**2. Peripheral pathophysiology change mechanisms**

A large body of studies has been accumulated during the last two decades to characterize and clarify mechanisms at the base of neuropathic pain development and maintenance. Peripheral

© 2013 Maione et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nerve injury causes axon and myelin sheath degradation associated with macrophage, neutrophil and T cell infiltrations [9, 10].

Spinal microglia respond quickly to injury, up-regulating cell surface proteins and increasing synthesis and the release of inflammatory mediators, including cytokines and proteases that can sensitize neurons, thereby establishing positive feedback which helps to facilitate noci‐ ceptive signalling [35]. Accordingly, the inhibition of microglia targets can reduce hypersen‐

New Insights on Neuropathic Pain Mechanisms as a Source for Novel Therapeutical Strategies

http://dx.doi.org/10.5772/55276

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The signals responsible for neuron-microglia and/or astrocyte communication are being extensively investigated since they may represent new targets for chronic pain management.

The first candidates are substances released by activated nociceptive primary afferent fibers, such as glutamate and SP, which are capable to activate microglia [36, 37]. Glutamate activates microglia by stimulating NMDA receptors [37], although other mechanisms involving metabotropic glutamate receptors (mGluRs) cannot be ruled out since it has been shown that mGluRs are expressed on microglial cells [38, 39]. SP acts mostly by activating microglia neurokinin-1 (NK1) receptors. Many mechanisms have been proposed for neuron-microglia crosstalk. Among these, the fractalkine (FKN, CX3CL1), a member of CX3C class of chemokines and its receptor CX3CR1 have been extensively investigated [39]. FKN is constitutively expressed by spinal cord and sensory neurons in the dorsal root ganglia (DRGs) [40-42], while CX3CR1 is exclusively expressed by microglia cells and, after peripheral nerve injury it is largely up-regulated in activated microglia [41]. FKN produces nociceptive behaviour by activating CX3CR1 on microglia and p38 mitogen-activated protein kinase (MAPK)-mediated pathways [42, 43]. A pathway for the cleavage of FKN from the membrane of neurons, has been elegantly demonstrated [42]. Briefly, neuronal FKN is cleaved by Cathepsin S (CatS), a proteolitic enzyme which is synthesized and released by activated microglia. Despite the CX3CL1/CX3CR1 pathway represents a pro-nociceptive non adaptive process, seems to

perform also a neuro-protective action in neurodegenerative diseases [44].

Another chemokine implicated in neuron-glia communication is the chemokine (C-C motif) ligand 2 (CCL2, MCP-1), which is *de novo* expressed by sensory neurons as early as a day after peripheral injury [44]. Once released, CCL2 activates microglia via interaction with CCR2 receptors, and, accordingly, mice lacking CCR2 receptors display a reduction in nerve injuryinduced tactile allodynia [45]. The action of the monocyte chemoattractant protein 1 (MCP-1) at the spinal level has also been demonstrated by the intrathecal administration of an MCP-1 neutralizing antibody, which proved able to inhibit neuropathic pain symptoms [44]. Another important candidate for neuronal-microglial cross-talk is ATP, that is produced by neurons as well as by glial cells. ATP exerts its effect on microglia by activating the purinergic ionotropic P2X4 and P2X7, as well as the metabotropic P2Y6 and P2Y12, receptors which are up and/or down-regulated in several conditions [46]. The stimulation of the P2X4 channel seems to be involved in the development of neuropathic pain by inducing the release of brain derived neurotrophic factor (BDNF) [47, 48]. In particular, this mechanism is believed to be responsible of the appearance of the tactile allodynia by inverting the ionic gradient of the GABAergic interneurons following the down-regulation of the KKC2 calcium transporter [47]. P2X4 receptor activation occurs earlier than that of P2X7 channel due to the greater affinity of ATP to bind to P2X4 receptor. Indeed, P2X7 is involved in the maintenance of microglial activation. The P2X7 receptor appears to be a functionally unique ionotropic receptor among the P2X

sitivity in neuropathic pain states.

The release of proinflammatory cytokines (interleukins, tumor necrosis factor-α) and media‐ tors (bradykinins and prostaglandins) and growth factors (nerve growth factor) leads to peripheral sensitization and hypersensitivity to innocuous and noxious stimuli [11, 12]. Bradykinins and prostaglandins potentiates, among other things, the activity of transient receptor potential vanilloid type 1 (TRPV1) channel, highly expressed on Aδ and C fibers, whose activity and expression is potentiated in neuropathic pain models [13-16]. Neuropathic pain causes also over-expression of voltage gated sodium channels and increases sodium currents leading to spontaneous discharges of Aδ and C fibers [17-19]. Peripheral sensitization is also associated with increased voltage gated Ca2+ channels [20, 21]. Increased intracellular Ca2+ elevates substance P (SP) and glutamate release thus exacerbating pain transmission.
