Calbindin neurons in the hamster suprachiasmatic nucleus do not exhibit a circadian variation in spontaneous firing rate

A subclass of evening cells promotes the switch from arousal to sleep at dusk

Animals exhibit rhythmic patterns of behavior that are shaped by an internal circadian clock and the external environment. Although light intensity varies across the day, there are particularly robust differences at twilight (dawn/dusk). These periods are also associated with major changes in behavioral states, such as the transition from arousal to sleep. However, the neural mechanisms by which time and environmental conditions promote these behavioral transitions are poorly defined. Here, we show that the E1 subclass of Drosophila evening clock neurons promotes the transition from arousal to sleep at dusk. We first demonstrate that the cell-autonomous clocks of E2 neurons primarily drive and adjust the phase of evening anticipation, the canonical behavior associated with "evening" clock neurons. We next show that conditionally silencing E1 neurons causes a significant delay in sleep onset after dusk. However, rather than simply promoting sleep, activating E1 neurons produces time- and light-dependent effects on behavior. Activation of E1 neurons has no effect early in the day but then triggers arousal before dusk and induces sleep after dusk. Strikingly, these activation-induced phenotypes depend on the presence of light during the day. Despite their influence on behavior around dusk, in vivo voltage imaging of E1 neurons reveals that their spiking rate and pattern do not significantly change throughout the day. Moreover, E1-specific clock ablation has no effect on arousal or sleep. Thus, we suggest that, rather than specifying "evening" time, E1 neurons act, in concert with other rhythmic neurons, to promote behavioral transitions at dusk.

A subclass of evening cells promotes the switch from arousal to sleep at dusk

Animals exhibit rhythmic patterns of behavior that are shaped by an internal circadian clock and the external environment. While light intensity varies across the day, there are particularly robust differences at twilight (dawn/dusk). These periods are also associated with major changes in behavioral states, such as the transition from arousal to sleep. However, the neural mechanisms by which time and environmental conditions promote these behavioral transitions are poorly defined. Here, we show that the E1 subclass of Drosophila evening clock neurons promotes the transition from arousal to sleep at dusk. We first demonstrate that the cell-autonomous clocks of E2 neurons alone are required to drive and adjust the phase of evening anticipation, the canonical behavior associated with "evening" clock neurons. We next show that conditionally silencing E1 neurons causes a significant delay in sleep onset after dusk. However, rather than simply promoting sleep, activating E1 neurons produces time- and light- dependent effects on behavior. Activation of E1 neurons has no effect early in the day, but then triggers arousal before dusk and induces sleep after dusk. Strikingly, these phenotypes critically depend on the presence of light during the day. Despite their influence on behavior around dusk, in vivo voltage imaging of E1 neurons reveals that their spiking rate does not vary between dawn and dusk. Moreover, E1-specific clock ablation has no effect on arousal or sleep. Thus, we suggest that, rather than specifying "evening" time, E1 neurons act, in concert with other rhythmic neurons, to promote behavioral transitions at dusk.

Diurnal properties of tonic and synaptic GABAA receptor-mediated currents in suprachiasmatic nucleus neurons

Synaptic and extrasynaptic GABA_A receptor (GABA_AR)-mediated neurotransmission is a critical component of the suprachiasmatic nucleus (SCN) neuronal network. However, the properties of the GABA_A tonic current (I_tonic) and its origin remain unexplored. Spontaneous GABA_A postsynaptic currents (sGPSCs) and I_tonic were recorded from SCN neurons with the whole cell voltage-clamp technique at different times of the day. GABA_AR antagonists (bicuculline, gabazine, and picrotoxin) inhibited sGPSC and induced an outward shift of the holding current, which defined the I_tonic amplitude. The sGPSC frequency, synaptic charge transfer, and I_tonic amplitude all demonstrated significant diurnal rhythms, with peaks in the middle of the day [zeitgeber time (ZT)7-8] and nadirs at night (ZT19-20). The I_tonic amplitude increased proportionally with the sGPSC frequency and synaptic charge transfer during the day and required action potential-mediated GABA release, which was confirmed by TTX application. The activation of presynaptic GABA_B receptors by baclofen did not significantly alter the I_tonic of neurons with low-frequency sGPSC. The equilibrium potential (Eq) for I_tonic was similar to the Eq for chloride and GABA_A receptor-activated currents. I_tonic showed outward rectification at membrane potentials over the range of -70 to -10 mV and then was linear at voltages greater than -10 mV. GABA_AR containing α4-, α5-, and δ-subunits were expressed in SCN, and their contribution to I_tonic was confirmed by application of the GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and the GABA_AR inverse agonist 11,12,13,13a-tetrahydro-7-methoxy-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c][1,4]benzodiazepine-1-carboxylic acid ethyl ester (L655,708). Thus, the I_tonic was mediated by extrasynaptic GABA_ARs activated predominantly by GABA diffusing out of GABAergic synapses.NEW & NOTEWORTHY A tonic current (I_tonic) mediated by GABA_A receptors (GABA_ARs) containing α4-, α5- and δ-subunits was observed in the suprachiasmatic nucleus. The I_tonic amplitude strongly depended on the action potential-mediated synaptic release of GABA. The equilibrium potential for I_tonic corresponds to that for GABA_A currents. The frequency of GABA_A postsynaptic currents and I_tonic amplitude increased during the day, with peak in the middle of the day, and then gradually declined with a nadir at night, thus showing a diurnal rhythm.

The Excitatory Effects of GABA within the Suprachiasmatic Nucleus: Regulation of Na-K-2Cl Cotransporters (NKCCs) by Environmental Lighting Conditions

