Blood flow directionality establishes the SCN as source and OVLT as target within a new vascular portal pathway

The suprachiasmatic nucleus (SCN) is the locus of a brain clock that sets the phase of oscillation in cells throughout the brain and body. Anatomical evidence reveals a portal system linking the SCN and the OVLT (here termed SCN-OVLTp). This discovery begs the question of the direction of blood flow and the nature of diffusible signals that flow in this specialized vasculature. Here we show unequivocally that the direction of blood flow is from the SCN to the OVLT, that the rate of flow is under circadian regulation, and that vasopressin (AVP) is present in portal vessels following systemic injection. These findings highlight a previously unknown CNS communication pathway. It is well established that the SCN is required for circadian regulation of AVP in the CSF and that the OVLT bears AVP receptors. Specifically, SCN neurons are necessary for time-stamped signals such as the peptide AVP, that can travel via portal veins to a target in the OVLT. The OVLT, a circumventricular organ offering a “window to the brain,” can relay neural and diffusible signals to broad brain areas via its efferent connections and via the CSF. We conclude that the SCN-OVLTp, like that of the pituitary portal system, discovered almost a century ago, allows neurosecretions to reach nearby specialized target sites, thereby avoiding dilution in the systemic blood. In both of these brain portal pathways, the target site, namely the pituitary and OVLT respectively, relay signals broadly, to both the brain and the rest of the body. ### Competing Interest Statement The authors have declared no competing interest.