Chronotype-Dependent Sleep Loss Is Associated with a Lower Amplitude in Circadian Rhythm and a Higher Fragmentation of REM Sleep in Young Healthy Adults

In modern society, the time and duration of sleep on workdays are primarily determined by external factors, e.g., the alarm clock. This can lead to a misalignment of the intrinsically determined sleep timing, which is dependent on the individual chronotype, resulting in reduced sleep quality. Although this is highly relevant given the high incidence of sleep disorders, little is known about the effect of this misalignment on sleep architecture. Using Fitbit trackers and questionnaire surveys, our study aims to elucidate sleep timing, sleep architecture, and subjective sleep quality in young healthy adults (n = 59) under real-life conditions (average of 82.4 ± 9.7 days). Correlations between variables were calculated to identify the direction of relationships. On workdays, the midpoint of sleep was earlier, the sleep duration was shorter, and tiredness upon waking was higher than on free days. A higher discrepancy between sleep duration on workdays and free days was associated with a lower stability of the circadian rhythm of REM sleep and also with a higher fragmentation of REM sleep. Similarly, a higher tiredness upon waking on free days, thus under intrinsically determined sleep timing conditions, was associated with a lower proportion and a higher fragmentation of REM sleep. This suggests that the misalignment between extrinsically and intrinsically determined sleep timing affects the architecture of sleep stages, particularly REM sleep, which is closely connected to sleep quality.

Acute Circadian Disruption Due to Constant Light Promotes Caspase 1 Activation in the Mouse Hippocampus

In mammals, the circadian system controls various physiological processes to maintain metabolism, behavior, and immune function during a daily 24 h cycle. Although driven by a cell-autonomous core clock in the hypothalamus, rhythmic activities are entrained to external cues, such as environmental lighting conditions. Exposure to artificial light at night (ALAN) can cause circadian disruption and thus is linked to an increased occurrence of civilization diseases in modern society. Moreover, alterations of circadian rhythms and dysregulation of immune responses, including inflammasome activation, are common attributes of neurodegenerative diseases, including Alzheimer', Parkinson's, and Huntington's disease. Although there is evidence that the inflammasome in the hippocampus is activated by stress, the direct effect of circadian disruption on inflammasome activation remains poorly understood. In the present study, we aimed to analyze whether exposure to constant light (LL) affects inflammasome activation in the mouse hippocampus. In addition to decreased circadian power and reduced locomotor activity, we found cleaved caspase 1 significantly elevated in the hippocampus of mice exposed to LL. However, we did not find hallmarks of inflammasome priming or cleavage of pro-interleukins. These findings suggest that acute circadian disruption leads to an assembled "ready to start" inflammasome, which may turn the brain more vulnerable to additional aversive stimuli.

Effects of the Light/Dark Phase and Constant Light on Spatial Working Memory and Spine Plasticity in the Mouse Hippocampus

Circadian rhythms in behavior and physiology such as rest/activity and hormones are driven by an internal clock and persist in the absence of rhythmic environmental cues. However, the period and phase of the internal clock are entrained by the environmental light/dark cycle. Consequently, aberrant lighting conditions, which are increasing in modern society, have a strong impact on rhythmic body and brain functions. Mice were exposed to three different lighting conditions, 12 h light/12 h dark cycle (LD), constant darkness (DD), and constant light (LL), to study the effects of the light/dark cycle and aberrant lighting on the hippocampus, a critical structure for temporal and spatial memory formation and navigation. Locomotor activity and plasma corticosterone levels were analyzed as readouts for circadian rhythms. Spatial working memory via Y-maze, spine morphology of Golgi-Cox-stained hippocampi, and plasticity of excitatory synapses, measured by number and size of synaptopodin and GluR1-immunreactive clusters, were analyzed. Our results indicate that the light/dark cycle drives diurnal differences in synaptic plasticity in hippocampus. Moreover, spatial working memory, spine density, and size and number of synaptopodin and GluR1 clusters were reduced in LL, while corticosterone levels were increased. This indicates that acute constant light affects hippocampal function and synaptic plasticity.

Sleep Duration on Workdays Is Correlated with Subjective Workload and Subjective Impact of High Workload on Sleep in Young Healthy Adults

Psychosocial stress is widespread worldwide and particularly affects young adults. There is a close and bidirectional relationship between sleep quality and mental health. Sleep duration, which is an important feature of sleep quality, shows both intra-individual variations and inter-individual differences. Internal clocks control individual sleep timing, which, in turn, defines the chronotype. On workdays, however, the end and duration of sleep are largely limited by external factors, such as alarm clocks, especially in later chronotypes. The aim of this study is to investigate whether there is a relationship between sleep timing and duration on workdays and measures for psychosocial stress, such as anxiety and depression; subjective workload; and the subjective impact of a high workload on sleep. We used a combination of Fitbit wearable actigraphy data and a questionnaire survey of young, healthy medical students and calculated correlations between the respective variables. We found that a shorter sleep duration on workdays is associated with a higher subjective workload and a higher subjective impact of a high workload on sleep, which, in turn, are associated with higher measures of anxiety and depression. Our study contributes to understanding the importance of sleep timing/duration and their regularity on weekdays for subjectively perceived psychosocial stress.

Radiotherapy of the Hepatocellular Carcinoma in Mice Has a Time-Of-Day-Dependent Impact on the Mouse Hippocampus

Chronic liver diseases including hepatocellular carcinoma (HCC) create a state of chronic inflammation that affects the brain via the liver-brain axis leading to an alteration of neurotransmission and cognition. However, little is known about the effects of HCC on the hippocampus, the key brain region for learning and memory. Moreover, radiotherapy used to treat HCC has severe side effects that impair patients' life quality. Thus, designing optimal strategies, such as chronotherapy, to enhance the efficacy and reduce the side effects of HCC treatment is critically important. We addressed the effects of HCC and the timed administration of radiotherapy in mice on the expression of pro-inflammatory cytokines, clock genes, markers for glial activation, oxidative stress, neuronal activity and proliferation in the hippocampal neurogenic niche. Our data showed that HCC induced the upregulation of genes encoding for pro-inflammatory cytokines, altered clock gene expressions and reduced proliferation in the hippocampus. Radiotherapy, in particular when applied during the light/inactive phase enhanced all these effects in addition to glial activation, increased oxidative stress, decreased neuronal activity and increased levels of phospho(p)-ERK. Our results suggested an interaction of the circadian molecular clockwork and the brain's innate immune system as key players in liver-brain crosstalk in HCC and that radiotherapy when applied during the light/inactive phase induced the most profound alterations in the hippocampus.

The Role of the Melatoninergic System in Circadian and Seasonal Rhythms—Insights From Different Mouse Strains

The melatoninergic system comprises the neurohormone melatonin and its molecular targets. The major source of melatonin is the pineal organ where melatonin is rhythmically produced during darkness. In mammals, melatonin biosynthesis is controlled by the central circadian rhythm generator in the suprachiasmatic nucleus (SCN) and photoreceptors in the retina. Melatonin elicits its function principally through two specific receptors called MT1 and MT2. MT1 is highly expressed in the SCN and the hypophysial pars tuberalis (PT), an important interface for control of seasonal functions. The expression of the MT2 is more widespread. The role of the melatoninergic system in the control of seasonal functions, such as reproduction, has been known for more than 4 decades, but investigations on its impact on the circadian system under normal (entrained) conditions started 2 decades later by comparing mouse strains with a fully functional melatoninergic system with mouse strains which either produce insufficient amounts of melatonin or lack the melatonin receptors MT1 and MT2. These studies revealed that an intact melatoninergic system is not required for the generation or maintenance of rhythmic behavior under physiological entrained conditions. As shown by jet lag experiments, the melatoninergic system facilitated faster re-entrainment of locomotor activity accompanied by a more rapid adaptation of the molecular clock work in the SCN. This action depended on MT2. Further studies indicated that the endogenous melatoninergic system stabilizes the locomotor activity under entrained conditions. Notably, these effects of the endogenous melatoninergic system are subtle, suggesting that other signals such as corticosterone or temperature contribute to the synchronization of locomotor activity. Outdoor experiments lasting for a whole year indicate a seasonal plasticity of the chronotype which depends on the melatoninergic system. The comparison between mice with an intact or a compromised melatoninergic system also points toward an impact of this system on sleep, memory and metabolism.

