Neurohypophysial hormones and circadian rhythm.

In conclusion, neurohypophysial hormone levels in human plasma and in the plasma, pituitary, and hypothalamus of the rat show clear rhythms over the 24-hour period. These are not true circadian rhythms, as they may be modified by a number of factors, including the stage of the estrous cycle and hydrational status. There is evidence to suggest that the patterns observed may in part reflect the action of the pineal product melatonin. Pinealectomy influences the pattern of hormone release, while melatonin and its analogues inhibit hormone release, although the major circulating metabolite has no effect.