GABAA, GABAC, and NMDA receptor subunit expression in the suprachiasmatic nucleus and other brain regions

Postoperative Delirium in Neurosurgical Patients: Recent Insights into the Pathogenesis

Postoperative delirium (POD) is a complication characterized by disturbances in attention, awareness, and cognitive function that occur shortly after surgery or emergence from anesthesia. Since it occurs prevalently in neurosurgical patients and poses great threats to the well-being of patients, much emphasis is placed on POD in neurosurgical units. However, there are intricate theories about its pathogenesis and limited pharmacological interventions for POD. In this study, we review the recent insights into its pathogenesis, mainly based on studies within five years, and the five dominant pathological theories that account for the development of POD, with the intention of furthering our understanding and boosting its clinical management.

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

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 Phase shifts in these peripheral explants were withdrawn due to the recovery period after the anesthetic treatment, which suggests that peripheral tissues require the assistance of related tissues or organs to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

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

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

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

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Sevoflurane weakened the bioluminescence intensity of all explant types.

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

Systematic review of drugs that modify the circadian system's phase-shifting responses to light exposure

We searched PubMed for primary research quantifying drug modification of light-induced circadian phase-shifting in rodents. This search, conducted for work published between 1960 and 2018, yielded a total of 146 papers reporting results from 901 studies. Relevant articles were those with any extractable data on phase resetting in wildtype (non-trait selected) rodents administered a drug, alongside a vehicle/control group, near or at the time of exposure. Most circadian pharmacology experiments were done using drugs thought to act directly on either the brain's central pacemaker, the suprachiasmatic nucleus (SCN), the SCN's primary relay, the retinohypothalamic tract, secondary pathways originating from the medial/dorsal raphe nuclei and intergeniculate leaflet, or the brain's sleep-arousal centers. While the neurotransmitter systems underlying these circuits were of particular interest, including those involving glutamate, gamma-aminobutyric acid, serotonin, and acetylcholine, other signaling modalities have also been assessed, including agonists and antagonists of receptors linked to dopamine, histamine, endocannabinoids, adenosine, opioids, and second-messenger pathways downstream of glutamate receptor activation. In an effort to identify drugs that unduly influence circadian responses to light, we quantified the net effects of each drug class by ratioing the size of the phase-shift observed after administration to that observed with vehicle in a given experiment. This allowed us to organize data across the literature, compare the relative efficacy of one mechanism versus another, and clarify which drugs might best suppress or potentiate phase resetting. Aggregation of the available data in this manner suggested that several candidates might be clinically relevant as auxiliary treatments to suppress ectopic light responses during shiftwork or amplify the circadian effects of timed bright light therapy. Future empirical research will be necessary to validate these possibilities.

The Effects of General Anaesthesia and Light on Behavioural Rhythms and GABAA Receptor Subunit Expression in the Mouse SCN

General anaesthesia (GA) is known to affect the circadian clock. However, the mechanisms that underlie GA-induced shifting of the clock are less well understood. Activation of γ-aminobutyric acid (GABA)_-type A receptors (GABA_AR) in the suprachiasmatic nucleus (SCN) can phase shift the clock and thus GABA and its receptors represent a putative pathway via which GA exerts its effect on the clock. Here, we investigated the concurrent effects of the inhalational anaesthetic, isoflurane, and light, on mouse behavioural locomotor rhythms and on α1, β3, and γ2 GABA_AR subunit expression in the SCN of the mouse brain. Behavioural phase shifts elicited by exposure of mice to four hours of GA (2% isoflurane) and light (400 lux) (n = 60) were determined by recording running wheel activity rhythms in constant conditions (DD). Full phase response curves for the effects of GA + light on behavioural rhythms show that phase shifts persist in anaesthetized mice exposed to light. Daily variation was detected in all three GABA_AR subunits in LD 12:12. The γ2 subunit expression was significantly increased following GA in DD (compared to light alone) at times of large behavioural phase delays. We conclude that the phase shifting effect of light on the mouse clock is not blocked by GA administration, and that γ2 may potentially be involved in the phase shifting effect of GA on the clock. Further analysis of GABA_AR subunit expression in the SCN will be necessary to confirm its role.

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.

Melatonin recovers sleep phase delayed by MK-801 through the melatonin MT2 receptor- Ca2+ -CaMKII-CREB pathway in the ventrolateral preoptic nucleus

Melatonin (MLT) is widely used to treat sleep disorders although the underlying mechanism is still elusive. In mice, using wheel-running detection, we found that exogenous MLT could completely recover the period length prolonged by N-methyl-D-aspartate receptor (NMDAR) impairment due to the injection of the NMDAR antagonist MK-801, a preclinical model of psychosis. The analysis of the possible underlying mechanisms indicated that MLT could regulate the homeostatic state in the ventrolateral preoptic nucleus (VLPO) instead of the circadian process in the suprachiasmatic nucleus (SCN). In addition, our data showed that MK-801 decreased Ca²⁺ -related CaMKII expression and CREB phosphorylation levels in the VLPO, and MLT could rescue these intracellular impairments but not NMDAR expression levels. Accordingly, Gcamp6 AAV virus was injected in-vivo to further monitor intracellular Ca²⁺ levels in the VLPO, and MLT demonstrated a unique ability to increase Ca²⁺ fluorescence compared with MK-801-injected mice. Additionally, using the selective melatonin MT₂ receptor antagonist 4-phenyl-2-propionamidotetralin (4P-PDOT), we discovered that the pharmacological effects of MLT upon NMDAR impairments were mediated by melatonin MT₂ receptors. Using electroencephalography/electromyography (EEG/EMG) recordings, we observed that the latency to the first nonrapid eye movement (NREM) sleep episode was delayed by MK-801, and MLT was able to recover this delay. In conclusion, exogenous MLT by acting upon melatonin MT₂ receptors rescues sleep phase delayed by NMDAR impairment via increasing intracellular Ca²⁺ signaling in the VLPO, suggesting a regulatory role of the neurohormone on the homeostatic system.

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.

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.

GRIN2D/GluN2D NMDA receptor: Unique features and its contribution to pediatric developmental and epileptic encephalopathy

N-methyl-d-aspartate receptors (NMDARs), a subset of ligand-gated ionotropic glutamate receptors, are critical for learning, memory, and neuronal development. However, when NMDAR subunits are mutated, a host of neuropathological conditions can occur, including epilepsy. Recently, genetic variation within the GRIN2D gene, which encodes the GluN2D subunit of the NMDAR, has been associated with a set of early-onset neurological diseases, notably developmental and epileptic encephalopathy (DEE). Importantly, patients with GRIN2D variants are largely refractory to conventional anti-epileptic drug (AED) treatment, highlighting the need to further understand the distinctive characteristics of GluN2D in neurological and pathological functions. In this review, we first summarize GluN2D's unique spatial and temporal expression patterns, electrophysiological profiles, and contributions to both pre- and postsynaptic signaling. Next, we review thirteen unique case studies from DEE patients harboring ten different causal GRIN2D variants. These patients are highly heterogenous, manifesting multiple seizure types, electroencephalographic recordings, and neurological and developmental outcomes. Lastly, this review concludes by highlighting the difficulty in treating patients with DEE-associated GRIN2D variants, and stresses the need for selective therapeutic agents delivered within a precise time window.

