ALTERATIONS IN OPIATE RECEPTOR-BINDING IN THE HIPPOCAMPUS OF AGED LONG-EVANS RATS

Quantitative in vitro autoradiography was used to examine [H-3]D-Ala(2 ), MePhe(4), Gly-Ol(5) enkephalin (DAGO) (mu-agonist) and [H-3]dipreno rphine (general opiate antagonist) binding sites in the hippocampal fo rmation of young (6-8 months) and aged (25-28 months) Long-Evans rats. Age-related changes in these binding sites were restricted to specifi c regions but were not generally dependent on the ligand used. No reli able age-related changes in opiate binding were observed in the CA1 fi eld or subicular region. In contrast, a decrease in the density of bin ding was found in both dorsal and ventral hippocampus within the CA3 f ield of aged brains. An age-related decrease in opiate binding within the dentate gyrus differed in its topography at dorsal and ventral lev els of the hippocampus. A uniform decrease of opiate receptor binding was found throughout the dorsal dentate gyrus, while a more localized decrease of these sites occurred in hilar and granular layers of the v entral dentate gyrus. These results indicate that a decrease of opiate binding in the hippocampal formation is largely localized to the CA3 region and dentate gyrus of aged rats. These findings are discussed wi th reference to age-related changes in hippocampal pathways containing opioid peptides. The implications for hippocampal opioid function in learning and age-related cognitive decline are also considered.