Nerve growth factor-mediated peripheral mechanisms are involved in mirror-image pain induction and maintenance

• Main Authors: Chau-Fu Cheng, 鄭昭甫
• Other Authors: Meei-Ling Tsaur
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/23884984557897241452

博士 === 國立陽明大學 === 神經科學研究所 === 102 === Mirror-image pain is characterized by mechanical hypersensitivity on the uninjured mirror-image side. Recent reports favor central mechanisms, but whether peripheral mechanisms are involved remains unclear. We used unilateral spinal nerve ligation (SNL) to induce mirror-image pain in rats as early as day 1 after surgery. On the mirror-image (contralateral) side, we found that at day 1 after ligation, satellite glia in the dorsal root ganglion (DRG) were activated, whereas macrophages/Schwann cells in the DRG and astrocytes/oligodendrocytes/microglia in the dorsal spinal cord were not. Subsequently, a rise of nerve growth factor (NGF) was detected in the contralateral DRG, and NGF immunoreactivity was concentrated in activated satellite glia. These phenomena were abolished if fluorocitrate (a glial inhibitor) was intrathecally injected before SNL. Intrathecal injection of NGF into naïve rats induced long-lasting mechanical hypersensitivity, similar to SNL-evoked mirror-image pain. Anti-NGF effectively relieved SNL-evoked mirror-image pain. In the contralateral DRG, SNL-evoked tumor necrosis factor alpha (TNFα) increase, which was started later than that in the ipsilateral DRG and cerebrospinal fluid, occurred earlier than satellite glial activation and NGF rise. Intrathecal injection of TNF-α into naïve rats not only activated satellite glia to elevate NGF levels in the DRG but also evoked mechanical hypersensitivity, which could be attenuated by anti-NGF injection. Thus, TNF-α-evoked satellite glial activation leads to NGF increase in contralateral DRG, which mediates mirror-image pain induction. In addition, neurite sprouting occurred in the contralatearl DRG at day 7 post-SNL. Using intrathecal injection of anti-NGF antibodies, we found that NGF is required for inducing ectopic innervation of DRG by sympathetic axons and intra-DRG sprouting of nociceptors expressing either calcitonin gene-related peptide (CGRP) or Kv4.3 potassium channel, and intra-DRG ectopic synaptogenesis. Downregulation of the postsynaptic protein PSD95 with a specific shRNA largely eliminated these ectopic synapses, suppressed activities of Kv4.3+ but not CGRP+ nociceptors, and attenuated mirror-image pain. Immunostaining confirmed that CGRP+ neurons are presynaptic to Kv4.3+ neurons. Furthermore, persistent mirror-image pain at 2 weeks post-injury was not affected by eliminating NGF, but significantly attenuated by downregulating PSD95. Thus, spinal nerve injury-elevated NGF triggers ectopic synaptogenesis within DRG by both sympathetic and DRG neurons, leading to persistent mirror-image pain.