Essay About Thoracic Spinal Cord Transection And Succinate Dehydrogenase Activity Are Independent

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
increase in both the activation and loading levels
induced by the functional overloading of a muscle
via synergist removal results in an increase in muscle
fiber size, a shift toward “slower” physiological properties,
and an enhancement of oxidative-related enzymes,
such as succinate dehydrogenase (SDH) and
citrate synthase. Furthermore, a chronic decrease or
elimination of neuromuscular activity results in muscle
fiber atrophy, a shift toward “faster” physiological
properties, and a decrease in the levels of oxidativerelated
enzymes (with the possible exception of predominantly
slow muscles such as the soleus). In contrast,
the role that the level and pattern of
neuromuscular activity has in determining the size
and metabolic properties of the cell body of motoneurons
in adult animals is less clear.28,42
We previously reported no change in the size or
SDH activity of a sample of motoneurons from the
lumbar spinal cord 6 months after a complete spinal
cord transection (ST) or spinal cord isolation (SI) in
adult cats.8 We did not, however, identify individual
motor pools nor did we separate the effects on –
Abbreviations: AHP, afterhyperpolarization; EDTA, ethylene-diamine tetraacetic
acid; EMG, electromyography; SDH, succinate dehydrogenase; SI,
spinal cord isolation; ST, spinal cord transection; TTX, tetrodotoxin
Key words: inactivity; motoneuron number; motoneuron plasticity; motoneuron
soma size; motoneuron soma succinate dehydrogenase activity
Correspondence to: R. R. Roy; e-mail: [email protected]
Щ 2007 Wiley Periodicals, Inc.
Published online 14 May 2007 in Wiley InterScience (www.interscience.wiley.
com). DOI 10.1002/mus.20810
234 Morphology of Inactive Motoneurons MUSCLE & NERVE August 2007
and -motoneurons. In addition, the cat is a sedentary
species