Neuropeptide Y rapidly reduces Period 1 and Period 2 mRNA levels in the hamster suprachiasmatic nucleus

Differences in recovery processes of circadian oscillators in various tissues after sevoflurane treatment in vivo

The inhalation anesthetic sevoflurane reversibly suppresses Period2 (Per2) mRNA expression in the suprachiasmatic nucleus (SCN). However, a discrepancy exists in phase shifting of the Per2 expression rhythm between sevoflurane application in rats (in vivo application) and explants (ex vivo application). This investigation aimed to resolve this issue. First, tissues from the SCN, choroid plexus in the lateral ventricle (CP-LV), and choroid plexus in the fourth ventricle (CP–4V), which are robust circadian oscillators, and pineal gland (PG) tissue, which is a circadian influencer, were prepared from Per2::dLuc transgenic rats. Significant phase responses of bioluminescence rhythms for different preparation times were monitored in the four tissue explant types. Second, tissue explants were prepared from anesthetized rats immediately after sevoflurane treatment, and bioluminescence rhythms were compared with those from non-anesthetized rats at various preparation times. Regarding bioluminescence rhythm phases, in vivo application of sevoflurane induced phase shifts in CP-LV, CP-4V, and PG explants according to the times that rats were administered anesthesia and the explants were prepared. Phase shifts in these peripheral explants were withdrawn due to the recovery period after the anesthetic treatment, which suggests that peripheral tissues require the assistance of related tissues or organs to correct phase shifts. In contrast, no phase shifts were observed in SCN explants. These results indicated that SCN explants can independently correct bioluminescence rhythm phase. The bioluminescence intensity of explants was also decreased after in vivo sevoflurane application. The suppressive effects on SCN explants were withdrawn due to a recovery day after the anesthetic treatment. In contrast, the suppressive effects on the bioluminescence intensities of CP-LV, CP-4V, and PG explants remained at 30 days after anesthesia administration. These results suggest that anesthetic suppression is imprinted within the peripheral tissues.

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We monitored bioluminescence in explants from Per2::dLuc rats after anesthesia.

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Sevoflurane induced phase shifts in peripheral explants but not in the SCN.

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Phase shifts in peripheral explants were withdrawn due to recovery period.

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Sevoflurane weakened the bioluminescence intensity of all explant types.

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The suppressive effects on peripheral explants were remained in a week later.

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.

Mammalian circadian networks mediated by the suprachiasmatic nucleus

The brain has a complex structure composed of hundreds of regions, forming networks to cooperate body functions. Therefore, understanding how various brain regions communicate with each other and with peripheral organs is important to understand human physiology. The suprachiasmatic nucleus (SCN) in the brain is the circadian pacemaker. The SCN receives photic information from the environment and conveys this to other parts of the brain and body to synchronize all circadian clocks. The circadian clock is an endogenous oscillator that generates daily rhythms in metabolism and physiology in almost all cells via a conserved transcriptional-translational negative feedback loop. So, the information flow from the environment to the SCN to other tissues synchronizes locally distributed circadian clocks to maintain homeostasis. Thus, understanding the circadian networks and how they adjust to environmental changes will better understand human physiology. This review will focus on circadian networks mediated by the SCN to understand how the environment, brain, and peripheral tissues form networks for cooperation.

Distinct feedback actions of behavioural arousal to the master circadian clock in nocturnal and diurnal mammals

The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus provides a temporal pattern of sleep and wake that - like many other behavioural and physiological rhythms - is oppositely phased in nocturnal and diurnal animals. The SCN primarily uses environmental light, perceived through the retina, to synchronize its endogenous circadian rhythms with the exact 24 h light/dark cycle of the outside world. The light responsiveness of the SCN is maximal during the night in both nocturnal and diurnal species. Behavioural arousal during the resting period not only perturbs sleep homeostasis, but also acts as a potent non-photic synchronizing cue. The feedback action of arousal on the SCN is mediated by processes involving several brain nuclei and neurotransmitters, which ultimately change the molecular functions of SCN pacemaker cells. Arousing stimuli during the sleeping period differentially affect the circadian system of nocturnal and diurnal species, as evidenced by the different circadian windows of sensitivity to behavioural arousal. In addition, arousing stimuli reduce and increase light resetting in nocturnal and diurnal species, respectively. It is important to address further question of circadian impairments associated with shift work and trans-meridian travel not only in the standard nocturnal laboratory animals but also in diurnal animal models.

The day/night difference in the circadian clock's response to acute lipopolysaccharide and the rhythmic Stat3 expression in the rat suprachiasmatic nucleus

The circadian clock in the suprachiasmatic nucleus (SCN) regulates daily rhythms in physiology and behaviour and is an important part of the mammalian homeostatic system. Previously, we have shown that systemic inflammatory stimulation with lipopolysaccharide (LPS) induced the daytime-dependent phosphorylation of STAT3 in the SCN. Here, we demonstrate the LPS-induced Stat3 mRNA expression in the SCN and show also the circadian rhythm in Stat3 expression in the SCN, with high levels during the day. Moreover, we examined the effects of LPS (1mg/kg), applied either during the day or the night, on the rhythm in locomotor activity of male Wistar rats. We observed that recovery of normal locomotor activity patterns took longer when the animals were injected during the night. The clock genes Per1, Per2 and Nr1d1, and phosphorylation of kinases ERK1/2 and GSK3β are sensitive to external cues and function as the molecular entry for external signals into the circadian clockwork. We also studied the immediate changes in these clock genes expressions and the phosphorylation of ERK1/2 and GSK3β in the suprachiasmatic nucleus in response to daytime or night-time inflammatory stimulation. We revealed mild and transient changes with respect to the controls. Our data stress the role of STAT3 in the circadian clock response to the LPS and provide further evidence of the interaction between the circadian clock and immune system.