The Effects of Light and the Circadian System on Rhythmic Brain Function

Life on earth has evolved under the influence of regularly recurring changes in the environment, such as the 24 h light/dark cycle. Consequently, organisms have developed endogenous clocks, generating 24 h (circadian) rhythms that serve to anticipate these rhythmic changes. In addition to these circadian rhythms, which persist in constant conditions and can be entrained to environmental rhythms, light drives rhythmic behavior and brain function, especially in nocturnal laboratory rodents. In recent decades, research has made great advances in the elucidation of the molecular circadian clockwork and circadian light perception. This review summarizes the role of light and the circadian clock in rhythmic brain function, with a focus on the complex interaction between the different components of the mammalian circadian system. Furthermore, chronodisruption as a consequence of light at night, genetic manipulation, and neurodegenerative diseases is briefly discussed.

Adult Neurogenesis under Control of the Circadian System

The mammalian circadian system is a hierarchically organized system, which controls a 24-h periodicity in a wide variety of body and brain functions and physiological processes. There is increasing evidence that the circadian system modulates the complex multistep process of adult neurogenesis, which is crucial for brain plasticity. This modulatory effect may be exercised via rhythmic systemic factors including neurotransmitters, hormones and neurotrophic factors as well as rhythmic behavior and physiology or via intrinsic factors within the neural progenitor cells such as the redox state and clock genes/molecular clockwork. In this review, we discuss the role of the circadian system for adult neurogenesis at both the systemic and the cellular levels. Better understanding of the role of the circadian system in modulation of adult neurogenesis can help develop new treatment strategies to improve the cognitive deterioration associated with chronodisruption due to detrimental light regimes or neurodegenerative diseases.

Does timing matter in radiotherapy of hepatocellular carcinoma? An experimental study in mice

This study investigates whether a chronotherapeutic treatment of hepatocellular carcinoma (HCC) may improve treatment efficacy and mitigate side effects on non-tumoral liver (NTL). HCC was induced in Per2::luc mice which were irradiated at four time points of the day. Proliferation and DNA-double strand breaks were analyzed in irradiated and nonirradiated animals by detection of Ki67 and γ-H2AX. Prior to whole animal experiments, organotypic slice cultures were investigated to determine the dosage to be used in whole animal experiments. Irradiation was most effective at the proliferation peaks in HCC at ZT02 (early inactivity phase) and ZT20 (late activity phase). Irradiation effects on NTL were minimal at ZT20. As compared with NTL, nonirradiated HCC revealed disruption in daily variation and downregulation of all investigated clock genes except Per1. Irradiation affected rhythmic clock gene expression in NTL and HCC at all ZTs except at ZT20 (late activity phase). Irradiation at ZT20 had no effect on total leukocyte numbers. Our results indicate ZT20 as the optimal time point for irradiation of HCC in mice at which the ratio between efficacy of tumor treatment and toxic side effects was maximal. Translational studies are now needed to evaluate whether the late activity phase is the optimal time point for irradiation of HCC in man.

P2Y2 deficiency impacts adult neurogenesis and related forebrain functions

Adult neurogenesis occurs particularly in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ) of the lateral ventricle. This continuous addition of neurons to pre-existing neuronal networks is essential for intact cognitive and olfactory functions, respectively. Purinergic signaling modulates adult neurogenesis, however, the role of individual purinergic receptor subtypes in this dynamic process and related cognitive performance is poorly understood. In this study, we analyzed the role of P2Y₂ receptor in the neurogenic niches and in related forebrain functions such as spatial working memory and olfaction using mice with a targeted deletion of the P2Y₂ receptor (P2Y2^-/- ). Proliferation, migration, differentiation, and survival of neuronal precursor cells (NPCs) were analyzed by BrdU assay and immunohistochemistry; signal transduction pathway components were analyzed by immunoblot. In P2Y2^-/- mice, proliferation of NPCs in the SGZ and the SVZ was reduced. However, migration, neuronal fate decision, and survival were not affected. Moreover, p-Akt expression was decreased in P2Y2^-/- mice. P2Y2^-/- mice showed an impaired performance in the Y-maze and a higher latency in the hidden food test. These data indicate that the P2Y₂ receptor plays an important role in NPC proliferation as well as in hippocampus-dependent working memory and olfactory function.

Relationship between locomotor activity rhythm and corticosterone levels during HCC development, progression, and treatment in a mouse model

Cancer-related fatigue (CRF) and stress are common symptoms in cancer patients and represent early side effects of cancer treatment which affect the life quality of the patients. CRF may partly depend on disruption of the circadian rhythm. Locomotor activity and corticosterone rhythms are two important circadian outputs which can be used to analyze possible effects on the circadian function during cancer development and treatment. The present study analyzes the relationship between locomotor activity rhythm, corticosterone levels, hepatocellular carcinoma (HCC) development, and radiotherapy treatment in a mouse model. HCC was induced in mice by single injection of diethylnitrosamine (DEN) and chronic treatment of phenobarbital in drinking water. Another group received chronic phenobarbital treatment only. Tumor bearing animals were divided randomly into four groups irradiated at four different Zeitgeber time points. Spontaneous locomotor activity was recorded continuously; serum corticosterone levels and p-ERK immunoreaction in the suprachiasmatic nucleus (SCN) were investigated. Phenobarbital treated mice showed damped corticosterone levels and a less stable 24 hours activity rhythm as well as an increase in activity during the light phase, reminiscent of sleep disruption. The tumor mice showed an increase in corticosterone level during the inactive phase and decreased activity during the dark phase, reminiscent of CRF. After irradiation, corticosterone levels were further increased and locomotor activity rhythms were disrupted. Lowest corticosterone levels were observed after irradiation during the early light phase; thus, this time might be the best to apply radiotherapy in order to minimize side effects.

Does a Red House Affect Rhythms in Mice with a Corrupted Circadian System?

The circadian rhythms of body functions in mammals are controlled by the circadian system. The suprachiasmatic nucleus (SCN) in the hypothalamus orchestrates subordinate oscillators. Time information is conveyed from the retina to the SCN to coordinate an organism's physiology and behavior with the light/dark cycle. At the cellular level, molecular clockwork composed of interlocked transcriptional/translational feedback loops of clock genes drives rhythmic gene expression. Mice with targeted deletion of the essential clock gene Bmal1 (Bmal1-/-) have an impaired light input pathway into the circadian system and show a loss of circadian rhythms. The red house (RH) is an animal welfare measure widely used for rodents as a hiding place. Red plastic provides light at a low irradiance and long wavelength-conditions which affect the circadian system. It is not known yet whether the RH affects rhythmic behavior in mice with a corrupted circadian system. Here, we analyzed whether the RH affects spontaneous locomotor activity in Bmal1-/- mice under standard laboratory light conditions. In addition, mPER1- and p-ERK-immunoreactions, as markers for rhythmic SCN neuronal activity, and day/night plasma corticosterone levels were evaluated. Our findings indicate that application of the RH to Bmal1-/- abolishes rhythmic locomotor behavior and dampens rhythmic SCN neuronal activity. However, RH had no effect on the day/night difference in corticosterone levels.

Time-dependent changes in proliferation, DNA damage and clock gene expression in hepatocellular carcinoma and healthy liver of a transgenic mouse model

Hepatocellular carcinoma (HCC) is highly resistant to anticancer therapy and novel therapeutic strategies are needed. Chronotherapy may become a promising approach because it may improve the efficacy of antimitotic radiation and chemotherapy by considering timing of treatment. To date little is known about time-of-day dependent changes of proliferation and DNA damage in HCC. Using transgenic c-myc/transforming growth factor (TGFα) mice as HCC animal model, we immunohistochemically demonstrated Ki67 as marker for proliferation and γ-H2AX as marker for DNA damage in HCC and surrounding healthy liver (HL). Core clock genes (Per1, Per2, Cry1, Cry2, Bmal 1, Rev-erbα and Clock) were examined by qPCR. Data were obtained from samples collected ex vivo at four different time points and from organotypic slice cultures (OSC). Significant differences were found between HCC and HL. In HCC, the number of Ki67 immunoreactive cells showed two peaks (ex vivo: ZT06 middle of day and ZT18 middle of night; OSC: CT04 and CT16). In ex vivo samples, the number of γ-H2AX positive cells in HCC peaked at ZT18 (middle of the night), while in OSC their number remained high during subjective day and night. In both HCC and HL, clock gene expression showed a time-of-day dependent expression ex vivo but no changes in OSC. The expression of Per2 and Cry1 was significantly lower in HCC than in HL. Our data support the concept of chronotherapy of HCC. OSC may become useful to test novel cancer therapies.

