Periodic Reporting for period 4 - SleepCirc (Claustrum, Brainstem and Sleep: Mechanisms and Function)
Reporting period: 2024-01-01 to 2024-06-30
We address fundamental questions about sleep mechanisms, function and evolution, and about the role of the claustrum, a forebrain area with hypothetical roles in attention and consciousness. These results are interesting in part because the claustrum has an ill-defined evolutionary origin and because its potential involvement in sleep has, to our knowledge, not been reported previously.
Our proposal exploits three new and previously unrelated results from the reptilian brain. (i) We discovered recently that, in the lizard Pogona vitticeps, slow-wave and REM sleep alternate in a clock-like fashion, suggesting the existence of regular sleep pattern generators in the brainstem. (ii) More recently, a single-cell RNA sequencing study of the reptilian brain by our laboratory, hinted that a small pallial area may be homologous to the mammalian claustrum. This homology acquired functional importance in a third, independent finding, based on electrophysiological recordings: (iii) sharp-wave ripples, a hallmark of Pogona slow-wave sleep, can be generated autonomously from an area that corresponds precisely to the transcriptomically-identified claustrum. This convergence provides a potential clue about claustrum function and evolution, especially because, in mammals at least, the claustrum is densely interconnected with the rest of the brain, especially cortex. Since our results were published on claustrum and slow wave sleep were published, two reports have appeared indicating similar results in mice, and more recently, in humans.
Our project exploits the unique advantages of Pogona sleep, combining scRNA-seq, tract-tracing, optogenetic, behavioral and electrophysiological approaches.
Our proposal exploits three new and previously unrelated results from the reptilian brain. (i) We discovered recently that, in the lizard Pogona vitticeps, slow-wave and REM sleep alternate in a clock-like fashion, suggesting the existence of regular sleep pattern generators in the brainstem. (ii) More recently, a single-cell RNA sequencing study of the reptilian brain by our laboratory, hinted that a small pallial area may be homologous to the mammalian claustrum. This homology acquired functional importance in a third, independent finding, based on electrophysiological recordings: (iii) sharp-wave ripples, a hallmark of Pogona slow-wave sleep, can be generated autonomously from an area that corresponds precisely to the transcriptomically-identified claustrum. This convergence provides a potential clue about claustrum function and evolution, especially because, in mammals at least, the claustrum is densely interconnected with the rest of the brain, especially cortex. Since our results were published on claustrum and slow wave sleep were published, two reports have appeared indicating similar results in mice, and more recently, in humans.
Our project exploits the unique advantages of Pogona sleep, combining scRNA-seq, tract-tracing, optogenetic, behavioral and electrophysiological approaches.
In an earlier reporting period for this grant, we completed a study identifying the anterior medial DVR as a reptilian claustrum homolog and characterized its role in generating sharp-wave ripple activity during non-REM sleep. This study combined behavioral, electrophysiological, viral pathway tracing and single-cell RNA sequencing techniques and resulted in a major publication (Norimoto, Fenk et al., Nature 2020).
We continued our scRNAseq approaches on the reptilian brain by focusing on the entire telencephalon, diencephalon, midbrain and cerebellum, and comparing it to mammalian transcriptomes. Our main areas of focus are the hypothalamus and thalamus, because they deviate most from expectations with respect to similarity with their mammalian counterparts. This work was completed and published in Science in September 2022 (Hain, Gallego-Flores et al., Science 2022).
We initiated the construction of an atlas of the reptilian brain, combining microCT, traditional histological sections (DAPI, Nissl etc), immunohistochemical sections and high-dimensional in situs (>100 probes). This atlas is in the final stages of assembly and will be published both as a paper and as a searchable and updatable database.
Our work on sleep and the involvement of the claustrum in sleep control has also led us to investigate bilateral coordination during sleep. Despite the absence of communication between claustra, the two sides are very precisely coordinated between REM sleep, with tantalizing results on phase relationships. We demonstrated that while the two sides of the brain operate independently during slow wave sleep, they are precsisely coordinated during REM sleep, thanks to winner take all interactions between midbrain nuclei (the isthmus) that drive REM activity in the claustrum. These midbrain nuclei happen to be ones that are known in birds to underlie winner-take-all competition for unilateral gaze control, thus revealing a totally unexpected convergence of circuits for vision and brain activity during REM. These results were published in Nature late last year (Fenk, Riquelme and Laurent, Nature, 2023). We have studied the properties of the networks underlying the ultradian sleep rhythm. Exploiting the high frequency and regularity of a lizard’s biphasic sleep, we used perturbation experiments to probe this ultradian rhythm and test the hypothesis that it originates in a central-pattern-generator circuit. Such circuits are typically susceptible to phase-dependent reset and entrainment by external stimuli. Using light pulses, we found that Pogona’s ultradian rhythm 8 could be reset in a phase-dependent manner, with a critical transition from phase delay to phase advance in the middle of NREM. The ultradian rhythm frequency could be decreased or increased, within limits, by entrainment with light pulses. During entrainment, REM could be shortened but not lengthened, whereas NREM could be more flexibly dilated. In awake animals, a few alternating light/dark epochs matching the durations of sleep’s natural REM and NREM entrained an ultradian-like brain rhythm that persisted for one to a few cycles after the alternating visual stimuli had ceased, suggesting the transient activation of an ultradian rhythm generator. In sleeping animals, a short light pulse delivered to a single eye caused an immediate ultradian-rhythm reset, but only of the contralateral hemisphere; both sides resynchronized spontaneously within a few sleep cycles, indicating that sleep is controlled by paired rhythm-generating circuits linked by functional excitation. Together, these results indicate that central pattern generators of a type usually known to control motor rhythms may also organize the ultradian sleep rhythm in an amniote vertebrate. These results were recently submitted for publication.