Circadian clock-gastrointestinal peptide interaction in peripheral tissues and the brain

Food intake and sleep are two mutually exclusive behaviors and both are normally confined to opposing phases of the diurnal cycle. The temporal coordination of behavior and physiology along the 24-h day-night cycle is organized by a network of circadian clocks that orchestrate transcriptional programs controlling cellular physiology. Many of the peptide hormones of the gastrointestinal tract are not only secreted in a circadian fashion, they can also affect circadian clock function in peripheral metabolic tissues and the brain, thus providing metabolic feedback to metabolic and neurobehavioral circuits. In this review, we summarize the current knowledge on this gastrointestinal peptide crosstalk and its potential role in the coordination of nutrition and the maintenance of metabolic homeostasis.

Circadian regulation of mouse suprachiasmatic nuclei neuronal states shapes responses to orexin

Our knowledge of how circadian and homeostatic brain circuits interact to temporally organize physiology and behavior is limited. Progress has been made with the determination that lateral hypothalamic orexin (OXA) neurons control arousal and appetitive states, while suprachiasmatic nuclei (SCN) neurons function as the master circadian clock. During the day, SCN neurons exhibit heterogeneity in spontaneous resting membrane potential (RMP), with some neurons becoming severely depolarized (hyperexcited) and ceasing to fire action potentials (APs), while other neurons rest at moderate RMP and fire APs. Intriguingly, the day phase is when the SCN clock is most readily influenced by arousal, but it is unclear if and how heterogeneity in the excitability state of SCN neurons shapes their response to arousal signals, such as OXA. In whole‐cell recordings we show that during the day OXA recruits GABA‐GABA_A receptor signaling to suppress the RMP of hyperexcited silent as well as moderately hyperpolarized AP‐firing SCN neurons. In the AP‐firing neurons, OXA hyperpolarized and silenced these SCN cells, while in the hyperexcited silent neurons OXA suppressed the RMP of these cells and evoked either AP‐firing, depolarized low‐amplitude membrane oscillations, or continued silence at a reduced RMP. These results demonstrate how the resting state of SCN neurons determines their response to OXA, and illustrate that the inhibitory action of this neurochemical correlate of arousal can trigger paradoxical AP firing.

Melatonin Signal Transduction Pathways Require E-Box-Mediated Transcription of Per1 and Per2 to Reset the SCN Clock at Dusk

Melatonin is released from the pineal gland into the circulatory system at night in the absence of light, acting as “hormone of darkness” to the brain and body. Melatonin also can regulate circadian phasing of the suprachiasmatic nucleus (SCN). During the day-to-night transition, melatonin exposure advances intrinsic SCN neural activity rhythms via the melatonin type-2 (MT₂) receptor and downstream activation of protein kinase C (PKC). The effects of melatonin on SCN phasing have not been linked to daily changes in the expression of core genes that constitute the molecular framework of the circadian clock. Using real-time RT-PCR, we found that melatonin induces an increase in the expression of two clock genes, Period 1 (Per1) and Period 2 (Per2). This effect occurs at CT 10, when melatonin advances SCN phase, but not at CT 6, when it does not. Using anti-sense oligodeoxynucleotides (α ODNs) to Per 1 and Per 2, as well as to E-box enhancer sequences in the promoters of these genes, we show that their specific induction is necessary for the phase-altering effects of melatonin on SCN neural activity rhythms in the rat. These effects of melatonin on Per1 and Per2 were mediated by PKC. This is unlike day-active non-photic signals that reset the SCN clock by non-PCK signal transduction mechanisms and by decreasing Per1 expression. Rather, this finding extends roles for Per1 and Per2, which are critical to photic phase-resetting, to a nonphotic zeitgeber, melatonin, and suggest that the regulation of these clock gene transcripts is required for clock resetting by diverse regulatory cues.

Neurobiology of the Sleep-Wake Cycle: Sleep Architecture, Circadian Regulation, and Regulatory Feedback

This mini-review article presents the remarkable progress that has been made in the past decade in our understanding of the neural circuitry underlying the regulation of sleep-wake states and circadian control of behaviors. Following a brief introduction to sleep architecture and physiology, the authors describe the neural circuitry and neurotransmitters that regulate sleep and cortical arousal (i.e., wakefulness). They next examine how sleep and wakefulness are regulated by mutual inhibition between sleep-and arousal-promoting circuitry and how this interaction functions analogously to an electronic “flip-flop” switch that ensures behavioral state stability. The authors then discuss the role of circadian and homeostatic processes in the consolidation of sleep, including the physiologic basis of homeostatic sleep drive (i.e., wake-dependent increase in sleep propensity) and the role of the SCN in the circadian regulation of sleep-wake cycles. Finally, they describe the hypothalamic circuitry for the integration of photic and nonphotic environmental time cues and how this integration allows organisms to sculpt patterns of rest-activity and sleep-wake cycles that are optimally adaptive.

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.

Acute morphine affects the rat circadian clock via rhythms of phosphorylated ERK1/2 and GSK3β kinases and Per1 expression in the rat suprachiasmatic nucleus

BACKGROUND AND PURPOSE

Opioids affect the circadian clock and may change the timing of many physiological processes. This study was undertaken to investigate the daily changes in sensitivity of the circadian pacemaker to an analgesic dose of morphine, and to uncover a possible interplay between circadian and opioid signalling.

EXPERIMENTAL APPROACH

A time-dependent effect of morphine (1 mg·kg(-1) , i.p.) applied either during the day or during the early night was followed, and the levels of phosphorylated ERK1/2, GSK3β, c-Fos and Per genes were assessed by immunohistochemistry and in situ hybridization. The effect of morphine pretreatment on light-induced pERK and c-Fos was examined, and day/night difference in activity of opioid receptors was evaluated by [(35) S]-GTPγS binding assay.

