伤害性感受器突触前NMDA自受体介导慢性痛敏化的突触前易化机制

It is well assumed that postsynaptic NMDA receptors play crucial roles in the synaptic plasticity and chronic pain. However, implication of NMDA receptors antagonists as analgesics in clinical trials has been hindered by their severe central side effects. Amounting evidence accumulated that NMDA receptors are widely expressed in the presynaptic region of primary afferent synapse---central terminals of nociceptors. Upon injury or inflammation, presynaptic NMDA receptors display dramatic upregulation. Most recently, our preliminary data showed that presynaptic NMDA receptors in nociceptor terminals exert an activity-dependent regulation of primary afferent synaptic transmission. In the basal state, presynaptic NMDA receptors depress synaptic transmission, while facilitate synaptic transmission in the pathological state. However, the cellular and molecular mechanisms underlying its activity-dependent regulation of spinal synaptic transmission and plasticity as well as pain hypersensitivity have remained elusive. Therefore, using the Cre-loxP system, we generated nociceptor specific knockout mice lacking NMDA receptors specifically in presynaptic terminals of nociceptors in the spinal cord, but not in postsynaptic neurons or elsewhere. By utilizing molecular biological assays, electrophysiological recordings, optical imaging as well as behavioral surveys, the present study aims to explore whether presynaptic NMDA receptors located in nociceptor terminals display an activity-dependent regulation of primary afferent synaptic transmission and its underlying mechanisms. Understanding the above questions may help to unravel the functional significance of presynaptic NMDA receptors in nociceptor terminals in the primary afferent synaptic transmission and plasticity as well as pain chronicity. New insights derived from these advances are expected to expediate development of novel analgesics acting at presynaptic NMDA receptors in nociceptor terminals for the treatment of chronic pain.

中枢神经系统突触后NMDA受体在突触可塑性和慢性痛中的作用不言而喻，但是严重的中枢毒副作用限制了其拮抗剂作为镇痛药在临床上的广泛应用。令人欣慰的是，研究表明NMDA受体还以自受体的形式大量表达于初级传入突触伤害性感受器突触前部位，并且在炎症或损伤后其功能活动显著增强。我们最新预实验显示在生理状态下突触前NMDA自受体对初级传入突触发挥显著的突触前抑制作用，而在病理状态下则逆转为强烈的突触前易化作用。但是，其发挥状态依赖性调控作用和介导慢性痛敏化的细胞分子机制尚不清楚。为此，本项目拟利用条件性敲除伤害性感受器突触前NMDA自受体动物模型，结合分子生物学、电生理学、光学成像及行为学等方法深入研究伤害性感受器突触前NMDA自受体的突触前调控作用及其作用机制，以期阐明其在初级传入突触可塑性及慢性痛敏化中的功能意义，为研发针对伤害性感受器突触前NMDA自受体的新型镇痛药提供重要的科学依据和理论基础。

慢性痛是严重危害人类身心健康和生活质量的慢性疾病，是严重的公共健康问题和社会问题。大量研究已经证实中枢神经系统突触后NMDA受体在诱致突触可塑性和慢性痛中的重要作用不言而喻。然而，临床上NMDA受体拮抗剂作为镇痛药的应用却因严重的中枢毒副作用而大大受限，导致NMDA受体在突触可塑性和慢性痛中的研究和治疗一时陷入窘境。令人欣慰的是，研究表明NMDA受体还以自受体的形式大量表达于初级传入突触伤害性感受器突触前部位，并且在炎症或损伤后其功能活动显著增强。我们最新预实验显示在生理状态下突触前NMDA自受体对初级传入突触发挥显著的突触前抑制作用，而在病理状态下则逆转为强烈的突触前易化作用。但是，其发挥状态依赖性调控作用和介导慢性痛敏化的细胞分子机制尚不清楚。为此，本项目我们通过构建特异性敲除伤害性感受器突触前NMDA受体的转基因小鼠，结合应用全细胞膜片钳记录、GCaMP钙成像、特异性病毒标记等方法，首次发现突触前NMDA受体对脊髓背角突触传递发挥状态依赖性的调控作用，即在正常生理状态下突触前NMDA受体发挥突触前抑制作用，而在炎症或损伤诱致的病理状态下则转变为突触前易化作用。进一步的机制研究揭示突触前NMDA受体通过激活SK通道介导了正常状态下的突触前抑制作用，而病理状态下NMDA受体发生显著上调，通过激活PKG-I-BDNF信号通路进一步抑制SK通道的抑制作用从而发挥突触前易化作用。行为学研究进一步阐明了伤害性感受器突触前NMDA受体在慢性痛尤其是中枢敏化中的致痛敏作用，特异性敲除伤害性感受器突触前NMDA受体可以显著缓解中枢敏化和痛敏的发生。据此提出伤害性感受器突触前NMDA受体在痛觉信息传递和可塑性改变以及疼痛持续慢性化过程中的工作假说，将为防治慢性痛这一人类健康主要杀手提供新的思路和治疗靶点，为慢性痛治疗找到“源头堵截”的最佳措施。