Aim: 

We have evaluated the serotonin circadian rhythms in plasma and different brain regions of rats.

Methods: 

Male Wistar rats of 14 2 weeks of age (n = 120) were used. They were maintained under conditions of 12 hours light and 12 dark, and food and water ad libitum. The animals were killed by decapitation. Blood was obtained from the neck vessels, and the different brain regions (pineal, hypothalamus, hippocampus, striatum, and cerebellum) were extracted. The serotonin levels were measured by ELISA every hour at night (20:00–08:00 h), and every 4 hours during the daytime (08:00–20:00 h).

Results: 

Ours results show that the maximum levels of serotonin in plasma were obtained at 09:00 and 22:00 and a minor peak at 01:00 hour. In hypothalamus there are a significant peak at 22:00 and two minor peaks at 17:00 and 02:00 hours; the same occur in hippocampus with a significant peak at 21:00, and two secondary peaks at 24:00 and 05:00 hours; in cerebellum there are two peaks at 21:00 and 02:00h, while in striatum and pineal there is only a peak at 21:00h at 23:00

Conclusion: 

We conclude that the highest levels of serotonin were during the phase of darkness, but vary depending on the region in which it is measured.