Gastrin-releasing peptide phase-shifts suprachiasmatic nuclei neuronal rhythms in vitro

Diverse genetic alteration dysregulates neuropeptide and intracellular signalling in the suprachiasmatic nuclei

In mammals, intrinsic 24 h or circadian rhythms are primarily generated by the suprachiasmatic nuclei (SCN). Rhythmic daily changes in the transcriptome and proteome of SCN cells are controlled by interlocking transcription-translation feedback loops (TTFLs) of core clock genes and their proteins. SCN cells function as autonomous circadian oscillators, which synchronize through intercellular neuropeptide signalling. Physiological and behavioural rhythms can be severely disrupted by genetic modification of a diverse range of genes and proteins in the SCN. With the advent of next generation sequencing, there is unprecedented information on the molecular profile of the SCN and how it is affected by genetically targeted alteration. However, whether the expression of some genes is more readily affected by genetic alteration of the SCN is unclear. Here, using publicly available datasets from recent RNA-seq assessments of the SCN from genetically altered and control mice, we evaluated whether there are commonalities in transcriptome dysregulation. This was completed for four different phases across the 24 h cycle and was augmented by Gene Ontology Molecular Function (GO:MF) and promoter analysis. Common differentially expressed genes (DEGs) and/or enriched GO:MF terms included signalling molecules, their receptors, and core clock components. Finally, examination of the JASPAR database indicated that E-box and CRE elements in the promoter regions of several commonly dysregulated genes. From this analysis, we identify differential expression of genes coding for molecules involved in SCN intra- and intercellular signalling as a potential cause of abnormal circadian rhythms.

Circadian dysregulation and Alzheimer’s disease: A comprehensive review

Alzheimer’s disease (AD), the foremost variant of dementia, has been associated with a menagerie of risk factors, many of which are considered to be modifiable. Among these modifiable risk factors is circadian rhythm, the chronobiological system that regulates sleep‐wake cycles, food consumption timing, hydration timing, and immune responses amongst many other necessary physiological processes. Circadian rhythm at the level of the suprachiasmatic nucleus (SCN), is tightly regulated in the human body by a host of biomolecular substances, principally the hormones melatonin, cortisol, and serotonin. In addition, photic information projected along afferent pathways to the SCN and peripheral oscillators regulates the synthesis of these hormones and mediates the manner in which they act on the SCN and its substructures. Dysregulation of this cycle, whether induced by environmental changes involving irregular exposure to light, or through endogenous pathology, will have a negative impact on immune system optimization and will heighten the deposition of Aβ and the hyperphosphorylation of the tau protein. Given these correlations, it appears that there is a physiologic association between circadian rhythm dysregulation and AD. This review will explore the physiology of circadian dysregulation in the AD brain, and will propose a basic model for its role in AD‐typical pathology, derived from the literature compiled and referenced throughout.

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.

Genesis of the Master Circadian Pacemaker in Mice

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central circadian clock of mammals. It is responsible for communicating temporal information to peripheral oscillators via humoral and endocrine signaling, ultimately controlling overt rhythms such as sleep-wake cycles, body temperature, and locomotor activity. Given the heterogeneity and complexity of the SCN, its genesis is tightly regulated by countless intrinsic and extrinsic factors. Here, we provide a brief overview of the development of the SCN, with special emphasis on the murine system.

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

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

Circuit development in the master clock network of mammals

Daily rhythms are generated by the circadian timekeeping system, which is orchestrated by the master circadian clock in the suprachiasmatic nucleus (SCN) of mammals. Circadian timekeeping is endogenous and does not require exposure to external cues during development. Nevertheless, the circadian system is not fully formed at birth in many mammalian species and it is important to understand how SCN development can affect the function of the circadian system in adulthood. The purpose of the current review is to discuss the ontogeny of cellular and circuit function in the SCN, with a focus on work performed in model rodent species (i.e., mouse, rat, and hamster). Particular emphasis is placed on the spatial and temporal patterns of SCN development that may contribute to the function of the master clock during adulthood. Additional work aimed at decoding the mechanisms that guide circadian development is expected to provide a solid foundation upon which to better understand the sources and factors contributing to aberrant maturation of clock function.

Characterization of NMB, GRP and their receptors (BRS3, NMBR and GRPR) in chickens

The two structurally and functionally related peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB) play critical roles in many physiological/pathological processes in mammals. However, the information regarding the expression and functionality of avian NMB, GRP and their receptors is limited. Here, we characterized cNMB, cGRP and their receptors (cNMBR, cGRPR and cBRS3) in chickens. Our results showed that: (1) cNMBR and cGRPR expressed in CHO cells could be potently activated by cNMB and cGRP, respectively, as monitored by cell-based luciferase reporter assays, indicating that cNMBR and cGRPR are cNMB- and cGRP-specific receptors; strikingly, BRS3 of chickens (/spotted gars), which is orthologous to mouse bombesin receptor subtype-3 (BRS3), could be potently activated by GRP and NMB, demonstrating that both peptides are the endogenous ligands for chicken (/spotted gar) BRS3; (2) quantitative real-time PCR (qPCR) revealed that cGRPR is widely expressed in chicken tissues with abundant expression in the ovary, pancreas, proventriculus, spinal cord and brain, whereas cNMB, cNMBR and cBRS3 are mainly expressed in the brain and testes; (3) interestingly, qPCR, Western blot and immunostaining revealed that cGRP is predominantly expressed in the anterior pituitary and mainly localized to LH-cells, suggesting that cGRP is likely a novel pituitary hormone in chickens. In summary, our data help to uncover the roles of GRP, NMB and their receptors in birds, and provide the first persuasive evidence from an evolutionary prospective that in vertebrates, GRP and NMB are the endogenous ligands for BRS3, an orphan receptor that has puzzled endocrinologists for more than two decades.

