Neuropathic pain conditions are exceedingly difficult to treat.1-4 Novel non-narcotic analgesics are desperately needed. Our receptomic and unbiased transcriptomic approaches identified the orphan G-protein coupled receptor (oGPCR), GPR160, as a major oGPCR whose transcript is significantly increased in the dorsal horn of the spinal cord (DH-SC) ipsilateral to nerve injury, in a model of traumatic nerve-injury induced neuropathic pain caused by constriction of the sciatic nerve in rats (CCI).5 No changes were found in the dorsal root ganglia. The role of GPR160 signaling in nociceptive processing is not known. Small molecule inhibitors of GPR160 are not yet available. Intrathecal (i.th.) injections of Gpr160 mRNA-targeted small interfering RNA (siGpr160) or neutralizing GPR160 antibody significantly blocked and reversed CCI-induced mechano- allodynia with no observable side-effects. Similar findings were obtained in another model of traumatic nerve injury (spared nerve injury model)6 and in a model of chemotherapy-induced neuropathic pain,7-10 underscoring generalization of findings. GPR160 inhibition had no effect in acute pain (hot plate/tail flick). Based on the existing literature anti-GPR160 approaches are unlikely to interfere with chemotherapies or promote cancer growth.11-13 We also de-orphanized GPR160 by identifying cocaine- and amphetamine-regulated transcript peptide (CARTp)14-16 as a ligand. CARTp is so-named because its level is increased following exposure to cocaine and amphetamine and other substances of abuse (i.e. morphine).16 Existing literature suggests that CARTp signaling activates protein kinase A (PKA),17,18 extracellular signal-regulated kinase (ERK)16-22 and cAMP-response element binding protein (CREB) pathways.18,23 These pathways are crucial to persistent pain sensitization.24,25 A recent study has linked CARTp-induced PKA/ERK/CREB to a cyclic AMP-independent, pertussis-toxin sensitive G?i/o-linked receptor.18 Guided by strong preliminary data, the following hypothesis will be tested: ?CARTp/GPR160 signaling in the spinal cord is essential for the development and maintenance of neuropathic pain states?.
Two Specific Aims will be pursued in male and female rodents across multiple models of neuropathic pain states. A multidisciplinary approach will be carried out in independent labs that have extensive experience in GPCR signaling and pain, using in vitro and in vivo studies that include genetic, behavioral, biochemical, molecular and immunohistochemical tests as well as targeted mass spectrometry. Proposed studies in Aim 1 will validate GPR160 as a non-opioid receptor target for therapeutic intervention in neuropathic pain whereas proposed studies in Aim 2 will characterize GPR160 coupling and downstream molecular signaling pathways. Impact: Discovery and validation of novel candidate targets for development of non-addictive therapeutics will have a huge impact in the treatment of chronic pain patients. Our discovery and proposed studies will provide foundational insights of CARTp/GPR160 signaling in the central nervous system and will validate GPR160 as a target for therapeutic intervention.
We have uncovered the role of a novel orphan GPCR (GPR160) in the central nervous system that contributes to neuropathic pain. In this proposal, we will use a multidisciplinary approach that will define the molecular mechanisms whereby activation of the GPR160 signaling pathway drive neuropathic pain. Our studies are anticipated to validate GPR160 as a target for therapeutic intervention with GPR160 antagonists addressing a huge unmet medical need with major socioeconomic consequences.