Retinal 5-Methoxytryptamine and 5-Methoxyindole-3-acetic Acid in the Rat and Quail: Diurnal Rhythms and Interspecies Differences

Endogenous 5-methoxytryptamine (5MT) and its biosynthetic oxidation product, 5-methoxyindole-3-acetic acid (5MIAA), were successfully identified and measured in the retina of the rat and quail by gas chromatography/electron-capture negative ion chemical ionization mass spectrometry (GC/EC-NICI-MS). In the rat retina, diurnal rhythms of 5MT and 5MIAA, with high levels at mid-light and opposite to that of melatonin, were observed. In the quail, high levels of retinal 5MT and 5MIAA were found at mid-dark, and in phase to that of melatonin. Biosynthetic pathways for retinal 5MT and 5MIAA in the rat and quail were discussed in relation to the diurnal rhythms observed. Our results indicate that the biosynthesis and physiological functions of retinal 5MT and 5MIAA could be species dependent.