Involvement of hippocampal serotonergic activity in age-related changes in exploratory behavior

The effect of aging on exploratory behavior was investigated in adult (5 month) and aged (28 month) CB6F1 mice. During the first 10 minutes of the test session, aged mice made fewer head dip responses and spent less time exploring the novel stimuli. Because the aged mice were observed to subsequently increase their duration of exploration, it was suggested that the initial differences in exploration reflected a suppression of exploratory behavior by the aged mice. Immediately after behavioral testing, mice were sacrificed and hippocampal serotonin (5HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined. While 5HT concentrations were not found to be significantly altered with age, significant increases in 5-HIAA concentrations and in the ratio of 5-HIAA/5HT were observed in the aged mice. Further, the elevation in the 5-HIAA/5HT ratio was found to be significantly correlated with age-related differences in the duration, but not the frequency, of head dip responses. In view of this finding, it was suggested that alterations in serotonergic function with age may selectively affect specific aspects of an animal's response to novel stimuli.