Direct Midbrain Dopamine Input to the Suprachiasmatic Nucleus Accelerates Circadian Entrainment

Food reward induction of rhythmic clock gene expression in the prefrontal cortex of rats is accompanied by changes in miR-34a-5p expression

The current study is focused on mechanisms by which the peripheral circadian oscillator in the prefrontal cortex (PFC) participates in food reward-induced activity. The experimental group of male Wistar rats was trained to receive a food reward with a low hedonic and caloric value. Afterwards, animals were exposed to a 5 h phase advance. Experimental animals could access a small food reward as they had been accustomed to, while control rats were exposed to the same phase shift without access to a food reward. When synchronisation to a new light:dark cycle was accompanied by intake of food reward, animals exerted more exact phase shift compared to the controls. In rats with access to a food reward, a rhythm in dopamine receptors types 1 and 2 in the PFC was detected. Rhythmic clock gene expression was induced in the PFC of rats when a food reward was provided together with a phase shift. The per2 and clock genes are predicted targets of miR-34a-5p. The precursor form of miR-34a-5p (pre-miR-34a-5p) showed a daily rhythm in expression in the PFC of the control and experimental groups. On the other hand, the mature form of miR-34a-5p exerted an inverted rhythm compared to pre-miR-34a-5p and negative correlation with per and clock genes expression only in the PFC of rewarded rats. A difference in the pattern of mature and pre-miR-34a-5p values was not related to expression of enzymes drosha, dicer and dgcr8. A role of the clock genes and miR-34a-5p in reward-facilitated synchronisation has been hypothesised.

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.

Methamphetamine and sleep impairments: neurobehavioral correlates and molecular mechanisms

Methamphetamine is a potent and highly addictive psychostimulant, and one of the most widely used illicit drugs. Over recent years, its global usage and seizure have been on a rapid rise, with growing detrimental effects on mental and physical health, and devastating psychosocial impact pressing for intervention. Among the unwanted effects of methamphetamine, acute and long-term sleep impairments are of major concern, posing a significant therapeutic challenge, and a cause of addiction relapse. Unraveling mechanisms and functional correlates of methamphetamine-related sleep and circadian disruption are, therefore, of key relevance to translational and clinical psychiatry. In this article, we review the mounting evidence for the acute and long-term impairements of sleep-wake behavior and circadian activity caused by single or recurring methamphetamine usage and withdrawal. Factors contributing to the severity of sleep loss and related cognitive deficit, with risks of relapse are discussed. Key molecular players mediating methamphetamine-induced dopamine release and neuromodulation are considered, with wake-promoting effects in mesolimbic circuits. The effects on various sleep phases and related changes in dopamine levels in selected subcortical structures are reviewed and compared to other psychostimulants with similar action mechanisms. A critical appraisal is presented of the therapeutic use of modafinil, countering sleep, and circadian rhythm impairments. Finally, emerging knowledge gaps and methodical limitations are highlighted along with the areas for future research and therapeutic translation.

Chronic methamphetamine uncovers a circadian rhythm in multiple-unit neural activity in the dorsal striatum which is independent of the suprachiasmatic nucleus

The dorsal striatum forms part of the basal ganglia circuit that is a major regulator of voluntary motor behavior. Dysfunction in this circuit is a critical factor in the pathology of neurological (Parkinson's and Huntington's disease) as well as psychiatric disorders. In this study, we employed in vivo real-time monitoring of multiple unit neural activity (MUA) in the dorsal striatum of freely moving mice. We demonstrate that the striatum exhibits robust diurnal and circadian rhythms in MUA that peak in the night. These rhythms are dependent upon the central circadian clock located in the suprachiasmatic nucleus (SCN) as lesions of this structure caused the loss of rhythmicity measured in the striatum. Nonetheless, chronic treatment of methamphetamine (METH) makes circadian rhythms appear in MUA recorded from the striatum of SCN-lesioned mice. These data demonstrate that the physiological properties of neurons in the dorsal striatum are regulated by the circadian system and that METH drives circadian rhythms in striatal physiology in the absence of the SCN. The finding of SCN-driven circadian rhythms in striatal physiology has important implications for an understanding of the temporal regulation of motor control as well as revealing how disease processes may disrupt this regulation.

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Dorsal striatum exhibits robust circadian rhythms in MUA in freely moving animals.

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Suprachiasmatic nucleus (SCN) lesions caused the loss of rhythmicity measured in the striatum.

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METH treatment made newly striatal MUA rhythms appear after SCN lesions.

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METH treatment reduced the amplitude and delayed the offset of SCN rhythms.

Blunted diurnal firing in lateral habenula projections to dorsal raphe nucleus and delayed photoentrainment in stress-susceptible mice

Daily rhythms are disrupted in patients with mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the 15-day chronic social defeat stress (CSDS) paradigm and in vitro slice electrophysiology, we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cells^LHb→DRN) and unlabeled DRN cells. We also performed optogenetic experiments to investigate if increased firing in cells^LHb→DRN during exposure to a weak 7-day social defeat stress (SDS) paradigm induces stress-susceptibility. Last, we investigated whether exposure to CSDS affected the ability of mice to photoentrain to a new light–dark (LD) cycle. The cells^LHb→DRN and unlabeled DRN cells of stress-susceptible mice express greater blunted diurnal firing compared to stress-näive (control) and stress-resilient mice. Daytime optogenetic activation of cells^LHb→DRN during SDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that exposure to strong stressors induces blunted daily rhythms in firing in cells^LHb→DRN, DRN cells and decreases the initial rate of photoentrainment in susceptible-mice. In contrast, resilient-mice may undergo homeostatic adaptations that maintain daily rhythms in firing in cells^LHb→DRN and also show rapid photoentrainment to a new LD cycle.