KEY RESULTS

Morphine stimulated a rise in pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus (SCN) when applied during the day but significantly reduced both kinases when applied during the night. Morphine at night transiently induced Period1 but not Period2 in the SCN and did not attenuate the light-induced level of pERK1/2 and c-Fos in the SCN. The activity of all three principal opioid receptors was high during the day but decreased significantly at night, except for the δ receptor. Finally, we demonstrated daily profiles of pERK1/2 and pGSK3β levels in the rat ventrolateral and dorsomedial SCN.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that the phase-shifting effect of opioids may be mediated via post-translational modification of clock proteins by means of activated ERK1/2 and GSK3β.

Activation of growth hormone secretagogue receptor induces time-dependent clock phase delay in mice

Early studies have reported a phase-shifting effect of growth hormone secretagogues (GHSs). This study aimed to determine the mechanism of action of GHSs. We examined the response of the hypothalamic suprachiasmatic nuclei (SCN) to growth hormone releasing peptide-6 (GHRP-6) by assessing effects on the phase of locomotor activity rhythms, SCN neuronal discharges, and the potential signaling pathways involved in the drug action on circadian rhythms. The results showed that bolus administration of GHRP-6 (100 μg/kg ip) at the beginning of subjective night (CT12) induced a phase delay of the free-running rhythms in male C57BL/6J mice under constant darkness, but did not elicit phase shift at other checked circadian time (CT) points. The phase-delay effect of GHRP-6 was abolished by d-(+)-Lys-GHRP-6 (GHS receptor antagonist), KN-93 [calcium/calmodulin-dependent protein kinase II (CaMK) II inhibitor], or anti-phosphorylated (p)-cAMP response element-binding protein (CREB) antibody. Further analyses demonstrated that GHRP-6 at CT12 induced higher calcium mobilization and neuronal discharge in the SCN compared with that at CT6, decreased the levels of glutamate and γ-aminobutyric acid, increased the levels of p-CaMKII, p-CREB, and period 1, and delayed the circadian expressions of circadian locomotor output cycles kaput, Bmal1, and prokineticin 2 in the SCN; these signaling changes resulted in behavioral phase delay. Collectively, GHRP-6 induces a CT-dependent phase delay via activating GHS receptor and the downstream signaling, which is partially similar to the signaling cascade of light-induced phase delay at early night. These novel observations may help to better understand the role of GHSs in circadian physiology.

The circadian factor Period 2 modulates p53 stability and transcriptional activity in unstressed cells

Period 2 forms a trimeric complex with p53 and Mdm2. As a result, p53’s transcriptional activity and stability are modulated in unstressed cells, ensuring that basal levels are present if a p53-mediated response is needed. These data provide evidence of cross-talk between circadian and checkpoint components, adding a level of regulation to the checkpoint.

Human Period 2 (hPer2) is a transcriptional regulator at the core of the circadian clock mechanism that is responsible for generating the negative feedback loop that sustains the clock. Its relevance to human disease is underlined by alterations in its function that affect numerous biochemical and physiological processes. When absent, it results in the development of various cancers and an increase in the cell's susceptibility to genotoxic stress. Thus we sought to define a yet-uncharacterized checkpoint node in which circadian components integrate environmental stress signals to the DNA-damage response. We found that hPer2 binds the C-terminal half of human p53 (hp53) and forms a stable trimeric complex with hp53’s negative regulator, Mdm2. We determined that hPer2 binding to hp53 prevents Mdm2 from being ubiquitinated and targeting hp53 by the proteasome. Down-regulation of hPer2 expression directly affects hp53 levels, whereas its overexpression influences both hp53 protein stability and transcription of targeted genes. Overall our findings place hPer2 directly at the heart of the hp53-mediated response by ensuring that basal levels of hp53 are available to precondition the cell when a rapid, hp53-mediated, transcriptional response is needed.

Acute suppressive and long-term phase modulation actions of orexin on the mammalian circadian clock

Circadian and homeostatic neural circuits organize the temporal architecture of physiology and behavior, but knowledge of their interactions is imperfect. For example, neurons containing the neuropeptide orexin homeostatically control arousal and appetitive states, while neurons in the suprachiasmatic nuclei (SCN) function as the brain's master circadian clock. The SCN regulates orexin neurons so that they are much more active during the circadian night than the circadian day, but it is unclear whether the orexin neurons reciprocally regulate the SCN clock. Here we show both orexinergic innervation and expression of genes encoding orexin receptors (OX1 and OX2) in the mouse SCN, with OX1 being upregulated at dusk. Remarkably, we find through in vitro physiological recordings that orexin predominantly suppresses mouse SCN Period1 (Per1)-EGFP-expressing clock cells. The mechanisms underpinning these suppressions vary across the circadian cycle, from presynaptic modulation of inhibitory GABAergic signaling during the day to directly activating leak K(+) currents at night. Orexin also augments the SCN clock-resetting effects of neuropeptide Y (NPY), another neurochemical correlate of arousal, and potentiates NPY's inhibition of SCN Per1-EGFP cells. These results build on emerging literature that challenge the widely held view that orexin signaling is exclusively excitatory and suggest new mechanisms for avoiding conflicts between circadian clock signals and homeostatic cues in the brain.

How to fix a broken clock

Fortunate are those who rise out of bed to greet the morning light well rested with the energy and enthusiasm to drive a productive day. Others, however, depend on hypnotics for sleep and require stimulants to awaken lethargic bodies. Sleep/wake disruption is a common occurrence in healthy individuals throughout their lifespan and is also a comorbid condition to many diseases (neurodegenerative) and psychiatric disorders (depression and bipolar). There is growing concern that chronic disruption of the sleep/wake cycle contributes to more serious conditions including diabetes (type 2), cardiovascular disease, and cancer. A poorly functioning circadian system resulting in misalignments in the timing of clocks throughout the body may be at the root of the problem for many people. In this article we discuss environmental (light therapy) and lifestyle changes (scheduled meals, exercise, and sleep) as interventions to help fix a broken clock. We also discuss the challenges and potential for future development of pharmacological treatments to manipulate this key biological system.