The suprachiasmatic nucleus (SCN) contains a pacemaker that generates circadian rhythms and entrains them with the 24-h light-dark cycle (LD). The SCN is composed of 16,000 to 20,000 heterogeneous neurons in bilaterally paired nuclei. γ-amino butyric acid (GABA) is the primary neurochemical signal within the SCN and plays a key role in regulating circadian function. While GABA is the primary inhibitory neurotransmitter in the brain, there is now evidence that GABA can also exert excitatory effects in the adult brain. Cation chloride cotransporters determine the effects of GABA on chloride equilibrium, thereby determining whether GABA produces hyperpolarizing or depolarizing actions following activation of GABA_A receptors. The activity of Na-K-2Cl cotransporter1 (NKCC1), the most prevalent chloride influx cotransporter isoform in the brain, plays a critical role in determining whether GABA has depolarizing effects. In the present study, we tested the hypothesis that NKCC1 protein expression in the SCN is regulated by environmental lighting and displays daily and circadian changes in the intact circadian system of the Syrian hamster. In hamsters housed in constant light (LL), the overall NKCC1 immunoreactivity (NKCC1-ir) in the SCN was significantly greater than in hamsters housed in LD or constant darkness (DD), although NKCC1 protein levels in the SCN were not different between hamsters housed in LD and DD. In hamsters housed in LD cycles, no differences in NKCC1-ir within the SCN were observed over the 24-h cycle. NKCC1 protein in the SCN was found to vary significantly over the circadian cycle in hamsters housed in free-running conditions. Overall, NKCC1 protein was greater in the ventral SCN than in the dorsal SCN, although no significant differences were observed across lighting conditions or time of day in either subregion. These data support the hypothesis that NKCC1 protein expression can be regulated by environmental lighting and circadian mechanisms within the SCN.

Membrane Currents, Gene Expression, and Circadian Clocks

Neuronal circadian oscillators in the mammalian and Drosophila brain express a circadian clock comprised of interlocking gene transcription feedback loops. The genetic clock regulates the membrane electrical activity by poorly understood signaling pathways to generate a circadian pattern of action potential firing. During the day, Na⁺ channels contribute an excitatory drive for the spontaneous activity of circadian clock neurons. Multiple types of K⁺ channels regulate the action potential firing pattern and the nightly reduction in neuronal activity. The membrane electrical activity possibly signaling by changes in intracellular Ca²⁺ and cyclic adenosine monophosphate (cAMP) regulates the activity of the gene clock. A decline in the signaling pathways that link the gene clock and neural activity during aging and disease may weaken the circadian output and generate significant impacts on human health.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

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.

SCN Organization Reconsidered

The SCN has long had organizational schemas imposed on it. In most, the SCN is dichotomized, with one region typically associated with the presence of vasopressin cells and the other associated with cells containing vasoactive intestinal polypeptide and certain afferent terminal fields. If assumed to be accurate, the schemas that have been intended to simplify and conceptually organize the known anatomy may actually interfere with the understanding of how various cell types and input pathways contribute to circadian rhythm regulation. This review describes inadequacies of existing schemas and notes several practical difficulties that undermine their usefulness. These include “static” versus “dynamic” anatomy, generalizations about SCN organization in relation to the plane or level of section, and the concept of differential density, all of which contribute to a view in which the SCN is substantially more complex than typically depicted in oversimplified line drawings. The need for accurate topographical description is emphasized.

The Suprachiasmatic Nucleus: A Clock of Multiple Components

Although impressive progress has been made in understanding the molecular basis of pacemaker function in the suprachiasmatic nucleus (SCN), fundamental questions about cellular and regional heterogeneity within the SCN, andhowthis heterogeneity might contribute toSCNpacemaker function at a tissue level, have remained unresolved. To reexamine cellular and regional heterogeneity within the SCN, the authors have focused on two key questions: which SCN cells are endogenously rhythmic and/or directly light responsive? Observations of endogenous rhythms of electrical activity, gene/protein expression, and protein phosphorylation suggest that the SCN in mammals examined to dateis composed of anatomically distinct rhythmic and nonrhythmic components. Endogenously rhythmic neurons are primarily found in rostral, dorsomedial, and ventromedial portions of the nucleus; at mid and caudal levels, the distribution of endogenously rhythmic cells in the SCN has the appearance of a “shell.” The majority of nonrhythmic cells, by contrast, are located in a central “core” region of the SCN, which is complementary to the shell. The location of light-responsive cells, defined by direct retinohypothalamic input and light-induced gene expression, largely overlaps the location of nonrhythmic cells in the SCN core, although, in hamsters and mice light-responsive cells are also present in the ventral portion of the rhythmic shell. While the relative positions of rhythmic and light-responsive components of the SCN are similar between species, the precise boundaries of these components, and neurochemical phenotype of cells within them, are variable. Intercellular communication between these components may bea key featurer esponsiblefor theuniquepace maker properties of the SCN observed at a tissue and whole animal level.

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.

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.

Distinct Firing Properties of Vasoactive Intestinal Peptide-Expressing Neurons in the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) regulates daily rhythms in physiology and behavior. Previous studies suggest a critical role for neurons expressing vasoactive intestinal peptide (VIP) in coordinating rhythmicity and synchronization in the SCN. Here we examined the firing properties of VIP-expressing SCN neurons in acute brain slices. Active and passive membrane properties were measured in VIP and in non-VIP neurons during the day and at night. Current-clamp recordings revealed that both VIP and non-VIP neurons were spontaneously active, with higher firing rates during the day than at night. Average firing frequencies, however, were higher in VIP neurons (3.1 ± 0.2 Hz, day and 2.4 ± 0.2 Hz, night) than in non-VIP neurons (1.8 ± 0.2 Hz, day and 0.9 ± 0.2 Hz, night), both day and night. The waveforms of individual action potentials in VIP and non-VIP neurons were also distinct. Action potential durations (APD50) were shorter in VIP neurons (3.6 ± 0.1 ms, day and 2.9 ± 0.1 ms, night) than in non-VIP neurons (4.4 ± 0.3 ms, day and 3.5 ± 0.2 ms, night) throughout the light-dark cycle. In addition, afterhyperpolarization (AHP) amplitudes were larger in VIP neurons (21 ± 0.8 mV, day and 24.9 ± 0.9 mV, night) than in non-VIP neurons (17.2 ± 1.1 mV, day and 20.5 ± 1.2 mV, night) during the day and at night. Furthermore, significant day/night differences were observed in APD50 and AHP amplitudes in both VIP and non-VIP SCN neurons, consistent with rhythmic changes in ionic conductances that contribute to shaping the firing properties of both cell types. The higher day and night firing rates of VIP neurons likely contribute to synchronizing electrical activity in the SCN.

