A-fiber sprouting in spinal cord dorsal horn is attenuated by proximal nerve stump encapsulation

Peripheral nerve transection in the rat alters the spinal cord dorsal horn central projections from both small and large DRG neurons. Injured neurons with C-fibers exhibit transganglionic degeneration of their terminations within lamina II of the spinal cord dorsal horn, while peripheral nerve injury of medium to large neurons induces collateral sprouting of myelinated A-fibers from lamina I and III/IV into lamina II in rats, cats, and primates. To date, it is not known what sequelae are responsible for the collateral sprouting of A-fibers after peripheral nerve injury, although target-derived factors are thought to play an important role. To determine whether target-derived factors are necessary for changes in A-fiber laminar terminations in rat spinal cord dorsal horn, we unilaterally transected the sciatic nerve and ensheathed the proximal nerve stump in a silicone cap. Three days before sacrifice of rat, the injured sciatic nerve was injected with cholera toxin beta-subunit conjugated to horseradish peroxidase (betaHRP) that effectively labels both peripheral and central A-fiber axons. The effect of the ligature, axotomy, and silicone cap treatment was evaluated by analyzing the extent of betaHRP-, Substance P-(SP-), and isolectin B4- (IB4-) immunoreactive (ir) fibers in the somatotopically appropriate spinal cord dorsal horn regions. In all animals, 2-5 weeks after nerve transection (treated or otherwise), IB4- and SP-ir is absent from lamina II. Animals without nerve cap treatment exhibited robust fiber sprouting into lamina II at 2 weeks. In sharp contrast, animals treated with silicone caps did not exhibit betaHRP-ir fibers in lamina II at 2 weeks. This observation was extended up to 5 weeks postinjury. These results suggest that axotomy-induced expansion of betaHRP-ir primary afferent central terminations in the spinal cord dorsal horn is dependent on factors produced in the injury site milieu. While our understanding of local repair mechanisms of injured peripheral nerves is incomplete, it is clear that the time-dependent production of growth factors in the nerve injury microenvironment favor nerve fiber outgrowth, both peripherally and centrally.