Daily rhythms are disrupted in patients suffering from mood disorders, and it is known that the lateral habenula and dorsal raphe nucleus contribute to circadian timekeeping and regulate mood. This study shows that stress-susceptible mice have blunted and inverted diurnal firing rhythms in lateral habenula cells that project to the dorsal raphe nucleus, and have a slow rate of photoentrainment to a new light cycle.

Type 1 dopamine receptor (D1R)-independent circadian food anticipatory activity in mice

Circadian rhythms are entrained by light and influenced by non-photic stimuli, such as feeding. The activity preceding scheduled mealtimes, food anticipatory activity (FAA), is elicited in rodents fed a limited amount at scheduled times. FAA is thought to be the output of an unidentified food entrained oscillator. Previous studies, using gene deletion and receptor pharmacology, implicated dopamine type receptor 1 (D1R) signaling in the dorsal striatum as necessary for FAA in mice. To further understand the role of D1R in promoting FAA, we utilized the Cre-lox system to create cell type-specific deletions of D1R, conditionally deleting D1R in GABA neurons using Vgat-ires-Cre line. This conditional deletion mutant had attenuated FAA, but the amount was higher than expected based on prior results using a constitutive knockout of D1R, D1R KO^Drago. This result prompted us to re-test the original D1R KO^Drago line, which expressed less FAA than controls, but only moderately so. To determine if genetic drift had diminished the effect of D1R deletion on FAA, we re-established the D1R KO^Drago knockout line from cryopreserved samples. The reestablished D1R KO^Drago-cryo had a clear impairment of FAA compared to controls, but still developed increased activity preceding mealtime across the 4 weeks of timed feeding. Finally, we tested a different deletion allele of D1R created by the Knockout Mouse Project. This line of D1R KO^KOMP mice had a significant impairment in the acquisition of FAA, but eventually reached similar levels of premeal activity compared to controls after 4 weeks of timed feeding. Taken together, our results suggest that D1R signaling promotes FAA, but other dopamine receptors likely contribute to FAA given that mice lacking the D1 receptor still retain some FAA.

The use of melatonin to mitigate the adverse metabolic side effects of antipsychotics

Antipsychotic drugs are efficacious first-line treatments for many individuals diagnosed with a psychiatric illness. However, their adverse metabolic side-effect profile, which resembles the metabolic syndrome, represents a significant clinical problem that increases morbidity and limits treatment adherence. Moreover, the mechanisms involved in antipsychotic-induced adverse metabolic effects (AMEs) are unknown and mitigating strategies and interventions are limited. However, recent clinical trials show that nightly administration of exogenous melatonin may mitigate or even prevent antipsychotic-induced AMEs. This clinical evidence in combination with recent preclinical data implicate the circadian system in antipsychotic-induced AMEs and their mitigation. In this chapter, we provide an overview on the circadian system and its involvement in antipsychotic-induced AMEs, as well as the potential beneficial effect of nightly melatonin administration to mitigate them.

Rhythms, Reward, and Blues: Consequences of Circadian Photoperiod on Affective and Reward Circuit Function

Circadian disruptions, along with altered affective and reward states, are commonly associated with psychiatric disorders. In addition to genetics, the enduring influence of environmental factors in programming neural networks is of increased interest in assessing the underpinnings of mental health. The duration of daylight or photoperiod is known to impact both the serotonin and dopamine systems, which are implicated in mood and reward-based disorders. This review first examines the effects of circadian disruption and photoperiod in the serotonin system in both human and preclinical studies. We next highlight how brain regions crucial for the serotoninergic system (i.e., dorsal raphe nucleus; DRN), and dopaminergic (i.e., nucleus accumbens; NAc and ventral tegmental area; VTA) system are intertwined in overlapping circuitry, and play influential roles in the pathology of mood and reward-based disorders. We then focus on human and animal studies that demonstrate the impact of circadian factors on the dopaminergic system. Lastly, we discuss how environmental factors such as circadian photoperiod can impact the neural circuits that are responsible for regulating affective and reward states, offering novel insights into the biological mechanisms underlying the pathophysiology, systems, and therapeutic treatments necessary for mood and reward-based disorders.

Potential Pathways for Circadian Dysfunction and Sundowning-Related Behavioral Aggression in Alzheimer's Disease and Related Dementias

Patients with Alzheimer's disease (AD) and related dementias are commonly reported to exhibit aggressive behavior and other emotional behavioral disturbances, which create a tremendous caretaker burden. There has been an abundance of work highlighting the importance of circadian function on mood and emotional behavioral regulation, and recent evidence demonstrates that a specific hypothalamic pathway links the circadian system to neurons that modulate aggressive behavior, regulating the propensity for aggression across the day. Such shared circuitry may have important ramifications for clarifying the complex interactions underlying "sundowning syndrome," a poorly understood (and even controversial) clinical phenomenon in AD and dementia patients that is characterized by agitation, aggression, and delirium during the late afternoon and early evening hours. The goal of this review is to highlight the potential output and input pathways of the circadian system that may underlie circadian dysfunction and behavioral aggression associated with sundowning syndrome, and to discuss possible ways these pathways might inform specific interventions for treatment. Moreover, the apparent bidirectional relationship between chronic disruptions of circadian and sleep-wake regulation and the pathology and symptoms of AD suggest that understanding the role of these circuits in such neurobehavioral pathologies could lead to better diagnostic or even preventive measures.

