Correlative association between N-methyl-D-aspartate receptor-mediated expression of period genes in the suprachiasmatic nucleus and phase shifts in behavior with photic entrainment of clock in hamsters

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.

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.

Circadian Rhythm Disruption and Alzheimer's Disease: The Dynamics of a Vicious Cycle

All mammalian cells exhibit circadian rhythm in cellular metabolism and energetics. Autonomous cellular clocks are modulated by various pathways that are essential for robust time keeping. In addition to the canonical transcriptional translational feedback loop, several new pathways of circadian timekeeping - non-transcriptional oscillations, post-translational modifications, epigenetics and cellular signaling in the circadian clock - have been identified. The physiology of circadian rhythm is expansive, and its link to the neurodegeneration is multifactorial. Circadian rhythm disruption is prevelant in contamporary society where light-noise, shift-work, and transmeridian travel are commonplace, and is also reported from the early stages of Alzheimer's disease (AD). Circadian alignment by bright light therapy in conjunction with chronobiotics is beneficial for treating sundowning syndrome and other cognitive symptoms in advanced AD patients. We performed a comprehensive analysis of the clinical and translational reports to review the physiology of the circadian clock, delineate its dysfunction in AD, and unravel the dynamics of the vicious cycle between two pathologies. The review delineates the role of putative targets like clock proteins PER, CLOCK, BMAL1, ROR, and clock-controlled proteins like AVP, SIRT1, FOXO, and PK2 towards future approaches for management of AD. Furthermore, the role of circadian rhythm disruption in aging is delineated.

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.

Evaluating Circadian Dysfunction in Mouse Models of Alzheimer's Disease: Where Do We Stand?

Circadian dysfunction has been described in patients with symptomatic Alzheimer’s disease (AD), as well as in presymptomatic phases of the disease. Modeling this circadian dysfunction in mouse models would provide an optimal platform for understanding mechanisms and developing therapies. While numerous studies have examined behavioral circadian function, and in some cases clock gene oscillation, in mouse models of AD, the results are variable and inconsistent across models, ages, and conditions. Ultimately, circadian changes observed in APP/PS1 models are inconsistent across studies and do not always replicate circadian phenotypes observed in human AD. Other models, including the 3xTG mouse, tau transgenic lines, and the accelerated aging SAMP8 line, show circadian phenotypes more consistent with human AD, although the literature is either inconsistent or minimal. We summarize these data and provide some recommendations to improve and standardize future studies of circadian function in AD mouse models.

Insights About Circadian Clock and Molecular Pathogenesis in Gliomas

The circadian clock is an endogenous time-keeping system that has been discovered across kingdoms of life. It controls and coordinates metabolism, physiology, and behavior to adapt to variations within the day and the seasonal environmental cycles driven by earth rotation. In mammals, although circadian rhythm is controlled by a set of core clock genes that are present in both in suprachiasmatic nucleus (SCN) of the hypothalamus and peripheral tissues, the generation and control of the circadian rhythm at the cellular, tissue, and organism levels occurs in a hierarchal fashion. The SCN is central pacemaker comprising the principal circadian clock that synchronizes peripheral circadian clocks to their appropriate phase. Different epidemiological studies have shown that disruption of normal circadian rhythm is implicated in increasing the risk of developing cancers. In addition, deregulated expression of clock genes has been demonstrated in various types of cancer. These findings indicate a close association between circadian clock and cancer development and progression. Here, we review different evidences of this association in relation to molecular pathogenesis in gliomas.

Altered N-methyl D-aspartate receptor subunit expression causes changes to the circadian clock and cell phenotype in osteoarthritic chondrocytes

The chondrocyte circadian clock is altered in osteoarthritis. This change is implicated in the disease-associated changes in chondrocyte phenotype and cartilage loss. Why the clock is changed is unknown. N-methyl-D-aspartate receptors (NMDAR) are critical for regulating the hypothalamic clock. Chondrocytes also express NMDAR and the type of NMDAR subunits expressed changes in osteoarthritis.

OBJECTIVE

To determine if NMDAR regulate the chondrocyte clock and phenotype.

DESIGN

Chondrocytes isolated from macroscopically-normal (MN) and osteoarthritic human cartilage were treated with NMDAR antagonists or transfected with GRIN2A or GRIN2B-targetting siRNA. H5 chondrocytes were transfected with GluN2B-expression plasmids. Clock genes and chondrocyte phenotypic markers were measured by RT-qPCR.

RESULTS

PER2 amplitude was higher and BMAL1 amplitude lower in osteoarthritic compared to MN chondrocytes. In osteoarthritic chondrocytes, NMDAR inhibition restored PER2 and BMAL1 expression to levels similar to MN chondrocytes, and resulted in reduced MMP13 and COL10A1. Paradoxically, NMDAR inhibition in MN chondrocytes resulted in increased PER2, decreased BMAL1 and increased MMP13 and COL10A1. Osteoarthritic, but not MN chondrocytes expressed GluN2B NMDAR subunits. GluN2B knockdown in osteoarthritic chondrocytes restored expression of circadian clock components and phenotypic markers to levels similar to MN chondrocytes. Ectopic expression of GluN2B resulted in reduced BMAL1, increased PER2 and altered SOX9, RUNX2 and MMP13 expression. Knockdown of PER2 mitigated the effects of GluN2B on SOX9 and MMP13.

