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

Abstract

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.