NEONATAL SCIATIC-NERVE LESION TRIGGERS THE SPROUTING OF FIBERS IN THECONTRALATERAL VENTRAL ROOT OF THE RAT

We explored the possibility that unilateral neurectomy of the sciatic nerve of the rat at the neonatal stage triggers sprouting of afferent fibers in the contralateral ventral root. 3 months after neonatal scia tic neurectomy, the numbers of both myelinated and unmyelinated fibers in the L5 and L3 ventral roots were counted on electron micrographic montages. Age-matched littermates were used as unoperated controls. To identify regenerating axons, electron microscopic immunohistochemistr y was done on the ventral roots using antibody against growth-associat ed phosphoprotein (GAP-43). Neonatal sciatic neurectomy resulted in: ( 1) about a three-fold increase in the number of unmyelinated fibers in the contralateral L5 ventral root as compared with the unoperated con trol; (2) about a 25-fold increase in the number of unmyelinated fiber s in the ipsilateral L5 ventral root as compared with the control; (3) similar to 25% of the unmyelinated fibers in the contralateral L5 ven tral root expressing GAP-43; and (4) no significant change in the numb er of unmyelinated fibers in the L3 ventral root of either side as com pared with the control. The data suggest that a neonatal sciatic neure ctomy of the rat triggers sprouting of unmyelinated afferent fibers in the ventral root of the contralateral as well as the ipsilateral side . The sprouting is restricted, however, to spinal segments which recei ve inputs from the sciatic nerve.