Transneuronal and peripheral mechanisms for the induction of motor neuron sprouting.

After injury to the nerve to one cutaneous pectoris muscle of the frog, the intact nerve to the contralateral muscle sprouts and forms additional synaptic connections with already innervated muscle fibers. It has been suggested (Rotshenker, S. (1979) J. Physiol. (Lond.) 292: 535-547; Rotshenker, S., and F. Reichert (1980) J. Comp. Neurol. 193: 413-422) that axotomy initiates a signal for sprouting in the injured neurons that is transferred transneuronally across the spinal cord to intact motor neurons. The present study was designed to test the hypothesis that axotomy initiates the signal for sprouting by interfering with some trophic signaling between the injured neurons and denervated muscle. Colchicine therefore was applied to the nerve to the left muscles to inhibit axonal transport on whose integrity trophic interactions depend. Consequently, supernumerary innervation developed in contralateral right intact muscles much the same as after axotom. Surprisingly, axons that were exposed to the drug also sprouted and formed synapses. Furthermore, the sprouting response of axons that were exposed to the drug also was produced in nerve fibers that were separated from their cell bodies. These results suggest two ways in which colchicine may produce sprouting and synapse formation and thereby suggest two mechanisms by which motor neurons may be induced to sprout: (a) transneuronally, by presenting growth stimuli to their cell bodies and central processes in the central nervous system and (b) by presenting growth stimuli to their peripheral extensions.