Use of c-fos to identify activity-dependent spinal neurons after stepping in intact adult rats

Study design: An investigation of c-fos activation pattern in spinal neurons of intact adult rats after acute bouts of treadmill locomotion. Objectives: To map spinal neurons that are involved in quadrupedal treadmill stepping of intact adult rats by using c-fos as a marker. Settings: Los Angeles, CA, USA. Methods: Spinal cord sections of rats that were not stepped (n = 4) were used to map the FOS-positive (+) neurons under basal conditions. The stepped group (n = 16) was placed on a treadmill to step quadrupedally for varying durations to induce c-fos activity. Spinal cord sections of thoracic and lumbar segments of Stp and Nstp rats were processed using a c-fos antibody, choline acetyl transferase and heat shock protein 27 for identifying motoneurons. Results: Stepping induced a greater number of FOS+ neurons than was observed in rats that did not step on the treadmill. There was a rostrocaudal and a dorsoventral gradient of FOS labeled neurons. The number of FOS+ neurons increased with the duration of treadmill stepping. Significant increases in FOS+ neurons were in the most medial parts of laminae IV, V, and VII. FOS+ motoneurons increased with treadmill stepping, particularly in large motoneurons (>= 700 mu m(2)). Conclusion: These data suggest that FOS can be used to identify activity-dependent neuronal pathways in the spinal cord that are associated with treadmill stepping, specifically in lamina VII and in alpha motoneurons.