Effects of altered muscle activation on oxidative enzyme activity in rat alpha-motoneurons

Some controversy exists as to whether alpha-motoneurons adapt their oxidative metabolism to changes in chronic activity levels and to altered status of their end organs, as occurs in other neuron types in the central nervous system. We measured, using a personal computer-based image analysis system, succinate dehydrogenase (SDH) activity in rat hindlimb motoneurons under conditions of increased activity (daily voluntary exercise plus treadmill endurance training, the latter 2 h/day, 4 days/wk, for 12 wk) and in a condition of muscle disuse (tetrodotoxin-induced paralysis for 2 wk) in which muscle oxidative enzymes are reduced. Although exercise-trained medial gastrocnemius showed significant adaptations (increased mean SDH activity of type I and increased proportion and total SDH activity of type I and combined I + IIa fibers), SDH activity of innervating motoneurons (identified by retrograde tracing using fast blue) was unchanged. In addition, tetrodotoxin-induced disuse, which results in hindlimb atrophy and SDH decreases (30% decrease measured in medial gastrocnemius muscle homogenates), failed to alter soma SDH or size in unspecified lumbar motoneurons. These results, obtained over a wider range of activity levels than in previous reports, suggest that the oxidative enzymes of motoneurons do not change despite clear adaptations in the muscles they innervate.