Plasticity of dorsal root and descending serotoninergic projections after partial deafferentation of the adult rat spinal cord

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Abstract
Plasticity of dorsal root (DR) and descending serotoninergic (5‐HT) projections following dorsal rhizotomy fromL2 to S1 sparing L5 was studied by means of an intra‐animal comparison in the adult rat spinal cord. Projections of the chronically and acutely spared root were compared by cholera‐toxin conjugated horseradish peroxidase (CT‐HRP) injected into the sciatic nerves as the transganglionic tracer. Projections in unoperated controls, operated controls (acute bilateral spared root), and in experimental animals (chronic spared root on one side and acute spared root on the other) were mapped and the density was measured with an image analysis system. Labeled DRG cells and motor neurons were counted to determine if there were differences in the delivery of the label between the two sides. Measurements of the area of the dorsal horn and, separately, of the superficial laminae were made to control for shrinkage.DR projections were symmetrical in operated and unoperated controls, but a significant increase in DR projection density was found from L6 to L3 in the dorsal horn and Clarke's nucleus at L1 on the chronic spared root side in animals in which an equal number of DRG cells was labeled on the two sides. Density of 5‐HT immunoreactivity was symmetrical in controls. Ipsilateral to chronic spared root rhizotomy, the area fraction occupied by 5‐HT projections increased in Clarke's nucleus and in the superficial dorsal horn of all partially deafferented segments except L5, the spared root segment.Partial deafferentation of the adult rat lumbosacral spinal cord may therefore elicit sprouting from the spared dorsal root and, outside of the dorsal root projection zone, sprouting from the spared descending 5‐HT system. Plasticity of dorsal root projections and of 5‐HT projections occur in different regions; in regions of the increased spared root projection, no increase is seen in 5‐HT projections, suggesting that sprouting in the adult rat spinal cord is regulated, perhaps by competitive or hierarchical mechanisms.