Green, I. K. | Search bioRxiv

bioRxiv2023

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.

neuroscience↗

bioRxiv2022

The canonical HPA axis contributes to locomotion during photoadaptation but is not required

The hypothalamic-pituitary-adrenal (HPA) axis and its effector molecules--glucocorticoids--modulate diverse aspects of physiology in vertebrates ranging from metabolism to immune function, behavior, and circadian cycle. While the glucocorticoid receptor (nr3c1) is known to be involved in light adaptation (photoadaptation) of the retinal cells, the role of nr3c1 and other HPA axis signaling molecules in the behavioral phenotypes observed during photoadaptation have not been delineated. Therefore, we investigated locomotor adaptation to various light/dark durations using larval zebrafish that carry a mutated allele in key HPA axis receptors. First, we established baseline locomotion and compared WT and nr3c1 mutant larvae in constantly lit and dark conditions (for 12-hrs). WT larval zebrafish showed highest locomotor activity in the middle of the day and low activity during the early and later parts of the day, which was generally higher in the light. The baseline locomotion was depressed in nr3c1 mutants throughout the day in both environments with a more significant change noted in the light. Next, groups of larvae mutant in nr3c1, nr3c2 (mineralocorticoid receptor), or mc2r (melanocortin receptor type 2; adrenocorticotropic hormone (ACTH) receptor) along with their wildtype (WT) siblings were acclimated in the dark and underwent four cycles of dark-light illumination changes with different durations of illumination: 7.5, 6, 4, or 2 min. The nr3c1 and mc2r mutant fish showed significantly decreased locomotion during the dark phase when the illumination was provided for 4 or 2 min. However, with 4 min or longer illumination, if locomotor deficits were observed in mutant larvae, they demonstrated a "catch-up" phenotype with increasing locomotion during the later dark phases of the assay and ultimately reaching the swimming distances indistinguishable from their WT siblings in the assays with 7.5-min illumination. Finally, we looked at effects of light intensity, beginning with our short light exposure. A lower intensity of light failed to elicit any response even from WT fish after 1-min illumination. However, this dim light still evoked a robust response from nr3c1 mutants and their WT siblings after 7.5-min illumination. The nr3c2 mutant larvae consistently showed locomotor response similar to their WT siblings regardless of the length of the light phase. Thus, activation of the canonical HPA axis (i.e. nr3c1, mc2r) was necessary to induce a rapid locomotor adaptation following after light to dark transition with shorter light exposure times (~[≤]4 min), but it was not needed for locomotor responses observed after longer exposure to light (~> 4 min). Moreover, the locomotor response after transitioning to dark from a short (1 min) exposure was dependent on the intensity of light, whereas the longer exposure was not. Together, these findings suggest that locomotion after longer exposure to light is not dependent on the HPA axis, suggesting that either parallel independent pathway(s) are responsible for the locomotor response in these cases or that the HPA axis facilitates a primary pathway to increase sensitivity to light exposure.

neuroscience↗