In vivo determination of direction of blood flow in the newly discovered SCN-OVLT vascular portal system in rat

By transporting products directly from the capillary bed of one region to the capillary bed of another region, vascular portal pathways enable minute amounts of important secretions to reach their specialized targets in high concentrations, without dilution in the systemic circulatory system. For decades there has been only one known portal system in the mammalian brain - that of the pituitary gland, first identified in 1933 (Popa and Fielding, J. Anatomy 1933). This year, we described a second portal pathway in the mouse linking the capillary vessels of the brain's clock suprachiasmatic nucleus (SCN) to those of the organum vasculosum of the lamina terminalis (OVLT), a circumventricular organ (Yao et al., Nat. Comm. 2021). A caveat in this initial work was that the direction of blood flow was unknown. To determine whether the SCN signaled the OVLT or vice-versa, we performed in vivo 2-photon imaging in anesthetized eGFP-vasopressin (VP) rats using a recently developed approach (Roy et al., Cell Report 2021) to study blood flow in this portal system. To delineate the SCN microvasculature in vivo, we intravenously infused fluorescent dextrans in anesthetized rats. The SCN-OVLT portal system was identified as Alexa 633 (an artery/arteriole specific dye)-negative vessels originating from a dense SCN capillary network that run rostrally towards the OVLT. These vessels displayed a mean diameter of ~20 μm. Blood flow in the portal vessels was measured by monitoring red blood cell (RBC) movement after intravenous injections with Rho70 kDa. Using kymographs, we found that in all cases, RBCs flowed rostrally, from the SCN towards the OVLT. Importantly, we found than blood flow was significantly higher at night (ZT17-19) compared to daylight (ZT5-7) (p< 0.001), while directionality remained the same (SCN→OVLT). Taken together, our results support the presence of a functional SCN-OVLT portal system in the rat in which blood flows unidirectionally from the SCN towards the OVLT. Moreover, our studies support the notion that blood flow in this system can be regulated. This clock portal system points to entirely new routes and targets for secreted signals from the SCN, restructuring our understanding of its output pathways. Support: NIH HLBI R01HL162575 to JES, AHA916907 to RKR and NSF 1749500 to RS. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.