Neuropeptide Y-induced phase shifts of PER2::LUC rhythms are mediated by long-term suppression of neuronal excitability in a phase-specific manner

Endogenous circadian rhythms are entrained to the 24-h light/dark cycle by both light and nonphotic stimuli. During the day, nonphotic stimuli, such as novel wheel-induced exercise, produce large phase advances. Neuropeptide Y (NPY) release from the thalamus onto suprachiasmatic nucleus (SCN) neurons at least partially mediates this nonphotic signal. The authors examined the hypothesis that NPY-induced phase advances are accompanied by suppression of PER2 and are mediated by long-term depression of neuronal excitability in a phase-specific manner. First, it was found that NPY-induced phase advances in PER2::LUC SCN cultures are largest when NPY (2.35 µM) is given in the early part of the day (circadian time [CT] 0-6). In addition, PER2::LUC levels in NPY-treated (compared to vehicle-treated) samples were suppressed beginning 6-7 h after treatment. Similar NPY application to organotypic Per1::GFP SCN cultures resulted in long-term suppression of spike rate of green fluorescent protein-positive (GFP+) cells when slices were treated with NPY during the early or middle of the day (zeitgeber time [ZT] 2 or 6), but not during the late day (ZT 10). Furthermore, 1-h bath application of NPY to acute SCN brain slices decreased general neuronal activity measured through extracellular recordings. Finally, NPY-induced phase advances of PER2::LUC rhythms were blocked by latent depolarization with 34.5 mM K(+) 3 h after NPY application. These results suggest that NPY-induced phase advances may be mediated by long-term depression of neuronal excitability. This model is consistent with findings in other brain regions that NPY-induced persistent hyperpolarization underlies mechanisms of energy homeostasis, anxiety-related behavior, and thalamocortical synchronous firing.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

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.

On the intrinsic regulation of neuropeptide Y release in the mammalian suprachiasmatic nucleus circadian clock

Timing of the circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic and non-photic inputs. Of these, neuropeptide Y (NPY) signaling from the intergeniculate leaflet (IGL) to the SCN plays a prominent role. Although NPY is critical to clock regulation, neither the mechanisms modulating IGL NPY neuronal activity nor the nature of regulatory NPY signaling in the SCN clock are understood, as NPY release in the SCN has never been measured. Here, microdialysis procedures for in vivo measurement of NPY were used in complementary experiments to address these questions. First, neuronal release of NPY in the hamster SCN was rhythmic under a 14L : 10D photocycle, with the acrophase soon after lights-on and the nadir at midday. No rhythmic fluctuation in NPY occurred under constant darkness. Second, a behavioral phase-resetting stimulus (wheel-running at midday that induces IGL serotonin release) acutely stimulated SCN NPY release. Third, bilateral IGL microinjection of the serotonin agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) (another non-photic phase-resetting stimulant), at midday enhanced SCN NPY release. Conversely, similar application of the serotonin antagonist, metergoline, abolished wheel-running-induced SCN NPY release. IGL microinjection of the GABA agonist, muscimol, suppressed SCN NPY release. These results support an intra-IGL mechanism whereby behavior-induced serotonergic activity suppresses inhibitory GABAergic transmission, promoting NPY activity and subsequent phase resetting. Collectively, these results confirm IGL-mediated NPY release in the SCN and verify that its daily rhythm of release is dependent upon the 14L : 10D photocycle, and that it is modulated by appropriately-timed phase-resetting behavior, probably mediated by serotonergic activation of NPY units in the IGL.

Endogenous peptide discovery of the rat circadian clock: a focused study of the suprachiasmatic nucleus by ultrahigh performance tandem mass spectrometry

Understanding how a small brain region, the suprachiasmatic nucleus (SCN), can synchronize the body's circadian rhythms is an ongoing research area. This important time-keeping system requires a complex suite of peptide hormones and transmitters that remain incompletely characterized. Here, capillary liquid chromatography and FTMS have been coupled with tailored software for the analysis of endogenous peptides present in the SCN of the rat brain. After ex vivo processing of brain slices, peptide extraction, identification, and characterization from tandem FTMS data with <5-ppm mass accuracy produced a hyperconfident list of 102 endogenous peptides, including 33 previously unidentified peptides, and 12 peptides that were post-translationally modified with amidation, phosphorylation, pyroglutamylation, or acetylation. This characterization of endogenous peptides from the SCN will aid in understanding the molecular mechanisms that mediate rhythmic behaviors in mammals.

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.

Organization of cell and tissue circadian pacemakers: a comparison among species

In most animal species, a circadian timing system has evolved as a strategy to cope with 24-hour rhythms in the environment. Circadian pacemakers are essential elements of the timing system and have been identified in anatomically discrete locations in animals ranging from insects to mammals. Rhythm generation occurs in single pacemaker neurons and is based on the interacting negative and positive molecular feedback loops. Rhythmicity in behavior and physiology is regulated by neuronal networks in which synchronization or coupling is required to produce coherent output signals. Coupling occurs among individual clock cells within an oscillating tissue, among functionally distinct subregions within the pacemaker, and between central pacemakers and the periphery. Recent evidence indicates that peripheral tissues can influence central pacemakers and contain autonomous circadian oscillators that contribute to the regulation of overt rhythmicity. The data discussed in this review describe coupling and synchronization mechanisms at the cell and tissue levels. By comparing the pacemaker systems of several multicellular animal species (Drosophila, cockroaches, crickets, snails, zebrafish and mammals), we will explore general organizational principles by which the circadian system regulates a 24-hour rhythmicity.

