Cellular clocks in AVP neurons of the SCN are critical for interneuronal coupling regulating circadian behavior rhythm

Selective changes in vasopressin neurons and astrocytes in the suprachiasmatic nucleus of Prader-Willi syndrome subjects

The hypothalamic suprachiasmatic nucleus (SCN) hosts the central circadian pacemaker and regulates daily rhythms in physiology and behavior. The SCN is composed of peptidergic neuron populations expressing arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP), as well as glial cells. Patients with Prader-Willi Syndrome (PWS) commonly experience circadian disturbances, which are particularly evident in their sleep/wake patterns. Using publicly available single-cell RNA sequencing data, we assessed the cell-type specificity of PWS-causative genes in murine SCN, which revealed the differential presence of PWS-related genes in glial and neural subpopulations. We then investigated neurons and glial cells in the SCN using immunohistochemistry in the postmortem hypothalami of PWS subjects and matched controls. We profiled neural populations characterized by AVP and VIP, astroglia characterized by glial fibrillary acid protein (GFAP), and microglia marked by ionized calcium-binding adapter molecule 1 (Iba1) and NADPH oxidase 2 (NOX2). Our analysis revealed an increased total number, neuronal density, and relative staining intensity of AVP-containing neurons in the PWS compared to controls while VIP-containing cells were unaltered. In contrast, GFAP-expressing astroglial cells were significantly lower in PWS subjects. Moreover, we did not detect any differences in microglia between PWS subjects and controls. Collectively, our findings show that PWS selectively affects AVP-containing neurons and GFAP-expressing astrocytes in the SCN. As each of these cell populations can affect the daily rhythmicity of the SCN biological clock machinery, the disruption of these cells may contribute to the circadian disturbances in patients with PWS.

The Never Given 2022 Pittendrigh/Aschoff Lecture: The Clock Network in the Brain-Insights From Insects

My journey into chronobiology began in 1977 with lectures and internships with Wolfgang Engelmann and Hans Erkert at the University of Tübingen in Germany. At that time, the only known animal clock gene was Period, and the location and organization of the master circadian clock in the brain was completely unknown for the model insect Drosophila melanogaster. I was thus privileged to witness and participate in the research that led us from discovering the first clock gene to identifying the clock network in the fly brain and the putative pathways linking it to behavior and physiology. This article highlights my role in these developments and also shows how the successful use of D. melanogaster for studies of circadian rhythms has contributed to the understanding of clock networks in other animals. I also report on my experiences in the German scientific system and hope that my story will be of interest to some of you.

BMAL1 Overexpression in Suprachiasmatic Nucleus Protects from Retinal Neurovascular Deficits in Diabetes

The suprachiasmatic nucleus (SCN) regulates circadian rhythms and influences physiological and behavioral processes. Disruptions in circadian rhythms (CRD) are observed in type 2 diabetes (T2D), and importantly, CRD acts as an independent risk factor for T2D and its associated complications. BMAL1, a circadian clock gene, is vital for sustaining an optimal circadian rhythm and physiological function. However, the therapeutic potential of BMAL1 overexpression in the SCN to rectify the neurovascular deficits of T2D has yet to be investigated. In this study, db/db mice, a well-established model of T2D exhibiting arrhythmic behavior and the complications of diabetes, were injected stereotaxically with AAV8-Bmal1 or a control virus in the SCN to evaluate the protective effects of correcting the central clock on neurovascular deficits. Given the complex neurovascular network and the eye's unique accessibility as a transparent system, ocular complications were selected as a model to examine the neuronal functional, behavioral, and vascular benefits of correcting the central clock. BMAL1 overexpression normalized the circadian rhythms, as demonstrated by improvements in the free-running period. The retinal neuronal function improved on electroretinogram, along with optomotor behavior and visual acuity enhancements. Retinal vascular deficits were also significantly reduced. Notably, our approach helped decrease fat content in genetically predisposed obese animals. Since the SCN is known to regulate hepatic glucose production via sympathetic mechanisms, glycemic control, and pyruvate tolerance tests were conducted. Systemically, we observed improved glucose homeostasis in BMAL1-overexpressing mice alongside a substantial reduction in hepatic gluconeogenesis. BMAL1 overexpression lowered plasma norepinephrine and liver TH levels, indicating a protective regulation of adrenergic signaling. Thus, this study underscores the therapeutic potential of targeting circadian clock genes like BMAL1 in the SCN to alleviate metabolic and neurovascular deficits associated with T2D. Our research offers a compelling framework for integrating circadian rhythms into managing diabetes and its complications.

Central amygdala NPBWR1 neurons facilitate social novelty seeking and new social interactions

The formation of new social interactions is vital for social animals, but the underlying neural mechanisms remain poorly understood. We identified CeANpbwr1 neurons, a population in central amygdala expressing neuropeptide B/W receptor-1 (NPBWR1), that play a critical role in these interactions. CeANpbwr1 neurons were activated during encounters with unfamiliar, but not with familiar, mice. Manipulations of CeANpbwr1 neurons showed that their excitation is essential for maintaining physical interactions with novel conspecifics. Activation of CeANpbwr1 neurons alleviated social deficits induced by chronic social defeat stress, suggesting therapeutic potential. Conversely, overexpression of human NPBWR1 in CeANpbwr1 neurons reduced activity of these neurons and impaired social interactions with unfamiliar mice. This effect was absent in a polymorphic variant of the human NPBWR1 gene (404A>T). These findings highlight how CeANpbwr1 neurons promote social novelty seeking and reveal a complex interplay between NPBWR1 genetic variations and social behavior.

Influence of lifestyle and the circadian clock on reproduction

Background

The biological reproductive process requires the precise coordination of annual and daily signals to adapt to environmental shifts. Humans and animals have developed shared neuroendocrine systems that have adapted to process daily and seasonal light signals within the hypothalamic-pituitary -gonadal axis. However, the stability of circadian and seasonal biological processes is at risk due to industrialization and contemporary round-the-clock lifestyles. These threats include skipping breakfast, excessive artificial illumination during inappropriate hours because of irregular work schedules, nighttime urban lighting, and widespread environmental pollution from endocrine-disrupting chemicals. This review aimed to explore the interplay between lifestyle factors, circadian rhythms, and reproductive functions.

Methods

This review examined the reciprocal influences of circadian clocks on reproductive hormones, exploring the underlying mechanisms and their implications for fertility and reproductive health. We emphasized key findings regarding molecular clock components, endocrine pathways, and the critical importance of synchronizing circadian rhythms with hormonal cycles.

Main Findings

The intersection of reproductive endocrinology and circadian biology reveals complex interactions between hormonal regulation and circadian rhythms. Circadian rhythm misalignments due to environmental factors, including late-night work and skipping breakfast, negatively impact endocrine and reproductive functions.

Conclusions

More strategies are needed to mitigate the effects of circadian disruption on reproductive functions.

Prokineticin 2 protein is diurnally expressed in PER2-containing clock neurons in the mouse suprachiasmatic nucleus

Expression of prokineticin 2 (PK2) mRNA in the suprachiasmatic nucleus (SCN), also known as the brain's clock, exhibits circadian oscillations with peak levels midday, zeitgeber time (ZT) 4, and almost undetectable levels during night. This circadian expression profile has substantially contributed to the suggested role of PK2 as an SCN output molecule involved in transmitting circadian rhythm of behavior and physiology. Due to unreliable specificity of PK2 antibodies, the 81 amino acid protein has primarily been studied at the mRNA level and correlation between circadian oscillating mRNAs and protein products are infrequent. Hence, data on PK2 protein expression in the SCN is lacking. In this study a thorough validation of a commercial PK2 antibody for immunohistochemistry (IHC) was performed followed by fluorescence IHC on SCN mouse brain sections at six consecutive ZTs over a 24-h cycle (12:12 light-dark, ZT0 =light ON whereas ZT12 =light OFF). Data were visualized and processed using confocal microscopy. Results showed that PK2 protein expression diurnally oscillates with calculated peak expression ZT5:40 ± 1:40 h. Opposite than described for PK2 mRNA, PK2 immunoreactivity was detectable at all times during the 24-h cycle. PK2 was primarily located in neurons of the shell compartment and > 80 % of these neurons co-expressed the core clock protein PER2. In conclusion, PK2 protein expression oscillates as the mRNA, supporting the suggested role of PK2 as a SCN molecule involved in circadian rhythm regulation.

