Serotonin synthesis and its light–dark variation in the rat retina

Retinal circadian rhythms are driven by an intrinsic oscillator, using chemical signals such as melatonin, secreted by photoreceptor cells. The purpose of the present work was to identify the origin of serotonin, the precursor of melatonin, in the retina of adult rat, where no immunoreactivity for serotonin or tryptophan hydroxylase had ever been detected. To demonstrate local synthesis of serotonin in the rat retina, substrates of tryptophan hydroxylase, the first limiting enzyme in the serotonin pathway, have been used. Tryptophan, in the presence of an inhibitor of aromatic amino acid decarboxylase, enhanced 5-hydroxytryptophan levels, whereas alpha-methyltryptophan, a competitive substrate inhibitor, was hydroxylated into alpha-methyl-5-hydroxytryptophan. Tryptophan hydroxylase substrate concentration was higher in the dark period than in the light period, and formation of hydroxylated compounds was increased. The presence of tryptophan hydroxylase mRNA in the rat retina was confirmed by RT-PCR. Taken together, the results support the local synthesis of serotonin by tryptophan hydroxylation, this metabolic pathway being required more critically when 5-HT is used for melatonin synthesis.