CNS inputs to the suprachiasmatic nucleus of the rat

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.

Crosstalk between the subiculum and sleep-wake regulation: A review

The circuitry underlying the initiation, maintenance, and coordination of wakefulness, rapid eye movement sleep, and non-rapid eye movement sleep is not thoroughly understood. Sleep is thought to arise due to decreased activity in the ascending reticular arousal system, which originates in the brainstem and awakens the thalamus and cortex during wakefulness. Despite the conventional association of sleep-wake states with hippocampal rhythms, the mutual influence of the hippocampal formation in regulating vigilance states has been largely neglected. Here, we focus on the subiculum, the main output region of the hippocampal formation. The subiculum, particulary the ventral part, sends extensive monosynaptic projections to crucial regions implicated in sleep-wake regulation, including the thalamus, lateral hypothalamus, tuberomammillary nucleus, basal forebrain, ventrolateral preoptic nucleus, ventrolateral tegmental area, and suprachiasmatic nucleus. Additionally, second-order projections from the subiculum are received by the laterodorsal tegmental nucleus, locus coeruleus, and median raphe nucleus, suggesting the potential involvement of the subiculum in the regulation of the sleep-wake cycle. We also discuss alterations in the subiculum observed in individuals with sleep disorders and in sleep-deprived mice, underscoring the significance of investigating neuronal communication between the subiculum and pathways promoting both sleep and wakefulness.

Potential neural substrates underlying circadian and olfactory disruptions in preclinical Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia, with over 45 million patients worldwide, and poses significant economic and emotional burdens to both patients and caregivers, significantly raising the number of those affected. Unfortunately, much of the existing research on the disease only addresses a small subset of associated symptomologies and pathologies. In this review, we propose to target the earliest stages of the disease, when symptomology first arises. In these stages, before the onset of hallmark symptoms of AD such as cognitive impairments and memory loss, circadian and olfactory disruptions arise and are detectable. Functional similarities between circadian and olfactory systems provide a basis upon which to seek out common mechanisms in AD which may target them early on in the disease. Existing studies of interactions between these systems, while intriguing, leave open the question of the neural substrates underlying them. Potential substrates for such interactions are proposed in this review, such as indirect projections that may functionally connect the two systems and dopaminergic signaling. These substrates may have significant implications for mechanisms underlying disruptions to circadian and olfactory function in early stages of AD. In this review, we propose early detection of AD using a combination of circadian and olfactory deficits and subsequent early treatment of these deficits may provide profound benefits to both patients and caregivers. Additionally, we suggest that targeting research toward the intersection of these two systems in AD could uncover mechanisms underlying the broader set of symptoms and pathologies that currently elude researchers.

A brainstem to circadian system circuit links Tau pathology to sundowning-related disturbances in an Alzheimer's disease mouse model

Alzheimer's disease (AD) patients exhibit progressive disruption of entrained circadian rhythms and an aberrant circadian input pathway may underlie such dysfunction. Here we examine AD-related pathology and circadian dysfunction in the APPSwe-Tau (TAPP) model of AD. We show these mice exhibit phase delayed body temperature and locomotor activity with increases around the active-to-rest phase transition. Similar AD-related disruptions are associated with sundowning, characterized by late afternoon and early evening agitation and aggression, and we show TAPP mice exhibit increased aggression around this transition. We show such circadian dysfunction and aggression coincide with hyperphosphorylated Tau (pTau) development in lateral parabrachial (LPB) neurons, with these disturbances appearing earlier in females. Finally, we show LPB neurons, including those expressing dynorphin (LPBdyn), project to circadian structures and are affected by pTau, and LPBdyn ablations partially recapitulate the hyperthermia of TAPP mice. Altogether we link pTau in a brainstem circadian input pathway to AD-related disturbances relevant to sundowning.

Potential Interactions Between Cerebellar Dysfunction and Sleep Disturbances in Dystonia

Dystonia is the third most common movement disorder. It causes debilitating twisting postures that are accompanied by repetitive and sometimes intermittent co- or over-contractions of agonist and antagonist muscles. Historically diagnosed as a basal ganglia disorder, dystonia is increasingly considered a network disorder involving various brain regions including the cerebellum. In certain etiologies of dystonia, aberrant motor activity is generated in the cerebellum and the abnormal signals then propagate through a “dystonia circuit” that includes the thalamus, basal ganglia, and cerebral cortex. Importantly, it has been reported that non-motor defects can accompany the motor symptoms; while their severity is not always correlated, it is hypothesized that common pathways may nevertheless be disrupted. In particular, circadian dysfunction and disordered sleep are common non-motor patient complaints in dystonia. Given recent evidence suggesting that the cerebellum contains a circadian oscillator, displays sleep-stage-specific neuronal activity, and sends robust long-range projections to several subcortical regions involved in circadian rhythm regulation, disordered sleep in dystonia may result from cerebellum-mediated dysfunction of the dystonia circuit. Here, we review the evidence linking dystonia, cerebellar network dysfunction, and cerebellar involvement in sleep. Together, these ideas may form the basis for the development of improved pharmacological and surgical interventions that could take advantage of cerebellar circuitry to restore normal motor function as well as non-motor (sleep) behaviors in dystonia.

Effects of prenatal exposure to fluoxetine on circadian rhythmicity in the locomotor activity and neuropeptide Y and 5-HT expression in male and female adult Wistar rats

Serotonin (5-HT) reuptake inhibitors, such as fluoxetine, are the most prescribed antidepressant for maternal depression. In this sense, it exposes mothers and the brains of infants to increased modulatory and trophic effects of serotonergic neurotransmission. 5-HT promotes essential brain changes throughout its development, which include neuron migration, differentiation, and organization of neural circuitries related to emotional, cognitive, and circadian behavior. Early exposure to the SSRIs induces long-term effects on behavioral and neural serotonergic signalization. We have aimed to evaluate the circadian rhythm of locomotor activity and the neurochemical content, neuropeptide Y (NPY) and 5-HT in three brain areas: intergeniculate leaflet (IGL), suprachiasmatic nuclei (SCN) and raphe nuclei (RN), at two zeitgebers (ZT6 and ZT18), in male and female rat's offspring early exposed (developmental period GD13-GD21) to fluoxetine (20mg/kg). First, we have conducted daily records of the locomotor activity rhythm using activity sensors coupled to individual cages over four weeks. We have lastly evaluated the immunoreactivity of NPY in both SCN and IGL, and as well the 5-HT expression in the dorsal and medial RN. In summary, our results showed that (1) prenatal fluoxetine affects phase entrainment of the rest/activity rhythm at ZT6 and ZT18, more in male than female specimens, and (2) modulates the NPY and 5-HT expression. Here, we show male rats are more susceptible to phase entrainment and the NPY and 5-HT misexpression compared to female ones. The sex differences induced by early exposure to fluoxetine in both the circadian rhythm of locomotor activity and the neurochemical expression into SCN, IGL, and midbrain raphe are an important highlight in the present work. Thus, our results may help to improve the knowledge on neurobiological mechanisms of circadian rhythms and are relevant to understanding the "broken brains" and behavioral abnormalities of offspring early exposed to antidepressants.

Systematic review of drugs that modify the circadian system's phase-shifting responses to light exposure

We searched PubMed for primary research quantifying drug modification of light-induced circadian phase-shifting in rodents. This search, conducted for work published between 1960 and 2018, yielded a total of 146 papers reporting results from 901 studies. Relevant articles were those with any extractable data on phase resetting in wildtype (non-trait selected) rodents administered a drug, alongside a vehicle/control group, near or at the time of exposure. Most circadian pharmacology experiments were done using drugs thought to act directly on either the brain's central pacemaker, the suprachiasmatic nucleus (SCN), the SCN's primary relay, the retinohypothalamic tract, secondary pathways originating from the medial/dorsal raphe nuclei and intergeniculate leaflet, or the brain's sleep-arousal centers. While the neurotransmitter systems underlying these circuits were of particular interest, including those involving glutamate, gamma-aminobutyric acid, serotonin, and acetylcholine, other signaling modalities have also been assessed, including agonists and antagonists of receptors linked to dopamine, histamine, endocannabinoids, adenosine, opioids, and second-messenger pathways downstream of glutamate receptor activation. In an effort to identify drugs that unduly influence circadian responses to light, we quantified the net effects of each drug class by ratioing the size of the phase-shift observed after administration to that observed with vehicle in a given experiment. This allowed us to organize data across the literature, compare the relative efficacy of one mechanism versus another, and clarify which drugs might best suppress or potentiate phase resetting. Aggregation of the available data in this manner suggested that several candidates might be clinically relevant as auxiliary treatments to suppress ectopic light responses during shiftwork or amplify the circadian effects of timed bright light therapy. Future empirical research will be necessary to validate these possibilities.