Disrupted Sleep and Circadian Rhythms in Schizophrenia and Their Interaction With Dopamine Signaling

Sleep and circadian rhythm disruption (SCRD) is a common feature of schizophrenia, and is associated with symptom severity and patient quality of life. It is commonly manifested as disturbances to the sleep/wake cycle, with sleep abnormalities occurring in up to 80% of patients, making it one of the most common symptoms of this disorder. Severe circadian misalignment has also been reported, including non-24 h periods and phase advances and delays. In parallel, there are alterations to physiological circadian parameters such as body temperature and rhythmic hormone production. At the molecular level, alterations in the rhythmic expression of core clock genes indicate a dysfunctional circadian clock. Furthermore, genetic association studies have demonstrated that mutations in several clock genes are associated with a higher risk of schizophrenia. Collectively, the evidence strongly suggests that sleep and circadian disruption is not only a symptom of schizophrenia but also plays an important causal role in this disorder. The alterations in dopamine signaling that occur in schizophrenia are likely to be central to this role. Dopamine is well-documented to be involved in the regulation of the sleep/wake cycle, in which it acts to promote wakefulness, such that elevated dopamine levels can disturb sleep. There is also evidence for the influence of dopamine on the circadian clock, such as through entrainment of the master clock in the suprachiasmatic nuclei (SCN), and dopamine signaling itself is under circadian control. Therefore dopamine is closely linked with sleep and the circadian system; it appears that they have a complex, bidirectional relationship in the pathogenesis of schizophrenia, such that disturbances to one exacerbate abnormalities in the other. This review will provide an overview of the evidence for a role of SCRD in schizophrenia, and examine the interplay of this with altered dopamine signaling. We will assess the evidence to suggest common underlying mechanisms in the regulation of sleep/circadian rhythms and the pathophysiology of schizophrenia. Improvements in sleep are associated with improvements in symptoms, along with quality of life measures such as cognitive ability and employability. Therefore the circadian system holds valuable potential as a new therapeutic target for this disorder.

Bright light improves sleep in patients with Parkinson's disease: possible role of circadian restoration

Parkinson’s disease (PD) is one of the most common neurodegenerative disorders. Among the most common manifestations of PD are sleep problems, which are coupled with the adverse effects of dopaminergic therapies (DT). A non-pharmacological solution for these sleep problems has been sought to avoid additional pharmacological intervention. Here, we show that bright light therapy (BLT) is effective for improving sleep in Japanese PD patients receiving DT. Furthermore, experimental evaluation of peripheral clock gene expression rhythms revealed that most PD patients receiving DT who experienced improved sleep following BLT showed a circadian phase shift, indicating the existence of a correlation between circadian modulation and sleep improvement. Conversely, this result indicates that sleep problems in PD patients receiving DT may arise at least in part as a result of circadian dysfunction. Indeed, we found that chronic dopaminergic stimulation induced a rapid attenuation of autonomous oscillations of clock gene expression in ex vivo cultured mouse suprachiasmatic nucleus (SCN) at the single neuron level. In conclusion, BLT is a promising medical treatment for improving sleep in PD patients receiving DT. This BLT-induced improvement may be due to the restoration of circadian function.

Chocolate for breakfast prevents circadian desynchrony in experimental models of jet-lag and shift-work

Night-workers, transcontinental travelers and individuals that regularly shift their sleep timing, suffer from circadian desynchrony and are at risk to develop metabolic disease, cancer, and mood disorders, among others. Experimental and clinical studies provide evidence that food intake restricted to the normal activity phase is a potent synchronizer for the circadian system and can prevent the detrimental metabolic effects associated with circadian disruption. As an alternative, we hypothesized that a timed piece of chocolate scheduled to the onset of the activity phase may be sufficient stimulus to synchronize circadian rhythms under conditions of shift-work or jet-lag. In Wistar rats, a daily piece of chocolate coupled to the onset of the active phase (breakfast) accelerated re-entrainment in a jet-lag model by setting the activity of the suprachiasmatic nucleus (SCN) to the new cycle. Furthermore, in a rat model of shift-work, a piece of chocolate for breakfast prevented circadian desynchrony, by increasing the amplitude of the day-night c-Fos activation in the SCN. Contrasting, chocolate for dinner prevented re-entrainment in the jet-lag condition and favored circadian desynchrony in the shift-work models. Moreover, chocolate for breakfast resulted in low body weight gain while chocolate for dinner boosted up body weight. Present data evidence the relevance of the timing of a highly caloric and palatable meal for circadian synchrony and metabolic function.

Blunted Diurnal Firing in Lateral Habenula Projections to Dorsal Raphe Nucleus and Delayed Photoentrainment in Stress-Susceptible Mice.. [DOI: 10.1101/2020.03.19.998732] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Daily rhythms are disrupted in patients suffering from mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the chronic social defeat stress (CSDS) paradigm and in-vitro slice electrophysiology we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cellsLHb→DRN) and DRN cells alone. We also performed optogenetic experiments to investigate if increased firing in cellsLHb→DRN during exposure to subthreshold social defeat stress (SSDS), induces stress-susceptibility. Last we investigated whether exposure to CSDS affected the ability of mice to phototentrain to a new LD cycle. The cellsLHb→DRN and DRN cells alone of stress-susceptible mice express greater blunted diurnal firing compared to stress-naive (control) and stress-resilient mice. Day-time optogenetic activation of cellsLHb→DRN during SSDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that CSDS induces blunted daily rhythms in firing in cellsLHb→DRN and slow rate of photoentrainment in susceptible-mice. In contrast, resilientmice may undergo homeostatic adaptations that maintain daily rhythms in firing in cellsLHb→DRN and also show rapid photoentrainment to a new LD-cycle.

