Central nervous effects of leptin and insulin on hippocampal leptin and insulin receptor expression following a learning task in Wistar rats.

Insulin and leptin are well known to be involved in the regulation of food intake and body weight. Recent studies have suggested that both hormones may also affect memory and learning processes. We explored whether the intrahippocampal administration of insulin or leptin improved spatial memory formation in rats following a radial maze task and whether the insulin and leptin receptor expression in different areas of the hippocampus was affected. Animals without a learning task were also investigated in order to differentiate between the influence of spatial learning and hormone application on receptor expression. Spatial memory formation was found unaltered following the different applications. The application of insulin increased the expression of the long form of the leptin receptor in the Ammon's horn and dentate gyrus region of the hippocampus in the learning group only. These data clearly show that learning is a prerequisite for the effect of insulin on leptin receptor expression.