Protective effects of 4-aminopyridine in experimental optic neuritis and multiple sclerosis

Chronic disability in multiple sclerosis is linked to neuroaxonal degeneration. 4-aminopyridine (4-AP) is used and licensed as a symptomatic treatment to ameliorate ambulatory disability in multiple sclerosis. The presumed mode of action is via blockade of axonal voltage gated potassium channels, thereby enhancing conduction in demyelinated axons. In this study, we provide evidence that in addition to those symptomatic effects, 4-AP can prevent neuroaxonal loss in the CNS. Using in vivo optical coherence tomography imaging, visual function testing and histologic assessment, we observed a reduction in retinal neurodegeneration with 4-AP in models of experimental optic neuritis and optic nerve crush. These effects were not related to an anti-inflammatory mode of action or a direct impact on retinal ganglion cells. Rather, histology and in vitro experiments indicated 4-AP stabilization of myelin and oligodendrocyte precursor cells associated with increased nuclear translocation of the nuclear factor of activated T cells. In experimental optic neuritis, 4-AP potentiated the effects of immunomodulatory treatment with fingolimod. As extended release 4-AP is already licensed for symptomatic multiple sclerosis treatment, we performed a retrospective, multicentre optical coherence tomography study to longitudinally compare retinal neurodegeneration between 52 patients on continuous 4-AP therapy and 51 matched controls. In line with the experimental data, during concurrent 4-AP therapy, degeneration of the macular retinal nerve fibre layer was reduced over 2 years. These results indicate disease-modifying effects of 4-AP beyond symptomatic therapy and provide support for the design of a prospective clinical study using visual function and retinal structure as outcome parameters.

Impact of Targeted Deletion of the Circadian Clock Gene Bmal1 in Excitatory Forebrain Neurons on Adult Neurogenesis and Olfactory Function

The circadian system is an endogenous timekeeping system that synchronizes physiology and behavior with the 24 h solar day. Mice with total deletion of the core circadian clock gene Bmal1 show circadian arrhythmicity, cognitive deficits, and accelerated age-dependent decline in adult neurogenesis as a consequence of increased oxidative stress. However, it is not yet known if the impaired adult neurogenesis is due to circadian disruption or to loss of the Bmal1 gene function. Therefore, we investigated oxidative stress and adult neurogenesis of the two principle neurogenic niches, the hippocampal subgranular zone and the subventricular zone in mice with a forebrain specific deletion of Bmal1 (Bmal1 fKO), which show regular circadian rhythmicity. Moreover, we analyzed the morphology of the olfactory bulb, as well as olfactory function in Bmal1 fKO mice. In Bmal1 fKO mice, oxidative stress was increased in subregions of the hippocampus and the olfactory bulb but not in the neurogenic niches. Consistently, adult neurogenesis was not affected in Bmal1 fKO mice. Although Reelin expression in the olfactory bulb was higher in Bmal1 fKO mice as compared to wildtype mice (Bmal1 WT), the olfactory function was not affected. Taken together, the targeted deletion of Bmal1 in mouse forebrain neurons is associated with a regional increase in oxidative stress and increased Reelin expression in the olfactory bulb but does not affect adult neurogenesis or olfactory function.

Bmal1-deficiency affects glial synaptic coverage of the hippocampal mossy fiber synapse and the actin cytoskeleton in astrocytes

Bmal1 is an essential component of the molecular clockwork, which drives circadian rhythms in cell function. In Bmal1-deficient (Bmal1-/-) mice, chronodisruption is associated with cognitive deficits and progressive brain pathology including astrocytosis indicated by increased expression of glial fibrillary acidic protein (GFAP). However, relatively little is known about the impact of Bmal1-deficiency on astrocyte morphology prior to astrocytosis. Therefore, in this study we analysed astrocyte morphology in young (6-8 weeks old) adult Bmal1-/- mice. At this age, overall GFAP immunoreactivity was not increased in Bmal1-deficient mice. At the ultrastructural level, we found a decrease in the volume fraction of the fine astrocytic processes that cover the hippocampal mossy fiber synapse, suggesting an impairment of perisynaptic processes and their contribution to neurotransmission. For further analyses of actin cytoskeleton, which is essential for distal process formation, we used cultured Bmal1-/- astrocytes. Bmal1-/- astrocytes showed an impaired formation of actin stress fibers. Moreover, Bmal1-/- astrocytes showed reduced levels of the actin-binding protein cortactin (CTTN). Cttn promoter region contains an E-Box like element and chromatin immunoprecipitation revealed that Cttn is a potential Bmal1 target gene. In addition, the level of GTP-bound (active) Rho-GTPase (Rho-GTP) was reduced in Bmal1-/- astrocytes. In summary, our data demonstrate that Bmal1-deficiency affects morphology of the fine astrocyte processes prior to strong upregulation of GFAP, presumably because of impaired Cttn expression and reduced Rho-GTP activation. These morphological changes might result in altered synaptic function and, thereby, relate to cognitive deficits in chronodisruption.

Monitoring retinal changes with optical coherence tomography predicts neuronal loss in experimental autoimmune encephalomyelitis

Background

Retinal optical coherence tomography (OCT) is a clinical and research tool in multiple sclerosis, where it has shown significant retinal nerve fiber (RNFL) and ganglion cell (RGC) layer thinning, while postmortem studies have reported RGC loss. Although retinal pathology in experimental autoimmune encephalomyelitis (EAE) has been described, comparative OCT studies among EAE models are scarce. Furthermore, the best practices for the implementation of OCT in the EAE lab, especially with afoveate animals like rodents, remain undefined. We aimed to describe the dynamics of retinal injury in different mouse EAE models and outline the optimal experimental conditions, scan protocols, and analysis methods, comparing these to histology to confirm the pathological underpinnings.

Methods

Using spectral-domain OCT, we analyzed the test-retest and the inter-rater reliability of volume, peripapillary, and combined horizontal and vertical line scans. We then monitored the thickness of the retinal layers in different EAE models: in wild-type (WT) C57Bl/6J mice immunized with myelin oligodendrocyte glycoprotein peptide (MOG_35–55) or with bovine myelin basic protein (MBP), in TCR^2D2 mice immunized with MOG_35–55, and in SJL/J mice immunized with myelin proteolipid lipoprotein (PLP_139–151). Strain-matched control mice were sham-immunized. RGC density was counted on retinal flatmounts at the end of each experiment.

Results

Volume scans centered on the optic disc showed the best reliability. Retinal changes during EAE were localized in the inner retinal layers (IRLs, the combination of the RNFL and the ganglion cell plus the inner plexiform layers). In WT, MOG_35–55 EAE, progressive thinning of IRL started rapidly after EAE onset, with 1/3 of total loss occurring during the initial 2 months. IRL thinning was associated with the degree of RGC loss and the severity of EAE. Sham-immunized SJL/J mice showed progressive IRL atrophy, which was accentuated in PLP-immunized mice. MOG_35–55-immunized TCR^2D2 mice showed severe EAE and retinal thinning. MBP immunization led to very mild disease without significant retinopathy.

Conclusions

Retinal neuroaxonal damage develops quickly during EAE. Changes in retinal thickness mirror neuronal loss and clinical severity. Monitoring of the IRL thickness after immunization against MOG_35–55 in C57Bl/6J mice seems the most convenient model to study retinal neurodegeneration in EAE.

From in vitro to ex vivo: subcellular localization and uptake of graphene quantum dots into solid tumors

Among various nanoparticles tested for pharmacological applications over the recent years, graphene quantum dots (GQDs) seem to be promising candidates for the construction of drug delivery systems due to their superior biophysical and biochemical properties. The subcellular fate of incorporated nanomaterial is decisive for transporting pharmaceuticals into target cells. Therefore a detailed characterization of the uptake of GQDs into different breast cancer models was performed. The demonstrated accumulation inside the endolysosomal system might be the reason for the particles' low toxicity, but has to be overcome for cytosolic or nuclear drug delivery. Furthermore, the penetration of GQDs into precision-cut mammary tumor slices was studied. These constitute a far closer to reality model system than monoclonal cell lines. The constant uptake into the depth of the tissue slices underlines the systems' potential for drug delivery into solid tumors.

