Modern life comes with modern perils... International travel and night shifts both disrupt the circadian rhythm, which can lead to more serious health issues, such as depression and even cancer (1)(2) in the longer-term. Circadian rhythm control stems from the suprachiasmatic nucleus (SCN) in the hypothalamus, which releases various hormones and neuropeptides to set the pace. But fine-tuning comes from the retina – specifically from intrinsically photosensitive retinal ganglion cells (RGCs) that convey light information directly to the SCN where the neurotransmitter glutamate is released (3). But it turns out that this isn’t the whole story...

Now, a team of researchers has discovered another subpopulation of RGCs in rats that communicate directly with the SCN, signaling with the neuropeptide hormone vasopressin (4) – and are aptly named vasopressin-expressing RGCs (VP-RGCs). After showing that these cells project directly into the SCN, the team demonstrated that expression of Fos, a transcription factor implicated in the regulation of vasopressin synthesis, was significantly higher in VP-RGCs following light stimulation and that, in turn, light-evoked vasopressin release enhanced both the responses of SCN neurons to light, and the expression of c-fos in the SCN. The team reports, “Vasopressin, well known to be an important output of the SCN, is also a time-dependent mediator of light information from the retina to the SCN.”