Kynurenine promotes Calcitonin secretion and reduces cortisol in the Japanese flounder Paralichthys olivaceus

Abstract

Deep ocean water (DOW) exerts positive effects on the growth of marine organisms, suggesting the presence of unknown component(s) that facilitate their aquaculture. We observed that DOW suppressed plasma cortisol (i.e., a stress marker) concentration in Japanese flounder (Paralichthys olivaceus) reared under high-density condition. RNA-sequencing analysis of flounder brains showed that when compared to surface seawater (SSW)-reared fish, DOW-reared fish had lower expression of hypothalamic (i.e., corticotropin-releasing hormone) and pituitary (i.e., proopiomelanocortin, including adrenocorticotropic hormone) hormone-encoding genes. Moreover, DOW-mediated regulation of gene expression was linked to decreased blood cortisol concentration in DOW-reared fish. Our results indicate that DOW activated osteoblasts in fish scales and facilitated the production of Calcitonin, a hypocalcemic hormone that acts as an analgesic. We then provide evidence that the Calcitonin produced is involved in the regulatory network of genes controlling cortisol secretion. In addition, the indole component kynurenine was identified as the component responsible for osteoblast activation in DOW. Furthermore, kynurenine increased plasma Calcitonin concentrations in flounders reared under high-density condition, while it decreased plasma cortisol concentration. Taken together, we propose that kynurenine in DOW exerts a cortisol-reducing effect in flounders by facilitating Calcitonin production by osteoblasts in the scales.

Data on plasma cortisol levels in nibbler fish Girella punctata reared under high-density conditions in either surface seawater or deep ocean water

“…The plasma cortisol concentration of nibbler fish reared in SSW on day 10 was significantly higher than that on day 0, whereas those reared in DOW did not show significant difference on the respective days. The current data contributes to the generalization of the cortisol-reducing effect of DOW on fish, which has been proposed in Japanese flounder [1] . These data could be used for developing and designing experiments to analyze the mechanisms underlying the cortisol-reducing effects by using small fish such as zebrafish, a well-established animal model.…”

“…In aquaculture, measures to reduce stress should be considered to improve fish welfare [5] . Based on the current data and data from our recent study [1] , using DOW for rearing marine organisms is expected to solve problems associated with stress induced by high densities. Researchers and fish farmers working on optimizing fish-farming conditions and methodologies to reduce stress induced by high densities of farmed fish can benefit from our data.…”

“…In aquaculture, measures to reduce stress should be considered to improve fish welfare [5] . Based on the current data and data from our recent study [1] , using DOW for rearing marine organisms is expected to solve problems associated with stress induced by high densities.…”

“… Deep ocean water (DOW) is the water obtained from depth of >200 m below the surface of Earth's oceans and is characterized by rich nutrients and cleanliness [1 , 2] . We have recently reported that DOW suppresses the high-density-induced increase of plasma cortisol levels (i.e., a stress marker) in Japanese flounder ( Paralichthys olivaceus ) [1] .…”

“… Deep ocean water (DOW) is the water obtained from depth of >200 m below the surface of Earth's oceans and is characterized by rich nutrients and cleanliness [1 , 2] . We have recently reported that DOW suppresses the high-density-induced increase of plasma cortisol levels (i.e., a stress marker) in Japanese flounder ( Paralichthys olivaceus ) [1] . The current study aimed to examine whether the cortisol-reducing effect of DOW was observed in other marine organisms as well by comparing the plasma cortisol levels of nibbler fish Girella punctata reared under high-density conditions between surface seawater (SSW) and DOW.…”