Effect of intrahippocampal insulin injection on memory performance in normal and diabetic rats

ABSTRACT

This work was designed to demonstrate the effect of diabetes on memory performance in rats and to evaluate the effect of intrahippocampal administration of insulin on memory retention consolidation] in normal non diabetic and diabetic adult male rats. The study consisted of 48 adult Wistar male rats with weights ranging from [185-200] gm. Rats were divided into 2 groups group A consisted of 24 normal non diabetic rats and group B consisted of 24 diabetic rats. The study was divided into 2 experiments Experiment 1 aimed to compare between the performance of normal non diabetic and diabetic rats during water maze training. It was performed on 16 rats 8 normal rats from group A and 8 diabetic rats from group B. All rats were trained in the water maze for 5 consecutive days to reach a hidden platform in the pool. Each day consisted of 8 training sections. The time required and distance traveled to reach the hidden platform was measured and the time spent in the target quadrant of the pool was calculated [expressed as a percent of the total time spent in the pool] each day. Experiment 2 aimed to test effect of post training intrahippocampal insulin injection on memory retention [consolidation] in normal and diabetic rats. This experiment was performed on 32 rats 16 normal rats from group A, and 16 diabetic rats from group B. The normal rats were divided into an intrahippocampal saline injected group [n=8] and an intrahippocampal insulin injected group [n=8]. Similarly the diabetic rats were divided into intrahippocampal saline injected group [n=8] and an intrahippocampal insulin injected group [n=8]. Rats were trained for one day on the water maze; the training session consisted of 8 trials 60 seconds each. Retention testing, used to test consolidation, consisted or 60 seconds of a free swimming period without the platform. The percent of the time spent and the percent of the distance swum in the target quadrant were calculated and the number of escape trials from the edge of the pool was measured. Experiment 1 showed that the performance of diabetic rats was significantly impaired when compared with the control group. Diabetic rats took a significantly longer time, and swam a significantly longer distance to reach the hidden platform when compared with control rats in each of the 5 days of training. Furthermore, the percent of time spent in the target quadrant of the pool was significantly lower when compared with the control rats. Experiment 2 showed that Intrahippocampal insulin injection enhanced memory consolidation in insulin injected normal rats when compared with saline injected normal rats as evidenced by a significantly higher percent of time spent and distance swum in the target quadrant of the pool in the insulin injected normal rats during retention testing. Furthermore, the number of escape trials from the edge of the pool was significantly lower in insulin injected normal rats when compared with saline injected normal rats. Similarly, intrahippocampal insulin injection in diabetic rats enhanced memory consolidation. However, when comparing diabetic insulin injected rats with normal saline injected rats their performance was impaired indicating a partial improving effect of intrahippocampal insulin on memory consolidation in diabetic rats. The present work demonstrated that intrahippocampal administration of insulin has an enhancing effect on memory consolidation in normal rats and resulted in a partial improvement of memory consolidation in diabetic rats. Implications of this research point to a potential therapeutic role of central insulin administration through the intranasal route for enhancement of memory consolidation