NMDA-evoked calcium transients and currents in the suprachiasmatic nucleus: gating by the circadian system

Dynamic endocannabinoid-mediated neuromodulation of retinal circadian circuitry

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

Misalignment of Circadian Rhythms in Diet-Induced Obesity

The biological clocks of the circadian timing system coordinate cellular and physiological processes and synchronize them with daily cycles. While the central clock in the suprachiasmatic nucleus (SCN) is mainly synchronized by the light/dark cycles, the peripheral clocks react to other stimuli, including the feeding/fasting state, nutrients, sleep-wake cycles, and physical activity. During the disruption of circadian rhythms due to genetic mutations or social and occupational obligations, incorrect arrangement between the internal clock system and environmental rhythms leads to the development of obesity. Desynchronization between the central and peripheral clocks by altered timing of food intake and diet composition leads to uncoupling of the peripheral clocks from the central pacemaker and to the development of metabolic disorders. The strong coupling of the SCN to the light-dark cycle creates a situation of misalignment when food is ingested during the "wrong" time of day. Food-anticipatory activity is mediated by a self-sustained circadian timing, and its principal component is a food-entrainable oscillator. Modifying the time of feeding alone greatly affects body weight, whereas ketogenic diet (KD) influences circadian biology, through the modulation of clock gene expression. Night-eating behavior is one of the causes of circadian disruption, and night eaters have compulsive and uncontrolled eating with severe obesity. By contrast, time-restricted eating (TRE) restores circadian rhythms through maintaining an appropriate daily rhythm of the eating-fasting cycle. The hypothalamus has a crucial role in the regulation of energy balance rather than food intake. While circadian locomotor output cycles kaput (CLOCK) expression levels increase with high-fat diet-induced obesity, peroxisome proliferator-activated receptor-alpha (PPARα) increases the transcriptional level of brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like 1 (BMAL1) in obese subjects. In this context, effective timing of chronotherapies aiming to correct SCN-driven rhythms depends on an accurate assessment of the SCN phase. In fact, in a multi-oscillator system, local rhythmicity and its disruption reflects the disruption of either local clocks or central clocks, thus imposing rhythmicity on those local tissues, whereas misalignment of peripheral oscillators is due to exosome-based intercellular communication.Consequently, disruption of clock genes results in dyslipidemia, insulin resistance, and obesity, while light exposure during the daytime, food intake during the daytime, and sleeping during the biological night promote circadian alignment between the central and peripheral clocks. Thus, shift work is associated with an increased risk of obesity, diabetes, and cardiovascular diseases because of unusual eating times as well as unusual light exposure and disruption of the circadian rhythm.

Single cell model for re-entrainment to a shifted light cycle

Our daily 24-h rhythm is synchronized to the external light-dark cycle resulting from the Earth's daily rotation. In the mammalian brain, the suprachiasmatic nucleus (SCN) serves as the master clock and receives light-mediated input via the retinohypothalamic tract. Abrupt changes in the timing of the light-dark cycle (e.g., due to jet lag) cause a phase shift in the circadian rhythms in the SCN. Here, we investigated the effects of a 6-h delay in the light-dark cycle on PERIOD2::LUCIFERASE expression at the single-cell level in mouse SCN organotypic explants. The ensemble pattern in phase shift response obtained from individual neurons in the anterior and central SCN revealed a bimodal distribution; specifically, neurons in the ventrolateral SCN responded with a rapid phase shift, while neurons in the dorsal SCN generally did not respond to the shift in the light-dark cycle. We also stimulated the hypothalamic tract in acute SCN slices to simulate light-mediated input to the SCN; interestingly, we found similarities between the distribution and fraction of rapid shifting neurons (in response to the delay) and neurons that were excited in response to electrical stimulation. These results suggest that a subpopulation of neurons in the ventral SCN that have an excitatory response to light input, shift their clock more readily than dorsal located neurons, and initiate the SCN's entrainment to the new light-dark cycle. Thus, we propose that light-excited neurons in the anterior and central SCN play an important role in the organism's ability to adjust to changes in the external light-dark cycle.

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

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

AMPA Receptor Function in Hypothalamic Synapses

AMPA receptors (AMPARs) are critical for mediating glutamatergic synaptic transmission and plasticity, thus playing a major role in the molecular machinery underlying cellular substrates of memory and learning. Their expression pattern, transport and regulatory mechanisms have been extensively studied in the hippocampus, but their functional properties in other brain regions remain poorly understood. Interestingly, electrophysiological and molecular evidence has confirmed a prominent role of AMPARs in the regulation of hypothalamic function. This review summarizes the existing evidence on AMPAR-mediated transmission in the hypothalamus, where they are believed to orchestrate the role of glutamatergic transmission in autonomous, neuroendocrine function, body homeostasis, and social behavior.

Orchestration of the circadian clock and its association with Alzheimer's disease: Role of endocannabinoid signaling

Circadian rhythms are 24-hour natural rhythms regulated by the suprachiasmatic nucleus, also known as the "master clock". The retino-hypothalamic tract entrains suprachiasmatic nucleus with photic information to synchronise endogenous circadian rhythms with the Earth's light-dark cycle. However, despite the robustness of circadian rhythms, an unhealthy lifestyle and chronic photic disturbances cause circadian rhythm disruption in the suprachiasmatic nucleus's TTFL loops via affecting glutamate and γ-aminobutyric acid-mediated neurotransmission in the suprachiasmatic nucleus. Recently, considerable evidence has been shown correlating CRd with the incidence of Alzheimer's disease. The present review aims to identify the existence and signalling of endocannabinoids in CRd induced Alzheimer's disease through retino-hypothalamic tract- suprachiasmatic nucleus-cortex. Immunohistochemistry has confirmed the expression of cannabinoid receptor 1 in the suprachiasmatic nucleus to modulate the circadian phases of the master clock. Literature also suggests that cannabinoids may alter activity of suprachiasmatic nucleus by influencing the activity of their major neurotransmitter γ-aminobutyric acid or by interacting indirectly with the suprachiasmatic nucleus's two other major inputs i.e., the geniculo-hypothalamic tract-mediated release of neuropeptide Y and serotonergic inputs from the dorsal raphe nuclei. Besides, the expression of cannabinoid receptor 2 ameliorates cognitive deficits via reduction of tauopathy and microglial activation. In conclusion, endocannabinoids may be identified as a putative target for correcting CRd and decelerating Alzheimer's disease.

Light stimulation during postnatal development is not determinant for glutamatergic neurotransmission from the retinohypothalamic tract to the suprachiasmatic nucleus in rats

The hypothalamic suprachiasmatic nucleus (SCN) is the leading circadian pacemaker in mammals, which synchronizes with environmental light through the retinohypothalamic tract (RHT). Although the SCN regulates circadian rhythms before birth, postnatal synaptic changes are needed for the RHT-SCN pathway to achieve total functional development. However, it is unknown whether visual experience affects developmental maturation. Here, we studied the effects of constant darkness (DD) rearing on the physiology (at pre- and postsynaptic levels) of glutamatergic neurotransmission between RHT and SCN during postnatal development in rats. Upon recording spontaneous and evoked excitatory postsynaptic currents (EPSCs) by electrical stimulation of RHT fibers, we found that DD animals at early postnatal ages (P3-19) exhibited different frequencies of spontaneous EPSCs and lower synaptic performance (short-term depression, release sites, and recruitment of RHT fibers) when compared with their normal light/dark (LD) counterparts. At the oldest age evaluated (P30-35), there was a synaptic response strengthening (probability of release, vesicular re-filling rate, and reduced synaptic depression) in DD rats, which functionally equaled (or surmounted) that of LD animals. Control experiments evaluating EPSCs in ventral SCN neurons of LD rats during day and night revealed no significant differences in spontaneous or evoked EPSCs by high-frequency trains in the RHT at any postnatal age. Our results suggest that DD conditions induce a compensatory mechanism in the glutamatergic signaling of the circadian system to increase the chances of synchronization to light at adult ages, and that the synaptic properties of RHT terminals during postnatal development are not critically influenced by environmental light.

