Becquet Denis, Girardet Clémence, Guillaumond Fabienne, François-Bellan Anne-Marie, Bosler Olivier
CNRS-UMR 6544, IFR Jean-Roche, Faculté de médecine, Université de la Méditerranée, Marseille, France.
Glia. 2008 Feb;56(3):294-305. doi: 10.1002/glia.20613.
Circadian rhythms in mammals are synchronized to the light (L)/dark (D) cycle through messages relaying in the master clock, the suprachiasmatic nucleus of the hypothalamus (SCN). Here, we provide evidence that the SCN undergoes rhythmic ultrastructural rearrangements over the 24-h cycle characterized by day/night changes of the glial, axon terminal, and/or somato-dendritic coverage of neurons expressing arginine vasopressin (AVP) or vasoactive intestinal peptide (VIP), the two main sources of SCN efferents. At nighttime, we noted an increase in the glial coverage of the dendrites of the VIP neurons (+29%) that was concomitant with a decrease in the mean coverage of the somata (-36%) and dendrites (-43%) of these neurons by axon terminals. Conversely, glial coverage of the dendrites of AVP neurons decreased (-19%) with parallel increase in the extent of somatal (+96%) and dendritic (+52%) membrane appositions involving these neurons. These plastic events were concomitant with daily fluctuations in quantitative expression of glial fibrillary acidic protein (GFAP), which were then used as an index of structural plasticity. The GFAP rhythm appeared to be strictly dependent on light entrainment, indicating that structural reorganization of the SCN may subserve synchronization of the clock to the L/D cycle. Other results presented reinforced this view while showing that circulating glucocorticoid hormones, which are known to modulate photic entrainment, were required to maintain amplitude of the GFAP rhythm to normal values.
哺乳动物的昼夜节律通过在下丘脑视交叉上核（SCN）这个主时钟中传递的信息与光/暗周期同步。在此，我们提供证据表明，SCN在24小时周期内会经历有节律的超微结构重排，其特征是表达精氨酸加压素（AVP）或血管活性肠肽（VIP）的神经元的神经胶质、轴突终末和/或体树突覆盖范围出现昼夜变化，而AVP和VIP是SCN传出纤维的两个主要来源。在夜间，我们注意到VIP神经元树突的神经胶质覆盖增加（+29%），与此同时，这些神经元的胞体（-36%）和树突（-43%）的轴突终末平均覆盖减少。相反，AVP神经元树突的神经胶质覆盖减少（-19%），而涉及这些神经元的胞体（+96%）和树突（+52%）膜贴附范围则平行增加。这些可塑性事件与胶质纤维酸性蛋白（GFAP）定量表达的每日波动同时发生，GFAP随后被用作结构可塑性的指标。GFAP节律似乎严格依赖于光的同步，这表明SCN的结构重组可能有助于时钟与光/暗周期的同步。展示的其他结果强化了这一观点，同时表明已知可调节光同步的循环糖皮质激素是将GFAP节律的振幅维持在正常值所必需的。
