Abstract

The cellular basis of age-related behavioral decline remains obscure but alterations in synapses are likely candidates. Accordingly, the beneficial effects on neural function of caloric restriction and exercise, which are among the most effective anti-aging treatments known, might also be mediated by synapses. As a starting point in testing these ideas, we studied the skeletal neuromuscular junction (NMJ), a large, accessible peripheral synapse. Comparison of NMJs in young adult and aged mice revealed a variety of age-related structural alterations, including axonal swellings, sprouting, synaptic detachment, partial or complete withdrawal of axons from some postsynaptic sites, and fragmentation of the postsynaptic specialization. Alterations were significant by 18 mo of age and severe by 24 mo. A life-long calorie-restricted diet significantly decreased the incidence of pre- and postsynaptic abnormalities in 24-mo-old mice and attenuated age-related loss of motor neurons and turnover of muscle fibers. One month of exercise (wheel running) in 22-mo-old mice also reduced age-related synaptic changes but had no effect on motor neuron number or muscle fiber turnover. Time-lapse imaging in vivo revealed that exercise partially reversed synaptic alterations that had already occurred. These results demonstrate a critical effect of aging on synaptic structure and provide evidence that interventions capable of extending health span and lifespan can partially reverse these age-related synaptic changes.