Neuronal hyperexcitability in both injured and adjacent uninjured neurons is associated with states of chronic injury and pain and is likely subject to neuroinflammatory processes. Chronic inflammatory responses are largely orchestrated by chemokines. One chemokine, monocyte chemoattractant protein-1 (MCP-1), in the presence of its cognate receptor, the ␤ chemokine receptor 2 (CCR2), produces neural activity in dissociated neuronal cultures of neonatal dorsal root ganglion (DRG) neurons. Using a neuropathic pain model, chronic compression of the DRG (CCD), we compared anatomically separate populations of noncompressed lumbar DRG (L3͞L6) with compressed lumbar DRG (L4͞L5) for changes in the gene expression of CCR2. In situ hybridization revealed that CCR2 mRNA was up-regulated in neurons and nonneuronal cells present in both compressed L4͞L5 and ipsilateral noncompressed L3͞L6 DRGs at postoperative day 5 (POD5). The total percentages of compressed and noncompressed neurons exhibiting CCR2 mRNA transcripts in L3, L5, and L6 DRG were 33 ؎ 3.5%, 49 ؎ 6.2%, and 41 ؎ 5.6%, respectively, and included cell bodies of small, medium, and large size. In addition, the preferred CCR2 ligand, MCP-1, was up-regulated by POD5 in both compressed L4͞L5 and noncompressed L3͞L6 DRG neurons. Application of MCP-1 to the cell bodies of the intact formerly compressed DRG in vitro produced potent excitatory effects not observed in control ganglia. MCP-1͞CCR2 signaling is directly involved with a chronic compression injury and may contribute to associated neuronal hyperexcitability and neuropathic pain.hyperalgesia ͉ nerve injury ͉ neuropathic pain ͉ peripheral sensitization I nf lammation accompanying peripheral nerve injury frequently produces neuropathic pain symptoms, such as hyperalgesia and allodynia. This hyperalgesia may reflect ongoing or ectopic changes in the excitability of neurons in both injured and adjacent uninjured dorsal root ganglion (DRG) (1). Mechanisms that may contribute to the changes in neuronal activity include altered expression of ion channels, kinases, enzymes, neuropeptides, transcription factors, neurotrophins, and͞or the de novo presence of proinflammatory mediators such as cytokines, chemokines, and their respective receptors. However, current knowledge of the modification of molecular properties in both injured and noninjured adjacent ganglia is limited.Recent studies implicate the ␤ chemokine receptor 2 (CCR2) in the development and maintenance of pain (2-4). CCR2 is a G protein-coupled receptor that is related in structure to other CCRs (5, 6) and is largely thought to be a major regulator of induced macrophage migration (7-9). CCR2 is also constitutively expressed by different types of cells in the central nervous system, including neurons (10, 11), activated astrocytes (12, 13), microglia (3), and neural progenitor cells (14,15).Most CCRs, including CCR2, bind multiple chemokines (16, 17). CCR2 binds a family of closely related ␤-chemokines (C-C) called monocyte chemoattractant proteins (MCP), of whic...

Previous experimental results from our laboratory demonstrated that monocyte chemoattractant protein-1 (MCP-1) depolarizes or increases the excitability of nociceptive neurons in the intact dorsal root ganglion (DRG) after a chronic compression of the DRG (CCD), an injury that upregulates neuronal expression of both MCP-1 and mRNA for its receptor CCR2. We presently explore the ionic mechanisms underlying the excitatory effects of MCP-1. MCP-1 (100 nM) was applied, after CCD, to acutely dissociated small DRG neurons with nociceptive properties. Under current clamp, the proportion of neurons depolarized was similar to that previously observed for CCD-treated neurons in the intact ganglion, although the magnitude of depolarization was greater. MCP-1 induced a decrease in rheobase by 44 +/- 10% and some cells became spontaneously active at resting potential. Action potential width at a voltage equal to 10% of the peak height was increased from 4.94 +/- 0.23 to 5.90 +/- 0.47 ms. In voltage clamp, MCP-1 induced an inward current in 27 of 50 neurons held at -60 mV, which increased with concentration over the range of 3 to 300 nM (EC(50) = 45 nM). The MCP-1-induced current was not voltage dependent and had an estimated reversal potential of -27 mV. In addition, MCP-1 inhibited a voltage-dependent, noninactivating outward current, presumably a delayed rectifier type K(+) conductance. We conclude that MCP-1 enhances excitability in CCD neurons by, at least, two mechanisms: 1) activation of a nonvoltage-dependent depolarizing current with characteristics similar to a nonselective cation conductance and 2) inhibition of a voltage-dependent outward current.

P2X receptors on dorsal root ganglion (DRG) neurons have been strongly implicated in pathological nociception after peripheral nerve injuries or inflammation. However, nothing is known of a role for purinergic receptors in neuropathic pain produced by a chronic compression of DRG (CCD) -an injury that may accompany an intraforaminal stenosis, a laterally herniated disc or other disorders of the spine leading to radicular pain. In a rat model of DRG compression, hyperexcitable neurons retain functioning axonal connections with their peripheral targets. It is unknown whether such hyperexcitability might enhance chemically mediated nociceptive stimulation of the skin. In this study, CCD facilitated the nocifensive behavior and mechanical hyperalgesia-induced by the P2X 3 agonist, α,β-methylene ATP (α,β-meATP). An injection of α,β-meATP into the hind paw of CCD rats resulted in a significantly greater decrease in the mean threshold to von Frey stimuli and a greater duration of paw lifts than in sham-operated control rats. CCD also increased the levels of P2X 3 receptor protein and the number of P2X 3 immunoreactive, small diameter DRG neurons in the compressed ganglion. P2X 3 receptors were co-labeled with the isolectin IB4, consistent with a role in nociception. In addition, a α,β-meATP induced significantly larger fast-inactivating currents in CCD-than in sham-operated acutely dissociated DRG neurons. These currents were accompanied by the generation of action potentials -but only in the CCD neurons. U0126, a specific inhibitor of the MEK1/2, greatly down-regulated the enhanced current. Taken together, these observations suggest that enhanced purinergic responses after CCD are mediated by P2X 3 receptors.