Connexin30 and Connexin43 show a time-of-day dependent expression in the mouse suprachiasmatic nucleus and modulate rhythmic locomotor activity in the context of chronodisruption

Background

The astroglial connexins Cx30 and Cx43 contribute to many important CNS functions including cognitive behaviour, motoric capacity and regulation of the sleep-wake cycle. The sleep wake cycle, is controlled by the circadian system. The central circadian rhythm generator resides in the suprachiasmatic nucleus (SCN). SCN neurons are tightly coupled in order to generate a coherent circadian rhythm. The SCN receives excitatory glutamatergic input from the retina which mediates entrainment of the circadian system to the environmental light-dark cycle. Connexins play an important role in electric coupling of SCN neurons and astrocytic-neuronal signalling that regulates rhythmic SCN neuronal activity. However, little is known about the regulation of Cx30 and Cx43 expression in the SCN, and the role of these connexins in light entrainment of the circadian system and in circadian rhythm generation.

Methods

We analysed time-of-day dependent as well as circadian expression of Cx30 and Cx43 mRNA and protein in the mouse SCN by means of qPCR and immunohistochemistry. Moreover, we analysed rhythmic spontaneous locomotor activity in mice with a targeted deletion of Cx30 and astrocyte specific deletion of Cx43 (DKO) in different light regimes by means of on-cage infrared detectors.

Results

Fluctuation of Cx30 protein expression is strongly dependent on the light-dark cycle whereas fluctuation of Cx43 protein expression persisted in constant darkness. DKO mice entrained to the light-dark cycle. However, re-entrainment after a phase delay was slightly impaired in DKO mice. Surprisingly, DKO mice were more resilient to chronodisruption.

Conclusion

Circadian fluctuation of Cx30 and Cx43 protein expression in the SCN is differently regulated. Cx30 and astroglial Cx43 play a role in rhythm stability and re-entrainment under challenging conditions.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0370-2) contains supplementary material, which is available to authorized users.

Deficiency of the clock gene Bmal1 affects neural progenitor cell migration

We demonstrate the impact of a disrupted molecular clock in Bmal1-deficient (Bmal1-/-) mice on migration of neural progenitor cells (NPCs). Proliferation of NPCs in rostral migratory stream (RMS) was reduced in Bmal1-/- mice, consistent with our earlier studies on adult neurogenesis in hippocampus. However, a significantly higher number of NPCs from Bmal1-/- mice reached the olfactory bulb as compared to wild-type littermates (Bmal1+/+ mice), indicating a higher migration velocity in Bmal1-/- mice. In isolated NPCs from Bmal1-/- mice, not only migration velocity and expression pattern of genes involved in detoxification of reactive oxygen species were affected, but also RNA oxidation of catalase was increased and catalase protein levels were decreased. Bmal1+/+ migration phenotype could be restored by treatment with catalase, while treatment of NPCs from Bmal1+/+ mice with hydrogen peroxide mimicked Bmal1-/- migration phenotype. Thus, we conclude that Bmal1 deficiency affects NPC migration as a consequence of dysregulated detoxification of reactive oxygen species.

Sex-Dependent Effects of Bmal1-Deficiency on Mouse Cerebral Cortex Infarction in Response to Photothrombotic Stroke

Stroke is a leading cause of disability and death worldwide. There is increasing evidence that occurrence of ischemic stroke is affected by circadian system and sex. However, little is known about the effect of these factors on structural recovery after ischemic stroke. Therefore, we studied infarction in cerebral neocortex of male and female mice with deletion of the clock gene Bmal1 (Bmal1-/-) after focal ischemia induced by photothrombosis (PT). The infarct core size was significantly smaller 14 days (d) as compared to seven days after PT, consistent with structural recovery during the sub-acute phase. However, when sexes were analyzed separately 14 days after PT, infarct core was significantly larger in wild-type (Bmal1+/+) female as compared to male Bmal1+/+ mice, and in female Bmal1+/+, as compared to female Bmal1-/- mice. Volumes of reactive astrogliosis and densely packed microglia closely mirrored the size of infarct core in respective groups. Estradiol levels were significantly higher in female Bmal1-/- as compared to Bmal1+/+ mice. Our data suggests a sex-dependent effect and an interaction between sex and genotype on infarct size, the recruitment of astrocytes and microglia, and a relationship of these cells with structural recovery probably due to positive effects of estradiol during the subacute phase.

Impaired Photic Entrainment of Spontaneous Locomotor Activity in Mice Overexpressing Human Mutant α-Synuclein

Parkinson's disease (PD) is characterized by distinct motor and non-motor symptoms. Sleep disorders are the most frequent and challenging non-motor symptoms in PD patients, and there is growing evidence that they are a consequence of disruptions within the circadian system. PD is characterized by a progressive degeneration of the dorsal vagal nucleus and midbrain dopaminergic neurons together with an imbalance of many other neurotransmitters. Mutations in α-synuclein (SNCA), a protein modulating SNARE complex-dependent neurotransmission, trigger dominantly inherited PD variants and sporadic cases of PD. The A53T SNCA missense mutation is associated with an autosomal dominant early-onset familial PD. To test whether this missense mutation affects the circadian system, we analyzed the spontaneous locomotor behavior of non-transgenic wildtype mice and transgenic mice overexpressing mutant human A53T α-synuclein (A53T). The mice were subjected to entrained- and free-running conditions as well as to experimental jet lag. Furthermore, the vesicular glutamate transporter 2 (VGLUT2) in the suprachiasmatic nucleus (SCN) was analyzed by immunohistochemistry. Free-running circadian rhythm and, thus, circadian rhythm generation, were not affected in A53T mice. A53T mice entrained to the light⁻dark cycle, however, with an advanced phase angle of 2.65 ± 0.5 h before lights off. Moreover, re-entrainment after experimental jet lag was impaired in A53T mice. Finally, VGLUT2 immunoreaction was reduced in the SCN of A53T mice. These data suggest an impaired light entrainment of the circadian system in A53T mice.

Time-of-day-dependent expression of purinergic receptors in mouse suprachiasmatic nucleus

Purinergic P2X and P2Y receptors are involved in mediating intercellular signalling via purines such as adenosine triphosphate (ATP). P2X and P2Y receptors have been implicated in numerous body functions including learning, memory and sleep. All of these body functions show time-of–day-dependent variations controlled by the master circadian oscillator located in the suprachiasmatic nucleus (SCN). Evidence exists for a role of purinergic signalling in intercellular coupling within SCN. However, few studies have been performed on the expression of purinergic receptors in SCN. Therefore, we analyse the expression of seven P2X (P2X1–7) and eight P2Y (P2Y1–2, 4, 6, 11–14) receptors in mouse SCN and address their time-of-day-dependent variation by using immunohistochemistry and real-time polymerase chain reaction. At the early light phase, P2X and P2Y receptors show a low to moderate, homogenously distributed immunoreaction throughout SCN. P2Y13 reveals strong immunoreaction in fibres within the core region of SCN. From the fifteen analysed P2 receptors, seven exhibit a time-of-day-dependent variation in SCN. P2X1 immunoreaction is very low in the early light phase with a minor increase at the end of the dark phase. P2X4 immunoreaction strongly increases during the dark phase in soma cells in the core region and in a dense network of fibres in the shell region of SCN. P2X3 immunoreaction is moderately elevated during the dark phase. Conversely, immunoreaction for P2Y2, P2Y12 and P2Y14 moderately increases at the early light phase and P2Y6 immunoreaction displays a moderate increase at the mid-light phase. Thus, this study demonstrates a time-of-day-dependent variation of P2 receptors in mouse SCN.

