© 2008 The Authors Doi: 10.1111/j.1742-7843.2008.00333.x Journal compilation © 2008 Nordic Pharmacological Society . Basic & Clinical Pharmacology & Toxicology , 104, 52–59 Blackwell Publishing Ltd Circadian Levels of Serotonin in Plasma and Brain after Oral Administration of Tryptophan in Rats Soledad Sánchez Mateos, Cristina L. Sánchez, Sergio D. Paredes, Carmen Barriga and Ana B. Rodríguez Department of Physiology, Neuroimmunophysiology Research Group, Faculty of Science, University of Extremadura, Badajoz, Spain (Received July 3, 2008; Accepted September 2, 2008) Abstract: 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. Circadian rhythms are generated and maintained by the biological clock located in the suprachiasmatic nucleus [1,2]. Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter strongly implicated in the regulation of these rhythms [3]. It is also involved in many functions throughout the brain, playing an important role in behaviour, depression, mood regulation, cognitive function, anxiety, sleep, appetite, sexual function, blood flow to the brain [4], and also has a fundamental role of precursor in melatonin synthesis. The synthesis and release of 5-HT are subject to circadian variations. Barassin et al. [5] and Dudley et al. [6] described in suprachiasmatic nucleus of rats and hamsters, respectively, that the maximum levels in release of 5-HT were obtained at the light/dark transition, just at the beginning of the dark phase, after which output of 5-HT decreased to basal levels throughout the remainder of the night. In previous research [7], we noted in rats that the peaks of 5-HT in basal conditions in plasma and different brain regions (pineal gland, hypoth- alamus, hippocampus, cerebellum and striatum) appear at the beginning of the dark period. This pattern can be altered in vitro by the use of 5-HT agonists acting at the level of the 5-HT1A receptors, with advance of the diurnal phase and delay of the nocturnal phase [8]. It has been found not only that this pattern of production of 5-HT persists in conditions of constant darkness [9–11], but also that it is influenced by luminosity signals [11,12]. Both the synthesis and the diurnal release of this neurotransmitter are activated by sympathetic innervations from the superior cervical ganglion, because if this is extirpated, the rhythm of 5-HT synthesis is lost [9,10]. These processes are controlled by different receptors: increased synthesis is regulated by β-adrenergic receptors, and release is mediated by α-adrenergic receptors [11]. The decrease in 5-HT levels during the dark period is because in this period the neurotransmitter is converted into melatonin, a hormone which is mainly synthesized during darkness in the pineal gland. Tryptophan is an essential amino acid that acts as a precursor in the synthesis of 5-HT. This synthesis occurs in two reactions, beginning with the action of the enzyme tryptophan hydroxylase, the limiting enzyme in this process, which converts tryptophan into 5-hydroxytryptophan which is then decarboxylated by the action of the enzyme 5- hydroxytryptophan decarboxylase, resulting in the formation of 5-HT (5-hydroxytryptamine or serotonin) [13]. A crucial factor governing central 5-HT synthesis is the availability of tryptophan to the brain [14]. Tryptophan is transported into the central nervous system with the neutral amino acid carrier for which other large neutral amino acids (phenylalanine, leucine, isoleucine, tyrosine and valine) compete [15–17]. Serum concentrations of tryptophan, or the ratio of tryptophan to its competing amino acids, could yield useful information about changes in 5-HT metabolism, which possibly underlie disease states [18,19]. When serum tryptophan concentrations are increased in experimental animals or human beings, the brain availability of tryptophan is enhanced leading to increased 5-HT synthesis [15,20,21], not only in serotonergic neurons, but also in pinealocytes of the pineal gland, and enterochro- maffin cells of the gastrointestinal tract [22–24]. It is well documented that blood tryptophan concentrations partially Author for correspondence: Soledad Sánchez Mateos, Department of Physiology, Neuroimmunophysiology Research Group, Faculty of Science, University of Extremadura, Avda de Elvas S/N, 06071 Badajoz, Spain (fax +34 924 289388, e-mail solsan@unex.es).