Remodeling of neuromuscular junctions in adult mouse soleus

The stability of neuromuscular junctions (NMJs) was studied in soleus muscles of adult mice by labeling acetylcholine receptors in vivo with rhodamine alpha-bungarotoxin. Identified NMJs were examined in living animals by low-light-level fluorescence microscopy on 2 or 3 occasions separated by up to 6 months. Many NMJs appeared identical each time they were viewed except for overall enlargement probably related to growth of the animal. Forty-four percent of NMJs, however, changed their shape over 6 months; these changes consisted mostly of small deletions or additions to part of the initial configuration. NMJs in adult soleus appeared to be less malleable than suggested by earlier studies but more plastic than NMJs in another muscle, the mouse sternomastoid, in which virtually no remodeling was observed using similar methods to the present study (Lichtman et al., 1987a). Thus, the degree of remodeling at NMJs may vary among different muscles, perhaps depending on their pattern of use.