Exosomes from Human Periapical Cyst-MSCs: Theranostic Application in Parkinson's Disease

The scientific community continuously strives to get new disease models, to discover early markers or novel therapeutic approaches, improving the diagnosis and prognosis of several human pathologies. Parkinson's Disease (PD) is characterized by a long asymptomatic phase, characterized by a selective loss of dopaminergic neurons. Recently, the human Periapical Cyst-Mesenchymal Stem Cells (hPCy-MSCs) have been differentiated in functional dopaminergic neurons: such oral-derived MSCs and the hPCy-MSCs-derived exosomes may represent a strategic and useful in vitro study-model, as well as intriguing therapeutic carriers. Circadian rhythm (CR) alteration variously impacts on PD pathways: an interesting research target is represented by the analysis of the exosomes released by dopaminergic neurons, derived from neural-differentiated hPCy-MSCs, after having reproduced in-vitro PD-like conditions. This review aims to describe the crosstalk among some aspects of circadian rhythm related to the onset of PD and the exosomes released by cells of PD patients. More in detail: the first part of this article will describe the main characteristics of circadian rhythm and the involvement of the exosomes found to be effective in the pathogenesis of PD. Finally, the authors will suggest how those exosomes derived from dopaminergic neurons, obtained by oral-derived stem cells (hPCy-MSCs) may represent a smart model for the in vitro research on PD, to find new biomarkers, to test new drugs or, fatally, to find new pathways applicable in future therapeutic approaches.

The p75 neurotrophin receptor in AgRP neurons is necessary for homeostatic feeding and food anticipation

Networks of neurons control feeding and activity patterns by integrating internal metabolic signals of energy balance with external environmental cues such as time-of-day. Proper circadian alignment of feeding behavior is necessary to prevent metabolic disease, and thus it is imperative that molecular players that maintain neuronal coordination of energy homeostasis are identified. Here, we demonstrate that mice lacking the p75 neurotrophin receptor, p75NTR, decrease their feeding and food anticipatory behavior (FAA) in response to daytime, but not nighttime, restricted feeding. These effects lead to increased weight loss, but do not require p75NTR during development. Instead, p75NTR is required for fasting-induced activation of neurons within the arcuate hypothalamus. Indeed, p75NTR specifically in AgRP neurons is required for FAA in response to daytime restricted feeding. These findings establish p75NTR as a novel regulator gating behavioral response to food scarcity and time-of-day dependence of circadian food anticipation.

Dopamine Signaling in the Suprachiasmatic Nucleus Enables Weight Gain Associated with Hedonic Feeding

The widespread availability of energy-dense, rewarding foods is correlated with the increased incidence of obesity across the globe. Overeating during mealtimes and unscheduled snacking disrupts timed metabolic processes, which further contribute to weight gain. The neuronal mechanism by which the consumption of energy-dense food restructures the timing of feeding is poorly understood. Here, we demonstrate that dopaminergic signaling within the suprachiasmatic nucleus (SCN), the central circadian pacemaker, disrupts the timing of feeding, resulting in overconsumption of food. D1 dopamine receptor (Drd1)-null mice are resistant to diet-induced obesity, metabolic disease, and circadian disruption associated with energy-dense diets. Conversely, genetic rescue of Drd1 expression within the SCN restores diet-induced overconsumption, weight gain, and obesogenic symptoms. Access to rewarding food increases SCN dopamine turnover, and elevated Drd1-signaling decreases SCN neuronal activity, which we posit disinhibits downstream orexigenic responses. These findings define a connection between the reward and circadian pathways in the regulation of pathological calorie consumption.

DAILY-A Personalized Circadian Zeitgeber Therapy as an Adjunctive Treatment for Alcohol Use Disorder Patients: Study Protocol for a Randomized Controlled Trial

Background: Hallmarks of alcohol use disorder (AUD) are disturbances of circadian rhythms and everyday structures. While circadian rhythms dictate the timing of daily recurring activities such as sleep, activity, and meals, conversely, these activities represent time cues, so called Zeitgebers, that the circadian system uses to synchronize with the environment. Here we present a study protocol for our newly developed therapy approach for AUD patients, in which we take advantage of this mutual influence and stabilize and strengthen their circadian system by creating strict daily schedules for daily Zeitgeber activities. Since every person has a circadian system with its own characteristics and is subject to social obligations, the daily plans are personalized for each test person. Our hypothesis is that a regular exposure to Zeitgebers stabilizes behavioral and physiological circadian rhythms and thereby reduces the risk of alcohol relapses and depressive symptoms and facilitates physical recovery in AUD patients during the 1st weeks of their addiction therapy. Methods/design: The study is a 6-weeks single site trial with a controlled, randomized, single-blinded, parallel-group design including patients with a diagnosis of AUD. The study runs parallel to the standard addiction therapy of the clinic. Patients are randomly assigned to either an intervention group (DAILY) or a sham control group (placebo treatment). Questionnaires and physiological assessments of both groups are conducted before and immediately after the intervention or control treatment. According to our hypothesis, the primary outcomes of this study are improvements of regularity, alcohol consumption, and relapse rate in AUD patients compared to AUD patients receiving control treatment. Secondary outcomes are reduced depressive symptoms and increased physical recovery. Discussion: This study is a randomized controlled trial to investigate the efficacy of a personalized circadian Zeitgeber therapy as an adjunctive treatment for alcohol use disorder patients. The overall goal of this and more extended future studies is the development of an adjunctive therapy for AUD patients that is uncomplicated in its use and easy to implement in the clinical and everyday routine. Trial registration: This study is registered at the German Clinical Trial Register with the trial number DRKS00019093 on November 28, 2019.

