Malisch Jessica L, Breuner Creagh W, Gomes Fernando R, Chappell Mark A, Garland Theodore
Department of Biology, University of California, Riverside, CA 92521, USA.
Gen Comp Endocrinol. 2008 Apr 1;156(2):210-7. doi: 10.1016/j.ygcen.2008.01.020. Epub 2008 Feb 10.
In vertebrates, baseline glucocorticoid concentrations vary predictably on a diel basis, usually peaking shortly before the onset of activity. Presumably, circadian patterns in glucocorticoid secretion have evolved to match predictable rises in energetic need. In mice from lines selectively bred for high voluntary wheel-running, previous studies have reported that baseline plasma corticosterone concentrations at two different times during the photophase are elevated twofold above those of non-selected control lines. Here, we tested the hypothesis that the elevated daytime corticosterone levels could be explained by a shift in the circadian pattern of corticosterone levels. We measured baseline total plasma corticosterone levels, corticosteroid-binding globulin (CBG) capacity, and calculated free corticosterone levels (corticosterone not bound to corticosteroid-binding globulin and potentially biologically active) at six points during the 24-hour cycle in males on a 12:12 photoperiod. We also examined the daily pattern of both wheel-running and home-cage activity. Based on combined analysis of all six points, the circadian pattern of total corticosterone, corticosteroid-binding globulin, and free corticosterone levels did not significantly differ between high-runner and control mice (linetype * time interaction P=0.56, 0.45, and 0.55, respectively); however, all varied with time (all P<0.0001) and mice from the selected lines had significantly elevated total (P=0.0125) and free (P=0.0140) corticosterone, with no difference in CBG binding capacity (P=0.77). All mice were active primarily during the dark phase, and the factorial increase in activity of selected relative to controls lines was 2.33 for total daily wheel revolutions and 2.76 for total daily home-cage activity. The onset of the active period for both measures of locomotor activity coincided with peak total and free corticosterone levels in both selected and control lines. These findings lend support to our hypothesis that elevated circulating corticosterone levels have evolved as an adaptation to support increased locomotor activity in the selected lines.
在脊椎动物中，糖皮质激素的基础浓度在昼夜节律的基础上呈现可预测的变化，通常在活动开始前不久达到峰值。据推测，糖皮质激素分泌的昼夜节律模式已经进化，以适应能量需求的可预测增加。在经过选择性培育以增加自愿性轮转运动的品系小鼠中，先前的研究报告称，在光照期的两个不同时间点，基础血浆皮质酮浓度比未选择的对照品系高出两倍。在此，我们检验了以下假设：白天皮质酮水平升高可由皮质酮水平的昼夜节律模式转变来解释。我们在12:12光周期的雄性小鼠的24小时周期内的六个时间点测量了基础血浆总皮质酮水平、皮质类固醇结合球蛋白（CBG）容量，并计算了游离皮质酮水平（未与皮质类固醇结合球蛋白结合且可能具有生物活性的皮质酮）。我们还研究了轮转运动和笼内活动的每日模式。基于对所有六个时间点的综合分析，高运动量小鼠和对照小鼠之间总皮质酮、皮质类固醇结合球蛋白和游离皮质酮水平的昼夜节律模式没有显著差异（线型*时间交互作用P分别为0.56、0.45和0.55）；然而，所有这些都随时间变化（所有P<0.0001），并且选择品系的小鼠总皮质酮（P=0.0125）和游离皮质酮（P=0.0140）显著升高，而CBG结合能力没有差异（P=0.77）。所有小鼠主要在黑暗期活动，选择品系相对于对照品系的活动因子增加，总日轮转次数为2.33，总日笼内活动为2.76。两种运动活动测量指标的活跃期开始时间与选择品系和对照品系中总皮质酮和游离皮质酮水平的峰值一致。这些发现支持了我们的假设，即循环皮质酮水平升高是作为一种适应进化而来的，以支持选择品系中增加的运动活动。
