Presence of immunoreactive melatonin in different tissues of the pigeon (Columba livia)

The distribution of melatonin was studied at midday and midnight in pigeon plasma, pineal, hypothalamus, eyes, Harderian glands, and duodenum. Tissue samples were homogenized, extracted with chloroform, evaporated, and measured by our melatonin radioimmunoassay (RIA). All the extracts gave dilution curves parallel to that of synthetic melatonin and eluted in reverse-phase high-performance liquid chromatography (HPLC) as synthetic melatonin indicating that the immunoreactivity was due to endogenous melatonin in pigeon tissues. Melatonin was found to be present in pigeon plasma, pineal, hypothalamus, eyes, Harderian glands, and duodenum. In all tissues it had a clear diurnal variation with low levels at midday and high levels at midnight. The highest amounts of melatonin were found in the eyes, duodenum, and the pineal so that extrapineal melatonin exceeded pineal melatonin. Further studies are needed to evaluate the significance of extrapineal melatonin.