Neurogenetic basis for circadian regulation of metabolism by the hypothalamus

Circadian rhythms are driven by a transcription-translation feedback loop that separates anabolic and catabolic processes across the Earth's 24-h light-dark cycle. Central pacemaker neurons that perceive light entrain a distributed clock network and are closely juxtaposed with hypothalamic neurons involved in regulation of sleep/wake and fast/feeding states. Gaps remain in identifying how pacemaker and extrapacemaker neurons communicate with energy-sensing neurons and the distinct role of circuit interactions versus transcriptionally driven cell-autonomous clocks in the timing of organismal bioenergetics. In this review, we discuss the reciprocal relationship through which the central clock drives appetitive behavior and metabolic homeostasis and the pathways through which nutrient state and sleep/wake behavior affect central clock function.

Circadian insights into the biology of depression: Symptoms, treatments and animal models

In depression, symptoms range from loss of motivation and energy to suicidal thoughts. Moreover, in depression alterations might be also observed in the sleep-wake cycle and in the daily rhythms of hormonal (e.g., cortisol, melatonin) secretion. Both, the sleep-wake cycle and hormonal rhythms, are regulated by the internal biological clock within the hypothalamic suprachiasmatic nucleus (SCN). Therefore, a dysregulation of the internal mechanism of the SCN might lead in the disturbance of temporal physiology and depression. Hence, circadian symptoms in mood disorders can be used as important biomarkers for the prevention and treatment of depression. Disruptions of daily rhythms in physiology and behavior are also observed in animal models of depression, giving thus an important tool of research for the understanding of the circadian mechanisms implicated in mood disorders. This review discusses the alterations of daily rhythms in depression, and how circadian perturbations might lead in mood changes and depressive-like behavior in humans and rodents respectively. The use of animal models with circadian disturbances and depressive-like behaviors will help to understand the central timing mechanisms underlying depression, and how treating the biological clock(s) it may be possible to improve mood.

Generation of circadian rhythms in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is remarkable. Despite numbering only about 10,000 neurons on each side of the third ventricle, the SCN is our principal circadian clock, directing the daily cycles of behaviour and physiology that set the tempo of our lives. When this nucleus is isolated in organotypic culture, its autonomous timing mechanism can persist indefinitely, with precision and robustness. The discovery of the cell-autonomous transcriptional and post-translational feedback loops that drive circadian activity in the SCN provided a powerful exemplar of the genetic specification of complex mammalian behaviours. However, the analysis of circadian time-keeping is moving beyond single cells. Technical and conceptual advances, including intersectional genetics, multidimensional imaging and network theory, are beginning to uncover the circuit-level mechanisms and emergent properties that make the SCN a uniquely precise and robust clock. However, much remains unknown about the SCN, not least the intrinsic properties of SCN neurons, its circuit topology and the neuronal computations that these circuits support. Moreover, the convention that the SCN is a neuronal clock has been overturned by the discovery that astrocytes are an integral part of the timepiece. As a test bed for examining the relationships between genes, cells and circuits in sculpting complex behaviours, the SCN continues to offer powerful lessons and opportunities for contemporary neuroscience.

Theabrownin from Pu-erh tea together with swinging exercise synergistically ameliorates obesity and insulin resistance in rats

PURPOSE

Theabrownin (TB)-containing Pu-erh tea has been shown to be hypolipidemic in rats fed a high-fat diet. Physical exercise such as swinging is also known to reduce obesity. We hypothesized that TB in combination with swinging can synergistically ameliorate obesity and insulin resistance in rats with metabolic syndrome.

METHODS

TB, rosiglitazone, or lovastatin (controls) was administered by gavage to rats fed a diet high in fat, sugar, and salt. A subgroup of the rats was subjected to a 30-min daily swinging exercise regimen, whereas the other rats did not exercise.

RESULTS

Theabrownin in combination with swinging was found to significantly improve serum lipid status and prevent development of obesity and insulin resistance in rats. Liver transcriptomics data suggested that theabrownin activated circadian rhythm, protein kinase A, the adenosine monophosphate-activated protein kinase, and insulin signaling pathways by enhancing cyclic adenosine monophosphate levels and, hence, accelerating nutrient metabolism and the consumption of sugar and fat. The serum dopamine levels in rats increased significantly after exercise. In parallel work, intraperitoneal dopamine injections were shown to significantly reduce weight gain and prevent the elevation in triglyceride levels that would otherwise be induced by the high fat-sugar-salt diet. Theabrownin prevented obesity and insulin resistance mainly by affecting the circadian rhythm, while swinging exercise stimulated the overproduction of dopamine to accelerate metabolism of glucose and lipid.

CONCLUSIONS

Theabrownin and exercise synergistically ameliorated metabolic syndrome in rats and effectively prevented obesity.

Future Directions for Understanding Adolescent Bipolar Spectrum Disorders: A Reward Hypersensitivity Perspective

The idea that bipolar spectrum disorders (BSDs) are characterized by enhanced sensitivity to rewarding stimuli is at the core of the reward hypersensitivity model, one of the most prominent and well-supported theories of BSDs. In this article, we present the reward hypersensitivity model of BSDs, review evidence supporting it, discuss its relevance to explaining why BSDs typically begin and consolidate during the period of adolescence, and consider three major unresolved issues for this model that provide important directions for future research. Finally, we present integrations of the reward hypersensitivity model with circadian rhythm and immune system models that should provide greater understanding of the mechanisms involved in BSDs, and then suggest additional directions for future research deriving from these integrated models.

Dopamine Signaling in Circadian Photoentrainment: Consequences of Desynchrony

Circadian rhythms, or biological oscillations of approximately 24 hours, impact almost all aspects of our lives by regulating the sleep-wake cycle, hormone release, body temperature fluctuation, and timing of food consumption. The molecular machinery governing these rhythms is similar across organisms ranging from unicellular fungi to insects, rodents, and humans. Circadian entrainment, or temporal synchrony with one's environment, is essential for survival. In mammals, the central circadian pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus and mediates entrainment to environmental conditions. While the light:dark cycle is the primary environmental cue, arousal-inducing, non-photic signals such as food consumption, exercise, and social interaction are also potent synchronizers. Many of these stimuli enhance dopaminergic signaling suggesting that a cohesive circadian physiology depends on the relationship between circadian clocks and the neuronal circuits responsible for detecting salient events. Here, we review the inner workings of mammalian circadian entrainment, and describe the health consequences of circadian rhythm disruptions with an emphasis on dopamine signaling.