Role of G-Substrate in the NO/cGMP/PKG Signal Transduction Pathway for Photic Entrainment of the Hamster Circadian Clock

The mammalian circadian clock at the hypothalamic suprachiasmatic nuclei (SCN) entrains biological rhythms to the 24-h cyclic environment, by encoding light-dark transitions in SCN neurons. Light pulses induce phase shifts in the clock and in circadian rhythms; photic signaling for circadian phase advances involves a nitric oxide (NO)/cyclic guanosine monophosphate (cGMP)/cGMP-dependent protein kinase (PKG) pathway, increasing the expression of Period (Per) genes. Effectors downstream of PKG remain unknown. Here we investigate the role of G-substrate (GS), a PKG substrate, in the hamster SCN. GS and phosphorylated G-substrate (p-GS) were present in a subset of SCN cells. Moreover, GS phosphorylation (p-GS/GS ratio) increased in SCN homogenates after light pulses delivered at circadian time (CT) 18 and intraperitoneal treatment with sildenafil, an inhibitor of phosphodiesterase 5 (a cGMP-specific phosphodiesterase). On the other hand, intracerebroventricular treatment with the PKG inhibitor KT5823, reduced photic phosphorylation of GS to basal levels. Since p-GS could act as a protein phosphatase 2 A (PP2A) inhibitor, we demonstrated physical interaction between p-GS and PP2A in SCN homogenates, and also a light-pulse dependent decrease of PP2A activity. Intracerebroventricular treatment with okadaic acid, a PP2A inhibitor, increased the magnitude of light-induced phase advances of locomotor rhythms. We provide evidence on the physiological phosphorylation of GS as a new downstream effector in the NO/cGMP/PKG photic pathway in the hamster SCN, including its role as a PP2A inhibitor.

Electrophysiological Approaches to Studying the Suprachiasmatic Nucleus

In mammals, the part of the nervous system responsible for most circadian behavior can be localized to a bilaterally paired structure in the hypothalamus known as the suprachiasmatic nucleus (SCN). Understanding the mammalian circadian system will require a detailed multilevel analysis of neural SCN circuits ex vivo and in vivo. Many of the techniques and approaches that are used for the analysis of the circuitry driving circadian oscillations in the SCN are similar to those employed in other brain regions. There is, however, one fundamental difference that needs to be taken into consideration, that is, the physiological, cell, and molecular properties of SCN neurons vary with the time of day. In this chapter, we will consider the preparations and electrophysiological techniques that we have used to analyze the SCN circuit focusing on the acute brain slice and intact, freely moving animal.

SynGAP is expressed in the murine suprachiasmatic nucleus and regulates circadian-gated locomotor activity and light-entrainment capacity

The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master circadian clock. The phasing of the SCN oscillator is locked to the daily solar cycle, and an intracellular signaling cassette from the small GTPase Ras to the p44/42 mitogen-activated protein kinase (ERK/MAPK) pathway is central to this entrainment process. Here, we analyzed the expression and function of SynGAP-a GTPase-activating protein that serves as a negative regulator of Ras signaling-within the murine SCN. Using a combination of immunohistochemical and Western blotting approaches, we show that SynGAP is broadly expressed throughout the SCN. In addition, temporal profiling assays revealed that SynGAP expression is regulated over the circadian cycle, with peak expression occurring during the circadian night. Further, time-of-day-gated expression of SynGAP was not observed in clock arrhythmic BMAL1 null mice, indicating that the daily oscillation in SynGAP is driven by the inherent circadian timing mechanism. We also show that SynGAP phosphorylation at serine 1138-an event that has been found to modulate its functional efficacy-is regulated by clock time and is responsive to photic input. Finally, circadian phenotypic analysis of Syngap1 heterozygous mice revealed enhanced locomotor activity, increased sensitivity to light-evoked clock entrainment, and elevated levels of light-evoked MAPK activity, which is consistent with the role of SynGAP as a negative regulator of MAPK signaling. These findings reveal that SynGAP functions as a modulator of SCN clock entrainment, an effect that may contribute to sleep and circadian abnormalities observed in patients with SYNGAP1 gene mutations.

TRESK is a key regulator of nocturnal suprachiasmatic nucleus dynamics and light adaptive responses

The suprachiasmatic nucleus (SCN) is a complex structure dependent upon multiple mechanisms to ensure rhythmic electrical activity that varies between day and night, to determine circadian adaptation and behaviours. SCN neurons are exposed to glutamate from multiple sources including from the retino-hypothalamic tract and from astrocytes. However, the mechanism preventing inappropriate post-synaptic glutamatergic effects is unexplored and unknown. Unexpectedly we discovered that TRESK, a calcium regulated two-pore potassium channel, plays a crucial role in this system. We propose that glutamate activates TRESK through NMDA and AMPA mediated calcium influx and calcineurin activation to then oppose further membrane depolarisation and rising intracellular calcium. Hence, in the absence of TRESK, glutamatergic activity is unregulated leading to membrane depolarisation, increased nocturnal SCN firing, inverted basal calcium levels and impaired sensitivity in light induced phase delays. Our data reveals TRESK plays an essential part in SCN regulatory mechanisms and light induced adaptive behaviours.

The suprachiasmatic nucleus (SCN) ensures rhythmic electrical activity that varies between day and night to determine circadian behaviours. The authors show that TRESK channels provide a feedback mechanism to maintain the SCN in the appropriate state for nocturnal light-induced behavioural changes.

Molecular Mechanisms Underlying the Circadian Rhythm of Blood Pressure in Normotensive Subjects

PURPOSE OF REVIEW

Blood pressure (BP) follows a circadian rhythm (CR) in normotensive subjects. BP increases in the morning and decreases at night. This review aims at providing an up-to-date overview regarding the molecular mechanisms underlying the circadian regulation of BP.

