One of the important physiological functions of melatonin, the principal hormone secreted by the pineal gland, is its immunoregulatory action (Skwarlo-Sonta, 1996; Rodríguez et al. 1999). The hormone is synthesized during the period of darkness from tryptophan, an essential amino acid in the diet. We therefore studied whether the oral administration of tryptophan influences either circulating melatonin levels or the innate immune response by evaluating the phagocytic activity of macrophages.
To this end, we used 4-month old male Wistar rats subjected to a 12 h light-12 h dark cycle. The experimental group consisted of animals (n = 6) administered tryptophan (125 mg kg-1) via an orogastric cannula half an hour before the start of the dark period (19.30 h). The control group consisted of animals under identical conditions but administered saline solution. Plasma was isolated from blood samples taken from the tail at 09.00, 21.00, and 02.00 h at the beginning of the treatment, half-way through (day 11), and at the end (day 21). On the last day, when the blood sample was taken, the animals were killed by decapitation, and macrophages were collected from the peritoneal cavity. All experiments were carried out according to the guidelines of the European Community Council Directive 86/6091 EEC. Data are expressed as mean values ± S.D. and were compared by Student’s unpaired t test. Values of P ²le³ 0.05 were considered significant. Plasma melatonin levels were assayed by RIA (IBL) and the macrophage phagocytic activity was evaluated as the phagocytosis index (PI), i.e. the number of latex beads ingested per 100 macrophages, the phagocytosis percentage (PP), i.e. the percentage of cells that had phagocytosed at least one latex bead, and the phagocytosis efficiency (PE), i.e. the ratio PI:PP which indicates how effectively the phagocytes ingested the particles.
The results showed the phagocytic activity to be higher in the tryptophan group than in the controls, with maximum activity at 02.00 h (night) (PI tryptophan group 620 ± 51; PI control group 244 ± 26). The 02.00 h melatonin levels were higher in the tryptophan group both half-way through the treatment (100 pg ml-1) and at the conclusion of the experiment (100 pg ml-1) than in the control group (75 pg ml-1). We may therefore conclude that tryptophan in the diet can raise nocturnal melatonin levels and enhance phagocytic activity during the period of darkness.
The authors thank Elena Circujano Vadillo for technical assistance. This research was supported by the Fundación Valhondo Calaff and the Consejería de Bienestar Social – Fondo Social Europeo (Junta de Extremadura, 2002).