Spatial learning and the hippocampal corticosterone receptor system of old rats: effect of the ACTH4-9 analogue ORG 2766.

Old (26 months) and young (6 months) male Wistar rats were treated chronically for 2 weeks with ORG 2766 or with vehicle, delivered via subcutaneously implanted minipumps (0.5 microgram peptide/0.5 microliter/h). Learning of a spatial task was not impaired in the old animals, except for one measure, i.e. the latency to find the goal box. In neither age group did ORG 2766 influence behavioral performance. The number of corticosterone receptor sites was decreased in the hippocampus of senescent rats, but restored to the level observed in young rats following ORG 2766 treatment. It is concluded that the number of hippocampal corticosterone receptor sites is a sensitive index of brain aging and effectiveness of ORG 2766.