Melatonin Receptor 1 Deficiency Affects Feeding Dynamics and Pro-Opiomelanocortin Expression in the Arcuate Nucleus and Pituitary of Mice

BACKGROUND/METHODS

Melatonin, the neurohormone for darkness, mediates photoperiod-dependent changes in physiology and behavior by targeting specific membrane-bound receptors (MT1 and MT2). In the present study, we investigated the impact of MT1 receptor deficiency on feeding behavior, locomotor activity and mRNA expression levels encoding for the polypeptide pro-opiomelanocortin (Pomc) and neuropeptide Y (Npy) in the hypothalamic arcuate nucleus (ARC) and the adenohypophysis [pars distalis (PD) and pars intermedia (PI)] in a comparison between wild-type (WT) and MT1-deficient (MT1-/-) mice.

RESULTS

The MT1-/- mice spent significantly more time feeding than the WT mice, while the general locomotor behavior, body weight and the total amount of food consumed did not differ between both genotypes. The nocturnal expression levels of Pomc in the ARC and PD were significantly higher in WT as compared to MT1-/- mice and exogenous melatonin administered during the light phase stimulated Pomc expression in WT mice only. No differences were found between WT and MT1-/- mice with regard to Pomc expression levels in the PI.

CONCLUSION

Thus, the MT1-mediated signaling stimulates Pomc expression in a region-specific pattern. Since the MT1-mediated changes in Pomc expression do not elicit direct orexigenic or anorexigenic effects, such effects are obviously mediated by regulatory systems downstream of the Pomc mRNA (e.g. cleavage and release of POMC derivatives), which are independent of MT1 signaling.

Perturbation of the molecular clockwork in the SCN of non-obese diabetic mice prior to diabetes onset

Circadian disruption is associated with the development of diabetes. Non-obese diabetic (NOD) mice show abnormal diurnal profiles in energy balance and locomotor activity suggesting circadian misalignment. Therefore, we analyzed cFos and mPER1 as markers for rhythmic neuronal activity within the suprachiasmatic nucleus (SCN) of wildtype (WT) and non-diabetic (nNOD) as well as acutely diabetic NOD (dNOD) mice. cFos levels show a day/night difference in both WT and nNOD but not in dNOD. mPER1 levels did not show a day/night difference in both nNOD and dNOD. This suggests that disruption of SCN rhythmicity in NOD mice precedes the actual onset of diabetes.

Heat Shock Factor 1 Deficiency Affects Systemic Body Temperature Regulation

INTRODUCTION

Heat shock factor 1 (HSF1) is a ubiquitous heat-sensitive transcription factor that mediates heat shock protein transcription in response to cellular stress, such as increased temperature, in order to protect the organism against misfolded proteins. In this study, we analysed the effect of HSF1 deficiency on core body temperature regulation.

MATERIALS AND METHODS

Body temperature, locomotor activity, and food consumption of wild-type mice and HSF1-deficient mice were recorded. Prolactin and thyroid-stimulating hormone levels were measured by ELISA. Gene expression in brown adipose tissue was analysed by quantitative real-time PCR. Hypothalamic HSF1 and its co-localisation with tyrosine hydroxylase was analysed using confocal laser scanning microscopy.

RESULTS

HSF1-deficient mice showed an increase in core body temperature (hyperthermia), decreased overall locomotor activity, and decreased levels of prolactin in pituitary and blood plasma reminiscent of cold adaptation. HSF1 could be detected in various hypothalamic regions involved in temperature regulation, suggesting a potential role of HSF1 in hypothalamic thermoregulation. Moreover, HSF1 co-localises with tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, suggesting a potential role of HSF1 in the hypothalamic control of prolactin release. In brown adipose tissue, levels of prolactin receptor and uncoupled protein 1 were increased in HSF1-deficient mice, consistent with an up-regulation of heat production.

CONCLUSION

Our data suggest a role of HSF1 in systemic thermoregulation.

Premature aging of the hippocampal neurogenic niche in adult Bmal1-deficient mice

Hippocampal neurogenesis undergoes dramatic age-related changes. Mice with targeted deletion of the clock geneBmal1 (Bmal1(-/-)) show disrupted regulation of reactive oxygen species homeostasis, accelerated aging, neurodegeneration and cognitive deficits. As proliferation of neuronal progenitor/precursor cells (NPCs) is enhanced in young Bmal1(-/-) mice, we tested the hypothesis that this results in premature aging of hippocampal neurogenic niche in adult Bmal1(-/-) mice as compared to wildtype littermates. We found significantly reduced pool of hippocampal NPCs, scattered distribution, enhanced survival of NPCs and an increased differentiation of NPCs into the astroglial lineage at the expense of the neuronal lineage. Immunoreaction of the redox sensitive histone deacetylase Sirtuine 1, peroxisomal membrane protein at 70 kDa and expression of the cell cycle inhibitor p21(Waf1/CIP1) were increased in adult Bmal1(-/-) mice. In conclusion, genetic disruption of the molecular clockwork leads to accelerated age-dependent decline in adult neurogenesis presumably as a consequence of oxidative stress.

Owls and larks in mice

Humans come in different chronotypes and, particularly, the late chronotype (the so-called owl) has been shown to be associated with several health risks. A number of studies show that laboratory mice also display various chronotypes. In mice as well as in humans, the chronotype shows correlations with the period length and rhythm stability. In addition, some mouse models for human diseases show alterations in their chronotypic behavior, which are comparable to those humans. Thus, analysis of the behavior of mice is a powerful tool to unravel the molecular and genetic background of the chronotype and the prevalence of risks and diseases that are associated with it. In this review, we summarize the correlation of chronotype with free-running period length and rhythm stability in inbred mouse strains, in mice with a compromised molecular clockwork, and in a mouse model for neurodegeneration.

Chronotype and stability of spontaneous locomotor activity rhythm in BMAL1-deficient mice

Behavior, physiological functions and cognitive performance change over the time of the day. These daily rhythms are either externally driven by rhythmic environmental cues such as the light/dark cycle (masking) or controlled by an internal circadian clock, the suprachiasmatic nucleus (SCN), which can be entrained to the light/dark cycle. Within a given species, there is genetically determined variability in the temporal preference for the onset of the active phase, the chronotype. The chronotype is the phase of entrainment between external and internal time and is largely regulated by the circadian clock. Genetic variations in clock genes and environmental influences contribute to the distribution of chronotypes in a given population. However, little is known about the determination of the chronotype, the stability of the locomotor rhythm and the re-synchronization capacity to jet lag in an animal without a functional endogenous clock. Therefore, we analyzed the chronotype of BMAL1-deficient mice (BMAL1-/-) as well as the effects of repeated experimental jet lag on locomotor activity rhythms. Moreover, light-induced period expression in the retina was analyzed to assess the responsiveness of the circadian light input system. In contrast to wild-type mice, BMAL1-/- showed a significantly later chronotype, adapted more rapidly to both phase advance and delay but showed reduced robustness of rhythmic locomotor activity after repeated phase shifts. However, photic induction of Period in the retina was not different between the two genotypes. Our findings suggest that a disturbed clockwork is associated with a late chronotype, reduced rhythm stability and higher vulnerability to repeated external desynchronization.

Melatonin-receptor-1-deficiency affects neurogenic differentiation factor immunoreaction in pancreatic islets and enteroendocrine cells of mice

Neurogenic differentiation factor (NeuroD) is a transcription factor involved in the differentiation of neurons and in the control of energy balance and metabolism. It plays a key role in type 1 and type 2 diabetes. Melatonin is an important rhythmic endocrine signal within the circadian system of mammals and modulates insulin secretion and glucose metabolism. In the mouse pars tuberalis, NeuroD mRNA levels show day/night variation, which is independent of the molecular clock gene mPER1 but depends on the functional melatonin receptor 1 (MT1). So far, little is known about the effect of melatonin on NeuroD synthesis in the gastrointestinal tract. Thus, NeuroD protein levels and cellular localization were analyzed by immunohistochemistry in pancreatic islets and duodenal enteroendocrine cells of MT1- and mPER1-deficienct mice. In addition, the localization of NeuroD-positive cells was analyzed by double-immunofluorescence and confocal laser microscopy. In duodenal enteroendocrine cells and pancreatic islets of WT and PER1-deficient mice, NeuroD immunoreaction showed a peak during the early subjective night. In contrast, this peak was absent in MT1-deficent mice. These data suggest that melatonin, by acting on MT1 receptors, affects NeuroD expression in the gastrointestinal tract and thus might contribute to circadian regulation in metabolic functions.

