Altered Light Sensitivity of Circadian Clock in Shank3+/- Mouse

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairment in communication and social interaction, repetitive or stereotypical behaviors, altered sensory perception, and sleep disorders. In general, the causes of ASD remain unknown, but in Phelan-McDermid syndrome, it is known that the disorder is related to the haploinsufficiency of the Shank3 gene. We used an autism model with compromised glutamatergic signaling, the Shank3+/- mouse, to study the circadian rhythm architecture of locomotion behavior and its entrainment to light. We also analyzed the synapse between the retinohypothalamic tract (RHT) and the suprachiasmatic nucleus (SCN), employing tract tracing and immunohistochemical techniques. We found that Shank3+/- mice were not impaired in the SCN circadian clock, as indicated by a lack of differences between groups in the circadian architecture in entrained animals to either long or short photoperiods. Circadian rhythm periodicity (tau) was unaltered between genotypes in constant darkness (DD, dim red light). Similar results were obtained in the re-entrainment to shifts in the light-dark cycle and in the entrainment to a skeleton photoperiod from DD. However, Shank3+/- mice showed larger phase responses to light pulses, both delays and advances, and rhythm disorganization induced by constant bright light. Immunohistochemical analyses indicated no differences in the RHT projection to the SCN or the number of SCN neurons expressing the N-methyl-D-aspartate (NMDA) receptor subunit NR2A, whereas the Shank3+/- animals showed decreased c-Fos induction by brief light pulses at CT14, but increased number of vasoactive intestinal polypeptide (VIP)-positive neurons. These results indicate alterations in light sensitivity in Shank3+/- mice. Further studies are necessary to understand the mechanisms involved in such increased light sensitivity, probably involving VIP neurons.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

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

Diurnal properties of voltage-gated Ca2+ currents in suprachiasmatic nucleus and roles in action potential firing

KEY POINTS

Circadian oscillations in spontaneous action potential firing in the suprachiasmatic nucleus (SCN) translate time-of-day throughout the mammalian brain. The ion channels that regulate the circadian pattern of SCN firing have not been comprehensively identified. Ca²⁺ channels regulate action potential activity across many types of excitable cells, and the activity of L-, N-, P/Q- and R-type channels are required for normal daytime firing frequency in SCN neurons and circuit rhythms. Only the L-type Ca²⁺ current exhibits a day versus night difference in current magnitude, providing insight into the mechanism that produces rhythmic action potential firing in SCN.

ABSTRACT

The mammalian circadian clock encodes time via rhythmic action potential activity in the suprachiasmatic nucleus (SCN) of the hypothalamus, which governs daily rhythms in physiology and behaviour. SCN neurons exhibit 24 h oscillations in spontaneous firing, with higher firing during day compared to night. Several ionic currents have been identified that regulate SCN firing, including voltage-gated Ca²⁺ currents, but the circadian regulation of distinct voltage-gated Ca²⁺ channel (VGCC) components has not been comprehensively addressed. In this study, whole-cell L- (nimodipine-sensitive), N- and P/Q- (ω-agatoxin IVA, ω-conotoxin GVIA, ω-conotoxin MVIIC-sensitive), R- (Ni²⁺ -sensitive) and T-type (TTA-P2-sensitive) currents were recorded from day and night SCN slices. Using standard voltage protocols, Ni²⁺ -sensitive currents comprised the largest proportion of total VGCC current, followed by nimodipine-, ω-agatoxin IVA-, ω-conotoxin GVIA- and TTA-P2-sensitive currents. Only the nimodipine-sensitive current exhibited a diurnal difference in magnitude, with daytime current larger than night. No diurnal variation was observed for the other Ca²⁺ current subtypes. The difference in nimodipine-sensitive current was due to larger peak current activated during the day, not differences in inactivation, and was eliminated by Bay K8644. Blocking L-type channels decreased firing selectively during the day, consistent with higher current magnitudes, and reduced SCN circuit rhythmicity recorded by multi-electrode arrays. Yet blocking N-, P/Q- and R-type channels also decreased daytime firing, with little effect at night, and decreased circuit rhythmicity. These data identify a unique diurnal regulation of L-type current among the major VGCC subtypes in SCN neurons, but also reveal that diurnal modulation is not required for time-of-day-specific effects on firing and circuit rhythmicity.

