Fish Scales Produce Cortisol upon Stimulation with ACTH

SIMPLE SUMMARY: Cortisol is the most commonly studied acute stress indicator in fish. Recently, it has been shown that cortisol can be measured in fish scales, and its concentration can reliably indicate a response to chronic stress. The mechanism of the increase of cortisol concentration in scales has been proposed to be accumulation through circulation; however, no study has so far examined whether the scales can produce and release cortisol. Therefore, in the present study the ability of isolated fish scales to produce and secrete cortisol after incubation with cortisol-stimulating ACTH was examined. Results show that ACTH administration increased cortisol release in a dose-dependent manner, an effect that was reversed when scales were incubated with an inhibitor of cortisol production, i.e., metyrapone. This is the first study to report peripheral cortisol producing abilities in fish scales. ABSTRACT: Cortisol concentration in fish scales is a novel and reliable indicator of chronic stress. However, until now cortisol in scales has been considered to be accumulated through the circulation and it has not yet been studied whether it can be de novo produced from cells found in the scales. In the current study, scales of European sea bass, Dicentrarchus labrax, were stimulated in-vitro with a range of concentrations of adrenocorticotropic hormone (ACTH) to investigate if they can produce and release cortisol. Moreover, scales were exposed to a combination of ACTH and metyrapone, an inhibitor of cortisol production, to examine whether cortisol was actually produced in the scales. Results from ACTH administration showed that scales increased their cortisol release in a dose-dependent manner. This effect was reversed when scales were co-incubated with ACTH and metyrapone, indicating that cortisol was produced de novo and not released only upon stimulation with ACTH.