Adult Ontogeny of Rat Working Memory of Social Interactions

Background:  Social memory has features that may make it uniquely appropriate for studying normal aging. We used a cross-section experimental design with an animal model to survey the lifetime adult ontogeny of memory of a brief social interaction.

Methods:  Groups of healthy adult male rats representing young adulthood (5 months), middle age (10 months), or old age (19-27 months) were tested weekly over a month in two paradigms. Basic social memory and social interference memory were quantified by differences between investigation times of a juvenile rat during a 5-min interaction (acquisition trial) and a second exposure (recall testing), with interexposure intervals (IEI) ranging from 15 min to 24 h.

Results:  Although basic social memory of all age groups was similar at the brief or longer IEI, there were memory declines at intermediate IEI delays in older males, especially the oldest groups. Decrements to working memory appeared as early as middle-age when an unfamiliar juvenile was inserted between acquisition and recall testing. Nonetheless, our healthy old animals retained a robust ability to recall identity of a conspecific that a minute by minute comparison suggested involves similar behavioral means of gathering social information.

Conclusions:  Normative aging of working social memory in male rats can be characterized as being more fragile, beginning at middle age but without significant further decline until near the end of the life span. Functional impact of these age-dependent changes in social memory, on the other hand, may be minimal for all but the very oldest animals in the social group.