RECENT FINDINGS

The suprachiasmatic nucleus (SCN) is the regulatory center for CRs. In SCN astrocytes, the phosphorylated glycogen synthase kinase-3β (pGSK-3β) also follows a CR and its expression reaches a maximum in the morning and decreases at night. pGSK-3β induces the β-catenin migration to the nucleus. During the daytime, the nuclear β-catenin increases the expression of the glutamate excitatory amino acid transporter 2 (EAAT2) and glutamine synthetase (GS). In SCN, EAAT2 removes glutamate from the synaptic cleft of glutamatergic neurons and transfers it to the astrocyte cytoplasm where GS converts glutamate into glutamine. Thus, glutamate decreases in the synaptic cleft. This decreases the stimulation of the glutamate receptors AMPA-R and NMDA-R located on glutamatergic post-synaptic neurons. Consequently, activation of NTS is decreased and BP increases. The opposite occurs at night. Despite several studies resulting from animal studies, the circadian regulation of BP appears largely controlled in normotensive subjects by the canonical WNT/β-catenin pathway involving the SCN, astrocytes, and glutamatergic neurons.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

A Symphony of Signals: Intercellular and Intracellular Signaling Mechanisms Underlying Circadian Timekeeping in Mice and Flies

The central pacemakers of circadian timekeeping systems are highly robust yet adaptable, providing the temporal coordination of rhythms in behavior and physiological processes in accordance with the demands imposed by environmental cycles. These features of the central pacemaker are achieved by a multi-oscillator network in which individual cellular oscillators are tightly coupled to the environmental day-night cycle, and to one another via intercellular coupling. In this review, we will summarize the roles of various neurotransmitters and neuropeptides in the regulation of circadian entrainment and synchrony within the mammalian and Drosophila central pacemakers. We will also describe the diverse functions of protein kinases in the relay of input signals to the core oscillator or the direct regulation of the molecular clock machinery.

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.

Glycogen synthase kinase 3 regulates photic signaling in the suprachiasmatic nucleus

Glycogen synthase kinase 3 (GSK3) is a serine-threonine kinase that regulates mammalian circadian rhythms at the behavioral, molecular and neurophysiological levels. In the central circadian pacemaker, the suprachiasmatic nucleus (SCN), inhibitory phosphorylation of GSK3 exhibits a rhythm across the 24 h day. We have recently shown that GSK3 is capable of influencing both the molecular clock and SCN neuronal activity rhythms. However, it is not known whether GSK3 regulates the response to environmental cues such as light. The goal of this study was to test the hypothesis that GSK3 activation mediates light-induced SCN excitability and photic entrainment. Immunofluorescence staining in the SCN of mice showed that late-night light exposure significantly increased GSK3 activity (decreased pGSK3β levels) 30-60 min after the light-pulse. In addition, pharmacological inhibition of GSK3 blocked the expected light-induced excitability in SCN neurons; however, this effect was not associated with changes in resting membrane potential or input resistance. Behaviorally, mice with constitutively active GSK3 (GSK3-KI) re-entrained to a 6-h phase advance in the light-dark cycle in significantly fewer days than WT control animals. Furthermore, the behavioral and SCN neuronal activity of GSK3-KI mice was phase-advanced compared to WT, in both normal and light-exposed conditions. Finally, GSK3-KI mice exhibited normal negative-masking behavior and electroretinographic responses to light, suggesting that the enhanced photic entrainment is not due to an overall increased sensitivity to light in these animals. Taken together, these results provide strong evidence that GSK3 activation contributes to light-induced phase-resetting at both the neurophysiological and behavioral levels.

Circadian Rhythms in Diet-Induced Obesity

The biological clocks of the circadian timing system coordinate cellular and physiological processes and synchronizes these with daily cycles, feeding patterns also regulates circadian clocks. The clock genes and adipocytokines show circadian rhythmicity. Dysfunction of these genes are involved in the alteration of these adipokines during the development of obesity. Food availability promotes the stimuli associated with food intake which is a circadian oscillator outside of the suprachiasmatic nucleus (SCN). Its circadian rhythm is arranged with the predictable daily mealtimes. Food anticipatory activity is mediated by a self-sustained circadian timing and its principal component is food entrained oscillator. However, the hypothalamus has a crucial role in the regulation of energy balance rather than food intake. Fatty acids or their metabolites can modulate neuronal activity by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. The timing of three-meal schedules indicates close association with the plasma levels of insulin and preceding food availability. Desynchronization between the central and peripheral clocks by altered timing of food intake and diet composition can lead to uncoupling of peripheral clocks from the central pacemaker and to the development of metabolic disorders. Metabolic dysfunction is associated with circadian disturbances at both central and peripheral levels and, eventual disruption of circadian clock functioning can lead to obesity. While CLOCK expression levels are increased with high fat diet-induced obesity, peroxisome proliferator-activated receptor (PPAR) alpha increases the transcriptional level of brain and muscle ARNT-like 1 (BMAL1) in obese subjects. Consequently, disruption of clock genes results in dyslipidemia, insulin resistance and obesity. Modifying the time of feeding alone can greatly affect body weight. Changes in the circadian clock are associated with temporal alterations in feeding behavior and increased weight gain. Thus, shift work is associated with increased risk for obesity, diabetes and cardio-vascular diseases as a result of unusual eating time and disruption of circadian rhythm.

Calcium Dynamics and Circadian Rhythms in Suprachiasmatic Nucleus Neurons

The hypothalamic suprachiasmatic nucleus (SCN) has a pivotal role in the mammalian circadian clock. SCN neurons generate circadian rhythms in action potential firing frequencies and neurotransmitter release, and the core oscillation is thought to be driven by “clock gene” transcription-translation feedback loops. Cytosolic Ca2+mobilization followed by stimulation of various receptors has been shown to reset the gene transcription cycles in SCN neurons, whereas contribution of steady-state cytosolic Ca2+levels to the rhythm generation is unclear. Recently, circadian rhythms in cytosolic Ca2+levels have been demonstrated in cultured SCN neurons. The circadian Ca2+rhythms are driven by the release of Ca2+from ryanodine-sensitive internal stores and resistant to the blockade of action potentials. These results raise the possibility that gene translation/transcription loops may interact with autonomous Ca2+oscillations in the production of circadian rhythms in SCN neurons.

Role of vasoactive intestinal peptide in the light input to the circadian system

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in a subset of neurons in the ventral region of the suprachiasmatic nucleus (SCN). While VIP is known to be important for the synchronization of the SCN network, the role of VIP in photic regulation of the circadian system has received less attention. In the present study, we found that the light-evoked increase in electrical activity in vivo was unaltered by the loss of VIP. In the absence of VIP, the ventral SCN still exhibited N-methyl-d-aspartate-evoked responses in a brain slice preparation, although the absolute levels of neural activity before and after treatment were significantly reduced. Next, we used calcium imaging techniques to determine if the loss of VIP altered the calcium influx due to retinohypothalamic tract stimulation. The magnitude of the evoked calcium influx was not reduced in the ventral SCN, but did decline in the dorsal SCN regions. We examined the time course of the photic induction of Period1 in the SCN using in situ hybridization in VIP-mutant mice. We found that the initial induction of Period1 was not reduced by the loss of this signaling peptide. However, the sustained increase in Period1 expression (after 30 min) was significantly reduced. Similar results were found by measuring the light induction of cFOS in the SCN. These findings suggest that VIP is critical for longer-term changes within the SCN circuit, but does not play a role in the acute light response.

