Objectives: Diabetes mellitus is one of the most prevalent diseases in the world. High incidence of some impairment in cognitive tasks and memory formation in diabetics has resulted in many studies on the effect of insulin on central nervous system in recent years. There are evidences that indicate presence of numerous insulin receptors in some areas of central nervous system including cerebral cortex, olfactory bulb, hippocampus, cerebellum, and hypothalamus. Nevertheless, reports on the effects of insulin on memory formation are inconsistent and require more investigation to be clarified. Because, hippocampus is an important area for memory formation, the present study is scheduled to investigate the effect of insulin injection in CA1region of hippocampus on memory formation. Methods: Fifty male Wistar rats were divided into 5 equal groups. 1. Control 2. Sham operation 3. Insulin 4. diabetic/ saline 5. diabetic/ insulin. Groups 4 and 5 were made diabetic by treatment with streptozotocine (STZ) (50 mg/kg) intrapritoneally. In all but the control group, two canulae were stereotaxically implanted in CA1 region of hippocampus. Learning was tested and compared between groups through passive avoidance test.
Results: Results showed that in the insulin group the latency increased and the time for staying in the dark compartment decreased as compared to control and sham groups (p< 0.05). Compared to diabetic/ insulin group no significant difference was found with diabetic/saline group.
Conclusion: In conclusion, according to the results obtained in this study, insulin facilitates memory in intact rats but not in diabetic animals. Increased latency in diabetic rats shows that probably their reaction to new environment or their sensitivity to pain has exaggerated.