Gao Shi-Hao, Wen Hui-Zhong, Shen Lin-Lin, Zhao Yan-Dong, Ruan Huai-Zhen
Department of Neurobiology, College of Basic Medical Science, Chongqing Key Laboratory of Neurobiology, Third Military Medical University, Chongqing 400038, China.
Department of Neurology, Daping Hospital, Third Military Medical University, Chongqing 400042, China.
Neuropharmacology. 2016 Jun;105:361-377. doi: 10.1016/j.neuropharm.2016.01.036. Epub 2016 Jan 29.
Neuronal hyperexcitability in the anterior cingulate cortex (ACC) is considered as one of the most important pathological changes responsible for the chronification of neuropathic pain. However, the underlying mechanisms remain elusive. In the present study, we investigated the possible mechanisms using a rat model of chronic constriction injury (CCI) to the sciatic nerve. We found a substantial decrease in hyperpolarization-activated/cyclic nucleotide-gated (HCN) currents in layer 5 pyramidal neurons (L5 PNs) in ACC slices, which dramatically increased the excitability of these neurons. This effect could be mimicked in sham slices by activating group 1 metabotropic glutamate receptors, and be blocked in CCI slices by inhibiting metabotropic glutamate receptor subtype 1 (mGluR1). Next, the inhibition of HCN currents was reversed by a protein kinase C (PKC) inhibitor, followed by a reduced neuronal hyperexcitability. Furthermore, HCN channel subtype 1 (HCN1) level was significantly reduced after CCI, whereas mGluR1 level increased. These changes were mainly observed in L5 of the ACC, where HCN1 and mGluR1 were highly colocalized. For behavioral tests, intra-ACC microinjection of mGluR1-shRNA suppressed the CCI-induced behavioral hypersensitivity, particularly thermal hyperalgesia, but not aversive behavior, and this effect was attenuated by the pre-blockade of HCN channels. Taken together, the neuronal hyperexcitability of ACC L5 PNs likely results from an upregulation of mGluR1 and a downstream pathway involving PKC activation and a downregulation of HCN1 in the early phase of neuropathic pain. These alterations may at least in part contribute to the development of behavioral hypersensitivity in CCI rats.
前扣带回皮质（ACC）中的神经元兴奋性过高被认为是导致神经性疼痛慢性化的最重要病理变化之一。然而，其潜在机制仍不清楚。在本研究中，我们使用坐骨神经慢性压迫损伤（CCI）大鼠模型研究了可能的机制。我们发现ACC切片中第5层锥体神经元（L5 PNs）的超极化激活/环核苷酸门控（HCN）电流大幅降低，这显著增加了这些神经元的兴奋性。在假手术切片中激活1型代谢型谷氨酸受体可模拟这种效应，而在CCI切片中抑制代谢型谷氨酸受体亚型1（mGluR1）可阻断这种效应。接下来，蛋白激酶C（PKC）抑制剂可逆转HCN电流的抑制，随后神经元兴奋性过高降低。此外，CCI后HCN通道亚型1（HCN1）水平显著降低，而mGluR1水平升高。这些变化主要在ACC的L5中观察到，其中HCN1和mGluR1高度共定位。在行为测试中，向ACC内微量注射mGluR1-shRNA可抑制CCI诱导的行为超敏反应，尤其是热痛觉过敏，但不影响厌恶行为，并且这种效应可被预先阻断HCN通道所减弱。综上所述，ACC L5 PNs的神经元兴奋性过高可能是由于mGluR1上调以及在神经性疼痛早期涉及PKC激活和HCN1下调的下游通路所致。这些改变可能至少部分促成了CCI大鼠行为超敏反应的发展。