The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleus

The use of glycine as a therapeutic option for improving sleep quality is a novel and safe approach. However, despite clinical evidence of its efficacy, the details of its mechanism remain poorly understood. In this study, we investigated the site of action and sleep-promoting mechanisms of glycine in rats. In acute sleep disturbance, oral administration of glycine-induced non-rapid eye movement (REM) sleep and shortened NREM sleep latency with a simultaneous decrease in core temperature. Oral and intracerebroventricular injection of glycine elevated cutaneous blood flow (CBF) at the plantar surface in a dose-dependent manner, resulting in heat loss. Pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists AP5 and CGP78608 but not the glycine receptor antagonist strychnine inhibited the CBF increase caused by glycine injection into the brain. Induction of c-Fos expression was observed in the hypothalamic nuclei, including the medial preoptic area (MPO) and the suprachiasmatic nucleus (SCN) shell after glycine administration. Bilateral microinjection of glycine into the SCN elevated CBF in a dose-dependent manner, whereas no effect was observed when glycine was injected into the MPO and dorsal subparaventricular zone. In addition, microinjection of D-serine into the SCN also increased CBF, whereas these effects were blocked in the presence of L-701324. SCN ablation completely abolished the sleep-promoting and hypothermic effects of glycine. These data suggest that exogenous glycine promotes sleep via peripheral vasodilatation through the activation of NMDA receptors in the SCN shell.

Circadian gating of neuronal functionality: a basis for iterative metaplasticity

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

The trouble with circadian clock dysfunction: multiple deleterious effects on the brain and body

This review consolidates research employing human correlational and experimental work across brain and body with experimental animal models to provide a more complete representation of how circadian rhythms influence almost all aspects of life. In doing so, we will cover the morphological and biochemical pathways responsible for rhythm generation as well as interactions between these systems and others (e.g., stress, feeding, reproduction). The effects of circadian disruption on the health of humans, including time of day effects, cognitive sequelae, dementia, Alzheimer's disease, diet, obesity, food preferences, mood disorders, and cancer will also be discussed. Subsequently, experimental support for these largely correlational human studies conducted in non-human animal models will be described.

Activation of TRPC4β by Gαi subunit increases Ca2+ selectivity and controls neurite morphogenesis in cultured hippocampal neuron

The ubiquitous transient receptor potential canonical (TRPC) channels function as non-selective, Ca(2+)-permeable channels. TRPC channels are activated by stimulation of Gαq-PLC-coupled receptors. Here, we report that TRPC4/TRPC5 can be activated by Gαi. We studied the essential role of Gαi subunits in TRPC4 activation and investigated changes in ion selectivity and pore dilation of the TRPC4 channel elicited by the Gαi2 subunit. Activation of TRPC4 by Gαi2 increased Ca2+ permeability and Ca2+ influx through TRPC4 channels. Co-expression of the muscarinic receptor (M2) and TRPC4 in HEK293 cells induced TRPC4-mediated Ca2+ influx. Moreover, both TRPC4β and the TRPC4β-Gαi2 signaling complex induced inhibition of neurite growth and arborization in cultured hippocampal neurons. Cells treated with KN-93, a CaMKII inhibitor, prevented TRPC4- and TRPC4-Gαi2(Q205L)-mediated inhibition of neurite branching and growth. These findings indicate an essential role of Gαi proteins in TRPC4 activation and extend our knowledge of the functional role of TRPC4 in hippocampal neurons.

The role of PPARβ/δ in the regulation of glutamatergic signaling in the hamster suprachiasmatic nucleus

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily and function as transcription factors that regulate gene expression in numerous biological processes. Although the PPARβ/δ subtype is highly expressed in the brain, its physiological roles in neuronal function remain to be elucidated. In this study, we examined the presence of PPARβ/δ in the master circadian clock of the Syrian hamster and investigated its putative functional role in this structure. In mammals, the central circadian clock, located in the suprachiasmatic nucleus (SCN), is entrained by the light-dark (LD) cycle via photic6 signals conveyed by a direct pathway whose terminals release glutamate. Using immunocytochemical and qRT-PCR analysis, we demonstrated that the rhythmic expression of PPAR β/δ within the SCN of hamsters raised under an LD cycle was detectable only at the transcriptional level when the hamsters were maintained under constant darkness (DD). The increase in the number of immunoreactive PPARβ/δ cells observed under DD after light stimulation during the early subjective night (CT14), but not during the subjective day (CT06), demonstrated that the expression of PPARβ/δ can be up-regulated according to the photosensitive phase of the circadian clock. All of the PPARβ/δ-positive cells in the SCN also expressed the glutamate receptor NMDAR1. Moreover, we demonstrated that at the photosensitive point (CT14), the administration of L-16504, a specific agonist of PPARβ/δ, amplified the phase delay of the locomotor response induced by a light pulse. Taken together, these data suggest that PPARβ/δ activation modulates glutamate release that mediates entrainment of the circadian clock by light.

cGMP-phosphodiesterase inhibition enhances photic responses and synchronization of the biological circadian clock in rodents

The master circadian clock in mammals is located in the hypothalamic suprachiasmatic nuclei (SCN) and is synchronized by several environmental stimuli, mainly the light-dark (LD) cycle. Light pulses in the late subjective night induce phase advances in locomotor circadian rhythms and the expression of clock genes (such as Per1-2). The mechanism responsible for light-induced phase advances involves the activation of guanylyl cyclase (GC), cGMP and its related protein kinase (PKG). Pharmacological manipulation of cGMP by phosphodiesterase (PDE) inhibition (e.g., sildenafil) increases low-intensity light-induced circadian responses, which could reflect the ability of the cGMP-dependent pathway to directly affect the photic sensitivity of the master circadian clock within the SCN. Indeed, sildenafil is also able to increase the phase-shifting effect of saturating (1200 lux) light pulses leading to phase advances of about 9 hours, as well as in C57 a mouse strain that shows reduced phase advances. In addition, sildenafil was effective in both male and female hamsters, as well as after oral administration. Other PDE inhibitors (such as vardenafil and tadalafil) also increased light-induced phase advances of locomotor activity rhythms and accelerated reentrainment after a phase advance in the LD cycle. Pharmacological inhibition of the main downstream target of cGMP, PKG, blocked light-induced expression of Per1. Our results indicate that the cGMP-dependent pathway can directly modulate the light-induced expression of clock-genes within the SCN and the magnitude of light-induced phase advances of overt rhythms, and provide promising tools to design treatments for human circadian disruptions.

Linking neural activity and molecular oscillations in the SCN

Neurons in the suprachiasmatic nucleus (SCN) function as part of a central timing circuit that drives daily changes in our behaviour and underlying physiology. A hallmark feature of SCN neuronal populations is that they are mostly electrically silent during the night, start to fire action potentials near dawn and then continue to generate action potentials with a slow and steady pace all day long. Sets of currents are responsible for this daily rhythm, with the strongest evidence for persistent Na(+) currents, L-type Ca(2+) currents, hyperpolarization-activated currents (I(H)), large-conductance Ca(2+) activated K(+) (BK) currents and fast delayed rectifier (FDR) K(+) currents. These rhythms in electrical activity are crucial for the function of the circadian timing system, including the expression of clock genes, and decline with ageing and disease. This article reviews our current understanding of the ionic and molecular mechanisms that drive the rhythmic firing patterns in the SCN.

