Decreased horseradish peroxidase labeling in deafferented spinal motoneurons of the rat

Abstract

It has been suggested 29,50 that the incorporation and retrograde transport of horseradish peroxidase (HRP) were linked to the level of neuronal activity. Therefore one could postulate that the motor impairment resulting from dorsal rhizotomy affects the HRP labeling of spinal motoneurons in the absence of morphological damage to the motor system. This hypothesis was tested in the adult rat by sectioning bilaterally the L 3-L 5 dorsal roots. 2–18 months after surgery, the L 4 radicular nerve was immersed in a solution of HRP. Labeled motoneurons were counted together with the motor axons of the L 4 ventral root and results were compared with values obtained in paired controls. Deafferentation resulted in a crippling deficit of lower movements with disuse atrophy of muscle fibers but had no effect on the fiber population of the sciatic nerve and the L 4 ventral root. Whereas in normal animals the L 4 HRP-labeled motoneurons represented 71.9–98.3% (average 85.4) of the motor axonal counts, in animals studied 4, 12 and 18 months after dorsal rhizotomy, the number of motoneurons containing HRP granules constituted only 20.1–55.7% (average 46.2) of the number of motor axons and many of the labeled cells were faintly stained. These findings, which may reflect either a decreased retrograde transport of HRP in deafferented motoneurons or an increased turnover of the enzyme in the cell body, call attention to the possibility that the degree of activity in neuronal pathways influences HRP labeling.