Distribution of synaptic specializations along isolated motor units formed in Xenopus nerve-muscle cultures

The capacity of individual muscle cells and neurons to establish synaptic specializations along the entire extent of their neurite-muscle contacts was assessed in culture. Spinal cord neurons derived from embryos of Xenopus laevis were plated at low density in cultures of Xenopus myotomal muscle cells in order to obtain isolated motor units whose neuron and muscle cells were not contacted by any other neuron. These isolated motor units were examined for localization of acetylcholine receptors (AChRs) after staining with fluorescent alpha-bungarotoxin and, in some cases, for localization of a synaptic vesicle antigen by immunofluorescence. The neurite-muscle contacts formed by competent neurons exhibited discontinuous sites of AChR localization occupying about 25% of the contact length as compared with 2% for incompetent neurons. Competent neurons, unlike incompetent ones, established these neurite-associated receptor patches (NARPs) on virtually all the muscle cells they contacted and functionally innervated them. These and other observations on the distribution of NARPs throughout the isolated motor units indicate that the capacity of competent neurons to establish NARPs extends to the limits of growth of most if not all of their neurites, that this capacity is least in the most proximal portions of initial neuritic segments, and that the overall capacity of muscle cells to generate NARPs can be saturated by long lengths of neurite-muscle contact. The results also suggest that even in the absence of competitive interactions between neurons there are spatial discontinuities in neuritic action and/or muscle response during the establishment of NARPs. Synaptic vesicle antigen patches (SVAPs) occurred along the neuritic arbor of all neurons, but their distribution in competent neurons (those which established NARPs) and in incompetent ones differed. For competent neurons the percentage of neurite length occupied by SVAPs was 4.8-fold greater on muscle cells than off, whereas the corresponding value for incompetent neurons was only 1.5-fold. This large preferential localization of SVAPs along neurite-muscle contacts of competent neurons was further associated with a colocalization of SVAPs and NARPs that was greater than predicted by chance. These results suggest that muscle cells are much more effective in influencing the distribution of synaptic vesicles along the neuritic arbor of competent neurons than along the arbor of incompetent neurons and that this influence is greatest at sites of postsynaptic differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)