Summary:Circadian clocks are endogenous timers adjusting behaviour and physiology with the solar day 1 .Synchronized circadian clocks improve fitness 2 and are crucial for our physical and mental wellbeing 3 . Visual and non-visual photoreceptors are responsible for synchronizing circadian clocks to light 4,5 , but clock-resetting is also achieved by alternating day and night temperatures with only 2°-4°C difference [6][7][8] . This temperature sensitivity is remarkable considering that the circadian clock period (~24 h) is largely independent of surrounding ambient temperatures 1,8 .Here we show that Drosophila Ionotropic Receptor 25a (IR25a) is required for behavioural synchronization to low-amplitude temperature cycles. This channel is expressed in sensory neurons of internal stretch receptors previously implicated in temperature synchronization of the circadian clock 9 . IR25a is required for temperature-synchronized clock protein oscillations in subsets of central clock neurons. Extracellular leg nerve recordings reveal temperature -and IR25a-dependent sensory responses, and IR25a mis-expression confers temperature-dependent firing of heterologous neurons. We propose that IR25a is part of an input pathway to the circadian clock that detects small temperature differences. This pathway operates in the absence of known 'hot' and 'cold' sensors in the Drosophila antenna 10,11 , revealing the existence of novel periphery-to-brain temperature signalling channels. Main Text:In Drosophila, daily activity rhythms are controlled by a network of ~150 clock neurons expressing the clock genes period (per) and timeless (tim). These encode repressor proteins that 3 negatively feedback on their own promoters resulting in 24 h oscillations of clock molecules.Temperature cycles (TC) synchronise molecular clocks present in peripheral appendages in a tissue-autonomous manner 9,12 , while synchronization of clock neurons in the brain largely depends on peripheral temperature receptors located in the chordotonal organs (ChO) and the ChO-expressed gene nocte 9,12,13 .To discover novel factors involved in temperature entrainment, we identified NOCTEinteracting proteins by co-immunoprecipitation and mass-spectrometry (Extended Data Tab. 1) 14 . We focused on IR25a, a member of a divergent subfamily of ionotropic glutamate receptors and verified the interaction by co-immunoprecipitation after overexpressing IR25a and NOCTE in all clock cells using tim-gal4 (Extended Data Fig. 1a). IR25a is expressed in different populations of sensory neurons, including those in the antenna and labellum [15][16][17] . In the olfactory system IR25a acts as a co-receptor with different odour-sensing IRs 15 .To investigate if IR25a is co-expressed with nocte in ChO we analysed IR25a expression in femur and antennal ChO using an IR25a-gal4 line 15 (Extended Data Fig. 2a). IR25a-gal4 driven mCD8-GFP labelled subsets of ChO neurons in the femur, overlapping substantially with nompC-QF driven QUAS-Tomato signals (Fig. 1 a-c). nompC-QF is expressed in larv...
A sensitivity of the circadian clock to light/dark cycles ensures that biological rhythms maintain optimal phase relationships with the external day. In animals, the circadian clock neuron network (CCNN) driving sleep/activity rhythms receives light input from multiple photoreceptors, but how these photoreceptors modulate CCNN components is not well understood. Here we show that the HofbauerBuchner eyelets differentially modulate two classes of ventral lateral neurons (LNvs) within the Drosophila CCNN. The eyelets antagonize Cryptochrome (CRY)-and compound-eye-based photoreception in the large LNvs while synergizing CRY-mediated photoreception in the small LNvs. Furthermore, we show that the large LNvs interact with subsets of "evening cells" to adjust the timing of the evening peak of activity in a day length-dependent manner. Our work identifies a peptidergic connection between the large LNvs and a group of evening cells that is critical for the seasonal adjustment of circadian rhythms.
How do the pioneer networks in the axial core of the vertebrate nervous system first develop? Fundamental to understanding any full-scale neuronal network is knowledge of the constituent neurons, their properties, synaptic interconnections, and normal activity. Our novel strategy uses basic developmental rules to generate model networks that retain individual neuron and synapse resolution and are capable of reproducing correct, whole animal responses. We apply our developmental strategy to young Xenopus tadpoles, whose brainstem and spinal cord share a core vertebrate plan, but at a tractable complexity. Following detailed anatomical and physiological measurements to complete a descriptive library of each type of spinal neuron, we build models of their axon growth controlled by simple chemical gradients and physical barriers. By adding dendrites and allowing probabilistic formation of synaptic connections, we reconstruct network connectivity among up to 2000 neurons. When the resulting "network" is populated by model neurons and synapses, with properties based on physiology, it can respond to sensory stimulation by mimicking tadpole swimming behavior. This functioning model represents the most complete reconstruction of a vertebrate neuronal network that can reproduce the complex, rhythmic behavior of a whole animal. The findings validate our novel developmental strategy for generating realistic networks with individual neuron-and synapse-level resolution. We use it to demonstrate how early functional neuronal connectivity and behavior may in life result from simple developmental "rules," which lay out a scaffold for the vertebrate CNS without specific neuron-to-neuron recognition.
We have characterized a light-input pathway regulating Drosophila clock neuron excitability. The molecular clock drives rhythmic electrical excitability of clock neurons, and we show that the recently discovered light-input factor Quasimodo (Qsm) regulates this variation, presumably via an Na A ll organisms are subject to predictable but drastic daily environmental changes caused by the earth's rotation around the sun. It is critical for the fitness and well-being of an individual to anticipate these changes, and this anticipation is done by circadian timekeeping systems (clocks). These regulate changes in behavior, physiology, and metabolism to ensure they occur at certain times during the day, thereby adapting the organism to its environment (1). The circadian system consists of three elements: the circadian clock to keep time, inputs that allow entrainment, and outputs that influence physiology and behavior (2). Like a normal clock, circadian clocks run at a steady pace (24 h) and can be reset. In nature this environmental synchronization is done via daily light and temperature cycles, food intake, and social interactions (3).In Drosophila the central clock comprises 75 neuron pairs grouped into identifiable clusters that subserve different circadian functions (Fig. 1A). The molecular basis of the circadian clock is remarkably conserved from Drosophila to mammals (4). This intracellular molecular clock drives clock neurons to express circadian rhythms in electrical excitability, including variation in membrane potential and spike firing. Clock neurons are depolarized and fire more during the day than at night, and circadian changes in the expression of clock-controlled genes encoding membrane proteins such as ion channels and transporters likely contribute to these rhythms (5-8). Such cyclical variations in activity play a critical role in synchronizing different clock neurons and conveying circadian signals to other parts of the nervous system and body (9, 10). Furthermore, they provide positive feedback to the molecular clock, which can dampen rapidly without such feedback (7,11,12).Light resets the circadian clock every morning to synchronize the clock to the environment via Timeless (Tim) degradation after activation of the blue-light photoreceptor Cryptochrome (Cry), Quasimodo (Qsm), and potentially also visual photoreceptors (13-17). Qsm acts either independently or downstream of Cry and also is able to affect clock protein stability in Qsmnegative neurons by an unknown non-cell-autonomous mechanism (16). Recently Cry has been shown to regulate clock neuron excitability via the redox sensor of the Hyperkinetic voltage-gated potassium (K V )-β subunit (Hk) (18,19), and here we ask if Qsm affects the clock neurons in a similar way.Membrane potential is important for control of circadian behavior, and manipulation of Shaw and the Narrow Abdomen (NA) channels, both of which are expressed and function within clock neurons influence neuronal electrical activity, the circadian clock, and clockcontrolled behavi...
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