Peripheral nerve lesion leads to the production of interleukin 6 (IL‐6)‐related neuropoietic cytokines involved in nerve protection and regeneration. This family of cytokines mainly signal through the signal transducer and activator of transcription (STAT) pathway that is locally activated in injured nerves. IL‐6 is also involved in pain that frequently arises from peripheral nerve lesion. We investigated the possible activation of this major IL‐6 signaling system in the spinal cord after peripheral nerve injury and its role in neuropathic pain. Ligation of L5–L6 spinal nerves (SNL) evoked an accumulation of active, phosphorylated form of STAT3 in microglial cells of dorsal spinal cord mostly in projection areas of injured nerves. SNL resulted also in a massive induction of IL‐6 mRNA expression in dorsal root ganglia and increased concentration of IL‐6 in dorsal spinal cord. Intrathecal injection of anti‐rat IL‐6 antibodies prevented the SNL‐induced accumulation of phospho‐STAT3 in the spinal cord. STAT3 pathway blockade with Janus kinase 2 inhibitor AG490 attenuated both mechanical allodynia and thermal hyperalgesia in SNL rats. These data show that in response to SNL injury Janus kinase/STAT3 system is activated mainly through IL‐6 signaling in spinal microglia and that this transduction pathway participates in development of pain associated with nerve alteration.

CCL2 chemokine and its receptor CCR2 may contribute to neuropathic pain development. We tested the hypothesis that injury to peripheral nerves triggers CCL2 release from afferents in the dorsal horn spinal cord (DHSC), leading to pronociceptive effects, involving the production of proinflammatory factors, in particular. Consistent with the release of CCL2 from primary afferents, electron microscopy showed the CCL2 immunoreactivity in glomerular boutons and secretory vesicles in the DHSC of naive rats. Through the ex vivo superfusion of DHSC slices, we demonstrated that the rate of CCL2 secretion was much lower in neonatal capsaicin-treated rats than in controls. Thus, much of the CCL2 released in the DHSC originates from nociceptive fibers bearing TRPV1 ( . These pathological pain-associated changes in the DHSC were mimicked by the intrathecal injection of exogenous CCL2 in naive rats and were prevented by the administration of INCB3344 or ERK inhibitor (PD98059). Finally, mechanical allodynia, which was fully developed 2 weeks after SN-CCI in rats, was attenuated by the intrathecal injection of INCB3344. Our data demonstrate that CCL2 has the typical characteristics of a neuronal mediator involved in nociceptive signal processing and that antagonists of its receptor are promising agents from treating neuropathic pain.

A better understanding of the mechanisms linked to chemokine pronociceptive effects is essential for the development of new strategies to better prevent and treat chronic pain. Among chemokines, MCP‐1/CCL2 involvement in neuropathic pain processing is now established. However, the mechanisms by which MCP‐1/CCL2 exerts its pronociceptive effects are still poorly understood. In the present study, we demonstrate that MCP‐1/CCL2 can alter pain neurotransmission in healthy rats. Using immunohistochemical studies, we first show that CCL2 is constitutively expressed by primary afferent neurons and their processes in the dorsal horn of the spinal cord. We also observe that CCL2 is co‐localized with pain‐related peptides (SP and CGRP) and capsaicin receptor (VR1). Accordingly, using in vitro superfusion system of lumbar dorsal root ganglion and spinal cord explants of healthy rats, we show that potassium or capsaicin evoke calcium‐dependent release of CCL2. In vivo, we demonstrate that intrathecal administration of CCL2 to healthy rats produces both thermal hyperalgesia and sustained mechanical allodynia (up to four consecutive days). These pronociceptive effects of CCL2 are completely prevented by the selective CCR2 antagonist (INCB3344), indicating that CCL2‐induced pain facilitation is elicited via direct spinal activation of CCR2 receptor. Therefore, preventing the activation of CCR2 might provide a fruitful strategy for treating pain.

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