Optogenetic interrogation reveals separable G-protein-dependent and -independent signalling linking G-protein-coupled receptors to the circadian oscillator

BACKGROUND

Endogenous circadian oscillators distributed across the mammalian body are synchronised among themselves and with external time via a variety of signalling molecules, some of which interact with G-protein-coupled receptors (GPCRs). GPCRs can regulate cell physiology via pathways originating with heterotrimeric G-proteins or β-arrestins. We applied an optogenetic approach to determine the contribution of these two signalling modes on circadian phase.

RESULTS

We employed a photopigment (JellyOp) that activates Gαs signalling with better selectivity and higher sensitivity than available alternatives, and a point mutant of this pigment (F112A) biased towards β-arrestin signalling. When expressed in fibroblasts, both native JellyOp and the F112A arrestin-biased mutant drove light-dependent phase resetting in the circadian clock. Shifts induced by the two opsins differed in their circadian phase dependence and the degree to which they were associated with clock gene induction.

CONCLUSIONS

Our data imply separable G-protein and arrestin inputs to the mammalian circadian clock and establish a pair of optogenetic tools suitable for manipulating Gαs- and β-arrestin-biased signalling in live cells.

Collective timekeeping among cells of the master circadian clock

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

Modeling the Behavior of Coupled Cellular Circadian Oscillators in the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) in the hypothalamus is the site of the master circadian clock in mammals, a complex tissue composed of multiple, coupled, single-cell circadian oscillators. Mathematical modeling is now providing insights on how individual SCN cells might interact and assemble to create an integrated pacemaker that governs the circadian behavior of whole animals. In this article, we will discuss the neurobiological constraints for modeling SCN behavior, system precision, implications of cellular heterogeneity, and analysis of heterogeneously coupled oscillator networks. Mathematical approaches will be critical for better understanding intercellular interactions within the SCN.

Circadian rhythms and attention deficit hyperactivity disorder: The what, the when and the why

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental condition characterised by impulsivity, inattention and hyperactivity. Aside from these core psychopathologies, sleep disturbances are found to be highly comorbid with ADHD, and indeed dysregulated sleep may contribute to some of the symptoms of the disorder. It is not clear how sleep disturbances come to be so common in ADHD, but one putative mechanism is through the circadian timekeeping system. This system underpins the generation of near 24-hour rhythms in a host of physiological, behavioural and psychological parameters, and is a key determinant of the sleep/wake cycle. In this paper we review the evidence for sleep and circadian rhythm disturbance in ADHD, examine the possible mechanistic links between these factors and the disorder and discuss future directions through which the circadian clock can be targetted for ADHD symptom relief.

Phase shifts to light are altered by antagonists to neuropeptide receptors

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is a heterogeneous structure. Two key populations of cells that receive retinal input and are believed to participate in circadian responses to light are cells that contain vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP). VIP acts primarily through the VPAC2 receptor, while GRP works primarily through the BB2 receptor. Both VIP and GRP phase shift the circadian clock in a manner similar to light when applied to the SCN, both in vivo and in vitro, indicating that they are sufficient to elicit photic-like phase shifts. However, it is not known if they are necessary signals for light to elicit phase shifts. Here we test the hypothesis that GRP and VIP are necessary signaling components for the photic phase shifting of the hamster circadian clock by examining two antagonists for each of these neuropeptides. The BB2 antagonist PD176252 had no effect on light-induced delays on its own, while the BB2 antagonist RC-3095 had the unexpected effect of significantly potentiating both phase delays and advances. Neither of the VIP antagonists ([d-p-Cl-Phe6, Leu17]-VIP, or PG99-465) altered phase shifting responses to light on their own. When the BB2 antagonist PD176252 and the VPAC2 antagonist PG99-465 were delivered together to the SCN, phase delays were significantly attenuated. These results indicate that photic phase shifting requires participation of either VIP or GRP; phase shifts to light are only impaired when signalling in both pathways are inhibited. Additionally, the unexpected potentiation of light-induced phase shifts by RC-3095 should be investigated further for potential chronobiotic applications.

In synch but not in step: Circadian clock circuits regulating plasticity in daily rhythms

The suprachiasmatic nucleus (SCN) is a network of neural oscillators that program daily rhythms in mammalian behavior and physiology. Over the last decade much has been learned about how SCN clock neurons coordinate together in time and space to form a cohesive population. Despite this insight, much remains unknown about how SCN neurons communicate with one another to produce emergent properties of the network. Here we review the current understanding of communication among SCN clock cells and highlight a collection of formal assays where changes in SCN interactions provide for plasticity in the waveform of circadian rhythms in behavior. Future studies that pair analytical behavioral assays with modern neuroscience techniques have the potential to provide deeper insight into SCN circuit mechanisms.