Circuit development in the master clock network of mammals

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

Foxd1 is required for terminal differentiation of anterior hypothalamic neuronal subtypes

Although the hypothalamus functions as a master homeostat for many behaviors, little is known about the transcriptional networks that control its development. To investigate this question, we analyzed mice deficient for the Forkhead domain transcription factor Foxd1. Foxd1 is selectively expressed in neuroepithelial cells of the prethalamus and hypothalamus prior to the onset of neurogenesis, and is later restricted to neural progenitors of the prethalamus and anterior hypothalamus. During early stages of neurogenesis, we observed that Foxd1-deficient mice showed reduced expression of Six3 and Vax1 in anterior hypothalamus, but overall patterning of the prethalamus and hypothalamus is unaffected. After neurogenesis is complete, however, a progressive reduction and eventual loss of expression of molecular markers of the suprachiasmatic, paraventricular and periventricular hypothalamic is observed. These findings demonstrate that Foxd1 acts in hypothalamic progenitors to allow sustained expression of a subset of genes selectively expressed in mature neurons of the anterior hypothalamus.

Functional Significance of the Excitatory Effects of GABA in the Suprachiasmatic Nucleus

Over 90% of neurons within the suprachiasmatic nucleus (SCN) express γ-aminobutyric acid (GABA). Although GABA is primarily an inhibitory neurotransmitter, in vitro studies suggest that the activation of GABA_A receptors (GABA_AR) elicits excitation in the adult SCN. The ratio of excitatory to inhibitory responses to GABA depends on the balance of chloride influx by Na⁺-K⁺-Cl^- cotransporter 1 (NKCC1) and chloride efflux by K⁺-Cl^- cotransporters (KCCs). Excitatory responses to GABA can be blocked by inhibition of the inward chloride cotransporter, NKCC1, with the loop diuretic bumetanide. Here we investigated the role of NKCC1 activity in phase shifting the circadian pacemaker in response to photic and nonphotic signals in male Syrian hamsters housed in constant darkness. In the early subjective night (CT 13.5), injection of bumetanide into the SCN reduced light-induced phase delays. However, during the late subjective night (CT 19), bumetanide administration did not alter light-induced phase advances. Injection of bumetanide during the subjective day (CT 6) did not alter the phase of free-running circadian rhythms but attenuated phase advances induced by injection of the GABA_AR agonist muscimol into the SCN. These data support the hypothesis that the excitatory effects of endogenously released GABA contribute to the ability of light to induce phase delays, thereby contributing to the most important function of the circadian system, its entrainment with the day-night cycle. Further, the finding that bumetanide inhibits the phase-advancing effects of muscimol during the subjective day supports the hypothesis that the excitatory responses to GABA also contribute to the ability of nonphotic stimuli to phase shift the circadian pacemaker.

Triheteromeric GluN1/GluN2A/GluN2C NMDARs with Unique Single-Channel Properties Are the Dominant Receptor Population in Cerebellar Granule Cells

NMDA-type glutamate receptors (NMDARs) are ligand-gated ion channels that mediate excitatory neurotransmission in the CNS. Here we describe functional and single-channel properties of triheteromeric GluN1/GluN2A/GluN2C receptors, which contain two GluN1, one GluN2A, and one GluN2C subunits. This NMDAR has three conductance levels and opens in bursts similar to GluN1/GluN2A receptors but with a single-channel open time and open probability reminiscent of GluN1/GluN2C receptors. The deactivation time course of GluN1/GluN2A/GluN2C receptors is intermediate to GluN1/GluN2A and GluN1/GluN2C receptors and is not dominated by GluN2A or GluN2C. We show that triheteromeric GluN1/GluN2A/GluN2C receptors are the predominant NMDARs in cerebellar granule cells and propose that co-expression of GluN2A and GluN2C in cerebellar granule cells occludes cell surface expression of diheteromeric GluN1/GluN2C receptors. This new insight into neuronal GluN1/GluN2A/GluN2C receptors highlights the complexity of NMDAR signaling in the CNS.

Delayed Cryptochrome Degradation Asymmetrically Alters the Daily Rhythm in Suprachiasmatic Clock Neuron Excitability

Suprachiasmatic nuclei (SCN) neurons contain an intracellular molecular circadian clock and the Cryptochromes (CRY1/2), key transcriptional repressors of this molecular apparatus, are subject to post-translational modification through ubiquitination and targeting for proteosomal degradation by the ubiquitin E3 ligase complex. Loss-of-function point mutations in a component of this ligase complex, Fbxl3, delay CRY1/2 degradation, reduce circadian rhythm strength, and lengthen the circadian period by ∼2.5 h. The molecular clock drives circadian changes in the membrane properties of SCN neurons, but it is unclear how alterations in CRY1/2 stability affect SCN neurophysiology. Here we use male and female Afterhours mice which carry the circadian period lengthening loss-of-function Fbxl3^Afh mutation and perform patch-clamp recordings from SCN brain slices across the projected day/night cycle. We find that the daily rhythm in membrane excitability in the ventral SCN (vSCN) was enhanced in amplitude and delayed in timing in Fbxl3^Afh/Afh mice. At night, vSCN cells from Fbxl3^Afh/Afh mice were more hyperpolarized, receiving more GABAergic input than their Fbxl3^+/+ counterparts. Unexpectedly, the progression to daytime hyperexcited states was slowed by Afh mutation, whereas the decline to hypoexcited states was accelerated. In long-term bioluminescence recordings, GABA_A receptor blockade desynchronized the Fbxl3^+/+ but not the Fbxl3^Afh/Afh vSCN neuronal network. Further, a neurochemical mimic of the light input pathway evoked larger shifts in molecular clock rhythms in Fbxl3^Afh/Afh compared with Fbxl3^+/+ SCN slices. These results reveal unanticipated consequences of delaying CRY degradation, indicating that the Afh mutation prolongs nighttime hyperpolarized states of vSCN cells through increased GABAergic synaptic transmission.

