CNS modulation of central terminal sensitization of trigeminal afferents in pain

描述(申请人提供)：慢性口腔面部疼痛综合征影响大约四分之一的人口，据信经常与创伤或牙科手术导致的三叉神经损伤有关。三叉神经病理性疼痛通常以损伤部位的原发性痛觉过敏和损伤区外广泛或继发性的痛觉过敏为特征，是治疗上的难题。人们认识到，原发性痛敏的发展既包括初级传入伤害性感受器的敏化，也包括神经损伤后脊髓背角和脊髓上结构水平的神经元过度活动，称为中枢敏化。继发性痛觉过敏也归因于中枢敏感化。最近，在大鼠三叉神经眶下神经慢性压迫性损伤模型中表现出的原发性和继发性痛觉过敏已被用来研究急性神经病理性疼痛向慢性神经病理性疼痛转化的机制。这些结果表明，下行的5-羟色胺和背角5-HT3A受体(5-HT3AR)参与了维持继发性痛觉过敏的中枢机制。瞬时受体电位香草酸1型(TRPV1)是一种非选择性阳离子通道，主要表达于初级伤害性神经元的外周和中枢终末。外周终端TRPV1检测各种有害的、物理和化学刺激作为外周感觉整合因子，并解释外周敏化和炎性疼痛放大。然而，对中枢终末TRPV1在痛觉传递和调制中的生理相关性和功能知之甚少。作为5-羟色胺受体家族中唯一的兴奋性配体门控离子通道，5-HT3Rs在初级伤害性感受器的中央终末表达。目前尚不清楚主动下行促进是否通过激活突触前5-HT3ARs来驱动中枢终末TRPV1的敏化，并有助于神经病理性疼痛状态的维持。在本研究中，我们将结合分子、遗传学、新近发展的终末钙显象、电生理记录和行为药理学方法，重点研究三叉神经损伤后中枢初级传入终末敏化和高兴奋性维持疼痛放大的中枢神经系统调控机制。我们假设，口面部神经损伤可导致继发性痛敏的维持，包括初级传入伤害性感受器的中央终末TRPV1的敏化，并通过激活下行的5-羟色胺和突触前5-HT3R机制对其进行调制。这三个目的将验证这些假说：1)确定神经损伤后Vc内是否存在TRPV1表达的动态变化及其参与继发性痛觉过敏的维持。2)确定中枢终末TRPV1是否在神经损伤后变得敏感，并参与继发性痛敏维持过程中突触兴奋性输入的增强。3)探讨下行5-羟色胺和VC5-HT3ARs在中枢终末TRPV1敏化和突触前高兴奋性突触前兴奋性持续性继发性痛敏中的作用。深入了解神经损伤后初级传入中枢终末敏化及其对中枢神经系统的调控机制，对于制定预防神经病理性疼痛慢性化的新策略具有重要意义。

DESCRIPTION (provided by applicant): Chronic orofacial pain syndromes affect approximately one fourth of the population and are believed to be frequently related to trigeminal nerve damage as a result of trauma or dental surgery. Trigeminal neuropathic pain is usually characterized by primary hyperalgesia at the site of injury and widespread or secondary hyperalgesia outside the injured zone, and presents a difficult therapeutic challenge. It is recognized that the development of primary hyperalgesia involves both sensitization of primary afferent nociceptors and neuronal hyperactivity referred to as central sensitization at the levels of the spinal dorsal horn and supra-spinal structures after nerve injury. Secondary hyperalgesia is also attributed to central sensitization. Recently, primary and secondary hyperalgesia demonstrated in a rat trigeminal neuropathic model involving the chronic constriction injury of the infraorbital nerve have been developed to investigate the mechanisms of transition of acute to chronic neuropathic pain. The findings indicate that descending 5-HT and dorsal horn 5-HT3A receptors (5- HT3AR) are involved in central mechanisms underlying the maintenance of secondary hyperalgesia. Transient Receptor Potential Vanilloid Type 1 (TRPV1) is a non-selective cation channel and is primarily expressed in peripheral and central terminals of primary nociceptive neurons. Peripheral terminal TRPV1 detects a variety of noxious, physical and chemical stimuli as peripheral sensory integrators and accounts for peripheral sensitization and inflammatory pain amplification. However, the physiological relevance and function of central terminal TRPV1 in pain transmission and modulation are poorly understood. As the only excitatory ligand-gated ion channel of the 5-HT receptor family, 5-HT3Rs are expressed in central terminals of primary nociceptors. It is unclear whether active descending facilitation drives central terminal TRPV1 sensitization via activation of presynaptic 5-HT3ARs and contributes to the maintenance of the neuropathic pain state. In this proposal, by combining molecular, genetic, newly developed terminal calcium imaging, electrophysiological recording and behavioral pharmacological approaches, we will focus on studying CNS modulation of primary afferent central terminal sensitization and hyperexcitability underlying the maintenance of pain amplification after trigeminal nerve injury. We hypothesize that nerve injury in the orofacial region can lead to the maintenance of secondary hyperalgesia that involves sensitization of central terminal TRPV1 of primary afferent nociceptors and its modulation by activation of descending 5-HT and presynaptic 5-HT3R mechanisms. The three aims will test these hypotheses: 1) To determine whether dynamic change of TRPV1 expression exists in the Vc after nerve injury and its involvement in the maintenance of secondary hyperalgesia. 2) To determine whether central terminal TRPV1 sensitizes after nerve injury and is involved in enhanced synaptic excitatory input during the maintenance of secondary hyperalgesia. 3) To determine the involvement of descending 5-HT and Vc 5- HT3ARs in central terminal TRPV1 sensitization and presynaptic hyperexcitability underlying persistent secondary hyperalgesia. A thorough understanding of the mechanisms of primary afferent central terminal sensitization and its CNS modulation after nerve injury will be important for the development of new strategies to prevent chronicity of neuropathic pain.