CONCLUSIONS

NMDAR regulate the chondrocyte clock and phenotype suggesting NMDAR may also regulate clocks in other peripheral tissues. GluN2B expression in osteoarthritis may contribute to pathology by altering the chondrocyte clock.

Neurobiological Functions of the Period Circadian Clock 2 Gene, Per2

Most organisms have adapted to a circadian rhythm that follows a roughly 24-hour cycle, which is modulated by both internal (clock-related genes) and external (environment) factors. In such organisms, the central nervous system (CNS) is influenced by the circadian rhythm of individual cells. Furthermore, the period circadian clock 2 (Per2) gene is an important component of the circadian clock, which modulates the circadian rhythm. Per2 is mainly expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus as well as other brain areas, including the midbrain and forebrain. This indicates that Per2 may affect various neurobiological activities such as sleeping, depression, and addiction. In this review, we focus on the neurobiological functions of Per2, which could help to better understand its roles in the CNS.

The cell adhesion molecule EphA4 is involved in circadian clock functions

Circadian (∼24 h) rhythms of cellular network plasticity in the central circadian clock, the suprachiasmatic nucleus (SCN), have been described. The neuronal network in the SCN regulates photic resetting of the circadian clock as well as stability of the circadian system during both entrained and constant conditions. EphA4, a cell adhesion molecule regulating synaptic plasticity by controlling connections of neurons and astrocytes, is expressed in the SCN. To address whether EphA4 plays a role in circadian photoreception and influences the neuronal network of the SCN, we have analyzed circadian wheel-running behavior of EphA4 knockout (EphA4^-/- ) mice under different light conditions and upon photic resetting, as well as their light-induced protein response in the SCN. EphA4^-/- mice exhibited reduced wheel-running activity, longer endogenous periods under constant darkness and shorter periods under constant light conditions, suggesting an effect of EphA4 on SCN function. Moreover, EphA4^-/- mice exhibited suppressed phase delays of their wheel-running activity following a light pulse during the beginning of the subjective night (CT15). Accordingly, light-induced c-FOS (FBJ murine osteosarcoma viral oncogene homolog) expression was diminished. Our results suggest a circadian role for EphA4 in the SCN neuronal network, affecting the circadian system and contributing to the circadian response to light.

Astrocytes Control Circadian Timekeeping in the Suprachiasmatic Nucleus via Glutamatergic Signaling

The suprachiasmatic nucleus (SCN) of the hypothalamus orchestrates daily rhythms of physiology and behavior in mammals. Its circadian (∼24 hr) oscillations of gene expression and electrical activity are generated intrinsically and can persist indefinitely in temporal isolation. This robust and resilient timekeeping is generally regarded as a product of the intrinsic connectivity of its neurons. Here we show that neurons constitute only one “half” of the SCN clock, the one metabolically active during circadian daytime. In contrast, SCN astrocytes are active during circadian nighttime, when they suppress the activity of SCN neurons by regulating extracellular glutamate levels. This glutamatergic gliotransmission is sensed by neurons of the dorsal SCN via specific pre-synaptic NMDA receptor assemblies containing NR2C subunits. Remarkably, somatic genetic re-programming of intracellular clocks in SCN astrocytes was capable of remodeling circadian behavioral rhythms in adult mice. Thus, SCN circuit-level timekeeping arises from interdependent and mutually supportive astrocytic-neuronal signaling.

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SCN neurons are active during circadian day, but SCN astrocytes are active at night

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Astrocytes direct circadian cycles of extracellular glutamate to inhibit SCN neurons

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Astrocyte-derived inhibition is mediated by NMDAR2C complexes on dorsal SCN neurons

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Genetic re-programming of the clock in SCN astrocytes reshapes circadian behavior

Genetics of Circadian Rhythms

Nearly all organisms exhibit time-dependent behavior and physiology across a 24-hour day known as circadian rhythms. These outputs are manifestations of endogenous cyclic gene expression patterns driven by the activity of a core transcription/translation feedback loop. Cyclic gene expression determines highly tissue-specific functional activity regulating such processes as metabolic state, endocrine activity, and neural excitability. Entrainment of these cellular clocks is achieved through exogenous daily inputs, such as light and food. Dysregulation of the transcription/translation feedback loop has been shown to result in a wide range of disorders and diseases driving increased interest in circadian therapies.

Effects of Photoperiod Extension on Clock Gene and Neuropeptide RNA Expression in the SCN of the Soay Sheep

In mammals, changing daylength (photoperiod) is the main synchronizer of seasonal functions. The photoperiodic information is transmitted through the retino-hypothalamic tract to the suprachiasmatic nuclei (SCN), site of the master circadian clock. To investigate effects of day length change on the sheep SCN, we used in-situ hybridization to assess the daily temporal organization of expression of circadian clock genes (Per1, Per2, Bmal1 and Fbxl21) and neuropeptides (Vip, Grp and Avp) in animals acclimated to a short photoperiod (SP; 8h of light) and at 3 or 15 days following transfer to a long photoperiod (LP3, LP15, respectively; 16h of light), achieved by an acute 8-h delay of lights off. We found that waveforms of SCN gene expression conformed to those previously seen in LP acclimated animals within 3 days of transfer to LP. Mean levels of expression for Per1-2 and Fbxl21 were nearly 2-fold higher in the LP15 than in the SP group. The expression of Vip was arrhythmic and unaffected by photoperiod, while, in contrast to rodents, Grp expression was not detectable within the sheep SCN. Expression of the circadian output gene Avp cycled robustly in all photoperiod groups with no detectable change in phasing. Overall these data suggest that synchronizing effects of light on SCN circadian organisation proceed similarly in ungulates and in rodents, despite differences in neuropeptide gene expression.