Insights into bombesin receptors and ligands: Highlighting recent advances

This following article is written for Prof. Abba Kastin's Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof. Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in Prof. Kastin's Handbook of Biological Active Peptides [137,138,331].

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.

The suprachiasmatic nuclei as a seasonal clock

In mammals, the suprachiasmatic nucleus (SCN) contains a central clock that synchronizes daily (i.e., 24-h) rhythms in physiology and behavior. SCN neurons are cell-autonomous oscillators that act synchronously to produce a coherent circadian rhythm. In addition, the SCN helps regulate seasonal rhythmicity. Photic information is perceived by the SCN and transmitted to the pineal gland, where it regulates melatonin production. Within the SCN, adaptations to changing photoperiod are reflected in changes in neurotransmitters and clock gene expression, resulting in waveform changes in rhythmic electrical activity, a major output of the SCN. Efferent pathways regulate the seasonal timing of breeding and hibernation. In humans, seasonal physiology and behavioral rhythms are also present, and the human SCN has seasonally rhythmic neurotransmitter levels and morphology. In summary, the SCN perceives and encodes changes in day length and drives seasonal changes in downstream pathways and structures in order to adapt to the changing seasons.

Lhx1 controls terminal differentiation and circadian function of the suprachiasmatic nucleus

Vertebrate circadian rhythms are organized by the hypothalamic suprachiasmatic nucleus (SCN). Despite its physiological importance, SCN development is poorly understood. Here, we show that Lim homeodomain transcription factor 1 (Lhx1) is essential for terminal differentiation and function of the SCN. Deletion of Lhx1 in the developing SCN results in loss of SCN-enriched neuropeptides involved in synchronization and coupling to downstream oscillators, among other aspects of circadian function. Intact, albeit damped, clock gene expression rhythms persist in Lhx1-deficient SCN; however, circadian activity rhythms are highly disorganized and susceptible to surprising changes in period, phase, and consolidation following neuropeptide infusion. Our results identify a factor required for SCN terminal differentiation. In addition, our in vivo study of combinatorial SCN neuropeptide disruption uncovered synergies among SCN-enriched neuropeptides in regulating normal circadian function. These animals provide a platform for studying the central oscillator's role in physiology and cognition.

Site-specific effects of gastrin-releasing peptide in the suprachiasmatic nucleus

The effects of gastrin-releasing peptide (GRP) on the circadian clock in the suprachiasmatic nucleus (SCN) are dependent on the activation of N-methyl-d-aspartate (NMDA) receptors in the SCN. In this study, the interaction between GRP, glutamate and serotonin in the regulation of circadian phase in Syrian hamsters was evaluated. Microinjection of GRP into the third ventricle induced c-fos and p-ERK expression throughout the SCN. Coadministration of an NMDA antagonist or 8-hydroxy-2-di-n-propylamino-tetralin [a serotonin (5-HT)1A,7 agonist, DPAT] with GRP limited c-fos expression in the SCN to a region dorsal to GRP cell bodies. Similar to the effects of NMDA antagonists, DPAT attenuated GRP-induced phase shifts in the early night, suggesting that the actions of serotonin on the photic phase shifting mechanism occur downstream from retinorecipient cells. c-fos and p-ERK immunoreactivity in the supraoptic (SON) and paraventricular hypothalamic nuclei also increased following ventricular microinjection of GRP. Because of this finding, a second set of experiments was designed to test a potential role for the SON in the regulation of clock function. Syrian hamsters were given microinjections of GRP into the peri-SON during the early night. GRP-induced c-fos activity in the SCN was similar to that following ventricular administration of GRP. GRP or bicuculline (a γ-aminobutyric acidA antagonist) administered near the SON during the early night elicited phase delays of circadian activity rhythms. These data suggest that GRP-induced phase-resetting is dependent on levels of glutamatergic and serotonergic neurotransmission in the SCN and implicate activity in the SON as a potential regulator of photic signaling in the SCN.

NMDA and PACAP receptor signaling interact to mediate retinal-induced scn cellular rhythmicity in the absence of light

The "core" region of the suprachiasmatic nucleus (SCN), a central clock responsible for coordinating circadian rhythms, shows a daily rhythm in phosphorylation of extracellular regulated kinase (pERK). This cellular rhythm persists under constant darkness and, despite the absence of light, is dependent upon inputs from the eye. The neural signals driving this rhythmicity remain unknown and here the roles of glutamate and PACAP are examined. First, rhythmic phosphorylation of the NR1 NMDA receptor subunit (pNR1, a marker for receptor activation) was shown to coincide with SCN core pERK, with a peak at circadian time (CT) 16. Enucleation and intraocular TTX administration attenuated the peak in the pERK and pNR1 rhythms, demonstrating that activation of the NMDA receptor and ERK in the SCN core at CT16 are dependent on retinal inputs. In contrast, ERK and NR1 phosphorylation in the SCN shell region were unaffected by these treatments. Intraventricular administration of the NMDA receptor antagonist MK-801 also attenuated the peak in SCN core pERK, indicating that ERK phosphorylation in this region requires NMDA receptor activation. As PACAP is implicated in photic entrainment and is known to modulate glutamate signaling, the effects of a PAC1 receptor antagonist (PACAP 6-38) on SCN core pERK and pNR1 also were examined. PACAP 6-38 administration attenuated SCN core pERK and pNR1, suggesting that PACAP induces pERK directly, and indirectly via a modulation of NMDA receptor signaling. Together, these data indicate that, in the absence of light, retinal-mediated NMDA and PAC1 receptor activation interact to induce cellular rhythms in the SCN core. These results highlight a novel function for glutamate and PACAP release in the hamster SCN apart from their well-known roles in the induction of photic circadian clock resetting.

