Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Abstract

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue—adrenocorticotropic hormone (ACTH)—as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Reference Intervals for Blood Biomarkers in Farmed Atlantic Salmon, Coho Salmon and Rainbow Trout in Chile: Promoting a Preventive Approach in Aquamedicine

“…The plasma COR concentration in our study was significantly higher in seawater than in freshwater regardless of species, although the highest concentration was observed in Atlantic salmon. Overall, these values are slightly higher than those described in salmonids in experimental studies [84,85], included during the smoltification process [86], but are similar to those described in field conditions [30]. Under field conditions in Chile, higher plasma cortisol concentrations have been described in Atlantic salmon infested by parasitic copepod Caligus rogercresseyi [87].…”

Effects of seawater and freshwater challenges on the Gh/Igf system in the saline-tolerant blackchin tilapia (Sarotherodon melanotheron)

“…Osmoregulation is an essential physiological adaptation required for health and reproduction of numerous euryhaline fish species ( 23 , 47 – 51 ). The blackchin tilapia is an euryhaline species, which has evolved particular physiological adaptations to drastic salinity changes ( 52 ).…”

New Insights Into the Evolution of Corticotropin-Releasing Hormone Family With a Special Focus on Teleosts

“…The authors showed that crh1ba (in their article crf1b1) was the most abundant in the post-smolt brain but that the four paralogs may be involved in the response to various stress exposures (hypoxia, chasing, combination of both and confinement) (23). Crh1ba (in their article crf-b1) was also suggested to be involved in the activation of the pituitary interrenal axis leading to the elevation of cortisol at smoltification (53).…”