Environmental & Life Sciences Graduate Program, Trent University, Peterborough, ON K9J 0G2, Canada.
Ontario Ministry of Natural Resources and Forestry, Trent University, DNA Building, Peterborough, ON K9J 0G2, Canada.
Gen Comp Endocrinol. 2018 Sep 15;266:78-86. doi: 10.1016/j.ygcen.2018.04.026. Epub 2018 May 12.
Southern flying squirrels have higher circulating cortisol levels than most vertebrates. However, regulation of tissue exposure to cortisol by the hormone's carrier protein, corticosteroid-binding globulin (CBG), appears to be altered due to lower-than-expected CBG expression levels, and a reduced affinity for cortisol. To assess the capacity of flying squirrels to regulate acute stress-mediated cortisol levels, we used the dexamethasone (DEX) suppression test followed by the adrenocorticotropic hormone (ACTH) stimulation test in both the breeding and non-breeding seasons, and quantified resultant changes in plasma cortisol and relative CBG levels. Regulation of cortisol via negative feedback, and the acute stress response appeared to function as they do in other vertebrates during the breeding season, but response to DEX in the non-breeding season showed that the sensitivity of the negative feedback mechanism changed across seasons. The relatively high concentrations of DEX required to induce negative feedback suggests that southern flying squirrels have a reduced sensitivity to cortisol compared with other vertebrates, and that high circulating cortisol levels may be required to compensate for low target tissue responsiveness in this species. Cortisol, but not CBG levels, were higher during the non-breeding than breeding season, and females had higher cortisol and CBG levels than males. Our data suggest that flying squirrel cortisol levels are regulated by negative feedback at a higher set point than in related species. Seasonal changes in cortisol levels, target tissue sensitivity to DEX, and in the capacity to respond to stressors appear to be part of the underlying physiology of southern flying squirrels, and may be required to maximize fitness in the face of tradeoffs between survival and reproduction.
南方飞鼠的循环皮质醇水平高于大多数脊椎动物。然而，由于 CBG 表达水平低于预期，以及对皮质醇的亲和力降低，皮质醇载体蛋白（CBG）对组织暴露于皮质醇的调节似乎发生了改变。为了评估飞鼠调节急性应激介导的皮质醇水平的能力，我们在繁殖和非繁殖季节都使用地塞米松（DEX）抑制试验和促肾上腺皮质激素（ACTH）刺激试验，定量分析血浆皮质醇和相对 CBG 水平的变化。通过负反馈调节皮质醇，以及急性应激反应，在繁殖季节似乎与其他脊椎动物的作用方式相同，但在非繁殖季节对 DEX 的反应表明，负反馈机制的敏感性在不同季节发生了变化。需要相对较高浓度的 DEX 来诱导负反馈，这表明南方飞鼠对皮质醇的敏感性低于其他脊椎动物，并且高水平的循环皮质醇可能需要补偿该物种中靶组织反应性低的问题。非繁殖季节的皮质醇，但不是 CBG 水平，高于繁殖季节，并且雌性的皮质醇和 CBG 水平高于雄性。我们的数据表明，飞鼠的皮质醇水平通过负反馈调节在高于相关物种的设定点上进行。皮质醇水平、DEX 靶组织敏感性以及对应激源的反应能力的季节性变化似乎是南方飞鼠生理机能的一部分，并且可能需要在生存和繁殖之间的权衡中最大限度地提高适应性。