Degeneration of the cerebellum in Huntington's disease (HD): possible relevance for the clinical picture and potential gateway to pathological mechanisms of the disease process

Huntington's disease (HD) is a polyglutamine disease and characterized neuropathologically by degeneration of the striatum and select layers of the neo- and allocortex. In the present study, we performed a systematic investigation of the cerebellum in eight clinically diagnosed and genetically confirmed HD patients. The cerebellum of all HD patients showed a considerable atrophy, as well as a consistent loss of Purkinje cells and nerve cells of the fastigial, globose, emboliform and dentate nuclei. This pathology was obvious already in HD brains assigned Vonsattel grade 2 striatal atrophy and did not correlate with the extent and distribution of striatal atrophy. Therefore, our findings suggest (i) that the cerebellum degenerates early during HD and independently from the striatal atrophy and (ii) that the onset of the pathological process of HD is multifocal. Degeneration of the cerebellum might contribute significantly to poorly understood symptoms occurring in HD such as impaired rapid alternating movements and fine motor skills, dysarthria, ataxia and postural instability, gait and stance imbalance, broad-based gait and stance, while the morphological alterations (ie ballooned neurons, torpedo-like axonal inclusions) observed in the majority of surviving nerve cells may represent a gateway to the unknown mechanisms of the pathological process of HD.

Tafa-3 encoding for a secretory peptide is expressed in the mouse pars tuberalis and is affected by melatonin 1 receptor deficiency

The hypophysial pars tuberalis (PT) is an important interface between neuroendocrine brain centers (hypothalamus, pineal organ) and the anterior lobe of the hypophysis (PD). The best investigated role of the PT is the control of seasonally changing functions. In mammals, melatonin secreted from the pineal organ represents a major input signal to the PT. By acting upon melatonin type 1 receptors (MT1) melatonin controls the functional activity of the PT. Most interestingly, the PT sends its output signals in two directions: via a "retrograde" pathway to the hypothalamus and via an "anterograde" pathway to the PD. TSH has been identified as "retrograde" messenger, while endocannabinoids function as messengers of the "anterograde" pathway. Here we show in mice that the PT expresses Tafa-3 encoding for a secretory peptide. In the PT of wild type mice Tafa-3 mRNA levels varied between day and night: they were low at mid-day and high at mid-night. This day/night difference was not observed in the PT of mice with a targeted deletion of the MT1 receptor indicating that Tafa-3 mRNA expression in the PT is controlled by melatonin acting through the MT1 receptor. Notably, Tafa-3 expression was not restricted to the PT, but was also found in other brain regions, such as the hippocampus, the habenular and thalamic nuclei. In these regions, Tafa-3 expression did not display a day/night difference and was not affected by MT1-deficiency. Thus, Tafa-3 expression appears to be controlled by region-specific mechanisms. Our data suggest that TAFA-3 is a signaling molecule from the PT and provides further evidence for the emerging concept that the PT rather than relying upon highly organ-specific messengers employs a cocktail of signaling molecules that also operate in other brain systems.

The endogenous melatonin (MT) signal facilitates reentrainment of the circadian system to light-induced phase advances by acting upon MT2 receptors

The indolamine melatonin is an important rhythmic endocrine signal in the circadian system. Exogenous melatonin can entrain circadian rhythms in physiology and behavior, but the role of endogenous melatonin and the two membrane-bound melatonin receptor types, MT1 and MT2, in reentrainment of daily rhythms to light-induced phase shifts is not understood. The present study analyzed locomotor activity rhythms and clock protein levels in the suprachiasmatic nuclei (SCN) of melatonin-deficient (C57BL/6J) and melatonin-proficient (C3H/HeN) mice, as well as in melatonin-proficient (C3H/HeN) mice with targeted deletion of the MT1, MT2, or both receptors, to determine effects associated with phase delays or phase advances of the light/dark (LD) cycle. In all mouse strains and genotypes, reentrainment of locomotor activity rhythms was significantly faster after a 6-h phase delay than a 6-h phase advance. Reentrainment after the phase advance was, however, significantly slower than in melatonin-deficient animals and in mice lacking functional MT2 receptors than melatonin-proficient animals with intact MT2 receptors. To investigate whether these behavioral differences coincide with differences in reentrainment of clock protein levels in the SCN, mPER1, mCRY1 immunoreactions were compared between control mice kept under the original LD cycle and killed at zeitgeber time 04 (ZT04) or at ZT10, respectively, and experimental mice subjected to a 6-h phase advance of the LD cycle and sacrificed at ZT10 on the third day after phase advance. This ZT corresponds to ZT04 of the original LD cycle. Under the original LD cycle, the numbers of mPER1- and mCRY1-immunoreactive cell nuclei were low at ZT04 and high at ZT10 in the SCN of all mouse strains and genotypes investigated. Notably, mouse strains with intact melatonin signaling and functional MT2 receptors showed a significant increase in the number of mPER1- and mCRY1-immunoreactive cell nuclei at the new ZT10 as compared to the former ZT04. These data suggest the endogenous melatonin signal facilitates reentrainment of the circadian system to phase advances on the level of the SCN molecular clockwork by acting upon MT2 receptors.

When does it start ticking? Ontogenetic development of the mammalian circadian system

Circadian rhythms in physiology and behavior ensure that vital functions are temporally synchronized with cyclic environmental changes. In mammals, the circadian system is conducted by a central circadian rhythm generator that resides in the hypothalamic suprachiasmatic nucleus (SCN) and controls multiple subsidiary circadian oscillators in the periphery. The molecular clockwork in SCN and peripheral oscillators consists of autoregulatory transcriptional/translational feedback loops of clock genes. The adult circadian system is synchronized to the astrophysical day by light whereas the fetal and neonatal circadian system entrains to nonphotic rhythmic maternal signals. This chapter reviews maturation and entrainment of the central circadian rhythm generator in the SCN and of peripheral oscillators during ontogenetic development.

Melatonin receptor 1-dependent gene expression in the mouse pars tuberalis as revealed by cDNA microarray analysis and in situ hybridization

Melatonin is an important rhythmic endocrine signal within the circadian system of mammals. The hypophyseal pars tuberalis (PT) is a major target for melatonin and shows a high density of melatonin type 1 receptors (MT1). Melatonin affects expression of clock genes in PT cells which encode for transcriptional regulators of rhythmic gene expression. In this study, microarray analysis was performed to screen for genes coding for transcriptional regulators under the control of MT1 receptors in the mouse PT. Gene expression levels were compared between melatonin-proficient mice deficient for MT1 (MT1-/-) and the corresponding wild-type (WT) during mid-subjective day (CT06, low endogenous melatonin levels) and mid-subjective night (CT18, high endogenous melatonin levels). In situ hybridization was used to validate the data obtained by microarray analysis to analyze the acute effect of daytime melatonin application on gene expression in the wild-type PT. In the wild-type PT, expression of Tim was higher during day as compared to night. These day/night differences in gene expression were not observed in the PT of MT1-/- mice, demonstrating the impact of MT1-mediated signal transduction pathway. Day-time application of melatonin acutely affected Tim and Cry1 expression but not Neurod1 and Npas4 expression. We conclude that melatonin regulates expression of genes coding for transcriptional regulators in the PT through MT1 receptors by either acute or long-term mechanisms.

Impact of melatonin and molecular clockwork components on the expression of thyrotropin beta-chain (Tshb) and the Tsh receptor in the mouse pars tuberalis

Photoperiodic regulation of reproduction in birds and mammals involves thyrotropin beta-chain (TSHb), which is secreted from the pars tuberalis (PT) and controls the expression of deiodinase type 2 and 3 in the ependymal cell layer of the infundibular recess (EC) via TSH receptors (TSHRs). To analyze the impact of melatonin and the molecular clockwork on the expression of Tshb and Tshr, we investigated melatonin-proficient C3H wild-type (WT), melatonin receptor 1-deficient (MT1-/-) or clockprotein PERIOD1-deficient (mPER1-/-) mice. Expression of Tshb and TSHb immunoreactivity in PT were low during day and high during the night in WT, high during the day and low during the night in mPER1-deficient, and equally high during the day and night in MT1-deficient mice. Melatonin injections into WT acutely suppressed Tshb expression. Transcription assays showed that the 5' upstream region of the Tshb gene could be controlled by clockproteins. Tshr levels in PT were low during the day and high during the night in WT and mPER1-deficient mice and equally low in MT1-deficient mice. Tshr expression in the EC did not show a day/night variation. Melatonin injections into WT acutely induced Tshr expression in PT but not in EC. TSH stimulation of hypothalamic slice cultures of WT induced phosphorylated cAMP response element-binding protein in PT and EC and deiodinase type 2 in the EC. Our data suggest that Tshb expression in PT is controlled by melatonin and the molecular clockwork and that melatonin activates Tshr expression in PT but not in EC. They also confirm the functional importance of TSHR in the PT and EC.

