Abnormalities of sympathetic innervation in the area of an injured peripheral nerve in a rat model of neuropathic pain

Using the rat model that we have developed, a potential underlying mechanism for sympathetically maintained neuropathic pain (SMP) was explored. In rats showing neuropathic pain behaviors after a tight ligation of the L₅ spinal nerve, putative sympathetic postganglionic fibers were examined in the injured spinal nerve and the dorsal root ganglion (DRG), using immunohistochemical staining with antibody against tyrosine hydroxylase (TH). In the neuropathic rats, there was an increase in the number of TH-immunolabeled fibers in the spinal nerve, and some DRG cells were surrounded by the labeled fibers. These abnormalities of sympathetic postganglionic innervation of the injured spinal nerve or the DRG may be a part of the mechanisms underlying the development of SMP.