SIGNIFICANCE STATEMENT The intracellular molecular clock drives changes in SCN neuronal excitability, but it is unclear how mutations affecting post-translational modification of molecular clock proteins influence the temporal expression of SCN neuronal state or intercellular communication within the SCN network. Here we show for the first time, that a mutation that prolongs the stability of key components of the intracellular clock, the cryptochrome proteins, unexpectedly increases in the expression of hypoexcited neuronal state in the ventral SCN at night and enhances hyperpolarization of ventral SCN neurons at this time. This is accompanied by increased GABAergic signaling and by enhanced responsiveness to a neurochemical mimic of the light input pathway to the SCN. Therefore, post-translational modification shapes SCN neuronal state and network properties.

Temporal Regulation of GABAA Receptor Subunit Expression: Role in Synaptic and Extrasynaptic Communication in the Suprachiasmatic Nucleus

Recent molecular studies suggest that the expression levels of δ and γ2 GABA_A receptor (GABA_AR) subunits regulate the balance between synaptic and extrasynaptic GABA neurotransmission in multiple brain regions. We investigated the expression of GABA_Aδ and GABA_Aγ2 and the functional significance of a change in balance between these subunits in a robust local GABA network contained within the suprachiasmatic nucleus of the hypothalamus (SCN). Muscimol, which can activate both synaptic and extrasynaptic GABA_ARs, injected into the SCN during the day phase advanced the circadian pacemaker, whereas injection of the extrasynaptic GABA_A superagonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) had no effect on circadian phase. In contrast, injection of either THIP or muscimol during the night was sufficient to block the phase shifting effects of light. Gene expression analysis of the whole SCN revealed different temporal patterns in GABA_Aδ and GABA_Aγ2 mRNA expression. When examined across all subregions of the SCN, quantitative immunohistochemical analysis found no significant variations in GABA_Aδ protein immunoreactivity (IR) but did find significant variations in GABA_Aγ2 protein-IR in hamsters housed in either LD cycles or in constant darkness. Remarkably, significant interactions in the ratio of GABA_Aδ:GABA_Aγ2 subunits between lighting condition and circadian phase occurred only within one highly discrete anatomical area of the SCN; a region that functions as the input for lighting information from the retina. Taken together, these data support the hypothesis that the balance between synaptic and extrasynaptic GABA_ARs determines the functional response to GABA, and that this balance is differentially regulated in a region-specific manner.

Astrocytes Control Circadian Timekeeping in the Suprachiasmatic Nucleus via Glutamatergic Signaling

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

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

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Astrocytes direct circadian cycles of extracellular glutamate to inhibit SCN neurons

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

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

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.

Effect of Day and Night Desflurane Anaesthesia on Melatonin Levels in Rats

OBJECTIVE

The aim of this study is to investigate the effect of day and night administration of desflurane anaesthesia on melatonin levels in rats.

METHODS

Twenty-four 15-day-old rats were included in the study and were divided into four groups. The rats were anaesthetised between 19:00-01:00 (night group) and 07:00-13:00 (day group) with 5.7% desflurane concentration in 6 L min^-1 100% oxygen. 6 L min^-1 oxygen was administered to the control groups. At the end of 6 h of anaesthesia, blood samples were taken, and rats were sacrificed. Blood samples were centrifuged and melatonin levels from plasma samples were measured with radioimmunoassay.

RESULTS

There was a statistically significant difference between the groups (p=0.007). Between group day control and group night control there was a statistically significant difference (p=0.042). Further, there was a significant difference between group day control and night desfluran as well (p=0.024). We could not find any difference between other groups.

CONCLUSION

This study showed that 6 hours of 5.7% desflurane anaesthesia during day and night hours did not significantly change melatonin levels.

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.

Enhanced GABAergic inhibition in the premammillary nucleus of photorefractory turkey hens via GABAA receptor upregulation

The premammillary nucleus (PMM) of the turkey mediobasal hypothalamus, where dopamine-melatonin (DA-Mel) neurons are localized, is a site for photoreception and photoperiodic time measurement, which is essential for the initiation of avian reproductive seasonality. In addition, this area could also be responsible for the onset and maintenance of photorefractoriness at the end of the breeding season due to the enhanced inhibitory effect of γ-aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter in the central nervous system which interferes with the photosexual response in the turkey, a seasonally breeding bird. Here, we further characterized the GABAA receptor subunits in the PMM DA-Mel neurons related to reproductive seasonality and the onset of photorefractoriness. GABAA receptor subunits and GABA synthesis enzymes in the PMM of photosensitive and photorefractory turkey hens were identified using real-time qRT-PCR. The upregulation of GABAA receptor α1-3, β2-3, γ1-3, ρ1-3, δ, and θ mRNA expression were observed in the PMM of photorefractory birds when compared to those of photosensitive ones while there is no change observed in the GABA synthesis enzymes, glutamate decarboxylase 1 and 2. Those upregulated GABAA receptor subunits were further examined using immunohistochemical staining and they appeared to be co-localized within the PMM DA-Mel neurons. The upregulation of GABAA receptor subunits observed in the PMM of photorefractory birds coincides with a lack of responsiveness to a light stimulus provided during the photosensitive phase. This is supported by the absence of c-fos induction and TH upregulation in the PMM and a subsequence inhibition of c-fos and GnRH-I expression in the nucleus commissurae pallii. The augmented GABAA receptor subunits expression may mediate an enhancement of inhibitory GABAergic neurotransmission and the subsequent interference with the photosexual response. This could contribute to the state of photorefractoriness and the termination of breeding activities in the turkey, a temperate zone bird.

Epigenetic suppression of mouse Per2 expression in the suprachiasmatic nucleus by the inhalational anesthetic, sevoflurane

BACKGROUND

We previously reported that sevoflurane anesthesia reversibly suppresses the expression of the clock gene, Period2 (Per2), in the mouse suprachiasmatic nucleus (SCN). However, the molecular mechanisms underlying this suppression remain unclear. In this study, we examined the possibility that sevoflurane suppresses Per2 expression via epigenetic modification of the Per2 promoter.

METHODS

Mice were anesthetized with a gas mixture of 2.5% sevoflurane/40% oxygen at a 6 L/min flow for 1 or 4 h. After termination, brains were removed and samples of SCN tissue were derived from frozen brain sections. Chromatin immunoprecipitation (ChIP) assays using anti-acetylated-histone antibodies were performed to investigate the effects of sevoflurane on histone acetylation of the Per2 promoter. Interaction between the E'-box (a cis-element in the Per2 promoter) and CLOCK (the Clock gene product) was also assessed by a ChIP assay using an anti-CLOCK antibody. The SCN concentration of nicotinamide adenine dinucleotide (NAD(+)), a CLOCK regulator, was assessed by liquid chromatography-mass spectrometry.

