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Effects of hindlimb unloading on neuromuscular development of neonatal rats
B L Huckstorf1, G R Slocum, J L Bain
1Department of Cell Biology, Neurobiology and Anatomy, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA.
Insights
Hindlimb suspension in neonatal rats disrupts muscle development and motor nerve endings in soleus muscles. Unloading affects muscle fiber types and innervation complexity, highlighting the role of loading in neuromuscular development.
Area of Science:
- Neuromuscular Biology
- Developmental Physiology
- Muscle Fiber Type Analysis
Background:
- Neonatal hindlimb suspension is a model to study unloading effects on muscle development.
- Understanding neuromuscular plasticity is crucial for developmental biology and rehabilitation.
Purpose of the Study:
- To investigate the impact of hindlimb suspension unloading on neonatal rat soleus and extensor digitorum longus (EDL) muscles.
- To analyze changes in muscle fiber type composition and motor innervation patterns.
Main Methods:
- Neonatal rats (8 days old) underwent hindlimb suspension for 9 days with controlled nursing periods.
- Muscle fiber types were identified using myofibrillar ATPase histochemistry and immunostaining for specific isomyosins.
- Motor innervation, including multiple innervation and motor terminal ending complexity, was assessed via silver/cholinesterase staining.
Main Results:
- Unloading accelerated fast IIA myosin production and delayed slow myosin expression in soleus muscles.
- Muscle weight and fiber size increases were retarded in suspended soleus muscles.
- While multiple innervation loss was not delayed, motor nerve ending complexity was reduced in unloaded soleus muscles.
- EDL muscles showed retarded transition from embryonic to fast myosin expression, potentially due to suspension-related foot drop.
Conclusions:
- Neuromuscular development in both weight-bearing and non-weight-bearing muscles is modulated by hindlimb loading.
- Motor neuron activity influences the resolution of multiple innervation.
- Muscle fiber postsynaptic elements control endplate size, regulating motor terminal arborization.
Abstract:
We hypothesized that hindlimb suspension unloading of 8-day-old neonatal rats would disrupt the normal development of muscle fiber types and the motor innervation of the antigravity (weightbearing) soleus muscles but not extensor digitorum longus (EDL) muscles. Five rats were suspended 4.5 h and returned 1.5 h to the dam for nursing on a 24 h cycle for 9 days. To control for isolation from the dam, the remaining five littermates were removed on the same schedule but not suspended. Another litter of 10 rats housed in the same room provided a vivarium control. Fibers were typed by myofibrillar ATPase histochemistry and immunostaining for embryonic, slow, fast IIA and fast IIB isomyosins. The percentage of multiple innervation and the complexity of singly-innervated motor terminal endings were assessed in silver/cholinesterase stained sections. Unique to the soleus, unloading accelerated production of fast IIA myosin, delayed expression of slow myosin and retarded increases in standardized muscle weight and fiber size. Loss of multiple innervation was not delayed. However, fewer than normal motor nerve endings achieved complexity. Suspended rats continued unloaded hindlimb movements. These findings suggest that motor neurons resolve multiple innervation through nerve impulse activity, whereas the postsynaptic element (muscle fiber) controls endplate size, which regulates motor terminal arborization. Unexpectedly, in the EDL of unloaded rats, transition from embryonic to fast myosin expression was retarded. Suspension-related foot drop, which stretches and chronically loads EDL, may have prevented fast fiber differentiation. These results demonstrate that neuromuscular development of both weightbearing and non-weightbearing muscles in rats is dependent upon and modulated by hindlimb loading.