Vasoactive intestinal polypeptide (VIP)-expressing neurons in the suprachiasmatic nucleus provide sparse GABAergic outputs to local neurons with circadian regulation occurring distal to the opening of postsynaptic GABAA ionotropic receptors

GABAergic synaptic transmission plays an important role in resetting and synchronizing circadian rhythms in the suprachiasmatic nucleus (SCN). Although the circadian modulation of intrinsic membrane currents and biochemical signaling have been examined in the SCN, the modulation of specific synaptic pathways within the SCN is unexplored. In addition, little is known about the functional properties of these pathways, including which ones involve GABAA receptors (GABAA-Rs). In brain slices obtained from mice, we examined synaptic responses originating from the SCN neurons expressing vasoactive intestinal peptide (VIP+ neurons). Focusing on the local projection within the ventromedial SCN, we found that VIP+ afferents provided input onto 49% of neurons with a preference for VIP-negative (VIP-) neurons. Responses were mediated by GABAA-Rs. The projection was sparsely connected and preferentially targeted a subset of SCN neurons unrelated to postsynaptic VIP expression. For most aspects of VIP+ network output, there was no circadian regulation. Excitability and spontaneous firing of the presynaptic VIP+ neurons were unchanged between day and night, and their network connectivity and synaptic function up through the evoked synaptic conductance were also unchanged. On the other hand, VIP+ input onto VIP- neurons became less inhibitory at night suggesting a postsynaptic alteration in the coupling of GABAA-R conductances to action potential firing. These data suggest that components of the VIP network and its synaptic output up through GABAA-R opening are invariant during the circadian cycle, but the effect on action potential firing is modulated by postsynaptic processes occurring after GABAA-R channel opening.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

GABAB receptor-mediated frequency-dependent and circadian changes in synaptic plasticity modulate retinal input to the suprachiasmatic nucleus

Light is the most important environmental signal that entrains the circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). The retinohypothalamic tract (RHT) was stimulated to simulate the light intensity-dependent discharges of intrinsically photosensitive retinal ganglion cells projecting axons to the hypothalamus. EPSCs were evoked by paired-pulse stimulation or by application of stimulus trains, and recorded from SCN neurons in rat brain slices. Initial release probability (Pr) and synaptic plasticity changes depended on the strength of GABAB receptor (GABABR)-mediated presynaptic inhibition and could be different at the same GABABR agonist concentration. Facilitation caused by frequency-dependent relief of GABABR-mediated inhibition was observed when the initial Pr was decreased to less than 15% of control during strong activation of presynaptic GABAB receptors by (±)baclofen (10 μm), GABA (2 mm) or by GABA uptake inhibitor nipecotic acid (5 mm). In contrast, short-term synaptic depression appeared during baclofen (10 μm) application when initial Pr was greater than 30% of control. Block of 4-aminopyridine-sensitive K(+) currents increased the amplitude and time constant of decay of evoked EPSCs (eEPSCs), and decreased the GABABR-mediated presynaptic inhibition. The GABAB receptor antagonist CGP55845 (3 μm) increased the eEPSCs amplitude 30% throughout the light-dark cycle. During light and dark phases the RHT inputs to 55% and 33% of recorded neurons, respectively, were under GABAB inhibitory control indicating that the tonic inhibition induced by local changes of endogenous GABA concentration contributes to the circadian variation of RHT transmitter release. We conclude that GABABR-mediated presynaptic inhibition decreased with increasing frequency and broadening of presynaptic action potentials, and depended on the sensitivity of RHT terminals to GABABR agonists, and diurnal changes of the extracellular GABA concentration around RHT axon terminals in the SCN.

Characterization of orderly spatiotemporal patterns of clock gene activation in mammalian suprachiasmatic nucleus

Because we can observe oscillation within individual cells and in the tissue as a whole, the suprachiasmatic nucleus (SCN) presents a unique system in the mammalian brain for the analysis of individual cells and the networks of which they are a part. While dispersed cells of the SCN sustain circadian oscillations in isolation, they are unstable oscillators that require network interactions for robust cycling. Using cluster analysis to assess bioluminescence in acute brain slices from PERIOD2::Luciferase (PER2::LUC) knockin mice, and immunochemistry of SCN from animals harvested at various circadian times, we assessed the spatiotemporal activation patterns of PER2 to explore the emergence of a coherent oscillation at the tissue level. The results indicate that circadian oscillation is characterized by a stable daily cycle of PER2 expression involving orderly serial activation of specific SCN subregions, followed by a silent interval, with substantial symmetry between the left and right side of the SCN. The biological significance of the clusters identified in living slices was confirmed by co-expression of LUC and PER2 in fixed, immunochemically stained brain sections, with the spatiotemporal pattern of LUC expression resembling that revealed in the cluster analysis of bioluminescent slices. We conclude that the precise timing of PER2 expression within individual neurons is dependent on their location within the nucleus, and that small groups of neurons within the SCN give rise to distinctive and identifiable subregions. We propose that serial activation of these subregions is the basis of robustness and resilience of the daily rhythm of the SCN.

Multiple hypothalamic cell populations encoding distinct visual information

Environmental illumination profoundly influences mammalian physiology and behaviour through actions on a master circadian oscillator in the suprachiasmatic nuclei (SCN) and other hypothalamic nuclei. The retinal and central mechanisms that shape daily patterns of light-evoked and spontaneous activity in this network of hypothalamic cells are still largely unclear. Similarly, the exact nature of the sensory information conveyed by such cells is unresolved. Here we set out to address these issues, through multielectrode recordings from the hypothalamus of red cone knockin mice (Opn1mwR). With this powerful mouse model, the photoreceptive origins of any response can be readily identified on the basis of their relative sensitivity to short and long wavelength light. Our experiments revealed that the firing pattern of many hypothalamic cells was influenced by changes in light levels and/or according to the steady state level of illumination. These ‘contrast' and ‘irradiance' responses were driven primarily by cone and melanopsin photoreceptors respectively, with rods exhibiting a much more subtle influence. Individual hypothalamic neurons differentially sampled from these information streams, giving rise to four distinct response types. The most common response phenotype in the SCN itself was sustained activation. Cells with this behaviour responded to all three photoreceptor classes in a manner consistent with their distinct contributions to circadian photoentrainment. These ‘sustained' cells were also unique in our sample in expressing circadian firing patterns with highest activity during the mid projected day. Surprisingly, we also found a minority of SCN neurons that lacked the melanopsin-derived irradiance signal and responded only to light transitions, allowing for the possibility that rod–cone contrast signals may be routed to SCN output targets without influencing neighbouring circadian oscillators. Finally, an array of cells extending throughout the periventricular hypothalamus and ventral thalamus were excited or inhibited solely according to the activity of melanopsin. These cells appeared to convey a filtered version of the visual signal, suitable for modulating physiology/behaviour purely according to environmental irradiance. In summary, these findings reveal unexpectedly widespread hypothalamic cell populations encoding distinct qualities of visual information.

