The role of cortisol in the physiological response to toxicants in fish

当生物体暴露在应激源、有害或潜在有害的刺激下时，就会产生生理应激反应。皮质醇是一种皮质类固醇激素，是鱼类组织水平反应的关键效应器。尽管在包括环境毒物在内的许多物种和不同的应激源中对应激反应进行了研究，但基本的问题仍然存在：在暴露于毒物期间，应激反应对生物体是有益的还是有害的；对毒物的脆弱性是否与应激反应的稳健性有关；特定物种的敏感性与皮质醇应激反应的模式之间是否存在联系？这项拟议研究的目的是阐明皮质醇在鱼类对毒物的生理反应中的作用。我们将测试这样的假设，即血浆皮质醇的强劲但短暂的上升使有机体不太容易受到毒物的影响，以及皮质醇在急性潜在致命暴露下具有有益影响，但在长期暴露下是有害的，会产生代谢成本。将采用毒理学和应激生理学相结合的比较方法，针对对两种测试化学应激源--硒和草甘膦农药Roundup以及非化学应激源--温度和缺氧具有不同敏感度的黑头鳕鱼、虹鱼和河豚进行比较。拟议研究的一个中心焦点是皮质醇应激反应的特征及其对鱼类的毒理学后果。为了克服经典应激研究的局限性，将用一种非侵入性的方法对每条鱼进行多次皮质醇测量，方法是通过鳃将激素从血液释放到保持水分。血液皮质醇的基线，峰值的大小和持续时间，以及在暴露于测试毒物期间恢复到基线的时间，将在每条鱼身上表现出特征。在三种鱼类模型中，将使用多元统计分析确定一系列终点(包括生存、生长、能量储备、激素状态、氧化应激)的毒理学脆弱性(以EC50表示，引起50%反应的毒物浓度)与皮质醇反应模式之间的关系。下一步，应激反应将被药理学操纵，要么通过合成皮质醇(氢化可的松)治疗来提高血浆皮质醇水平，要么通过11B水解酶阻滞剂美替拉酮来阻断皮质醇的分泌，或者通过类固醇拮抗剂RU 486来阻断皮质醇受体，并将评估皮质醇滴度与毒物易感性之间的关系。最后，为了进一步评估皮质醇反应模式与应激源易感性之间的联系，非化学应激源(温度、低氧)将用于愚头鱼(暖水物种)和虹鲑鱼(冷水物种)。这项整合了毒理学和应激生理学的研究，将为皮质醇在应激源的生理反应中的作用以及调节物种对应激源敏感性差异的机制提供新的理解。

The physiological stress response is elicited when organisms are exposed to stressors, harmful or potentially harmful stimuli. Cortisol, a corticosteroid hormone, is a key effector of the response at tissue level in fish. Even though the stress response has been investigated in many species and diverse stressors, including environmental toxicants, fundamental questions remain: is the stress response beneficial or detrimental to the organism during exposure to toxicants; is vulnerability to toxicants related to robustness of the stress response; is there a link between species-specific sensitivity and the pattern of the cortisol stress response? The goal of the proposed research is to elucidate the role of cortisol in the physiological response to toxicants in fish. We will test the hypotheses that a robust but brief elevation of plasma cortisol makes the organism less vulnerable to a toxicant and that cortisol has beneficial effects under acute potentially lethal exposures, but is detrimental, engendering metabolic costs, under chronic exposures. A comparative approach integrating toxicology and stress physiology will be used, with fathead minnow, rainbow trout and brook trout, species with different sensitivities to two test chemical stressors, selenium and glyphosate pesticide Roundup, and non chemical stressors, temperature and hypoxia. A central focus of the proposed studies is the characterization of the cortisol stress response and its toxicological consequences for the fish. To overcome the limitations of classical stress studies, multiple cortisol measurements on individual fish will be made with a non invasive method using release of the hormone across gills from blood to holding water. The baseline of blood cortisol, the magnitude and duration of the peak, and return to baseline during exposure to test toxicants will be characterized in individual fish. The relationship between toxicological vulnerability (expressed as EC50, concentration of toxicants eliciting 50% of a response) in a suite of endpoints including survival, growth, energy reserves, hormonal status, oxidative stress, and the pattern of the cortisol response will be determined in the three fish models, using multivariate statistical analyses. Next, the stress response will be manipulated pharmacologically to either elevate plasma cortisol mimicking high stress scenario by treatment with synthetic cortisol (hydrocortisone), block cortisol secretion by metyrapone (blocker of 11B hydrolase), or block cortisol receptors by RU 486 (steroid antagonist), and the relationship between cortisol titres and and vulnerability to the toxicants will be evaluated. Last, to further evaluate the link between the pattern of the cortisol response and vulnerability to a stressor, non chemical stressors (temperature, hypoxia) will be used in fathead minnow (warm water species) and rainbow trout (cold water species). The proposed research, integrating toxicology and stress physiology, will provide new understanding of the role cortisol in the physiological response to stressors, and of the mechanisms mediating species differences in sensitivity to stressors.