Effect of circadian rhythm disturbance on morphine preference and addiction in male rats: Involvement of period genes and dopamine D1 receptor

It is claimed that a correlation exists between disturbance of circadian rhythms by factors such as alteration of normal light-dark cycle and the development of addiction. However, the exact mechanisms involved in this relationship are not much understood. Here we have studied the effect of constant light on morphine voluntary consumption and withdrawal symptoms and also investigated the involvement of Per1, Per2 and dopamine D1 receptor in these processes. Male wistar rats were kept under standard (LD) or constant light (LL) conditions for one month. The plasma concentration of melatonin was evaluated by enzyme-linked immunosorbent assay (ELISA). Real-time PCR was used to determine the mRNA expression of Per1, Per2 and dopamine D1 receptor in the striatum and prefrontal cortex. Morphine preference (50mg/L) was evaluated in a two-bottle-choice paradigm for 10 weeks and withdrawal symptoms were recorded after administration of naloxone (3mg/kg). One month exposure to constant light resulted in a significant decrease of melatonin concentration in the LL group. In addition, mRNA levels of Per2 and dopamine D1 receptor were up-regulated in both the striatum and prefrontal cortex of the LL group. However, expression of Per1 gene was only up-regulated in the striatum of LL rats in comparison to LD animals. Furthermore, after one month exposure to constant light, morphine consumption and preference ratio and also severity of naloxone-induced withdrawal syndrome were significantly greater in LL animals. It is concluded that exposure to constant light by up-regulation of Per2 and dopamine D1 receptor in the striatum and prefrontal cortex and up-regulation of Per1 in the striatum and the possible involvement of melatonin makes animals vulnerable to morphine preference and addiction.

Synchronization of Biological Clock Neurons by Light and Peripheral Feedback Systems Promotes Circadian Rhythms and Health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.

Alcohol and lithium have opposing effects on the period and phase of the behavioral free-running activity rhythm

Bipolar patients have a high prevalence of comorbid alcohol use and abuse disorders, while chronic alcohol drinking may increase the presence and severity of certain symptoms of bipolar disorder. As such, there may be many individuals that are prescribed lithium to alleviate the manic symptoms of bipolar disorder, but also drink alcohol concurrently. In addition, both alcoholics and individuals with bipolar disorder often exhibit disruptions to their sleep-wake cycles and other circadian rhythms. Interestingly, both ethanol and lithium are known to alter both the period and the phase of free-running rhythms in mammals. While lithium is known to lengthen the period, ethanol seems to shorten the period and attenuate the responses to acute light pulses. Therefore, the present study aimed to determine whether ethanol and lithium have opposing effects on the circadian pacemaker when administered together. C57BL/6J mice were provided drinking solutions containing lithium, alcohol, or both, and their free-running rhythms along with their response to photic phase shifts were investigated. Mice treated with lithium displayed period lengthening, which was almost completely negated when ethanol was added. Moreover, ethanol significantly attenuated light-induced phase delays while the addition of lithium partially restored this response. These results indicate that alcohol and lithium have opposing effects on behavioral circadian rhythms. Individuals with bipolar disorder who are prescribed lithium and who drink alcohol might be inadvertently altering their sleep and circadian cycles, which may exacerbate their symptoms.

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.

Day or night administration of ketamine and pentobarbital differentially affect circadian rhythms of pineal melatonin secretion and locomotor activity in rats

BACKGROUND

Surgery with general anesthesia disturbs circadian rhythms, which may lead to postoperative sleep disorders and delirium in patients. However, it is unclear how circadian rhythms are affected by different anesthetics administered at different times during the rest-activity cycle. We hypothesized that pentobarbital (an agonist at the γ-aminobutyric acid A receptors) and ketamine (an antagonist at the N-methyl-d-aspartate receptors) would have differential effects on circadian rhythms, and these effects would also be influenced by the time of their administration (the active versus resting phase).

METHODS

Rats were divided into 4 groups according to the anesthetic administered (pentobarbital or ketamine) and the timing of intraperitoneal administration (active/night phase or resting/day phase). Using online pineal microdialysis, we analyzed pineal melatonin secretion and locomotor activity rhythms in rats under a light/dark (12/12-hour) cycle for 5 days after anesthesia and microdialysis catheter implantation. The data were analyzed for rhythmicity by cosinor analysis.

RESULTS

Ketamine administered during the resting phase produced 65- and 153-minute phase advances, respectively, in melatonin secretion and locomotor activity rhythms on the first day after anesthesia. In contrast, ketamine administered during the active phase produced 43- and 235-minute phase delays. Pentobarbital had no effect on the phase of either melatonin secretion or locomotor activity, irrespective of the timing of administration. When administered during the active phase, both anesthetics decreased the amplitude of melatonin secretion on the day after anesthesia; when administered during the resting phase, however, neither anesthetic affected the amplitude. The amplitude of locomotor activity decreased in all animals for 3 days after anesthesia.

CONCLUSION

Ketamine has opposite phase-shifting effects on circadian rhythms according to the time of administration, whereas pentobarbital has no effect. Furthermore, both anesthetics decrease the postoperative amplitude of pineal melatonin secretion if administered during the active, but not the resting, phase of the 24-hour rest-activity cycle.

