Synapse Involvement of the Dorsal Horn in Experimental Lumbar Nerve Root Compression

Study design This study was aimed at investigating changes in the dorsal horn of the lumbar cord induced by mechanical compression using an in vivo model. Objective To determine the effect of axonal flow disturbance in the dorsal horns induced by nerve root compression. Summary of background data Few studies have looked at changes of synapses within the dorsal horn caused by disturbance of axonal flow and the axon reaction as a result of mechanical compression of the dorsal root. Methods In mongrel dogs, the 7th lumbar nerve root was compressed for 1 week, or 3 weeks using a clip. After intravenous injection of Evans blue albumin, they were observed under a fluorescence microscope for the purpose of clarifying the function of the blood-spinal cord barrier. Morphologic changes of the synapses in the dorsal horns secondary to the nerve fiber degeneration were examined by light and electron microscope. Changes on immuno-staining for substance P, calcitonin gene-related peptide, and somatostatin in the dorsal horn were also examined. Results Light microscope observation conducted 1 week after compression of the nerve roots revealed Wallerian degeneration of the myelinated nerve in the dorsal horn, and fluorescence microscope observation of these areas demonstrated edema formation resulting from damage of the blood-spinal cord barrier. Three weeks after the compression, electron microscope observation revealed shrinkage of the axon terminals, ubiquitous presence of high electron density degeneration and presence of synapses whose contact with synapses was disrupted. Immuno-histochemical studies showed a marked decrease of substance P, calcitonin gene-related peptide, and somatostatin staining in substance gelatinosa with Wallerian degeneration after compression of nerve root. Conclusion It is important to recognize that compressive disturbance of the nerve roots caused Wallerian degeneration not only at the site of compression of nerve roots but also at the synapses of spinal cord dorsal horns.