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In animals, overt circadian rhythms of physiology and behavior are centrally regulated by a circadian clock located in specific brain regions. In the fruit fly Drosophila and in mammals, these clocks rely on single-cell oscillators, but critical for their function as central circadian pacemakers are network properties that change dynamically throughout the circadian cycle as well as in response to environmental stimuli. 1 , 2 , 3 In the fly, this plasticity involves circadian rhythms of expansion and retraction of clock neuron fibers. 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 Whether these drastic structural changes are a universal property of central neuronal pacemakers is unknown. To address this question, we studied neurons of the mouse suprachiasmatic nucleus (SCN) that express vasoactive intestinal polypeptide (VIP), which are critical for the SCN to function as a central circadian pacemaker. By targeting the expression of the fluorescent protein tdTomato to these neurons and using tissue clearing techniques to visualize all SCN VIPergic neurons and their fibers, we show that, similar to clock neurons in the fly, VIPergic fibers undergo a daily rhythm of expansion and retraction, with maximal branching during the day. This rhythm is circadian, as it persists under constant environmental conditions and is present in both males and females. We propose that circadian structural remodeling of clock neurons represents a key feature of central circadian pacemakers that is likely critical to regulate network properties, the response to environmental stimuli, and the regulation of circadian outputs., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

We show that nocturnal aversive stimuli presented to mice while they are eating and drinking outside of their safe nest can entrain circadian behaviors, leading to a shift toward daytime activity. We also show that the canonical molecular circadian clock is necessary for fear entrainment and that an intact molecular clockwork in the suprachiasmatic nucleus, the site of the central circadian pacemaker, is necessary but not sufficient to sustain fear entrainment of circadian rhythms. Our results demonstrate that entrainment of a circadian clock by cyclic fearful stimuli can lead to severely mistimed circadian behavior that persists even after the aversive stimulus is removed. Together, our findings support the interpretation that circadian and sleep symptoms associated with fear and anxiety disorders are, in part, the output of a fear-entrained clock, and provide a mechanistic insight into this clock., Competing Interests: Competing interests statement:The authors declare no competing interest.

The suprachiasmatic nucleus (SCN) of the hypothalamus is the site of a central circadian clock that orchestrates overt rhythms of physiology and behavior. Circadian timekeeping requires intercellular communication among SCN neurons, and multiple signaling pathways contribute to SCN network coupling. Gamma-aminobutyric acid (GABA) is produced by virtually all SCN neurons, and previous work demonstrates that this transmitter regulates coupling in the adult SCN but is not essential for the nucleus to sustain overt circadian rhythms. Here, we show that the deletion of the gene that codes for the GABA vesicular transporter Vgat from neuromedin-S (NMS) + neurons-a subset of neurons critical for SCN function-causes arrhythmia of locomotor activity and sleep. Further, NMS- Vgat deletion impairs intrinsic clock gene rhythms in SCN explants cultured ex vivo. Although vasoactive intestinal polypeptide (VIP) is critical for SCN function, Vgat deletion from VIP-expressing neurons did not lead to circadian arrhythmia in locomotor activity rhythms. Likewise, adult SCN-specific deletion of Vgat led to mild impairment of behavioral rhythms. Our results suggest that while the removal of GABA release from the adult SCN does not affect the pacemaker's ability to sustain overt circadian rhythms, its removal from a critical subset of neurons within the SCN throughout development removes the nucleus ability to sustain circadian rhythms. Our findings support a model in which SCN GABA release is critical for the developmental establishment of intercellular network properties that define the SCN as a central pacemaker., Competing Interests: Competing interests statement:The authors declare no competing interest.