Morning and Evening Circadian Pacemakers Independently Drive Premotor Centers via a Specific Dopamine Relay

Many animals exhibit morning and evening peaks of locomotor behavior. In Drosophila, two corresponding circadian neural oscillators-M (morning) cells and E (evening) cells-exhibit a corresponding morning or evening neural activity peak. Yet we know little of the neural circuitry by which distinct circadian oscillators produce specific outputs to precisely control behavioral episodes. Here, we show that ring neurons of the ellipsoid body (EB-RNs) display spontaneous morning and evening neural activity peaks in vivo: these peaks coincide with the bouts of locomotor activity and result from independent activation by M and E pacemakers. Further, M and E cells regulate EB-RNs via identified PPM3 dopaminergic neurons, which project to the EB and are normally co-active with EB-RNs. These in vivo findings establish the fundamental elements of a circadian neuronal output pathway: distinct circadian oscillators independently drive a common pre-motor center through the agency of specific dopaminergic interneurons.

Eating Rewards the Gears of the Clock

Eating behavior is regulated by metabolic and hedonic brain networks, which interact with each other to balance the physiological regulation of hunger and satiety. The daily balance of this regulation is controlled by the central circadian clock. Importantly, metabolic and reward properties of food impact the functioning of circadian clocks, altering the oscillatory activity of the molecular clockwork and circadian rhythms. However, when feeding (metabolic or reward) is timed, the whole circadian system is entrained. Furthermore, besides synchronizing the clock, the timing of both metabolic and reward eating might be crucial for health, to improve circadian physiology, as well as to treat metabolic (e.g., diabetes, obesity) and neurological diseases (e.g., mental, neurodegenerative).

Off the Clock: From Circadian Disruption to Metabolic Disease

Circadian timekeeping allows appropriate temporal regulation of an organism’s internal metabolism to anticipate and respond to recurrent daily changes in the environment. Evidence from animal genetic models and from humans under circadian misalignment (such as shift work or jet lag) shows that disruption of circadian rhythms contributes to the development of obesity and metabolic disease. Inappropriate timing of food intake and high-fat feeding also lead to disruptions of the temporal coordination of metabolism and physiology and subsequently promote its pathogenesis. This review illustrates the impact of genetically or environmentally induced molecular clock disruption (at the level of the brain and peripheral tissues) and the interplay between the circadian system and metabolic processes. Here, we discuss some mechanisms responsible for diet-induced circadian desynchrony and consider the impact of nutritional cues in inter-organ communication, with a particular focus on the communication between peripheral organs and brain. Finally, we discuss the relay of environmental information by signal-dependent transcription factors to adjust the timing of gene oscillations. Collectively, a better knowledge of the mechanisms by which the circadian clock function can be compromised will lead to novel preventive and therapeutic strategies for obesity and other metabolic disorders arising from circadian desynchrony.

Loss of function of NCOR1 and NCOR2 impairs memory through a novel GABAergic hypothalamus-CA3 projection

Nuclear receptor corepressor 1 (NCOR1) and NCOR2 (also known as SMRT) regulate gene expression by activating histone deacetylase 3 through their deacetylase activation domain (DAD). We show that mice with DAD knock-in mutations have memory deficits, reduced anxiety levels, and reduced social interactions. Mice with NCOR1 and NORC2 depletion specifically in GABAergic neurons (NS-V mice) recapitulated the memory deficits and had reduced GABA_A receptor subunit α2 (GABRA2) expression in lateral hypothalamus GABAergic (LH^GABA) neurons. This was associated with LH^GABA neuron hyperexcitability and impaired hippocampal long-term potentiation, through a monosynaptic LH^GABA to CA3^GABA projection. Optogenetic activation of this projection caused memory deficits, whereas targeted manipulation of LH^GABA or CA3^GABA neuron activity reversed memory deficits in NS-V mice. We describe de novo variants in NCOR1, NCOR2 or HDAC3 in patients with intellectual disability or neurodevelopmental disorders. These findings identify a hypothalamus-hippocampus projection that may link endocrine signals with synaptic plasticity through NCOR-mediated regulation of GABA signaling.

Rhythms of life: circadian disruption and brain disorders across the lifespan

Many processes in the human body - including brain function - are regulated over the 24-hour cycle, and there are strong associations between disrupted circadian rhythms (for example, sleep-wake cycles) and disorders of the CNS. Brain disorders such as autism, depression and Parkinson disease typically develop at certain stages of life, and circadian rhythms are important during each stage of life for the regulation of processes that may influence the development of these disorders. Here, we describe circadian disruptions observed in various brain disorders throughout the human lifespan and highlight emerging evidence suggesting these disruptions affect the brain. Currently, much of the evidence linking brain disorders and circadian dysfunction is correlational, and so whether and what kind of causal relationships might exist are unclear. We therefore identify remaining questions that may direct future research towards a better understanding of the links between circadian disruption and CNS disorders.

The Intertwined Roles of Circadian Rhythms and Neuronal Metabolism Fueling Drug Reward and Addiction

Drug addiction is a highly prevalent and devastating disorder with few effective treatments, resulting in enormous burdens on family and society. The cellular and behavioral effects of drugs of abuse are related to their abilities to elevate synaptic dopamine levels. Midbrain dopaminergic neurons projecting from the ventral tegmental area to the nucleus accumbens play crucial roles in substance-induced neural and behavioral plasticity. Significantly, increasing work suggests that interplay between the brain circadian system and the cellular bioenergetic machinery in these dopamine neurons plays a critical role in mediating the actions of drugs of abuse. Here, we describe recent progress in elucidating the interconnections between circadian and metabolic systems at the molecular and cellular levels and their relationships to modulation of drug reward and addiction.

