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Ocular surface diseases are among the most frequent ocular pathologies, with prevalence ranging from 20% of the general population. In addition, ocular pain following corneal injury is frequently observed in clinic. The aim of the study was to characterize the peripheral and central neuroinflammatory process in the trigeminal pathways in response to cornea alteration induced by chronic topical instillations of 0.2% benzalkonium chloride (BAC) in male C57BL/6 J mice. In vitro BAC induced neurotoxicity and increases neuronal (FOS, ATF3) and pro-inflammatory (IL-6) markers in primary mouse trigeminal ganglion culture. BAC-treated mice exhibited 7 days after the treatment reduced aqueous tear production and increased inflammatory cell infiltration in the cornea. Hypertonic saline-evoked eye wipe behavior was enhanced in BAC-treated animals that exhibited increased FOS, ATF3 and Iba1 immunoreactivity in the trigeminal ganglion. Ocular inflammation is associated with a significant increase in IL-6 and TNF-α mRNA expression in the trigeminal ganglion. We reported a strong increase in FOS and Iba1 positive cells in particular in the sensory trigeminal complex at the ipsilateral interpolaris/caudalis (Vi/Vc) transition and Vc/upper cervical cord (Vc/C1) regions. In addition, activated microglial cells were tightly wrapped around activated FOS neurons in both regions and phosphorylated p38 mitogen-activated protein kinase was markedly enhanced specifically in microglial cells during ocular inflammation. Similar data were obtained in the facial motor nucleus. These neuroanatomical data correlated with the increase in mRNA expression of pro-inflammatory (TNF-α, IL-6, CCL2) and neuronal (FOS and ATF3) markers. Interestingly, the suppression of corneal inflammation 10 days following the end of BAC treatment resulted in a marked attenuation of peripheral and central changes observed in pathological conditions.This study provides the first demonstration that corneal inflammation induces activation of neurons and microglial p38 MAPK pathway within sensory trigeminal complex. These results suggest that this altered activity in intracellular signaling caused by ocular inflammation might play a priming role in the central sensitization of ocular related brainstem circuits, which represents a significant factor in ocular pain development.

Neuropathic pain resulting from peripheral nerve injury involves many persistent neuroinflammatory processes including inflammatory chemokines that control leukocyte trafficking and activate resident cells. Several studies have shown that CCL2 chemokine, a potent attractant of monocytes, and its cognate receptor, CCR2, play a critical role in regulating nociceptive processes during neuropathic pain. However, the role of CCL2 in peripheral leukocyte infiltration-associated neuropathic pain remains poorly understood. In particular, the contribution of individual CCL2-expressing cell populations (i.e. stromal and leukocytes) to immune cell recruitment into the injured nerve has not been established. Here, in preclinical model of peripheral neuropathic pain (i.e. chronic constriction injury of the sciatic nerve), we have demonstrated that, CCL2 content was increased specifically in nerve fibers. This upregulation of CCL2 correlated with local monocyte/macrophage infiltration and pain processing. Furthermore, sciatic intraneural microinjection of CCL2 in naïve animals triggered long-lasting pain behavior associated with local monocyte/macrophage recruitment. Using a specific CCR2 antagonist and mice with a CCL2 genetic deletion, we have also established that the CCL2/CCR2 axis drives monocyte/macrophage infiltration and pain hypersensitivity in the CCI model. Finally, specific deletion of CCL2 in stromal or immune cells respectively using irradiated bone marrow-chimeric CCI mice demonstrated that stromal cell-derived CCL2 (in contrast to CCL2 immune cell-derived) tightly controls monocyte/macrophage recruitment into the lesion and plays a major role in the development of neuropathic pain. These findings demonstrate that in chronic pain states, CCL2 expressed by sciatic nerve cells predominantly drove local neuro-immune interactions and pain-related behavior through CCR2 signaling., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Functional interactions between the chemokine receptor CXCR4 and opioid receptors have been reported in the brain, leading to a decreased morphine analgesic activity. However the cellular mechanisms responsible for this loss of opioid analgesia are largely unknown. Here we examined whether Src family-kinases (SFK)-linked mechanisms induced by CXCR4 contributed to the loss of acute morphine analgesia and could represent a new physiological anti-opioid signaling pathway. In this way, we showed by immunohistochemistry and western blot that CXCL12 rapidly activated SFK phosphorylation in vitro in primary cultured lumbar rat dorsal root ganglia (DRG) but also in vivo in the DRG and the spinal cord. We showed that SFK activation occurred in a sub population of sensory neurons, in spinal microglia but also in spinal nerve terminals expressing mu-(MOR) and delta-opioid (DOR) receptor. In addition we described that CXCR4 is detected in MOR- and DOR-immunoreactive neurons in the DRG and spinal cord. In vivo, we demonstrated that an intrathecal administration of CXCL12 (1μg) significantly attenuated the subcutaneous morphine (4mg/kg) analgesia. Conversely, pretreatment with a potent CXCR4 antagonist (5μg) significantly enhanced morphine analgesia. Similar effects were obtained after an intrathecal injection of a specific SFK inhibitor, PP2 (10μg). Furthermore, PP2 abrogated CXCL12-induced decrease in morphine analgesia by suppressing SFK activation in the spinal cord. In conclusion, our data highlight that CXCL12-induced loss of acute morphine analgesia is linked to Src family kinases activation., (Copyright © 2013 Elsevier Inc. All rights reserved.)