Circadian Levels of Serotonin in Plasma and Brain after Oral Administration of Tryptophan in Rats

Serotonin, one of the most important neurotransmitters in the central nervous system, is synthesized by the amino acid, tryptophan. Given that this essential amino acid is consumed in the diet, the aim of this study was to evaluate the effect of orally administered L-tryptophan (125 mg/kg) on circadian variations in the levels of serotonin in brain and plasma. We used male Wistar rats of 14 +/- 2 weeks of age (n = 240), maintained under conditions of a 12-hr light:dark cycle, and food and water ad libitum. Tryptophan administration was by gavage in a daily single dose at 7 p.m. for 7 days. The serotonin levels were measured by ELISA every hour at night (8 p.m. to 8 a.m.) and every 4 hr during daytime (8 a.m. to 8 p.m.). The results show that in both the tryptophan-treated and untreated groups the highest values appeared during the beginning of the darkness with a peak at 9, 10 and 11 p.m. in controls, and at 9 p.m. in the tryptophan-treated group. After tryptophan administration, the levels of serotonin were significantly higher in the plasma and all the brain regions analysed than in the control group. This increase of serotonin levels was greater in the pineal gland than in other brain regions, and the least in plasma. In conclusion, oral administration of tryptophan during 7 days enhances serotonin levels over a 24-hr period, and produces an advance in the peak of serotonin in both plasma and different brain regions.