Fractalkine (CX3CL1) and fractalkine receptor (CX3CR1) distribution in spinal cord and dorsal root ganglia under basal and neuropathic pain conditions

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Abstract
Fractalkine is a unique chemokine reported to be constitutively expressed by neurons. Its only receptor, CX3CR1, is expressed by microglia. Little is known about the expression of fractalkine and CX3CR1 in spinal cord. Given that peripheral nerve inflammation and/or injury gives rise to neuropathic pain, and neuropathic pain may be partially mediated by spinal cord glial activation and consequent glial proinflammatory cytokine release, there must be a signal released by affected neurons that triggers the activation of glia. We sought to determine whether there is anatomical evidence implicating spinal fractalkine as such a neuron‐to‐glia signal. We mapped the regional and cellular localization of fractalkine and CX3CR1 in the rat spinal cord and dorsal root ganglion, under basal conditions and following induction of neuropathic pain, employing both an inflammatory (sciatic inflammatory neuropathy; SIN) as well as a traumatic (chronic constriction injury; CCI) model. Fractalkine immunoreactivity and mRNA were observed in neurons, but not glia, in the rat spinal cord and dorsal root ganglia, and levels did not change following either CCI or SIN. By contrast, CX3CR1 was expressed by microglia in the basal state, and the microglial cellular concentration was up‐regulated in a regionally specific manner in response to neuropathy. CX3CR1‐expressing cells were identified as microglia by their cellular morphology and positive OX‐42 and CD4 immunostaining. The cellular distribution of fractalkine and CX3CR1 in the spinal circuit associated with nociceptive transmission supports a potential role in the mechanisms that contribute to the exaggerated pain state in these models of neuropathy.