Increased and decreased activity elicits specific morphological adaptations of the neuromuscular junction.

The objective of this investigation was to compare the effects of decreased vs. increased activity on the neuromuscular system. Twenty-four young adult (7 weeks old) Sprague-Dawley rats were randomly assigned to three treatment groups (N=8/group). Increased activity was achieved by treadmill running for up to 1 h/day. Decreased activity was induced by muscle unloading via the hindlimb suspension model. Control animals engaged in normal weight bearing and ambulatory activity. At the end of the 10 week intervention period, animals were killed and soleus muscles were removed, quickly frozen, and examined using cytofluorescent (neuromuscular junctions) and histochemical (myofibers) procedures. Pre-synaptic morphology was quantified by measuring nerve terminal branching, and post-synaptic assessment was conducted by staining acetylcholine receptors at the endplate. Myofiber profiles of solei were compiled by determining fiber size (cross-sectional area) and fiber type composition. Results show that exercise training significantly (P