Microglial activation in the visual pathway in experimental glaucoma: spatiotemporal characterization and correlation with axonal injury

Glia are the main cellular CNS elements initiating defense mechanisms against destructive influences and promoting regenerative processes. The aim of the current work was to characterize the microglial response within the visual pathway in a rat model of experimental glaucoma and to correlate the microglial response with the severity of axonal degeneration. Experimental glaucoma was induced in each right eye of adult Sprague-Dawley rats by translimbal laser photocoagulation of the trabecular meshwork. Rats were subsequently killed at various times from 3 days to 6 weeks. Tissue sections were obtained from globes, optic nerves, chiasmata, and optic tracts for immunohistochemistry and toluidine blue staining. This model of experimental glaucoma led to a marked activation of microglia in the retina, optic nerve, and tract. Indeed, microglial activity remained elevated, even after intraocular pressure returned to basal levels. It is postulated that this process accompanies ongoing axonal degeneration. The degree of activation in the optic nerve correlated with axonal damage. Activation was characterized by increased density and morphologic changes. Both major histocompatibility complex (MHC) class I and MHC class II surface proteins were persistently upregulated in optic nerves and localized to microglial cells; however, this did not correlate with any significant T-cell infiltration. Interestingly, MHC class II expression was not detected in the retina. The present data may have implications for the study of the pathology associated with the visual pathway in diseases such as glaucoma.