Electrical stimulation restores the specificity of sensory axon regeneration

Electrical stimulation at the time of nerve repair promotes motoneurons to reinnervate appropriate pathways leading to muscle [Al-Majed, A.A., Neumann, C.M., Brushart, T.M., Gordon, T., 2000. Brief electrical stimulation promotes the speed and accuracy of motor axonal regeneration. J. Neurosci. 20, 2602-2608] and stimulates sensory neurons to regenerate [Geremia, N.M., Gordon, T., Al-Majed, A.A., Brushart, T.M., Verge, V.M., 2002. Brief electrical stimulation promotes regeneration of sensory fibers into cutaneous and muscle branches of femoral nerve. Neurosci. Abstr. 535.14]. The present experiments examine the effects of electrical stimulation on the specificity of sensory axon regeneration. The unoperated rat femoral cutaneous branch is served by 2-3 times more DRG neurons than is the muscle branch [Brushart, T.M., 1988. Preferential reinnervation of motor nerves by regenerating motor axons. J. Neurosci. 8, 1026-1031]. After transection and repair of the femoral trunk, equal numbers of DRG neurons project to both branches. However, 1 h of electrical stimulation restores the normal proportion of DRG neurons reinnervating skin and muscle. To ask if the redistribution of stimulated neurons results from enhanced specificity of target reinnervation, we developed a new technique of sequential double labeling. DRG neurons projecting to the femoral muscle branch were prelabeled with Fluoro Gold 2 weeks before the nerve was transected proximally and repaired with or without 1 h of 20-Hz electrical stimulation. Three weeks after repair, the muscle nerve was labeled a second time with Fluororuby. The percentage of regenerating neurons that both originally served muscle and returned to muscle after nerve repair increased from 40% without stimulation to 75% with stimulation. Electrical stimulation thus dramatically alters the distribution of regenerating sensory axons, replacing normally random behavior with selective reinnervation of tissue-specific targets. If the enhanced regeneration specificity resulting from electrical stimulation is found to improve function in a large animal model, this convenient and safe technique may be a useful adjunct to clinical nerve repair.