Aging Influences Adaptations of the Neuromuscular Junction to Endurance Training

This investigation sought to determine if aging affected adaptations of the neuromuscular junction (NMJ) to exercise training. Twenty young adult (8 months) and 20 aged (24 months) rats were assigned to either a program of treadmill exercise, or sedentary conditions. Following the 10-week experimental period, rats were euthanized, and soleus and plantaris muscles were removed and frozen. Longitudinal sections of the muscles were fluorescently stained to visualize pre-synaptic nerve terminals and post-synaptic endplates on both slow- and fast-twitch fibers. Images were collected with confocal microscopy and quantified. Muscle cross-sections were histochemically stained to assess muscle fiber profiles (size and fiber type). Our analysis of NMJs revealed a high degree of specificity and sensitivity to aging, exercise training, and their interaction. In the soleus, slow-twitch NMJs demonstrated significant (P ≤ 0.05) training-induced adaptations in young adult, but not aged rats. In the fast-twitch NMJs of the soleus, aging, but not training, was associated with remodeling. In the plantaris, aging, but not training, remodeled the predominant fast-twitch NMJs, but only pre-synaptically. In contrast, the slow-twitch NMJs of the plantaris displayed morphologic adaptations to both aging and exercise in pre- and post-synaptic components. Muscle fiber profiles indicated that changes in NMJ size were unrelated to adaptations of their fibers. Our data show that aging interferes with the ability of NMJs to adapt to exercise training. Results also reveal complexity in the coordination of synaptic responses among different muscles, and different fiber types within muscles, in their adaptation to aging and exercise training.