We continued our scRNAseq approaches on the reptilian brain by focusing on the entire telencephalon, diencephalon, midbrain and cerebellum, and comparing it to mammalian transcriptomes. Our main areas of focus are the hypothalamus and thalamus, because they deviate most from expectations with respect to similarity with their mammalian counterparts. This work was completed and published in Science in September 2022 (Hain, Gallego-Flores et al., Science 2022).
We initiated the construction of an atlas of the reptilian brain, combining microCT, traditional histological sections (DAPI, Nissl etc), immunohistochemical sections and high-dimensional in situs (>100 probes). This atlas is in the final stages of assembly and will be published both as a paper and as a searchable and updatable database.
Our work on sleep and the involvement of the claustrum in sleep control has also led us to investigate bilateral coordination during sleep. Despite the absence of communication between claustra, the two sides are very precisely coordinated between REM sleep, with tantalizing results on phase relationships. We demonstrated that while the two sides of the brain operate independently during slow wave sleep, they are precsisely coordinated during REM sleep, thanks to winner take all interactions between midbrain nuclei (the isthmus) that drive REM activity in the claustrum. These midbrain nuclei happen to be ones that are known in birds to underlie winner-take-all competition for unilateral gaze control, thus revealing a totally unexpected convergence of circuits for vision and brain activity during REM. These results were published in Nature late last year (Fenk, Riquelme and Laurent, Nature, 2023). We have studied the properties of the networks underlying the ultradian sleep rhythm. Exploiting the high frequency and regularity of a lizard’s biphasic sleep, we used perturbation experiments to probe this ultradian rhythm and test the hypothesis that it originates in a central-pattern-generator circuit. Such circuits are typically susceptible to phase-dependent reset and entrainment by external stimuli. Using light pulses, we found that Pogona’s ultradian rhythm 8 could be reset in a phase-dependent manner, with a critical transition from phase delay to phase advance in the middle of NREM. The ultradian rhythm frequency could be decreased or increased, within limits, by entrainment with light pulses. During entrainment, REM could be shortened but not lengthened, whereas NREM could be more flexibly dilated. In awake animals, a few alternating light/dark epochs matching the durations of sleep’s natural REM and NREM entrained an ultradian-like brain rhythm that persisted for one to a few cycles after the alternating visual stimuli had ceased, suggesting the transient activation of an ultradian rhythm generator. In sleeping animals, a short light pulse delivered to a single eye caused an immediate ultradian-rhythm reset, but only of the contralateral hemisphere; both sides resynchronized spontaneously within a few sleep cycles, indicating that sleep is controlled by paired rhythm-generating circuits linked by functional excitation. Together, these results indicate that central pattern generators of a type usually known to control motor rhythms may also organize the ultradian sleep rhythm in an amniote vertebrate. These results were recently submitted for publication.
Our project made very significant progress so far in the following directions:
1. Identification of a claustrum outside of the mammalian lineage.
2. Identification of a role for the claustrum in slow wave sleep, since confirmed in mice and humans by other labs.
3. First comprehensive brain transcriptomic comparative study across vertebrates, showing that one can identify likely homologies and points of divergence in brain evolution, and that neuron types in extant species evolved from common, putative ancestral populations, and diverged idiosyncratically in each lineage. One can thus find in reptiles and mammals, neuron types that remain very similar, despite 320M years of separate evolution, and others that evolved separately and diverged, although their common ancestry can be traced transcriptomically to putative ancestral cell populations.
4. Detailed analysis of bilateral interactions between brain cerebral hemispheres during the two phases of sleep, revealing unexpected structure and involvement of midbrain tegmentum and tectum (isthmic circuits) in winner-take-all competition between the two sides of the brain during REM (in which each side dominates for a while in alteration with the other), but not during slow-wave (NREM) sleep.
5. Identification of oscillator dynamics, phase-dependent reset and entrainment of the REM/NREM circuits of Pogona, suggesting that the circuits underlying sleep’s ultradian rhythm are similar to central pattern generating circuits present in the hindbrain, such as thos underlying respiration, sighing or vocalization.
6. Characterization of the interactions between circadian control circuits and ultradian sleep rhythm, with special attention to the role of melatonin as a potential chemical trigger for the oscillator circuit mentioned in 5.
1. Identification of a claustrum outside of the mammalian lineage.
2. Identification of a role for the claustrum in slow wave sleep, since confirmed in mice and humans by other labs.
3. First comprehensive brain transcriptomic comparative study across vertebrates, showing that one can identify likely homologies and points of divergence in brain evolution, and that neuron types in extant species evolved from common, putative ancestral populations, and diverged idiosyncratically in each lineage. One can thus find in reptiles and mammals, neuron types that remain very similar, despite 320M years of separate evolution, and others that evolved separately and diverged, although their common ancestry can be traced transcriptomically to putative ancestral cell populations.
4. Detailed analysis of bilateral interactions between brain cerebral hemispheres during the two phases of sleep, revealing unexpected structure and involvement of midbrain tegmentum and tectum (isthmic circuits) in winner-take-all competition between the two sides of the brain during REM (in which each side dominates for a while in alteration with the other), but not during slow-wave (NREM) sleep.
5. Identification of oscillator dynamics, phase-dependent reset and entrainment of the REM/NREM circuits of Pogona, suggesting that the circuits underlying sleep’s ultradian rhythm are similar to central pattern generating circuits present in the hindbrain, such as thos underlying respiration, sighing or vocalization.
6. Characterization of the interactions between circadian control circuits and ultradian sleep rhythm, with special attention to the role of melatonin as a potential chemical trigger for the oscillator circuit mentioned in 5.