HIPPOCAMPAL CORTICOSTEROID RECEPTOR MESSENGER-RNA EXPRESSION AND SPATIAL-LEARNING IN THE AGED WISTAR RAT

The expression of mineralocorticoid (MR) and glucocorticoid receptor (GR) mRNA in the hippocampus of aged (18-24 months) and young (7 months) Wistar rats in relation to spatial learning in the water maze was investigated. All aged rats showed impaired performance of the task to locate a hidden platform compared to young controls, but the Wistar strain as a whole showed inferior performance to a group of young Lister hooded controls (added for the purposes of comparison) with even young Wistar rats apparently failing to learn platform location. The aged Wistar rats spent nearly 70% of their time near the side walls of the pool throughout the 5 days of training, while the young Wistar controls showed a gradual reduction in the percentage time spent there. MR and GR mRNA expression were unaltered in any hippocampal subregion in aged rats compared to young controls. Plasma corticosterone levels were also unchanged in aged rats. These results show that alterations in hippocampal corticosteroid receptor gene expression are not an inevitable consequence of aging in the rat, but that aging may be associated with strategy changes in the performance of a spatial learning task that are independent of glucocorticoid hypersecretion or alterations in hippocampal corticosteroid receptor mRNA expression.