The brain clock portal system: SCN-OVLT

Vascular portal systems are key structures, necessary for transporting products directly from the capillary bed of one region to the capillary bed of another region in high concentrations, without first returning to the heart. The only known portal systems in the brain is the hypophyseal-pituitary portal system, a structure necessary for survival and reproduction. Secretions from specific populations of hypothalamic neurons travel into fenestrated capillaries of the median eminence (ME) and thence drain into portal veins which break up into the secondary capillary plexus of the anterior pituitary. Neurons of the hypothalamic suprachiasmatic nucleus (SCN), locus of the brain’s master clock, also produces secretions deeply implicated in health and survival (Karatsoreos, 2019; Musiek & Holtzman, 2016). Here we describe a portal system connecting the SCN and organum vasculosum of the lamina terminalis (OVLT) - a circumventricular organ (CVO). CVOs lie around ventricles and lack a blood-brain barrier, enabling communication between the blood, brain, and cerebrospinal fluid. This “clock portal system” points to entirely new routes and targets for secreted signals, restructuring our understanding of brain communication pathways. Whether any of the remaining six CVOs in the mammalian brain bear portal systems is yet to be determined.