Expression of 5-HT7 receptor mRNA in the hamster brain: effect of aging and association with calbindin-D28K expression

Aging affects several processes modulated by the 5-HT(7) receptor subtype, including circadian rhythms, learning and memory, and depression. Previously, we showed that aging induces a decrease in the hamster dorsal raphe (DRN) in both 5-HT(7) receptor binding and circadian phase resetting responses to 8-OH-DPAT microinjection. To elucidate the mechanisms underlying the aging decrease in 5-HT(7) receptors, we investigated aging modulation of 5-HT(7) receptor mRNA expression in the DRN, brain regions afferent to the DRN, and brain regions regulating circadian rhythms or memory. In situ hybridization for 5-HT(7) receptor mRNA was performed on coronal sections prepared from the brains of young, middle-aged, and old male Syrian hamsters. 5-HT(7) receptor mRNA expression was quantified by densitometry of X-ray film autoradiograms. The results showed that aging did not significantly affect 5-HT(7) receptor mRNA expression in the DRN or most other brain regions examined, with the exception of the cingulate cortex and paraventricular thalamic nucleus. Within the suprachiasmatic nucleus, the site of the master circadian pacemaker in mammals, 5-HT(7) receptor mRNA expression was localized in a discrete subregion resembling the calbindin subnucleus previously described. A second experiment using adjacent tissue sections showed that 5-HT(7) receptor mRNA and calbindin mRNAs were concentrated in the same region of the SCN, and as well as in the same region of several other brain structures. The localization of 5-HT(7) receptors and calbindin mRNAs within the same regions suggests that the proteins they encode may interact to modulate processes such as circadian timekeeping.

Daily restricted feeding resets the circadian clock in the suprachiasmatic nucleus of CS mice

Circadian rhythms in clock gene expressions in the suprachiasmatic nucleus (SCN) of CS mice and C57BL/6J mice were measured under a daily restricted feeding (RF) schedule in continuous darkness (DD), and entrainment of the SCN circadian pacemaker to RF was examined. After 2-3 wk under a light-dark cycle with free access to food, animals were released into DD and fed for 3 h at a fixed time of day for 3-4 wk. Subsequently, they returned to having free access to food for 2-3 wk. In CS mice, wheel-running rhythms entrained to RF with a stable phase relationship between the activity onset and feeding time, and the rhythms started to free run from the feeding time after the termination of RF. mPer1, mPer2, and mBMAL1 mRNA rhythms in the SCN showed a fixed phase relationship with feeding time, indicating that the circadian pacemaker in the SCN entrained to RF. On the other hand, in C57BL/6J mice, wheel-running rhythms free ran under RF, and clock gene expression rhythms in the SCN showed a stable phase relation not to feeding time but to the behavioral rhythms, indicating that the circadian pacemaker in the SCN did not entrain. These results indicate that the SCN circadian pacemaker of CS mice is entrainable to RF under DD and suggest that CS mice have a circadian clock system that can be reset by a signal associated with feeding time.

Light and GABAAreceptor activation alterPeriodmRNA levels in the SCN of diurnal Nile grass rats

We examined Period (Per) mRNA rhythms in the suprachiasmatic nucleus (SCN) of a diurnal rodent and assessed how phase-shifting stimuli acutely affect SCN Per mRNA using semiquantitative in situ hybridization. First, Per1 and Per2 varied rhythmically in the SCN over the course of one circadian cycle in constant darkness: Per1 mRNA was highest in the early to mid-subjective day, while Per2 mRNA levels peaked in the late subjective day. Second, acute light exposure in the early subjective night significantly increased both Per1 and Per2 mRNA. Third, Per2 but not Per1 levels decreased 1 and 2 h after injection of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol into the SCN during the subjective day. Fourth, muscimol also reduced the light-induced Per2 in the early subjective night, but Per1 induction by light was not significantly affected. Consistent with previous studies, these data demonstrate that diurnal and nocturnal animals show very similar daily patterns of Per mRNA and light-induced Per increases in the SCN. As with light, muscimol alters circadian phase, and daytime phase alterations induced by muscimol are associated with significant decreases in Per2 mRNA. In diurnal animals, muscimol-induced decreases in Per are associated with phase delays rather than advances. The direction of the daytime phase shift may be determined by the relative suppression of Per1 vs. Per2 in SCN cells. As in nocturnal animals, changes in Per1 and Per2 mRNA by photic and non-photic stimuli appear to be associated with circadian phase alteration.

GABAAreceptor activation suppressesPeriod 1mRNA andPeriod 2mRNA in the suprachiasmatic nucleus during the mid-subjective day

The mammalian circadian clock can be entrained by photic and nonphotic environmental time cues. gamma-aminobutyric acid (GABA) is a nonphotic stimulus that induces phase advances in the circadian clock during the middle of the subjective day. Several nonphotic stimuli suppress Period 1- and Period 2 mRNA expression in the suprachiasmatic nucleus (SCN); however, the effect of GABA on Period mRNA is unknown. In the present study we demonstrate that microinjection of the GABA(A) receptor agonist muscimol into the SCN region suppresses the expression of Period 1 mRNA in the hamster. A significant suppression of Period 2 mRNA following microinjection of muscimol was not observed in free-running conditions. However, Period 2 mRNA was significantly reduced following muscimol treatment when animals were maintained under a light cycle and transferred to constant darkness 42 h prior to treatment. An additional study investigated the maximum behavioural phase advance inducible by GABA(A) receptor activation.Together, these data indicate that, like other nonphotic stimuli, GABA suppresses Period 1- and Period 2 mRNA in the SCN.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Potentiation of the resetting effects of light on circadian rhythms of hamsters using serotonin and neuropeptide Y receptor antagonists

