Rat О±- And ОÑ-- Motoneurones Soma Size And Succinate Dehydrogenase Activity Are Independent Of Neuromuscular Activity Level

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: 234вЂ"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: rrr@ucla.edu

Ð'© 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