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Alterations in the function of the hypothalamic suprachiasmatic nucleus (SCN) with age have been reported. As serotonin is an important regulator of the circadian clock located in SCN, this work studied the changes produced in the synthesis of serotonin with age using the accumulation of 5-HTP after decarboxylase inhibition as a measure of serotonin synthesis in the brain in vivo, in young and old ring doves at the onset of lights-on and lights-off. A diurnal cycle in tryptophan hydroxylation was observed in young animals, with an increased daylight synthesis and metabolism of 5-HT in hippocampus, neostriatum and hypothalamus. A single dose of melatonin (1 mg/kg, i.p., 1 h) at lighttime produced an inhibitory effect on the synthesis of 5-HT. In contrast, differences in 5-HT synthesis and metabolism between day and night disappeared in old animals indicating an absence of a circadian rhythm in 5-HT synthesis and metabolism. The administration of L-tryptophan (240 mg/kg, i.p.) strongly increased the 5-HT synthesis in young animals only during lights-off time while it increased in old ones irrespective of the administration time. These results suggest that the supplemental administration of tryptophan might aid to improve the descent in 5-HT that normally occurs, as animals get old.

Three different lactation experiments have been tested in a double blind procedure for 3 weeks, to improve sleep-wake patterns in infants. In a control experiment, standard infant commercial milk (1.5% tryptophan) was administered without changes during the day. In a second control (inverse), enriched milk (3.4% tryptophan) was given during light-time (06.00-18.00 h), and standard commercial milk during night-time (18.00-06.00 h). During the experimental week, the infants received standard milk during light-time and tryptophan enriched milk during night-time. The infants receiving the enriched formula during dark time showed improvements in the sleep parameters studied, and no statistical differences were found between the two control lactations. The urinary metabolites of serotonin suggest that the observed improvements were due to an increased use of serotonin to melatonin synthesis. In conclusion, the chronobiological changes in the normal components of the diet can improve infantile development of sleep/wake rhythms. [ABSTRACT FROM AUTHOR]