Opioid use for chronic pain is limited by antinociceptive tolerance, opioid-induced hyperalgesia (OIH), physical dependence and addiction.1-3 The triggering mechanisms remain elusive. Our published work4,5 and preliminary data suggest for the first time that dysregulation of sphingolipid metabolism in the central nervous system (CNS) leading to exaggerated production of sphingosine 1-phosphate (S1P) and activation of an astrocyte-based S1P receptor subtype 1 (S1PR1) signaling pathway is central to these processes. Oral administration of CNS penetrant S1PR1 competitive and functional antagonists, including FTY720 (Gilenya),6 blocked morphine and oxycodone-induced antinociceptive tolerance and OIH in rodents of both genders, as well as morphine-induced dependence and reward. FTY720 is already FDA-approved6 and therefore, rapid clinical translation of the expected finding is very feasible. We also discovered that sustained opioid-unwanted actions do not develop in conditional S1PR1-knockout mice lacking S1PR1 in spinal astrocytes and that the beneficial effects of S1PR1-targeted agents are attenuated by at least 90% in conditional S1PR1-knockdown mice of both genders, which lack one S1pr1 allele in spinal astrocytes when compared to their littermate controls. These results unravel the importance of astrocyte-based S1PR1 signaling and suggest that astrocytes are a cellular target for anti-S1PR1 activity. Increased levels of S1P in CNS were associated with increased 1) astrocyte reactivity, 2) expression of the Nod-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome (critical in IL1? formation and signaling)7 and 3) formation of inflammatory/ neuroexcitatory cytokines. Blocking S1PR1 inhibited these processes. In contrast, IL10 (an important anti- inflammatory and neuroprotective cytokine) increased significantly. Intrathecal delivery of a neutralizing anti- IL10 antibody blocked the effects of S1PR1 antagonists suggesting that their beneficial effects are driven by an endogenous IL10 pathway. A multidisciplinary plan builds on our strong preliminary data to test our hypothesis: An astrocyte-based SphK1/S1P/S1PR1 signaling pathway driven by NLRP3-induced neuroinflammation in the CNS underlies the development of morphine-induced antinociceptive tolerance/OIH and reward. Targeting S1PR1 provides a novel approach for therapeutic intervention.
Three aims will test our hypothesis: 1) Establish S1PR1 as a molecular target for sustained opioid intervention, 2) Examine opioid-induced alterations of sphingolipid metabolism and SphK1/S1P/S1PR1 signaling and 3) Determine molecular and biochemical pathways engaged downstream of S1PR1 activation. Impact: Our results will unravel a previously unrecognized role for an astrocyte based SphK1/S1P/S1PR1- signaling pathway in sustained opioid use and will provide the foundation for clinical evaluation of S1PR1- targeted therapeutics as adjunct to opioids.
Opioids are often inadequate for long-term treatment of chronic non-cancer pain. Tolerance and increased pain sensitivities lead to dosing escalation, over-prescription, physical dependence, addiction and abuse. The molecular underpinnings remain poorly understood. Our findings suggest that sphingosine 1-phosphate (S1P) signaling in the central nervous system is germane to these processes and establish the S1P receptor subtype 1, S1PR1, as a novel molecular target to address the major socioeconomic problem created by opioid abuse.