Morphological and Functional Roles of Macrophages and Postganglionic Sympathetic Fibers in Dorsal Root Ganglions of a Rat Neuropathic Pain Model.

Abstract

The induction and functional involvement of macrophages and postganglionic sympathetic fibers have been reported in rat neuropathic pain models. In the present study, we examined the morphology and structural relationship among macrophages, postganglionic sympathetic fibers, the dorsal root ganglion (DRG) neurons, and the satellite cells by light and electron microscopic examination. We used the rat spinal nerve ligation model reported by Kim & Chung. On postoperative day (POD) 1, when sympathetically maintained pain was confirmed, macrophages invaded from the nerve ligation site and reached the DRG neuronal area. Macrophages might have induced the demyelination of the spinal nerve and the degeneration of the satellite cells, and then the DRG neurons, from which the satellite cells were detached, lost the insulation from other neurons or nerve fibers. After the invasion of macrophages, postganglionic sympathetic fibers with TH-like immunoreactivity migrated into the same area, and occupied sites around the neuronal soma which were surrounded by satellite cells under normal conditions. At POD 180, the sympathetically maintained pain had disappeared, and the DRG neurons and spinal nerve showed normal structure without any invasive macrophages or postganglionic sympathetic fibers. The above changes in the morphological arrangements of macrophages and postganglionic sympathetic fibers in the DRG were thus elucidated and we suggest that this process may be part of the mechanisms underlying the development of the sympathetically maintained pain.