Evidence for invasion of regenerated ventral root afferents into the spinal cord of the rat subjected to sciatic neurectomy during the neonatal period

Sectioning the sciatic nerve of experimental animals at the neonatal stage triggers growth of afferent fibers in the ventral root. The present study examined the possibility that the regenerating fiber terminals grow into the spinal cord. The sciatic nerve on one side was cut in neonatal rats. After the rats were fully grown, either an electrophysiological or a histochemical study was performed. The results of electrophysiological experiments showed that stimulation of certain loci in the L5 spinal cord evoked antidromic potentials in the L5 ventral root with a long latency. Various evidence suggests that the long latency potentials are due to activation of C fibers. These C-fiber potentials were on average bigger and were elicited from more numerous loci on the side ipsilateral to the sciatic nerve lesion than on the contralateral side. Furthermore, stimulation of the spinal cord of unoperated normal rats rarely evoked such potentials. For the histochemical study, horseradish peroxidase (HRP) was injected into the L5 spinal cord after cutting the L4-L6 dorsal roots. A lot more cells in the L5 dorsal root ganglion (DRG) on the side ipsilateral to the sciatic nerve lesion were labeled with HRP transported retrogradely through the L5 ventral root than on the contralateral side. Control experiments showed that few DRG cells are labeled with HRP in normal unoperated rats. The combined results of the electrophysiological and histochemical studies suggest invasion of ventral root afferents into the spinal cord, given enough postoperative time. It is not known whether or not these terminals make functional synaptic contacts in the spinal cord.