DIFFERENCES IN SYMPATHETIC INNERVATION OF MOUSE DRG FOLLOWING PROXIMAL OR DISTAL NERVE LESIONS

Nerve injury leads to novel sympathetic innervation of the dorsal root ganglion (DRG). We have hypothesized previously that the degenerating nerve increases the sympathetic sprouting in the DRG and pain after c hronic sciatic constriction injury (CCI) by virtue of its influence on sensory and sympathetic axons spared by the injury. However, L5 spina l nerve ligation and transection (SNL) results in the complete isolati on of the L5 DRG from the degenerating stump, yet sympathetic axons in vade the ganglion, and sympathetically dependent pain develops. We inv estigated the role of Wallerian degeneration in both sympathetic sprou ting and neuropathic pain in these two models of painful peripheral ne uropathy by comparing responses of normal C57B1/6J and C57B1/Wld(s) mi ce in which degeneration is impaired. After CCI, Wld(s) mice, unlike 6 J mice, did not develop thermal or mechanoallodynia or sympathetic inn ervation of the L5 DRG. After SNL, both strains developed mechanoallod ynia and sympathetic sprouts in L5, but only 6J mice developed thermal allodynia. Observation of the origins of the invading sympathetic axo ns revealed that after CCI, sympathetics innervating blood vessels and dura (probably intact) sprouted into the ganglion, but after SNL symp athetics (probably axotomized) invaded from the injured spinal nerve. Based on these findings, we hypothesize that there are two mechanisms for sympathetic sprouting into DRG, differentially dependent on Waller ian degeneration. Analysis of pain behavior in these animals reveals t hat (i) mechanoallodynia and sympathetic innervation of the DRG tend t o coincide and (ii) thermal allodynia and Wallerian degeneration, but not sympathetic innervation of the DRG tend to coincide. (C) 1998 Acad emic Press.