Radiation-induced modulation of the microglial population in the normal and injured mature spinal cord

Recent attempts by other investigators to enhance repair processes in the spinal cord have involved the administration of X rays to spinal cord injury sites. Although some functional improvement has been reported, the underlying cellular changes within the irradiated spinal cords are not clear. Studies initiated recently in this laboratory examined the potential of X rays to modulate nonneuronal cell populations associated with an injury site in adult mammalian spinal cords. These studies revealed a unique and previously unreported radiosensitivity of the microglial cell population. Administration of X radiation to a unilateral dorsal lesion cavity in the cervical spinal cord revealed a significant decrease (approximately half) in numbers of microglia associated with the cavity. Even more unexpected were the significant decreases in microglial cells observed on the nonlesioned side of the spinal cord or in sham-operated spinal cords in irradiated rats. In contrast to reports of others, densitometric quantification of GFAP immunoreactive cells and processes indicated no differences in the astrocytic reactions associated with the lesion cavities between nonirradiated and irradiated groups in our studies. The demonstration that exposure of a spinal cord injury site to radiation modifies the responses of certain components of the glial environment to injury may offer a noninvasive approach for direct treatment of that site. Studies are in progress to determine if this altered glial environment enhances the extension of regrowing axons from a peripheral nerve graft across the interface with the irradiated lesion cavity and into the spinal cord parenchyma.