RESULTS

Acetylation of histone H4 in the proximal region of the Per2 promoter was significantly reduced by sevoflurane. This change in the epigenetic profile of the Per2 gene was observed prior to suppression of Per2 expression. Simultaneously, a reduction in the CLOCK-E'-box interaction in the Per2 promoter was observed. Sevoflurane treatment did not affect the concentration of NAD(+) in the SCN.

CONCLUSIONS

Independent of NAD(+) concentration in the SCN, sevoflurane decreases CLOCK binding to the Per2 promoter E'-box motif, reducing histone acetylation and leading to suppression of Per2 expression.

Direct and specific effect of sevoflurane anesthesia on rat Per2 expression in the suprachiasmatic nucleus

Background

Our previous studies revealed that application of the inhalation anesthetic, sevoflurane, reversibly repressed the expression of Per2 in the mouse suprachiasmatic nucleus (SCN). We aimed to examine whether sevoflurane directly affects the SCN.

Methods

We performed in vivo and in vitro experiments to investigate rat Per2 expression under sevoflurane-treatment. The in vivo effects of sevoflurane on rPer2 expression were examined by quantitative in situ hybridization with a radioactively-labeled cRNA probe. Additionally, we examined the effect of sevoflurane anesthesia on rest/activity rhythms in the rat. In the in vitro experiments, we applied sevoflurane to SCN explant cultures from Per2-dLuc transgenic rats, and monitored luciferase bioluminescence, representing Per2 promoter activity. Bioluminescence from two peripheral organs, the kidney cortex and the anterior pituitary gland, were also analyzed.

Results

Application of sevoflurane in rats significantly suppressed Per2 expression in the SCN compared with untreated animals. We observed no sevoflurane-induced phase-shift in the rest/activity rhythms. In the in vitro experiments, the intermittent application of sevoflurane repressed the increase of Per2-dLuc luminescence and led to a phase delay in the Per2-dLuc luminescence rhythm. Sevoflurane treatment did not suppress bioluminescence in the kidney cortex or the anterior pituitary gland.

Conclusion

The suppression of Per2-dLuc luminescence by sevoflurane in in vitro SCN cultures isolated from peripheral inputs and other nuclei suggest a direct action of sevoflurane on the SCN itself. That sevoflurane has no such effect on peripheral organs suggests that this action might be mediated through a neuron-specific cellular mechanism or a regulation of the signal transduction between neurons.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

RNA editing of the IQ domain in Ca(v)1.3 channels modulates their Ca²⁺-dependent inactivation

Adenosine-to-inosine RNA editing is crucial for generating molecular diversity, and serves to regulate protein function through recoding of genomic information. Here, we discover editing within Ca(v)1.3 Ca²⁺ channels, renown for low-voltage Ca²⁺-influx and neuronal pacemaking. Significantly, editing occurs within the channel's IQ domain, a calmodulin-binding site mediating inhibitory Ca²⁺-feedback (CDI) on channels. The editing turns out to require RNA adenosine deaminase ADAR2, whose variable activity could underlie a spatially diverse pattern of Ca(v)1.3 editing seen across the brain. Edited Ca(v)1.3 protein is detected both in brain tissue and within the surface membrane of primary neurons. Functionally, edited Ca(v)1.3 channels exhibit strong reduction of CDI; in particular, neurons within the suprachiasmatic nucleus show diminished CDI, with higher frequencies of repetitive action-potential and calcium-spike activity, in wild-type versus ADAR2 knockout mice. Our study reveals a mechanism for fine-tuning Ca(v)1.3 channel properties in CNS, which likely impacts a broad spectrum of neurobiological functions.

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

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

Na(V)1.1 channels are critical for intercellular communication in the suprachiasmatic nucleus and for normal circadian rhythms

Na(V)1.1 is the primary voltage-gated Na(+) channel in several classes of GABAergic interneurons, and its reduced activity leads to reduced excitability and decreased GABAergic tone. Here, we show that Na(V)1.1 channels are expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus. Mice carrying a heterozygous loss of function mutation in the Scn1a gene (Scn1a(+/-)), which encodes the pore-forming α-subunit of the Na(V)1.1 channel, have longer circadian period than WT mice and lack light-induced phase shifts. In contrast, Scn1a(+/-) mice have exaggerated light-induced negative-masking behavior and normal electroretinogram, suggesting an intact retina light response. Scn1a(+/-) mice show normal light induction of c-Fos and mPer1 mRNA in ventral SCN but impaired gene expression responses in dorsal SCN. Electrical stimulation of the optic chiasm elicits reduced calcium transients and impaired ventro-dorsal communication in SCN neurons from Scn1a(+/-) mice, and this communication is barely detectable in the homozygous gene KO (Scn1a(-/-)). Enhancement of GABAergic transmission with tiagabine plus clonazepam partially rescues the effects of deletion of Na(V)1.1 on circadian period and phase shifting. Our report demonstrates that a specific voltage-gated Na(+) channel and its associated impairment of SCN interneuronal communication lead to major deficits in the function of the master circadian pacemaker. Heterozygous loss of Na(V)1.1 channels is the underlying cause for severe myoclonic epilepsy of infancy; the circadian deficits that we report may contribute to sleep disorders in severe myoclonic epilepsy of infancy patients.

The effects of glutamate receptor agonists and antagonists on mouse hypothalamic and hippocampal neuronal activity shown through manganese enhanced MRI

Manganese enhanced MRI (MEMRI) is an imaging paradigm that can be used to assess neuronal activity in vivo. Here we investigate, through the use of MEMRI, the influence of receptor dynamics on neuronal activity in the hypothalamus and hippocampus focusing on the glutamate receptor signalling system. We demonstrate that intraperitoneal (i.p.) administration of monosodium glutamate (MSG) and the ionotropic glutamate receptor (iGluR) agonists NMDA and AMPA resulted in significantly increased signal intensity (SI) in the arcuate nucleus (ARC), the suprachiasmatic nucleus (SCN) and the CA3 region of the hippocampus of mice consistent with increased neuronal activity. Administration of the NMDA receptor antagonist MK-801 resulted in significantly decreased SI in the paraventricular nucleus (PVN) consistent with decreased neuronal activity. Co-administration of MSG and the AMPA receptor antagonist NBQX attenuated the increase in SI observed in the ARC from MSG alone, suggesting MEMRI may be applicable to the study of receptor dynamics in vivo. We also observed that administration of the various iGluR agonists and antagonists modulated SI in the lateral ventricle and that high dose MSG (300 mg) caused a hitherto unseen enhancement in SI in the entire cortical/subarachnoid region. In conclusion, MEMRI reveals changes in neuronal activity in response to iGluR agonists and antagonists in the CNS in vivo as well as revealing multifaceted effects beyond those attributable to neuronal activity alone.

