Endocrine Abstracts

Circadian rhythms are a ubiquitous feature in living organisms. They are not just a passive response to daily cyclic fluctuations in the environment, but are instead driven by endogenous and self-sustained clocks. More than 40 years ago, the suprachiasmatic nucleus (SCN) of the hypothalamus was identified as the location of an endogenous circadian clock driving internal rhythmicity and this SCN oscillator was long thought to be the only mammalian clock, functioning as a master pacemaker. It is now well recognized that peripheral circadian oscillators are present in virtually every organ and cell of the body and that optimal functioning of the circadian system is dependent on the tight synchronization of the central and peripheral pacemakers. The tempo between central and peripheral clocks is transmitted by multiple pathways, including the hard-wired autonomous nervous system, the release of hormonal signals in the peripheral circulation (notably glucocorticoids and melatonin) and behaviors such as sleeping and eating. Circadian misalignment refers to abnormal timing between SCN rhythms, peripheral rhythms and behavior (e.g. sleeping during the biological day, eating during the biological night) and the environment (e.g. exposure to light during the biological night). Conditions of circadian misalignment abound in modern society and are invariably associated with metabolic and sleep disturbances. In the present talk, we will review the impact of impaired light perception (as occurs in diabetic retinopathy) and abnormal rhythmicity of circulating corticosteroids (as occurs in adrenal insufficiency) on endocrine and metabolic function. Novel therapeutic approaches, including timed melatonin administration and chronophysiological delivery of exogenous glucocorticoids will be discussed.