The Role of the Central Histaminergic System in Behavioral State Control

Histamine is a small monoamine signaling molecule that plays a role in many peripheral and central physiological processes, including the regulation of wakefulness. The tuberomammillary nucleus is the sole neuronal source of histamine in the brain, and histamine neurons are thought to promote wakefulness and vigilance maintenance - under certain environmental and/or behavioral contexts - through their diffuse innervation of the cortex and other wake-promoting brain circuits. Histamine neurons also contain a number of other putative neurotransmitters, although the functional role of these co-transmitters remains incompletely understood. Within the brain histamine operates through three receptor subtypes that are located on pre- and post-synaptic membranes. Some histamine receptors exhibit constitutive activity, and hence exist in an activated state even in the absence of histamine. Newer medications used to reduce sleepiness in narcolepsy patients in fact enhance histamine signaling by blunting the constitutive activity of these histamine receptors. In this chapter, we provide an overview of the central histamine system with an emphasis on its role in behavioral state regulation and how drugs targeting histamine receptors are used clinically to treat a wide range of sleep-wake disorders.

The impact of stress and stress hormones on endogenous clocks and circadian rhythms

In mammals, daily rhythms in physiology and behavior are under control of a circadian pacemaker situated in the suprachiasmatic nucleus (SCN). This master clock receives photic input from the retina and coordinates peripheral oscillators present in other tissues, maintaining all rhythms in the body synchronized to the environmental light-dark cycle. In line with its function as a master clock, the SCN appears to be well protected against unpredictable stressful stimuli. However, available data indicate that stress and stress hormones at certain times of day are capable of shifting peripheral oscillators in, e.g., liver, kidney and heart, which are normally under control of the SCN. Such shifts of peripheral oscillators may represent a temporary change in circadian organization that facilitates adaptation to repeated stress. Alternatively, these shifts of internal rhythms may represent an imbalance between precisely orchestrated physiological and behavioral processes that may have severe consequences for health and well-being.

Whole-brain mapping of afferent projections to the suprachiasmatic nucleus of the tree shrew

The suprachiasmatic nucleus (SCN) is essential for the neural control of mammalian circadian timing system. The circadian activity of the SCN is modulated by its afferent projections. In the present study, we examine neuroanatomical characteristics and afferent projections of the SCN in the tree shrew (Tupaia belangeri chinensis) using immunocytochemistry and retrograde tracer Fluoro-Gold (FG). Distribution of the vasoactive intestinal peptide was present in the SCN from rostral to caudal, especially concentrated in its ventral part. FG-labeled neurons were observed in the lateral septal nucleus, septofimbrial nucleus, paraventricular thalamic nucleus, posterior hypothalamic nucleus, posterior complex of the thalamus, ventral subiculum, rostral linear nucleus of the raphe, periaqueductal gray, mesencephalic reticular formation, dorsal raphe nucleus, pedunculopontine tegmental nucleus, medial parabrachial nucleus, locus coeruleus, parvicellular reticular nucleus, intermediate reticular nucleus, and ventrolateral reticular nucleus. In summary, the morphology of the SCN in tree shrews is described from rostral to caudal. In addition, our data demonstrate for the first time that the SCN in tree shrews receives inputs from numerous brain regions in the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. This comprehensive knowledge of the afferent projections of the SCN in tree shrews provides further insights into the neural organization and physiological processes of circadian rhythms.

Doublecortin-like expressing astrocytes of the suprachiasmatic nucleus are implicated in the biosynthesis of vasopressin and influences circadian rhythms

We have recently identified a novel plasticity protein, doublecortin-like (DCL), that is specifically expressed in the shell of the mouse suprachiasmatic nucleus (SCN). DCL is implicated in neuroplastic events, such as neurogenesis, that require structural rearrangements of the microtubule cytoskeleton, enabling dynamic movements of cell bodies and dendrites. We have inspected DCL expression in the SCN by confocal microscopy and found that DCL is expressed in GABA transporter-3 (GAT3)-positive astrocytes that envelope arginine vasopressin (AVP)-expressing cells. To investigate the role of these DCL-positive astrocytes in circadian rhythmicity, we have used transgenic mice expressing doxycycline-induced short-hairpin (sh) RNA's targeting DCL mRNA (DCL knockdown mice). Compared with littermate wild type (WT) controls, DCL-knockdown mice exhibit significant shorter circadian rest-activity periods in constant darkness and adjusted significantly faster to a jet-lag protocol. As DCL-positive astrocytes are closely associated with AVP-positive cells, we analyzed AVP expression in DCL-knockdown mice and in their WT littermates by 3D reconstructions and transmission electron microscopy (TEM). We found significantly higher numbers of AVP-positive cells with increased volume and more intensity in DCL-knockdown mice. We found alterations in the numbers of dense core vesicle-containing neurons at ZT8 and ZT20 suggesting that the peak and trough of neuropeptide biosynthesis is dampened in DCL-knockdown mice compared to WT littermates. Together, our data suggest an important role for the astrocytic plasticity in the regulation of circadian rhythms and point to the existence of a specific DCL⁺ astrocyte-AVP⁺ neuronal network located in the dorsal SCN implicated in AVP biosynthesis.

GABAergic Neurons in the Dorsal-Intermediate Lateral Septum Regulate Sleep-Wakefulness and Anesthesia in Mice

BACKGROUND

The γ-aminobutyric acid-mediated (GABAergic) inhibitory system in the brain is critical for regulation of sleep-wake and general anesthesia. The lateral septum contains mainly GABAergic neurons, being cytoarchitectonically divided into the dorsal, intermediate, and ventral parts. This study hypothesized that GABAergic neurons of the lateral septum participate in the control of wakefulness and promote recovery from anesthesia.

METHODS

By employing fiber photometry, chemogenetic and optogenetic neuronal manipulations, anterograde tracing, in vivo electrophysiology, and electroencephalogram/electromyography recordings in adult male mice, the authors measured the role of lateral septum GABAergic neurons to the control of sleep-wake transition and anesthesia emergence and the corresponding neuron circuits in arousal and emergence control.

RESULTS

The GABAergic neurons of the lateral septum exhibited high activities during the awake state by in vivo fiber photometry recordings (awake vs. non-rapid eye movement sleep: 3.3 ± 1.4% vs. -1.3 ± 1.2%, P < 0.001, n = 7 mice/group; awake vs. anesthesia: 2.6 ± 1.2% vs. -1.3 ± 0.8%, P < 0.001, n = 7 mice/group). Using chemogenetic stimulation of lateral septum GABAergic neurons resulted in a 100.5% increase in wakefulness and a 51.2% reduction in non-rapid eye movement sleep. Optogenetic activation of these GABAergic neurons promoted wakefulness from sleep (median [25th, 75th percentiles]: 153.0 [115.9, 179.7] s to 4.0 [3.4, 4.6] s, P = 0.009, n = 5 mice/group) and accelerated emergence from isoflurane anesthesia (514.4 ± 122.2 s vs. 226.5 ± 53.3 s, P < 0.001, n = 8 mice/group). Furthermore, the authors demonstrated that the lateral septum GABAergic neurons send 70.7% (228 of 323 cells) of monosynaptic projections to the ventral tegmental area GABAergic neurons, preferentially inhibiting their activities and thus regulating wakefulness and isoflurane anesthesia depth.

CONCLUSIONS

The results uncover a fundamental role of the lateral septum GABAergic neurons and their circuit in maintaining awake state and promoting general anesthesia emergence time.

EDITOR’S PERSPECTIVE

Activation State of the Supramammillary Nucleus Regulates Body Composition and Peripheral Fuel Metabolism

Whole body fuel metabolism and energy balance are controlled by an interactive brain neuronal circuitry involving multiple brain centers regulating cognition, circadian rhythms, reward, feeding and peripheral biochemical metabolism. The hypothalamic supramammillary nucleus (SuMN) comprises an integral node having connections with these metabolically relevant centers, and thus could be a key central coordination center for regulating peripheral energy balance. This study investigated the effect of chronically diminishing or increasing SuMN neuronal activity on body composition and peripheral fuel metabolism. The influence of neuronal activity level at the SuMN area on peripheral metabolism was investigated via chronic (2-4 week) direct SuMN treatment with agents that inhibit neuronal activity (GABAa receptor agonist [Muscimol] and AMPA plus NMDA glutamate receptor antagonists [CNQX plus dAP5, respectively]) in high fat fed animals refractory to the obesogenic effects of high fat diet. Such treatment reduced SuMN neuronal activity and induced metabolic syndrome, and likewise did so in animals fed low fat diet including inducement of glucose intolerance, insulin resistance, hyperinsulinemia, hyperleptinemia, and increased body weight gain and fat mass coupled with both increased food consumption and feed efficiency. Consistent with these results, circadian-timed activation of neuronal activity at the SuMN area with daily local infusion of glutamate receptor agonists, AMPA or NMDA at the natural daily peak of SuMN neuronal activity improved insulin resistance and obesity in high fat diet-induced insulin resistant animals. These studies are the first of their kind to identify the SuMN area as a novel brain locus that regulates peripheral fuel metabolism.