Suprachiasmatic nucleus neurons display endogenous resistance to excitotoxicity

A comprehensive understanding of neuroprotective pathways is essential to progress in the battle against numerous neurodegenerative conditions. The hypothalamic suprachiasmatic nucleus (SCN) is endogenously resistant to glutamate (Glu) excitotoxicity in vivo. This study was designed to determine whether immortalized SCN neurons (SCN2.2 cells) retain this characteristic. We first established that SCN2.2 cells retained the ability to respond to Glu. SCN2.2 cells expressed N-methyl-d-aspartate (NMDA) receptor subtypes NR1 and NR2A/2B, suggesting the presence of functional receptors. mRNA for the NMDA receptor subunits NR2A and NR2B were higher in the SCN2.2 than in the control hypothalamic neurons (GT1-7). Specific NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate and d-(-)-2-amino-5-phosphonovaleric acid blocked Glu-induced activation of gene expression. SCN2.2 cells were resistant to Glu excitotoxicity compared with GT1-7 neurons as assessed with a mitochondrial function assay, cell death by trypan blue exclusion and apoptosis by terminal deoxynucleotidyl transferase dUTP nick end labeling. SCN2.2 resistance to Glu excitoxicity was retained in the presence of the broad spectrum Glu transport inhibitor, l-trans-pyrrolidine-2,4 dicarboxylate, excluding glial Glu uptake as a major neuroprotective mechanism. Collectively, these observations demonstrate endogenous neuroprotection in SCN2.2 cells; this cell line is resistant to excitotoxicity under conditions that are toxic to other immortalized cell lines. Thus, the SCN2.2 cell line may provide insights into the molecular mechanisms that confer endogenous neuroprotection in the SCN.

Stoichiometry of N-methyl-D-aspartate receptors within the suprachiasmatic nucleus

The circadian pacemaker within the suprachiasmatic nucleus (SCN) confers daily rhythms to bodily functions. In nature, the circadian clock will adopt a 24-h period by synchronizing to the solar light/dark cycle. This light entrainment process is mediated, in part, at glutamatergic synapses formed between retinal ganglion afferents and SCN neurons. N-methyl-D-aspartate receptors (NMDARs) located on SCN neurons gate light-induced phase resetting. Despite their importance in circadian physiology, little is known about their functional stoichiometry. We investigated the NR2-subunit composition with whole cell recordings of SCN neurons within the murine hypothalamic brain slice using a combination of subtype-selective NMDAR antagonists and voltage-clamp protocols. We found that extracellular magnesium ([Mg](o)) strongly blocks SCN NMDARs exhibiting affinities and voltage sensitivities associated with NR2A and NR2B subunits. These NMDAR currents were inhibited strongly by NR2B-selective antagonists, Ro 25-6981 (3.5 microM, 55.0 +/- 9.0% block; mean +/- SE) and ifenprodil (10 microM, 55.8 +/- 3.0% block). The current remaining showed decreased [Mg](o) affinities reminiscent of NR2C and NR2D subunits but was highly sensitive to [Zn](o), a potent NR2A blocker, showing a approximately 44.2 +/- 1.1% maximal inhibition at saturating concentrations with an IC(50) of 7.8 +/- 1.1 nM. Considering the selectivity, efficacy, and potency of the drugs used in combination with [Mg](o)-block characteristics of the NMDAR, our data show that both diheteromeric NR2B NMDARs and triheteromeric NR2A NMDARs (paired with an NR2C or NR2D subunits) account for the vast majority of the NMDAR current within the SCN.

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

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

Ethanol modulates mammalian circadian clock phase resetting through extrasynaptic GABA receptor activation

Ethanol modulates the actions of multiple neurotransmitter systems, including GABA. However, its enhancing effects on GABA signaling typically are seen only at high concentrations. In contrast, although GABA is a prominent neurotransmitter in the circadian clock of the suprachiasmatic nucleus (SCN), we see ethanol modulation of clock phase resetting at low concentrations (<50 mM). A possible explanation is that ethanol enhances GABAergic signaling in the SCN through activating GABA(A) receptors that contain the delta subunit (GABA(Adelta) receptors), which are sensitive to low ethanol concentrations. Therefore, we investigated whether ethanol acts on GABA(Adelta) receptors in the SCN. Here we show that acute application of the GABA(Adelta) receptor antagonist, RO15-4513, to mouse hypothalamic slices containing the SCN prevents ethanol inhibition of nighttime glutamate-induced (photic-like) phase delays of the circadian clock. Diazepam, which enhances activity of GABA(A) receptors containing the gamma subunit (GABA(Agamma) receptors), does not modulate these phase shifts. Moreover, we find that RO15-4513 prevents ethanol enhancement of daytime serotonergic (non-photic) phase advances of the circadian clock. Furthermore, diazepam phase-advances the SCN circadian clock when applied alone in the daytime, while ethanol has no effect by itself at that time. These data support the hypothesis that ethanol acts on GABA(Adelta) receptors in the SCN to modulate photic and non-photic circadian clock phase resetting. They also reveal distinct modulatory roles of different GABA(A) receptor subtypes in circadian clock phase regulation.

The GABAergic network in the suprachiasmatic nucleus as a key regulator of the biological clock: does it change during senescence?

GABA is the main neurotransmitter of the hypothalamic suprachiasmatic nucleus (SCN) and plays a key role in the function of this master circadian pacemaker. Despite the evidence that disturbances of biological rhythms are common during aging, little is known about the GABAergic network in the SCN of the aging brain. We here provide a brief overview of the GABAergic structures and the role of GABA in the SCN. We also review some age-related changes of the GABAergic system occurring in the brain outside the SCN. Finally, we present preliminary data on the GABAergic system within the SCN comparing young and aging mice. In particular, our study on age-related changes in the SCN focused on the daily expression of the alpha3 subunit of the GABA(A) receptor and on the density of GABAergic axon terminals. Interestingly, our preliminary findings point to alterations of the GABAergic network in the biological clock during senescence.