Conserved expression of the glutamate NMDA receptor 1 subunit splice variants during the development of the Siberian hamster suprachiasmatic nucleus

Glutamate neurotransmission and the N-methyl-D-aspartate receptor (NMDAR) are central to photic signaling to the master circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). NMDARs also play important roles in brain development including visual input circuits. The functional NMDAR is comprised of multiple subunits, but each requiring the NR1 subunit for normal activity. The NR1 can be alternatively spliced to produce isoforms that confer different functional properties on the NMDAR. The SCN undergoes extensive developmental changes during postnatal life, including synaptogenesis and acquisition of photic signaling. These changes are especially important in the highly photoperiodic Siberian hamster, in which development of sensitivity to photic cues within the SCN could impact early physiological programming. In this study we examined the expression of NR1 isoforms in the hamster at different developmental ages. Gene expression in the forebrain was quantified by in situ hybridization using oligonucleotide probes specific to alternatively spliced regions of the NR1 heteronuclear mRNA, including examination of anterior hypothalamus, piriform cortex, caudate-putamen, thalamus and hippocampus. Gene expression analysis within the SCN revealed the absence of the N1 cassette, the presence of the C2 cassette alone and the combined absence of C1 and C2 cassettes, indicating that the dominant splice variants are NR1-2a and NR1-4a. Whilst we observe changes at different developmental ages in levels of NR1 isoform probe hybridization in various forebrain structures, we find no significant changes within the SCN. This suggests that a switch in NR1 isoform does not underlie or is not produced by developmental changes within the hamster SCN. Consistency of the NR1 isoforms would ensure that the response of the SCN cells to photic signals remains stable throughout life, an important aspect of the function of the SCN as a responder to environmental changes in quality/quantity of light over the circadian day and annual cycle.

The expression of NR2B subunit of NMDA receptor in the suprachiasmatic nucleus of Wistar rats and its role in glutamate-induced CREB and ERK1/2 phosphorylation

Most behavioral and physiological processes in living organisms exhibit periodic circadian rhythmicity. In mammals, these rhythms are coordinated by the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. In order to precisely synchronize free-running circadian oscillations to the 24h solar cycle, signals from the external environment, primarily the light/dark cycle, must reach the circadian clock within the SCN. A light pulse elevates intracellular Ca(2+) levels, and activates signaling cascades, leading to transcriptional activation of the clock genes mPer1 and mPer2 via phosphorylation of extracellular-signal-regulated kinases 1/2 (ERK1/2) and cyclic AMP-responsive element binding protein (CREB). Glutamate is the primary excitatory transmitter in retinal terminals in the SCN, and NMDA receptors (NMDAR) are the principal glutamate receptors that mediate the effect of light on resetting the circadian clock. Here we show the circadian rhythm in mRNA expression and protein level of the NMDAR 2B subunit (NR2B) in the SCN, with a peak at night. Also, we demonstrate ifenprodil inhibition of glutamate-induced phosphorylation of CREB (pCREB) and ERK1/2 (pERK1/2), and support thus the evidence for NR2B role in activation of signaling cascade involved in photic resetting of the circadian clock.

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.

The adjustment and manipulation of biological rhythms by light, nutrition, and abused drugs

Daily restricted feeding entrains the circadian rhythm of mouse clock gene expression in the central nervous system, excluding the suprachiasmatic nucleus (SCN), as well as in the peripheral tissues such as the liver, lung, and heart. In addition to entrainment of the clock genes, daily restricted feeding induces a locomotor activity increase 2-3h before the restricted feeding time initiates. The increase in activity is called the food-anticipatory activity (FAA). In addition to FAA, daily restricted feeding can also entrain peripheral circadian clocks in other organs such as liver, lung, and heart. This type of oscillator is called the food-entrainable peripheral oscillator (FEPO). At present, the mechanisms for restricted feeding-induced entrainment of locomotor activity (FAA) and/or peripheral clock (FEPO) are still unknown. In this review, we describe the role of the central nervous system and peripheral tissues in FAA performance and also in the entrainment of clock gene expression. In addition, the mechanism for entrainment of circadian oscillators by the abuse of drugs, such as methamphetamine, is discussed.

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.

Regulation of gene expression in brain tissues of rats repeatedly treated by the highly abused opioid agonist, oxycodone: microarray profiling and gene mapping analysis

Although oxycodone is the most often used opioid agonist, it remains one of the most understudied drugs. We used microarray analysis to better understand the global changes in gene expression in brain tissues of rats repeatedly treated with oxycodone. Many genes were significantly regulated by oxycodone (e.g., Fkbp5, Per2, Rt1.Dalpha, Slc16a1, and Abcg2). Validation of the microarray data by quantitative real-time-polymerase chain reaction (Q-PCR) indicated that there was a strong significant correlation (r = 0.979, p < 0.0000001) between the Q-PCR and the microarray data. Using MetaCore (a computational platform), many biological processes were identified [e.g., organic anion transport (p = 7.251 x 10(-4)) and regulation of immune response (p = 5.090 x 10(-4))]. Among the regulated genes, Abcg2 mRNA was up-regulated by 2.1-fold, which was further confirmed by immunoblotting (1.8-fold up-regulation). Testing the Abcg2 affinity status of oxycodone using an Abcg2 ATPase assay suggests that oxycodone behaves as an Abcg2 substrate only at higher concentrations (> or = 500 microM). Furthermore, brain uptake studies demonstrated that oxycodone-induced Abcg2 up-regulation resulted in a significant (p < 0.05) decrease (approximately 2-fold) in brain/plasma ratios of mitoxantrone. These results highlight markers/mediators of neuronal responses and identify regulatory pathways involved in the pharmacological action of oxycodone. These results also identify genes that potentially modulate tolerance, dependence, immune response, and drug-drug interactions. Finally, our findings suggest that oxycodone-induced up-regulation of Abcg2 enhanced the efflux of the Abcg2 substrate, mitoxantrone, limiting its brain accumulation and resulting in an undesirable drug-drug interaction. Extrapolating these results to other Abcg2 substrates (e.g., daunorubicin and doxorubicin) indicates that the brain uptake of these agents may be affected if they are administered concomitantly with oxycodone.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

