Potter Liam E, Paylor John W, Suh Jee Su, Tenorio Gustavo, Caliaperumal Jayalakshmi, Colbourne Fred, Baker Glen, Winship Ian, Kerr Bradley J
Neuroscience and Mental Health Institute, University of Alberta, Edmonton, AB, T6G 2E1, Canada.
Department of Anesthesiology and Pain Medicine, University of Alberta, Clinical Sciences Building, 8-120, Edmonton, AB, T6G 2G3, Canada.
J Neuroinflammation. 2016 Jun 10;13(1):142. doi: 10.1186/s12974-016-0609-4.
Chronic neuropathic pain is a common symptom of multiple sclerosis (MS). MOG35-55-induced experimental autoimmune encephalomyelitis (EAE) has been used as an animal model to investigate the mechanisms of pain in MS. Previous studies have implicated sensitization of spinal nociceptive networks in the pathogenesis of pain in EAE. However, the involvement of supraspinal sites of nociceptive integration, such as the primary somatosensory cortex (S1), has not been defined. We therefore examined functional, structural, and immunological alterations in S1 during the early stages of EAE, when pain behaviors first appear. We also assessed the effects of the antidepressant phenelzine (PLZ) on S1 alterations and nociceptive (mechanical) sensitivity in early EAE. PLZ has been shown to restore central nervous system (CNS) tissue concentrations of GABA and the monoamines (5-HT, NA) in EAE. We hypothesized that PLZ treatment would also normalize nociceptive sensitivity in EAE by restoring the balance of excitation and inhibition (E-I) in the CNS.
We used in vivo flavoprotein autofluorescence imaging (FAI) to assess neural ensemble responses in S1 to vibrotactile stimulation of the limbs in early EAE. We also used immunohistochemistry (IHC), and Golgi-Cox staining, to examine synaptic changes and neuroinflammation in S1. Mechanical sensitivity was assessed at the clinical onset of EAE with Von Frey hairs.
Mice with early EAE exhibited significantly intensified and expanded FAI responses in S1 compared to controls. IHC revealed increased vesicular glutamate transporter (VGLUT1) expression and disrupted parvalbumin+ (PV+) interneuron connectivity in S1 of EAE mice. Furthermore, peri-neuronal nets (PNNs) were significantly reduced in S1. Morphological analysis of excitatory neurons in S1 revealed increased dendritic spine densities. Iba-1+ cortical microglia were significantly elevated early in the disease. Chronic PLZ treatment was found to normalize mechanical thresholds in EAE. PLZ also normalized S1 FAI responses, neuronal morphologies, and cortical microglia numbers and attenuated VGLUT1 reactivity-but did not significantly attenuate the loss of PNNs.
These findings implicate a pro-excitatory shift in the E-I balance of the somatosensory CNS, arising early in the pathogenesis EAE and leading to large-scale functional and structural plasticity in S1. They also suggest a novel antinociceptive effect of PLZ treatment.
慢性神经性疼痛是多发性硬化症（MS）的常见症状。髓鞘少突胶质细胞糖蛋白35-55诱导的实验性自身免疫性脑脊髓炎（EAE）已被用作研究MS疼痛机制的动物模型。先前的研究表明脊髓伤害性网络的敏化在EAE疼痛的发病机制中起作用。然而，伤害性整合的脊髓上部位，如初级体感皮层（S1）的参与情况尚未明确。因此，我们在EAE早期疼痛行为首次出现时，研究了S1的功能、结构和免疫变化。我们还评估了抗抑郁药苯乙肼（PLZ）对EAE早期S1变化和伤害性（机械）敏感性的影响。已证明PLZ可恢复EAE中枢神经系统（CNS）组织中GABA和单胺（5-HT、NA）的浓度。我们假设PLZ治疗还可通过恢复CNS中兴奋与抑制（E-I）的平衡来使EAE中的伤害性敏感性正常化。
我们使用体内黄素蛋白自发荧光成像（FAI）来评估EAE早期S1对肢体振动触觉刺激的神经群体反应。我们还使用免疫组织化学（IHC）和高尔基-考克斯染色来检查S1中的突触变化和神经炎症。在EAE临床发病时用von Frey毛发评估机械敏感性。
与对照组相比，早期EAE小鼠在S1中表现出明显增强和扩大的FAI反应。IHC显示EAE小鼠S1中囊泡谷氨酸转运体（VGLUT1）表达增加，小白蛋白阳性（PV+）中间神经元连接性破坏。此外，S1中的神经元周围网络（PNN）显著减少。S1中兴奋性神经元的形态学分析显示树突棘密度增加。疾病早期Iba-1+皮质小胶质细胞显著升高。发现慢性PLZ治疗可使EAE中的机械阈值正常化。PLZ还使S1的FAI反应、神经元形态和皮质小胶质细胞数量正常化，并减弱VGLUT1反应性，但并未显著减弱PNN的丧失。
这些发现表明体感CNS的E-I平衡向促兴奋转变，这种转变在EAE发病机制早期出现，并导致S1中大规模的功能和结构可塑性。它们还提示了PLZ治疗的一种新的抗伤害作用。
