Characterization of Cognitive Impairment in Type 2 Diabetic Rats

Among all the secondary complications of type 2 diabetes, cognitive impairment associated with diabetes is less well studied and characterized. We made an attempt to investigate the molecular mechanisms involved in cognitive decline in high fat diet-streptozotocin model of type 2 diabetes. High-fat diet and a single low dose streptozotocin (25 mg/kg) were used to induce diabetes in Sprague-Dawley rats. Developments of cognitive deficit in these animals were measured using change in transfer latency in elevated plus maze test and step-down latency in passive avoidance test. This was followed by biochemical estimation of acetylcholinesterase and reduced glutathione levels followed by histological evaluation of cortical and hippocampal slices. The high fat diet-streptozotocin animals developed type 2 diabetes, characterized by an increase in fasting blood glucose levels >250 mg/kg, marked increase (P<0.001) in triglyceride (207±7.2) and total cholesterol level (125.3±2.7) with a decrease (P<0.001) in high-density lipoproteins (14.27±1.1) when compared to control animals. After 4 weeks of development of type 2 diabetes high fat diet-streptozotocin animals showed signs of cognitive deficit with a significant decline in step-down latency (P<0.001) and a significant increase in transfer latency (P<0.001) compared to controls. The high fat diet-streptozotocin animals also showed significant decrease (P<0.001) in cortico-hippocampal glutathione levels and increase in (P<0.001) acetylcholinesterase activity as compared to control animals suggesting type 2 diabetes-associated oxidative stress and decrease in hippocampal Ach activity. Immunohistochemical studies in these animals showed pronounced astrogliosis and microglial activation in the cortical and hippocampal slices suggesting neuroinflammatory changes. We also found significant loss (P<0.001) of hippocampal neurons at CA1 and CA3 region in the diabetic rats as compared with healthy controls. Antidiabetic compound glibenclamide (10 mg/kg for 3 weeks) partly reversed the above neurobehavioral, biochemical and histological changes in the diabetic rat brain. Hence we show that high fat diet-streptozotocin type 2 diabetic rats may develop signs of cognitive impairment. This cognitive decline was found to be associated with an increase in acetylcholinesterase activity and loss of hippocampal neurons. Oxidative stress and neuroinflammatory changes in the brain of these animals may be responsible for hippocampal neuronal loss, which could be partly reversed upon glibenclamide treatment.