A multicellular model for differential regulation of circadian signals in the core and shell regions of the suprachiasmatic nucleus

We developed a multicellular model of the mammalian circadian clock characterized by a high degree of heterogeneity with respect to single cell periodicity and behavior (intrinsic and driven oscillators), neurotransmitter release (VIP, GABA and glutamate synthesis) and spatial organization (core and shell regions), mimicking structural patterns within the suprachiasmatic nucleus (SCN) associated with distinct circadian functions. We simulated the SCN core and shell separately utilizing experimentally derived connectivity schemes for the two subdivisions as observed within the rat SCN. The core was modeled via a small world network characterized by VIP and GABA co-localization, whereas the shell was simulated as a nearest neighbor network promoting local GABAergic connections. To study the function of the axonal plexus extending from the densely innervated ventrolateral region to distal areas across the dorsomedial SCN, directed long range links from the core to the shell were gradually introduced via a probability p(cs) that ranged from 0 to 1. A probability value of 0 excluded core-shell interactions, whereas p(cs)=1 achieved maximal connectivity between the two regions. Our model exhibited a threshold in the number of core-to-shell links required for sufficient cell-to-cell coordination to maintain periodicity and rhythmic behavior across the entire model network (including both shell and core populations) in constant darkness as well as 12:12h light-dark cycles. By contrast, constant light was shown to increase phase synchronization across the shell while core populations remained poorly synchronized, suggesting differential light response across the two SCN compartments. We further simulated increasing percentages of intrinsic oscillators and demonstrated a negative correlation between the number of intrinsic oscillators distributed across the SCN and the ability of the system to produce synchronized signals. Simulations that differed with respect to the placement of intrinsic oscillators supported the hypothesis that improved synchronization is achieved with networks characterized by localized intrinsic oscillators placed exclusively within the shell versus networks containing uniformly distributed intrinsic oscillators in both SCN compartments. This study has successfully reproduced a number of spatiotemporal and behavioral attributes of the SCN, providing a useful computational tool to correlate observed circadian phenotypes with distinct chemoarchitectural properties of spatially localized neural populations.

Fast delayed rectifier potassium current: critical for input and output of the circadian system

The ability to generate intrinsic circadian rhythms in electrical activity appears to be a key property of central pacemaker neurons and one essential to the function of the circadian timing system. Previous work has demonstrated that suprachiasmatic nucleus (SCN) neurons express the fast delayed rectifier (FDR) potassium current and raise questions about the function of this current. Here, we report that mice lacking both Kcnc1 and Kcnc2 genes [double knock-out (dKO)] fail to express the Kv3.1 and 3.2 channels in the SCN as well as exhibit a greatly reduced FDR current. SCN neurons from these dKO mice exhibit reduced spontaneous activity during the day as well as reduced NMDA-evoked excitatory responses during the night. Interestingly, the daily rhythm in PER2 expression in the SCN was not altered in the dKO mice, although the photic induction of c-Fos was attenuated. Behaviorally, the dKO mice exhibited extremely disrupted daily rhythms in wheel-running behavior. In a light/dark cycle, some of the dKO mice were arrhythmic, whereas others expressed a diurnal rhythm with low amplitude and significant activity during the day. When placed in constant darkness, the dKO mice exhibited low-amplitude, fragmented rhythms and attenuated light responses. Together, these data are consistent with the hypothesis that the FDR current is critical for the generation of robust circadian rhythms in behavior as well as the synchronization of the circadian system to the photic environment.

Circadian alteration in neurobiology during protracted opiate withdrawal in rats

Protracted opiate withdrawal can extend for months of disrupted hormonal circadian rhythms. We examined rodent behaviors and these circadian disturbances in hormone and peptide levels as well as brain clock gene expression during 60 days of protracted withdrawal. Our behavioral tests included open field, elevated plus maze, and sucrose preference tests at 36 h, 10, 30, and 60 days after stopping chronic morphine. At these four assessment points, we collected samples every 4 h for 24 h to examine circadian rhythms in blood hormone and peptide levels and brain expression of rPER1, rPER2, and rPER3 clock genes. Decreased locomotor activity and elevated adrenocorticotropic hormone and melatonin levels persisted for 2 months after morphine withdrawal, but corticosterone was elevated only at 36 h and 10 days after withdrawal. Orexin levels were high at 36 h after withdrawal, but then reversed during protracted withdrawal to abnormally low levels. Beta-endorphin (β-EP) levels showed no differences from normal. However, circadian rhythms were blunted for all of these hormones. Corticosterone, adrenocorticotropic hormone, and orexin blunting persisted at least for 60 days. The blunted circadian rhythm of β-EP and melatonin recovered by day 60, but the peak phase of β-EP was delayed about 8 h. Blunted circadian rhythms and reduced expression of rPER1, rPER2, and rPER3 persisted at least for 60 days in the suprachiasmatic nucleus, prefrontal cortex, nucleus accumbens core, central nucleus of the amygdala, Hippocampus, and ventral tegmental area. Circadian rhythms of rPER1 in the nucleus accumbens shell and basolateral nucleus of the amygdala and of rPER2 in the central nucleus of the amygdala were reversed. Disrupted circadian rhythms of rPER1, rPER 2, and rPER3 expression in reward-related brain circuits and blunted circadian rhythms in peripheral hormones and peptides may play a role in protracted opiate withdrawal and contribute to relapse.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Stoichiometry of N-methyl-D-aspartate receptors within the suprachiasmatic nucleus

The circadian pacemaker within the suprachiasmatic nucleus (SCN) confers daily rhythms to bodily functions. In nature, the circadian clock will adopt a 24-h period by synchronizing to the solar light/dark cycle. This light entrainment process is mediated, in part, at glutamatergic synapses formed between retinal ganglion afferents and SCN neurons. N-methyl-D-aspartate receptors (NMDARs) located on SCN neurons gate light-induced phase resetting. Despite their importance in circadian physiology, little is known about their functional stoichiometry. We investigated the NR2-subunit composition with whole cell recordings of SCN neurons within the murine hypothalamic brain slice using a combination of subtype-selective NMDAR antagonists and voltage-clamp protocols. We found that extracellular magnesium ([Mg](o)) strongly blocks SCN NMDARs exhibiting affinities and voltage sensitivities associated with NR2A and NR2B subunits. These NMDAR currents were inhibited strongly by NR2B-selective antagonists, Ro 25-6981 (3.5 microM, 55.0 +/- 9.0% block; mean +/- SE) and ifenprodil (10 microM, 55.8 +/- 3.0% block). The current remaining showed decreased [Mg](o) affinities reminiscent of NR2C and NR2D subunits but was highly sensitive to [Zn](o), a potent NR2A blocker, showing a approximately 44.2 +/- 1.1% maximal inhibition at saturating concentrations with an IC(50) of 7.8 +/- 1.1 nM. Considering the selectivity, efficacy, and potency of the drugs used in combination with [Mg](o)-block characteristics of the NMDAR, our data show that both diheteromeric NR2B NMDARs and triheteromeric NR2A NMDARs (paired with an NR2C or NR2D subunits) account for the vast majority of the NMDAR current within the SCN.