Copper in the suprachiasmatic circadian clock: A possible link between multiple circadian oscillators

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is very robust, able to coordinate our daily physiological and behavioral rhythms with exquisite accuracy. Simultaneously, the SCN clock is highly sensitive to environmental timing cues such as the solar cycle. This duality of resiliency and sensitivity may be sustained in part by a complex intertwining of three cellular oscillators: transcription/translation, metabolic/redox, and membrane excitability. We suggest here that one of the links connecting these oscillators may be forged from copper (Cu). Cellular Cu levels are highly regulated in the brain and peripherally, and Cu affects cellular metabolism, redox state, cell signaling, and transcription. We have shown that both Cu chelation and application induce nighttime phase shifts of the SCN clock in vitro and that these treatments affect glutamate, N-methyl-D-aspartate receptor, and associated signaling processes differently. More recently we found that Cu induces mitogen-activated protein kinase-dependent phase shifts, while the mechanisms by which Cu removal induces phase shifts remain unclear. Lastly, we have found that two Cu transporters are expressed in the SCN, and that one of these transporters (ATP7A) exhibits a day/night rhythm. Our results suggest that Cu homeostasis is tightly regulated in the SCN, and that changes in Cu levels may serve as a time cue for the circadian clock. We discuss these findings in light of the existing literature and current models of multiple coupled circadian oscillators in the SCN.

The circadian clock: A tale of genetic-electrical interplay and synaptic integration

Pioneering work in Drosophila uncovered the building blocks of the molecular clock, consisting of transcription-translation feedback loops (TTFLs). Subsequent experimental work demonstrated that the mammalian TTFL is localized in cells and tissues throughout the brain and body. Further research established that neuronal activity forms an essential aspect of clock function. However, how the membrane electrical activity of clock neurons of the suprachiasmatic nucleus collaborate with the TTFL to drive circadian behaviors remains mostly unknown. Intercellular communication synchronizes the individual circadian oscillators to produce a precise and coherent circadian output. Here, we briefly review significant research that is increasing our understanding of the critical interactions between the TTFL and neuronal and glial activity in the generation of circadian timing signals.

Calcium-regulated chloride channel anoctamin-1 is present in the suprachiasmatic nuclei of rats

Calcium-regulated chloride channel (CaCC) anoctamin-1 has been recently identified in neurons. In neurons, which express the Na-K-2Cl cotransporter, activation of CaCCs increases firing frequency, by reversion between the Cl equilibrium potential and the membrane resting potential, leading to membrane depolarization by Cl extrusion from the cell. Although there are no reports of CaCCs present in the suprachiasmatic nuclei (SCN), the fact that Na-K-2Cl cotransporter is present in SCN neurons, where it has been shown to be involved in the excitatory effects of γ-aminobutyric acid, together with the increase of neuronal firing rate induced by release of intracellular Ca after administration of 100 nM ryanodine, leads us to determine whether CaCCs are present in the SCN. Immunohistochemistry and western blots show the expression of the CaCCs anoctamin-1 protein. Quantitative PCR demonstrated the expression of anoctamin-1 mRNA in the SCN. These results clearly indicate the presence of CaCC in SCN of rats.

Neuronal oscillations on an ultra-slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

Neuronal oscillations of the brain, such as those observed in the cortices and hippocampi of behaving animals and humans, span across wide frequency bands, from slow delta waves (0.1 Hz) to ultra-fast ripples (600 Hz). Here, we focus on ultra-slow neuronal oscillators in the hypothalamic suprachiasmatic nuclei (SCN), the master daily clock that operates on interlocking transcription-translation feedback loops to produce circadian rhythms in clock gene expression with a period of near 24 h (< 0.001 Hz). This intracellular molecular clock interacts with the cell's membrane through poorly understood mechanisms to drive the daily pattern in the electrical excitability of SCN neurons, exhibiting an up-state during the day and a down-state at night. In turn, the membrane activity feeds back to regulate the oscillatory activity of clock gene programs. In this review, we emphasise the circadian processes that drive daily electrical oscillations in SCN neurons, and highlight how mathematical modelling contributes to our increasing understanding of circadian rhythm generation, synchronisation and communication within this hypothalamic region and across other brain circuits.

