Molecular mechanisms of phenotypic change in intact nociceptor

We have previously shown that regenerated cutaneous afferents exhibit sensitization. These changes were correlated with increased expression of neurotrophic factors in the affected skin, and receptors and ion channels in the dorsal root ganglia (DRG). Here we found that saphenous nerve injury resulted in similar alterations in gene expression in the skin innervated by the uninjured contralateral saphenous nerve. Several Nerve Growth Factor (NGF) and GDNF ligands were upregulated in the dorsal hindpaw hairy skin 7-21 days following contralateral saphenous nerve axotomy. This increase in expression was correlated with upregulation of several receptors and ion channels in the L2,3 DRGs, including GFRa1, nucleotide receptors P2X3, P2Y1, and P2Y2, heat transducing channel TRPV1, and acid-sensing ion channels 1 (ASIC1) and 3 (ASIC3). Characterization of intact saphenous afferent fibers revealed that 4-5 weeks after contralateral injury, C-polymodal population had significantly reduced heat thresholds. There was also a significant increase in the proportion of mechanically insensitive-heat sensitive (CH) neurons. These findings provide evidence of peripheral mechanisms that may be involved in the development of mirror-image pain, a clinical phenomenon in which patients experience pain in the healthy region of the body contralateral to the injury site.;The changes in the response properties of intact afferents at 4-5 weeks following contralateral injury were correlated with upregulation of P2Y1 and TRPV1 in the DRGs at 28d post-injury, and we investigated the role of these two targets in the functional plasticity observed by utilizing an in vivo small interfering RNA (siRNA)-mediated knockdown strategy to target the upregulation of P2Y1 and TRPV1 in the L2,3 DRGs at 28 days following contralateral injury. We found that blocking P2Y1 upregulation prevented the heat threshold reduction in C-polymodal afferents and conclude that P2Y1 plays a critical role in the heat sensitivity of these fibers. We then found that blocking the upregulation of TRPV1 blocked the recruitment of C-heat fibers. Taken together, our findings suggest that chronic exposure of intact cutaneous sensory neurons to increased endogenous levels of neurotrophic factors results in upregulation of target receptors and ion channels in the DRG and sensitization and phenotypic changes in intact sensory neurons.