Microglial Activation Is a Very Early Event in Diabetic Retinopathy of Alloxan–Induced Diabetic Mice

Abstract

Abstract: : Purpose: Neuroglial changes have been reported in the retina of diabetic patients and rat models prior to detectable vascular changes. Since both types of changes are not observed consistently in streptozotocin–induced diabetic mice, they were examined in another mouse model of diabetic retinopathy, the alloxan–induced diabetic mouse. Methods: Diabetes was triggered in 10 C57/Bl6 male mice of two months by alloxan injections while another group was used as controls. The blood glucose level was measured every week in all mice and diabetic mice were injected with 3UI insulin (insuline zinc, Lilly, France) every day. All mice were investigated by electoretinographic recordings and scanner laser ophthalmoscope (SLO) examination at 15 days, 1 month and 3 months after the onset of diabetes. Retinal sections were processed to visualize micro and macroglial cells and apoptotic cells. Results: As a consequence of the alloxan injection the mean blood glucose level immediately increased from 10.35 to 31.03 mmol/L despite insulin injections. Diabetes was also attested by the weight loss and the increase in glycated haemoglobin. After 3 months of diabetes, the b/a wave amplitude ratio of the electroretinogram was reduced at the highest intensities while oscillatory potentials were delayed in diabetic mice (p<0.05, t–test). Retinal fundus and vessels remained unchanged during the study as assessed by SLO examination and fluorescein angiography. After 3 months of diabetes, no TUNEL staining was detected in vertical and horizontal sections of the retina. No increase GFAP immunostaining indicative of a glial reaction was present in Müller cells. By contrast, microglial cells had changed morphology and their dendrites exhibited reduced length (total cumulated length 1.74 vs 2.63mm; p<0.01, t–test) in diabetic retinas. Conclusions: In alloxan–induced diabetic mice, neuroglial changes were limited to electroretinographic alteration and microglial changes after 3 months of diabetes whereas no vascular changes were detected. These observations indicate that the microglial reaction could be a very early event in the progression of diabetic retinopathy.

Keywords: diabetic retinopathy • electroretinography: non-clinical • microglia