Background: Microglial activation has been recognized as a neuropathological feature in diabetic retinopathy. But the early spatiotemporal characterization of microglial activation in the retina and the optic nerve of diabetic animals has not been fully investigated. The purpose of this study was to investigate early sequential changes of microglia in the retinas of rats with streptozotocin-induced diabetes. Microglia in the optic nerves of rats with streptozotocin-induced diabetes were also studied.
Methods: In 4-week, 8-week, and 12-week diabetic and normal control rats, microglial activation in the retinas and optic nerves was evaluated by immunolabeling with OX-42 antibody. Density, proportion of activation, and laminar distribution of retinal microglia were quantified. The retinal mRNA level of Iba-1, a microglial-specific marker, was measured by real-time PCR.
Results: The density of retinal microglia was not different between diabetic and control rats, but the proportion of activated microglia increased significantly in diabetic rats at each time point. The proportion of microglia increased obviously in the nerve fiber layer and the ganglion cell layer while decreasing in the inner plexiform layer in 12-week diabetic rats. Moreover, retinal Iba-1 mRNA expression increased in 8-week and 12-week diabetic rats. Processes of microglia in the optic nerves of control rats were aligned with the long axis of nerve fibers, while the alignment was disturbed in diabetic rats.
Conclusions: Morphology, proportion of activation, distribution, and mRNA expression of retinal microglia changed characteristically with the progression of the disease in early-stage diabetic rats.