Chronic ethanol attenuates circadian photic phase resetting and alters nocturnal activity patterns in the hamster

Acute ethanol (EtOH) administration impairs circadian clock phase resetting, suggesting a mode for the disruptive effect of alcohol abuse on human circadian rhythms. Here, we extend this research by characterizing the chronobiological effects of chronic alcohol consumption. First, daily profiles of EtOH were measured in the suprachiasmatic nucleus (SCN) and subcutaneously using microdialysis in hamsters drinking EtOH. In both cases, EtOH peaked near lights-off and declined throughout the dark-phase to low day-time levels. Drinking bouts preceded EtOH peaks by approximately 20 min. Second, hamsters chronically drinking EtOH received a light pulse during the late dark phase [Zeitgeber time (ZT) 18.5] to induce photic phase advances. Water controls had shifts of 1.2 +/- 0.2 h, whereas those drinking 10% and 20% EtOH had much reduced shifts (0.5 +/- 0.1 and 0.3 +/- 0.1 h, respectively; P < 0.001 vs. controls). Third, incremental decreases in light intensity (270 lux to 0.5 lux) were used to explore chronic EtOH effects on photic entrainment and rhythm stability. Activity onset was unaffected by 20% EtOH at all light intensities. Conversely, the 24-h pattern of activity bouts was disrupted by EtOH under all light intensities. Finally, replacement of chronic EtOH with water was used to examine withdrawal effects. Water controls had photic phase advances of 1.1 +/- 0.3 h, while hamsters deprived of EtOH for 2-3 days showed enhanced shifts (2.1 +/- 0.3 h; P < 0.05 vs. controls). Thus, in chronically drinking hamsters, brain EtOH levels are sufficient to inhibit photic phase resetting and disrupt circadian activity. Chronic EtOH did not impair photic entrainment; however, its replacement with water potentiated photic phase resetting.

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.

The dorsomedial hypothalamic nucleus is not necessary for food-anticipatory circadian rhythms of behavior, temperature or clock gene expression in mice

Circadian rhythms in mammals are regulated by a light-entrainable circadian pacemaker in the hypothalamic suprachiasmatic nucleus and food-entrainable oscillators located elsewhere in the brain and body. The dorsomedial hypothalamic nucleus (DMH) has been proposed to be the site of oscillators driving food-anticipatory circadian rhythms, but this is controversial. To further evaluate this hypothesis, we measured clock gene, temperature and activity rhythms in intact and DMH-ablated mice. A single 4-h midday feeding after an overnight fast induced mPer1 and mPer2 mRNA expression in the DMH, arcuate nucleus, nucleus of the solitary tract and area postrema, and reset daily rhythms of mPer1, mPer2 and mBMAL1 in the DMH, arcuate and neocortex. These rhythms persisted during 2 days of food deprivation after 12 days of scheduled daytime feeding. Acute induction of DMH mPer1 and mPer2 was N-methyl-D-aspartate (NMDA) receptor-dependent, whereas rhythmic expression after 6 days of restricted feeding was not. Thermal DMH lesions did not affect acute induction or rhythmic expression of clock genes in other brain regions in response to scheduled daytime feeding. DMH lesions attenuated mean daily activity levels and nocturnality but did not affect food-anticipatory rhythms of activity and body temperature in either light-dark or constant darkness. These results confirm that the DMH and other brain regions express circadian clock gene rhythms sensitive to daytime feeding schedules, but do not support the hypothesis that DMH oscillations drive food-anticipatory behavioral or temperature rhythms.

Acute ethanol impairs photic and nonphotic circadian phase resetting in the Syrian hamster

Disrupted circadian rhythmicity is associated with ethanol (EtOH) abuse, yet little is known about how EtOH affects the mammalian circadian clock of the suprachiasmatic nucleus (SCN). Clock timing is regulated by photic and nonphotic inputs to the SCN involving glutamate release from the retinohypothalamic tract and serotonin (5-HT) from the midbrain raphe, respectively. Our recent in vitro studies in the SCN slice revealed that EtOH blocks photic phase-resetting action of glutamate and enhances the nonphotic phase-resetting action of the 5-HT1A,7 agonist, 8-OH-DPAT. To explore the basis of these effects in the whole animal, we used microdialysis to characterize the pharmacokinetics of intraperitoneal injection of EtOH in the hamster SCN extracellular fluid compartment and then studied the effects of such EtOH treatment on photic and serotonergic phase resetting of the circadian locomotor activity rhythm. Peak EtOH levels (approximately 50 mM) from a 2 g/kg injection occurred within 20-40 min with a half-life of approximately 3 h. EtOH treatment dose-dependently attenuated photic phase advances but had no effect on phase delays and, contrary to in vitro findings, markedly attenuated 8-OH-DPAT-induced phase advances. In a complementary experiment using reverse microdialysis to deliver a timed SCN perfusion of EtOH during a phase-advancing light pulse, the phase advances were blocked, similar to systemic EtOH treatment. These results are evidence that acute EtOH significantly affects photic and nonphotic phase-resetting responses critical to circadian clock regulation. Notably, EtOH inhibition of photic signaling is manifest through direct action in the SCN. Such actions could underlie the disruption of circadian rhythmicity associated with alcohol abuse.