Phenobarbital blockade of the preovulatory luteinizing hormone surge: association with phase-advanced circadian clock and altered suprachiasmatic nucleus Period1 gene expression

The suprachiasmatic nucleus (SCN) controls the timing of the preovulatory luteinizing hormone (LH) surge in laboratory rodents. Barbiturate administration during a critical period on proestrus delays the surge and prolongs the estrous cycle 1 day. Because a nonphotic timing signal (zeitgeber) during the critical period that phase advances activity rhythms can also induce the latter effect, we hypothesized that barbiturates delay the LH surge by phase-advancing its circadian timing signal beyond the critical period. In experiment 1, locomotor rhythms and estrous cycles were monitored in hamsters for 2-3 wk preinjection and postinjection of vehicle or phenobarbital and after transfer to darkness at zeitgeber time (ZT) 6 on proestrus. Phenobarbital delayed estrous cycles in five of seven hamsters, which exhibited phase shifts that averaged twofold greater than those exhibited by vehicle controls or phenobarbital-injected hamsters with normal cycles. Experiment 2 used a similar protocol, but injections were at ZT 5, and blood samples for LH determination were collected from 1200 to 1800 on proestrus and the next day via jugular cannulae inserted the day before proestrus. Phenobarbital delayed the LH surge 1 day in all six hamsters, but it occurred at an earlier circadian time, supporting the above hypothesis. Experiment 3 investigated whether phenobarbital, like other nonphotic zeitgebers, suppresses SCN Period1 and Period2 transcription. Two hours postinjection, phenobarbital decreased SCN expression of only Period1 mRNA, as determined by in situ hybridization. These results suggest that phenobarbital advances the SCN pacemaker, governing activity rhythms and hormone release in part by decreasing its Period1 gene expression.

Regulation of light's action in the mammalian circadian clock: role of the extrasynaptic GABAA receptor

GABA(A) receptor agonists act in the suprachiasmatic nucleus (SCN) to reset circadian rhythms during the day but inhibit the ability of light to reset rhythms during the night. In the present study, we examined whether these paradoxical differences in the effect of GABA(A) receptor stimulation on the circadian system are mediated by separate GABA(A) receptor subtypes. 4,5,6,7-Tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), a GABA(A) receptor agonist, preferentially activates GABA(A) receptors in extrasynaptic locations. THIP, muscimol (a GABA(A) agonist), or vehicle were microinjected into the SCN region of Syrian hamsters free-running in constant darkness during the mid-subjective day, early subjective night, or late subjective night. The subjective night injections were followed by a light pulse or sham control. Behavioral phase shifts of wheel running rhythms and both Period1 (Per1) and Per2 mRNA levels in the SCN were assessed. Animals that received THIP during the subjective day did not exhibit significant phase alterations. During the early and late subjective night, however, THIP abolished the phase-shifting effects of light and the ability of light to increase Per1 and Per2 mRNA levels. The ability of N-methyl-d-aspartic acid to phase-shift wheel running rhythms was also attenuated by THIP. Together these data demonstrate that THIP does not produce phase shifts during the subjective day, but does inhibit the ability of light to produce phase shifts. Thus, extrasynaptic GABA(A) receptors appear to play a role in regulating light input to the SCN, while a different population of GABA(A) receptors appears to be responsible for daytime effects of GABA.

Decline of the presynaptic network, including GABAergic terminals, in the aging suprachiasmatic nucleus of the mouse

Biological rhythms, and especially the sleep/wake cycle, are frequently disrupted during senescence. This draws attention to the study of aging-related changes in the hypothalamic suprachiasmatic nucleus (SCN), the master circadian pacemaker. The authors here compared the SCN of young and old mice, analyzing presynaptic terminals, including the gamma-aminobutyric acid (GABA)ergic network, and molecules related to the regulation of GABA, the main neurotransmitter of SCN neurons. Transcripts of the alpha3 subunit of the GABAA receptor and the GABA-synthesizing enzyme glutamic acid decarboxylase isoform 67 (GAD67) were analyzed with real-time RT-PCR and GAD67 protein with Western blotting. These parameters did not show significant changes between the 2 age groups. Presynaptic terminals were identified in confocal microscopy with synaptophysin immunofluorescence, and the GABAergic subset of those terminals was revealed by the colocalization of GAD67 and synaptophysin. Quantitative analysis of labeled synaptic endings performed in 2 SCN subregions, where retinal afferents are known to be, respectively, very dense or very sparse, revealed marked aging-related changes. In both subregions, the evaluated parameters (the number of and the area covered by presynaptic terminals and by their GABAergic subset) were significantly decreased in old versus young mice. No significant differences were found between SCN tissue samples from animals sacrificed at different times of day, in either age group. Altogether, the data point out marked reduction in the synaptic network of the aging biological clock, which also affects GABAergic terminals. Such alterations could underlie aging-related SCN dysfunction, including low-amplitude output during senescence.

Role for the NR2B subunit of the N-methyl-D-aspartate receptor in mediating light input to the circadian system

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells that utilize glutamate as a neurotransmitter. A variety of evidence suggests that the release of glutamate then activates N-methyl-D-aspartate (NMDA) receptors within the SCN and triggers a signaling cascade that ultimately leads to phase shifts in the circadian system. In this study, we first sought to explore the role of the NR2B subunit in mediating the effects of light on the circadian system of hamsters and mice. We found that localized microinjection of the NR2B subunit antagonist ifenprodil into the SCN region reduces the magnitude of light-induced phase shifts of the circadian rhythm in wheel-running activity. Next, we found that the NR2B message and levels of phospho-NR2B vary with time of day in SCN tissue using semiquantitative real-time polymerase chain reaction and western blot analysis, respectively. Functionally, we found that blocking the NR2B subunit with ifenprodil significantly reduced the magnitude of NMDA currents recorded in SCN neurons. Ifenprodil also significantly reduced the magnitude of NMDA-induced Ca2+ changes in SCN cells. Together, these results demonstrate that the NR2B subunit is an important component of NMDA receptor-mediated responses within SCN neurons and that this subunit contributes to light-induced phase shifts of the mammalian circadian system.

Neurotransmitters of the suprachiasmatic nuclei

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.

Circadian Rhythmicity in AVP Secretion and GABAergic Synaptic Transmission in the Rat Suprachiasmatic Nucleus

A variety of physiological and behavioral functions exhibit circadian changes and these circadian rhythms are driven by oscillatory expression of clock genes in the suprachiasmatic nuclei (SCN). It is still unknown how this molecular clockwork is controlled by extracellular neurohormones and neurotransmitters and which membrane receptors undergo circadian modulation. Circadian rhythm can be measured as a secretion of arginine vasopressin (AVP) in organotypic SCN culture for several weeks. Melatonin applied directly to the SCN late in the day induces a phase advance, when applied late at night or at the beginning of the day melatonin causes a phase delay. The time window for phase advance corresponds with the highest level of melatonin receptors in the SCN but the mechanism of melatonin-induced phase delay is unknown. The principal neurotransmitter on SCN synapses is gamma-aminobutyric acid (GABA), which acts at postsynaptic GABA(A) receptors. Spontaneous release of GABA from presynaptic nerve terminals, recorded as miniature inhibitory postsynaptic currents in the presence of TTX, does not change, but zinc sensitivity of exogenous GABA-induced currents varies during the day and night, possibly due to changes in subunit composition of GABA(A) receptors. We conclude that there is daily variation in the postsynaptic, but not presynaptic, function in the SCN.