The connections of Locus Coeruleus with hypothalamus: potential involvement in Alzheimer's disease

The hypothalamus and Locus Coeruleus (LC) share a variety of functions, as both of them take part in the regulation of the sleep/wake cycle and in the modulation of autonomic and homeostatic activities. Such a functional interplay takes place due to the dense and complex anatomical connections linking the two brain structures. In Alzheimer's disease (AD), the occurrence of endocrine, autonomic and sleep disturbances have been associated with the disruption of the hypothalamic network; at the same time, in this disease, the occurrence of LC degeneration is receiving growing attention for the potential roles it may have both from a pathophysiological and pathogenetic point of view. In this review, we summarize the current knowledge on the anatomical and functional connections between the LC and hypothalamus, to better understand whether the impairment of the former may be responsible for the pathological involvement of the latter, and whether the disruption of their interplay may concur to the pathophysiology of AD. Although only a few papers specifically explored this topic, intriguingly, some pre-clinical and post-mortem human studies showed that aberrant protein spreading and neuroinflammation may cause hypothalamus degeneration and that these pathological features may be linked to LC impairment. Moreover, experimental studies in rodents showed that LC plays a relevant role in modulating the hypothalamic sleep/wake cycle regulation or neuroendocrine and systemic hormones; in line with this, the degeneration of LC itself may partly explain the occurrence of hypothalamic-related symptoms in AD.

Genesis of the Master Circadian Pacemaker in Mice

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central circadian clock of mammals. It is responsible for communicating temporal information to peripheral oscillators via humoral and endocrine signaling, ultimately controlling overt rhythms such as sleep-wake cycles, body temperature, and locomotor activity. Given the heterogeneity and complexity of the SCN, its genesis is tightly regulated by countless intrinsic and extrinsic factors. Here, we provide a brief overview of the development of the SCN, with special emphasis on the murine system.

Resilience in the suprachiasmatic nucleus: Implications for aging and Alzheimer's disease

Many believe that the circadian impairments associated with aging and Alzheimer's disease are, simply enough, a byproduct of tissue degeneration within the central pacemaker, the suprachiasmatic nucleus (SCN). However, the findings that have accumulated to date examining the SCNs obtained postmortem from the brains of older individuals, or those diagnosed with Alzheimer's disease upon autopsy, suggest only limited atrophy. We review this literature as well as a complementary one concerning fetal-donor SCN transplant, which established that many circadian timekeeping functions can be maintained with rudimentary (structurally limited) representations of the SCN. Together, these corpora of data suggest that the SCN is a resilient brain region that cannot be directly (or solely) implicated in the behavioral manifestations of circadian disorganization often witnessed during aging as well as early and late progression of Alzheimer's disease. We complete our review by suggesting future directions of research that may bridge this conceptual divide and briefly discuss the implications of it for improving health outcomes in later adulthood.

Gender difference in circadian clock responses for social interaction with conspecific of the opposite-sex

Social cues are potent non-photic modulators of the circadian clock and play a vital role in resetting the endogenous clock. Several lines of evidence strongly suggest a functional link between olfactory cues and the circadian clock. However, there is a paucity of information on the effects of social interaction with the conspecifics of the opposite sex on the circadian clock. Hence, we studied the effect of social cues of sexually mature naïve opposite sex of the conspecific on the phase resetting of the circadian clock at various circadian times (CT) and molecular changes at the suprachiasmatic nuclei (SCN) and odor responsive structure in the brain of mice. Sexually naïve adult male and female free-running mice (designated as 'runners') were exposed to the conspecifics of the opposite-sex ('strangers') for 30 min at CT3, CT9, CT15, and CT21. Both male and female 'runners' exhibited a phase advance at CT3, delay at CT21, and no response at CT9. However, at CT15 only the male 'runners' exhibited phase advance but not the female 'runners'. Control mice did not elicit any significant phase shifts at all CTs. Social interactions with conspecifics of the opposite-sex up-regulated c-fos/C-FOS, omp in the olfactory bulb, per-1/PER-1 in the SCN, C-FOS, and PER-1 in the piriform cortex of both male and female runners at CT3. However, at CT15 up-regulation of variables only occurred in male but not in female runners. Together, the present investigation has shown the gender difference in circadian clock responses for social cues with conspecific of the opposite-sex in mice.

The mechanistic link between selective vulnerability of the locus coeruleus and neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is neuropathologically characterized by the intracellular accumulation of hyperphosphorylated tau and the extracellular deposition of amyloid-β plaques, which affect certain brain regions in a progressive manner. The locus coeruleus (LC), a small nucleus in the pons of the brainstem, is widely recognized as one of the earliest sites of neurofibrillary tangle formation in AD. Patients with AD exhibit significant neuronal loss in the LC, resulting in a marked reduction of its size and function. The LC, which vastly innervates several regions of the brain, is the primary source of the neurotransmitter norepinephrine (NE) in the central nervous system. Considering that NE is a major modulator of behavior, contributing to neuroprotection and suppression of neuroinflammation, degeneration of the LC in AD and the ultimate dysregulation of the LC-NE system has detrimental effects in the brain. In this review, we detail the neuroanatomy and function of the LC, its essential role in neuroprotection, and how this is dysregulated in AD. We discuss AD-related neuropathologic changes in the LC and mechanisms by which LC neurons are selectively vulnerable to insult. Further, we elucidate the neurotoxic effects of LC de-innervation both locally and at projection sites, and how this augments disease pathology, progression and severity. We summarize how preservation of the LC-NE system could be used in the treatment of AD and other neurodegenerative diseases affected by LC degeneration.

Aberrant functional connectivity between the suprachiasmatic nucleus and the superior temporal gyrus: Bridging RORA gene polymorphism with diurnal mood variation in major depressive disorder

Diurnal mood variation (DMV), a common symptom of major depressive disorder (MDD), is associated with circadian related genes and dysregulation of the suprachiasmatic nucleus (SCN). Previous research confirmed that the RORA gene is involved in the regulation of circadian rhythms. In this study, we hypothesized that polymorphisms of RORA may affect DMV symptoms of MDD through functional changes in the SCN. A total of 208 patients diagnosed with depression and 120 control subjects were enrolled and underwent a resting-state functional magnetic resonance imaging (rs-fMRI). Blood samples were collected and genotyping of 9 RORA gene SNPs were performed using next-generation sequencing technology. Patients were categorized as an AA genotype or C allele carriers based on RORA rs72752802 polymorphism. SCN-seed functional connectivity (FC) was compared between the two groups and correlation with severity of DMV was analyzed. Finally, a mediation analysis was performed to further determine FC intermediary effects. We observed that rs72752802 was significantly associated with patients' DMV symptoms. C allele carriers of rs72752802 showed significantly decreased FC between the right SCN and right superior temporal gyrus (rSTG). This was also correlated with DMV symptoms. In addition, the rs72752802 SNP influenced DMV symptoms through intermediary effects of SCN-rSTG connectivity. The study presented here provides a neurological and genetic basis for understanding depressed patients experiencing DMV.

Time is of the essence: Coupling sleep-wake and circadian neurobiology to the antidepressant effects of ketamine

Several studies have demonstrated the effectiveness of ketamine in rapidly alleviating depression and suicidal ideation. Intense research efforts have been undertaken to expose the precise mechanism underlying the antidepressant action of ketamine; however, the translation of findings into new clinical treatments has been slow. This translational gap is partially explained by a lack of understanding of the function of time and circadian timing in the complex neurobiology around ketamine. Indeed, the acute pharmacological effects of a single ketamine treatment last for only a few hours, whereas the antidepressant effects peak at around 24 hours and are sustained for the following few days. Numerous studies have investigated the acute and long-lasting neurobiological changes induced by ketamine; however, the most dramatic and fundamental change that the brain undergoes each day is rarely taken into consideration. Here, we explore the link between sleep and circadian regulation and rapid-acting antidepressant effects and summarize how diverse phenomena associated with ketamine's antidepressant actions - such as cortical excitation, synaptogenesis, and involved molecular determinants - are intimately connected with the neurobiology of wake, sleep, and circadian rhythms. We review several recently proposed hypotheses about rapid antidepressant actions, which focus on sleep or circadian regulation, and discuss their implications for ongoing research. Considering these aspects may be the last piece of the puzzle necessary to gain a more comprehensive understanding of the effects of rapid-acting antidepressants on the brain.