Chronic sleep deprivation disturbs energy balance modulated by suprachiasmatic nucleus efferents in mice

BACKGROUND

Epidemiologic researches show that short sleep duration may affect feeding behaviors resulting in higher energy intake and increased risk of obesity, but the further mechanisms that can interpret the causality remain unclear. The circadian rhythm is fine-tuned by the suprachiasmatic nucleus (SCN) as the master clock, which is essential for driving rhythms in food intake and energy metabolism through neuronal projections to the arcuate nucleus (ARC) and paraventricular nucleus (PVN).

RESULTS

We showed that chronic SD-induced aberrant expressions of AgRP/NPY and POMC attributed to compromised JAK/STAT3 signals and reduced energy expenditure in the mice, which can be rescued with AAV-genetic overexpression of BMAL1 into SCN. The potential mechanism may be related to the disruptions of SCN efferent mediated by BMAL1.

CONCLUSIONS

Chronic SD impairs energy balance through directly dampening BMAL1 expression, probably in the transcription level, in the SCN, which in turn affects the neuron projections to ARC and PVN. Remarkably, we provide evidence that may explain the causal mechanisms associated with sleep curtailment and obesity in adolescents.

Chemogenetic activation of astrocytes modulates sleep-wakefulness states in a brain region-dependent manner

Study Objectives

Astrocytes change their intracellular calcium (Ca2+) concentration during sleep/wakefulness states in mice. Furthermore, the Ca2+ dynamics in astrocytes vary depending on the brain region. However, it remains unclear whether alterations in astrocyte activity can affect sleep-wake states and cortical oscillations in a brain region-dependent manner.

Methods

Astrocyte activity was artificially manipulated in mice using chemogenetics. Astrocytes in the hippocampus and pons, which are 2 brain regions previously classified into different clusters based on their Ca2+ dynamics during sleep-wakefulness, were focused on to compare whether there are differences in the effects of astrocytes from different brain regions.

Results

The chemogenetic activation of astrocytes in the hippocampus significantly decreased the total time of wakefulness and increased the total time of sleep. This had little effect on cortical oscillations in all sleep-wakefulness states. On the other hand, the activation of astrocytes in the pons substantially suppressed rapid eye movement (REM) sleep in association with a decreased number of REM episodes, indicating strong inhibition of REM onset. Regarding cortical oscillations, the delta wave component during non-REM sleep was significantly enhanced.

Conclusions

These results suggest that astrocytes modulate sleep-wakefulness states and cortical oscillations. Furthermore, the role of astrocytes in sleep-wakefulness states appears to vary among brain regions.

The Functional Connectome Mediating Circadian Synchrony in the Suprachiasmatic Nucleus

Circadian rhythms in mammals arise from the spatiotemporal synchronization of ~20,000 neuronal clocks in the Suprachiasmatic Nucleus (SCN). While anatomical, molecular, and genetic approaches have revealed diverse cell types and signaling mechanisms, the network wiring that enables SCN cells to communicate and synchronize remains unclear. To overcome the challenges of revealing functional connectivity from fixed tissue, we developed MITE (Mutual Information & Transfer Entropy), an information theory approach that infers directed cell-cell connections with high fidelity. By analyzing 3447 hours of continuously recorded clock gene expression from 9011 cells in 17 mice, we found that the functional connectome of SCN was highly conserved bilaterally and across mice, sparse, and organized into a dorsomedial and a ventrolateral module. While most connections were local, we discovered long-range connections from ventral cells to cells in both the ventral and dorsal SCN. Based on their functional connectivity, SCN cells can be characterized as circadian signal generators, broadcasters, sinks, or bridges. For example, a subset of VIP neurons acts as hubs that generate circadian signals critical to synchronize daily rhythms across the SCN neural network. Simulations of the experimentally inferred SCN networks recapitulated the stereotypical dorsal-to-ventral wave of daily PER2 expression and ability to spontaneously synchronize, revealing that SCN emergent dynamics are sculpted by cell-cell connectivity. We conclude that MITE provides a powerful method to infer functional connectomes, and that the conserved architecture of cell-cell connections mediates circadian synchrony across space and time in the mammalian SCN.

GABAergic signalling in the suprachiasmatic nucleus is required for coherent circadian rhythmicity

The suprachiasmatic nucleus is the circadian pacemaker of the mammalian brain. Suprachiasmatic nucleus neurons display synchronization of their firing frequency on a circadian timescale, which is required for the pacemaker function of the suprachiasmatic nucleus. However, the mechanisms by which suprachiasmatic nucleus neurons remain synchronized in vivo are poorly understood, although synaptic communication is considered indispensable. Suprachiasmatic nucleus neurons contain the neurotransmitter GABA and express GABA receptors. This has inspired the hypothesis that GABA signalling may play a central role in network synchronization, although this remains untested in vivo. Here, using local genetic deletion, we show that disruption of GABA synaptic transmission within the suprachiasmatic nucleus of adult mice results in the eventual deterioration of physiological and behavioural rhythmicity in vivo and concomitant cellular desynchrony in vitro. These findings suggest that intercellular GABA signalling is essential for behavioural rhythmicity and cellular synchrony of the suprachiasmatic nucleus neural network.

Dynamic endocannabinoid-mediated neuromodulation of retinal circadian circuitry

Circadian rhythms are biological rhythms that originate from the "master circadian clock," called the suprachiasmatic nucleus (SCN). SCN orchestrates the circadian rhythms using light as a chief zeitgeber, enabling humans to synchronize their daily physio-behavioral activities with the Earth's light-dark cycle. However, chronic/ irregular photic disturbances from the retina via the retinohypothalamic tract (RHT) can disrupt the amplitude and the expression of clock genes, such as the period circadian clock 2, causing circadian rhythm disruption (CRd) and associated neuropathologies. The present review discusses neuromodulation across the RHT originating from retinal photic inputs and modulation offered by endocannabinoids as a function of mitigation of the CRd and associated neuro-dysfunction. Literature indicates that cannabinoid agonists alleviate the SCN's ability to get entrained to light by modulating the activity of its chief neurotransmitter, i.e., γ-aminobutyric acid, thus preventing light-induced disruption of activity rhythms in laboratory animals. In the retina, endocannabinoid signaling modulates the overall gain of the retinal ganglion cells by regulating the membrane currents (Ca2+, K+, and Cl- channels) and glutamatergic neurotransmission of photoreceptors and bipolar cells. Additionally, endocannabinoids signalling also regulate the high-voltage-activated Ca2+ channels to mitigate the retinal ganglion cells and intrinsically photosensitive retinal ganglion cells-mediated glutamate release in the SCN, thus regulating the RHT-mediated light stimulation of SCN neurons to prevent excitotoxicity. As per the literature, cannabinoid receptors 1 and 2 are becoming newer targets in drug discovery paradigms, and the involvement of endocannabinoids in light-induced CRd through the RHT may possibly mitigate severe neuropathologies.

A critical role of Ca2+/calmodulin-dependent protein kinase II in coupling between evening and morning circadian oscillators in the suprachiasmatic nucleus

Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) is widely expressed in the brain and is involved in various functions, including memory formation, mood and sleep. We previously reported that CaMKIIα is involved in the circadian molecular clock. Mice lacking functional CaMKIIα (K42R mice) exhibited a gradual increase in activity time (α decompression) of running-wheel (RW) activity due to a lengthened circadian period (τ) of activity offset under constant darkness (DD). In the present study, to investigate the functional roles of CaMKIIα in behavioural rhythms, we measured RW and general movements simultaneously under prolonged DD. Tau became longer as the relative intensity of behaviour activity within an activity time shifted from activity onset towards activity offset. In some K42R mice, α was gradually expanded with a marked reduction of RW activity, while general movements persisted without noticeable decline, which was followed by an abrupt shortening of α (α compression) with differential phase shifts of the activity onset and offset and recovery of RW activity. These results suggest that an internal coupling between the oscillators controlling activity onset and offset is bidirectional but with different strengths. The α compression occurred recurrently in 38% of K42R mice examined with an average interval of 37 days in association with attenuation of RW activity but never in the wild-type (WT) mice. Consistent with behavioural rhythms, the circadian period of the PER2::LUC rhythm in the cultured suprachiasmatic nucleus (SCN) slice was significantly longer in K42R than in WT. These findings are best interpreted by assuming that a loss of functional CaMKIIα attenuates the coupling between the onset and offset oscillators.

