Aim: 

We analyzed the effect of administration of tryptophan on circadian levels of serotonin in plasma and brain of rats.

Methods: 

We used male Wistar rats of 14 2 weeks of age (n = 240), maintained under conditions of 12 hours light and 12 dark. Animals were treated for 7 consecutive days with a single daily oral dose (125 mg / 1 ml saline solution) of L-tryptophan at 19:00 (one hour before light turn off) using a gavage needle. Control rats received only 1 ml of saline solution with the same schedule as the tryptophan-treated animals. The tryptophan was administered for 7 days. The serotonin levels in plasma and brain were measured by ELISA every hour at night (20:00–08:00h), and every 4 hours during the daytime (08:00–20:00h).

Results: 

The results show that, in both the tryptophan treated and the untreated groups the highest values appear during the beginning of the darkness with a peak at 21:00, 22:00, and 23:00 hours in controls, and at 21:00 hours in the tryptophan-treated group. After tryptophan administration the levels of serotonin were significantly higher in all places analyzed than in the control group. This increase of serotonin levels was greatest in the pineal gland than in other brain regions, and least in plasma.

Conclusion: 

In sum, the oral administration of tryptophan during 7 days enhances serotonin levels over a 24 hours period, and produces an advance in the peak of serotonin in both plasma and different brain regions.