Circadian clocks: regulators of endocrine and metabolic rhythms

Daily and seasonal rhythms in the endocrine system are co-ordinated by a hypothalamic pacemaker, the suprachias-matic nuclei (SCN) that is synchronised to solar tirne by direct retinal afferents. Individual SCN neurons are circadian clocks, their intrinsic oscillator consisting of a series of interfinked autoregulatory transcriptioinal/post-translatioiiaI feedback loops incorporating Period (Per) and Cryptochrome (Cry) genes. Mutations that alter the rate of transcription of Per and Cry genes or the stability of Per and Cry proteins affect clock speed. Molecular timekeeping in SCN neurons is synchronised and sustained by interneuronal neuropeptider-gic signals. A molecular clock mechanism comparable to that of the SCN is present in most major organ systems. These tissue clocks are synchronised by endocrine, autonomic and behavioural cues that are dependent on the SCN, and in turn they drive the circadian expression of local transcriptomes, thereby co-ordinating circadian metabolism and physiology. Rhythmic glucocorticoid signalling is a prominent mediator of SCN output and internal synchroniser. The role of local SCN-syrichronised clocks in controlling vital processes, including xenobiotic detoxification, cell division and nutrient metabolism, is essential to health, and disturbances to circadian timing arising from modern working schedules are becoming recognised as an increasingly relevant factor in major systemic illness. Moreover, the newly identified molecular components of circadian control systems provide novel avenues for therapeutic intervention.