The suprachiasmatic nucleus (SCN) is the master circadian clock of mammals, generating and transmitting an internal representation of environmental time that is produced by the cell-autonomous transcriptional/post-translational feedback loops (TTFLs) of the 10,000 neurons and 3500 glial cells. Recently, we showed that TTFL function in SCN astrocytes alone is sufficient to drive circadian timekeeping and behavior, raising questions about the respective contributions of astrocytes and neurons within the SCN circuit. We compared their relative roles in circadian timekeeping in mouse SCN explants, of either sex. Treatment with the glial-specific toxin fluorocitrate revealed a requirement for metabolically competent astrocytes for circuit-level timekeeping. Recombinase-mediated genetically complemented Cryptochrome (Cry) proteins in Cry1-deficient and/or Cry2-deficient SCNs were used to compare the influence of the TTFLs of neurons or astrocytes in the initiation of de novo oscillation or in pacemaking. While neurons and astrocytes both initiated de novo oscillation and lengthened the period equally, their kinetics were different, with astrocytes taking twice as long. Furthermore, astrocytes could shorten the period, but not as potently as neurons. Chemogenetic manipulation of Gi- and Gq-coupled signaling pathways in neurons acutely advanced or delayed the ensemble phase, respectively. In contrast, comparable manipulations in astrocytes were without effect. Thus, astrocytes can initiate SCN rhythms and bidirectionally control the SCN period, albeit with lower potency than neurons. Nevertheless, their activation does not influence the SCN phase. The emergent SCN properties of high-amplitude oscillation, initiation of rhythmicity, pacemaking, and phase are differentially regulated: astrocytes and neurons sustain the ongoing oscillation, but its phase is determined by neurons.

SIGNIFICANCE STATEMENT The hypothalamic suprachiasmatic nucleus (SCN) encodes and disseminates time-of-day information to allow mammals to adapt their physiology to daily environmental cycles. Recent investigations have revealed a role for astrocytes, in addition to neurons, in the regulation of this rhythm. Using pharmacology, genetic complementation, and chemogenetics, we compared the abilities of neurons and astrocytes in determining the emergent SCN properties of high-amplitude oscillation, initiation of rhythmicity, pacemaking, and determination of phase. These findings parameterize the circadian properties of the astrocyte population in the SCN and reveal the types of circadian information that astrocytes and neurons can contribute within their heterogeneous cellular network.

视交叉上核 (SCN) 是哺乳动物的主要生物钟，生成并传输环境时间的内部表示，该内部表示由 10,000 个神经元和 3500 个神经胶质细胞的细胞自主转录/翻译后反馈环路 (TTFL) 产生。最近，我们发现，仅 SCN 星形胶质细胞中的 TTFL 功能就足以驱动昼夜节律计时和行为，这引发了关于星形胶质细胞和神经元在 SCN 回路中各自贡献的问题。我们比较了它们在小鼠 SCN 外植体（无论性别）的昼夜节律计时中的相对作用。用神经胶质特异性毒素氟柠檬酸盐治疗表明，星形胶质细胞需要具有代谢能力的星形胶质细胞来进行电路级计时。使用 Cry1 缺陷和/或 Cry2 缺陷 SCN 中重组酶介导的基因互补隐花色素 (Cry) 蛋白来比较神经元或星形胶质细胞 TTFL 对从头振荡启动或起搏的影响。虽然神经元和星形胶质细胞都启动从头振荡并同等地延长周期，但它们的动力学不同，星形胶质细胞花费的时间是其两倍。此外，星形胶质细胞可以缩短该周期，但不如神经元那么有效。神经元中 Gi 和 Gq 耦合信号通路的化学遗传学操作分别急剧提前或延迟了整体阶段。相比之下，星形胶质细胞中的类似操作没有效果。因此，星形胶质细胞可以启动 SCN 节律并双向控制 SCN 周期，尽管其效力低于神经元。然而，它们的激活并不影响 SCN 阶段。 高振幅振荡、节律启动、起搏和相位等新兴的 SCN 特性受到不同的调节：星形胶质细胞和神经元维持持续的振荡，但其相位由神经元决定。

意义声明下丘脑视交叉上核 (SCN) 编码并传播一天中的时间信息，使哺乳动物能够调整其生理机能以适应日常环境周期。最近的研究揭示了除了神经元之外，星形胶质细胞在调节这种节律中的作用。利用药理学、遗传互补和化学遗传学，我们比较了神经元和星形胶质细胞在确定高振幅振荡、节律启动、起搏和相位确定等突发 SCN 特性方面的能力。这些发现参数化了 SCN 中星形胶质细胞群的昼夜节律特性，并揭示了星形胶质细胞和神经元在其异质细胞网络中可以贡献的昼夜节律信息类型。