Department of Biology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada.
Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada.
Gen Comp Endocrinol. 2020 Sep 1;295:113524. doi: 10.1016/j.ygcen.2020.113524. Epub 2020 Jun 8.
Early life stages are sensitive to environmental insults and changes during critical developmental periods; this can often result in altered adult behaviour and physiology. Examining the development of the hypothalamus-pituitary-interrenal (HPI) axis and its responsiveness, or lack thereof, during development are important for understanding the short- and long-term impacts of stressors on embryonic and larval fish. We examined the ontogeny of the HPI axis in embryonic (21, 38, 63, 83 and 103 days post-fertilisation (dpf)) and larval (1, 2, 3 and 4 weeks post-hatch (wph)) lake whitefish (Coregonus clupeaformis) by quantifying changes in mRNA levels of several genes associated with HPI axis functioning and whole animal cortisol levels throughout development and in response to a severe or mild hypoxic stress. Cortisol, and crh, crhbp1, pomc and star transcripts were detected from the earliest embryonic age studied. Cortisol levels in control embryos decreased between 21 and 63 dpf, suggesting the utilisation of maternal cortisol deposits. However, by 83 dpf (70% developed) endogenous de novo synthesis had generated a 4.5-fold increase in whole embryo cortisol. Importantly, we provide novel data showing that the HPI axis can be activated even earlier. Whole body cortisol increased in eyed lake whitefish embryos (38 dpf; ~32% developed) in response to hypoxia stress. Coincident with this hypoxia-induced increase in cortisol in 38 dpf embryos were corresponding increases in crh, crhbp1, pomc and star transcript levels. Beyond 38 dpf, the HPI axis in lake whitefish embryos was hyporesponsive to hypoxia stress at all embryonic ages examined (63, 83 and 103 dpf; 54, 72 and 85% developed, respectively). Post-hatch, larvae responded to hypoxia with an increase in cortisol levels and HPI axis genes at 1 wph, but this response was lost and larvae appeared hyporesponsive at subsequent ages (2, 3 and 4 wph). Collectively our work demonstrates that during fish embryogenesis and the larval stage there are windows where the HPI axis is responsive and windows where it is truly hyporesponsive; both could be beneficial in ensuring undisrupted development particularly in the face of increasing environmental changes.
早期生命阶段对外界环境的干扰和关键发育阶段的变化很敏感；这通常会导致成年后的行为和生理发生变化。研究下丘脑-垂体-肾上腺（HPI）轴的发育以及其在发育过程中的反应能力或缺乏反应能力，对于了解应激源对胚胎和幼鱼的短期和长期影响非常重要。我们通过定量检测 HPI 轴功能相关基因的 mRNA 水平和整个发育过程中以及对严重或轻度缺氧应激的反应中整个动物的皮质醇水平，研究了胚胎（受精后 21、38、63、83 和 103 天（dpf））和幼鱼（孵化后 1、2、3 和 4 周（wph））白鲑鱼（Coregonus clupeaformis）的 HPI 轴的个体发生。在研究的最早胚胎年龄就检测到了皮质醇和 crh、crhbp1、pomc 和 star 转录本。对照胚胎中的皮质醇水平在 21 至 63 dpf 之间下降，表明利用了母体皮质醇储存。然而，到 83 dpf（70%发育）时，内源从头合成产生了整个胚胎皮质醇的 4.5 倍增加。重要的是，我们提供了新的数据，表明 HPI 轴甚至可以更早被激活。在受缺氧胁迫的 38 dpf（约 32%发育）的白鲑鱼胚胎中，整个身体的皮质醇增加。与 38 dpf 胚胎中皮质醇诱导性增加相一致的是，crh、crhbp1、pomc 和 star 转录本水平相应增加。超过 38 dpf 后，在所有检查的胚胎年龄（63、83 和 103 dpf；分别为 54%、72%和 85%发育）下，白鲑鱼胚胎的 HPI 轴对缺氧应激反应迟钝。孵化后，1 周龄的幼鱼对缺氧应激会增加皮质醇水平和 HPI 轴基因，但这种反应消失，随后幼鱼的反应变得迟钝（2、3 和 4 周龄）。总的来说，我们的工作表明，在鱼类胚胎发生和幼鱼阶段，存在 HPI 轴反应的窗口和真正反应迟钝的窗口；这两者都有助于确保发育不受干扰，特别是在面对日益增加的环境变化时。
