Current evidence suggests that exercise and glial cell line-derived neurotrophic factor (GDNF) independently cause significant morphological changes in the neuromuscular system. The aim of the current study was to determine if increased physical activity regulates GDNF protein content in rat skeletal muscle. Extensor Digitorum Longus (EDL) and Soleus (SOL) hindlimb skeletal muscles were analyzed following 2 weeks of involuntary exercise and 4 hours of field stimulation or stretch in muscle bath preparations. GDNF protein content was measured via ELISA. Two weeks of exercise increased GDNF protein content in SOL as compared to sedentary controls (4.4 ±0.3 pg GDNF/mg tissue and 3.1 ±0.6 pg GDNF/mg tissue, respectively) and decreased GDNF protein content in EDL as compared to controls (1.0 ±0.1 pg GDNF/mg tissue and 2.3 ±0.7 pg GDNF/mg tissue, respectively). GDNF protein content in the EDL decreased following both field stimulation (56% ±18% decrease from controls) and stretch (66% ±10% decrease from controls). SOL responded to field stimulation with a 38% ±7% increase from controls in GDNF protein content, but showed no change following stretch. Pre-treatment with α-bungarotoxin abolished the effects of field stimulation in both muscles and blocked the effect of stretch in EDL. α-bungarotoxin pre-treatment and stretch increased GDNF protein content to 240% ±10% of controls in the SOL. Exposure to carbamylcholine decreased GDNF protein content to 51% ±28% of controls in the EDL but not SOL. These results suggest that GDNF protein content in skeletal muscle may be controlled by stretch, where it may increase GDNF protein content, and membrane depolarization/ACh which acts to decrease GDNF protein content. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Keywords NIH Public Access Author ManuscriptNeuroscience. Author manuscript; available in PMC 2012 February 3.Published in final edited form as: Neuroscience. 2011 February 3; 174: 234-244. doi:10.1016/j.neuroscience.2010. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript Effect of exercise on the peripheral nervous systemIncreased physical activity has been shown to alter the structure and function of the neuromuscular junction (NMJ). Exercise increases the size and degree of branching of motor nerve terminals at the NMJ (Andonian and Fahim, 1987), increases total area of both preand postsynaptic elements (Deschenes et al., 1993), and increases quantal content of acetylcholine (ACh) release (Dorlochter et al., 1991). The dispersion of acetylcholine receptors (AChRs), including both endplate peri...

Glial cell line‐derived neurotrophic factor (GDNF) is a potent survival factor for motor neurons. Over expression of GDNF in skeletal muscle has been shown to cause a hyperinnervation of muscle fibers. We have previously shown that exercise increases GDNF expression in skeletal muscle. In our previous studies GDNF content was measured 24 hours after the last bout of exercise. For the current study we wanted to determine if GDNF content of muscle differed in muscles removed immediately following exercise (0h) compared to those taken 24 hours (24h) later. Following 2 weeks of exercise soleus (SOL) and extensor digitorium longus (EDL) were removed at 0h or 24h and analyzed for GDNF content by enzyme‐linked immunosorbant assay. The results showed a significant difference (p≤0.05) in GDNF levels between SOL 24h and control but not in any other time point or tissue. These results may indicate that production of GDNF in skeletal muscle could be regulated in an activity dependent manner and that, GDNF levels are increasing subsequent to the last bout of exercise for a duration of at least 24 hr. The deferential expression of GDNF in skeletal muscle with exercise could have implications in the aging and diseased NMJ's of humans. This work was supported by NIH Grant 1R15AG022908‐01A2, MSU‐KCMS, and Western Michigan University.

Skeletal muscle produces GDNF, which is a neurotrophic factor that is critical for the development and maintenance of neural tissue. GNDF is a potent survival factor that can rescue motor neurons from programmed and axotomy induced cell death. The present study investigated the relationship between exercise and GDNF expression in skeletal muscle comparing 11 and 17 week old (n=6) Fischer 344 rats. The 11 week old animals were exercised for 2 weeks, half of the group was euthanized immediately after exercise and the remaining were euthanized 24 hours later. The extensor hallucis longus (EHL) muscle was removed and analyzed for GDNF content using enzyme‐linked immunosorbant assay (ELISA). GDNF content was significantly higher in animals that were exercised and euthanized 24 hours later compared to age matched sedentary controls (n=6). Immunohistochemical staining for skeletal slow muscle fiber and GDNF suggested an increase in GDNF content in the slow fibers. Labeled α‐bungarotoxin localized GDNF to the motor end plates in the skeletal muscle. Animals euthanized immediately after exercise showed a decrease in frequency of end plates staining positively for GDNF compared to controls. These results suggest physical activity modulates GDNF expression in skeletal muscle. This work was supported by NIH Grant 1R15AG022908‐01A2, MSU‐KCMS, and Western Michigan University.

Glial‐cell line derived neurotrophic factor (GDNF) is a neurotrophic factor produced by skeletal muscle. GDNF has been shown to be potent survival factor for motor neurons. We have previously shown that GDNF content in skeletal muscle is altered following exercise and GDNF levels 24 hours after the last bout of exercise are significantly different than those measured immediately after exercise. We sought to determine how long after exercise do GDNF protein levels remain altered. We removed extensor digitorum longus (EDL) and Soleus (SOL), 24 hr, 72 hr and 1 week following the last bout of involuntary exercise. Muscles were processed for GDNF protein content using enzyme linked immunosorbant assay. Contralateral EDL and SOL were removed at the same time points for immunohistochemical analysis. The results showed that 72 hr following exercise there was a significant increase of GDNF protein content in SOL and a significant decrease in EDL. The results suggest exercise causes a relatively long‐lived effect on GDNF protein content and that expression of GDNF protein in type I skeletal muscle maybe regulated differently that that in type II skeletal muscle.This work was supported by NIH Grant 1R15AG022908‐01A2, MSU‐KCMS, and Western Michigan University.

Glial cell line‐derived neurotrophic factor (GDNF) is a neurotrophic factor found in skeletal muscle that affects peripheral motor neurons. The purpose of our experiment was to examine changes in GDNF protein expression in skeletal muscle from rats that have undergone different durations of exercise. GDNF protein content of skeletal muscle was examined following involuntary exercise on a running wheel for 2, 4, and 6 weeks. Muscles examined included diaphragm (dia), pectoralis (pec) and extensor hallucis longus (ehl). GDNF protein content was measured via enzyme‐linked immunosorbant assay. The results show that pec and ehl muscles responded to exercise with an early decrease in GDNF content (2 weeks), followed by a return to control levels after 4 and 6 weeks of exercise. Dia muscle showed a significant increase in GDNF protein content following 6 weeks of training. If GDNF expression in skeletal muscle is regulated by physical activity this could help to explain beneficial effects of exercise on motor nervous system structure and function. This work was supported by NIH grant 1 R15 AG022908‐01A2, The Monroe‐Brown Foundation, Western Michigan University, and MSU‐KCMS.