Suprachiasmatic VIP neurons are required for normal circadian rhythmicity and comprised of molecularly distinct subpopulations

The hypothalamic suprachiasmatic (SCN) clock contains several neurochemically defined cell groups that contribute to the genesis of circadian rhythms. Using cell-specific and genetically targeted approaches we have confirmed an indispensable role for vasoactive intestinal polypeptide-expressing SCN (SCN^VIP) neurons, including their molecular clock, in generating the mammalian locomotor activity (LMA) circadian rhythm. Optogenetic-assisted circuit mapping revealed functional, di-synaptic connectivity between SCN^VIP neurons and dorsomedial hypothalamic neurons, providing a circuit substrate by which SCN^VIP neurons may regulate LMA rhythms. In vivo photometry revealed that while SCN^VIP neurons are acutely responsive to light, their activity is otherwise behavioral state invariant. Single-nuclei RNA-sequencing revealed that SCN^VIP neurons comprise two transcriptionally distinct subtypes, including putative pacemaker and non-pacemaker populations. Altogether, our work establishes necessity of SCN^VIP neurons for the LMA circadian rhythm, elucidates organization of circadian outflow from and modulatory input to SCN^VIP cells, and demonstrates a subpopulation-level molecular heterogeneity that suggests distinct functions for specific SCN^VIP subtypes.

Cell groups in the hypothalamic suprachiasmatic clock contribute to the genesis of circadian rhythms. The authors identified two populations of vasoactive intestinal polypeptide-expressing neurons in the suprachiasmatic nucleus which regulate locomotor circadian rhythm in mice.

Circumventricular Organs and Parasite Neurotropism: Neglected Gates to the Brain?

Circumventricular organs (CVOs), neural structures located around the third and fourth ventricles, harbor, similarly to the choroid plexus, vessels devoid of a blood-brain barrier (BBB). This enables them to sense immune-stimulatory molecules in the blood circulation, but may also increase chances of exposure to microbes. In spite of this, attacks to CVOs by microbes are rarely described. It is here highlighted that CVOs and choroid plexus can be infected by pathogens circulating in the bloodstream, providing a route for brain penetration, as shown by infections with the parasites Trypanosoma brucei. Immune responses elicited by pathogens or systemic infections in the choroid plexus and CVOs are briefly outlined. From the choroid plexus trypanosomes can seed into the ventricles and initiate accelerated infiltration of T cells and parasites in periventricular areas. The highly motile trypanosomes may also enter the brain parenchyma from the median eminence, a CVO located at the base of the third ventricle, by crossing the border into the BBB-protected hypothalamic arcuate nuclei. A gate may, thus, be provided for trypanosomes to move into brain areas connected to networks of regulation of circadian rhythms and sleep-wakefulness, to which other CVOs are also connected. Functional imbalances in these networks characterize human African trypanosomiasis, also called sleeping sickness. They are distinct from the sickness response to bacterial infections, but can occur in common neuropsychiatric diseases. Altogether the findings lead to the question: does the neglect in reporting microbe attacks to CVOs reflect lack of awareness in investigations or of gate-opening capability by microbes?

Whole-Brain Monosynaptic Afferent Projections to the Cholecystokinin Neurons of the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) is the principal pacemaker driving the circadian rhythms of physiological behaviors. The SCN consists of distinct neurons expressing neuropeptides, including arginine vasopressin (AVP), vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), cholecystokinin (CCK), and so on. AVP, VIP, and GRP neurons receive light stimulation from the retina to synchronize endogenous circadian clocks with the solar day, whereas CCK neurons are not directly innervated by retinal ganglion cells and may be involved in the non-photic regulation of the circadian clock. To better understand the function of CCK neurons in non-photic circadian rhythm, it is vital to clarify the direct afferent inputs to CCK neurons in the SCN. Here, we utilized a recently developed rabies virus- and Cre/loxP-based, cell type-specific, retrograde tracing system to map and quantitatively analyze the whole-brain monosynaptic inputs to SCN CCK neurons. We found that SCN CCK neurons received direct inputs from 29 brain nuclei. Among these nuclei, paraventricular nucleus of the hypothalamus (PVH), paraventricular nucleus of the thalamus (PVT), supraoptic nucleus (SON), ventromedial nucleus of the hypothalamus, and seven other nuclei sent numerous inputs to CCK neurons. Moderate inputs originated from the zona incerta, periventricular hypothalamic nucleus, and five other nuclei. A few inputs to CCK neurons originated from the orbital frontal cortex, prelimbic cortex, cingulate cortex, claustrum, and seven other nuclei. In addition, SCN CCK neurons were preferentially innervated by AVP neurons of the ipsilateral PVH and SON rather than their contralateral counterpart, whereas the contralateral PVT sent more projections to CCK neurons than to its ipsilateral counterpart. Taken together, these results expand our knowledge of the specific innervation to mouse SCN CCK neurons and provide an important indication for further investigations on the function of CCK neurons.

Brain-derived neurotrophic factor acts at neurons of the subfornical organ to influence cardiovascular function

Brain-derived neurotrophic factor (BDNF), a neurotrophin traditionally associated with neural plasticity, has more recently been implicated in fluid balance and cardiovascular regulation. It is abundantly expressed in both the central nervous system (CNS) and peripheral tissue, and is also found in circulation. Studies suggest that circulating BDNF may influence the CNS through actions at the subfornical organ (SFO), a circumventricular organ (CVO) characterized by the lack of a normal blood-brain barrier (BBB). The SFO, well-known for its involvement in cardiovascular regulation, has been shown to express BDNF mRNA and mRNA for the TrkB receptor at which BDNF preferentially binds. This study was undertaken to determine if: (1) BDNF influences the excitability of SFO neurons in vitro; and (2) the cardiovascular consequences of direct administration of BDNF into the SFO of anesthetized rats. Electrophysiological studies revealed that bath application of BDNF (1 nmol/L) influenced the excitability of the majority of neurons (60%, n = 13/22), the majority of which exhibited a membrane depolarization (13.8 ± 2.5 mV, n = 9) with the remaining affected cells exhibiting hyperpolarizations (-11.1 ± 2.3 mV, n = 4). BDNF microinjections into the SFO of anesthetized rats caused a significant decrease in blood pressure (mean [area under the curve] AUC = -364.4 ± 89.0 mmHg × sec, n = 5) with no effects on heart rate (mean AUC = -12.2 ± 3.4, n = 5). Together these observations suggest the SFO to be a CNS site at which circulating BDNF could exert its effects on cardiovascular regulation.

Pupil Size Coupling to Cortical States Protects the Stability of Deep Sleep via Parasympathetic Modulation

During wakefulness, pupil diameter can reflect changes in attention, vigilance, and cortical states. How pupil size relates to cortical activity during sleep, however, remains unknown. Pupillometry during natural sleep is inherently challenging since the eyelids are usually closed. Here, we present a novel head-fixed sleep paradigm in combination with infrared back-illumination pupillometry (iBip) allowing robust tracking of pupil diameter in sleeping mice. We found that pupil size can be used as a reliable indicator of sleep states and that cortical activity becomes tightly coupled to pupil size fluctuations during non-rapid eye movement (NREM) sleep. Pharmacological blocking experiments indicate that the observed pupil size changes during sleep are mediated via the parasympathetic system. We furthermore found that constrictions of the pupil during NREM episodes might play a protective role for stability of sleep depth. These findings reveal a fundamental relationship between cortical activity and pupil size, which has so far been hidden behind closed eyelids.

