Relation between age-related changes in hyper-emotionality and serotonergic neuronal activities in the rat limbic system.

To examine the relationship between higher concentrations of 5-HT and 5-HIAA in the aged rat brain and their hyper-emotionality, behavioral and neurochemical studies were performed. In the behavioral studies, under novel circumstances, the aged rats showed an significant increase in defecation. Furthermore, their locomotor activity was much less than that of young rats, and the numbers of head-dips of aged rats were significantly decreased compared to those of young rats. In the neurochemical studies, the concentrations of 5-HT and 5-HIAA in the limbic system (e.g., hippocampus, amygdala and septal area) of aged rats were much higher than those of young rats (P < 0.001) under normal conditions. The possibility that the hyper-emotionality of aged rats might be related to the hyper-activities of serotonergic neurons in their limbic system were taken into consideration when the results were analyzed.