Circadian rhythms are entrained by light/dark cycles. In hamsters, the effects of light on circadian rhythms can be modulated by serotonergic input to the suprachiasmatic nucleus from the raphe nuclei and by neuropeptide Y containing afferents to the suprachiasmatic nucleus from the intergeniculate leaflet in the thalamus. In this study we measured effects of compounds acting on serotonergic 1A and neuropeptide Y Y5 receptors to determine if combined serotonergic-neuropeptide Y inhibition could synergistically potentiate effects of light on rhythms. We used mixed serotonergic agonist/antagonists BMY 7378 or NAN-190 as well as a neuropeptide Y Y5 antagonist CP-760,542. Both BMY 7378 and NAN-190 are thought to block serotonin release via acting as agonists at the 5-hydroxytryptamine 1A (5-HT1A) autoreceptors on cells in the raphe, and also block response of target cells by acting as antagonists at post-synaptic 5-HT1A receptors, for example, in the suprachiasmatic nuclei or the intergeniculate leaflet. Replicating prior work, we found that pretreatment with either drug alone increased the phase shift to light at circadian time 19. The combined effect of BMY 7378 and CP-760,542 given prior to light at circadian time 19 was to further potentiate the subsequent phase shift in wheel-running rhythms (the phase shift was 317% of controls; light alone: 1.35 h phase shift vs. BMY 7378, CP-760,542, and light: 4.27 h phase shift). Combined treatment with NAN-190 and CP-760,542 produced a light-induced phase shift 576% of controls (phase shift to light alone: 1.23 h vs. NAN-190, CP-760,542, and light: 7.1 h phase shift). These results suggest that the resetting effects of light on circadian rhythms can be greatly potentiated in hamsters by using pharmacological treatments that block both serotonergic and neuropeptide Y afferents to the suprachiasmatic nuclei.

Effect of long-term exposure to constant dim light on the circadian system of rats

The newly discovered multi-oscillatory nature of the mammalian circadian clock system and the cloning of the genes involved in the molecular mechanism that generates circadian rhythmicity have opened new approaches for understanding how mammals are temporally organized and how the mammalian circadian system reacts to the lack of normal synchronization cues. In the present study we investigated the effects of long-term exposure to constant red dim light on the pattern of the expression of Period 1 in the suprachiasmatic nuclei of the hypothalamus and of Arylalkylamine N-acetyltransferase(Aa-nat) in the retina and pineal gland. Our data demonstrate that Period 1 mRNA expression in the suprachiasmatic nuclei of the hypothalamus was not affected by exposure to constant red dim light for 60 days, whereas Aa-nat mRNA expression in the retina and in the pineal gland was significantly affected, since in some animals (20-30%) Aa-nat mRNA levels were found to be higher during the subjective day. A circadian rhythm of serum melatonin and locomotor activity was present in all the animals tested. In 4 animals serum melatonin levels were high during the subjective day. Our data suggest that long-term exposure to constant red dim light may induce desynchronization between the circadian rhythm of locomotor activity and serum melatonin levels.

AMPA/kainate receptor antagonist DNQX blocks the acute increase of Per2 mRNA levels in most but not all areas of the SCN

The daily light:dark cycle synchronizes the circadian timing system by resetting the phase of the circadian pacemaker on a daily basis. Light acutely increases mRNA levels of the clock genes Per1 and Per2 in the suprachiasmatic nucleus (SCN), the site of the primary circadian pacemaker in mammals. Light is conveyed to the SCN through the retinohypothalamic tract (RHT), an efferent projection from retinal ganglion cells that releases the excitatory amino acid (EAA) neurotransmitter glutamate in the SCN. EAA receptor activation in the SCN is critical for the ability of light to phase-shift the circadian pacemaker. In a previous study, we demonstrated that EAA receptor activation is necessary and sufficient for light to acutely increase Per1 mRNA levels in the SCN. In the current study, we determined whether EAA receptor activation in the SCN is necessary for the ability of light to increase Per2 mRNA levels in the SCN in Syrian hamsters. The NMDA receptor antagonist AP5 and the AMPA/kainate receptor antagonist DNQX inhibited the ability of light and NMDA to acutely increase Per2 mRNA levels in the SCN. In hamsters injected with DNQX, Per1 and Per2 mRNA levels remained slightly elevated in the ventrolateral SCN, suggesting that AMPA/kainate receptor activation in this region is not critical for the effects of light on the circadian pacemaker.

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 dark exposure in the middle of the day on Period1, Period2, and arylalkylamine N-acetyltransferase mRNA levels in the rat suprachiasmatic nucleus and pineal gland

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus contains a central circadian pacemaker, which adjusts circadian rhythms within the body to environmental light-dark cycles. It has been shown that dark exposure in the day causes phase shifts in circadian rhythms, but it does not induce changes in the melatonin levels in the pineal gland. In this study, we examined the effect of dark exposure on two "circadian clock" genes Period1 and Period2 mRNA levels in the rat SCN, and on Period1, Period2, and arylalkylamine N-acetyltransferase (Aa-Nat, the rate-limiting enzyme in melatonin synthesis) gene expression in the pineal gland. Period1 and Period2 mRNA levels were significantly decreased in the SCN after 0.5 and 2 h, respectively, therefore suggesting that changes in those mRNA levels may be the part of the mechanisms of dark-induced phase shifts. Period1 and Aa-Nat mRNA levels in the pineal gland were not affected by darkness, but Period2 was moderately affected. Since Period1 and Aa-Nat mRNA levels in the pineal gland did not respond to dark stimulation, we further examined whether the pineal gland itself is capable of responding to adrenergic stimulation at this time of the day. Isoproterenol significantly induced Period1 and Aa-Nat mRNA levels; however, it did not affect Period2. Although previous studies have reported that during the day the SCN "gates" the dark information reaching the pineal, our data demonstrate that dark information may reach the pineal during the daytime.

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.

The role of Period1 in non-photic resetting of the hamster circadian pacemaker in the suprachiasmatic nucleus

Non-photic stimuli, such as diurnal wheel running in rodents, phase shift the circadian clock and suppress the expression of Per1 in the suprachiasmatic nucleus (SCN). The goal of the present study was to directly decrease Per1 expression using antisense (AS) oligodeoxynucleotides to determine if such suppression produced non-photic phase shifts. Injections of Per1-AS suppressed expression of Per1 within the SCN and produced phase shifts similar to those resulting from other non-photic manipulation, with large phase advances to injections during the subjective day. These results indicate that the decrease in expression of Per1 is a cause rather than a consequence of non-photic phase shifts.