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Infrared back-illumination allows accurate pupillometry in sleeping mice

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Brain activity and pupil diameter are tightly coupled during sleep

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The parasympathetic system is the main driver of pupillary changes during NREM sleep

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Pupillary constrictions might have a protective function to stabilize deep sleep

Long-lasting antinociceptive effects of green light in acute and chronic pain in rats

Treatments for chronic pain are inadequate, and new options are needed. Nonpharmaceutical approaches are especially attractive with many potential advantages including safety. Light therapy has been suggested to be beneficial in certain medical conditions such as depression, but this approach remains to be explored for modulation of pain. We investigated the effects of light-emitting diodes (LEDs), in the visible spectrum, on acute sensory thresholds in naive rats as well as in experimental neuropathic pain. Rats receiving green LED light (wavelength 525 nm, 8 h/d) showed significantly increased paw withdrawal latency to a noxious thermal stimulus; this antinociceptive effect persisted for 4 days after termination of last exposure without development of tolerance. No apparent side effects were noted and motor performance was not impaired. Despite LED exposure, opaque contact lenses prevented antinociception. Rats fitted with green contact lenses exposed to room light exhibited antinociception arguing for a role of the visual system. Antinociception was not due to stress/anxiety but likely due to increased enkephalins expression in the spinal cord. Naloxone reversed the antinociception, suggesting involvement of central opioid circuits. Rostral ventromedial medulla inactivation prevented expression of light-induced antinociception suggesting engagement of descending inhibition. Green LED exposure also reversed thermal and mechanical hyperalgesia in rats with spinal nerve ligation. Pharmacological and proteomic profiling of dorsal root ganglion neurons from green LED-exposed rats identified changes in calcium channel activity, including a decrease in the N-type (CaV2.2) channel, a primary analgesic target. Thus, green LED therapy may represent a novel, nonpharmacological approach for managing pain.

Temporal variations in presynaptic release probability in the lateral habenula

Rhythmicity plays an important role in a number of biological systems. The habenular complex is reported to contain an intrinsic molecular clock and to show rhythmic expression of circadian clock genes and proteins including per2/PER2. In this study, we observed that there is a temporal rhythmicity in the presynaptic efficacy of the lateral habenula (LHb) neurons. We collected a substantial number of recordings at different time points of the day during the light phase. The frequency and amplitude of spontaneous excitatory transmission were increased in the afternoon compared to recordings performed in the morning. In addition, the paired-pulse ratio and the success rate of minimal stimulation were also significantly different depending on the time of the recording. We did not see any significant differences in recordings obtained from pyramidal neurons of the hippocampus in the same brain slices. Taken together, our data indicates that the LHb exhibits intrinsic temporal oscillation in basal neurotransmission and in presynaptic release probability. Given the rapidly growing interest on the function of the LHb, more careful examination of synaptic transmission in the LHb is thus required.

[Research advances in circadian rhythm of epileptic seizures]

The time phase of epileptic seizures has attracted more and more attention. Epileptic seizures have their own circadian rhythm. The same type of epilepsy has different seizure frequencies in different time periods and states (such as sleeping/awakening state and natural day/night cycle). The circadian rhythm of epileptic seizures has complex molecular and endocrine mechanisms, and currently there are several hypotheses. Clarification of the circadian rhythm of epileptic seizures and prevention and administration according to such circadian rhythm can effectively control seizures and reduce the adverse effects of drugs. The research on the circadian rhythm of epileptic seizures provides a new idea for the treatment of epilepsy.

Ultradian rhythms in mammalian physiology and behavior

Diverse mammalian ultradian rhythms (URs) with periods in the 1-6h range, are omnipresent at multiple levels of biological organization and of functional and adaptive significance. Specification of neuroendocrine substrates that generate URs remains elusive. The suprachiasmatic (SCN) and arcuate (ARC) nuclei of the rodent hypothalamus subserve several behavioral URs. Recently, in a major advance, dopaminergic signaling in striatal circuitry, likely at D2 receptors, has been implicated in behavioral and thermoregulatory URs of mice. We propose a neural network in which reciprocal communication among the SCN, the ARC and striatal dopaminergic circuitry modulates the period and waveform of behavioral and physiological URs.

Sleep architecture and homeostasis in mice with partial ablation of melanin-concentrating hormone neurons

Recent reports support a key role of tuberal hypothalamic neurons secreting melanin concentrating-hormone (MCH) in the promotion of Paradoxical Sleep (PS). Controversies remain concerning their concomitant involvement in Slow-Wave Sleep (SWS). We studied the effects of their selective loss achieved by an Ataxin 3-mediated ablation strategy to decipher the contribution of MCH neurons to SWS and/or PS. Polysomnographic recordings were performed on male adult transgenic mice expressing Ataxin-3 transgene within MCH neurons (MCH(Atax)) and their wild-type littermates (MCH(WT)) bred on two genetic backgrounds (FVB/N and C57BL/6). Compared to MCH(WT) mice, MCH(Atax) mice were characterized by a significant drop in MCH mRNAs (-70%), a partial loss of MCH-immunoreactive neurons (-30%) and a marked reduction in brain density of MCH-immunoreactive fibers. Under basal condition, such MCH(Atax) mice exhibited higher PS amounts during the light period and a pronounced SWS fragmentation without any modification of SWS quantities. Moreover, SWS and PS rebounds following 4-h total sleep deprivation were quantitatively similar in MCH(Atax)vs. MCH(WT) mice. Additionally, MCH(Atax) mice were unable to consolidate SWS and increase slow-wave activity (SWA) in response to this homeostatic challenge as observed in MCH(WT) littermates. Here, we show that the partial loss of MCH neurons is sufficient to disturb the fine-tuning of sleep. Our data provided new insights into their contribution to subtle process managing SWS quality and its efficiency rather than SWS quantities, as evidenced by the deleterious impact on two powerful markers of sleep depth, i.e., SWS consolidation/fragmentation and SWA intensity under basal condition and under high sleep pressure.

Constructing the suprachiasmatic nucleus: a watchmaker's perspective on the central clockworks

The circadian system constrains an organism's palette of behaviors to portions of the solar day appropriate to its ecological niche. The central light-entrained clock in the suprachiasmatic nucleus (SCN) of the mammalian circadian system has evolved a complex network of interdependent signaling mechanisms linking multiple distinct oscillators to serve this crucial function. However, studies of the mechanisms controlling SCN development have greatly lagged behind our understanding of its physiological functions. We review advances in the understanding of adult SCN function, what has been described about SCN development to date, and the potential of both current and future studies of SCN development to yield important insights into master clock function, dysfunction, and evolution.

Impaired circadian photosensitivity in mice lacking glutamate transmission from retinal melanopsin cells

Intrinsically photoreceptive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin and convey retinal light inputs to the circadian system via the retinohypothalamic tract (RHT) projection to the suprachiasmatic nucleus (SCN). The principal neurotransmitter of this projection is glutamate, and ipRGCs use the vesicular glutamate transporter 2 (VGLUT2) to package glutamate into synaptic vesicles. However, these neurons contain other potential neurotransmitters, such as pituitary adenylate cyclase activating polypeptide (PACAP). To test the role of glutamate in mediating ipRGC light inputs into the SCN, we crossed mice in which Cre-recombinase expression is driven by the melanopsin promotor (Opn4(Cre/+)) with mice in which the second exon of VGLUT2 is flanked by loxP sites (VGLUT2(fl/fl)), producing ipRGCs that are unable to package glutamate into synaptic vesicles. Such mice had free-running circadian rhythms that did not entrain to a 12:12 light-dark (12:12 LD) cycle, nor did they show a phase delay after a 45-min light pulse administered at circadian time (CT) 14. A small subset of the mice did appear to entrain to the 12:12 LD cycle with a positive phase angle to lights-off; a similar entrainment pattern could be achieved in free-running mice if they were exposed to a 12:12 LD cycle with light of a greater intensity. Glutamate transmission from the ipRGCs is necessary for normal light entrainment of the SCN at moderate (0.35 W/m(2)) light levels, but residual transmission (possibly by PACAP in ipRGCs or by other RGCs) can weakly entrain animals, particularly at very high (6.53 W/m(2)) light levels, although it may be less effective at suppressing locomotor activity (light masking).

Circadian Insights into Motivated Behavior

For an organism to be successful in an evolutionary sense, it and its offspring must survive. Such survival depends on satisfying a number of needs that are driven by motivated behaviors, such as eating, sleeping, and mating. An individual can usually only pursue one motivated behavior at a time. The circadian system provides temporal structure to the organism's 24 hour day, partitioning specific behaviors to particular times of the day. The circadian system also allows anticipation of opportunities to engage in motivated behaviors that occur at predictable times of the day. Such anticipation enhances fitness by ensuring that the organism is physiologically ready to make use of a time-limited resource as soon as it becomes available. This could include activation of the sympathetic nervous system to transition from sleep to wake, or to engage in mating, or to activate of the parasympathetic nervous system to facilitate transitions to sleep, or to prepare the body to digest a meal. In addition to enabling temporal partitioning of motivated behaviors, the circadian system may also regulate the amplitude of the drive state motivating the behavior. For example, the circadian clock modulates not only when it is time to eat, but also how hungry we are. In this chapter we explore the physiology of our circadian clock and its involvement in a number of motivated behaviors such as sleeping, eating, exercise, sexual behavior, and maternal behavior. We also examine ways in which dysfunction of circadian timing can contribute to disease states, particularly in psychiatric conditions that include adherent motivational states.