Period gene expression in the diurnal degu (Octodon degus) differs from the nocturnal laboratory rat (Rattus norvegicus)

Recent data suggest that both nocturnal and diurnal mammals generate circadian rhythms using similarly phased feedback loops involving Period genes in the suprachiasmatic nuclei (SCN) of the hypothalamus. These molecular oscillations also exist in the brain outside of the SCN, but the relationship between SCN and extra-SCN oscillations is unclear. We hypothesized that a comparison of "diurnal" and "nocturnal" central nervous system Per rhythms would uncover differences in the underlying circadian mechanisms between these two chronotypes. Therefore, this study compared the 24-h oscillatory patterns of Per1 and Per2 mRNA in the SCN and putative striatum and cortex of Octodon degus (degu), a diurnal hystricognath rodent, with those of the nocturnal laboratory rat, Rattus norvegicus. The brains of adult male degus and rats were collected at 2-h intervals across 24 h in entrained light-dark and constant darkness conditions, and sections were analyzed via in situ hybridization. In the SCN, degu Per1 and Per2 hybridization signal exhibited 24-h oscillatory patterns similar in phasing to those seen in other rodents, with peaks occurring during the light period and troughs during the dark period. However, Per1 remained elevated for five fewer hours in the degu than in the rat, and Per2 remained elevated for two fewer hours in the degu. In brain areas outside of the SCN, the phase of Per2 hybridization signal rhythms in the degu were 180 degrees out of phase with those found in the rat, and Per1 hybridization signal lacked significant rhythmicity. These results suggest that, while certain basic components of the transcriptional-translational feedback loop generating circadian rhythms are similar in diurnal and nocturnal mammals, there are variations that may reflect adaptations to circadian niche.

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.

Interactions of GABA A receptor activation and light on period mRNA expression in the suprachiasmatic nucleus

Activation of gamma-aminobutyric acid (GABA) A receptors in the suprachiasmatic nucleus (SCN) resets the circadian clock during the day and inhibits the ability of light to reset the clock at night. Light in turn acts during the day to inhibit the phase-resetting effects of GABA. Some evidence suggests that Period mRNA changes in the SCN are responsible for these interactions between light and GABA. Here, the hypothesis that light and the GABA A receptor interact by altering the expression of Period 1 and/or Period 2 mRNA in the SCN is tested. The GABA A agonist muscimol was injected near the SCN just prior to a light pulse, during the mid-subjective day and the early and late subjective night. Changes in Period 1 and Period 2 mRNA were measured in the SCN by in situ hybridization. Light-induced Period 1 mRNA was inhibited by GABA A receptor activation in the early and late subjective night, while Period 2 mRNA was only inhibited during the late night. During the subjective day, light had no effect on the ability of muscimol to suppress Period 1 mRNA hybridization signal. Thus, light and GABA A receptor activation inhibit each other's ability to induce behavioral phase shifts throughout the subjective day and night. However, only in the late night are these behavioral effects correlated with changes in Period gene expression. Together, our data support the hypothesis that the interacting effects of light and GABA are the result of the opposing actions of these stimuli on Period mRNA, but only during the subjective night.

Gastrin-releasing peptide mediates light-like resetting of the suprachiasmatic nucleus circadian pacemaker through cAMP response element-binding protein and Per1 activation

Circadian rhythmicity in the primary mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus, is maintained by transcriptional and translational feedback loops among circadian clock genes. Photic resetting of the SCN pacemaker involves induction of the clock genes Period1 (Per1) and Period2 (Per2) and communication among distinct cell populations. Gastrin-releasing peptide (GRP) is localized to the SCN ventral retinorecipient zone, from where it may communicate photic resetting signals within the SCN network. Here, we tested the putative role of GRP as an intra-SCN light signal at the behavioral and cellular levels, and we also tested whether GRP actions are dependent on activation of the cAMP response element-binding protein (CREB) pathway and Per1. In vivo microinjections of GRP to the SCN regions of Per1::green fluorescent protein (GFP) mice during the late night induced Per1::GFP throughout the SCN, including a limited population of arginine vasopressin-immunoreactive (AVP-IR) neurons. Blocking spike-mediated communication with tetrodotoxin did not disrupt overall Per1::GFP induction but did reduce induction within AVP-IR neurons. In vitro GRP application resulted in persistent increases in the spike frequency of Per1::GFP-induced neurons. Blocking endogenous Per1 with antisense oligodeoxynucleotides inhibited GRP-induced increases in spike frequency. Furthermore, inhibition of CREB-mediated gene activation with decoy oligonucleotides blocked GRP-induced phase shifts of PER2::luciferase rhythms in SCN slices. Altogether, these results indicate that GRP communicates phase resetting signals within the SCN network via both spike-dependent and spike-independent mechanisms, and that activation of the CREB pathway and Per1 are key steps in mediating downstream events in GRP resetting of SCN neurons.

