200 kD neurofilament protein and synapse elimination in the rat soleus muscle.

We studied the distribution and appearance of the phosphorylated form of the 200 kD neurofilament protein in the rat soleus muscle during the period of postnatal synapse elimination. Unlike many muscles, the appearance of singly innervated muscle cells in soleus occurs well after myogenesis has been completed, so that synapses are eliminated from a stable population of muscle cells. Immunoreactivity to the 200 kD neurofilament protein is present in the terminals of neuromuscular synapses of animals at all postnatal ages from 0 to 21 days. Before postnatal day 10, when physiological studies indicate that all soleus muscle cells receive more than one synaptic input, as many as 30% of soleus muscle cells contain phosphorylated 200 kD neurofilament protein immunoreactivity in only one synaptic terminal. At older ages the number of polyneuronally innervated muscle cells observed using immunostaining is similar to that observed physiologically. These findings suggest that not all developing neuromuscular synapses contain phosphorylated 200 kD neurofilament protein, and that those terminals lacking it comprise most of those eliminated early in the postnatal period. We conclude that the presence of phosphorylated 200 kD neurofilament protein might be highly correlated with the survival of motor nerve terminals during postnatal neuromuscular synapse elimination.