Aging rat neuromuscular junctions: a morphometric study of cholinesterase-stained whole mounts and ultrastructure

Previous morphological studies of the adult rat soleus neuromuscular junction (NMJ) indicate that its ultrastructure changes with age, a consequence of a dynamic, ongoing remodeling of pre- and postsynaptic elements. With aging, the balance between axonal sprouting and withdrawal appears to shift in favor of denervation. Here we have examined the extent and direction of the remodeling process during the adult life of the rat (5-111 weeks) by quantification and morphometry of NMJ structure as seen in electron microscopic composites and in cholinesterase (ChE) stained whole mounts. We have found that the net effect of axonal sprouting and withdrawal on the entire NMJ region varies with age: at 5-12 weeks there is rapid growth and maturation; at 12-48 weeks slower growth occurs, and at 48-82 weeks there is a gradual shift in direction to result in progressive loss of synaptic contact in individual NMJs; finally, at 82-111 weeks, there is loss of entire NMJs with some continued reinnervation at others. The diaphragm NMJ also exhibits continuous structural remodeling; however, between 82-111 weeks many changes in the ChE staining pattern appear abruptly.