Gastrin-releasing peptide acts via postsynaptic BB2 receptors to modulate inward rectifier K+ and TRPV1-like conductances in rat paraventricular thalamic neurons

Gastrin-releasing peptide (GRP) is a bombesin-like peptide with a widespread distribution in mammalian CNS, where it has a role in food intake, circadian rhythm generation, fear memory, itch sensation and sexual behaviour. While it has been established that GRP predominantly excites neurons, details of the membrane mechanism involved in this action remain largely undefined. We used perforated patch clamp recording in acute brain slice preparations to investigate GRP-affected receptors and ionic conductances in neurons of the rat paraventricular thalamic nucleus (PVT). PVT is a component of the midline and intralaminar thalamus that participates in arousal, motivational drives and stress responses, and exhibits a prominence of GRP-like immunoreactive fibres. Exposure of PVT neurons to low nanomolar concentrations of GRP induced sustained TTX-resistant membrane depolarizations that could trigger rhythmic burst discharges or tonic firing. Membrane current analyses in voltage clamp revealed an underlying postsynaptic bombesin type 2 receptor-mediated inward current that resulted from the simultaneous suppression of a Ba(2+)-sensitive inward rectifier K(+) conductance and activation of a non-selective cation conductance with biophysical and pharmacological properties reminiscent of transient receptor potential vanilloid (TRPV) 1. A role for a TRPV1-like conductance was further implied by a significant suppressant influence of a TRPV1 antagonist on GRP-induced membrane depolarization and rhythmic burst or tonic firing. The results provide a detailed picture of the receptor and ionic conductances that are involved in GRP's excitatory action in midline thalamus.

Quantitative peptidomics for discovery of circadian-related peptides from the rat suprachiasmatic nucleus

In mammals the suprachiasmatic nucleus (SCN), the master circadian clock, is sensitive to light input via the optic chiasm and synchronizes many daily biological rhythms. Here we explore variations in the expression levels of neuropeptides present in the SCN of rats using a label-free quantification approach that is based on integrating peak intensities between daytime, Zeitgeber time (ZT) 6, and nighttime, ZT 18. From nine analyses comparing the levels between these two time points, 10 endogenous peptides derived from eight prohormones exhibited significant differences in their expression levels (adjusted p-value <0.05). Of these, seven peptides derived from six prohormones, including GRP, PACAP, and CART, exhibited ≥ 30% increases at ZT 18, and the VGRPEWWMDYQ peptide derived from proenkephalin A showed a >50% increase at nighttime. Several endogenous peptides showing statistically significant changes in this study have not been previously reported to alter their levels as a function of time of day, nor have they been implicated in prior functional SCN studies. This information on peptide expression changes serves as a resource for discovering unknown peptide regulators that affect circadian rhythms in the SCN.

The clock shop: coupled circadian oscillators

Daily rhythms in neural activity underlie circadian rhythms in sleep-wake and other daily behaviors. The cells within the mammalian suprachiasmatic nucleus (SCN) are intrinsically capable of 24-h timekeeping. These cells synchronize with each other and with local environmental cycles to drive coherent rhythms in daily behaviors. Recent studies have identified a small number of neuropeptides critical for this ability to synchronize and sustain coordinated daily rhythms. This review highlights the roles of specific intracellular and intercellular signals within the SCN that underlie circadian synchrony.

Cell autonomy and synchrony of suprachiasmatic nucleus circadian oscillators

The suprachiasmatic nucleus (SCN) of the hypothalamus is the site of the master circadian pacemaker in mammals. The individual cells of the SCN are capable of functioning independently from one another and therefore must form a cohesive circadian network through intercellular coupling. The network properties of the SCN lead to coordination of circadian rhythms among its neurons and neuronal subpopulations. There is increasing evidence for multiple interconnected oscillators within the SCN, and in this review we will highlight recent advances in our knowledge of the complex organization and function of the cellular and network-level SCN clock. Understanding the way in which synchrony is achieved between cells in the SCN will provide insight into the means by which this important nucleus orchestrates circadian rhythms throughout the organism.

Gastrin-releasing peptide modulates fast delayed rectifier potassium current in Per1-expressing SCN neurons

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) drives and maintains 24-h physiological rhythms, the phases of which are set by the local environmental light-dark cycle. Gastrin-releasing peptide (GRP) communicates photic phase setting signals in the SCN by increasing neurophysiological activity of SCN neurons. Here, the ionic basis for persistent GRP-induced changes in neuronal activity was investigated in SCN slice cultures from Per1::GFP reporter mice during the early night. Recordings from Per1 -fluorescent neurons in SCN slices several hours after GRP treatment revealed a significantly greater action potential frequency, a significant increase in voltage-activated outward current at depolarized potentials, and a significant increase in 4-aminopyridine-sensitive fast delayed rectifier (fDR) potassium currents when compared to vehicle-treated slices. In addition, the persistent increase in spike rate following early-night GRP application was blocked in SCN neurons from mice deficient in Kv3 channel proteins. Because fDR currents are regulated by the clock and are elevated in amplitude during the day, the present results support the model that GRP delays the phase of the clock during the early night by prolonging day-like membrane properties of SCN cells. Furthermore, these findings implicate fDR currents in the ionic basis for GRP-mediated entrainment of the primary mammalian circadian pacemaker.

