Rat and mouse strains differ in behavioral and physiological characteristics, and such differences can contribute to explain discrepant results between laboratories and better select the most appropriate strain for a particular purpose. Differences in the activity of the hypothalamic-pituitary-adrenal (HPA) axis are particularly important given the pivotal role of this system in determining consequences of exposure to stressors. In this regard, Long-Evans (LE) rats are widely used in stress research, but there is no specific study aiming at thoroughly characterizing HPA activity in LE versus other extensively used strains. In a first experiment, LE showed higher resting ACTH and corticosterone levels only at certain points of the circadian rhythm, but much greater ACTH responsiveness to stressors (novel environment and forced swim) than Sprague-Dawley (SD) rats. Accordingly, enhanced corticotropin-releasing hormone (CRH) expression in the paraventricular nucleus of the hypothalamus and reduced expression of glucocorticoid receptors were observed in the hippocampal formation. Additionally, they are hyperactive in novel environments, and prone to adopt passive-like behavior when compared to SD rats. Supporting that altered HPA function has a marked physiological impact, we observed in another set of animals much lower thymus weight in LE than SD rats. Finally, to demonstrate that LE rats are likely to have higher HPA responsiveness to stressors than most strains, we studied resting and stress levels of HPA hormones in LE versus Wistar and Fischer rats, the latter considered an example of high HPA responsiveness. Again, LE showed higher resting and stress levels of ACTH than both Wistar and Fischer rats. As ACTH responsiveness to stressors in LE rats is stronger than that previously reported when comparing other rat strains and they are commercially available, they could be an appropriate model for studying the behavioral and physiological implications of a hyper-active HPA axis under normal and pathological conditions.

大鼠和小鼠品系的行为和生理特征不同，这种差异有助于解释实验室之间的差异结果，并更好地为特定目的选择最合适的品系。鉴于该系统在确定暴露于压力源的后果方面的关键作用，下丘脑-垂体-肾上腺 (HPA) 轴活动的差异尤为重要。在这方面，Long-Evans (LE) 大鼠被广泛用于应激研究，但没有专门的研究旨在彻底表征 LE 与其他广泛使用的菌株的 HPA 活性。在第一个实验中，LE 仅在昼夜节律的某些点显示出较高的静息 ACTH 和皮质酮水平，但与 Sprague-Dawley (SD) 大鼠相比，ACTH 对压力源（新环境和强迫游泳）的反应要大得多。因此，在海马结构中观察到下丘脑室旁核中促肾上腺皮质激素释放激素 (CRH) 表达增强，糖皮质激素受体表达减少。此外，与 SD 大鼠相比，它们在新环境中过度活跃，并且容易采取被动行为。支持改变HPA功能具有显着的生理影响，我们在另一组动物中观察到 LE 的胸腺重量远低于 SD 大鼠。最后，为了证明 LE 大鼠可能比大多数品系对压力源具有更高的 HPA 反应性，我们研究了 LE 与 Wistar 和 Fischer 大鼠中 HPA 激素的静息和应激水平，后者认为是高 HPA 反应性的一个例子。同样，LE 显示出比 Wistar 和 Fischer 大鼠更高的 ACTH 静息和压力水平。由于 LE 大鼠的 ACTH 对压力源的反应比之前报告的其他大鼠品系更强，而且它们是可商购的，因此它们可能是研究正常和病理条件下过度活跃的 HPA 轴的行为和生理影响的合适模型.