Is sympathetic sprouting in the dorsal root ganglia responsible for the production of neuropathic pain in a rat model?

Partial peripheral nerve injury often results in neuropathic pain that is a ggravated by sympathetic excitation and induces sympathetic nerve sprouting in both the injured nerve and corresponding dorsal root ganglia (DRGs). Pr esently, the functional mechanisms of the interactions between the sproutin g and injured somatic afferents remain uncertain. This study was performed to see whether the sprouting in the DRGs plays a key role in the developmen t of neuropathic pain. To this aim, we compared two groups of rats, both of which were subjected to unilateral transection of the superior and inferio r caudal trunks at the level between the S1 and S2 spinal nerves, with resp ect to sympathetic fiber sprouting; one group showed well-developed neuropa thic pain behaviors (i.e. mechanical, cold and warm allodynia signs) and th e other group showed poorly-developed ones. Immuno-histochemical staining w ith tyrosine hydroxylase (TH) antibody of the injured S1 DRG taken from bot h groups of rats after behavioral tests revealed that the magnitude of pene tration of TH-positive fibers into the S1 DRG was not significantly differe nt between the two groups. These results suggest that sympathetic nerve spr outing in the injured DRG is not a key factor in the development of neuropa thic pain. (C) 1999 Elsevier Science ireland Ltd. All rights reserved.