MrgprC, A New Target for the Treatment of Neuropathic Pain

描述(申请人提供)：神经性疼痛的治疗继续挑战临床医生，部分原因是现有治疗方法的疗效有限，并且缺乏针对疼痛的特定药物靶点。MAS相关G蛋白偶联受体(MRGPR)是一类新的G蛋白偶联受体(GPCRs)家族，在小直径感觉神经元中特异表达。我们的初步工作表明，一些MRGPR，特别是MRGPRC，可能构成了一种新的持续性疼痛抑制系统。鞘内注射多肽(BAM 8-22)和小分子非肽激动剂MRgprC激动剂(化合物58)均可减轻大鼠和野生型小鼠的神经病理性疼痛。然而，BAM 8-22在mrpgr突变小鼠的神经病理性疼痛中失去了这种止痛作用。利用行为、电生理和分子生物学的辅助手段，我们的目标是评估作为治疗神经病理性疼痛的新靶点的MRgprC的治疗价值，更重要的是，研究MRgprC介导的疼痛抑制的细胞和分子机制。具体地说，目标1将详细描述鞘内注射两种MRGprC激动剂对L5脊髓神经损伤后大鼠触觉过敏、机械性痛敏、热痛敏和自发性疼痛的影响。然后，我们将研究MRgprC siRNA治疗是否降低了BAM 8-22的止痛效果。我们还将研究BAM 8-22的早期干预是否延长了神经病理性疼痛的缓解，增强了神经病理性疼痛的吗啡止痛，并防止了大鼠手术后的疼痛。在目标2中，我们将使用体外电生理和分子生物学方法来研究MRgprC介导的背根节(DRG)神经元痛抑制的细胞和分子机制。我们将同时使用功能丧失和功能获得策略来确定MRgprC激活是否减弱DRG细胞中的高压门控(HVA)钙电流。我们将进一步确定神经损伤是否上调未损伤的DRG神经元中MRGprC的表达和功能。在目标3中，我们将研究MRgprC介导的中枢神经系统疼痛抑制的神经生理学基础。我们将进行体内电生理研究，以确定MRgprC激动剂是否减弱背角神经元对重复伤害性输入的敏感性，并使由神经损伤引起的已建立的神经元超兴奋性正常化。我们将研究神经损伤是否改变BAM 22的表达，BAM 22是一种内源性MRGPR配体，在脊髓和DRG中的表达。我们推测，在神经病理性疼痛状态下，MRGprC激动剂可能具有抗痛觉过敏的作用。由于mrgprC是人类mrgprX1的同源基因，我们的发现可能对开发新的药物先导和基于机制的治疗策略具有重要意义，以管理几乎没有副作用的神经性疼痛。

DESCRIPTION (provided by applicant): The treatment of neuropathic pain continues to challenge clinicians, in part because of limited efficacy of available treatments and a lack of pain-specific drug targets. Mas-related G-protein-coupled receptors (Mrgprs) represent a novel family of G-protein-coupled receptors (GPCRs) that are specifically expressed in small-diameter sensory neurons. Our preliminary work suggests that some Mrgprs, in particular MrgprC, may constitute a novel inhibitory system for persistent pain. Intrathecal administration of a peptide (BAM 8-22) and a small molecule non-peptide MrgprC agonist (compound 58) both attenuated neuropathic pain in rats and in wild type mice. However, BAM 8-22 lost this analgesic action on neuropathic pain in Mrpgr mutant mice. Using complementary behavioral, electrophysiological, and molecular biological approaches, we aim to assess the therapeutic value of MrgprC as a novel target in the treatment of neuropathic pain, and importantly, to examine the cellular and molecular mechanisms underlying MrgprC-mediated pain inhibition. Specifically, Aim 1 will characterize in detail the effects of intrathecal administration of the two MrgprC agonists on tactile allodynia, mechanical hyperalgesia, heat hyperalgesia and spontaneous pain in rats after an L5 spinal nerve injury. We will then examine whether MrgprC siRNA treatment reduces the analgesic efficacy of BAM 8-22. We will also investigate whether an early intervention with BAM 8-22 prolongs attenuation of neuropathic pain, enhances morphine analgesia in neuropathic pain, and prevents postsurgical pain in rats. In Aim 2, we will employ in vitro electrophysiological and molecular biological approaches to study the cellular and molecular mechanisms underlying MrgprC-mediated pain inhibition in the dorsal root ganglion (DRG) neurons. We will use both loss-of-function and gain-of-function strategies to determine if MrgprC activation attenuates high-voltage-gated (HVA) calcium current in DRG cells. We will further determine whether nerve injury up regulates the expression and function of MrgprC in uninjured DRG neurons. In Aim 3 we will study the neurophysiologic basis for MrgprC-mediated pain inhibition in the central nervous system. We will conduct in vivo electrophysiological studies to determine if MrgprC agonist attenuates dorsal horn neuronal sensitization to repetitive noxious inputs and normalizes the established neuronal hyperexcitability induced by the nerve injury. We will investigate whether nerve injury alters the expression of BAM 22, an endogenous Mrgpr ligand, in spinal cord and in DRGs. We postulate that MrgprC agonists may function as anti-hyperalgesic agents during the neuropathic pain state. Because MrgprC is an ortholog to the human MrgprX1, our findings may have important implications for developing new drug leads and mechanism-based treatment strategies for managing neuropathic pain with few side effects.