A multiscale model to investigate circadian rhythmicity of pacemaker neurons in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is a multicellular system that drives daily rhythms in mammalian behavior and physiology. Although the gene regulatory network that produces daily oscillations within individual neurons is well characterized, less is known about the electrophysiology of the SCN cells and how firing rate correlates with circadian gene expression. We developed a firing rate code model to incorporate known electrophysiological properties of SCN pacemaker cells, including circadian dependent changes in membrane voltage and ion conductances. Calcium dynamics were included in the model as the putative link between electrical firing and gene expression. Individual ion currents exhibited oscillatory patterns matching experimental data both in current levels and phase relationships. VIP and GABA neurotransmitters, which encode synaptic signals across the SCN, were found to play critical roles in daily oscillations of membrane excitability and gene expression. Blocking various mechanisms of intracellular calcium accumulation by simulated pharmacological agents (nimodipine, IP3- and ryanodine-blockers) reproduced experimentally observed trends in firing rate dynamics and core-clock gene transcription. The intracellular calcium concentration was shown to regulate diverse circadian processes such as firing frequency, gene expression and system periodicity. The model predicted a direct relationship between firing frequency and gene expression amplitudes, demonstrated the importance of intracellular pathways for single cell behavior and provided a novel multiscale framework which captured characteristics of the SCN at both the electrophysiological and gene regulatory levels.

Circadian rhythms are ∼24 hour cycles in biochemical, physiological and behavioral processes observed in a diverse range of organisms including Cyanobacteria, Neurospora, Drosophila, mice and humans. In mammals, the dominant circadian pacemaker that drives daily rhythms is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is composed of a highly connected network of ∼20,000 neurons. Within each individual SCN neuron core clock genes and proteins interact through intertwined regulatory loops to generate circadian oscillations on the molecular level. These neurons express daily rhythmicity in their firing frequency and other electrophysiological properties. The mechanisms by which the core clock produces synchronized rhythms in neural firing and gene expression are postulated to involve intracellular calcium, a second messenger that regulates many cellular processes. The interaction between the various clock components however remains unknown. In this paper, we present a single cell model that incorporates the circadian gene regulatory pathway, cellular electrophysiological properties, and cytosolic calcium dynamics. Our results suggest a possible system architecture that accounts for the robustness of the circadian clock at the single cell level. Our simulations predict a dual role for intracellular pathways instigated by intracellular calcium and VIP: maintaining the periodicity and amplitude of the core clock genes as well as the firing frequency oscillations.

Retinohypothalamic tract synapses in the rat suprachiasmatic nucleus demonstrate short-term synaptic plasticity

The master circadian pacemaker located in the suprachiasmatic nucleus (SCN) is entrained by light intensity-dependent signals transmitted via the retinohypothalamic tract (RHT). Short-term plasticity at glutamatergic RHT-SCN synapses was studied using stimulus frequencies that simulated the firing of light sensitive retinal ganglion cells. The evoked excitatory postsynaptic current (eEPSC) was recorded from SCN neurons located in hypothalamic brain slices. The eEPSC amplitude was stable during 0.08 Hz stimulation and exhibited frequency-dependent short-term synaptic depression (SD) during 0.5 to 100 Hz stimulus trains in 95 of 99 (96%) recorded neurons. During SD the steady-state eEPSC amplitude decreased, whereas the cumulative charge transfer increased in a frequency-dependent manner and saturated at 20 Hz. SD was similar during subjective day and night and decreased with increasing temperature. Paired-pulse stimulation (PPS) and voltage-dependent Ca(2+) channel (VDCC) blockers were used to characterize a presynaptic release mechanism. Facilitation was present in 30% and depression in 70% of studied neurons during PPS. Synaptic transmission was reduced by blocking both N- and P/Q-type presynaptic VDCCs, but only the N-type channel blocker significantly relieved SD. Aniracetam inhibited AMPA receptor desensitization but did not alter SD. Thus we concluded that SD is the principal form of short-term plasticity at RHT synapses, which presynaptically and frequency-dependently attenuates light-induced glutamatergic RHT synaptic transmission protecting SCN neurons against excessive excitation.

The role of the neuropeptides PACAP and VIP in the photic regulation of gene expression in the suprachiasmatic nucleus

Previously, we have shown that mice deficient in either vasoactive intestinal peptide (VIP) or pituitary adenylate cyclase-activating polypeptide (PACAP) exhibit specific deficits in the behavioral response of their circadian system to light. In this study, we investigated how the photic regulation of the molecular clock within the suprachiasmatic nucleus (SCN) is altered by the loss of these closely-related peptides. During the subjective night, the magnitude of the light-induction of FOS and phosphorylated mitogen-activated protein kinase (p-MAPK) immunoreactive cells within the SCN was significantly reduced in both VIP- and PACAP-deficient mice when compared with wild-type mice. The photic induction of the clock gene Period1 (Per1) in the SCN was reduced in the VIP- but not in the PACAP-deficient mice. Baselines levels of FOS, p-MAPK or Per1 in the night were not altered by the loss of these peptides. In contrast, during the subjective day, light exposure increased the levels of FOS, p-MAPK and Per1 in the SCN of VIP-deficient mice, but not in the other genotypes. During this phase, baseline levels of these markers were reduced in the VIP-deficient mice compared with untreated controls. Finally, the loss of either neuropeptide reduced the magnitude of the light-evoked increase in Per1 levels in the adrenals in the subjective night without any change in baseline levels. In summary, our results indicate that both VIP and PACAP regulate the responsiveness of cells within the SCN to the effects of light. Furthermore, VIP, but not PACAP, is required for the appropriate temporal gating of light-induced gene expression within the SCN.

Suprachiasmatic nucleus: cell autonomy and network properties

The suprachiasmatic nucleus (SCN) is the primary circadian pacemaker in mammals. Individual SCN neurons in dispersed culture can generate independent circadian oscillations of clock gene expression and neuronal firing. However, SCN rhythmicity depends on sufficient membrane depolarization and levels of intracellular calcium and cAMP. In the intact SCN, cellular oscillations are synchronized and reinforced by rhythmic synaptic input from other cells, resulting in a reproducible topographic pattern of distinct phases and amplitudes specified by SCN circuit organization. The SCN network synchronizes its component cellular oscillators, reinforces their oscillations, responds to light input by altering their phase distribution, increases their robustness to genetic perturbations, and enhances their precision. Thus, even though individual SCN neurons can be cell-autonomous circadian oscillators, neuronal network properties are integral to normal function of the SCN.

Expression of clock genes in the suprachiasmatic nucleus: effect of environmental lighting conditions

The suprachiasmatic nucleus (SCN) is the anatomical substrate for the principal circadian clock coordinating daily rhythms in a vast array of behavioral and physiological responses. Individual SCN neurons are cellular oscillators and are organized into a multi-oscillator network following unique spatiotemporal patterns. The rhythms generated in the SCN are generally entrained to the environmental light dark cycle, which is the most salient cue influencing the network organization of the SCN. The neural network in the SCN is a heterogeneous structure, containing two major compartments identified by applying physiological and functional criteria, namely the retinorecipient core region and the highly rhythmic shell region. Changes in the environmental lighting condition are first detected and processed by the core region, and then conveyed to the rest of the SCN, leading to adaptive responses of the entire network. This review will focus on the studies that explore the responses of the SCN network by examining the expression of clock genes, under various lighting paradigms, such as acute light exposure, lighting schedules or exposure to different light durations. The results will be discussed under the framework of functionally distinct SCN sub regions and oscillator groups. The evidence presented here suggests that the environmental lighting conditions alter the spatiotemporal organization of the cellular oscillators within the SCN, which consequently affect the overt rhythms in behavior and physiology. Thus, information on how the SCN network elements respond to environmental cues is key to understanding the human health problems that stem from circadian rhythm disruption.