Circadian gating of neuronal functionality: a basis for iterative metaplasticity

Brain plasticity, the ability of the nervous system to encode experience, is a modulatory process leading to long-lasting structural and functional changes. Salient experiences induce plastic changes in neurons of the hippocampus, the basis of memory formation and recall. In the suprachiasmatic nucleus (SCN), the central circadian (~24-h) clock, experience with light at night induces changes in neuronal state, leading to circadian plasticity. The SCN's endogenous ~24-h time-generator comprises a dynamic series of functional states, which gate plastic responses. This restricts light-induced alteration in SCN state-dynamics and outputs to the nighttime. Endogenously generated circadian oscillators coordinate the cyclic states of excitability and intracellular signaling molecules that prime SCN receptivity to plasticity signals, generating nightly windows of susceptibility. We propose that this constitutes a paradigm of ~24-h iterative metaplasticity, the repeated, patterned occurrence of susceptibility to induction of neuronal plasticity. We detail effectors permissive for the cyclic susceptibility to plasticity. We consider similarities of intracellular and membrane mechanisms underlying plasticity in SCN circadian plasticity and in hippocampal long-term potentiation (LTP). The emerging prominence of the hippocampal circadian clock points to iterative metaplasticity in that tissue as well. Exploring these links holds great promise for understanding circadian shaping of synaptic plasticity, learning, and memory.

Non-peptide oxytocin receptor ligands and hamster circadian wheel running rhythms

The synchronization of circadian rhythms in sleep, endocrine and metabolic functions with the environmental light cycle is essential for health, and dysfunction of this synchrony is thought to play a part in the development of many neurological disorders. There is a demonstrable need to develop new therapeutics for the treatment of neurological disorders such as depression and schizophrenia, and oxytocin is currently being investigated for this purpose. There are no published reports describing activity of oxytocin receptor ligands on mammalian circadian rhythms and that, then, is the purpose of this study. Non-peptide oxytocin receptor ligands that cross the blood brain barrier were systemically injected in hamsters to determine their ability to modulate light-induced phase advances and delays of circadian wheel running rhythms. The oxytocin receptor agonist WAY267464 (10 mg/kg) inhibited light induced phase advances of wheel running rhythms by 55%, but had no effect on light-induced phase delays. In contrast, the oxytocin receptor antagonist WAY162720 (10 mg/kg) inhibited light-induced phase delays by nearly 75%, but had no effect on light-induced phase advances. Additionally, WAY162720 was able to antagonize the inhibitory effects of WAY267464 on light-induced phase advances. These results are consistent for a role of oxytocin in the phase-delaying effects of light on circadian activity rhythms early in the night. Therefore, oxytocin may prove to be useful in developing therapeutics for the treatment of mood disorders with a concomitant dysfunction in circadian rhythms. Investigators should also be cognizant that oxytocin ligands may negatively affect circadian rhythms during clinical trials for other conditions.

Anatomical, molecular and pathological consideration of the circumventricular organs

BACKGROUND AND PURPOSE

Circumventricular organs (CVOs) are a diverse group of specialised structures characterized by peculiar vascular and position around the third and fourth ventricles of the brain. In humans, these organs are present during the fetal period and some become vestigial after birth. Some, such as the pineal gland (PG), subcommissural organ (SCO) and organum vasculosum of the lamina terminalis (OVLT), which are located around the third ventricle, might be the site of origin of periventricular tumours. In contrast to humans, CVOs are present in the adult rat and can be dissected by laser capture microdissection (LCM).

METHODS

In this study, we used LCM and microarrays to analyse the transcriptomes of three CVOs, the SCO, the subfornical organ (SFO) and the PG and the third ventricle ependyma of the adult rat, in order to better characterise these organs at the molecular level. Furthermore, an immunohistochemical study of Claudin-3 (CLDN3), a membrane protein involved in forming cellular tight junctions, was performed at the level of the SCO.

RESULTS

This study highlighted some potentially new or already described specific markers of these structures as Erbb2 and Col11a1 in ependyma, Epcam and CLDN3 in the SCO, Ren1 and Slc22a3 in the SFO and Tph, Anat and Asmt in the PG. Moreover, we found that CLDN3 expression was restricted to the apical pole of ependymocytes in the SCO.

Temporal modulation of the canonical clockwork in the suprachiasmatic nucleus and olfactory bulb by the mammary pheromone 2MB2 in pre-visual rabbits

During the early stages of development, the olfactory system plays a vital role in the survival of altricial mammals. One remarkable example is the Oryctolagus cuniculus, whose mother-young interaction greatly depends on the 2-methylbut-2-enal (2MB2) pheromone that triggers nipple search and grasping behaviors. Olfactory stimulation with 2MB2 regulates the expression of the core body temperature and locomotor activity rhythms in rabbit pups, indicating the modulation of the circadian system by this volatile cue. To address this issue, in the present study, we determined the effect of stimulation with pulses of 2MB2 on the molecular circadian clockwork in the suprachiasmatic nucleus (SCN) and in the main olfactory bulb (MOB). For this purpose, 7-day-old rabbits were stimulated with distilled water (CON), with ethyl isobutyrate (ETHYL) or with the pheromone (2MB2) at different times of the cycle, and 1h later, the expression of the activity marker C-FOS and of the clock proteins PER1, CRY1 and BMAL1 was evaluated in the SCN and in the three layers of the MOB. The clock proteins were abundantly expressed in both structures; nevertheless these showed diurnal rhythmicity only in the MOB, confirming that central pacemakers exhibit a heterochronical development of the molecular clockwork. C-FOS expression in the SCN and in the MOB was modulated by exposure to ETHYL and to 2MB2 only when these stimulants were presented at ZT00 and at ZT18. In contrast, the clock proteins were essentially modulated by 2MB2 at ZT00 and at ZT06 in both structures. In addition, the PER1 and CRY1 proteins exhibited differential responses to stimulation in the three layers of the MOB. For the first time, we report a modulatory and time-dependent effect of the mammary pheromone 2MB2 on the expression of the core clock proteins in the SCN and in the MOB in rabbits during pre-visual stages of development.

Lhx1 controls terminal differentiation and circadian function of the suprachiasmatic nucleus

Vertebrate circadian rhythms are organized by the hypothalamic suprachiasmatic nucleus (SCN). Despite its physiological importance, SCN development is poorly understood. Here, we show that Lim homeodomain transcription factor 1 (Lhx1) is essential for terminal differentiation and function of the SCN. Deletion of Lhx1 in the developing SCN results in loss of SCN-enriched neuropeptides involved in synchronization and coupling to downstream oscillators, among other aspects of circadian function. Intact, albeit damped, clock gene expression rhythms persist in Lhx1-deficient SCN; however, circadian activity rhythms are highly disorganized and susceptible to surprising changes in period, phase, and consolidation following neuropeptide infusion. Our results identify a factor required for SCN terminal differentiation. In addition, our in vivo study of combinatorial SCN neuropeptide disruption uncovered synergies among SCN-enriched neuropeptides in regulating normal circadian function. These animals provide a platform for studying the central oscillator's role in physiology and cognition.

Sadness might isolate you in a non-smelling world: olfactory perception and depression

Major depressive disorder (MDD) occurs with a high prevalence among mental illnesses. MDD patients experience sadness and hopelessness, with blunted affective reactivity. However, such depressive episodes are also key symptoms in other depressive disorders, like Bipolar Disorder (BPD) or Seasonal Affective Disorder (SAD). Moreover, depressive symptoms can also be found in healthy individuals, but are experienced as less severe or for a shorter duration than in patients. Here, it is aimed to summarize studies investigating odor perception in depression, including depressive states in healthy individuals and patient populations. Odor perception in depression has been assessed with psychophysical methods (olfactory sensitivity, odor identification, and discrimination), and odor ratings (intensity, emotional valence, familiarity). In addition, some studies investigated affective reactions to odors, and physiological and anatomical correlates of odor perception in depression. The summary reveals that MDD is associated with reduced olfactory sensitivity. However, odor identification and discrimination scores seem to be unaffected by depression. The reduced olfactory sensitivity might be associated with a reduced ability to encode olfactory information and a reduced volume of the olfactory bulb. While similar processes seem to occur in healthy individuals experiencing depressive states, they have not been observed in BPD or SAD patients. However, in order to conclude that the reduced olfactory sensitivity is directly linked to depression, it is suggested that studies should implement control measures of cognitive performances or perceptual abilities in other stimulus modalities. It is concluded that the reduced olfactory performance in MDD patients seems to be disorder-, modality-, and test-specific, and that the application of an appropriate olfactory and cognitive test-battery might be highly useful in the differential diagnosis of MDD.

