Memory, opiate receptors, and aging

Abstract

In a series of studies, we investigated differences in retention of avoidance responses in young and aged rats. The aged rats showed poorer retention performance of an inhibitory avoidance response, but better performance of an active avoidance response. In a swim escape task, when training trials were given close together in time, the aged rats showed impaired acquisition as compared to younger animals. We then investigated the effects of naloxone on learning and retention performance in young and old rats. Our findings indicated that in some cases, the direction of the opiate antagonist effect depends upon the age of the animal. With low footshock, naloxone either impairs or has no effect on the retention performance of old rats, but with high footshock, naloxone enhances their retention performance indicating that age-related changes in opioid systems may underly some behavioral differences between young and aged animals Therefore, we examined, in vitro, opiate receptor binding in various brain regions of young and old rats. Analysis of dihydromorphine binding indicated that opiate receptor concentrations in aged rats are reduced in some brain regions and that these changes are different in aged male and female rats. Studies of in vivo bindings of opiate receptors have yielded relationships between receptor changes and differences in response to footshock in the flinch-jump test. These findings suggest that age-related alterations in brain opioid systems may underlie some of the changes in learning and memory processes that occur with age.