Clocks within the Master Gland: Hypophyseal Rhythms and Their Physiological Significance

Various aspects of mammalian endocrine physiology show a time-of-day variation with a period of 24 h, which represents an adaptation to the daily environmental fluctuations resulting from the rotation of the earth. These 24-h rhythms in hormone abundance and consequently hormone function may rely on rhythmic signals produced by the master circadian clock, which resides in the suprachiasmatic nucleus and is thought to chiefly dictate the pattern of rest and activity in mammals in conjunction with the light/dark (LD) cycle. However, it is likely that clocks intrinsic to elements of the endocrine axes also contribute to the 24-h rhythms in hormone function. Here we review the evidence for rhythm generation in the endocrine master gland, the pituitary, and its physiological significance in the context of endocrine axes regulation and function.