Arginine vasopressin: Critical regulator of circadian homeostasis

Circadian rhythms optimally regulate numerous physiological processes in an organism and synchronize them with the external environment. The suprachiasmatic nucleus (SCN), the center of the circadian clock in mammals, is composed of multiple cell types that form a network that provides the basis for the remarkable stability of the circadian clock. Among the neuropeptides expressed in the SCN, arginine vasopressin (AVP) has attracted much attention because of its deep involvement in the function of circadian rhythms, as elucidated in particular by studies using genetically engineered mice. This review briefly summarizes the current knowledge on the peptidergic distribution and topographic neuronal organization in the SCN, the molecular mechanisms of the clock genes, and the relationship between the SCN and peripheral clocks. With respect to the physiological roles of AVP and AVP-expressing neurons, in addition to a sex-dependent action of AVP in the SCN, studies using AVP receptor knockout mice and mice genetically manipulated to alter the clock properties of AVP neurons are summarized here, highlighting its importance in maintaining circadian homeostasis and its potential as a target for therapeutic interventions.

Blood flows from the SCN toward the OVLT within a new brain vascular portal pathway

The suprachiasmatic nucleus (SCN) sets the phase of oscillation throughout the brain and body. Anatomical evidence reveals a portal system linking the SCN and the organum vasculosum of the lamina terminalis (OVLT), begging the question of the direction of blood flow and the nature of diffusible signals that flow in this specialized vasculature. Using a combination of anatomical and in vivo two-photon imaging approaches, we unequivocally show that blood flows unidirectionally from the SCN to the OVLT, that blood flow rate displays daily oscillations with a higher rate at night than in the day, and that circulating vasopressin can access portal vessels. These findings highlight a previously unknown central nervous system communication pathway, which, like that of the pituitary portal system, could allow neurosecretions to reach nearby target sites in OVLT, avoiding dilution in the systemic blood. In both of these brain portal pathways, the target sites relay signals broadly to both the brain and the rest of the body.

The Suprachiasmatic Nucleus at 50: Looking Back, Then Looking Forward

It has been 50 years since the suprachiasmatic nucleus (SCN) was first identified as the central circadian clock and 25 years since the last overview of developments in the field was published in the Journal of Biological Rhythms. Here, we explore new mechanisms and concepts that have emerged in the subsequent 25 years. Since 1997, methodological developments, such as luminescent and fluorescent reporter techniques, have revealed intricate relationships between cellular and network-level mechanisms. In particular, specific neuropeptides such as arginine vasopressin, vasoactive intestinal peptide, and gastrin-releasing peptide have been identified as key players in the synchronization of cellular circadian rhythms within the SCN. The discovery of multiple oscillators governing behavioral and physiological rhythms has significantly advanced our understanding of the circadian clock. The interaction between neurons and glial cells has been found to play a crucial role in regulating these circadian rhythms within the SCN. Furthermore, the properties of the SCN network vary across ontogenetic stages. The application of cell type-specific genetic manipulations has revealed components of the functional input-output system of the SCN and their correlation with physiological functions. This review concludes with the high-risk effort of identifying open questions and challenges that lie ahead.

Circadian rhythm mechanism in the suprachiasmatic nucleus and its relation to the olfactory system

Animals need sleep, and the suprachiasmatic nucleus, the center of the circadian rhythm, plays an important role in determining the timing of sleep. The main input to the suprachiasmatic nucleus is the retinohypothalamic tract, with additional inputs from the intergeniculate leaflet pathway, the serotonergic afferent from the raphe, and other hypothalamic regions. Within the suprachiasmatic nucleus, two of the major subtypes are vasoactive intestinal polypeptide (VIP)-positive neurons and arginine-vasopressin (AVP)-positive neurons. VIP neurons are important for light entrainment and synchronization of suprachiasmatic nucleus neurons, whereas AVP neurons are important for circadian period determination. Output targets of the suprachiasmatic nucleus include the hypothalamus (subparaventricular zone, paraventricular hypothalamic nucleus, preoptic area, and medial hypothalamus), the thalamus (paraventricular thalamic nuclei), and lateral septum. The suprachiasmatic nucleus also sends information through several brain regions to the pineal gland. The olfactory bulb is thought to be able to generate a circadian rhythm without the suprachiasmatic nucleus. Some reports indicate that circadian rhythms of the olfactory bulb and olfactory cortex exist in the absence of the suprachiasmatic nucleus, but another report claims the influence of the suprachiasmatic nucleus. The regulation of circadian rhythms by sensory inputs other than light stimuli, including olfaction, has not been well studied and further progress is expected.

Integrated neural tracing and in-situ barcoded sequencing reveals the logic of SCN efferent circuits in regulating circadian behaviors

The circadian clock coordinates rhythms in numerous physiological processes to maintain organismal homeostasis. Since the suprachiasmatic nucleus (SCN) is widely accepted as the circadian pacemaker, it is critical to understand the neural mechanisms by which rhythmic information is transferred from the SCN to peripheral clocks. Here, we present the first comprehensive map of SCN efferent connections and suggest a molecular logic underlying these projections. The SCN projects broadly to most major regions of the brain, rather than solely to the hypothalamus and thalamus. The efferent projections from different subtypes of SCN neurons vary in distance and intensity, and blocking synaptic transmission of these circuits affects circadian rhythms in locomotion and feeding to different extents. We also developed a barcoding system to integrate retrograde tracing with in-situ sequencing, allowing us to link circuit anatomy and spatial patterns of gene expression. Analyses using this system revealed that brain regions functioning downstream of the SCN receive input from multiple neuropeptidergic cell types within the SCN, and that individual SCN neurons generally project to a single downstream brain region. This map of SCN efferent connections provides a critical foundation for future investigations into the neural circuits underlying SCN-mediated rhythms in physiology. Further, our new barcoded tracing method provides a tool for revealing the molecular logic of neuronal circuits within heterogeneous brain regions.

Status and trends of RGS16 based on data visualization analysis: A review

G-protein signaling regulator 16 (RGS16) has been confirmed that RGS16 is associated with cancer, neurodegenerative diseases, and cardiovascular diseases. Moreover, many studies have shown that RGS16 can be used as a biomarker for cancer diagnosis and prognosis. We used CiteSpace and VOS viewer software to perform a bibliometric analysis of 290 publications in the core collection of Web of Science. All the articles come from 399 institutions, including 618 authors, 179 journals, 40 countries, 115 keywords, 1 language, two types of papers, and reviews. The United States has the largest number of publications. The Research Center of Allergy and Infectious Diseases (NIAID) publishes the most papers, Emory University is the most recent of all institutions with the most recent results in the RGS16 study. Cell biology is the most studied discipline, and the most studied topic is migration. Drury published RGS16-related articles with the most citations (n = 15), and Berman published articles with the most citations (n = 106). The biological applications of RGS16 are currently a hot area of RGS16 research, including inflammation, cancer, ulcerative colitis, metabolic acidosis, platelet activation, and thrombosis. The current scientometrics study provides an overview of RGS16 research from 1995 to 2022. This study provides an overview of current and potential future research hotspots in the field of RGS16 and can be used as a resource for interested researchers.

Phenotyping of light-activated neurons in the mouse SCN based on the expression of FOS and EGR1

Light-sensitive neurons are located in the ventral and central core of the suprachiasmatic nucleus (SCN), whereas stably oscillating clock neurons are found mainly in the dorsal shell. Signals between the SCN core and shell are believed to play an important role in light entrainment. Core neurons express vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), and Neuroglobin (Ngb), whereas the shell neurons express vasopressin (AVP), prokineticin 2, and the VIP type 2 (VPAC2) receptor. In rodents, light has a phase-shifting capacity at night, which induces rapid and transient expression of the EGR1 and FOS in the SCN. Methods: The present study used immunohistochemical staining of FOS, EGR1, and phenotypical markers of SCN neurons (VIP, AVP, Ngb) to identify subtypes/populations of light-responsive neurons at early night. Results: Double immunohistochemistry and cell counting were used to evaluate the number of SCN neurons expressing FOS and EGR1 in the SCN. The number of neurons expressing either EGR1 or FOS was higher than the total number of neurons co-storing EGR1 and FOS. Of the total number of light-responsive cells, 42% expressed only EGR1, 43% expressed only FOS, and 15% expressed both EGR1 and FOS. Light-responsive VIP neurons represented only 31% of all VIP neurons, and EGR1 represents the largest group of light-responsive VIP neurons (18%). VIP neurons expressing only FOS represented 1% of the total light-responsive VIP neurons. 81% of the Ngb neurons in the mouse SCN were light-responsive, and of these neurons expressing only EGR1 after light stimulation represented 44%, whereas 24% expressed FOS. Although most light-responsive neurons are found in the core of the SCN, 29% of the AVP neurons in the shell were light-responsive, of which 8% expressed EGR1, 10% expressed FOS, and 11% co-expressed both EGR1 and FOS after light stimulation. Discussion: Our analysis revealed cell-specific differences in light responsiveness between different peptidergic and Ngb-expressing neurons in different compartments of the mouse SCN, indicating that light activates diverse neuronal networks in the SCN, some of which participate in photoentrainment.