Intrinsic regulation of spatiotemporal organization within the suprachiasmatic nucleus

The mammalian pacemaker in the suprachiasmatic nucleus (SCN) contains a population of neural oscillators capable of sustaining cell-autonomous rhythms in gene expression and electrical firing. A critical question for understanding pacemaker function is how SCN oscillators are organized into a coherent tissue capable of coordinating circadian rhythms in behavior and physiology. Here we undertake a comprehensive analysis of oscillatory function across the SCN of the adult PER2::LUC mouse by developing a novel approach involving multi-position bioluminescence imaging and unbiased computational analyses. We demonstrate that there is phase heterogeneity across all three dimensions of the SCN that is intrinsically regulated and extrinsically modulated by light in a region-specific manner. By investigating the mechanistic bases of SCN phase heterogeneity, we show for the first time that phase differences are not systematically related to regional differences in period, waveform, amplitude, or brightness. Furthermore, phase differences are not related to regional differences in the expression of arginine vasopressin and vasoactive intestinal polypeptide, two key neuropeptides characterizing functionally distinct subdivisions of the SCN. The consistency of SCN spatiotemporal organization across individuals and across planes of section suggests that the precise phasing of oscillators is a robust feature of the pacemaker important for its function.

Phase misalignment between suprachiasmatic neuronal oscillators impairs photic behavioral phase shifts but not photic induction of gene expression

The ability of the circadian pacemaker within the suprachiasmatic nucleus (SCN) to respond to light stimulation in a phase-specific manner constitutes the basis for photic entrainment of circadian rhythms. The neural basis for this phase specificity is unclear. We asked whether a lack of synchrony between SCN neurons, as reflected in phase misalignment between dorsomedial (dmSCN) and ventrolateral (vlSCN) neuronal oscillators in the rat, would impact the ability of the pacemaker to respond to phase-resetting light pulses. Light pulses delivered at maximal phase misalignment between the vlSCN and dmSCN oscillators increased expression of Per1 mRNA, regardless of the circadian phase of the dmSCN. However, phase shifts of locomotor activity were only observed when the vlSCN and dmSCN were phase aligned at the time of stimulation. Our results fit a model in which a vlSCN oscillator phase gates its own response to light and in turn relays light information to a dmSCN oscillator. This model predicts that the phase misalignment that results from circadian internal desynchronization could preserve the ability of light to induce gene expression within the master circadian clock but impair its ability to induce behavioral phase shifts.

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.

Modeling the spontaneous activity in suprachiasmatic nucleus neurons: role of cation single channels

A population of interconnected neurons of the mammalian suprachiasmatic nuclei (SCN) controls circadian rhythms in physiological functions. In turn, a circadian rhythm of individual neurons is driven by intracellular processes, which via activation of specific membrane channels, produce circadian modulation of electrical firing rate. Yet the membrane target(s) of the cellular clock have remained enigmatic. Previously, subthreshold voltage-dependent cation (SVC) channels have been proposed as the membrane target of the cellular clock responsible for circadian modulation of the firing rate in SCN neurons. We tested this hypothesis with computational modeling based on experimental results from on-cell recording of SVC channel openings in acutely isolated SCN neurons and long-term continuous recording of activity from dispersed SCN neurons in a multielectrode array dish (MED). The model reproduced the circadian behavior if the number of SVC channels or their kinetics were modulated in accordance with protein concentration in a model of the intracellular clock (Scheper et al., 1999. J. Neurosci. 19, 40-47). Such modulation changed the average firing rate of the model neuron from zero ("subjective-night" silence) up to 18 Hz ("subjective-day" peak). Furthermore, the variability of interspike intervals (ISI) and the circadian pattern of firing rate (i.e. silence-to-activity ratio and shape of circadian peaks) are in reasonable agreement with experimental data obtained in dispersed SCN neurons in MED. These results suggest that the variability of ISI in intact SCN neurons is mostly due to stochastic single-channel openings, and that the circadian pattern of the firing rate is specified by threshold properties of dependence of the spontaneous firing rate on the number of single channels (R-N relationship). This plausible mathematical modeling supports the hypothesis that SVC channels could be a critical element in circadian modulation of firing rate in SCN neurons.

Reorganization of suprachiasmatic nucleus networks under 24-h LDLD conditions

The suprachiasmatic nucleus (SCN), locus of the master circadian clock in the brain, is comprised of multioscillator neural networks that are highly plastic in responding to environmental lighting conditions. Under a 24-h light:dark:light:dark (LDLD) cycle, hamsters bifurcate their circadian locomotor activity such that wheel running occurs in each of the 2 daily dark periods with complete inactivity in between. In the present study, we explored the neural underpinning of this behavioral bifurcation. Using calbindin (CalB)- containing cells of the SCN as a regional marker, we characterized PER1 and c-FOS expression in the core and shell SCN subregions. In LD-housed animals, it is known that PER1 and c-FOS in the core and shell region are in phase with each other. In contrast, in behaviorally bifurcated animals housed in LDLD, the core and shell SCN exhibit antiphase rhythms of PER1. Furthermore, cells in the core show high FOS expression in each photophase of the LDLD cycle. The activation of FOS in the core is light driven and disappears rapidly when the photophase is replaced by darkness. The results suggest that bifurcated activity bouts in daytime and nighttime are associated with oscillating groups of cells in the core and shell subregions, respectively, and support the notion that reorganization of SCN networks underlies changes in behavioral responses under different environmental lighting conditions.

Phase organization of circadian oscillators in extended gate and oscillator models

The suprachiasmatic nuclei (SCN) control daily oscillations in physiology and behavior. The gate-oscillator model captures functional heterogeneity in SCN and has been successful in reproducing many features of SCN. This paper investigates the mechanism of phase organization in the gate-oscillator model and finds that only stable fixed points of the phase transition function are essential to phase organization. This obvious finding forms the basis for understanding the complex phase distribution in the gate-oscillator scheme. Extending the model with a dead zone of the phase transition function and the propagation delay of the gate signal which may represent the spatial structure of SCN, the author discusses how some features of experimentally reported phase distribution, such as the existence of anti-phase neurons and fixed phase difference between neurons, could be understood in the framework of the gate-oscillator model. The extended model shows clearly the way in which the interplay between the single-cell property and the property of the network organization influence the phase distribution of SCN neurons.