Daily and seasonal adaptation of the circadian clock requires plasticity of the SCN neuronal network

Circadian rhythms are an essential property of many living organisms, and arise from an internal pacemaker, or clock. In mammals, this clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus, and generates an intrinsic circadian rhythm that is transmitted to other parts of the CNS. We will review the evidence that basic adaptive functions of the circadian system rely on functional plasticity in the neuronal network organization, and involve a change in phase relation among oscillatory neurons. We will illustrate this for: (i) photic entrainment of the circadian clock to the light-dark cycle; and (ii) seasonal adaptation of the clock to changes in day length. Molecular studies have shown plasticity in the phase relation between the ventral and dorsal SCN during adjustment to a shifted environmental cycle. Seasonal adaptation relies predominantly on plasticity in the phase relation between the rostral and caudal SCN. Electrical activity is integrated in the SCN, and appears to reflect the sum of the differently phased molecular expression patterns. While both photic entrainment and seasonal adaptation arise from a redistribution of SCN oscillatory activity patterns, different neuronal coupling mechanisms are employed, which are reviewed in the present paper.

Vasoactive intestinal polypeptide requires parallel changes in adenylate cyclase and phospholipase C to entrain circadian rhythms to a predictable phase

Circadian oscillations in the suprachiasmatic nucleus (SCN) depend on transcriptional repression by Period (PER)1 and PER2 proteins within single cells and on vasoactive intestinal polypeptide (VIP) signaling between cells. Because VIP is released by SCN neurons in a circadian pattern, and, after photic stimulation, it has been suggested to play a role in the synchronization to environmental light cycles. It is not known, however, if or how VIP entrains circadian gene expression or behavior. Here, we tested candidate signaling pathways required for VIP-mediated entrainment of SCN rhythms. We found that single applications of VIP reset PER2 rhythms in a time- and dose-dependent manner that differed from light. Unlike VIP-mediated signaling in other cell types, simultaneous antagonism of adenylate cyclase and phospholipase C activities was required to block the VIP-induced phase shifts of SCN rhythms. Consistent with this, VIP rapidly increased intracellular cAMP in most SCN neurons. Critically, daily VIP treatment entrained PER2 rhythms to a predicted phase angle within several days, depending on the concentration of VIP and the interval between VIP applications. We conclude that VIP entrains circadian timing among SCN neurons through rapid and parallel changes in adenylate cyclase and phospholipase C activities.

Circadian integration of glutamatergic signals by little SAAS in novel suprachiasmatic circuits

Background

Neuropeptides are critical integrative elements within the central circadian clock in the suprachiasmatic nucleus (SCN), where they mediate both cell-to-cell synchronization and phase adjustments that cause light entrainment. Forward peptidomics identified little SAAS, derived from the proSAAS prohormone, among novel SCN peptides, but its role in the SCN is poorly understood.

Methodology/Principal Findings

Little SAAS localization and co-expression with established SCN neuropeptides were evaluated by immunohistochemistry using highly specific antisera and stereological analysis. Functional context was assessed relative to c-FOS induction in light-stimulated animals and on neuronal circadian rhythms in glutamate-stimulated brain slices. We found that little SAAS-expressing neurons comprise the third most abundant neuropeptidergic class (16.4%) with unusual functional circuit contexts. Little SAAS is localized within the densely retinorecipient central SCN of both rat and mouse, but not the retinohypothalamic tract (RHT). Some little SAAS colocalizes with vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide (GRP), known mediators of light signals, but not arginine vasopressin (AVP). Nearly 50% of little SAAS neurons express c-FOS in response to light exposure in early night. Blockade of signals that relay light information, via NMDA receptors or VIP- and GRP-cognate receptors, has no effect on phase delays of circadian rhythms induced by little SAAS.

Conclusions/Significance

Little SAAS relays signals downstream of light/glutamatergic signaling from eye to SCN, and independent of VIP and GRP action. These findings suggest that little SAAS forms a third SCN neuropeptidergic system, processing light information and activating phase-shifts within novel circuits of the central circadian clock.

