To assess the effects of external administration of L-tryptophan on the synthesis of serotonin and melatonin as well as on the immune function of Wistar rats, 300 mg of the amino acid were administered through an oral cannula either during daylight (08:00) or at night (20:00) for 5 days. Brain, plasma, and peritoneal macrophage samples were collected 4 h after the administration. The accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used to measure the rate of tryptophan hydroxylation in vivo. Circulating melatonin levels were determined by radioimmunoassay, and the phagocytic activity of macrophages was measured by counting, under oil-immersion phase-contrast microscopy, the number of particles ingested. The results showed a diurnal increase (p < 0.05) in the brain 5-HTP, serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindolacetic acid (5-HIAA) of the animals which had received tryptophan at 08:00 and were killed 4 h later. In the animals which received tryptophan during the dark period, the 5-HT declined but the 5-HT/5-HIAA ratio remained unchanged. There was also a significant increase (p < 0.05) in nocturnal circulating melatonin levels and in the innate immune response of the peritoneal macrophages in the animals which had received tryptophan at 20:00. The results indicated that the synthesis of serotonin and melatonin, as well as the innate immune response, can be modulated by oral ingestion of tryptophan.
Alzheimer's disease (AD) is the most common neurodegenerative disorder in the elderly. In the last years, abnormalities of lipid metabolism and in particular of docosahexaenoic acid (DHA) have been recently linked with the development of the disease. According to the recent studies showing how hydroxylation of fatty acids enhances their biological activity, here we show that chronic treatment with a hydroxylated derivative of DHA, the 2-hydroxy-DHA (2OHDHA) in the 5XFAD transgenic mice model of AD improves performance in the radial arm maze test and restores cell proliferation in the dentate gyrus, with no changes in the presence of beta amyloid (Aβ) plaques. These results suggest that 2OHDHA induced restoration of cell proliferation can be regarded as a major component in memory recovery that is independent of Aβ load thus, setting the starting point for the development of a new drug for the treatment of AD.
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