The central circadian clock of the suprachiasmatic nucleus as an ensemble of multiple oscillatory neurons.

Circadian rhythms are oscillations with approximately 24-h period that appear in most of physiological events in our body. The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the central circadian pacemaker in mammals and entrains to the environmental light/dark cycle. The SCN is a network structure composed of multiple types of γ-amino butyric acid (GABA)-ergic neurons and glial cells. Although individual SCN neurons have intracellular molecular machinery of circadian clock and the ability to oscillate cell-autonomously, interneuronal communications among these neurons are essential for the circadian pacemaking of the SCN. However, the mechanisms underlying the SCN network remain largely unknown. Here, I briefly review the molecular, cellular, and anatomical structures of the SCN and introduce recent studies aiming to understand the differential roles of multiple neuropeptides and neuropeptide-expressing neurons in the SCN network.