Increased oxidation of pineal serotonin as a possible explanation for reduced melatonin synthesis in the aging Djungarian hamster (Phodopus sungorus).

Groups of young adult (1.7 months) and old adult (9.7 months) female Djungarian hamsters were exposed to short-day photo-periods (SD; 8L:16D) or long-day photoperiods (LD; 16L:8D) for 6 weeks. Pineal and serum melatonin rhythms were dampened in old SD animals when compared with young SD animals. The melatonin precursor serotonin showed diurnal variations with opposite phases to melatonin synthesis. However, pineal serotonin levels were even lower in old SD animals than in young SD animals, contrary to the expected accumulation of the precursor. The synthesis of a metabolite of serotonin, 5-hydroxyindoleacetic acid (5-HIAA), was significantly lower in younger hamsters, under both photoperiods. These results indicate that the age-associated decline in the production of melatonin is a consequence of an increased oxidation of its precursors.