Circadian Clock Signals in Endocrine Glands

Abstract

Hormones secreted from endocrine glands relay circadian timing signals produced in the suprachiasmatic nucleus (SCN) to the organs in the whole body. The SCN controls endocrine organs directly via the hypothalamic neuroendocrine system and indirectly via the autonomic nervous system. Hypothalamic neuroendocrine hormones, including corticotropin-releasing hormones and thyrotropin-releasing hormones, control pituitary hormones. Pineal melatonin, strictly controlled by sympathetic rhythms that intimately reflect circadian SCN rhythms, is released into the circulation and into the cerebrospinal fluid, regulating circadian and seasonal physiology. Adrenal glucocorticoids, regulated by pituitary hormones and sympathetic nerve activity, transmit central rhythmic signals to peripheral organs, and coordinate the timing of cellular metabolism in the whole body. Adrenal aldosterone, another hormone of the adrenal cortex, is rhythmically produced by cells in the zona glomerulosa of the adrenal cortex and controls circulation rhythms to maintain blood pressure (BP). Thus, hormones are involved in the generation of 24-hour rhythms using a unique control system that corresponds to their function.