Sympathetic sprouting in the dorsal root ganglion after spinal nerve ligation: evidence of regenerative collateral sprouting

It is well documented that there is an increase in the number of sympatheti c fibers within the dorsal root ganglion (DRG) after a peripheral nerve inj ury. The present study examined the numbers and distribution of sympathetic fibers in the DRG and their sprouting routes by utilizing various surgical manipulations and retrograde tracing and immunohistochemical staining meth ods in spinal nerve-ligated neuropathic rats. The appearance of many double immunostained fibers with antibodies to tyrosine hydroxylase (TH) and grow th associated protein-43 (GAP-43) in the L5 DRG 1 week after L5 spinal nerv e ligation, indicated sprouting of sympathetic fibers. The confined locatio n of early sprouting sympathetic fibers in the distal half of the L5 DRG co nfirmed that sprouting fibers come primarily from the injured spinal nerve. A second cut proximal to the previously ligated L5 spinal nerve - a proces s which would transect the regenerating sympathetic fibers extending from t he injury site - did not change the density of sympathetic fibers in the L5 DRG. When retrograde tracers (fast blue and diamidino yellow) were injecte d into the L5 spinal nerve and DRG, respectively, the number of double-labe led sympathetic postganglionic neurons was greatly increased after spinal n erve ligation, suggesting the increased number of sympathetic neurons proje cting to both the spinal nerve and DRG. All these results indicate that man y sympathetic fibers in the DRG are regenerating branches that are sproutin g from the proximal part of the injured spinal nerve (regenerative collater al sprouting). (C) 2001 Elsevier Science B.V. All rights reserved.