SYMPATHETIC SPROUTING IN THE DORSAL-ROOT GANGLIA OF THE INJURED PERIPHERAL-NERVE IN A RAT NEUROPATHIC PAIN MODEL

The extent of the sprouting of sympathetic postganglionic fibers in th e dorsal root ganglion (DRG) and the peripheral nerves was examined in neuropathic rats at different postoperative times. After the L5 and L 6 spinal nerves were ligated on one side, three different pain behavio r tests (representing mechanical allodynia, cold allodynia, ongoing pa in exacerbated by cold stress) were performed at various time interval s. The sympathetic postganglionic fibers were visualized by immunostai ning with antibodies to tyrosine hydroxylase (TH). In the neuropathic rats, all three pain behaviors were fully developed within 3 days afte r the surgery, maintained up to 2 weeks, and then started to decline g radually afterward. At 20 weeks after neuropathic surgery, pain behavi ors were reduced significantly compared to the peak response, but were still higher than the presurgery levels. Sympathectomy, performed 4 d ays after neuropathic surgery, almost completely abolished the signs o f mechanical allodynia and ongoing pain behaviors, and it reduced the behaviors of cold allodynia to approximately half. The numerical densi ty of sympathetic fibers in the DRG of an injured segment was signific antly higher at 1, 4, and 20 weeks after neuropathic surgery as compar ed to the normal, suggesting that there is sprouting of sympathetic fi bers in the DRG after peripheral nerve injury. Sprouting of sympatheti c fibers in the DRG was extensive as early as 2 days after the spinal nerve ligation, and the sprouted fibers were almost completely elimina ted after sympathectomy. The data suggest that sympathetic innervation of the DRG may play an important role in the development and maintena nce of sympathetically maintained neuropathic pain. (C) 1996 Wiley-Lis s, Inc.