Characterization of tryptophan high affinity transport system in pinealocytes of the rat. Day-night modulation

Summary.  Tryptophan is required in the pineal gland for the formation of serotonin, precursor of melatonin biosynthesis. The level of this amino acid in the serum and in the pineal gland of the rat undergoes a circadian rhythm, and reduced plasma tryptophan concentration decreases secretion of melatonin in humans. Tryptophan is transported into the cells by the long chain neutral amine acid system T and by the aromatic amino acid system T. The high affinity component of [³H]tryptophan uptake was studied in pinealocytes of the rat. Inhibition was observed in the presence of phenylalanine or tyrosine, but not in the presence of neutral amino acids, alanine, glycine, serine, lysine or by 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid, a substrate specific for system L. The transport of tryptophan was temperature-dependent and trans-stimulated by phenylalanine and tyrosine, but was energy-, sodium-, chloride-, and pH-independent. In addition, the sulphydryl agent N-ethylmaleimide did not modify the high affinity transport of tryptophan in pinealocytes. The kinetic parameters were not significantly different at 12:00 as compared to 24:00 h. The treatment with the inhibitor of tryptophan hydroxylase, p-chlorophenylalanine, produced an increase in the maximal velocity of the uptake and a reduction in the affinity at 12:00, but not at 24:00 h, probably indicating that during the day, the formation of serotonin in the pineal gland is favoured by elevating the uptake of tryptophan, whereas at 24:00 h other mechanisms, such as induction of enzymes are taking place. High affinity tryptophan uptake in the rat pineal gland occurs through system T and is upregulated during the day when the availability of serotonin is reduced. Received March 15, 2001 Accepted July 8, 2002 Published online January 20, 2003 Acknowledgements This work was supported by the Grant S1-3490 from Consejo Nacional de Investigaciones Cientificas y Tecnológicas (CONICIT), Venezuela. We appreciate the secretarial assistance of Mrs. Isabel Otaegui. Carmen I. Gutiérrez is a PhD Student from Ciencias Fisiológicas, Facultad de Medicina, Universidad Central de Venezueta (UCV), Caracas, and supported by Universidad Francisco de Miranda, Coro, Falcón, Venezuela. Joseph Glykys is a Medical Student from Universidad de Carabobo, Valencia, Venezuela, and an Assistant Student of Centro de Estudios Avanzados, IVIC. Authors' address: Dr. Lucimey Lima, Laboratorio de Neuroquímica, Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas, Apdo. 21827, Caracas 1020-A, Venezuela, Fax: 58-212-504-1295, E-mail: llima@cbb.ivic.ve