The mammalian molecular clockwork controls rhythmic expression of its own input pathway components

The core molecular clockwork in the suprachiasmatic nucleus (SCN) is based on autoregulatory feedback loops of transcriptional activators (CLOCK/NPAS2 and BMAL1) and inhibitors (mPER1-2 and mCRY1-2). To synchronize the phase of the molecular clockwork to the environmental day and night condition, light at dusk and dawn increases mPer expression. However, the signal transduction pathways differ remarkably between the day/night and the night/day transition. Light during early night leads to intracellular Ca(2+) release by neuronal ryanodine receptors (RyRs), resulting in phase delays. Light during late night triggers an increase in guanylyl cyclase activity, resulting in phase advances. To date, it is still unknown how the core molecular clockwork regulates the availability of the respective input pathway components. Therefore, we examined light resetting mechanisms in mice with an impaired molecular clockwork (BMAL1(-/-)) and the corresponding wild type (BMAL1(+/+)) using in situ hybridization, real-time PCR, immunohistochemistry, and a luciferase reporter system. In addition, intracellular calcium concentrations (Ca(2+)(i)) were measured in SCN slices using two-photon microscopy. In the SCN of BMAL1(-/-) mice Ryr mRNA and RyR protein levels were reduced, and light-induced mPer expression was selectively impaired during early night. Transcription assays with NIH3T3 fibroblasts showed that Ryr expression was activated by CLOCK::BMAL1 and inhibited by mCRY1. The Ca(2+)(i) response of SCN cells to the RyR agonist caffeine was reduced in BMAL1(-/-) compared with BMAL1(+/+) mice. Our findings provide the first evidence that the mammalian molecular clockwork influences Ryr expression and thus controls its own photic input pathway components.

Differential maturation of circadian rhythms in clock gene proteins in the suprachiasmatic nucleus and the pars tuberalis during mouse ontogeny

Circadian rhythms of many body functions in mammals are controlled by a master pacemaker, residing in the hypothalamic suprachiasmatic nucleus (SCN), which synchronises peripheral oscillators. The SCN and peripheral oscillators share several components of the molecular clockwork and comprise transcriptional activators (BMAL1 and CLOCK/NPAS2) and inhibitors (mPER1/2 and mCRY1/2). Here we compared the ontogenetic maturation of the clockwork in the SCN and pars tuberalis (PT). The PT is a peripheral oscillator that strongly depends on rhythmic melatonin signals. Immunoreactions for clock gene proteins were determined in the SCN and PT at four different timepoints during four differential stages of mouse ontogeny: foetal (embryonic day 18), newborn (2-day-old), infantile (10-day-old), and adult. In the foetal SCN, levels of immunoreactions of all clock proteins were significantly lower than adult levels except for BMAL1. In the newborn SCN the clock protein immunoreactions had not yet reached adult levels, but the infantile SCN showed similar levels of immunoreactions as the adult. In contrast, immunoreactions for all clock gene proteins in the foetal PT were as intense as in newborn, infantile and adult, and showed the same phase. As the foetal pineal gland is not yet capable of rhythmic melatonin production, the rhythms in clock gene proteins in the foetal PT are presumably dependent on the maternal melatonin signal. Thus, our data provide the first evidence that maternal melatonin is important for establishing and maintaining circadian rhythms in a foetal peripheral oscillator.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

The impact of CREB and its phosphorylation at Ser142 on inflammatory nociception

Peripheral noxious stimulation leads to phosphorylation and thereby activation of the transcription factor CREB in the spinal cord. CREB phosphorylation occurs mainly at serine 133, but the phosphorylation site at serine 142 may also be important. We investigated the impact of spinal CREB protein levels and phosphorylation at Ser142 on the nociceptive behaviour in rat and mouse models of inflammatory nociception. Downregulation of total CREB protein in the rat spinal cord by antisense-oligonucleotides resulted in antinociceptive effects. After peripheral noxious stimulation CREB was phosphorylated in the spinal cord at serine 133 and 142 indicating a potential role of both residues in nociceptive processing. However, Ser142 mutant mice developed equal behavioural correlates of hyperalgesia as wild-type mice in different inflammatory models. Thus, our data confirm that CREB is essential for spinal nociceptive processing. However, prevention of phosphorylation only at serine 142 is not sufficient to modulate the nociceptive response.

Loss of responsiveness to melatonin in the aging mouse suprachiasmatic nucleus

Melatonin modulates circadian rhythms via the hypothalamic suprachiasmatic nucleus (SCN). One of the most robust assays for SCN melatonin receptor activation in mice is the inhibition of PACAP-induced phosphorylation of the transcription factor Ca(2+)/cAMP responsive element binding protein (CREB). To assess the effect of aging on responsiveness to melatonin, SCN slices from mice of different ages were prepared and treated with PACAP alone or PACAP plus melatonin. CREB phosphorylation state was assessed by immunohistochemistry. In SCN slices from young (2-4-month-old) mice, melatonin reduced the level of phospho-CREB immunoreactivity following PACAP treatment in a dose-dependent manner. In SCN slices from aged mice (19-22 months of age), PACAP alone induced comparable levels of phospho-CREB, but melatonin treatment failed to inhibit the PACAP-induced CREB phosphorylation. The results indicate an age-related loss of sensitivity to melatonin in the SCN. The findings are discussed in the context of the impact of endogenous and exogenous melatonin on sleep in elderly humans.

Melatonin Plays a Crucial Role in the Regulation of Rhythmic Clock Gene Expression in the Mouse Pars Tuberalis

Circadian rhythms in physiology and behavior are driven by a central clock residing within the hypothalamic suprachiasmatic nucleus (SCN). Molecularly, the biological clock is based on the transcriptional/translational feedback loop of clock genes (mPer, mCry, Clock, and Bmal1). Circadian expression of clock genes is not limited to the SCN, but is found in many peripheral tissues. Peripheral rhythms depend on neuroendocrine/neuronal output from the SCN. Melatonin, the hormone of darkness, represents an important neuroendocrine output of the circadian clock. The hypophyseal pars tuberalis (PT) is one of the main target regions for melatonin. The aim of the study was to test whether mPer, mCry, Clock, and Bmal1 are rhythmically expressed in the mouse PT and how the absence of melatonin receptors affects clock gene expression. We analyzed clock gene expression by in situ hybridization and compared wild-type (WT), melatonin 1 receptor knockout (MT1 ko), and melatonin 2 receptor knockout (MT2 ko) mice. mPer1, mCry1, Clock, and Bmal1, but not mPer2 and mCry2, were rhythmically expressed in the PT of WT and MT2 ko mice. In the PT of MT1 ko mice, expression of mPer1, mCry1, Clock, and Bmal1 was dramatically reduced. We conclude that melatonin, acting through the MT1 receptor, is an important regulator of rhythmic clock gene expression in the mouse PT.

