Evaluation of neonatal streptozotocin induced diabetic rat model for the development of cataract

Abstract

Type 2 diabetes (T2D) generally follows prediabetes (PD) conditions such as impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT). Although studies reported an association of IGT or IFG with cataract, the experimental basis for PD associated cataract is not known. Hence, we evaluated neonatal streptozotocin (nSTZ) induced rat model to study PD associated cataractogenesis by injecting STZ to two-day old rats. While majority (70%) of nSTZ injected pups developed IGT (nSTZ-PD) by two months but not cataract even after seven months, remaining (30%) nSTZ rats developed hyperglycemia (nSTZ-D) by two months and mature cataract by seven months. Lens biochemical analysis indicated increased oxidative stress as indicated by increased SOD activity, lipid peroxidation, and protein carbonyl levels in nSTZ-D cataractous lens. There was also increased polyol pathway as assessed by aldose reductase activity and sorbitol levels. Though nSTZ-PD animals have not shown any signs of lenticular opacity, insolubilization of proteins along with enhanced polyol pathway was observed in the lens. Further there was increased oxidative stress in lens of IGT animals. These results suggest that oxidative stress along with increased polyol pathway might play a role in IGT-associated lens abnormalities. In conclusion, nSTZ-PD rat model could aid to investigate IGT-associated lens abnormalities.

Figure 1

Plasma glucose and insulin levels during experiment. Fasting glucose levels (mg/dL) were estimated once in a week. Values represent mean ± SD (n = 7–16). The asterisk denotes that data are significantly different from control group and hash mark denotes significantly different from nSTZ-PD group.

Figure 2

Immunohistochemistry of insulin positive cells in pancreas. Pancreatic beta cells were identified by insulin monoclonal antibody (a) and the insulin positive cells were counted by Leica Laser Microdissection microscope (b). Arrow indicates islet. The values are mentioned as percentage beta cells against total islets cells (b). Values represent mean ± SD (n = 3). The asterisk denotes that data are significantly different from control group and hash mark denotes significantly different from nSTZ-PD group.

Figure 3

Glucose and insulin response during OGTT at two months of age. Plasma glucose (a) and insulin (b) levels in response to an oral glucose challenge in 2-month-old STZ injected (n = 7–16) and control (n = 7) SD rats following a 16 h fast. Values represent mean ± SD. The asterisk denotes that data are significantly different from control group.

Figure 4

Cataract development in nSTZ induced diabetic rats. Cataract formation was monitored weekly by slit-lamp microscope and the stage of cataract was scored according to the classification described in Section 2. Stages of cataract in each group were averaged at the given time and plotted as a function of time. Representative photographs of lens from each group at the end of experiment are shown in the inset. Values represent the mean ± SD. The asterisk denotes that data are significantly different from control group.

Figure 5

Lipid peroxidation (MDA) and protein carbonyl content in rat lens. Values are mean ± SD (n = 3). The asterisk denotes that data are significantly different from control group.

Figure 6

Activities of antioxidant enzymes superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione S-transferase (GST) in lens protein. Values are mean ± SD (n = 3). The asterisk denotes that data are significantly different from control group.