SCN VIP Neurons Are Essential for Normal Light-Mediated Resetting of the Circadian System

The suprachiasmatic nucleus (SCN) synchronizes circadian rhythms in behavior and physiology to the external light cycle, but the mechanisms by which this occurs are unclear. As the neuropeptide vasoactive intestinal peptide (VIP) is important for circadian light responses, we tested the hypothesis that rhythmic VIP-producing SCN neurons mediate circadian light responses in male and female mice. Using in vivo fiber photometry over multiple days, we found daily rhythms in spontaneous calcium events of SCN VIP neurons that peaked during the subjective day and were disrupted by constant light. The light-evoked calcium responses peaked around subjective dusk and were greater during the subjective night. Using novel VIP sensor cells, we found that the activity patterns in SCN VIP neurons correlated tightly with spontaneous and NMDA-evoked VIP release. Finally, in vivo hyperpolarization of VIP neurons attenuated light-induced shifts of daily rhythms in locomotion. We conclude that SCN VIP neurons exhibit circadian rhythms in spontaneous and light-responsive activity and are essential for the normal resetting of daily rhythms by environmental light.SIGNIFICANCE STATEMENT Daily rhythms in behavior and physiology, including sleep/wake and hormone release, are synchronized to local time by the master circadian pacemaker, the suprachiasmatic nucleus (SCN). The advent of artificial lighting and, consequently, light exposure at night, is associated with an increased risk of disease due to disrupted circadian rhythms. However, the mechanisms by which the SCN encodes normal and pathological light information are unclear. Here, we find that vasoactive intestinal peptide (VIP)-producing SCN neurons exhibit daily rhythms in neuronal activity and VIP release, and that blocking the activity of these neurons attenuates light-induced phase shifts. We conclude that rhythmic VIP neurons are an essential component of the circadian light transduction pathway.

Time-shifting effects of methylphenidate on daily rhythms in the diurnal rodent Arvicanthis ansorgei

People suffering of attention-deficit/hyperactivity disorder (ADHD) and treated with the psychostimulant methylphenidate (MPH) show sleep-wake cycle and daily rhythm alterations despite the beneficial effects of MPH on behavioral symptoms (i.e., hyperactivity, attention). In nocturnal rodents (i.e., mice), chronic exposure to MPH alters the neural activity of the circadian clock in the suprachiasmatic nucleus (SCN), behavioral rhythms, and the sleep-wake cycle. Here, we studied the effects of MPH on daily rhythms of behavior and body temperature of the diurnal rodent Arvicanthis ansorgei. Under a light-dark cycle, chronic exposure to MPH in drinking water delayed the onset of both activity and body temperature rhythms. Interestingly, delays were larger when MPH access was restricted to the first 6 h of the light phase (i.e., activity phase) of the 24-h cycle. Since MPH effects are dependent on animal's fluid intake, in a last experiment, we controlled the time and dose of MPH delivery in Arvicanthis using an intraperitoneal perfusion method. Similarly to the experiment with MPH in drinking water, Arvicanthis showed a delay in the onset of general activity and body temperature when MPH infusions, but not vehicle, were during the first 6 h of the light phase. This study indicates that MPH alters daily rhythms in a time-dependent manner and proposes the use of a diurnal rodent for the study of the effects of MPH on the circadian clock. Knowing the circadian modulation on the effects of MPH in behavior could give new insights in the treatment of ADHD.

Acoustically targeted chemogenetics for the non-invasive control of neural circuits

Neurological and psychiatric disorders are often characterized by dysfunctional neural circuits in specific regions of the brain. Existing treatment strategies, including the use of drugs and implantable brain stimulators, aim to modulate the activity of these circuits. However, they are not cell-type-specific, lack spatial targeting or require invasive procedures. Here, we report a cell-type-specific and non-invasive approach based on acoustically targeted chemogenetics that enables the modulation of neural circuits with spatiotemporal specificity. The approach uses ultrasound waves to transiently open the blood-brain barrier and transduce neurons at specific locations in the brain with virally encoded engineered G-protein-coupled receptors. The engineered neurons subsequently respond to systemically administered designer compounds to activate or inhibit their activity. In a mouse model of memory formation, the approach can modify and subsequently activate or inhibit excitatory neurons within the hippocampus, with selective control over individual brain regions. This technology overcomes some of the key limitations associated with conventional brain therapies.

Dorsal tegmental dopamine neurons gate associative learning of fear

Functional neuroanatomy of Pavlovian fear has identified neuronal circuits and synapses associating conditioned stimuli with aversive events. Hebbian plasticity within these networks requires additional reinforcement to store particularly salient experiences into long-term memory. Here, we have identified a circuit reciprocally connecting the ventral periaqueductal grey (vPAG)/dorsal raphe (DR) region and the central amygdala (CE) that gates fear learning. We found that vPAG/DR dopaminergic (vPdRD) neurons encode a positive prediction error in response to unpredicted shocks, and may reshape intra-amygdala connectivity via a dopamine-dependent form of long-term potentiation (LTP). Negative feedback from the CE to vPdRD neurons might limit reinforcement to events that have not been predicted. These findings add a new module to the midbrain DA circuit architecture underlying associative reinforcement learning and identify vPdRD neurons as critical component of Pavlovian fear conditioning. We propose that dysregulation of vPdRD neuronal activity may contribute to fear-related psychiatric disorders.