The transcription factor VAX1 in VIP neurons of the suprachiasmatic nucleus impacts circadian rhythm generation, depressive-like behavior, and the reproductive axis in a sex-specific manner in mice

Background

The suprachiasmatic nucleus (SCN) within the hypothalamus is a key brain structure required to relay light information to the body and synchronize cell and tissue level rhythms and hormone release. Specific subpopulations of SCN neurons, defined by their peptide expression, regulate defined SCN output. Here we focus on the vasoactive intestinal peptide (VIP) expressing neurons of the SCN. SCN VIP neurons are known to regulate circadian rhythms and reproductive function.

Methods

To specifically study SCN VIP neurons, we generated a novel knock out mouse line by conditionally deleting the SCN enriched transcription factor, Ventral Anterior Homeobox 1 (Vax1), in VIP neurons (Vax1Vip; Vax1fl/fl:VipCre).

Results

We found that Vax1Vip females presented with lengthened estrous cycles, reduced circulating estrogen, and increased depressive-like behavior. Further, Vax1Vip males and females presented with a shortened circadian period in locomotor activity and ex vivo SCN circadian period. On a molecular level, the shortening of the SCN period was driven, at least partially, by a direct regulatory role of VAX1 on the circadian clock genes Bmal1 and Per2. Interestingly, Vax1Vip females presented with increased expression of arginine vasopressin (Avp) in the paraventricular nucleus, which resulted in increased circulating corticosterone. SCN VIP and AVP neurons regulate the reproductive gonadotropin-releasing hormone (GnRH) and kisspeptin neurons. To determine how the reproductive neuroendocrine network was impacted in Vax1Vip mice, we assessed GnRH sensitivity to a kisspeptin challenge in vivo. We found that GnRH neurons in Vax1Vip females, but not males, had an increased sensitivity to kisspeptin, leading to increased luteinizing hormone release. Interestingly, Vax1Vip males showed a small, but significant increase in total sperm and a modest delay in pubertal onset. Both male and female Vax1Vip mice were fertile and generated litters comparable in size and frequency to controls.

Conclusion

Together, these data identify VAX1 in SCN VIP neurons as a neurological overlap between circadian timekeeping, female reproduction, and depressive-like symptoms in mice, and provide novel insight into the role of SCN VIP neurons.

Action Sequence Learning Is Impaired in Genetically Modified Mice with the Suppressed GABAergic Transmission from the Thalamic Reticular Nucleus to the Thalamus

The thalamic reticular nucleus (TRN) is a thin sheet of GABAergic neurons surrounding the thalamus, and it regulates the activity of thalamic relay neurons. The TRN has been reported to be involved in sensory gating, attentional regulation, and some other functions. However, little is known about the contribution of the TRN to sequence learning. In the present study, we examined whether the TRN is involved in reward-based learning of action sequence with no eliciting stimuli (operant conditioning), by analyzing the performance of male and female Avp-Vgat-/- mice (Vgatflox/flox mice crossed to an Avp-Cre driver line) on tasks conducted in an operant box having three levers. Our histological and electrophysiological data demonstrated that in adult Avp-Vgat-/- mice, vesicular GABA transporter (VGAT) was absent in most TRN neurons and the GABAergic transmission from the TRN to the thalamus was largely suppressed. The performance on a task in which mice needed to press an active lever for food reward showed that simple operant learning of lever pressing and learning of win-stay and lose-shift strategies are not affected in Avp-Vgat-/- mice. In contrast, the performance on a task in which mice needed to press three levers in a correct order for food reward showed that learning of the order of lever pressing (action sequence learning) was impaired in Avp-Vgat-/- mice. These results suggest that the TRN plays an important role in action sequence learning.

Differential responses to artificial photoperiods of the rising and falling phases of human melatonin rhythm are consistent with a dual oscillator hypothesis

Circadian rhythms and sleep-wake cycles were measured in volunteers staying singly in temporal isolation unit where they were exposed to artificial short and long light-dark (LD) cycles for 7 days. The long day consisted of 16-h light and 8-h dark (LD 16:8) and the short day consisted of 8-h light and 16-h dark (LD 8:16). During the light period, bright light of approximately 5,000 lux was given from the ceiling and during the dark period there was no illumination. Sleep was monitored by bed sensors, wrist actiwatch, and polysomnography (PSG) on the first and last nights of the schedule. Sleep length was significantly longer under LD 8:16 than under LD 16:8 and the sleep quality estimated by PSG was worse under LD 8:16 than under LD 16:8, which were comparable to natural seasonality in sleep. The circadian rhythm in plasma melatonin was measured in dim light (10 lux) before and after the LD exposures. The nocturnal melatonin secretion (NMS) was significantly longer after LD 8:16 than after LD 16:8 due to differential phase shifts of the rising and falling phases of NMS. After LD 8:16, the falling phase was much advanced than the rising phase, whereas after LD 16:8 the rising phase was much delayed than the falling phase, resulting in the NMS compression. These results indicate that the light sensitivity in terms of phase shifting is different in the two circadian phases, supporting a dual oscillator hypothesis with different phase-response curves for light in the human circadian system.NEW & NOTEWORTHY The present study demonstrated differential light responsiveness of the rising and falling phases of nocturnal melatonin secretion in human subjects exposed to artificial long (LD 16:8) and short days (LD 8:16) and suggested the involvement of different oscillators under these phases. The findings well mimicked the seasonality of the circadian rhythms in nature and consisted with the evening/morning dual oscillator hypothesis proposed originally for nocturnal rodents, providing a new concept for the human circadian system.

In vivo recording of the circadian calcium rhythm in Prokineticin 2 neurons of the suprachiasmatic nucleus

Prokineticin 2 (Prok2) is a small protein expressed in a subpopulation of neurons in the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals. Prok2 has been implicated as a candidate output molecule from the SCN to control multiple circadian rhythms. Genetic manipulation specific to Prok2-producing neurons would be a powerful approach to understanding their function. Here, we report the generation of Prok2-tTA knock-in mice expressing the tetracycline transactivator (tTA) specifically in Prok2 neurons and an application of these mice to in vivo recording of Ca2+ rhythms in these neurons. First, the specific and efficient expression of tTA in Prok2 neurons was verified by crossing the mice with EGFP reporter mice. Prok2-tTA mice were then used to express a fluorescent Ca2+ sensor protein to record the circadian Ca2+ rhythm in SCN Prok2 neurons in vivo. Ca2+ in these cells showed clear circadian rhythms in both light-dark and constant dark conditions, with their peaks around midday. Notably, the hours of high Ca2+ nearly coincided with the rest period of the behavioral rhythm. These observations fit well with the predicted function of Prok2 neurons as a candidate output pathway of the SCN by suppressing locomotor activity during both daytime and subjective daytime.

In vivo recording of suprachiasmatic nucleus dynamics reveals a dominant role of arginine vasopressin neurons in circadian pacesetting

The central circadian clock of the suprachiasmatic nucleus (SCN) is a network consisting of various types of neurons and glial cells. Individual cells have the autonomous molecular machinery of a cellular clock, but their intrinsic periods vary considerably. Here, we show that arginine vasopressin (AVP) neurons set the ensemble period of the SCN network in vivo to control the circadian behavior rhythm. Artificial lengthening of cellular periods by deleting casein kinase 1 delta (CK1δ) in the whole SCN lengthened the free-running period of behavior rhythm to an extent similar to CK1δ deletion specific to AVP neurons. However, in SCN slices, PER2::LUC reporter rhythms of these mice only partially and transiently recapitulated the period lengthening, showing a dissociation between the SCN shell and core with a period instability in the shell. In contrast, in vivo calcium rhythms of both AVP and vasoactive intestinal peptide (VIP) neurons in the SCN of freely moving mice demonstrated stably lengthened periods similar to the behavioral rhythm upon AVP neuron-specific CK1δ deletion, without changing the phase relationships between each other. Furthermore, optogenetic activation of AVP neurons acutely induced calcium increase in VIP neurons in vivo. These results indicate that AVP neurons regulate other SCN neurons, such as VIP neurons, in vivo and thus act as a primary determinant of the SCN ensemble period.

