Preserved LTP and water maze learning in hyperglycaemic-hyperinsulinemic ZDF rats

Previous investigations have demonstrated that cognitive deficits as well as hippocampal dysfunctions are generated in animals presenting manifestations of Type 1 diabetes (T1D) mellitus. The present study examined whether such deficits can also be reproduced in the Zucker Diabetic Fatty (ZDF) rats after they developed symptoms of Type 2 diabetes (T2D). Learning and memory assessments were performed using the Morris water maze 5 weeks after the animals presented symptoms of Type 1 diabetes for Experiment 1 (Exp 1) and after 8 weeks for Experiment 2 (Exp 2). Testing in the water maze revealed that ZDF rats learned the task normally, although control rats were found to swim significantly faster after 5 or 8 weeks of untreated diabetes. From an electrophysiological perspective, we observed that the integrity of synaptic function was also preserved in ZDF rats as no alterations in long-term potentiation (LTP) were observed in the area CA1 of hippocampal slices. It is concluded that hyperglycaemia is not the only factor influencing water maze learning and LTP in this animal model of Type 2 diabetes (T2D). The experiments suggest that the resistance of ZDF rats to cognitive and electrophysiological dysfunctions might be related to the protective action of hyperinsulinemia. Indeed, measurements of the plasma insulin level at the end of testing were significantly superior in ZDF rats in comparison to control rats.