Odor is a time cue for circadian behavior

Many physiological and behavioral processes such as sleep and wakefulness, hormone secretion, and olfactory sensitivity exhibit a 24-h rhythmicity that persists in constant conditions with a period close to (circa) 24 h. These circadian rhythms are driven by a network of endogenous clocks residing in various tissues, including the olfactory system, and are synchronized to the outside world by environmental time cues such as light, temperature, and food. In addition to having these well-known zeitgebers of circadian clocks, most environments consist of a multitude of odors that report, for example, the availability of food or the presence of predators--and often, they do so in a time-of-day-dependent manner. Considering the evolutionary significance of odors for various fitness-related behaviors such as mate choice, predator avoidance, and foraging strategies, we asked whether odors--similar to light, temperature, or food--might act as a circadian time cue able to influence circadian locomotor behavior in mammals. Administering individual air flow, periodically saturated with an artificial odor mix, to running wheel-equipped mouse cages, we found that rhythmic odor administrations significantly lengthened the period of circadian activity rhythms. Additionally, odor cues led to partial reemergence of circadian locomotor rhythmicity in suprachiasmatic nuclei (SCN)-lesioned mice, suggesting that the SCN as the central circadian pacemaker are not immediately required for odor-mediated effects on circadian behavior. However, odor-based modulation of circadian behavior did not occur in clock mutant (cry1(-/-) /cry2(-/-)) mice, indicating an odor-mediated mechanism that involves extra-SCN canonical clocks, such as the olfactory clock itself. Our results indicate not only that odor stimuli can act as a circadian time cue modulating circadian behavior but also that odor effects are even more pronounced in the absence of the SCN but nevertheless require the presence of a functional canonical clock.

Leptin-sensitive neurons in the arcuate nucleus integrate activity and temperature circadian rhythms and anticipatory responses to food restriction

Previously, we investigated the role of neuropeptide Y and leptin-sensitive networks in the mediobasal hypothalamus in sleep and feeding and found profound homeostatic and circadian deficits with an intact suprachiasmatic nucleus. We propose that the arcuate nuclei (Arc) are required for the integration of homeostatic circadian systems, including temperature and activity. We tested this hypothesis using saporin toxin conjugated to leptin (Lep-SAP) injected into Arc in rats. Lep-SAP rats became obese and hyperphagic and progressed through a dynamic phase to a static phase of growth. Circadian rhythms were examined over 49 days during the static phase. Rats were maintained on a 12:12-h light-dark (LD) schedule for 13 days and, thereafter, maintained in continuous dark (DD). After the first 13 days of DD, food was restricted to 4 h/day for 10 days. We found that the activity of Lep-SAP rats was arrhythmic in DD, but that food anticipatory activity was, nevertheless, entrainable to the restricted feeding schedule, and the entrained rhythm persisted during the subsequent 3-day fast in DD. Thus, for activity, the circuitry for the light-entrainable oscillator, but not for the food-entrainable oscillator, was disabled by the Arc lesion. In contrast, temperature remained rhythmic in DD in the Lep-SAP rats and did not entrain to restricted feeding. We conclude that the leptin-sensitive network that includes the Arc is required for entrainment of activity by photic cues and entrainment of temperature by food, but is not required for entrainment of activity by food or temperature by photic cues.

Characterization of the hypothalamus of Xenopus laevis during development. I. The alar regions

The patterns of expression of a set of conserved developmental regulatory transcription factors and neuronal markers were analyzed in the alar hypothalamus of Xenopus laevis throughout development. Combined immunohistochemical and in situ hybridization techniques were used for the identification of subdivisions and their boundaries. The alar hypothalamus was located rostral to the diencephalon in the secondary prosencephalon and represents the rostral continuation of the alar territories of the diencephalon and brainstem, according to the prosomeric model. It is composed of the supraoptoparaventricular (dorsal) and the suprachiasmatic (ventral) regions, and limits dorsally with the preoptic region, caudally with the prethalamic eminence and the prethalamus, and ventrally with the basal hypothalamus. The supraoptoparaventricular area is defined by the orthopedia (Otp) expression and is subdivided into rostral and caudal portions, on the basis of the Nkx2.2 expression only in the rostral portion. This region is the source of many neuroendocrine cells, primarily located in the rostral subdivision. The suprachiasmatic region is characterized by Dll4/Isl1 expression, and was also subdivided into rostral and caudal portions, based on the expression of Nkx2.1/Nkx2.2 and Lhx1/7 exclusively in the rostral portion. Both alar regions are mainly connected with subpallial areas strongly implicated in the limbic system and show robust intrahypothalamic connections. Caudally, both regions project to brainstem centers and spinal cord. All these data support that in terms of topology, molecular specification, and connectivity the subdivisions of the anuran alar hypothalamus possess many features shared with their counterparts in amniotes, likely controlling similar reflexes, responses, and behaviors.

Signals from the brainstem sleep/wake centers regulate behavioral timing via the circadian clock

Sleep-wake cycling is controlled by the complex interplay between two brain systems, one which controls vigilance state, regulating the transition between sleep and wake, and the other circadian, which communicates time-of-day. Together, they align sleep appropriately with energetic need and the day-night cycle. Neural circuits connect brain stem sites that regulate vigilance state with the suprachiasmatic nucleus (SCN), the master circadian clock, but the function of these connections has been unknown. Coupling discrete stimulation of pontine nuclei controlling vigilance state with analytical chemical measurements of intra-SCN microdialysates in mouse, we found significant neurotransmitter release at the SCN and, concomitantly, resetting of behavioral circadian rhythms. Depending upon stimulus conditions and time-of-day, SCN acetylcholine and/or glutamate levels were augmented and generated shifts of behavioral rhythms. These results establish modes of neurochemical communication from brain regions controlling vigilance state to the central circadian clock, with behavioral consequences. They suggest a basis for dynamic integration across brain systems that regulate vigilance states, and a potential vulnerability to altered communication in sleep disorders.

Effect of angiotensin II on rhythmic per2 expression in the suprachiasmatic nucleus and heart and daily rhythm of activity in Wistar rats

Endogenous daily rhythms are generated by the hierarchically organized circadian system predominantly synchronized by the external light (L): dark (D) cycle. During recent years several humoral signals have been found to influence the generation and manifestation of daily rhythm. Since most studies have been performed under in vitro conditions, the mechanisms employed under in vivo conditions need to be investigated. Our study focused on angiotensin II (angII)-mediated regulation of Per2 expression in the suprachiasmatic nuclei (SCN) and heart and spontaneous locomotor activity in Wistar rats under synchronized conditions. Angiotensin II was infused (100ng/kg/min) via subcutaneously implanted osmotic minipumps for 7 or 28days. Samples were taken in 4-h intervals during a 24hcycle and after a light pulse applied in the first and second part of the dark phase. Gene expression was measured using real time PCR. Locomotor activity was monitored using an infrared camera with a remote control installed in the animal facility. Seven days of angII infusion caused an increase in blood pressure and heart/body weight index and 28days of angII infusion also increased water intake in comparison with controls. We observed a distinct daily rhythm in Per2 expression in the SCN and heart of control rats and infused rats. Seven days of angII infusion did not influence Per2 expression in the heart. 28days of angII treatment caused significant phase advance and a decrease in nighttime expression of Per2 and influenced expression of clock controlled genes Rev-erb alpha and Dbp in the heart compared to the control. Four weeks of angII infusion decreased the responsiveness of Per2 expression in the SCN to a light pulse at the end of the dark phase of the 24hcycle. Expression of mRNA coding angiotensin-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2) showed a daily rhythm in the heart of control rats. Four weeks of angII infusion caused a decrease in amplitude of rhythmic expression of Ace, the disappearance of rhythm and an increase in Ace2 expression. The Ace/Ace2 ratio showed a rhythmic pattern in the heart of control rats with peak levels during the dark phase. Angiotensin II infusion decreased the mean Ace/Ace2 mRNA ratio in the heart. We observed a significant daily rhythm in expression of brain natriuretic peptide (BNP) in the heart of control rats. In hypertensive rats mean value of Bnp expression increased. Locomotor activity showed a distinct daily rhythm in both groups. Angiotensin II time dependently decreased ratio of locomotor activity in active versus passive phase of 24hcycle. To conclude, 28days of subcutaneous infusion of angII modulates the functioning of the central and peripheral circadian system measured at the level of Per2 expression and locomotor activity.

