Acute stress differentially affects the reproductive neuroendocrine systems of the European sea bass in a daily- and region-dependent manner

Abstract

Although the influence of stress on fish reproduction has been widely addressed, scarce information is available to date on the effects of acute stress challenges throughout the daily cycle on the main neuroendocrine systems involved in the control of fish reproduction. In this study, we analyzed how acute stress differentially affects the gonadotropin-inhibitory hormone (Gnih), gonadotropin-releasing hormone (Gnrh1, Gnrh2, Gnrh3) and kisspeptin (Kiss2) systems of the European sea bass (Dicentrarchus labrax) and how this response is related to brain regions and time of day. The experiments were conducted in December, under natural photoperiod and water temperature conditions. Sea bass were sampled over a 24-h cycle at Zeitgeber time (ZT) 0.5, 4, 7.5, 12, 16 and 20 h. At each time point, an untreated control group (CTRL) was sampled, while a stressed group was exposed to air for 1 min, returned to the tank and sampled 1 h later. Fish were sacrificed and brains dissected to collect telencephalon (olfactory bulbs and cerebral hemispheres), diencephalon (preoptic area and hypothalamus) and midbrain/hindbrain samples. In the different brain regions, the transcript levels of gnih and its receptor (gnihr), gnrh1, gnrh2, gnrh3 and/or kiss2 were determined by quantitative RT-PCR (qPCR). Our results showed significant effects of acute stress on the gnih (up-regulation), gnrh2 (up-regulation) and gnrh3 (down-regulation) gene expression of sea bass, which were dependent on the brain area analyzed. Furthermore, the stress response was significantly influenced by the time of day at which the stressor was applied, with most stress responses being higher at the end of the day and especially at night. The rhythmic profile of Gnih was markedly affected by the acute stressor, whereas the gnrh3 daily rhythm of transcript levels disappeared in the stress group, as determined by cosinor analysis. Our results reveal that the brain Gnih, Gnrh2 and Gnrh3 populations can respond to an acute stress challenge in sea bass in a region- and time-dependent manner. These results suggest that acute stress could have a significant impact on the reproductive axis of sea bass by affecting daily rhythms of expression of key reproductive neuroendocrine systems, with these effects being less pronounced at the beginning of the active phase in this generally-considered diurnal species.