Physiological responses of the circadian clock to acute light exposure at night

Circadian rhythms in physiological, endocrine and metabolic functioning are controlled by a neural clock located in the suprachiasmatic nucleus (SCN). This structure is endogenously rhythmic and the phase of this rhythm can be reset by light information from the eye. A key feature of the SCN is that while it is a small structure containing on the order of about 20,000 cells, it is amazingly heterogeneous. It is likely that anatomical heterogeneity reflects an underlying functional heterogeneity. In this review, we examine the physiological responses of cells in the SCN to light stimuli that reset the phase of the circadian clock, highlighting where possible the spatial pattern of such responses. Increases in intracellular calcium are an important signal in response to light, and this increase triggers many biochemical cascades that mediate responses to light. Furthermore, only some cells in the SCN are actually endogenously rhythmic, and these cells likely do not receive strong direct input from the retina. Therefore, this review also considers how light information is conveyed from the retinorecipient cells to the endogenously rhythmic cells that track circadian phase. A number of neuropeptides, including vasoactive intestinal polypeptide, gastrin-releasing peptide and substance P, may be particularly important in relaying such signals, but other neurochemicals such as GABA and nitric oxide may participate as well. A thorough understanding of the intracellular and intercellular responses to light, as well as the spatial arrangements of such responses may help identify important pharmacological targets for therapeutic interventions to treat sleep and circadian disorders.

Circadian modulation of melanopsin-driven light response in rat ganglion-cell photoreceptors

Intrinsically photosensitive retinal ganglion cells (ipRGCs) project to the suprachiasmatic nucleus (SCN) and are essential for normal photic entrainment of global circadian rhythms in physiology and behavior. The effect of light on the central clock is dependent on circadian phase, and the retina itself contains intrinsic circadian oscillators that can alter its sensitivity to light. This raises the possibility that the ipRGCs, and hence the photoentraining signals in the retinohypothalamic tract, are subject to circadian modulation. Although the ipRGC photopigment melanopsin reportedly exhibits circadian variations in expression, there has been no direct test of the hypothesis that ipRGC sensitivity is under circadian control. Here, the authors provide such a test by measuring the sensitivity of intrinsic photoresponses of rat ipRGCs at 4 circadian times (CTs) using multielectrode array recording. There was little if any circadian modulation in the threshold of intrinsic ipRGC photoresponses. However, very bright light evoked significantly more spiking early in the subjective night (CT12-13) than at other circadian phases. Thus, the gain of the melanopsin-driven response is slightly increased in the early night, at roughly the circadian phase when melanopsin synthesis is thought to be elevated. However, this gain change is probably too modest to contribute much to shape the phase response curve (PRC) for behavioral photoentrainment.

Circadian regulation of a-type potassium currents in the suprachiasmatic nucleus

In mammals, the precise circadian timing of many biological processes depends on the generation of oscillations in neural activity of pacemaker cells in the suprachiasmatic nucleus (SCN) of the hypothalamus. Understanding the ionic mechanisms underlying these rhythms is an important goal of research in chronobiology. Previous work has shown that SCN neurons express A-type potassium currents (IAs), but little is known about the properties of this current in the SCN. We sought to characterize some of these properties, including the identities of IA channel subunits found in the SCN and the circadian regulation of IA itself. In this study, we were able to detect significant hybridization for Shal-related family members 1 and 2 (Kv4.1 and 4.2) within the SCN. In addition, we used Western blot to show that the Kv4.1 and 4.2 proteins are expressed in SCN tissue. We further show that the magnitude of the IA current exhibits a diurnal rhythm that peaks during the day in the dorsal region of the mouse SCN. This rhythm seems to be driven by a subset of SCN neurons with a larger peak current and a longer decay constant. Importantly, this rhythm in neurons in the dorsal SCN continues in constant darkness, providing an important demonstration of the circadian regulation of an intrinsic voltage-gated current in mammalian cells. We conclude that the anatomical expression, biophysical properties, and pharmacological profiles measured are all consistent with the SCN IA current being generated by Kv4 channels. Additionally, these data suggest a role for IA in the regulation of spontaneous action potential firing during the transitions between day/night and in the integration of synaptic inputs to SCN neurons throughout the daily cycle.

Circadian and developmental regulation of N-methyl-d-aspartate-receptor 1 mRNA splice variants and N-methyl-d-aspartate-receptor 3 subunit expression within the rat suprachiasmatic nucleus

The circadian rhythms of mammals are generated by the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Its intrinsic period is entrained to a 24 h cycle by external cues, mainly by light. Light impinging on the SCN at night causes either advancing or delaying phase shifts of the circadian clock. N-methyl-d-aspartate receptors (NMDAR) are the main glutamate receptors mediating the effect of light on the molecular clockwork in the SCN. They are composed of multiple subunits, each with specific characteristics whose mutual interactions strongly determine properties of the receptor. In the brain, the distribution of NMDAR subunits depends on the region and developmental stage. Here, we report the circadian expression of the NMDAR1 subunit in the adult rat SCN and depict its splice variants that may constitute the functional receptor channel in the SCN. During ontogenesis, expression of two of the NMDAR1 subunit splice variants, as well as the NMDAR3A and 3B subunits, exhibits developmental loss around the time of eye opening. Moreover, we demonstrate the spatial and developmental characteristics of the expression of the truncated splice form of NMDAR1 subunit NR1-E in the brain. Our data suggest that specific properties of the NMDAR subunits we describe within the SCN likely influence the photic transduction pathways mediating the clock entrainment. Furthermore, the developmental changes in NMDAR composition may contribute to the gradual postnatal maturation of the entrainment pathways.

Morphine withdrawal produces circadian rhythm alterations of clock genes in mesolimbic brain areas and peripheral blood mononuclear cells in rats

Previous studies have shown that clock genes are expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus, other brain regions, and peripheral tissues. Various peripheral oscillators can run independently of the SCN. However, no published studies have reported changes in the expression of clock genes in the rat central nervous system and peripheral blood mononuclear cells (PBMCs) after withdrawal from chronic morphine treatment. Rats were administered with morphine twice daily at progressively increasing doses for 7 days; spontaneous withdrawal signs were recorded 14 h after the last morphine administration. Then, brain and blood samples were collected at each of eight time points (every 3 h: ZT 9; ZT 12; ZT 15; ZT 18; ZT 21; ZT 0; ZT 3; ZT 6) to examine expression of rPER1 and rPER2 and rCLOCK. Rats presented obvious morphine withdrawal signs, such as teeth chattering, shaking, exploring, ptosis, and weight loss. In morphine-treated rats, rPER1 and rPER2 expression in the SCN, basolateral amygdala, and nucleus accumbens shell showed robust circadian rhythms that were essentially identical to those in control rats. However, robust circadian rhythm in rPER1 expression in the ventral tegmental area was completely phase-reversed in morphine-treated rats. A blunting of circadian oscillations of rPER1 expression occurred in the central amygdala, hippocampus, nucleus accumbens core, and PBMCs and rPER2 expression occurred in the central amygdala, prefrontal cortex, nucleus accumbens core, and PBMCs in morphine-treated rats compared with controls. rCLOCK expression in morphine-treated rats showed no rhythmic change, identical to control rats. These findings indicate that withdrawal from chronic morphine treatment resulted in desynchronization from the SCN rhythm, with blunting of rPER1 and rPER2 expression in reward-related neurocircuits and PBMCs.

