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Background: A chimeric murine retrovirus, FrCasE, causes a rapid noninflammatory spongiform neurodegenerative disease of the motor system with an incubation period of 15 to 16 days after neonatal inoculation. Neurovirulence is determined by the viral envelope gene, but the neurodegeneration is an indirect consequence of virus infection, because the neurons that degenerate appear not to be infected., Experimental Design: The current study was undertaken to compare the kinetics of lesion development and the expression of viral envelope protein in the central nervous system (CNS). Neonatal mice were inoculated with FrCasE intraperitoneally and were killed at various times for determination of the kinetics of the CNS infection, the distribution of lesion in the CNS, and the distribution of viral envelope protein. In addition, qualitative features of both viral envelope and gag proteins were followed by immunoblot analysis., Results: The lesions induced by FrCasE consisted of vacuolar degeneration but without associated astrocytosis, the lack of an astroglial response being a consequence of the rapidity of the disease process. Vacuoles were observed primarily in the neuropil of the motor centers of spinal cord, brain stem, and cerebral cortex. Lesions appeared in all of these areas during a narrow window of time (< or = 3 days). Cells in which viral envelope protein was detected by immunohistochemistry before the appearance of spongiform degeneration included premigratory cerebellar cortical granule neurons as well as vascular elements in the regions that would ultimately exhibit spongiform degeneration. Two forms of viral envelope protein were detected in the CNS. A 70-kilodalton species appeared first, followed by an approximately 64-kilodalton species, which was detected coincident with the first appearance of spongiform lesions., Conclusions: Astrocytosis is a secondary reaction to the neuronal cytopathology induced by FrCasE and appears to be dependent on the developmental state of the CNS. The abrupt, diffuse nature of lesion development in this disease suggests a global effect of the virus infection. Cells of the CNS vasculature (either endothelial cells, perivascular microglial cells, or both) as well as cerebellar granule neurons appear to be seminally involved in the pathogenesis of the spongiform degeneration. The two species of viral envelope protein appear to be expressed by different cell types in the CNS.