REST-mediated regulation of opioid receptors in chronic pain mouse models

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Project Summary Chronic neuropathic pain (CNP) is usually caused by disease or damage involving the somatosensory nervous system, adversely affecting millions of Americans. It is difficult to treat and remains a major clinical problem. Opioids, acting through opioid receptors (ORs: MOR for µ, KOR for κ, DOR for δ, NOP for nociceptin), trigger a complex signaling system and function as powerful analgesics. However, chronic opioid treatment causes hyperalgesia/analgesic tolerance and addiction, which have resulted in an opioid epidemic in the U.S. The opioid-induced hyperalgesia /analgesic tolerance (OIH/AT) and addiction can be modulated by many factors including OR expression levels and heteromer formation with different ORs (for example, MOR-DOR) or with other receptors such as the cannabinoid receptor (CNR1). Our long-term goal is to develop new strategies to enhance opioid analgesic effects and reduce opioid consumption for treatment of CNP. REST is a major epigenetic regulator. We and others have found that overexpression (OE) of REST in the dorsal root ganglion (DRG), causing repression of the MOR gene oprm1, is linked to the onset and maintenance of CNP. Our recent studies indicate that peripheral nerve injury in fact reduces opioid analgesia via the REST corepressor G9a-mediated chromatin repression of oprm1. Further, our preliminary studies suggest that MORs in DRG neurons are essential for OIH/AT. This would suggest that the REST-MOR axis in DRG neurons is a major mechanism regulating both CNP and OIH/AT. However, although the discovery of oprm1 as a REST target using a gene-by-gene approach is useful, it is unclear whether REST regulates the impacts of opioid analgesia in CNP or in OIH/AT by controlling the expression of other ORs or CNR1 or both of these processes. Our preliminary results suggest that REST differentially regulates expression of these receptors in DRG neurons. While it causes a decrease in the expression of MOR and DOR, it causes an increase in the expression of NOP and CNR1, perhaps by repressing the expression of an inhibitor of these genes such as a miRNA. To begin to generate comprehensive insights into the role of REST in CNP, we have now developed an innovative experimental system consisting of Rest conditional knock-out (cKO) mice and REST conditional OE (cOE) mice. Preliminary results indicate that whereas DRG-specific Rest cKO mice show attenuated pain hypersensitivity after nerve injury, DRG-specific REST cOE mice exhibit pain hypersensitivity even without nerve injury. Thus, the two contrasting mouse models recapitulate the chronic pain transition and provide a robust system in which to study mechanisms governing OR expression in primary sensory neurons in CNP and in OIH/AT. Here we propose to test the central hypothesis that REST in DRG neurons is involved in regulating opioid analgesia in CNP and in OIH/AT by governing ORs/CNR1 expression through epigenomic regulation of these genes. Thus, manipulation of REST in DRG neurons could be utilized to increase opioid analgesic efficacy and reduce opioid consumption. The project is responsive to PAR-18-742.