Sprouting and synapse formation produced by Carbocaine.

Injury to one cutaneous pectoris nerve of the frog induces sprouting of and synapse formation by the contralateral homologous intact nerve. It was previously suggested that axotomy initiates a signal for growth in the cell bodies of the injured motor neurons and that this signal is transferred transneuronally across the spinal cord to intact motor neurons (e.g., Rotshenker, S. (1982) J. Neurosci. 2: 1359-1368). The present study was designed to test the hypothesis that axotomy initiated the signal for growth by depriving neuronal somata of a trophic substance derived from the target muscle. The superficial layer of muscle fibers comprising one cutaneous pectoris muscle, the source of the hypothetical trophic substance, was removed by means of local application of the myotoxic local anesthetic Carbocaine. An increased supernumerary pattern of innervation developed in drug-treated muscles first and in contralateral intact muscles thereafter. These results raise the possibility that target muscle fibers play a role as regulators of a signal for growth in the cell bodies of their innervating motor neurons.