Desynchronization between Food Intake and Light Stimulations Induces Uterine Clock Quiescence in Female Mice

BACKGROUND

Dysmenorrhea is associated with breakfast skipping in young women, suggesting that fasting in the early active phase disrupts uterine functions.

OBJECTIVES

To investigate the possible involvement of the uterine clock system in fasting-induced uterine dysfunction, we examined core clock gene expressions in the uterus using a 28-h interval-fed mouse model.

METHODS

Young female mice (8 wk of age) were divided into 3 groups: group I (ad libitum feeding), group II (time-restricted feeding, initial 4 h of the active period every day), and group III (time-restricted feeding for 8 h with a 28-h cycle). Groups II and III have the same fasting interval of 20 h. After analyzing feeding and wheel running behaviors during 2 wk of dietary restriction, mice were sacrificed at 4-h intervals, and the expression profiles of clock genes in the uterus and liver were examined by qPCR.

RESULTS

The mice in group I took food mainly during the dark phase and those in group II during the initial 4 h of the dark phase, whereas those in group III delayed feeding time by 4 h per cycle. In all groups, spontaneous wheel running was observed during the dark phase. There was no difference in the quantity of feeding and the amount of running exercise among the 3 groups during the second week. The mRNA expressions of peripheral clock genes, Bmal1, Clock, Per1, Per2, Cry1, Nr1d1, and Dbp and a clock-controlled gene, Fabp1, in the uterus showed rhythmic oscillations with normal sequential expression cascade in groups I and II, whereas their expressions decreased and circadian cycles disappeared in group III. In contrast, liver core clock genes in group III showed clear circadian cycles.

CONCLUSIONS

Fluctuations in the timing of the first food intake impair the uterine clock oscillator system to reduce clock gene expressions and abolish their circadian rhythms.

Sex differences in afferents and efferents of vasopressin neurons of the bed nucleus of the stria terminalis and medial amygdala in mice

Steroid-sensitive vasopressin (AVP) neurons in the bed nucleus of the stria terminalis (BNST) and medial amygdala (MeA) have been implicated in the control of social behavior, but the connectional architecture of these cells is not well understood. Here we used a modified rabies virus (RV) approach to identify cells that provide monosynaptic input to BNST and MeA AVP cells, and an adeno-associated viral (AAV) anterograde tracer strategy to map the outputs of these cells. Although the location of in- and outputs of these cells generally overlap, we observed several sex differences with differences in density of outputs typically favoring males, but the direction of sex differences in inputs vary based on their location. Moreover, the AVP cells located in both the BNST and MeA are in direct contact with each other suggesting that AVP cells in these two regions act in a coordinated manner, and possibly differently by sex. This study represents the first comprehensive mapping of the sexually dimorphic and steroid-sensitive AVP neurons in the mouse brain.

Uncovering the brain-wide pattern of synaptic input to vasopressin-expressing neurons in the paraventricular nucleus of the hypothalamus

Arginine vasopressin (AVP) is a neuropeptide critical for the mammalian stress response and social behavior. AVP produced in the hypothalamus regulates water osmolality and vasoconstriction in the body, and in the brain, it regulates social behavior, aggression, and anxiety. However, the circuit mechanisms that link AVP to social behavior, homeostatic function, and disease are not well understood. This study investigates the circuit configurations of AVP-expressing neurons in the rodent hypothalamus and characterizes synaptic input from the entire brain. We targeted the paraventricular nucleus (PVN) using retrograde viral tracing techniques to identify direct afferent synaptic connections made onto AVP-expressing neurons. AVP neurons in the PVN display region-specific anatomical configurations that reflect their unique contributions to homeostatic function, motor behaviors, feeding, and affiliative behavior. The afferent connections identified were similar in both sexes and subsequent molecular investigation of these inputs shows that those local hypothalamic inputs are overwhelmingly nonpeptidergic cells indicating a potential interneuron nexus between hormone cell activation and broader cortical connection. This proposed work reveals new insights into the organization of social behavior circuits in the brain, and how neuropeptides act centrally to modulate social behaviors.

Structure and Function of Neuronal Circuits Linking Ventrolateral Preoptic Nucleus and Lateral Hypothalamic Area

To understand how sleep-wakefulness cycles are regulated, it is essential to disentangle structural and functional relationships between the preoptic area (POA) and lateral hypothalamic area (LHA), since these regions play important yet opposing roles in the sleep-wakefulness regulation. GABA- and galanin (GAL)-producing neurons in the ventrolateral preoptic nucleus (VLPO) of the POA (VLPOGABA and VLPOGAL neurons) are responsible for the maintenance of sleep, while the LHA contains orexin-producing neurons (orexin neurons) that are crucial for maintenance of wakefulness. Through the use of rabies virus-mediated neural tracing combined with in situ hybridization (ISH) in male and female orexin-iCre mice, we revealed that the vesicular GABA transporter (Vgat, Slc32a1)- and galanin (Gal)-expressing neurons in the VLPO directly synapse with orexin neurons in the LHA. A majority (56.3 ± 8.1%) of all VLPO input neurons connecting to orexin neurons were double-positive for Vgat and Gal Using projection-specific rabies virus-mediated tracing in male and female Vgat-ires-Cre and Gal-Cre mice, we discovered that VLPOGABA and VLPOGAL neurons that send projections to the LHA received innervations from similarly distributed input neurons in many brain regions, with the POA and LHA being among the main upstream areas. Additionally, we found that acute optogenetic excitation of axons of VLPOGABA neurons, but not VLPOGAL neurons, in the LHA of male Vgat-ires-Cre mice induced wakefulness. This study deciphers the connectivity between the VLPO and LHA, provides a large-scale map of upstream neuronal populations of VLPO→LHA neurons, and reveals a previously uncovered function of the VLPOGABA→LHA pathway in the regulation of sleep and wakefulness.SIGNIFICANCE STATEMENT We identified neurons in the ventrolateral preoptic nucleus (VLPO) that are positive for vesicular GABA transporter (Vgat) and/or galanin (Gal) and serve as presynaptic partners of orexin-producing neurons in the lateral hypothalamic area (LHA). We depicted monosynaptic input neurons of GABA- and galanin-producing neurons in the VLPO that send projections to the LHA throughout the entire brain. Their input neurons largely overlap, suggesting that they comprise a common neuronal population. However, acute excitatory optogenetic manipulation of the VLPOGABA→LHA pathway, but not the VLPOGAL→LHA pathway, evoked wakefulness. This study shows the connectivity of major components of the sleep/wake circuitry in the hypothalamus and unveils a previously unrecognized function of the VLPOGABA→LHA pathway in sleep-wakefulness regulation. Furthermore, we suggest the existence of subpopulations of VLPOGABA neurons that innervate LHA.

Cholecystokinin neurons in mouse suprachiasmatic nucleus regulate the robustness of circadian clock

The suprachiasmatic nucleus (SCN) can generate robust circadian behaviors in mammals under different environments, but the underlying neural mechanisms remained unclear. Here, we showed that the activities of cholecystokinin (CCK) neurons in the mouse SCN preceded the onset of behavioral activities under different photoperiods. CCK-neuron-deficient mice displayed shortened free-running periods, failed to compress their activities under a long photoperiod, and developed rapid splitting or became arrhythmic under constant light. Furthermore, unlike vasoactive intestinal polypeptide (VIP) neurons, CCK neurons are not directly light sensitive, but their activation can elicit phase advance and counter light-induced phase delay mediated by VIP neurons. Under long photoperiods, the impact of CCK neurons on SCN dominates over that of VIP neurons. Finally, we found that the slow-responding CCK neurons control the rate of recovery during jet lag. Together, our results demonstrated that SCN CCK neurons are crucial for the robustness and plasticity of the mammalian circadian clock.