Roles of light and serotonin in the regulation of gastrin-releasing peptide and arginine vasopressin output in the hamster SCN circadian clock

Daily timing of the mammalian circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic input from the retina via the retinohypothalamic tract. This signaling is mediated by glutamate, which activates SCN retinorecipient units communicating to pacemaker cells in part through the release of gastrin-releasing peptide (GRP). Efferent signaling from the SCN involves another SCN-containing peptide, arginine vasopressin (AVP). Little is known regarding the mechanisms regulating these peptides, as literature on in vivo peptide release in the SCN is sparse. Here, microdialysis-radioimmunoassay procedures were used to characterize mechanisms controlling GRP and AVP release in the hamster SCN. In animals housed under a 14/10-h light-dark cycle both peptides exhibited daily fluctuations of release, with levels increasing during the morning to peak around midday. Under constant darkness, this pattern persisted for AVP, but rhythmicity was altered for GRP, characterized by a broad plateau throughout the subjective night and early subjective day. Neuronal release of the peptides was confirmed by their suppression with reverse-microdialysis perfusion of calcium blockers and stimulation with depolarizing agents. Reverse-microdialysis perfusion with the 5-HT(1A,7) agonist 8-OH-DPAT ((±)-8-hydroxydipropylaminotetralin hydrobromide) during the day significantly suppressed GRP but had little effect on AVP. Also, perfusion with the glutamate agonist NMDA, or exposure to light at night, increased GRP but did not affect AVP. These analyses reveal distinct daily rhythms of SCN peptidergic activity, with GRP but not AVP release attenuated by serotonergic activation that inhibits photic phase-resetting, and activated by glutamatergic and photic stimulation that mediate this phase-resetting.

An NMDA antagonist inhibits light but not GRP-induced phase shifts when administered after the phase-shifting stimulus

Brief light pulses or microinjection of gastrin-releasing peptide (GRP) into the third ventricle or near the suprachiasmatic nucleus (SCN) induce phase shifts of circadian rhythms during the subjective night. It has previously been reported that these effects are strongly influenced by the activation of N-methyl-d-aspartate (NMDA) receptors and the availability of glutamate. We hypothesized that the photic signaling pathway in the SCN was dependent on glutamate neurotransmission even after the completion of a photic stimulus. Adult male Syrian hamsters equipped with a surgically implanted guide cannula aimed at the SCN region were housed in constant darkness until stable free-running rhythms of wheel-running activity were apparent. Light pulses administered in the early night induced phase delays of circadian rhythms which were attenuated by the co-administration of (+/-)-2-amino-5-phosphonopentanoic acid (AP5), an NMDA antagonist. Microinjection of AP5 also inhibited light-induced shifts, to a lesser extent, immediately after and 15 min after, but not 30 min after the light pulse. A second experiment was designed to test whether AP5 would be able to attenuate GRP-induced shifts 15 min following microinjection of GRP. Phase shifts of animals that received microinjection of AP5 15 min after the administration of GRP were not different from those that received microinjection of GRP and vehicle. These data suggest that glutamate signaling remains necessary for a full photic response in the SCN even after the termination of the photic signal, but that this dependency ends once GRP-dependent signaling is complete.

Gastrin-releasing peptide links stressor to cancer progression

INTRODUCTION

Gastrin-releasing peptide (GRP) plays an important role in cancer growth and metastasis; however, the mechanisms of how GRP affects cancer progression are not well understood. Recent studies revealed that chronic stress is a major risk factor for cancer progression, and this effect may be mediated by GRP. In this review, we will discuss the mechanisms and implications of GRP linking stressor to cancer progression.

MATERIALS AND METHODS

We retrieved the studies of the relationship between GRP, stress and cancers through PubMed using systematic methods to search, select, and evaluate the findings.

RESULTS

The results suggested that GRP can mediate the effects of stress on cancers at systemic, tissue and cellular levels: Stress elicits the secretion of GRP in the brain and GRP in turn activates the stress response pathways resulting in an elevation of stress hormones and GRP in the plasma and tissues. GRP in synergy with stress hormones stimulates the growth and invasion of cancer cells by suppressing the anti-tumor immune function and directly activating the pro-proliferative and pro-migratory signaling pathways in cancer cells.

CONCLUSION

GRP is a multi-functional peptide, which acts as a stress mediator as well as a growth factor linking stressor to cancer progression. GRP and its high-affinity receptor are useful targets for the diagnosis and treatment of cancers.

Circadian trafficking of calbindin-ir in fibers of SCN neurons

Calbindin-D28K (CalB)-containing cells form a distinct cluster within the core of the hamster suprachiasmatic nucleus (SCN). These cells are directly retinorecipient but lack detectable rhythms in clock gene expression or electrical activity. In studies exploring SCN connectivity using double-label immunochemistry, we previously reported an absence of contacts among CalB fibers and vasopressin (VP) cells in animals sacrificed during the day. Here, we explored circadian variations in CalB-immunoreactivity (-ir) and re-examined the connections between CalB and other SCN cell types at zeitgeber times (ZT) 4 and 14. The results reveal a circadian rhythm of CalB-ir in fibers of SCN cells with high expression during the night and subjective night and low expression during the day and subjective day. This circadian difference is not seen in the other brain regions studied. Significantly more appositions were detected between CalB fibers and VP cells during the night than during the day, while circadian variation in numbers of contacts was not seen between CalB fibers and vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), or gastrin-releasing peptide (GRP) cells. There was no detectable variation in appositions from any peptidergic fiber type onto CalB cells. The present findings suggest that CalB cells relay photic information to VP oscillator cells of the SCN shell in a temporally gated manner.

