Melatonin counteracts the 5-aminolevulinic acid-induced rise of rat forebrain tryptophan and serotonin concentrations at night.

When rats were exposed to extended light into the normal dark phase, the administration of 5-aminolevulinic acid (5-ALA) markedly stimulated the accumulation of tryptophan (TRP) and the synthesis of serotonin (5-HT) in the forebrain. When a single injection of melatonin (25 micrograms) was given near the end of the 5-ALA treatment, the rise in forebrain TRP levels was either totally prevented or reversed; likewise, the increased levels of 5-HT were also suppressed. Melatonin by itself also slightly suppressed forebrain TRP levels in non-5-ALA-treated rats. Whether these are pharmacological or physiological effects of melatonin remains to be determined.