PACAP and PDF signaling in the regulation of mammalian and insect circadian rhythms

Endogenous circadian clocks are inherent to all living organisms. They are needed to guarantee successful life since they regulate very important biological processes such as behavior and reproduction. Secretin-like G-protein coupled receptors are very important factors in the signal transduction pathways of circadian clocks. In this review, we will focus on the role of two secretin-like signaling pathways that play an important role in the regulation of the mammalian and the insect clock, respectively: the pituitary adenylate cyclase-activating polypeptide (PACAP) and pigment dispersing factor (PDF) signaling pathways. Both pathways are most likely related although their function in the biological clock differs.

Circadian rhythms in the development of obesity: potential role for the circadian clock within the adipocyte

Obesity is one of the most profound public health problems today, and simplistic explanations based on excessive nutritional consumption or lack of physical activity are inadequate to account for this dramatic and literal growth in our world population. Recent reports have suggested that disruptions in sleep patterns, often linked to our '24-h' lifestyle, are associated with increased body fat and altered metabolism, although the cause-effect relationship for these associations has yet to be elucidated. Abnormal sleep/wake patterns likely alter intracellular circadian clocks, which are molecular mechanisms that enable the cell/tissue/organism to anticipate diurnal variations in its environment. The environment may include circulating levels of nutrients (e.g. glucose, fatty acids and triglycerides) and various hormones (e.g. insulin, glucocorticoids). As such, alterations in this molecular mechanism, in particular within the adipocyte, likely induce metabolic changes that may potentiate disrupted metabolism, adipose accumulation and/or obesity. Although diurnal variations in adipokines and adipose tissue metabolism have been observed, little is known regarding the molecular mechanisms that influence these events.

Ethanol-induced hypnotic tolerance is absent in N-methyl-D-aspartate receptor epsilon 1 subunit knockout mice

Recent pharmacological studies suggest that N-methyl-D-aspartate (NMDA) receptors play an important role in neuroadaptive processes in the development of tolerance to addictive drugs, such as opioids, amphetamine, and cocaine. In the present study, we investigated the contribution of the NMDA receptor to ethanol-induced hypnotic tolerance using NMDA receptor epsilon1 subunit knockout mice. Hypnotic sensitivity to a single injection of 3, 3.5, and 4 g/kg ethanol was not significantly different between wild-type mice and NMDA receptor epsilon1 subunit knockout mice. In contrast, although wild-type mice displayed hypnotic tolerance after repeated administration of 4 g/kg ethanol for 4 consecutive days, no change in the duration of hypnosis was observed in knockout mice. No significant differences were observed in blood ethanol clearance between wild-type and knockout mice on day 4. Our results indicate epsilon1 subunit containing the NMDA receptor might be involved in the development of ethanol-induced hypnotic tolerance.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

A novel neuronal cell line derived from the ventrolateral region of the suprachiasmatic nucleus

The suprachiasmatic nucleus of the anterior hypothalamus is the center of an internal biological clock in mammals. Glutamate is the neurotransmitter of retino-hypothalamic tract responsible for mediating the circadian actions of light in rodents. N-methyl-d-aspartate receptors, particularly NR2B subunit are reported to be principally involved in photic resetting of the biological clock in vivo and in slice culture. But, the precise cellular mechanisms of the resetting are not elucidated, because no adequate neuronal cell lines derived from the suprachiasmatic nucleus have been established. We established a neuronal cell line, N14.5, derived from the suprachiasmatic nucleus of a transgenic rat harboring the temperature-sensitive simian virus 40 large T-antigen gene. When the cells were cultured at 39 degrees C, the morphological features were turned fibroblastic into neuronal round cell body with neurite extensions. These cells showed immunoreactivities for neuronal markers (betaIII-tubulin, microtubule-associated protein 2 and TAU2) and as well as for vasoactive intestinal peptide which is expressed in the ventrolateral region of the suprachiasmatic nucleus. The cells expressed N-methyl-d-aspartate receptors, particularly NR1 and NR2B subunits as revealed by quantitative PCR. N-methyl-d-aspartate activated phosphorylation of p44/42 mitogen-activated protein kinase and increased expression level of Per1 and Per2 mRNA. These results suggest that the N14.5 is a novel neuronal cell line derived from the ventrolateral region of the suprachiasmatic nucleus, and that N-methyl-d-aspartate receptors expressed in the cells are a functional receptor. The N14.5 cells may be a useful tool to elucidate numerous chronobiological processes, especially resetting mechanism induced by an external light signal.

The opioid fentanyl affects light input, electrical activity andPergene expression in the hamster suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) contain a major circadian pacemaker, which is regulated by photic and nonphotic stimuli. Although enkephalins are present in the SCN, their role in phase regulation of the pacemaker is largely unknown. The opioid agonist fentanyl, a homologue of morphine, is an addictive drug that induces phase shifts of circadian rhythms in hamsters. We observed that these phase shifts are blocked by naloxone, which is a critical test for true opioid receptor involvement, and conclude that opioid receptors are the sole mediators of the actions of fentanyl on the circadian timing system. A strong interaction between opioids and light input was shown by the ability of fentanyl and light to completely block each other's phase shifts of behavioural activity rhythms. Neuronal ensemble recordings in vitro provide first evidence that SCN cells show direct responses to fentanyl and react with a suppression of firing rate. Moreover, we show that fentanyl induces a strong attenuation of light-induced Syrian hamster Period 1 (shPer1) gene expression during the night. During the subjective day, we found no evidence for a role of shPer1 in mediation of fentanyl-induced phase shifts. Based on the present results, however, we cannot exclude the involvement of shPer2. Our data indicate that opioids can strongly modify the photic responsiveness of the circadian pacemaker and may do so via direct effects on SCN electrical activity and regulation of Per genes. This suggests that the pathways regulating addictive behaviour and the circadian clock intersect.