Light rescues circadian behavior and brain dopamine abnormalities in diurnal rodents exposed to a winter-like photoperiod

Seasonal affective disorder (SAD), beyond mood changes, is characterized by alterations in daily rhythms of behavior and physiology. The pathophysiological conditions of SAD involve changes in day length and its first-line treatment is bright light therapy. Animal models using nocturnal rodents have been studied to elucidate the neurobiological mechanisms of depression, but might be ill suited to study the therapeutic effects of light in SAD since they exhibit light-aversive responses. Here Arvicanthis ansorgei, a diurnal rodent, was used to determine behavioral, molecular and brain dopamine changes in response to exposure to a winter-like photoperiod consisting of a light-dark cycle with 8 h of light, under diminished light intensity, and 16 h of darkness. Furthermore, we evaluated whether timed-daily bright light exposure has an effect on behavior and brain physiology of winter-like exposed animals. Arvicanthis under a winter-like condition showed alterations in the synchronization of the locomotor activity rhythm to the light-dark cycle. Moreover, alterations in day-night activity of dopaminergic neurotransmission were revealed in the nucleus accumbens and the dorsal striatum, and in the day-night clock gene expression in the suprachiasmatic nucleus. Interestingly, whereas dopamine disturbances were reversed in animals exposed to daily light at early or late day, altered phase of the daily rhythm of locomotion was reverted only in animals exposed to light at the late day. Moreover, Per2 gene expression in the SCN was also affected by light exposure at late day in winter-like exposed animals. These findings suggest that light induces effects on behavior by mechanisms that rely on both circadian and rhythm-independent pathways influencing the dopaminergic circuitry. This last point might be crucial for understanding the mechanisms of non-pharmacological treatment in SAD.

Racing the clock: The role of circadian rhythmicity in addiction across the lifespan

Although potent effects of psychoactive drugs on circadian rhythms were first described over 30 years ago, research into the reciprocal relationship between the reward system and the circadian system - and the impact of this relationship on addiction - has only become a focus in the last decade. Nonetheless, great progress has been made in that short time toward understanding how drugs of abuse impact the molecular and physiological circadian clocks, as well as how disruption of normal circadian rhythm biology may contribute to addiction and ameliorate the efficacy of treatments for addiction. In particular, data have emerged demonstrating that disrupted circadian rhythms, such as those observed in shift workers and adolescents, increase susceptibility to addiction. Furthermore, circadian rhythms and addiction impact one another longitudinally - specifically from adolescence to the elderly. In this review, the current understanding of how the circadian clock interacts with substances of abuse within the context of age-dependent changes in rhythmicity, including the potential existence of a drug-sensitive clock, the correlation between chronotype and addiction vulnerability, and the importance of rhythmicity in the mesocorticolimbic dopamine system, is discussed. The primary focus is on alcohol addiction, as the preponderance of research is in this area, with references to other addictions as warranted. The implications of clock-drug interactions for the treatment of addiction will also be reviewed, and the potential of therapeutics that reset the circadian rhythm will be highlighted.

Circadian peak dopaminergic activity response at the biological clock pacemaker (suprachiasmatic nucleus) area mediates the metabolic responsiveness to a high-fat diet

Among vertebrate species of the major vertebrate classes in the wild, a seasonal rhythm of whole body fuel metabolism, oscillating from a lean to obese condition, is a common biological phenomenon. This annual cycle is driven in part by annual changes in the circadian dopaminergic signalling at the suprachiasmatic nuclei (SCN), with diminution of circadian peak dopaminergic activity at the SCN facilitating development of the seasonal obese insulin-resistant condition. The present study investigated whether such an ancient circadian dopamine-SCN activity system for expression of the seasonal obese, insulin-resistant phenotype may be operative in animals made obese amd insulin resistant by high-fat feeding and, if so, whether reinstatement of the circadian dopaminergic peak at the SCN would be sufficient to reverse the adverse metabolic impact of the high-fat diet without any alteration of caloric intake. First, we identified the supramammillary nucleus as a novel site providing the majority of dopaminergic neuronal input to the SCN. We further identified dopamine D2 receptors within the peri-SCN region as being functional in mediating SCN responsiveness to local dopamine. In lean, insulin-sensitive rats, the peak in the circadian rhythm of dopamine release at the peri-SCN coincided with the daily peak in SCN electrophysiological responsiveness to local dopamine administration. However, in rats made obese and insulin resistant by high-fat diet (HFD) feeding, these coincident circadian peak activities were both markedly attenuated or abolished. Reinstatement of the circadian peak in dopamine level at the peri-SCN by its appropriate circadian-timed daily microinjection to this area (but not outside this circadian time-interval) abrogated the obese, insulin-resistant condition without altering the consumption of the HFD. These findings suggest that the circadian peak of dopaminergic activity at the peri-SCN/SCN is a key modulator of metabolism and the responsiveness to adverse metabolic consequences of HFD consumption.

Circadian Rhythm Disturbances in Mood Disorders: Insights into the Role of the Suprachiasmatic Nucleus

Circadian rhythm disturbances are a common symptom among individuals with mood disorders. The suprachiasmatic nucleus (SCN), in the ventral part of the anterior hypothalamus, orchestrates physiological and behavioral circadian rhythms. The SCN consists of self-sustaining oscillators and receives photic and nonphotic cues, which entrain the SCN to the external environment. In turn, through synaptic and hormonal mechanisms, the SCN can drive and synchronize circadian rhythms in extra-SCN brain regions and peripheral tissues. Thus, genetic or environmental perturbations of SCN rhythms could disrupt brain regions more closely related to mood regulation and cause mood disturbances. Here, we review clinical and preclinical studies that provide evidence both for and against a causal role for the SCN in mood disorders.