Repetitive mild traumatic brain injury alters central and peripheral clock gene expression in the adolescent rat

Mild traumatic brain injury (mTBI) or concussion is a common injury worldwide leading to substantial medical costs and a high burden on society. In adolescents, falls and sports related trauma are often the causes of mTBI. Importantly, critical brain growth and development occurs during this sensitive period making the prospect of a brain injury a worrying phenomenon. Upwards of 70% of patients report circadian disruption following these injuries and this has been shown to impede recovery. Therefore, we sought to determine if core circadian clock gene expression was disrupted in rat model of repetitive mTBI (RmTBI). Male and female adolescent rats (n = 129) received sham or RmTBI. The animals were then euthanized at different times throughout the day and night. Tissue from the hypothalamus, cerebellum, hippocampus, liver, and small intestine were evaluated for the expression of per1, per2, cry1, clock, bmal1 and rev-erb-α. We found most clock genes varied across the day/night indicating circadian expression patterns. In the hypothalamus we found RmTBI altered the expression of cry1 and bmal1 in addition to sex differences in per2, cry1, clock, bmal1 and rev-erb- α. In the cerebellum, per1, per2, cry1, clock, bmal1 and rev-erb-α rhythms were all knocked out by RmTBI in addition to sex differences in cry1, clock and bmal1 expression. We also detected a significant decrease in overall expression of all clock genes in males in the middle of the night. In the hippocampus we found that RmTBI changed the rhythm of rev-erb-α expression in addition to sex differences in bmal1 expression. In the liver we detected strong rhythms in all genes examined, however only per2 expression was knocked out by RmTBI, in addition we also detected sex differences in per2 and cry1. We also detected an overall decrease in female clock gene expression in the early night. In the small intestine, RmTBI altered cry1 expression and there were sex differences in rev-erb-α. These results indicate that RmTBI alters core circadian clock gene expression in the central and peripheral nervous system in a time, tissue and sex dependent manner. This may be disrupting important phase relationships between the brain and peripheral nervous system and contributing to post-injury symptomology and also highlights the importance for time and sex dependent assessment of injury outcomes.

The REV-ERB Nuclear Receptors: Timekeepers for the Core Clock Period and Metabolism

REV-ERB nuclear receptors are potent transcriptional repressors that play an important role in the core mammalian molecular clock and metabolism. Deletion of both REV-ERBα and its largely redundant isoform REV-ERBβ in a murine tissue-specific manner have shed light on their specific functions in clock mechanisms and circadian metabolism. This review highlights recent findings that establish REV-ERBs as crucial circadian timekeepers in a variety of tissues, regulating overlapping and distinct processes that maintain normal physiology and protect from metabolic dysfunction.

Circadian rhythms in the blood-brain barrier: impact on neurological disorders and stress responses

Circadian disruption has become more prevalent in society due to the increase in shift work, sleep disruption, blue light exposure, and travel via different time zones. The circadian rhythm is a timed transcription-translation feedback loop with positive regulators, BMAL1 and CLOCK, that interact with negative regulators, CRY and PER, to regulate both the central and peripheral clocks. This review highlights the functions of the circadian rhythm, specifically in the blood-brain barrier (BBB), during both healthy and pathological states. The BBB is a highly selective dynamic interface composed of CNS endothelial cells, astrocytes, pericytes, neurons, and microglia that form the neurovascular unit (NVU). Circadian rhythms modulate BBB integrity through regulating oscillations of tight junction proteins, assisting in functions of the NVU, and modulating transporter functions. Circadian disruptions within the BBB have been observed in stress responses and several neurological disorders, including brain metastasis, epilepsy, Alzheimer's disease, and Parkinson's disease. Further understanding of these interactions may facilitate the development of improved treatment options and preventative measures.

Neural function of Bmal1: an overview

Bmal1 (Brain and muscle arnt-like, or Arntl) is a bHLH/PAS domain transcription factor central to the transcription/translation feedback loop of the biologic clock. Although Bmal1 is well-established as a major regulator of circadian rhythm, a growing number of studies in recent years have shown that dysfunction of Bmal1 underlies a variety of psychiatric, neurodegenerative-like, and endocrine metabolism-related disorders, as well as potential oncogenic roles. In this review, we systematically summarized Bmal1 expression in different brain regions, its neurological functions related or not to circadian rhythm and biological clock, and pathological phenotypes arising from Bmal1 knockout. This review also discusses oscillation and rhythmicity, especially in the suprachiasmatic nucleus, and provides perspective on future progress in Bmal1 research.

The Mammalian Circadian Time-Keeping System

Our physiology and behavior follow precise daily programs that adapt us to the alternating opportunities and challenges of day and night. Under experimental isolation, these rhythms persist with a period of approximately one day (circadian), demonstrating their control by an internal autonomous clock. Circadian time is created at the cellular level by a transcriptional/translational feedback loop (TTFL) in which the protein products of the Period and Cryptochrome genes inhibit their own transcription. Because the accumulation of protein is slow and delayed, the system oscillates spontaneously with a period of ∼24 hours. This cell-autonomous TTFL controls cycles of gene expression in all major tissues and these cycles underpin our daily metabolic programs. In turn, our innumerable cellular clocks are coordinated by a central pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. When isolated in slice culture, the SCN TTFL and its dependent cycles of neural activity persist indefinitely, operating as "a clock in a dish". In vivo, SCN time is synchronized to solar time by direct innervation from specialized retinal photoreceptors. In turn, the precise circadian cycle of action potential firing signals SCN-generated time to hypothalamic and brain stem targets, which co-ordinate downstream autonomic, endocrine, and behavioral (feeding) cues to synchronize and sustain the distributed cellular clock network. Circadian time therefore pervades every level of biological organization, from molecules to society. Understanding its mechanisms offers important opportunities to mitigate the consequences of circadian disruption, so prevalent in modern societies, that arise from shiftwork, aging, and neurodegenerative diseases, not least Huntington's disease.

Imaging the Hypothalamo-Neurohypophysial System

The hypothalamo-neurohypophysial system (HNS) is a brain peptidergic neurosecretory apparatus which is composed of arginine vasopressin (AVP) and oxytocin (OXT) magnocellular neurones and their neuronal processes in the posterior pituitary (PP). In response to specific stimuli, AVP and OXT are secreted into the systemic circulation at the neurovascular interface of the PP, where they act as hormones, but they can also behave as neurotransmitters when released at the somatodendritic compartment or by axon collaterals to other brain regions. Because these peptides are crucial for several physiological processes, including fluid homoeostasis and reproduction, it is of great importance to map the HNS connectome in its entirety in order to understand its functions. In recent years, advances in imaging technologies have provided considerable new information about the HNS. These approaches include the use of reporter proteins under the control of specific promoters, viral tracers, brain-clearing methods, genetically encoded indicators, sniffer cells, mass spectrometry imaging, and spatially resolved transcriptomics. In this review, we illustrate how these latest approaches have enhanced our understanding of the structure and function of the HNS and how they might contribute further in the coming years.

Circadian expression and specific localization of synaptotagmin17 in the suprachiasmatic nucleus, the master circadian oscillator in mammals

The localization and function of synaptotagmin (syt)17 in the suprachiasmatic nucleus (SCN) of the brain, which is the master circadian oscillator, were investigated. The Syt17 mRNA-containing neurons were mainly situated in the shell region while SYT17 immunoreactive cell bodies and neural fibers were detected in the core and shell of the SCN and the subparaventricular zone (SPZ). Further, electron microscopy analysis revealed SYT17 in the rough endoplasmic reticulum (rER), Golgi apparatus (G), and large and small vesicles of neurons. Syt17 mRNA expression in the SCN showed a circadian rhythm, and light exposure at night suppressed its expression. In addition, the free running period of locomotor activity rhythm was shortened in Syt17-deletion mutant mice. These findings suggest that SYT17 is involved in the regulation of circadian rhythms.