The substructure of the suprachiasmatic nucleus: Similarities between nocturnal and diurnal spiny mice

Evolutionary transitions between nocturnal and diurnal patterns of adaptation to the day-night cycle must have involved fundamental changes in the neural mechanisms that coordinate the daily patterning of activity, but little is known about how these mechanisms differ. One reason is that information on these systems in very closely related diurnal and nocturnal species is lacking. In this study, we characterize the suprachiasmatic nucleus (SCN), the primary brain structure involved in the generation and coordination of circadian rhythms, in two members of the genus Acomys with very different activity patterns, Acomys russatus (the golden spiny mouse, diurnal) and Acomys cahirinus (the common spiny mouse, nocturnal). Immunohistochemical techniques were used to label cell bodies containing vasoactive intestinal polypeptide (VIP), vasopressin (VP), gastrin-releasing peptide (GRP) and calbindin (CalB) in the SCN, as well as two sets of inputs to it, those containing serotonin (5-HT) and neuropeptide Y (NPY), respectively. All were present in the SCN of both species and no differences between them were seen. On the basis of neuronal phenotype, the SCN was organized into three basic regions that contained VIP-immunoreactive (-ir), CalB-ir and VP-ir cells, in the ventral, middle and dorsal SCN, respectively. In the rostral SCN, GRP-ir cells were in both the VIP and the CalB cell regions, and in the caudal area they were distributed across a portion of each of the other three regions. Fibers containing NPY-ir and serotonin (5-HT)-ir were most concentrated in the areas containing VIP-ir and CalB-ir cells, respectively. The details of the spatial relationships among the labeled cells and fibers seen here are discussed in relation to different approaches investigators have taken to characterize the SCN more generally.

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.

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.

Basis of robustness and resilience in the suprachiasmatic nucleus: individual neurons form nodes in circuits that cycle daily

How the cellular elements of the SCN are synchronized to each other is not well understood. We explore circadian oscillations manifest at the level of the cell, the tissue, and the whole animal to better understand intra-SCN synchrony and master clock function of the nucleus. At each level of analysis, responses to variations in operating environment (robustness), and following damage to components of the system (resilience), provide insight into the mechanisms whereby the SCN orchestrates circadian timing. Tissue level rhythmicity reveals circuits associated with an orderly spatiotemporal daily pattern of activity that is not predictable from their cellular elements. Specifically, in stable state, some SCN regions express low amplitude or undetectable rhythms in clock gene expression while others produce high amplitude oscillations. Within the SCN, clock gene expression follows a spatially ordered, repeated pattern of activation and inactivation. This pattern of activation is plastic and subserves responses to changes in external and internal conditions. Just as daily rhythms at the cellular level depend on sequential expression and interaction of clock genes, so too do rhythms at the SCN tissue level depend on sequential activation of local nodes. We hypothesize that individual neurons are organized into nodes that are themselves sequentially activated across the volume of the SCN in a cycle that repeats on a daily basis. We further propose that robustness is expressed in the ability of the SCN to sustain rhythmicity over a wide range of internal and external conditions, and that this reflects plasticity of the underlying nodes and circuits. Resilience is expressed in the ability of SCN cells to oscillate and to sustain activity-related rhythms at the behavioral level. Importantly, other aspects of pacemaker function remain to be examined.

GABAergic signaling induces divergent neuronal Ca2+ responses in the suprachiasmatic nucleus network

Intercellular communication between gamma-aminobutyric acid (GABA)ergic suprachiasmatic nucleus (SCN) neurons facilitates light-induced phase changes and synchronization of individual neural oscillators within the SCN network. We used ratiometric Ca(2+) imaging techniques to record changes in the intracellular calcium concentration ([Ca(2+)](i)) to study the role of GABA in interneuronal communication and the response of the SCN neuronal network to optic nerve stimulations that mimic entraining light signals. Stimulation of the retinohypothalamic tract (RHT) evoked divergent Ca(2+) responses in neurons that varied regionally within the SCN with a pattern that correlated with those evoked by pharmacological GABA applications. GABA(A) and GABA(B) receptor agonists and antagonists were used to evaluate components of the GABA-induced changes in [Ca(2+)](i). Application of the GABA(A) receptor antagonist gabazine induced changes in baseline [Ca(2+)](i) in a direction opposite to that evoked by GABA, and similarly altered the RHT stimulation-induced Ca(2+) response. GABA application induced Ca(2+) responses varied in time and region within the SCN network. The NKCC1 cotransporter blocker, bumetanide, and L-type calcium channel blocker, nimodipine, attenuated the GABA-induced rise of [Ca(2+)](i). These results suggest that physiological GABA induces opposing effects on [Ca(2+)](i) based on the chloride equilibrium potential, and may play an important role in neuronal Ca(2+) balance, synchronization and modulation of light input signaling in the SCN network.

Day-length encoding through tonic photic effects in the retinorecipient SCN region

The circadian clock in the suprachiasmatic nucleus (SCN) plays a critical role in seasonal processes by sensing ambient photoperiod. To explore how it measures day-length, we assessed the state of SCN oscillators using markers for neuronal activity (c-FOS) and the clock protein (PER1) in Syrian hamsters housed in long (LD, 16 : 8 h light : dark) vs. short days (SD, 8 : 16 h light : dark). During SD, there was no detectable phase dispersion across the rostrocaudal extent of the nucleus. In contrast, during LD, rhythms in the caudal SCN phase led those in the mid- and rostral SCN by 4-8 h and 8-12 h, respectively. Importantly, some neurons in the retinorecipient core SCN were unique in that they were FOS-positive during the dark phase in LD, but not SD. Transfer of LD animals to constant darkness or skeleton photoperiod revealed that dark-phase FOS expression depends on tonic light exposure rather than on intrinsic clock properties. By transferring animals from SD to LD, we next discovered that there are two separate populations of SCN cells, one responding to acute and the other to tonic light exposure. The results suggest that the seasonal encoding of day-length by the SCN entails reorganization of its constituent oscillators by a subgroup of neurons in the SCN core that respond to tonic photic cues.

