Regional concentrations of melatonin in the rat brain in the light and dark period.

The levels of melatonin in five brain regions, whole brain, pineal and serum samples were studied in rats adapted under a photoperiod of 12 h light and 12 h dark. It was found that the melatonin levels for all the tissues obtained in the dark period were significantly higher than those obtained in the light period. Regional study of melatonin levels in the brain in the light and dark period demonstrated a high level in the hypothalamus, intermediate levels in the mid-brain, cerebellum and pons-medulla and low level in the telencephalon. Our findings indicate that melatonin in the brain is unevenly distributed and that there are diurnal rhythms of melatonin in all the five brain regions studied.