Suprachiasmatic nucleus: cell autonomy and network properties

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

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

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

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

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

Circadian clock resetting in the mouse changes with age

The most widely recognised consequence of normal age-related changes in biological timing is the sleep disruption that appears in old age and diminishes the quality of life. These sleep disorders are part of the normal ageing process and consist primarily of increased amounts of wakefulness and reduced amounts of deep sleep. Changes in the amplitude and timing of the sleep-wake cycle appear to represent, at least in part, a loss of effective circadian regulation of sleep. Understanding alterations in the characteristics of stimuli that help to consolidate internal rhythms will lead to recommendations to improve synchronisation in old age. Converging evidence from both human and animal studies indicate that senescence is associated with alterations in the neural structure thought to be primarily responsible for the generation of the circadian oscillation, the suprachiasmatic nuclei (SCN). Work has shown that there are changes in the anatomy, physiology and ability of the clock to reset in response to stimuli with age. Therefore it is possible that at least some of the observed age-related changes in sleep and circadian timing could be mediated at the level of the SCN. The SCN contain a circadian clock whose activity can be recorded in vitro for several days. We have tested the response of the circadian clock to a number of neurochemicals that reset the clock in a manner similar to light, including glutamate, N-methyl-D-aspartate (NMDA), gastrin-releasing peptide (GRP) and histamine (HA). In addition, we have also tested agents which phase shift in a pattern similar to behavioural 'non-photic' signals, including neuropeptide Y (NPY), serotonin (5HT) and gamma-aminobutyric acid (GABA). These were tested on the circadian clock in young and older mice (approximately 4 and 15 months old). We found deficits in the response to specific neurochemicals but not to others in our older mice. These results indicate that some changes seen in the responsiveness of the circadian clock to light with age may be mediated at the level of the SCN. Further, the responsiveness of the circadian clock with age is attenuated to some, but not all stimuli. This suggests that not all clock stimuli lose their effectiveness with age, and that it may be possible to compensate for deficits in clock performance by enhancing the strength of those stimulus pathways which are intact.

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.

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.

rPer1 and rPer2 induction during phases of the circadian cycle critical for light resetting of the circadian clock

Photic resetting of a biological clock is one of the fundamental characteristics of circadian systems and allows living organisms to adjust to a particular environment. Nocturnal light induces the Per1 and Per2 genes, which leads to a resetting of the circadian clock in the suprachiasmatic nucleus (SCN), the mammalian circadian center. In our present study, we investigated whether light differentially induces the rat Per1 (rPer1) and Per2 (rPer2) genes to enable resetting of their circadian clocks. In a 24-hour LD cycle (12 h light:12 h dark), which is shorter than the normal free-running period for rats, Per1 alone showed strong induction in the ventrolateral region of the SCN (VLSCN) during the early day. In contrast, in a 25 hour LD cycle (12.5 h light:12.5 h dark), which is longer than the free running period for these animals, rPer2 alone was strongly induced in the VLSCN, at the end of the light phase and during the early dark periods. Our current findings therefore suggest that Per1 and Per2 are differentially regulated for daily entrainment to the LD cycle.

Investigating the role of substance P in photic responses of the circadian system: individual and combined actions with gastrin-releasing peptide

The suprachiasmatic nucleus (SCN) contains the master mammalian circadian pacemaker. It is comprised of several phenotypically distinct cell groups, some of which are situated in the weakly rhythmic retinoresponsive ventrolateral region while others are found in the rhythmic, non-retinoresponsive dorsomedial region. The mechanism by which retinorecipient cells convey photic information to the dorsomedial clock cells is unclear. The ventrolateral SCN core contains a variety of cell phenotypes. Two neuropeptides, namely substance P (SP) and gastrin-releasing peptide (GRP) extensively colocalize with calbindin D28K, a marker for SCN cells that are strongly light-responsive. Previous studies have implicated these neuropeptides in photic phase shifting of the circadian system. The present study examines how these peptides interact to regulate photic responses of the circadian system. It was observed that 55.5 +/- 9.1% of SP cells colocalized GRP. SP did not enhance GRP-induced phase shifts in the early-subjective night, while it significantly attenuated GRP-induced phase shifts during the late-subjective night. SP induced significant phase shifts that did not resemble light in the early-subjective night, but was not necessary for light-induced phase shifts and Fos expression at this time. SP induced significant Fos expression only in the late subjective night. SP may not be a necessary component in the pathway(s) involved in photic phase shifting during the early-subjective night, but may modulate phase shifts during the late-subjective night. Distinct biochemical mechanisms that underlie behavioral phase shifts may account for the differences observed in the early- vs. late-subjective night.

Vasoactive intestinal polypeptide entrains circadian rhythms in astrocytes

Many mammalian cell types show daily rhythms in gene expression driven by a circadian pacemaker. For example, cultured astrocytes display circadian rhythms in Period1 and Period2 expression. It is not known, however, how or which intercellular factors synchronize and sustain rhythmicity in astrocytes. Because astrocytes are highly sensitive to vasoactive intestinal polypeptide (VIP), a neuropeptide released by neurons and important for the coordination of daily cycling, the authors hypothesized that VIP entrains circadian rhythms in astrocytes. They used astrocyte cultures derived from knock-in mice containing a bioluminescent reporter of PERIOD2 (PER2) protein, to assess the effects of VIP on the rhythmic properties of astrocytes. VIP induced a dose-dependent increase in the peak-to-trough amplitude of the ensemble rhythms of PER2 expression with maximal effects near 100 nM VIP and threshold values between 0.1 and 1 nM. VIP also induced dose- and phase-dependent shifts in PER2 rhythms and daily VIP administration entrained bioluminescence rhythms of astrocytes to a predicted phase angle. This is the first demonstration that a neuropeptide can entrain glial cells to a phase predicted by a phase-response curve. The authors conclude that VIP potently entrains astrocytes in vitro and is a candidate for coordinating daily rhythms among glia in the brain.

