ADAPTATIONS OF RAT PHRENIC MOTONEURONS TO INACTIVITY 130

Abstract

Morphological and metabolic adaptations of phrenic motoneurons were assessed in response to two weeks of inactivity induced by cervical spinal cord hemisection at C2 level (spinal isolation; SI). The efficacy of SI was verified by the absence of spontaneous inspiratory-related EMG activity in right hemidiaphragm muscle. Phrenic motoneurons on the right side were retrogradely labeled using the fluorescence tracer, fast blue. After two weeks, the cervical cord and a segment of the right mid-costal diaphragm muscle were excised. Labeled phrenic motoneurons were processed using quantitative histochemical techniques for succinate dehydrogenase (SDH) activity. Type II muscle fibers in the diaphragm were identified by immunoreactivity for an anti-fast myosin heavy chain antibody. Nerve terminals on type I and II muscle fibers were then visualized by immunoreactivity for anti-protein gene product (PGP-7.5) and motor endplates were visualized by binding with tetramethlrhodamine α-bungarotoxin. The SDH activity and somal diameter of phrenic motoneurons, and the planner areas of nerve terminals and motor endplates were quantified using a computerized image-processing system. Phrenic motoneuron SDH activity was not significantly different between SI and control animals. Somal diameter of phrenic motoneurons in the SI group was smaller than in controls. In particular, the number of larger phrenic motoneurons (with somal diameters greater than 40μm) decreased in the SI group. These larger motoneurons putatively innervate fast-twitch motor units. The areas of nerve terminals and motor endplates on type II fibers increased in SI animals, while the areas of neuromuscular junctions on type I fiber were unchanged. Based on these results, we conclude that adaptations of phrenic motoneurons to inactivation are restricted to fast-twitch motor units.