Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC334
Effect of hindlimb suspension on the expressions of neurotrophins in rat soleus muscle
T. Hirose1, A. Tsutaki2, E. Ochi3, S. Min2, K. Nakazato2
1. Toin University of Yokohama, Yokohama, Japan. 2. Nippon Sports Science University, Setagaya, Tokyo, Japan. 3. Meiji Gakuin University, Minatoku, Tokyo, Japan.
The aim of this study was to examine the effect of hindlimb suspension (HS) on the expressions of neurotrophin (NGF, BDNF, NT-3) proteins and their receptors(TrkA, TrkB, TrkC, p75) in rat hindlimb muscles. Skeletal muscle shows the plasticity against changes in the mechanical environment. Decreased physical activity leads to muscle-fiber atrophy and muscle-fiber type shift from oxidative to glycolytic. HS has been reported to induce changes in muscular properties mainly in the slow muscles such as soleus muscle, in similar manner to space flight or simulated micro gravity (Edgerton & Roy, 1998; Ohira, 2000). Neural system as well as muscle fibers has been shown to be affected by HS. The afferent input from proprioceptors in steady state transitionally decreases after HS (Kawano et al. 2004). In another study, the afferent discharge of reflex was reported to increase during HS (De-Doncker et al.2003). Neurotrophins are family of members of proteins which induce the survival, development and function of neurons. In the present study, we hypothesized that HS would affect the expressions of neurotrophines and their receptors which are assumed to affect the neural potency in soleus muscle. 5-week-old male Wistar rats (n=32) were divided into HS (1 week; n=8, 2weeks; n=8) and control (1 week ;n=8, 2weeks; n=8) groups. During HS rats were suspended with their tails and free to access to food and water on their forearms. Rats were monitored via behavioural observations in addition to monitoring for general physical health. After 1 and 2 wk of HS, rats were anesthestized with isoflurane and killed by blood removal to excise soleus from both legs. After homogenizing soleus in RIPA buffer, westernblotting was performed to examine the expressions of NGF, BDNF, NT-3 proteins and TrkA, TrkB, TrkC, p75 receptors. Immunohistochemistry was examined to determine the loci of those proteins and receptors during HS. During the exposure to HS, NGF, BDNF, NT-3 and TrkA, TrkB, TrkC, expression decreased in the soleus muscle at day 7 and 14, except p75. P75 content interestingly increased during HS. The signals of immunohistochemistry of NGF and BDNF were located at motorneurons in soleus muscle. The immunohistochemistry of NT-3 and TrkC revealed their expressions around nerve-related tissues including muscle spindles and connective-tissues sheath. Localizations of neurotrophins were not altered after HS. The results indicate that the protein contents of neurotrophins and Trk receptors in the soleus muscle decrease in response to unloading. Mechanical stress affects the NT-3 and TrkC production, which may induce the change of function of muscle spindle. However, p75 protein abundance increased through HS, which might cause a different effect on skeletal muscle atrophy by mediating apoptosis. It is concluded that neuromuscular activities may be important for maintaining metabolism of neurotrohins which regulate the functions of efferent and afferent neuromuscular systems.
Where applicable, experiments conform with Society ethical requirements