Targeted mutation of the calbindin D28K gene disrupts circadian rhythmicity and entrainment

The suprachiasmatic nucleus (SCN) is the principal circadian pacemaker in mammals. A salient feature of the SCN is that cells of a particular phenotype are topographically organized; this organization defines functionally distinct subregions that interact to generate coherent rhythmicity. In Syrian hamsters (Mesocricetus auratus), a dense population of directly retinorecipient calbindin D(28K) (CalB) neurons in the caudal SCN marks a subregion critical for circadian rhythmicity. In mouse SCN, a dense cluster of CalB neurons occurs during early postnatal development, but in the adult CalB neurons are dispersed through the SCN. In the adult retina CalB colocalizes with melanopsin-expressing ganglion cells. In the present study, we explored the role of CalB in modulating circadian function and photic entrainment by investigating mice with a targeted mutation of the CalB gene (CalB-/- mice). In constant darkness (DD), CalB-/- animals either become arrhythmic (40%) or exhibit low-amplitude locomotor rhythms with marked activity during subjective day (60%). Rhythmic clock gene expression is blunted in these latter animals. Importantly, CalB-/- mice exhibit anomalies in entrainment revealed following transfer from a light : dark cycle to DD. Paradoxically, responses to acute light pulses measured by behavioral phase shifts, SCN FOS protein and Period1 mRNA expression are normal. Together, the developmental pattern of CalB expression in mouse SCN, the presence of CalB in photoresponsive ganglion cells and the abnormalities seen in CalB-/- mice suggest an important role for CalB in mouse circadian function.

Development of the mouse suprachiasmatic nucleus: determination of time of cell origin and spatial arrangements within the nucleus

The suprachiasmatic nucleus (SCN) in mammals functions as the principal circadian pacemaker synchronizing diverse physiological and behavioral processes to environmental stimuli. It consists of heterogeneous populations of cells with unique spatial organization that can vary among species, but are commonly discussed within a framework of two principal regions, the ventrolateral or dorsomedial halves of the nucleus or in other instances the core and shell. In both hamsters and rats, cells of different SCN regions have been shown to have different developmental histories. Using bromodeoxyuridine as a marker of cell division, the present study investigated the time of SCN cell origin in mice (C57BL/6) and their settling patterns within the nucleus. Results show that SCN cytogenesis occurs between embryonic days 12 and 15 and is complete 5 days prior to birth. Cells born on embryonic day 12 are mainly confined to a ventrolateral region of the mid-SCN, whereas cells produced later on embryonic days 13.5 and 14.5 form a cap around the cells produced first and extend into the posterior and anterior ends of the nucleus. These results suggest an ordered spatiotemporal program of SCN cytogenesis whereby a mid-SCN core is born first followed by a surrounding shell of later-born cells. Variations in cytogenesis could affect the relative sizes of different SCN regions and, thereby, affect its function. The relative contributions of a highly ordered program of cytogenesis and intercellular interactions after postmitotic cells leave the germinal epithelium remain to be determined.

Minireview: The neuroendocrinology of the suprachiasmatic nucleus as a conductor of body time in mammals

Circadian rhythms in physiology and behavior are regulated by a master clock resident in the suprachiasmatic nucleus (SCN) of the hypothalamus, and dysfunctions in the circadian system can lead to serious health effects. This paper reviews the organization of the SCN as the brain clock, how it regulates gonadal hormone secretion, and how androgens modulate aspects of circadian behavior known to be regulated by the SCN. We show that androgen receptors are restricted to a core SCN region that receives photic input as well as afferents from arousal systems in the brain. We suggest that androgens modulate circadian behavior directly via actions on the SCN and that both androgens and estrogens modulate circadian rhythms through an indirect route, by affecting overall activity and arousal levels. Thus, this system has multiple levels of regulation; the SCN regulates circadian rhythms in gonadal hormone secretion, and hormones feed back to influence SCN functions.

Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons

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

A role for androgens in regulating circadian behavior and the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is the locus of a master circadian clock controlling behavioral and physiological rhythms, including rhythmic secretion of gonadal hormones. Gonadectomy results in marked alteration of circadian behaviors, including lengthened free-running period, decreased precision of daily onset of running, and elimination of early-evening but not late-night activity bouts. Androgen replacement restores these responses. These aspects of rhythmicity are thought to be regulated by the brain clock, although the site of androgen action remains unknown. Anatomically, the rodent SCN is composed of a ventrolateral core and a dorsomedial shell, and the present studies show that androgen receptors (AR) are localized to the ventrolateral core SCN. Using a transgenic mouse bearing dual reporter molecules driven by the AR targeted to both membrane and nucleus, we find that projections of AR-containing cells form a dense plexus in the core, with their fibers appearing to exit the SCN dorsally. In a second transgenic strain, in which the retinorecipient gastrin-releasing peptide cells express a green fluorescent protein reporter, we show that gastrin-releasing peptide cells contain AR. Through immunocytochemistry, we also show that SCN AR cells express FOS after a light pulse. Importantly, gonadectomy reduces the FOS response after a phase-shifting light pulse, whereas androgen replacement restores levels to those in intact animals. Taken together, the results support previous findings of a hypothalamic neuroendocrine feedback loop. As such, the SCN regulates circadian rhythms in gonadal hormone secretion, and in turn, androgens act on their receptors within the SCN to alter circadian function.

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.

Calbindin D28K protein cells in a primate suprachiasmatic nucleus: localization, daily rhythm and age-related changes

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian pacemaker. The SCN controls daily rhythms and synchronizes the organism to its environment and especially to photic signals. Photic signals via the retinohypothalamic tract reach the ventral part of the SCN, where the majority of calbindin-containing neurons are located. Calbindin cells seem important for the control of circadian rhythmicity. As ageing leads to marked changes in the expression of circadian rhythms, we investigated in the mouse lemur, a nocturnal primate, age-related changes in the oscillation of calbindin protein expression in SCN neurons. We used immunohistochemistry and quantitative analysis of calbindin expression in the SCN of adult and aged mouse lemurs. In this primate, a dense cluster of calbindin-positive neurons was found in the ventral part of the SCN. In adult animals, calbindin-positive SCN neurons did not exhibit daily rhythms in their number or intensity, but exhibited significant daily variations in the percentage of cells with a calbindin-positive nucleus, characterized by high values during the daytime and low values during the night. Immunoreactive intensity peaked in the middle of the daytime. Calbindin expression in the nuclei of calbindin cells in the SCN tends to be modified by ageing. The amplitude of daily variation in calbindin expression was damped, with a lower immunointensity during the daytime and a delayed decrease during the night. These changes may affect the ability of the SCN to transmit rhythmic information to other SCN cells and thereby modify the synchronization of the different cell populations in the SCN.

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.