Differential contribution of Ca2+ sources to day and night BK current activation in the circadian clock

Large conductance K⁺ (BK) channels are expressed widely in neurons, where their activation is regulated by membrane depolarization and intracellular Ca²⁺ (Ca²⁺_i). To enable this regulation, BK channels functionally couple to both voltage-gated Ca²⁺ channels (VGCCs) and channels mediating Ca²⁺ release from intracellular stores. However, the relationship between BK channels and their specific Ca²⁺ source for particular patterns of excitability is not well understood. In neurons within the suprachiasmatic nucleus (SCN)-the brain's circadian clock-BK current, VGCC current, and Ca²⁺_i are diurnally regulated, but paradoxically, BK current is greatest at night when VGCC current and Ca²⁺_i are reduced. Here, to determine whether diurnal regulation of Ca²⁺ is relevant for BK channel activation, we combine pharmacology with day and night patch-clamp recordings in acute slices of SCN. We find that activation of BK current depends primarily on three types of channels but that the relative contribution changes between day and night. BK current can be abrogated with nimodipine during the day but not at night, establishing that L-type Ca²⁺ channels (LTCCs) are the primary daytime Ca²⁺ source for BK activation. In contrast, dantrolene causes a significant decrease in BK current at night, suggesting that nighttime BK activation is driven by ryanodine receptor (RyR)-mediated Ca²⁺_i release. The N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC causes a smaller reduction of BK current that does not differ between day and night. Finally, inhibition of LTCCs, but not RyRs, eliminates BK inactivation, but the BK β2 subunit was not required for activation of BK current by LTCCs. These data reveal a dynamic coupling strategy between BK channels and their Ca²⁺ sources in the SCN, contributing to diurnal regulation of SCN excitability.

Calcium Circadian Rhythmicity in the Suprachiasmatic Nucleus: Cell Autonomy and Network Modulation

Circadian rhythms of mammalian physiology and behavior are coordinated by the suprachiasmatic nucleus (SCN) in the hypothalamus. Within SCN neurons, various aspects of cell physiology exhibit circadian oscillations, including circadian clock gene expression, levels of intracellular Ca²⁺ ([Ca²⁺]_i), and neuronal firing rate. [Ca²⁺]_i oscillates in SCN neurons even in the absence of neuronal firing. To determine the causal relationship between circadian clock gene expression and [Ca²⁺]_i rhythms in the SCN, as well as the SCN neuronal network dependence of [Ca²⁺]_i rhythms, we introduced GCaMP3, a genetically encoded fluorescent Ca²⁺ indicator, into SCN neurons from PER2::LUC knock-in reporter mice. Then, PER2 and [Ca²⁺]_i were imaged in SCN dispersed and organotypic slice cultures. In dispersed cells, PER2 and [Ca²⁺]_i both exhibited cell autonomous circadian rhythms, but [Ca²⁺]_i rhythms were typically weaker than PER2 rhythms. This result matches the predictions of a detailed mathematical model in which clock gene rhythms drive [Ca²⁺]_i rhythms. As predicted by the model, PER2 and [Ca²⁺]_i rhythms were both stronger in SCN slices than in dispersed cells and were weakened by blocking neuronal firing in slices but not in dispersed cells. The phase relationship between [Ca²⁺]_i and PER2 rhythms was more variable in cells within slices than in dispersed cells. Both PER2 and [Ca²⁺]_i rhythms were abolished in SCN cells deficient in the essential clock gene Bmal1. These results suggest that the circadian rhythm of [Ca²⁺]_i in SCN neurons is cell autonomous and dependent on clock gene rhythms, but reinforced and modulated by a synchronized SCN neuronal network.

Ryanodine-sensitive intracellular Ca2+ channels are involved in the output from the SCN circadian clock

The suprachiasmatic nuclei (SCN) contain the major circadian clock responsible for generation of circadian rhythms in mammals. The time measured by the molecular circadian clock must eventually be translated into a neuronal firing rate pattern to transmit a meaningful signal to other tissues and organs in the animal. Previous observations suggest that circadian modulation of ryanodine receptors (RyR) is a key element of the output pathway from the molecular circadian clock. To directly test this hypothesis, we studied the effects of RyR activation and inhibition on real time expression of PERIOD2::LUCIFERASE, intracellular calcium levels and spontaneous firing frequency in mouse SCN neurons. Furthermore, we determined whether the RyR-2 mRNA is expressed with a daily variation in SCN neurons. We provide evidence that pharmacological manipulation of RyR in mice SCN neurons alters the free [Ca²⁺ ]_i in the cytoplasm and the spontaneous firing without affecting the molecular clock mechanism. Our data also show a daily variation in RyR-2 mRNA from single mouse SCN neurons with highest levels during the day. Together, these results confirm the hypothesis that RyR-2 is a key element of the circadian clock output from SCN neurons.

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

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

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.