Lynn Sharon E, Kern Michael D
The College of Wooster, Department of Biology, 931 College Mall, Wooster, OH 44691, USA.
Gen Comp Endocrinol. 2018 Oct 1;267:82-89. doi: 10.1016/j.ygcen.2018.06.003. Epub 2018 Jun 15.
Early life experiences can affect the function of the hypothalamo-pituitary-adrenal (HPA) axis of vertebrates, with potential fitness consequences later in life. In altricial species, for example, variation in parental behavior, e.g. brooding or feeding, can modify the activity of the HPA axis of the young by altering their exposure to noxious stimuli as the young develop in the nest. We have shown that a drop in the body temperature of eastern bluebird (Sialia sialis) chicks, such as occurs when females are away from the nest, elevates their blood corticosterone levels. If repeated during the early days of their development, cooling bouts also reduce the chicks' later corticosterone secretion in response to handling. Thus, variation in maternal behavior has the capacity to shape the function of the chicks' HPA axis. To better understand how maternal absence from the nest activates the HPA axis of bluebird chicks, we experimentally mimicked the cooling that occurs when the female is away from the nest, and investigated a) the age at which the HPA axis becomes capable of responding to cooling by increasing corticosterone secretion, b) whether corticosterone secretion remains elevated throughout long periods of cooling, and c) whether fasting (also potentially associated with maternal absence) interacts with cooling to affect corticosterone secretion. Cooling for 18 min significantly elevated circulating corticosterone levels of chicks as young as 4 days post-hatch, indicating that their HPA axis is sensitive to cooling very early in life. Corticosterone levels remained elevated throughout longer bouts of cooling. However, a 1-hr period of fasting had no effect on corticosterone secretion, regardless of whether chicks were cooled or not. Collectively, these data demonstrate that variation in maternal brooding behavior can substantially modify the corticosterone profiles of chicks during early postnatal development, and that chick temperature is likely the main driver of this.
早期生活经历会影响脊椎动物下丘脑 - 垂体 - 肾上腺（HPA）轴的功能，并可能对其后期生活的适应性产生影响。例如，在晚成雏物种中，亲代行为的变化，如育雏或喂食行为的变化，会通过改变幼雏在巢中发育时接触有害刺激的程度，从而改变幼雏HPA轴的活性。我们已经表明，东部蓝鸲（Sialia sialis）雏鸟的体温下降，比如当雌性离开巢穴时发生的情况，会使其血液中皮质酮水平升高。如果在雏鸟发育早期反复出现降温情况，那么降温期也会降低雏鸟后期对应处理时的皮质酮分泌。因此，母体行为的变化有能力塑造雏鸟HPA轴的功能。为了更好地理解母体离巢是如何激活蓝鸲雏鸟的HPA轴的，我们通过实验模拟了雌性离巢时发生的降温情况，并研究了：a）HPA轴能够通过增加皮质酮分泌来响应降温的年龄；b）皮质酮分泌在长时间降温过程中是否持续升高；c）禁食（这也可能与母体离巢有关）是否与降温相互作用以影响皮质酮分泌。18分钟的降温显著提高了孵化后4天的雏鸟的循环皮质酮水平，这表明它们的HPA轴在生命早期对降温非常敏感。在更长时间的降温过程中，皮质酮水平持续升高。然而，1小时的禁食对皮质酮分泌没有影响，无论雏鸟是否被降温。总体而言，这些数据表明，母体育雏行为的变化可以在雏鸟出生后的早期发育阶段显著改变其皮质酮水平，而雏鸟体温很可能是这一变化的主要驱动因素。
