Distribution of c-fos expressing dorsal horn neurons after electrical stimulation of low threshold sensory fibers in the chronically injured sciatic nerve.

The distribution of proto-oncogene c-Fos protein-immunoreactive cells in the spinal cord dorsal horn was studied after electrical stimulation at A alpha/A beta-fiber intensity of normal and previously injured sciatic nerves in urethane anesthetized rats. No or only occasional Fos protein-like immunoreactive cells were seen after stimulation of the normal uninjured nerve or after nerve transection without stimulation. Electrical nerve stimulation at 3, 12, and 21 days after sciatic nerve transection resulted in substantial increases in the numbers of Fos protein-like immunoreactive cell nuclei in each of Rexed's laminae I-V. Combined demonstration of Fos protein-like immunoreactivity and of glial fibrillary acidic protein-like immunoreactivity (astroglia) or OX-42 immunoreactivity (microglia), indicated that the observed Fos protein-like response was confined to neurons and not to astroglia or microglia. Combined demonstration in the spinal cord of Fos protein-like immunoreactive neurons and neurons labeled retrogradely with Fluoro-Gold from the gracile nucleus showed that some of the Fos protein-like immunoreactive neurons in Rexed's laminae III and IV contributed to the postsynaptic dorsal column pathway. The results indicate that stimulation at A alpha/A beta-fiber intensity of a previously injured nerve gives rise to an abnormally increased activation pattern of postsynaptic neurons in the dorsal horn, some of which contribute to the postsynaptic dorsal column pathway.