Gates and oscillators II: zeitgebers and the network model of the brain clock

Circadian rhythms in physiology and behavior are regulated by the SCN. When assessed by expression of clock genes, at least 2 distinct functional cell types are discernible within the SCN: nonrhythmic, light-inducible, retinorecipient cells and rhythmic autonomous oscillator cells that are not directly retinorecipient. To predict the responses of the circadian system, the authors have proposed a model based on these biological properties. In this model, output of rhythmic oscillator cells regulates the activity of the gate cells. The gate cells provide a daily organizing signal that maintains phase coherence among the oscillator cells. In the absence of external stimuli, this arrangement yields a multicomponent system capable of producing a self-sustained consensus rhythm. This follow-up study considers how the system responds when the gate cells are activated by an external stimulus, simulating a response to an entraining (or phase-setting) signal. In this model, the authors find that the system can be entrained to periods within the circadian range, that the free-running system can be phase shifted by timed activation of the gate, and that the phase response curve for activation is similar to that observed when animals are exposed to a light pulse. Finally, exogenous triggering of the gate over a number of days can organize an arrhythmic system, simulating the light-dependent reappearance of rhythmicity in a population of disorganized, independent oscillators. The model demonstrates that a single mechanism (i.e., the output of gate cells) can account for not only free-running and entrained rhythmicity but also other circadian phenomena, including limits of entrainment, a PRC with both delay and advance zones, and the light-dependent reappearance of rhythmicity in an arrhythmic animal.

Brain-derived neurotrophic factor and neurotrophin receptors modulate glutamate-induced phase shifts of the suprachiasmatic nucleus

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells. Previous work raised the possibility that brain-derived neurotrophic factor (BDNF) and its high-affinity tropomyosin-related receptor kinase may be important as modulators of this excitatory input into the SCN. In order to test this possibility, we used whole-cell patch-clamp methods to measure spontaneous excitatory currents in mouse SCN neurons. We found that the amplitude and frequency of these currents were increased by BDNF and decreased by the neurotrophin receptor inhibitor K252a. The neurotrophin also increased the magnitude of currents evoked by application of N-methyl-d-aspartate and amino-methyl proprionic acid. Next, we measured the rhythms in action potential discharge from the SCN brain slice preparation. We found that application of K252a dramatically reduced the magnitude of phase shifts of the electrical activity rhythm generated by the application of glutamate. By itself, BDNF caused phase shifts that resembled those produced by glutamate and were blocked by K252a. The results demonstrate that BDNF and neurotrophin receptors can enhance glutamatergic synaptic transmission within a subset of SCN neurons and potentiate glutamate-induced phase shifts of the circadian rhythm of neural activity 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.

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."

Diurnal regulation of the gastrin-releasing peptide receptor in the mouse circadian clock

In mammals, circadian rhythms are generated by the suprachiasmatic nuclei (SCN) of the hypothalamus. SCN neurons are heterogeneous and can be classified according to their function, anatomical connections, morphology and/or peptidergic identity. We focus here on gastrin-releasing peptide- (GRP) and on GRP receptor- (GRPr) expressing cells of the SCN. Pharmacological application of GRP in vivo or in vitro can shift the phase of circadian rhythms, and GRPr-deficient mice show blunted photic phase shifting. Given the in vivo and in vitro effects of GRP on circadian behavior and on SCN neuronal activity, we investigated whether the GRPr might be under circadian and/or diurnal control. Using in situ hybridization and autoradiographic receptor binding, we localized the GRPr in the mouse SCN and determined that GRP binding varies with time of day in animals housed in a light-dark cycle but not in conditions of constant darkness. The latter results were confirmed with Western blots of SCN tissue. Together, the present findings reveal that changes in GRPr are light driven and not endogenously organized. Diurnal variation in GRPr activity probably underlies intra-SCN signaling important for entrainment and phase shifting.

Signaling within the master clock of the brain: localized activation of mitogen-activated protein kinase by gastrin-releasing peptide

The circadian clock located in the mammalian suprachiasmatic nucleus (SCN) exhibits substantial heterogeneity in both its neurochemical and functional organization, with retinal input and oscillatory timekeeping functions segregated to different regions within the nucleus. Although it is clear that photic information must be relayed from directly retinorecipient cells to the population of oscillator cells within the nucleus, the intra-SCN signal (or signals) underlying such communication has yet to be identified. Gastrin-releasing peptide (GRP), which is found within calbindin-containing retinorecipient cells and causes photic-like phase shifts when applied directly to the SCN, is a candidate molecule. Here we examine the effect of GRP on both molecular and behavioral properties of the hamster circadian system. Within 30 min a third ventricle injection of GRP produces an increase in the number of cells expressing the phosphorylated form of extracellular signal-regulated kinases 1/2 (p-ERK1/2), localized in a discrete group of SCN cells that form a cap dorsal to calbindin cells and lateral to vasopressin cells. At 1 h after the peak of p-ERK expression these cap cells express c-fos, Period1, and Period2. Pharmacological blockade of ERK phosphorylation attenuates phase shifts to GRP. These data indicate that GRP is an output signal of retinorecipient SCN cells and activates a small cluster of SCN neurons. This novel cell group likely serves as a relay or integration point for communicating photic phase-resetting information to the rhythmic cells of the SCN. These findings represent a first step in deconstructing the SCN network constituting the brain clock.

Complex organization of mouse and rat suprachiasmatic nucleus

The suprachiasmatic nucleus, site of the dominant mammalian circadian clock, contains a variety of different neurons that tend to form groups within the nucleus. The present investigation used single and multiple label tract tracing and immunofluorescence methods to evaluate the relative locations of the neuron groups and to compare them with the distributions of the three major afferent projections, the retinohypothalamic tract, geniculohypothalamic tract and the serotonergic pathway from the median raphe nucleus. The suprachiasmatic nucleus has a complex order characterized by peptidergic cell groups (vasopressin, gastrin releasing peptide, vasoactive intestinal polypeptide, calbindin, calretinin, corticotrophin releasing factor and enkephalin) that, in most cases, substantially overlap. The retinohypothalamic tract projects bilaterally to virtually all the suprachiasmatic nucleus in both rat (predominantly contralateral) and mouse (symmetric) and its terminal field overlaps that for the geniculohypothalamic tract, but with distinctions visible according to density criteria; neither provides more than sparse innervation of the dorsomedial suprachiasmatic nucleus. In the mouse, the serotonergic terminal field is densest medially and ventrally, but is also distributed elsewhere with varying density. The serotonergic terminal plexus in the rat is densest centromedially and largely, but not completely, overlaps the complete distribution of retinal terminals with density much reduced in the lateral suprachiasmatic nucleus. The locations of vasopressin neurons, retinohypothalamic tract terminals and serotonergic (mouse, rat) or geniculohypothalamic tract (rat) provide evidence for three clear, but not exclusionary, sectors of the suprachiasmatic nucleus. The data, in conjunction with emerging knowledge concerning rhythmically dynamic changes in the size of regions of neuropeptide gene expression in suprachiasmatic nucleus cells, support the view that suprachiasmatic nucleus organization is more complex than a simple "core" and "shell" arrangement. While generalizations about suprachiasmatic nucleus organization can be made with respect to location of cell phenotypes or terminal fields, oversimplification may hinder, rather than facilitate, understanding of suprachiasmatic nucleus structure-function relationships.

