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Rat О±-And Оі-Motoneurones Soma Size and Succinate Dehydrogenase Activity Are Independent of Neuromuscular Activity Level

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ABSTRACT: The chronic level of neuromuscular activity, that is, activation

and loading, strongly influences the morphological, metabolic, phenotypic,

and physiological properties of skeletal muscles. The effects on the innervating

motoneurons, however, are less established. We determined and

compared the effects of 30 days of decreased activity (induced by a complete

mid-thoracic spinal cord transection, ST) or near inactivity (induced by

spinal cord isolation, SI) on the soma size and succinate dehydrogenase

(SDH) activity of motoneurons innervating a predominantly slow ankle extensor

(soleus) and a predominantly fast ankle flexor (tibialis anterior) muscle

of adult rats. Soleus and tibialis anterior motoneuron pools were labeled

retrogradely using nuclear yellow. The - and -motoneurons were classified

based on soma size. Mean number of labeled motoneurons, and mean

soma size and SDH activity for both - and -motoneurons were similar in

control, ST, and SI rats. Compared to previous reports showing significant

decreases in muscle fiber size and adaptations toward a “faster” metabolic

profile following ST and SI, the results indicate that, unlike the muscles they

innervate, the motoneurons are relatively unresponsive to chronic reductions

in neuromuscular activity. The implication of these results is that mean

size and SDH activity are independent of the number of action potentials

generated by both - and -motoneurons and that even the absence of

afferent input to the spinal cord has no influence on size and oxidative

metabolic potential of the motoneuron soma.

Muscle Nerve 36: 234–241, 2007

RAT - AND -MOTONEURON SOMA SIZE AND

SUCCINATE DEHYDROGENASE ACTIVITY ARE

INDEPENDENT OF NEUROMUSCULAR ACTIVITY LEVEL

ROLAND R. ROY, PhD,1 AKIKO MATSUMOTO, MS,2 HUI ZHONG, MD,1

AKIHIKO ISHIHARA, PhD,2 and V. REGGIE EDGERTON, PhD1,3

1 Brain Research Institute, 1320 Gonda Neuroscience and Genetics Building, University of California,

Los Angeles, Box 951761, Los Angeles, California 90095-1761, USA

2 Laboratory of Neurochemistry, Graduate School of Human and Environmental Studies,

Kyoto University, Kyoto, Japan

3 Department of Physiological Science, University of California, Los Angeles, California, USA

Accepted 22 March 2007

Mammalian skeletal muscles are highly responsive

to chronic changes in neuromuscular activity levels,

such as changes in activation or mechanical loading

levels and patterns.14,15,37,41 For example, a chronic

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