Emotional changes related to age in rats--a behavioral analysis.

The present study investigated age-related differences in the emotional behavior of rats using factor analysis to identify motivational factors influencing spontaneous behavior in open field with illuminated center (OF), plus maze (EPM) and social interactions test. Animals of the same strain, bred under the same conditions, formed two experimental groups: young adults (YA, N=20) tested at the age of 4 months and old rats (OA, N=16) tested at the age of 24 months. The computer video based tracking system EthoVision was used for automated acquisition and analysis of data. The results of each test were analyzed separately for YA and OA by factor analysis. Two main independent factors emerged from the analysis of OF measures-factor 1, which appeared to reflect motor activity, and factor 2, reflecting anxiety. The measures best reflecting motor activity (distance moved in the peripheral zone) and anxiety (time spent in central zone) decreased significantly with age. Factor analysis for EPM measures revealed, in both groups, three independent factors. In YA, factor 1 reflected motor activity, factor 2-anxiety, in OA measures of anxiety loaded on factor 1, measures of activity on factor 2. Factor 3 in both groups appeared to represent a decision making process. The number of entries to the closed arms declined significantly in OA, showing an age related decrease of motor activity. Also, the ratio of open arms entries in relation to the total number of entries decreased in OA, indicating a higher anxiety level. Three independent factors emerged from the analysis of social interaction measures. The pattern of factor loading was different in young and old animals, although the number and time of social interactions did not show age-related differences. In addition to a decrease of motor activity we conclude that old rats also differ from young animals in emotional and social behavior.