Regionality of short and long period oscillators in the suprachiasmatic nucleus and their manner of synchronization

In mammals, the center of the circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Many studies have suggested that there are multiple regions generating different circadian periods within the SCN, but the exact localization of the regions has not been elucidated. In this study, using a transgenic rat carrying a destabilized luciferase reporter gene driven by a regulatory element of Per2 gene (Per2::dLuc), we investigated the regional variation of period lengths in horizontal slices of the SCN. We revealed a distinct caudal medial region (short period region, SPR) and a rostro-lateral region (long period region, LPR) that generate circadian rhythms with periods shorter than and longer than 24 hours, respectively. We also found that the core region of the SCN marked by dense VIP (vasoactive intestinal peptide) mRNA-expressing neurons covered a part of LPR, and that the shell region of the SCN contains both SPR and the rest of the LPR. Furthermore, we observed how synchronization is achieved between regions generating distinct circadian periods in the SCN. We found that the longer circadian rhythm of the rostral region appears to entrain the circadian rhythm in the caudal region. Our findings clarify the localization of regionality of circadian periods and the mechanism by which the integrated circadian rhythm is formed in the SCN.

Functions of regulators of G protein signaling 16 in immunity, inflammation, and other diseases

Regulators of G protein signaling (RGS) act as guanosine triphosphatase activating proteins to accelerate guanosine triphosphate hydrolysis of the G protein α subunit, leading to the termination of the G protein-coupled receptor (GPCR) downstream signaling pathway. RGS16, which is expressed in a number of cells and tissues, belongs to one of the small B/R4 subfamilies of RGS proteins and consists of a conserved RGS structural domain with short, disordered amino- and carboxy-terminal extensions and an α-helix that classically binds and de-activates heterotrimeric G proteins. However, with the deepening of research, it has been revealed that RGS16 protein not only regulates the classical GPCR pathway, but also affects immune, inflammatory, tumor and metabolic processes through other signaling pathways including the mitogen-activated protein kinase, phosphoinositide 3-kinase/protein kinase B, Ras homolog family member A and stromal cell-derived factor 1/C-X-C motif chemokine receptor 4 pathways. Additionally, the RGS16 protein may be involved in the Hepatitis B Virus -induced inflammatory response. Therefore, given the continuous expansion of knowledge regarding its role and mechanism, the structure, characteristics, regulatory mechanisms and known functions of the small RGS proteinRGS16 are reviewed in this paper to prepare for diagnosis, treatment, and prognostic evaluation of different diseases such as inflammation, tumor, and metabolic disorders and to better study its function in other diseases.

Seasonal changes in day length induce multisynaptic neurotransmitter switching to regulate hypothalamic network activity and behavior

Seasonal changes in day length (photoperiod) affect numerous physiological functions. The suprachiasmatic nucleus (SCN)-paraventricular nucleus (PVN) axis plays a key role in processing photoperiod-related information. Seasonal variations in SCN and PVN neurotransmitter expression have been observed in humans and animal models. However, the molecular mechanisms by which the SCN-PVN network responds to altered photoperiod is unknown. Here, we show in mice that neuromedin S (NMS) and vasoactive intestinal polypeptide (VIP) neurons in the SCN display photoperiod-induced neurotransmitter plasticity. In vivo recording of calcium dynamics revealed that NMS neurons alter PVN network activity in response to winter-like photoperiod. Chronic manipulation of NMS neurons is sufficient to induce neurotransmitter switching in PVN neurons and affects locomotor activity. Our findings reveal previously unidentified molecular adaptations of the SCN-PVN network in response to seasonality and the role for NMS neurons in adjusting hypothalamic function to day length via a coordinated multisynaptic neurotransmitter switching affecting behavior.

Female fertility does not require Bmal1 in suprachiasmatic nucleus neurons expressing arginine vasopressin, vasoactive intestinal peptide, or neuromedin-S

Disruptions to the circadian system alter reproductive capacity, particularly in females. Mice lacking the core circadian clock gene, Bmal1, are infertile and have evidence of neuroendocrine disruption including the absence of the preovulatory luteinizing hormone (LH) surge and enhanced responsiveness to exogenous kisspeptin. Here, we explore the role of Bmal1 in suprachiasmatic nucleus (SCN) neuron populations known to project to the neuroendocrine axis. We generated four mouse lines using Cre/Lox technology to create conditional deletion of Bmal1 in arginine vasopressin (Bmal1fl/fl:Avpcre ), vasoactive intestinal peptide (Bmal1fl/fl:Vipcre ), both (Bmal1fl/fl:Avpcre+Vipcre ), and neuromedin-s (Bmal1fl/fl:Nmscre ) neurons. We demonstrate that the loss of Bmal1 in these populations has substantial effects on home-cage circadian activity and temperature rhythms. Despite this, we found that female mice from these lines demonstrated normal estrus cycles, fecundity, kisspeptin responsiveness, and inducible LH surge. We found no evidence of reproductive disruption in constant darkness. Overall, our results indicate that while conditional Bmal1 knockout in AVP, VIP, or NMS neurons is sufficient to disrupted locomotor activity, this disruption is insufficient to recapitulate the neuroendocrine reproductive effects of the whole-body Bmal1 knockout.

Vasopressin neurons in the paraventricular hypothalamus promote wakefulness via lateral hypothalamic orexin neurons

The sleep-wakefulness cycle is regulated by complicated neural networks that include many different populations of neurons throughout the brain. Arginine vasopressin neurons in the paraventricular nucleus of the hypothalamus (PVH^AVP) regulate various physiological events and behaviors, such as body-fluid homeostasis, blood pressure, stress response, social interaction, and feeding. Changes in arousal level often accompany these PVH^AVP-mediated adaptive responses. However, the contribution of PVH^AVP neurons to sleep-wakefulness regulation has remained unknown. Here, we report the involvement of PVH^AVP neurons in arousal promotion. Optogenetic stimulation of PVH^AVP neurons rapidly induced transitions to wakefulness from both NREM and REM sleep. This arousal effect was dependent on AVP expression in these neurons. Similarly, chemogenetic activation of PVH^AVP neurons increased wakefulness and reduced NREM and REM sleep, whereas chemogenetic inhibition of these neurons significantly reduced wakefulness and increased NREM sleep. We observed dense projections of PVH^AVP neurons in the lateral hypothalamus with potential connections to orexin/hypocretin (LH^Orx) neurons. Optogenetic stimulation of PVH^AVP neuronal fibers in the LH immediately induced wakefulness, whereas blocking orexin receptors attenuated the arousal effect of PVH^AVP neuronal activation drastically. Monosynaptic rabies-virus tracing revealed that PVH^AVP neurons receive inputs from multiple brain regions involved in sleep-wakefulness regulation, as well as those involved in stress response and energy metabolism. Moreover, PVH^AVP neurons mediated the arousal induced by novelty stress and a melanocortin receptor agonist melanotan-II. Thus, our data suggested that PVH^AVP neurons promote wakefulness via LH^Orx neurons in the basal sleep-wakefulness and some stressful conditions.

Paraventricular hypothalamic vasopressin neurons induce self-grooming in mice

Self-grooming plays an essential role in hygiene maintenance, thermoregulation, and stress response. However, the neural populations involved in self-grooming remain largely unknown. The paraventricular hypothalamic nucleus (PVH) has been implicated in the regulation of self-grooming. Arginine vasopressin-producing neurons are among the major neuronal populations in the PVH (PVH^AVP), which play important roles in water homeostasis, blood pressure regulation, feeding, and stress response. Here, we report the critical role of PVH^AVP neurons in the induction of self-grooming. Optogenetic activation of PVH^AVP neurons immediately induced self-grooming in freely moving mice. Chemogenetic activation of these neurons also increased time spent self-grooming. In contrast, their chemogenetic inhibition significantly reduced naturally occurring self-grooming, suggesting that PVH^AVP-induced grooming has physiological relevance. Notably, optogenetic activation of PVH^AVP neurons triggered self-grooming over other adaptive behaviors, such as voracious feeding induced by fasting and social interaction with female mice. Thus, our study proposes the novel role of PVH^AVP neurons in regulating self-grooming behavior and, consequently, hygiene maintenance and stress response. Furthermore, uncontrolled activation of these neurons may be potentially relevant to diseases characterized by compulsive behaviors and impaired social interaction, such as autism, obsessive–compulsive disorder, and anorexia nervosa.