Excitatory actions of GABA in the suprachiasmatic nucleus

Neurons in the suprachiasmatic nucleus (SCN) are responsible for the generation of circadian oscillations, and understanding how these neurons communicate to form a functional circuit is a critical issue. The neurotransmitter GABA and its receptors are widely expressed in the SCN where they mediate cell-to-cell communication. Previous studies have raised the possibility that GABA can function as an excitatory transmitter in adult SCN neurons during the day, but this work is controversial. In the present study, we first tested the hypothesis that GABA can evoke excitatory responses during certain phases of the daily cycle by broadly sampling how SCN neurons respond to GABA using extracellular single-unit recording and gramicidin-perforated-patch recording techniques. We found that, although GABA inhibits most SCN neurons, some level of GABA-mediated excitation was present in both dorsal and ventral regions of the SCN, regardless of the time of day. These GABA-evoked excitatory responses were most common during the night in the dorsal SCN region. The Na(+)-K(+)-2Cl(-) cotransporter (NKCC) inhibitor, bumetanide, prevented these excitatory responses. In individual neurons, the application of bumetanide was sufficient to change GABA-evoked excitation to inhibition. Calcium-imaging experiments also indicated that GABA-elicited calcium transients in SCN cells are highly dependent on the NKCC isoform 1 (NKCC1). Finally, Western blot analysis indicated that NKCC1 expression in the dorsal SCN is higher in the night. Together, this work indicates that GABA can play an excitatory role in communication between adult SCN neurons and that this excitation is critically dependent on NKCC1.

Role for the NR2B subunit of the N-methyl-D-aspartate receptor in mediating light input to the circadian system

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells that utilize glutamate as a neurotransmitter. A variety of evidence suggests that the release of glutamate then activates N-methyl-D-aspartate (NMDA) receptors within the SCN and triggers a signaling cascade that ultimately leads to phase shifts in the circadian system. In this study, we first sought to explore the role of the NR2B subunit in mediating the effects of light on the circadian system of hamsters and mice. We found that localized microinjection of the NR2B subunit antagonist ifenprodil into the SCN region reduces the magnitude of light-induced phase shifts of the circadian rhythm in wheel-running activity. Next, we found that the NR2B message and levels of phospho-NR2B vary with time of day in SCN tissue using semiquantitative real-time polymerase chain reaction and western blot analysis, respectively. Functionally, we found that blocking the NR2B subunit with ifenprodil significantly reduced the magnitude of NMDA currents recorded in SCN neurons. Ifenprodil also significantly reduced the magnitude of NMDA-induced Ca2+ changes in SCN cells. Together, these results demonstrate that the NR2B subunit is an important component of NMDA receptor-mediated responses within SCN neurons and that this subunit contributes to light-induced phase shifts of the mammalian circadian system.

Reciprocity between phase shifts and amplitude changes in the mammalian circadian clock

Circadian rhythms help organisms adapt to predictable daily changes in their environment. Light resets the phase of the underlying oscillator to maintain the organism in sync with its surroundings. Light also affects the amplitude of overt rhythms. At a critical phase during the night, when phase shifts are maximal, light can reduce rhythm amplitude to nearly zero, whereas in the subjective day, when phase shifts are minimal, it can boost amplitude substantially. To explore the cellular basis for this reciprocal relationship between phase shift and amplitude change, we generated a photoentrainable, cell-based system in mammalian fibroblasts that shares several key features of suprachiasmatic nucleus light entrainment. Upon light stimulation, these cells exhibit calcium/cyclic AMP responsive element-binding (CREB) protein phosphorylation, leading to temporally gated acute induction of the Per2 gene, followed by phase-dependent changes in phase and/or amplitude of the PER2 circadian rhythm. At phases near the PER2 peak, photic stimulation causes little phase shift but enhanced rhythm amplitude. At phases near the PER2 nadir, on the other hand, the same stimuli cause large phase shifts but dampen rhythm amplitude. Real-time monitoring of PER2 oscillations in single cells reveals that changes in both synchrony and amplitude of individual oscillators underlie these phenomena.

Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons

Glutamate released from retinohypothalamic tract (RHT) synapses with suprachiasmatic nucleus (SCN) neurons induces phase changes in the circadian clock presumably by using Ca2+ as a second messenger. We used electrophysiological and Ca2+ imaging techniques to simultaneously record changes in the membrane potential and intracellular calcium concentration ([Ca2+]i) in SCN neurons after stimulation of the RHT at physiologically relevant frequencies. Stimulation of the RHT sufficient to generate an EPSP did not produce detectable changes in [Ca2+]i, whereas EPSP-induced action potentials evoked an increase in [Ca2+]i, suggesting that the change in postsynaptic somatic [Ca2+]i produced by synaptically activated glutamate receptors was the result of membrane depolarization activating voltage-dependent Ca2+ channels. The magnitude of the Ca2+ response was dependent on the RHT stimulation frequency and duration, and on the SCN neuron action potential frequency. Membrane depolarization-induced changes in [Ca2+]i were larger and decayed more quickly in the dendrites than in the soma and were attenuated by nimodipine, suggesting a compartmentalization of Ca2+ signaling and a contribution of L-type Ca2+ channels. RHT stimulation at frequencies that mimicked the output of light-sensitive retinal ganglion cells (RGCs) evoked [Ca2+]i transients in SCN neurons via membrane depolarization and activation of voltage-dependent Ca2+ channels. These data suggest that for Ca2+ to induce phase advances or delays, light-induced signaling from RGCs must augment the underlying oscillatory somatic [Ca2+]i by evoking postsynaptic action potentials in SCN neurons during a period of slow spontaneous firing such as occurs during nighttime.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Shedding light on circadian clock resetting by dark exposure: differential effects between diurnal and nocturnal rodents

The master circadian clock in mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, is entrained by light and behavioural stimulation. In addition, the SCN can be reset by dark pulses in nocturnal rodents under constant light conditions. Here, the shifting effects of a dark pulse on the SCN clock were detailed at both a behavioural and molecular level in a nocturnal rodent (Syrian hamster), and were compared to those of a diurnal rodent (Arvicanthis ansorgei). Four-hour dark pulses led to phase advances in the circadian rhythm of locomotor activity from subjective midday to dusk in hamsters, but from subjective dusk to midnight in Arvicanthis. Moreover, dark pulses had no resetting effect during the middle of the subjective night in hamsters, while such a dead shifting zone occurred during most of the subjective day in Arvicanthis. The behavioural phase advances in both hamsters and Arvicanthis were most often accompanied by marked downregulation of the clock genes Per1 and/or Per2 in the SCN, and also by changes in the transforming growth factor-alpha expression, a neuropeptide that suppresses daytime activity in nocturnal mammals. Despite that both hamsters and Arvicanthis showed dark-induced phase advances at circadian time-12, Per1 gene and its protein PER1 were downregulated in Arvicanthis but not in hamsters. Altogether these results show that dark resetting of the SCN is always associated with downregulation of Per1 and/or Per2 expression, and mostly occurs during resting. Thus, the circadian window of sensitivity to dark differs between nocturnal and diurnal rodents.