Light Input to the Mammalian Circadian Clock.

Circadian rhythms are 24-h cycles in physiology and behavior that occur in virtually all organisms. These processes are not simply driven by changes in the external environment as they persist under constant conditions, providing evidence for an internal biological clock. In mammals, this clock is located in the hypothalamic suprachiasmatic nuclei (SCN) and is based upon an intracellular mechanism composed of a transcriptional-translational feedback loop composed of a number of core clock genes. However, a clock is of no use unless it can be set to the correct time. The primary time cue for the molecular clock in the SCN is light detected by the eye. The photoreceptors involved in this process include the rods and cones that mediate vision, as well as the recently identified melanopsin-expressing photosensitive retinal ganglion cells (pRGCs). Light information is conveyed to the SCN via the retinohypothalamic tract, resulting in an intracellular signaling cascade which converges on cAMP-response elements in the promoters of several key clock genes. Over the last two decades a number of studies have investigated the transcriptional response of the SCN to light stimuli with the aim of further understanding these molecular signaling pathways. Here we provide an overview of these studies and provide protocols for studying the molecular responses to light in the SCN clock.