Melatonin: Origin, Control of Circadian Rhythm and Site of Action

It has been postulated that melatonin could be secreted directly into the CSF as well as into the blood. Is the CSF route of secretion important? The pineal is a major source of circulating melatonin. Does circulating melatonin also originate from other structures? The alternating 24-hour light/dark cycle serves as the zeitgeber for the melatonin rhythm. What event is the cue? Does increasing the hours of darkness lead to enhanced melatonin release? What is the relation of the pineal circadian rhythm to other rhythmic phenomena including rhythms in other hormones, in temperature and in locomotion? Both membrane and cytosol binding of melatonin have been described. Which type of binding is most likely to represent the physiologic receptor?