Dark pulse resetting of the suprachiasmatic clock in Syrian hamsters: behavioral phase-shifts and clock gene expression

In mammals, the circadian clock in the suprachiasmatic nuclei (SCN) is mainly synchronized to photic cues provided by the daily light/dark cycle. Phase-shifts produced by light exposure during the night are correlated with rapid induction of two clock genes, Per1 and Per2, in the SCN. Nonphotic stimuli such as behavioral and pharmacological cues, when presented during the subjective day, induce behavioral phase-advances and a down-regulation of Per1 and Per2 expression in the SCN. When applied during the subjective day, dark pulses in continuous light also produce phase-advances. These phase-shifting effects have been interpreted as reflecting either a photic image mirror, nonphotic cues, or a combination of both. Here we evaluated in Syrian hamsters housed in constant light how dark pulses applied in late subjective day affect levels of Per1, Per2 and Cry1 mRNA. Four-hour dark pulses with no access to a wheel produced 1.2+/-0.4 h phase-advances of locomotor activity rhythm while control manipulation induced non-significant shifts (0.1+/-0.2 h). Dark pulses transiently down-regulated Per1 and Per2 mRNA levels in the SCN by 40 and 20% respectively, while the levels of Cry1 mRNA remained unaffected. In behaviorally split hamsters in which Per oscillations were asymmetric between the left and right sides of the SCN, dark pulses reduced Per expression in the half-SCN with high Per. This study shows that exposure during the late subjective day to dark pulses independent of wheel-running have nonphotic-like effects on the SCN clock at both behavioral and molecular levels.

Neuropeptide Y, GABA and circadian phase shifts to photic stimuli

Circadian rhythms can be phase shifted by photic and non-photic stimuli. The circadian clock, anatomically defined as the suprachiasmatic nucleus (SCN), can be phase delayed by light during the early subjective night and phase advanced during the late subjective night. Non-photic stimuli reset the clock when presented during the subjective day. A possible pathway for the non-photic resetting of the clock is thought to originate from the intergeniculate leaflet, which conveys information to the SCN through the geniculohypothalamic tract and utilizes among others neuropeptide Y (NPY) and GABA as neurotransmitters. Photic and non-photic stimuli have been shown to interact during the early and late subjective night. Microinjections of NPY or muscimol, a GABA(A) receptor agonist, into the region of the SCN can attenuate light-induced phase shifts during the early and late subjective night. The precise mechanism for these interactions is unknown. In the current study we investigate the involvement of a GABAergic mechanism in the interaction between NPY and light during the early and late subjective night. Microinjections of NPY significantly attenuated light-induced phase delays and inhibited phase advances (P<0.05). The administration of bicuculline during light exposure, before NPY microinjection did not alter the ability of NPY to attenuate light-induced phase delays and block photic phase advances. These results indicate that NPY attenuates photic phase shifts via a mechanism independent of GABA(A) receptor activation. Furthermore it is evident that NPY influences circadian clock function via differing cellular pathways over the course of a circadian cycle.

Pattern of retinal projections in the California ground squirrel (Spermophilus beecheyi): anterograde tracing study using cholera toxin

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.

Attenuation of circadian light induced phase advances and delays by neuropeptide Y and a neuropeptide Y Y1/Y5 receptor agonist

Circadian rhythms can be synchronised to photic and non-photic stimuli. The circadian clock, anatomically defined as the suprachiasmatic nucleus in mammals, can be phase shifted by light during the night. Non-photic stimuli reset the circadian rhythm during the day. Photic and non-photic stimuli have been shown to interact during the day and night. Precise mechanisms for these complex interactions are unknown. A possible pathway for non-photic resetting of the clock is thought to generate from the intergeniculate leaflet, which conveys information to the suprachiasmatic nucleus (SCN) through the geniculohypothalamic tract and utilises neuropeptide Y (NPY) as its primary neurotransmitter. Interactions between light and NPY were investigated during the early (2 h after activity onset) and late (6 h after activity onset) night in male Syrian hamsters. NPY microinjections into the region of the SCN significantly attenuated light-induced phase delay, during the early subjective night. Phase advances to light were completely inhibited by the administration of NPY during the late night. The precise mechanism by which NPY attenuates or blocks photic phase shifts is unclear, but the NPY Y5 receptor has been implicated in the mediation of this inhibitory effect. The NPY Y1/Y5 receptor agonist, [Leu(31),Pro(34)]NPY, was administered via cannula microinjections following light exposure during the early and late night. [Leu(31),Pro(34)]NPY significantly attenuated phase delays to light during the early night and blocked phase advances during the late night, in a manner similar to NPY. These results show the ability of NPY to attenuate phase shifts to light during the early night and block light-induced phase advances during the late night. Furthermore, this is the first in vivo study implicating the involvement of the NPY Y1/Y5 receptors in the complex interaction of photic and non-photic stimuli during the night. The alteration of photic phase shifts by NPY may influence photic entrainment within the circadian system.

Temporal lobe seizures alter the amplitude and timing of rat behavioral rhythms

Daily rhythms of spontaneous locomotor activity (SLA) in rats were studied before and after an episode of pilocarpine-induced convulsive status epilepticus (SE). A pronounced increase in activity levels during both the light and dark phases was found 1 week after SE as compared with baseline SLA and controls administered saline. Rats with bilateral lesions of the nucleus accumbens (shell) did not exhibit any significant change in SLA 1 week after SE compared with controls. We suggest that during the first week after SE the increase in SLA was induced by abnormal neuronal activity in the hippocampus driving a descending limbic-motor pathway via the nucleus accumbens. EEG recordings revealed high-amplitude interictal spikes in hippocampal CA1. During subsequent weeks, SLA rhythms of nonlesioned chronic epileptic rats remained elevated but progressively normalized over a period of 12 weeks. Although both chronic epileptic and control groups displayed near-24-hour activity patterns under light-dark conditions, significant delays (>4 hour) in acrophase were observed after spontaneous seizures had developed. The phase delay was positively correlated with seizure history and likely the result of postictal hyperactivity associated with seizures during the normal rest period. Despite these changes, cell density in the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) did not differ significantly between epileptic and control groups. In the absence of damage to brain areas directly involved with the regulation of behavioral rhythms, chronic seizure activity presumably alters the timing of activity patterns through a nonphotic mechanism, perhaps involving activation of the SCN or IGL during limbic seizures.

