Both basic fibroblast growth factor and ciliary neurotrophic factor promote the retention of polyneuronal innervation of developing skeletal muscle fibers.

At birth, nearly all rat muscle fibers receive synaptic inputs from more than one motoneuron at a single end-plate site. By the end of the third postnatal week all but one of these inputs has been eliminated. During this loss of polyneuronal innervation, developing neuromuscular synapses compete with one another. Although the nature of this competition is not known, it is commonly assumed that it is mediated through differential activity of the competing inputs. One means by which such differential activity might be translated into a biological signal would be if the synapses compete in an activity-dependent manner for a scarce supply of neurotrophic molecules. A prediction of this hypothesis is that excess quantities of such trophic molecules will reduce competition and thereby slow down or abolish the normal loss of polyneuronal innervation. In newborn rats, the effects of injection of either basic fibroblast growth factor (bFGF) or ciliary neurotrophic factor (CNTF) on the outcome of neuromuscular synapse elimination were investigated. Daily injections of either bFGF or CNTF were made for 1 week into the lateral gastrocnemius muscle beginning at the postnatal age of 2 days. The amount of polyneuronal innervation of fibers in trophic molecule-injected muscles and saline-injected contralateral muscles was assayed using intracellular recording methods. For both bFGF- and CNTF-injected muscles, an increase in the percentage of polyneuronally innervated fibers relative to saline-injected muscles was noted. For bFGF-injected muscles, the amount of polyneuronal innervation remained at nearly 60% as late as the postnatal age of 14 days (P14). This is the amount of polyneuronal innervation found at age 6 days in normal animals. Nearly 40% of the fibers of CNTF-injected muscles remained polyneuronally innervated at age P14, the amount expected at age 9 days. These results indicate that both bFGF and CNTF exert powerful and long-lasting effects on developing neuromuscular synapses.