Supraspinal mechanism of 5-HT-dependent descending facilitation in chronic pain

 描述(由申请人提供)：慢性疼痛是一个主要的公共卫生问题，对治疗提出了最困难的挑战。尽管慢性疼痛已被越来越多地认为是一种神经系统疾病，但其潜在机制在很大程度上仍不清楚。最近的研究表明，三叉神经和脊髓伤害性信息传递的关键结构--延髓头端腹内侧核(RVM)的下行促进作用增强或下行抑制减少，参与了慢性疼痛的脊髓上机制。新的证据表明，5-羟色胺依赖的右室下行易化是神经损伤后急性疼痛向慢性疼痛转变的原因。然而，突触和细胞信号转导机制对5-羟色胺依赖的下行促进作用知之甚少。瞬时受体电位香草酸1型通道是一种配体门控型非选择性阳离子通道，具有高钙通透性，在原代伤害性感觉神经元中显著表达，是伤害性热和炎性介质的分子感受器。最近，还发现TRPV1通道在大脑中表达，与其他突触蛋白相互作用，并作为下游信号的整合者，介导长时间的突触可塑性。尽管TRPV1通道分布于右室，但其在下行痛觉调制中的潜在生理和病理意义尚不清楚。我们探讨了神经损伤后RVM中神经胶质细胞-细胞因子-神经元的相互作用在下行疼痛易化和痛敏维持中的作用。因此，我们将进一步研究TRPV1通道在RVM中的功能特征，扩大我们对涉及含5-羟色胺的RVM神经元的超兴奋性和行为超敏的不同信号机制的研究。我们的假设是，钙离子通透性的TRPV1通道在RVM5-羟色胺能神经元中功能性地表达，并调节RVM5-HT神经元的兴奋性突触传递和神经元活动。我们进一步假设，RVMTRPV1的长期上调和敏化是在神经损伤后诱导的，并促进了神经胶质细胞高活性和神经元高兴奋性之间的细胞内信号转导。通过整合与细胞因子受体激活相关的下游信号和随后NMDA受体活性的增加，5-羟色胺神经元中的TRPV1敏化维持神经的过度兴奋性，并在维持神经病理性疼痛的基础上促进5-羟色胺依赖性下行疼痛的易化。以下具体目标将使用多学科方法来检验这些假设。#1：验证中枢TRPV1通道存在于RVM，并在投射到背角的含5-羟色胺的RVM神经元中功能性表达的假设。#2：验证这一假说，即TRPV1在RVM中的长期上调和敏化发生在神经损伤后，并介导神经病理性疼痛发生的基础的下行促进。#3：为了验证一种假说，即RVM中胶质细胞的过度活动释放的促炎细胞因子使神经损伤后局部的TRPV1功能变得敏感，这依赖于细胞内非典型蛋白激酶C/Mzeta信号的长期上调。#4：验证TRPV1激活通过增强RVM神经元的谷氨酸能突触传递和5-羟色胺神经元的超兴奋性而参与下行痛觉调制的假说。这一发现将扩大我们对中枢TRPV1通道在5-羟色胺依赖的下行易化的脊髓上机制中的新作用的理解，并为神经病理性疼痛的治疗提供新的治疗靶点。

 DESCRIPTION (provided by applicant): Chronic pain is a major public health problem and offers a most difficult challenge to therapy. Although chronic pain has been increasingly recognized as a disease of the nervous system, the underlying mechanisms remain largely unclear. Recently studies suggest that enhanced descending facilitation or reduced descending inhibition from the rostral ventromedial medulla (RVM), a pivotal structure of descending modulation of trigeminal and spinal nociceptive transmission, contribute to supraspinal mechanisms of chronic pain. New evidence shows that 5-HT-dependent descending facilitation from the RVM is responsible for the transition of acute pain to chronicity after nerve injury. However, the synaptic and cellular signaling mechanisms accounting for 5-HT-dependent descending facilitation are poorly understood. Transient receptor potential vanilloid type 1 (TRPV1) channels are ligand-gated nonselective cation channels with high Ca++ permeability and are prominently expressed in primary nociceptive sensory neurons as molecular sensors for noxious heat and inflammatory mediators. More recently, TRPV1 channels are also found to express in the brain, cross-talk with other synaptic proteins and act as an integrator for downstream signaling to mediate long-lasting synaptic plasticity. Although TRPV1 channels are distributed in the RVM, their potential physiological and pathological significance in descending pain modulation remains unknown. We have explored glia-cytokine-neuronal interaction in the RVM underlying descending pain facilitation and the maintenance of hyperalgesia after nerve injury. Thus, we will further study the functional signature of TRPV1 channels in the RVM, expanding our study of distinct signaling mechanisms involved in hyperexcitability of 5-HT-containing RVM neurons and behavioral hypersensitivity. Our hypothesis is that calcium-permeable TRPV1 channels are functionally expressed in RVM 5-HT-containing neurons and modulate excitatory synaptic transmission and neuronal activity in the RVM. We further hypothesize that the long-lasting upregulation and sensitization of RVM TRPV1 is induced after nerve injury and facilitate intracellular signal transduction between glial hyperactivity and neuronal hyperexcitability. Through integrating downstream signaling relevant to cytokine receptor activation and subsequent increase of NMDA receptor activity, TRPV1 sensitization in 5-HT-containing neurons maintains neural hyperexcitability and contributes to 5-HT-dependent descending pain facilitation underlying the maintenance of neuropathic pain. The following Specific Aims will test these hypotheses using multidisciplinary approaches. #1: To test the hypothesis that central TRPV1 channels are present in the RVM and functionally expressed in 5-HT-containing RVM neurons projecting to the dorsal horn. #2: To test the hypothesis that long-term upregulation and sensitization of TRPV1 in the RVM occur after nerve injury and mediate descending facilitation underlying the development of neuropathic pain. #3: To test the hypothesis that proinflammatory cytokines released upon glial hyperactivity in the RVM sensitize local TRPV1 function after nerve injury, which is dependent on long-lasting upregulation of intracellular atypical protein kinase C/Mzeta signaling. #4: To test the hypothesis that TRPV1 activation is involved in descending pain modulation through potentiation of glutamatergic synaptic transmission in RVM neurons and hyperexcitability of 5-HT-containing neurons. The findings from this proposal will expand our appreciation of a novel role of central TRPV1 channels in supraspinal mechanisms of 5-HT-dependent descending facilitation and provide new therapeutic targets for the treatment of neuropathic pain.