Changes in forebrain serotonin at the light-dark transition

In vivo microdialysis in the rat was used to determine whether changes in forebrain serotonin (5-HT) levels seen during the light-dark transition differ by area examined and whether these changes could be accounted for by complementary changes in behaviour. 5-HT levels increased significantly during the first half hour of dark phase in the hippocampus (45.9%), striatum (13.4%), amygdala (19.7%) and prefrontal cortex (18.4%), the increase in the hippocampus being significantly larger than those in the other areas. 5-HT levels co-varied significantly with changes in the levels of alert waking, a behavioural measure of the time spent in active waking, suggesting that this factor might account for the changes in 5-HT across the light-dark transition.