Central sensitization and network hyperexcitability of the nociceptive system is a basic mechanism of neuropathic pain. We hypothesize that development of cortical hyperexcitability underlying neuropathic pain may involve homeostatic plasticity in response to lesion-induced somatosensory deprivation and activity loss, and can be controlled by enhancing cortical activity. In a mouse model of neuropathic pain, in vivo two-photon imaging and patch clamp recording showed initial loss and subsequent recovery and enhancement of spontaneous firings of somatosensory cortical pyramidal neurons. Unilateral optogenetic stimulation of cortical pyramidal neurons both prevented and reduced pain-like behavior as detected by bilateral mechanical hypersensitivity of hindlimbs, but corpus callosotomy eliminated the analgesic effect that was ipsilateral, but not contralateral, to optogenetic stimulation, suggesting involvement of inter-hemispheric excitatory drive in this effect. Enhancing activity by focally blocking cortical GABAergic inhibition had a similar relieving effect on the pain-like behavior. Patch clamp recordings from layer V pyramidal neurons showed that optogenetic stimulation normalized cortical hyperexcitability through changing neuronal membrane properties and reducing frequency of excitatory postsynaptic events. We conclude that development of neuropathic pain involves abnormal homeostatic activity regulation of somatosensory cortex, and that enhancing cortical excitatory activity may be a novel strategy for preventing and controlling neuropathic pain.

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Keywords: Experimental autoimmune encephalomyelitis r Fasudil r Macrophage polarization r Rho kinase inhibitor r T-cell regulation Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionMultiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS), which is characterized by chronicCorrespondence: Prof. Bao-Guo Xiao e-mail: bgxiao@shmu.edu.cn inflammation, myelin destruction, axonal loss, and neurological disability [1]. The exact cause of MS is still unknown, but the pathogenesis of MS/experimental autoimmune encephalomyelitis (EAE) is known to involve the following: (i) myelin-specific T cells are initially activated, (ii) activated T cells migrate across the blood-brain barrier (BBB) and infiltrate the CNS parenchyma, (iii) infiltrated T cells produce proinflammatory cytokines, which [6]. Despite these detrimental roles of macrophages within the injured CNS, many studies have also reported beneficial roles of macrophages. Treatment with M2 macrophages resulted in an attenuation of EAE progression [7,8].Given that polarized macrophages are reversible in a well-defined microenvironment, the polarization of M1 cells and M2 cells has become a new therapeutic target for MS. Rho-kinase (ROCK), a serine/threonine kinase, plays the role of molecular switch by regulating cell migration, proliferation, and survival [7]. A series of studies have demonstrated that the expression and/or activity of ROCK were increased in different EAE models [9][10][11][12][13] due to the release of inhibitory molecules (such as Nogo A, MAG, and Omgp) from damaged CNS tissues that activated the Rho/ROCK signal transduction pathway [14]. Inhibition of ROCK resulted in accelerated regeneration and enhanced functional recovery, which has also proved to be efficacious in animal models of stroke [15], MS [9-13], ALS [16], Alzheimer's disease [17], and Parkinson's diseases [18,19]. ROCK is therefore considered a promising drug target for preventing neurodegeneration and stimulating neuroregeneration in several neurological diseases [20]. Previous investigations from our group and other labs have indicated that the ROCK inhibitor Fasudil ameliorates the clinical severity of EAE in different models and at different stages [9][10][11]13], accompanied by reduced demyelination and inhibition of neuroinflammation [9,10,12,13]. Mechanisms underlying the therapeutic potential of Fasudil in EAE models include (i) downregulation of Th17 and Th1 T cells [9,10], (ii) protection of BBB and blood-spinal cord barrier integrity [11], (iii) inhibition of TLR-4 and p-NF-κB/p65 inflammatory axis [12], and (iv) shift from M1 to M2 macrophages [13]. Although there are several publications describing the therapeutic effects and immunological changes of Fasudil in EAE models, little is known about its immunomodulatory effect on macrophages and T cells. In addition, given that Fasudil has certain limitations in clinical practice, including a relatively narrow safety window, it...