Cell Type-Specific Genetic Manipulation and Impaired Circadian Rhythms in ViptTA Knock-In Mice

The suprachiasmatic nucleus (SCN), the central circadian clock in mammals, is a neural network consisting of various types of GABAergic neurons, which can be differentiated by the co-expression of specific peptides such as vasoactive intestinal peptide (VIP) and arginine vasopressin (AVP). VIP has been considered as a critical factor for the circadian rhythmicity and synchronization of individual SCN neurons. However, the precise mechanisms of how VIP neurons regulate SCN circuits remain incompletely understood. Here, we generated Vip^tTA knock-in mice that express tetracycline transactivator (tTA) specifically in VIP neurons by inserting tTA sequence at the start codon of Vip gene. The specific and efficient expression of tTA in VIP neurons was verified using EGFP reporter mice. In addition, combined with Avp-Cre mice, Vip^tTA mice enabled us to simultaneously apply different genetic manipulations to VIP and AVP neurons in the SCN. Immunostaining showed that VIP is expressed at a slightly reduced level in heterozygous Vip^tTA mice but is completely absent in homozygous mice. Consistently, homozygous Vip^tTA mice showed impaired circadian behavioral rhythms similar to those of Vip knockout mice, such as attenuated rhythmicity and shortened circadian period. In contrast, heterozygous mice demonstrated normal circadian behavioral rhythms comparable to wild-type mice. These data suggest that Vip^tTA mice are a valuable genetic tool to express exogenous genes specifically in VIP neurons in both normal and VIP-deficient mice, facilitating the study of VIP neuronal roles in the SCN neural network.

It's About Time: The Circadian Network as Time-Keeper for Cognitive Functioning, Locomotor Activity and Mental Health

A variety of organisms including mammals have evolved a 24h, self-sustained timekeeping machinery known as the circadian clock (biological clock), which enables to anticipate, respond, and adapt to environmental influences such as the daily light and dark cycles. Proper functioning of the clock plays a pivotal role in the temporal regulation of a wide range of cellular, physiological, and behavioural processes. The disruption of circadian rhythms was found to be associated with the onset and progression of several pathologies including sleep and mental disorders, cancer, and neurodegeneration. Thus, the role of the circadian clock in health and disease, and its clinical applications, have gained increasing attention, but the exact mechanisms underlying temporal regulation require further work and the integration of evidence from different research fields. In this review, we address the current knowledge regarding the functioning of molecular circuits as generators of circadian rhythms and the essential role of circadian synchrony in a healthy organism. In particular, we discuss the role of circadian regulation in the context of behaviour and cognitive functioning, delineating how the loss of this tight interplay is linked to pathological development with a focus on mental disorders and neurodegeneration. We further describe emerging new aspects on the link between the circadian clock and physical exercise-induced cognitive functioning, and its current usage as circadian activator with a positive impact in delaying the progression of certain pathologies including neurodegeneration and brain-related disorders. Finally, we discuss recent epidemiological evidence pointing to an important role of the circadian clock in mental health.

Rapid eye movement sleep is initiated by basolateral amygdala dopamine signaling in mice

The sleep cycle is characterized by alternating non-rapid eye movement (NREM) and rapid eye movement (REM) sleeps. The mechanisms by which this cycle is generated are incompletely understood. We found that a transient increase of dopamine (DA) in the basolateral amygdala (BLA) during NREM sleep terminates NREM sleep and initiates REM sleep. DA acts on dopamine receptor D2 (Drd2)-expressing neurons in the BLA to induce the NREM-to-REM transition. This mechanism also plays a role in cataplectic attacks-a pathological intrusion of REM sleep into wakefulness-in narcoleptics. These results show a critical role of DA signaling in the BLA in initiating REM sleep and provide a neuronal basis for sleep cycle generation.

High-throughput Genetically Modified Animal Experiments Achieved by Next-generation Mammalian Genetics

Animal models are essential tools for modern scientists to conduct biological experiments and investigate their hypotheses in vivo. However, for the past decade, raising the throughput of such animal experiments has been a great challenge. Conventionally, in vivo high-throughput assay was achieved through large-scale mutagen-driven forward genetic screening, which took years to find causal genes. In contrast, reverse genetics accelerated the causal gene identification process, but its throughput was also limited by 2 barriers, that is, the genome modification step and the time-consuming crossing step. Defined as genetics without crossing, next-generation genetics is able to produce gene-modified animals that can be analyzed at the founder generation (F0). This method is or can be accomplished through recent technological advances in gene editing and virus-based efficient gene modifications. Notably, next-generation genetics has accelerated the process of cross-species studies, and it will be a useful technique during animal experiments as it can provide genetic perturbation at an individual level without crossing. In this review, we begin by introducing the history of animal-based high-throughput analysis, with a specific focus on chronobiology. We then describe ways that gene modification efficiency during animal experiments was enhanced and why crossing remained a barrier to reaching higher efficiency. Moreover, we mention the Triple CRISPR as a critical technique for achieving next-generation genetics. Finally, we discuss the potential applications and limitations of next-generation mammalian genetics.

Circadian Rhythms in the Neuronal Network Timing the Luteinizing Hormone Surge

For billions of years before electric light was invented, life on Earth evolved under the pattern of light during the day and darkness during the night. Through evolution, nearly all organisms internalized the temporal rhythm of Earth's 24-hour rotation and evolved self-sustaining biological clocks with a ~24-hour rhythm. These internal rhythms are called circadian rhythms, and the molecular constituents that generate them are called molecular circadian clocks. Alignment of molecular clocks with the environmental light-dark rhythms optimizes physiology and behavior. This phenomenon is particularly true for reproductive function, in which seasonal breeders use day length information to time yearly changes in fertility. However, it is becoming increasingly clear that light-induced disruption of circadian rhythms can negatively impact fertility in nonseasonal breeders as well. In particular, the luteinizing hormone surge promoting ovulation is sensitive to circadian disruption. In this review, we will summarize our current understanding of the neuronal networks that underlie circadian rhythms and the luteinizing hormone surge.

Cell-Type-Specific Circadian Bioluminescence Rhythms in Dbp Reporter Mice

Circadian rhythms are endogenously generated physiological and molecular rhythms with a cycle length of about 24 h. Bioluminescent reporters have been exceptionally useful for studying circadian rhythms in numerous species. Here, we report development of a reporter mouse generated by modification of a widely expressed and highly rhythmic gene encoding D-site albumin promoter binding protein (Dbp). In this line of mice, firefly luciferase is expressed from the Dbp locus in a Cre recombinase-dependent manner, allowing assessment of bioluminescence rhythms in specific cellular populations. A mouse line in which luciferase expression was Cre-independent was also generated. The Dbp reporter alleles do not alter Dbp gene expression rhythms in liver or circadian locomotor activity rhythms. In vivo and ex vivo studies show the utility of the reporter alleles for monitoring rhythmicity. Our studies reveal cell-type-specific characteristics of rhythms among neuronal populations within the suprachiasmatic nuclei ex vivo. In vivo studies show Dbp-driven bioluminescence rhythms in the liver of Albumin-Cre;Dbp^KI/+ "liver reporter" mice. After a shift of the lighting schedule, locomotor activity achieved the proper phase relationship with the new lighting cycle more rapidly than hepatic bioluminescence did. As previously shown, restricting food access to the daytime altered the phase of hepatic rhythmicity. Our model allowed assessment of the rate of recovery from misalignment once animals were provided with food ad libitum. These studies confirm the previously demonstrated circadian misalignment following environmental perturbations and reveal the utility of this model for minimally invasive, longitudinal monitoring of rhythmicity from specific mouse tissues.

SOX2 Regulates Neuronal Differentiation of the Suprachiasmatic Nucleus

In mammals, the hypothalamic suprachiasmatic nucleus (SCN) functions as the central circadian pacemaker, orchestrating behavioral and physiological rhythms in alignment to the environmental light/dark cycle. The neurons that comprise the SCN are anatomically and functionally heterogeneous, but despite their physiological importance, little is known about the pathways that guide their specification and differentiation. Here, we report that the stem/progenitor cell transcription factor, Sex determining region Y-box 2 (Sox2), is required in the embryonic SCN to control the expression of SCN-enriched neuropeptides and transcription factors. Ablation of Sox2 in the developing SCN leads to downregulation of circadian neuropeptides as early as embryonic day (E) 15.5, followed by a decrease in the expression of two transcription factors involved in SCN development, Lhx1 and Six6, in neonates. Thymidine analog-retention assays revealed that Sox2 deficiency contributed to reduced survival of SCN neurons during the postnatal period of cell clearance, but did not affect progenitor cell proliferation or SCN specification. Our results identify SOX2 as an essential transcription factor for the proper differentiation and survival of neurons within the developing SCN.