K/AKT intracellular signaling pathway for MT mice compared to saline mice in both the central and peripheral nervous system. The addition of the MT+SNL model saw larger gene expression increases or decreases in almost all genes, with no clear trend for gene expression changes both between and within the central and peripheral nervous system tissues. Additionally, there was decreased gene expression in all genes except Pten in the MT+SNL spinal cord. The daily administration of PI3K/AKT pathway inhibitors, thalidomide, SP600125, and quercetin to MT+SNL mice attenuated the development of morphine tolerance, suggesting PI3K/AKT pathway activity is positively correlated with morphine tolerance. Thus, our initial hypothesis that stated PI3K/AKT pathway downregulation may contribute to morphine tolerance was not supported by our data collected from this study. Overall, the PI3K/AKT intracellular signaling pathway is a potential target for reducing the development of morphine tolerance. Continued research into this pathway, including further protein analysis and studies utilizing knockout mice, will advance understanding of morphine tolerance and potentially contribute to the development of new analgesic drug therapies.