Regulation and signaling of human bombesin receptors and their biological effects

PURPOSE OF REVIEW

This review will highlight recent advances in the understanding of molecular mechanisms by which mammalian bombesin receptors are regulated and which intracellular signaling pathways have been characterized to mediate agonist-dependent receptor biological effects.

RECENT FINDINGS

Mammalian bombesin receptors have been demonstrated to be involved in a larger array of physiological and pathophysiological conditions than previously reported. Pharmacological experiments in vitro and in vivo as well as utilization of animals genetically deficient of the gastrin-releasing peptide receptor demonstrated roles in memory and fear behavior, lung development and injury, small intestinal cell repair, autocrine tumor growth, and mediating signals for pruritus and penile reflexes. Intracellular signaling studies predominantly of the gastrin-releasing peptide receptor owing to its frequent overexpression in some human malignancies showed that PI3 kinase activation is an important mechanism of cell proliferation. Tumor cell treatment including gastrin-releasing peptide receptor antagonists combined with inhibition of epidermal growth factor receptor resulted in an additive effect on blocking cell proliferation. Novel molecular mechanisms of the orphan bombesin receptor subtype-3 and gastrin-releasing peptide receptor gene regulation have been elucidated.

SUMMARY

Inhibition of gastrin-releasing peptide receptor signaling in human malignancies represents an attractive target for pharmacological treatment. Novel functions of bombesin related peptides have been identified including processes in the central nervous system, lung and intestinal tract.

Mammalian circadian signaling networks and therapeutic targets

Virtually all cells in the body have an intracellular clockwork based on a negative feedback mechanism. The circadian timekeeping system in mammals is a hierarchical multi-oscillator network, with the suprachiasmatic nuclei (SCN) acting as the central pacemaker. The SCN synchronizes to daily light-dark cycles and coordinates rhythmic physiology and behavior. Synchronization in the SCN and at the organismal level is a key feature of the circadian clock system. In particular, intercellular coupling in the SCN synchronizes neuron oscillators and confers robustness against perturbations. Recent advances in our knowledge of and ability to manipulate circadian rhythms make available cell-based clock models, which lack strong coupling and are ideal for target discovery and chemical biology.

Electrophysiology of the suprachiasmatic circadian clock

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

Nocturnal expression of phosphorylated-ERK1/2 in gastrin-releasing peptide neurons of the rat suprachiasmatic nucleus

Extracellular regulated kinase (ERK) signalling is believed to play roles in various aspects of circadian clock mechanisms. In this study, we show in rat that the nuclear versus cytoplasmic intracellular distribution of the phosphorylated forms of ERK1/2 (P-ERK1/2) in the central clock, namely the suprachiasmatic nucleus (SCN), is proportionally constant across the light/dark cycle while the spatial distribution and neurochemical phenotype of cells expressing these activated forms are time-regulated according to a daily rhythm and light-regulated. P-ERK1/2 was exclusively found in neuronal elements. At daytime, it was detected throughout the dorsoventral extent of the SCN, partly within neurons synthesizing either arginine-vasopressin or vasoactive intestinal peptide (VIP). At night time, it was segregated in the ventrolateral aspect of the nucleus, within a cluster of cells 45% of which were gastrin-releasing peptide (GRP) neurons with or without co-localization with VIP. After a light pulse at night, expression of P-ERK1/2 increased in GRP neurons but also appeared in a population of neurons that stained for VIP only. These data show that the GRP neurons are closely associated with ERK1/2 activation at night and point to the importance of ERK1/2 signalling not only in intra-SCN transmission of photic information but also in maintenance of neuronal rhythms in the SCN.

Exploring spatiotemporal organization of SCN circuits

Suprachiasmatic nucleus (SCN) neuroanatomy has been a subject of intense interest since the discovery of the SCN's function as a brain clock and subsequent studies revealing substantial heterogeneity of its component neurons. Understanding the network organization of the SCN has become increasingly relevant in the context of studies showing that its functional circuitry, evident in the spatial and temporal expression of clock genes, can be reorganized by inputs from the internal and external environment. Although multiple mechanisms have been proposed for coupling among SCN neurons, relatively little is known of the precise pattern of SCN circuitry. To explore SCN networks, we examine responses of the SCN to various photic conditions, using in vivo and in vitro studies with associated mathematical modeling to study spatiotemporal changes in SCN activity. We find an orderly and reproducible spatiotemporal pattern of oscillatory gene expression in the SCN, which requires the presence of the ventrolateral core region. Without the SCN core region, behavioral rhythmicity is abolished in vivo, whereas low-amplitude rhythmicity can be detected in SCN slices in vitro, but with loss of normal topographic organization. These studies reveal SCN circuit properties required to signal daily time.