Effect of Haloperidol on mPer1 Gene Expression in Mouse Suprachiasmatic Nuclei

The effect of a typical neuroleptic haloperidol (Hal) on mPer1 gene expression was investigated in mouse suprachiasmatic nuclei (SCN). Hal induced mPer1 mRNA levels both in vivo and in cultured SCN cells. For mechanisms underlying Hal-induced mPer1 expression, N-methyl-d-aspartate (NMDA) glutamate receptor subtype, the phosphorylation form of the transcription factor, and the Ser-133 phosphorylation form of cAMP-responsive element-binding protein (CREB) played an important role, because the induction of mPer1 mRNA significantly decreased after pretreatment with a non-competitive NMDA receptor antagonist, such as MK-801 or CREB antisense. These results suggest that Hal may increase CREB phosphorylation and mPer1 expression according to the activation of the NMDA receptor through the dopaminergic pathways. Although the injection of Hal during the light period increased the amplitude of mPer1 mRNA rhythmicity in a nondrug state, the injection of the drug during the dark period disturbed the rhythmic pattern of mPer1 mRNA. These results suggest that the rhythmicity of clock genes in SCN may be disturbed depending on the dosing time of Hal. On the other hand, because the induction of mPer1 mRNA by Hal seems to be at least partly caused by the NMDA receptor, showing a phase shift or resetting effect of the circadian clock, Hal may also cause such phase shift effects.

Light response of the neuronal firing activity in the suprachiasmatic nucleus of mice

To investigate the neural mechanisms underlying the mammalian photic entrainment of circadian rhythms, the response of neuronal extracellular firing activity to retinal light stimulation was investigated in the suprachiasmatic nucleus (SCN) of anesthetized mice during nighttime and daytime. In nighttime, most recorded SCN cells (83%) increased their firing frequency in response to retinal illumination. Some SCN cells (11%) responded by decreasing their firing rate. In daytime, the retinal illumination increased the firing rate in only 26% of the SCN cells, and no response was observed in the remaining cells. The light intensity threshold for the activation of SCN cells at zeitgeber time (ZT) 16 was approximately 3 x 10(11) photons cm(-2)s(-1) and the maximum response was observed at approximately 1 x 10(14) photons cm(-2)s(-1). Therefore, photic response in the firing of mouse SCN cells may be phase-dependent and have a higher threshold, which corresponds to properties of the photic entrainment in locomotor activity of mice.

Estrogen differentially regulates expression ofPer1 andPer2 genes between central and peripheral clocks and between reproductive and nonreproductive tissues in female rats

Although it has long been established that estrogen alters circadian rhythms in behavior, physiology, and reproductive functions in mammals, the molecular mechanism for these effects remains unknown. To explore the possibility that estrogen affects circadian rhythms by changing the expression of clock-related genes, we investigated the effects of chronic treatment with 17beta-estradiol (E2) on the expression of Per1 and Per2 genes in the brain (suprachiasmatic nucleus and cerebral cortex) and periphery (liver, kidney, and uterus) of ovariectomized rats by means of in situ hybridization and northern blotting. In the brain, E2 treatment advanced the peak of Per2 mRNA expression in the SCN; however, it failed to affect the rhythm of Per2 mRNA expression in the CX and Per1 mRNA expression in both the SCN and the CX. In nonreproductive peripheral tissues (liver and kidney), E2 delayed the phase and increased the amplitude of Per1 mRNA expression. In the reproductive tissues (uterus), biphasic rhythms in Per1 and Per2 mRNA were observed after E2 treatment. These findings suggest that the effects of estrogen are different between central and peripheral clock in the brain, and between reproductive and nonreproductive tissues in the periphery.

Let there be “more” light: enhancement of light actions on the circadian system through non-photic pathways

Circadian rhythms are internally generated circa 24 h rhythms. The phase of the circadian pacemaker in mammals can be adjusted by external stimuli such as the daily cycle of light, as well as by internal stimuli such as information related to the physiological and behavioral status of the organism, collectively called "non-photic stimuli". We review a large number of studies regarding photic-non-photic interactions on the circadian system, with special focus on two widely described neurotransmitters associated with non-photic input pathways: neuropeptide Y (NPY) and serotonin 5-HT. Both neurotransmitters are capable of phase advancing the master pacemaker oscillation when applied during the subjective day, as do several behavioral manipulations. Also, both are capable of inhibiting light-induced phase shifts during the subjective night, suggesting a dynamic interaction between photic and non-photic stimuli in the fine-tuning of the pacemaker function. Suppression of the NPYergic and/or serotonergic non-photic input pathways can in turn potentiate the phase-shifting effects of light. These findings pose new questions about the possibility of a physiological role for the dynamic interaction between photic and non-photic inputs. This might be particularly important in the case of circadian system adjustments under certain conditions, such as depression, shift work or jet lag.