Response of the mouse circadian system to serotonin 1A/2/7 agonists in vivo: surprisingly little

Serotonin (5-HT) is thought to play a role in regulating nonphotic phase shifts and modulating photic phase shifts of the mammalian circadian system, but results with different species (rats vs. hamsters) and techniques (in vivo vs. in vitro; systemic vs. intracerebral drug delivery) have been discordant. Here we examined the effects of the 5-HT1A/7 agonist 8-OH-DPAT and the 5-HT1/2 agonist quipazine on the circadian system in mice, with some parallel experiments conducted with hamsters for comparative purposes. In mice, neither drug, delivered systemically at a range of circadian phases and doses, induced phase shifts significantly different from vehicle injections. In hamsters, quipazine intraperitoneally (i.p.) did not induce phase shifts, whereas 8-OH-DPAT induced phase shifts after i.p. but not intra-SCN injections. In mice, quipazine modestly increased c-Fos expression in the SCN (site of the circadian pacemaker) during the subjective day, whereas 8-OH-DPAT did not affect SCN c-Fos. In hamsters, both drugs suppressed SCN c-Fos in the subjective day. In both species, both drugs strongly induced c-Fos in the paraventricular nucleus (within-subject positive control). 8-OH-DPAT did not significantly attenuate light-induced phase shifts in mice but did in hamsters (between-species positive control). These results indicate that in the intact mouse in vivo, acute activation of 5-HT1A/2/7 receptors in the circadian system is not sufficient to reset the SCN pacemaker or to oppose phase-shifting effects of light. There appear to be significant species differences in the susceptibility of the circadian system to modulation by systemically delivered serotonergics.

Recent developments in our understanding of the physiological role of PP-fold peptide receptor subtypes

The three peptides pancreatic polypeptide (PP), peptide YY (PYY), and neuropeptide Y (NPY) share a similar structure known as the PP-fold. There are four known human G-protein coupled receptors for the PP-fold peptides, namely Y1, Y2, Y4, and Y5, each of them being able to bind at least two of the three endogenous ligands. All three peptides are found in the circulation acting as hormones. Although NPY is only released from neurons, PYY and PP are primarily found in endocrine cells in the gut, where they exert such effects as inhibition of gall bladder secretion, gut motility, and pancreatic secretion. However, when PYY is administered in an experimental setting to animals, cloned receptors, or tissue preparations, it can mimic the effects of NPY in essentially all studies, making it difficult to study the effects of PP-fold peptides and to delineate what receptor and peptide accounts for a particular effect. Initial studies with transgenic animals confirmed the well-established action of NPY on metabolism, food-intake, vascular systems, memory, mood, neuronal excitability, and reproduction. More recently, using transgenic techniques and novel antagonists for the Y1, Y2, and Y5 receptors, NPY has been found to be a key player in the regulation of ethanol consumption and neuronal development.

Contrary to other non-photic cues, acute melatonin injection does not induce immediate changes of clock gene mRNA expression in the rat suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) contain the main clock of the mammalian circadian system. The endogenous oscillation machinery involves interactive positive and negative transcriptional and posttranslational feedback loops involving the clock genes Per1, Per2, Per3, Clock, Bmal1, Cry1 and Cry2. The SCN endogenous oscillation is entrained to 24 h by the light/dark cycle. Light induced regulation of Per1 and Per2 mRNA expression have been suggested to take part in the clock resetting. However, other factors have chronobiotic and synchronizing effects on SCN activity. Especially, the nocturnal pineal gland hormone, melatonin, which is involved in the regulation of both circadian and seasonal rhythms, is known to feedback on the SCN. Melatonin applied on SCN slices immediately phase-shifts their neuronal electrical activity, while daily injections of melatonin to free running rodents resynchronize their locomotor activity to 24 h. To determine whether melatonin feedback control on SCN activity implicates transcriptional regulation of the clock genes, we monitored the expression pattern of Per 1, 2, 3, Bmal1, Cry1 and AVP mRNAs after a single melatonin injection at the end of the subjective day. Results showed that melatonin injection affected none of the mRNA expression pattern during the first circadian night. Per1, Per3, Bmal1 and AVP expression patterns were, however, significantly but differentially affected, during the second subjective night after the melatonin injection. The present results strongly suggest that the immediate phase shifting effect of melatonin on the SCN molecular loop implicates rather post-translational than transcriptional mechanisms.

Attenuation of phase shifts to light by activity or neuropeptide Y: a time course study

Circadian rhythms in mammals can be synchronised to photic and non-photic stimuli. Interactions between photic and behavioural stimuli were investigated during the late subjective night, 6 h after activity onset in Syrian hamsters (CT18). Light pulses of 130 lx for 15 min at this time resulted in phase advance shifts. Novel wheel exposure, for a period of 3 h, following photic stimulation was able to attenuate the phase advancing effects of light. A time delay of up to 60 min between photic and behavioural stimuli also resulted in significant attenuation of light-induced phase shifts (P<0.05). A 90-min interval between stimuli resulted in no significant attenuation. Novel wheel exposure mediates its effects via the intergeniculate leaflet, which conveys information to the SCN and utilises neuropeptide Y (NPY) as its primary neurotransmitter. Phase shifts to light pulses given at CT18 were attenuated by NPY administration. Neuropeptide Y injections up to 60 min post-light exposure significantly attenuated phase shifts by 50% on average. However a 90-min interval between light and NPY microinjection did not significantly affect light-induced phase shifts. These results confirm previous work indicating that novel wheel exposure and NPY administration can modulate light-induced phase shifts during the late night. Further, they show for the first time that the time course for this interaction is similar between wheel running and NPY. Most significantly, our work indicates that the time course in vivo in the late night is similar to that shown previously in vitro during the early night.