The cortisol awakening response is blunted in patients with active Cushing's disease

INTRODUCTION

Cortisol awakening response (CAR) is a rapid increase of cortisol levels within 30-45 min after awakening.

OBJECTIVE

This study evaluates CAR compared with cortisol circadian rhythm in active and in remission Cushing's disease (CD).

MATERIALS AND METHODS

We evaluated healthy controls (HC, n=19), obese (OB, n=10), in remission (n=08), and active CD patients (n=10). Salivary free cortisol (SF) was determined at 0800, 1100, 1700, 2000, and 2300 h on the first day. CAR was obtained the next morning immediately upon awakening and at 15, 30, 45, and 60-min post-wake up.

RESULTS

We observed differences in SF levels throughout the day in HC, OB, and in remission CD (ANOVA P=0.0001) but not in active CD (P=0.2). We demonstrated SF increment after awakening in HC, OB, and in remission CD (ANOVA P=0.007), with no effect of time on SF in active CD. The relative increment of SF obtained at the peak after awakening (CARi%) in the active CD (67±57%) was lower than in HC (154±107%), OB (240±188%), and in remission CD (186±184%) patients (P=0.009). There was a negative correlation between the SF at awakening and the CARi% in HC (r=-0.8), OB (r=-0.78), and in remission CD (r=-0.74) but not in active CD (r=-0.35; P=0.31).

CONCLUSION

This study originally described a blunted CAR in active CD in contrast to its presence in HC, OB, and in remission CD. This subtle dysfunction of the hypothalamus-pituitary-adrenal axis may represent a distinct and additional physiopathological phenomenon superimposing the dysregulated cortisol circadian rhythm in this disease.

Occurrence of epilepsy at different zeitgeber times alters sleep homeostasis differently in rats

STUDY OBJECTIVES

Controversial sleep disruptions (e.g., poor nighttime sleep and daytime somnolence) are common in epilepsy patients. Sleep is known to be regulated by homeostatic factors, which mediate sleep propensity, and the circadian oscillator, a clocklike mechanism. However, it is unknown how epileptic episodes that occur at different zeitgeber times (ZTs) alter sleep regulation. This study was designed to elucidate the sleep disruptions associated with epilepsy and their underlying mechanisms by delivering kindled epilepsy at different ZTs: ZT0, ZT6, and ZT13.

DESIGN

Kindled epilepsy was induced at 3 different ZTs, and sleep-wake activities were analyzed before and after full-blown seizure. Ribonuclease protection assay, radioimmunoassay, and immunohistochemistry were respectively employed to determine the levels of interleukin-1 mRNA, corticosterone, and PER1 protein.

SETTING

The experiments were performed at Neurophysiology Laboratory at National Taiwan University. PARTICIPANT AND INTERVENTIONS: Male Sprague-Dawley rats were implanted with electroencephalograph (EEG) electrodes, a bipolar stimulating electrode, and a guide cannula. Kindling stimuli were delivered via a bipolar electrode placed in the right central nucleus of the amygdala.

MEASUREMENT AND RESULTS

Kindled epilepsy occurring at ZT0 and ZT13 predominantly affected homeostatic factors, whereas ZT6-kindling stimuli altered the circadian oscillator. ZT0-kindling decreased rapid eye movement (REM) and non-REM (NREM) sleep, which was mediated by corticotrophin-releasing hormone, but did not alter the rhythm of sleep-wake fluctuation. On the other hand, ZT13-kindling enhanced interleukin-1 and consequently increased NREM sleep without altering the sleep-wake fluctuation. Nevertheless, the expression of PER1 protein in suprachiasmatic nucleus of the hypothalamus and the circadian rhythm of sleep fluctuation were respectively advanced 6 h and 2 h when kindling stimulation was delivered at ZT6. Shifts of sleep circadian rhythm and PER1 oscillation induced by ZT6-kindling were blocked by administration of hypocretin receptor antagonist SB334867 into the SCN, indicating the involvement of hypocretin.

CONCLUSION

These observations suggest that the occurrence of epilepsy at different ZTs alters sleep processes differently.

CITATION

Yi PL; Chen YJ; Lin CT; Chang FC. Occurrence of epilepsy at different zeitgeber times alters sleep homeostasis differently in rats. SLEEP 2012;35(12):1651-1665.

Reconstructing mammalian sleep dynamics with data assimilation

Data assimilation is a valuable tool in the study of any complex system, where measurements are incomplete, uncertain, or both. It enables the user to take advantage of all available information including experimental measurements and short-term model forecasts of a system. Although data assimilation has been used to study other biological systems, the study of the sleep-wake regulatory network has yet to benefit from this toolset. We present a data assimilation framework based on the unscented Kalman filter (UKF) for combining sparse measurements together with a relatively high-dimensional nonlinear computational model to estimate the state of a model of the sleep-wake regulatory system. We demonstrate with simulation studies that a few noisy variables can be used to accurately reconstruct the remaining hidden variables. We introduce a metric for ranking relative partial observability of computational models, within the UKF framework, that allows us to choose the optimal variables for measurement and also provides a methodology for optimizing framework parameters such as UKF covariance inflation. In addition, we demonstrate a parameter estimation method that allows us to track non-stationary model parameters and accommodate slow dynamics not included in the UKF filter model. Finally, we show that we can even use observed discretized sleep-state, which is not one of the model variables, to reconstruct model state and estimate unknown parameters. Sleep is implicated in many neurological disorders from epilepsy to schizophrenia, but simultaneous observation of the many brain components that regulate this behavior is difficult. We anticipate that this data assimilation framework will enable better understanding of the detailed interactions governing sleep and wake behavior and provide for better, more targeted, therapies.

Mathematical models are developed to better understand interactions between components of a system that together govern the overall behavior. Mathematical models of sleep have helped to elucidate the neuronal cell groups that are involved in promoting sleep and wake behavior and the transitions between them. However, to be able to take full advantage of these models one must be able to estimate the value of all included variables accurately. Data assimilation refers to methods that allow the user to combine noisy measurements of just a few system variables with the mathematical model of that system to estimate all variables, including those originally inaccessible for measurement. Using these techniques we show that we can reconstruct the unmeasured variables and parameters of a mathematical model of the sleep-wake network. These reconstructed estimates can then be used to better understand the underlying neuronal behavior that results in sleep and wake activity. Because sleep is implicated in a wide array of neurological disorders from epilepsy to schizophrenia, we anticipate that this framework will enable better understanding of the link between sleep and the rest of the brain and provide for better, more targeted, therapies.

Tyrosine hydroxylase phosphorylation in catecholaminergic brain regions: a marker of activation following acute hypotension and glucoprivation

The expression of c-Fos defines brain regions activated by the stressors hypotension and glucoprivation however, whether this identifies all brain sites involved is unknown. Furthermore, the neurochemicals that delineate these regions, or are utilized in them when responding to these stressors remain undefined. Conscious rats were subjected to hypotension, glucoprivation or vehicle for 30, 60 or 120 min and changes in the phosphorylation of serine residues 19, 31 and 40 in the biosynthetic enzyme, tyrosine hydroxylase (TH), the activity of TH and/or, the expression of c-Fos were determined, in up to ten brain regions simultaneously that contain catecholaminergic cell bodies and/or terminals: A1, A2, caudal C1, rostral C1, A6, A8/9, A10, nucleus accumbens, dorsal striatum and medial prefrontal cortex. Glucoprivation evoked phosphorylation changes in A1, caudal C1, rostral C1 and nucleus accumbens whereas hypotension evoked changes A1, caudal C1, rostral C1, A6, A8/9, A10 and medial prefrontal cortex 30 min post stimulus whereas few changes were evident at 60 min. Although increases in pSer19, indicative of depolarization, were seen in sites where c-Fos was evoked, phosphorylation changes were a sensitive measure of activation in A8/9 and A10 regions that did not express c-Fos and in the prefrontal cortex that contains only catecholaminergic terminals. Specific patterns of serine residue phosphorylation were detected, dependent upon the stimulus and brain region, suggesting activation of distinct signaling cascades. Hypotension evoked a reduction in phosphorylation in A1 suggestive of reduced kinase activity. TH activity was increased, indicating synthesis of TH, in regions where pSer31 alone was increased (prefrontal cortex) or in conjunction with pSer40 (caudal C1). Thus, changes in phosphorylation of serine residues in TH provide a highly sensitive measure of activity, cellular signaling and catecholamine utilization in catecholaminergic brain regions, in the short term, in response to hypotension and glucoprivation.