Potential pathways for intercellular communication within the calbindin subnucleus of the hamster suprachiasmatic nucleus

In mammals, the suprachiasmatic nucleus (SCN) is the master circadian pacemaker. Within the caudal hamster SCN, a cluster of neurons containing the calcium binding protein, calbindin-D28K (CB), has been implicated in circadian locomotion. However, calbindin-immunoreactive (CB+) neurons in the calbindin subnucleus (CBsn) do not display a circadian rhythm in spontaneous firing [Eur J Neurosci 16 (2002) 2469]. Previously, we proposed that intercellular communication might be essential in integrating outputs from rhythmic (CB-) neurons and nonrhythmic (CB+) neurons to produce a circadian output in the intact animal. The primary aim of this study is to provide a neuroanatomical framework to better understand intercellular communication within the CBsn. Using reconstructions of previously recorded neurons, we demonstrate that CB+ neurons have significantly more dendrites than CB- neurons. In addition, CBsn neurons have dorsally oriented dendritic arbors. Using double-label confocal microscopy, we show that GABA colocalizes with CB+ neurons and GABA(A) receptor subunits make intimate contacts with neurons in the CBsn. Transforming growth factor alpha (TGFalpha), a substance shown to inhibit locomotion [Science 294 (2001) 2511], is present within the CBsn. In addition, neurons in this region express the epidermal growth factor receptor, the only receptor for TGFalpha. Lastly, we show that CB+ neurons are coupled to CB+ and CB- neurons by gap junctions. The current study provides a structural framework for synaptic communication, electrical coupling, and signaling via a growth factor within the CBsn of the hamster SCN. Our results reveal connections that have the potential for integrating cellular communication within a subregion of the SCN that is critically involved in circadian locomotion.

Day–night variations in zinc sensitivity of GABAA receptor-channels in rat suprachiasmatic nucleus

In the suprachiasmatic nucleus (SCN), electrical activity, secretion, and other cellular functions undergo profound rhythm during day-night cycle due to oscillatory expression of clock gene constituents. Although SCN is enriched with gamma-aminobutyric acid (GABA)-ergic neurons, it is unknown whether there are circadian changes in the GABAA receptor expression and/or function. Here we investigated the possible daily variations in zinc sensitivity of GABAA channels in rat SCN neurons maintained in brain slices. Extracellular zinc inhibited GABA-induced currents in all ventrolateral (VL) and dorsomedial (DM) SCN neurons studied, as well as in neurons of non-SCN regions. In SCN neurons, the currents evoked by 30 microM GABA were inhibited by Zn2+ with an IC50 of 50.3+/-3.2 microM, whereas currents evoked by 100 microM GABA were inhibited with an IC50 of 181.6+/-32.0 microM. The antagonist action of zinc saturated at 97.4+/-0.7% for 30 microM GABA and 91.6+/-2.7% for 100 microM GABA. These observations indicate that Zn2+ inhibits SCN GABAA receptor competitively and in part non-competitively. In SCN neurons, but not in other neurons, the zinc sensitivity varied with daily time. During the day, the calculated IC50 for zinc was significantly lower than during the night (43.9+/-4.7 microM vs. 58.6+/-3.8, respectively). These results indicate that native GABAA receptors in SCN neurons display pharmacological properties of receptors having and not having gamma subunit and that the proportionality of these receptors could change during the day and night.

DNA microarray analysis of functionally discrete human brain regions reveals divergent transcriptional profiles

Transcriptional profiles within discrete human brain regions are likely to reflect structural and functional specialization. Using DNA microarray technology, this study investigates differences in transcriptional profiles of highly divergent brain regions (the cerebellar cortex and the cerebral cortex) as well as differences between two closely related brain structures (the anterior cingulate cortex and the dorsolateral prefrontal cortex). Replication of this study across three independent laboratories, to address false-positive and false-negative results using microarray technology, is also discussed. We find greater than a thousand transcripts to be differentially expressed between cerebellum and cerebral cortex and very few transcripts to be differentially expressed between the two neocortical regions. We further characterized transcripts that were found to be specifically expressed within brain regions being compared and found that ontological classes representing signal transduction machinery, neurogenesis, synaptic transmission, and transcription factors were most highly represented.

GABA receptor-mediated inhibition of neuronal activity in rat SCN in vitro: pharmacology and influence of circadian phase

The effect of gamma-aminobutyric acid (GABA) on neuronal firing rate in rat suprachiasmatic nucleus (SCN) slices was examined using continuous recording methods. GABA inhibited neuronal discharge during both the subjective day and the subjective night in a concentration-dependent manner characterized by two apparent affinity states. The GABAA receptor agonist muscimol caused potent inhibition regardless of circadian time; repeated applications of the agonist did not reverse the direction of effect. The GABAA receptor antagonists bicuculline and picrotoxin increased excitability when applied during either subjective day or subjective night. A significant increase in GABAA receptor- mediated inhibition, as well as endogenous GABAergic tone, was observed on the second day after slice preparation. The GABAB receptor agonist baclofen inhibited cell firing during subjective day and night, but the GABAB antagonist phaclofen had no significant effect. These data provide additional strong support for a predominantly inhibitory role of GABA in the rat SCN, regardless of the time of application in relation to the circadian rhythm, and demonstrate an important level of plasticity of this system in vitro.

Presynaptic and postsynaptic GABAA receptors in rat suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN), the brain's circadian clock, is composed mainly of GABAergic neurons, that are interconnected via synapses with GABA(A) receptors. Here we report on the subcellular localization of these receptors in the SCN, as revealed by an extensively characterized antibody to the alpha 3 subunit of GABA(A) receptors in conjunction with pre- and postembedding electron microscopic immunocytochemistry. GABA(A) receptor immunoreactivity was observed in neuronal perikarya, dendritic processes and axonal terminals. In perikarya and proximal dendrites, GABA(A) receptor immunoreactivity was expressed mainly in endoplasmic reticulum and Golgi complexes, while in the distal part of dendrites, immunoreaction product was associated with postsynaptic plasma membrane. Many GABAergic axonal terminals, as revealed by postembedding immunogold labeling, displayed GABA(A) receptor immunoreactivity, associated mainly with the extrasynaptic portion of their plasma membrane. The function of these receptors was studied in hypothalamic slices using whole-cell patch-clamp recording of the responses to minimal stimulation of an area dorsal to the SCN. Analysis of the evoked inhibitory postsynaptic currents showed that either bath or local application of 100 microM of GABA decreased GABAergic transmission, manifested as a two-fold increase in failure rate. This presynaptic effect, which was detected in the presence of the glutamate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione and the selective GABA(B) receptor blocker CGP55845A, appears to be mediated via activation of GABA(A) receptors. Our results thus show that GABA(A) receptors are widely distributed in the SCN and may subserve both pre- and postsynaptic roles in controlling the mammalian circadian clock.