Rhythms in clock proteins in the mouse pars tuberalis depend on MT1 melatonin receptor signalling

Melatonin provides a rhythmic neuroendocrine output, driven by a central circadian clock that encodes information about phase and length of the night. In the hypophyseal pars tuberalis (PT), melatonin is crucial for rhythmic expression of the clock genes mPer1 and mCry1, and melatonin acting in the PT influences prolactin secretion from the pars distalis. To examine further the possibility of a circadian clockwork functioning in the PT, and the impact of melatonin on this tissue, we assessed circadian clock proteins by immunohistochemistry and compared the diurnal expression in the PT of wild type (WT), and MT1 melatonin receptor-deficient (MT1-/-) mice. While in the PT of WT mice mPER1, mPER2, and mCRY1 showed a pronounced rhythm, mCRY2, CLOCK, and BMAL1 were constitutively present. Despite reported differences in maximal levels and timing of mCry1, mPer1, and mPer2 RNAs, the corresponding protein levels peaked simultaneously during late day, suggesting a codependency for their stabilization and/or nuclear entry. MT1-/- mice had reduced levels of mPER1, mCRY1, CLOCK and BMAL1, consistent with the earlier reported reduction in mRNA expression of these clock genes. Surprisingly, mPER2-immunoreaction was constitutively low, although mPer2 was rhythmically expressed in the PT of MT1-/- mice. This suggests that mPER2 is degraded due to the reduced levels of its stabilizing interaction partners mPER1 and mCRY1. The results show that melatonin, acting through the MT1, determines availability of the circadian proteins mPER1, mPER2 and mCRY1 and thus plays a crucial role in regulating rhythmicity in PT cells.

Light does not degrade the constitutively expressed BMAL1 protein in the mouse suprachiasmatic nucleus

Biological rhythms in mammals are driven by a central circadian clock located in the suprachiasmatic nucleus (SCN). At the molecular level the biological clock is based on the rhythmic expression of clock genes. Two basic helix-loop-helix (bHLH)/PAS-containing transcription factors, CLOCK and BMAL1 (MOP3), provide the basic drive to the system by activating transcription of negative regulators through E box enhancer elements. A critical feature of circadian timing is the ability of the clockwork to be entrained to the environmental light/dark cycle. The light-resetting mechanism of the mammalian circadian clock is poorly understood. Light-induced phase shifts are correlated with the induction of the clock genes mPer1 and mPer2 and a subsequent increase in mPER1 protein levels. It has previously been suggested that rapid degradation of BMAL1 protein in the rat SCN is part of the resetting mechanism of the central pacemaker. Our study shows that BMAL1 and CLOCK proteins are continuously expressed at high levels in the mouse SCN, supporting the hypothesis that rhythmic negative feedback plays the major role in rhythm generation in the mammalian pacemaker. Using both immunocytochemistry and immunoblot analysis, our studies demonstrate that BMAL1 protein in the mouse SCN is not affected by a phase-resetting light pulse. These results indicate that rapid degradation of BMAL1 protein is not a consistent feature of resetting mechanisms in rodents.

Targeted disruption of the mouse Mel(1b) melatonin receptor

Two high-affinity, G protein-coupled melatonin receptor subtypes have been identified in mammals. Targeted disruption of the Mel(1a) melatonin receptor prevents some, but not all, responses to the hormone, suggesting functional redundancy among receptor subtypes (Liu et al., Neuron 19:91-102, 1997). In the present work, the mouse Mel(1b) melatonin receptor cDNA was isolated and characterized, and the gene has been disrupted. The cDNA encodes a receptor with high affinity for melatonin and a pharmacological profile consistent with its assignment as encoding a melatonin receptor. Mice with targeted disruption of the Mel(1b) receptor have no obvious circadian phenotype. Melatonin suppressed multiunit electrical activity in the suprachiasmatic nucleus (SCN) in Mel(1b) receptor-deficient mice as effectively as in wild-type controls. The neuropeptide, pituitary adenylyl cyclase activating peptide, increases the level of phosphorylated cyclic AMP response element binding protein (CREB) in SCN slices, and melatonin reduces this effect. The Mel(1a) receptor subtype mediates this inhibitory response at moderate ligand concentrations (1 nM). A residual response apparent in Mel(1a) receptor-deficient C3H mice at higher melatonin concentrations (100 nM) is absent in Mel(1a)-Mel(1b) double-mutant mice, indicating that the Mel(1b) receptor mediates this effect of melatonin. These data indicate that there is a limited functional redundancy between the receptor subtypes in the SCN. Mice with targeted disruption of melatonin receptor subtypes will allow molecular dissection of other melatonin receptor-mediated responses.

Organisation of the circadian system in melatonin-proficient C3H and melatonin-deficient C57BL mice: a comparative investigation

In all vertebrates melatonin is rhythmically synthesised in the pineal gland and functions as a hormonal message encoding for the duration of darkness. This review focuses on the role of melatonin in the circadian organisation of mammals by comparing signal transduction mechanisms in the pineal organ, the suprachiasmatic nucleus and the hypophyseal pars tuberalis in melatonin-proficient (C3H) and melatonin-deficient (C57BL) mice strains. Surprisingly, the major signal transduction cascades in the pineal organ did not differ between the two mouse strains. With regard to the suprachiasmatic nucleus, the site of the endogenous clock, it was found that melatonin at most sets the gain for clock error signals mediated via the retinohypothalamic tract, but has no effect on the rhythm generation itself or on the maintenance of the oscillation. In contrast, melatonin plays an essential role in the control of the hypophyseal pars tuberalis. Here it acts in concert with adenosine to elicit rhythms in clock gene expression. Melatonin opens a temporally restricted gate for adenosine to induce cyclic AMP (cAMP)-sensitive genes by sensitising the adenylyl cyclase. This interaction, which grants a temporally precise regulation of gene expression, may reflect the central role of melatonin, i.e. in synchronising peripheral clock cells that require unique phasing of output signals with the master clock in the brain.

Mammalian melatonin receptors: molecular biology and signal transduction

The pineal hormone, melatonin, is an important regulator of seasonal reproduction and circadian rhythms. Its effects are mediated via high-affinity melatonin receptors, located on cells of the pituitary pars tuberalis (PT) and suprachiasmatic nucleus (SCN), respectively. Two subtypes of mammalian melatonin receptors have been cloned and characterized, the MT1 (Mel(1a)) and the MT2 (Mel(1b)) melatonin receptor subtypes. Both subtypes are members of the seven-transmembrane G protein-coupled receptor family. By using recombinant melatonin receptors it has been shown that the MT1 melatonin receptor is coupled to different G proteins that mediate adenylyl cyclase inhibition and phospholipase C beta activation. The MT2 receptor is also coupled to inhibition of adenylyl cyclase and additionally it inhibits the soluble guanylyl cyclase pathway. In mice with a targeted deletion of the MT1 receptor, the acute inhibitory effects of melatonin on SCN multiunit activity are completely abolished, while the phase-shifting responses to melatonin (given in physiological concentrations) appear normal. Furthermore, melatonin inhibits the phosphorylation of the transcription factor cyclic AMP response element binding protein, induced by the pituitary adenylate cyclase-activating polypeptide in SCN cells predominantly via the MT1 receptor. However, a functional MT2 receptor in the rodent SCN is partially able to compensate for the absence of the MT1 receptor in MT1 receptor-deficient mice. These findings indicate redundant and non-redundant roles of the receptor subtypes in regulating SCN function. In the PT, a functional MT1 receptor is essential for the rhythmic synthesis of the clock gene product mPER1. Melatonin produces a long-lasting sensitization of adenylyl cyclase and thus amplifies cyclic AMP signaling when melatonin levels decline at dawn. This action of melatonin amplifies gene expression rhythms in the PT and provides a mechanism for reinforcing rhythmicity in peripheral tissues which themselves lack the capacity for self-sustained oscillation. Mice with targeted deletion of melatonin receptor subtypes provide an excellent model to understand cellular mechanisms through which melatonin modulates circadian and photoperiodic rhythmicity.

Phosphorylation of CREB Ser142 regulates light-induced phase shifts of the circadian clock

Biological rhythms are driven in mammals by a central circadian clock located in the suprachiasmatic nucleus (SCN). Light-induced phase shifting of this clock is correlated with phosphorylation of CREB at Ser133 in the SCN. Here, we characterize phosphorylation of CREB at Ser142 and describe its contribution to the entrainment of the clock. In the SCN, light and glutamate strongly induce CREB Ser142 phosphorylation. To determine the physiological relevance of phosphorylation at Ser142, we generated a mouse mutant, CREB(S142A), lacking this phosphorylation site. Light-induced phase shifts of locomotion and expression of c-Fos and mPer1 in the SCN are significantly attenuated in CREB(S142A) mutants. Our findings provide genetic evidence that CREB Ser142 phosphorylation is involved in the entrainment of the mammalian clock and reveal a novel phosphorylation-dependent regulation of CREB activity.