Hindlimb Immobilization Impairs Neuromuscular Junction Transmission in Young Rats

BackgroundImmobilization, bed rest, or illness rapidly lead to weakness out of proportion to muscle atrophy. Although the contribution of muscle wasting to weakness is well described, the role of neuromuscular junction (NMJ) dysfunction in early disuse-related weakness is not well understood. ObjectiveWe investigated whether short-term unilateral hindlimb immobilization (HLI) in rats impairs NMJ transmission and contributes to muscle weakness out of proportion to atrophy. MethodsFour-month-old male Fischer-344/Brown Norway rats underwent 10 days of unilateral HLI (n=6) or remained mobile (n=6). Neuromuscular excitability and transmission were assessed using compound muscle action potentials (CMAP), repetitive nerve stimulation (RNS), and single-fiber electromyography (SFEMG). Muscle contractility testing quantified tetanic torque, and post-mortem analysis measured muscle mass. ResultsTen days of HLI reduced gastrocnemius, soleus, and plantaris muscle mass by [~]20-35%. Plantarflexion peak tetanic torque normalized to body weight declined by 23%, and torque-time integral was reduced by 36%, indicating disproportionate functional loss (muscle size versus contractile output) and supporting underlying neural impairment. CMAP amplitude decreased from 69 mV to 53.23 mV (a 22.9% reduction; p = 0.0109), indicating a loss of summated neuromuscular excitability. Furthermore, both RNS and SFEMG indicated features consistent with NMJ transmission defects. RNS revealed CMAP decrement from [~]0% pre-HLI to -9.15% at 40 Hz stimulation and -8.63% at 50 Hz post-HLI. Similarly, SFEMG confirmed marked NMJ transmission defects, with jitter increasing 103% and blocking increased from <1% to >11% of fibers. ConclusionsOur findings suggest that short-term immobilization produces rapid and pronounced impairments in NMJ transmission that contribute to weakness beyond the degree of muscle atrophy. These findings identify the NMJ as an early and vulnerable site of disuse-induced dysfunction and highlight the potential for synaptic-targeted therapies to preserve muscle performance during immobilization and recovery.