Two antiphase oscillations occur in each suprachiasmatic nucleus of behaviorally split hamsters

The suprachiasmatic nuclei (SCNs) control circadian rhythms of numerous behavioral and physiological responses. In hamsters, constant light causes "splitting" of circadian rhythms, such that a single daily bout of activity separates into two components, 12 h apart, with antiphase circadian oscillations in the left and right SCN. Given the phenotypic and functional heterogeneity of the SCN, in which ventrolateral but not dorsomedial neurons are retinorecipient, we asked how these two compartments respond to the constant lighting conditions that produce splitting, using three different phase markers of neuronal activity: PER1 (Period 1), c-FOS, and pERK (phosphorylated extracellular signal-regulated kinase). We report the emergence of a coherent novel network in which each side of the SCN exhibits two antiphase oscillating subregions, here termed "core-like" and "shell-like," in addition to the known antiphase oscillation between the right and left SCN. The novel SCN response entails a coherent rhythm in a core-like region of the SCN, which otherwise is not cycling. A mathematical model is presented, and this model interprets the observed changes in the proportion of in-phase and antiphase populations of SCN oscillators and suggests novel testable hypotheses. Finally, the functional significance of this network was explored by investigating the adjacent hypothalamus. Activation of the paraventricular nucleus is in-phase with the ipsilateral core-like SCN, whereas activation of the lateral subparaventricular zone is in-phase with the ipsilateral shell-like SCN, pointing to a multiplicity of SCN output signals. These results suggest a neural basis for internal coincidence of SCN oscillators, and a novel mechanism of plasticity in SCN neural networks and outputs.

Dark pulse suppression of P-ERK and c-Fos in the hamster suprachiasmatic nuclei

It is well-established that light pulses regulate components of the extracellular signal-regulated kinases I/II (ERK) cascade in the suprachiasmatic nuclei (SCN) circadian clock. These events are important for photic-resetting of the circadian clock. The SCN circadian clock is also reset by pulses of dark, but it is unknown if this stimulus alters the activity of ERK, the transcription factor Elk-1 or expression of the immediate early gene c-fos in the SCN. Using Syrian hamsters free-running in constant light, we determined the effects of dark pulses on these factors in the SCN. In constant light, levels of phosphorylated ERK (P-ERK) showed significant circadian variation in the Syrian hamster SCN, while levels of c-Fos or phosphorylated Elk-1 (P-Elk-1) did not. A 6-h dark pulse beginning at circadian time (CT) 8 down-regulated expression of P-ERK and c-Fos, but not P-Elk-1, in the SCN. Following termination of the pulse, levels of c-Fos increased above time-matched control values, while P-ERK expression did not. When given at the beginning of the subjective night (CT13), a 6-h dark pulse did not phase-shift behavioural rhythms and failed to alter the expression of c-Fos, P-ERK, or P-Elk-1 in the SCN. At the level of the visual thalamus, expression of c-Fos in the intergeniculate leaflet was higher during the subjective night as compared to the subjective day, although dark pulses had no robust effects on expression of c-Fos or P-ELK-1 in this structure. We conclude that dark-pulse resetting of the circadian clock is complex and involves both non-photic and photic components.

Neurogenesis and ontogeny of specific cell phenotypes within the hamster suprachiasmatic nucleus

The hamster suprachiasmatic nucleus (SCN) is anatomically and functionally heterogeneous. A group of cells in the SCN shell, delineated by vasopressin-ergic neurons, are rhythmic with respect to Period gene expression and electrical activity but do not receive direct retinal input. In contrast, some cells in the SCN core, marked by neurons containing calbindin-D28k, gastrin-releasing peptide (GRP), substance P (SP), and vasoactive intestinal polypeptide (VIP), are not rhythmic with respect to Period gene expression and electrical activity but do receive direct retinal input. Examination of the timing of neurogenesis using bromodeoxyuridine indicates that SCN cells are born between embryonic day 9.5 and 12.5. Calbindin, GRP, substance P, and VIP cells are born only during early SCN neurogenesis, between embryonic days 9.5-11.0. Vasopressin cells are born over the whole period of SCN neurogenesis, appearing as late as embryonic day 12.5. Examination of the ontogeny of peptide expression in these cell types reveals transient expression of calbindin in a cluster of dorsolateral SCN cells on postnatal days 1-2. The adult pattern of calbindin expression is detected in a different ventrolateral cell cluster starting on postnatal day 2. GRP and SP expression appear on postnatal day 8 and 10, respectively, after the retinohypothalamic tract has innervated the SCN. In summary, the present study describes the ontogeny-specific peptidergic phenotypes in the SCN and compares these developmental patterns to previously identified patterns in the appearance of circadian functions. These comparisons suggest the possibility that these coincident appearances may be causally related, with the direction of causation to be determined.

Orchestrating time: arrangements of the brain circadian clock

Daily oscillations in physiology and behavior are regulated by a brain clock located in the suprachiasmatic nucleus (SCN). Individual cells within this nucleus contain an autonomous molecular clock. Recent discoveries that make use of new molecular and genetic data and tools highlight the conclusion that the SCN is a heterogeneous network of functionally and phenotypically differentiated cells. Neurons within SCN subregions serve distinctly separate functions in regulating the overall activity of the circadian clock: some cells within the SCN rhythmically express "clock" genes, whereas others exhibit induced expression of these genes after the organism has been exposed to a light pulse. The coordinated interaction of these functionally distinct cells is integral to the coherent functioning of the brain clock.