Electrophysiology of optic nerve input to suprachiasmatic nucleus neurons in rats and degus

Neurons in the mammalian suprachiasmatic nucleus (SCN), the principal pacemaker of the circadian system, receive direct retinal input. Some SCN neurons respond to retinal illumination or optic nerve stimulation with changes in firing rates. In nocturnal rodents, retinal illumination increases firing rates of a large majority and decreases firing rates of a minority of responsive neurons. In two species of diurnal rodent, these proportions are altered or even reversed. Since retinal input to the SCN has been reported to involve release of the excitatory neurotransmitter glutamate, the mechanism mediating suppressions is unknown. We studied responses of neurons in SCN slices from diurnal degus and nocturnal rats to optic nerve stimulation. To test whether suppressions are mediated indirectly by release of the inhibitory neurotransmitter GABA from SCN neurons that are first activated by glutamate release, we attempted to block suppressions by adding to the bath either APV, an antagonist for excitatory glutamate receptors, or bicuculline, a GABA(A) receptor antagonist. If glutamate is the only neurotransmitter released by optic nerves in the SCN, APV should prevent both activations and suppressions in response to optic nerve stimulation. We found that APV had little effect on suppressions although it effectively blocked activations. Bicuculline blocked most suppressions. These findings are inconsistent with a model in which the retina provides only excitatory glutamate input to the SCN via NMDA receptors. Since some retinal fibers in adult mammals contain GABA, it is possible that the retinal innervation of the SCN includes both glutamate- and GABA-containing axons.

GABA-induced current and circadian regulation of chloride in neurones of the rat suprachiasmatic nucleus

1. We have shown previously that GABA, the main neurotransmitter in the suprachiasmatic nucleus (SCN), has dual effects on SCN neurones, excitatory during the day and inhibitory at night. This duality has been attributed to changes in [Cl(-)](i) during the circadian cycle. To unravel the processes underlying these changes we investigated the biophysical properties of the GABAergic receptors and the regulation of [Cl(-)](i) in SCN neurones. 2. We used voltage-clamp methodology in conjunction with local application of GABA to characterise the current induced by GABA in SCN neurones within acute brain slices. This current, mediated via GABA(A) receptors, shows moderate voltage dependence, does not desensitise and can significantly alter [Cl(-)](i). 3. Loading or depletion of intracellular Cl(-) was induced by a train of GABA pulses. The recovery of intracellular Cl(-) was deduced from the change in [Cl(-)](i) calculated from the response to a test GABA pulse presented at different intervals after the conditioning train of GABA application. The time course of recovery was described by an exponential curve. Recovery following Cl(-) depletion was slower than recovery from Cl(-) loading and was further delayed during the subjective night. 4. We concluded that: (a) SCN neurones express a large number of somatic GABA(A) receptors, which give rise to a modifiable, tonic Cl(-) conductance that modulates cell excitability; (b) two Cl(-) transport mechanisms operate in SCN neurones, one that replenishes the cell with Cl(-) following Cl(-) depletion and another that removes Cl(-) after Cl(-) loading; (c) the efficiency of the replenishing mechanism is reduced during the subjective night; and (d) this reduction explains a lower [Cl(-)](i) during the night phase of the circadian cycle.

Rhythmic variation in gamma-aminobutyric acid(A)-receptor subunit composition in the circadian system and median eminence of Syrian hamsters

Temporal changes in the level of expression of gamma-aminobutyric acid (GABA)(A) receptor subunits alpha2, alpha5, beta1 and beta3 were characterized by Western blot analysis in the hamster suprachiasmatic nuclei, retina and median eminence. A nocturnal maximum in the level of GABA(A) receptor beta1 subunit at midday and midnight (12:00 and 00.00 h) was found in the suprachiasmatic nucleus (SCN), the retina and the median eminence of Syrian hamsters. Alpha2 and beta3 subunit levels peaked during the day in the median eminence. Finally, retinal alpha5 levels were maximal during the night. beta1 temporal changes in the SCN and median eminence, as well as alpha2 variations in the median eminence were maintained under constant dark conditions, suggesting an endogenous control, while the other variations were only observed under light-dark cycle conditions.

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

Because the rapid induction of Period (Per) genes is associated with the photic entrainment of the biological clock, we examined whether N-methyl-D-aspartate (NMDA) receptors were involved in the photic induction of Per genes in the hamster suprachiasmatic nucleus (SCN). In situ hybridization observation revealed that light during the early subjective night [circadian time (CT) 13.5] or the late subjective night (CT20) caused an induction of Per1 and Per2 but not Per3 mRNA in the SCN. Photic induction of Per mRNA at CT13.5 was observed especially in the ventrolateral SCN, whereas that at CT20 was more widespread from the ventrolateral to the dorsal SCN. A noncompetitive NMDA receptor antagonist, +MK801, dose-dependently (0. 1-5.0 mg/kg) suppressed only the ventrolateral part of Per1 and Per2 mRNA induction by light at CT13.5 or CT20 in the SCN. The suppressive effects of +MK801 on Per mRNA strongly correlated with the attenuating action of this compound on phase shifts by light at both CT13.5 and CT20. A competitive NMDA receptor antagonist, D-2-amino-5-phosphonovalerate (D-APV), also exhibited inhibitory actions on light (CT20)-induced Per1 and Per2 mRNA expression in the ventrolateral SCN. Furthermore, local injection of NMDA into the SCN resulted in the induction of Per1 and Per2 mRNA in the SCN. Among NMDA receptors, NR2B and NR2C mRNA were expressed in the ventrolateral and dorsal SCN, respectively. These results suggest that the activation of NMDA receptor is a critical step for photic induction of Per1 and Per2 transcripts in the SCN, which are linked to a photic behavioral entrainment.

N-methyl-D-aspartate receptor subtype 2C is not involved in circadian oscillation or photoic entrainment of the biological clock in mice

Ishida et al. [1994: Neurosci Lett 166: 211-215] reported the circadian change of N-methyl-D-aspartate (NMDA) receptor subtype 2C mRNA and photic induction of this receptor's mRNA in the suprachiasmatic nucleus (SCN). Therefore, we investigated the role of NMDA receptor subtypes in the biological clock using NMDA receptor 2A (NR2A)- or 2C (NR2C)-deficient mice. However, NR2C-/- mice showed normal light-dark (LD)-entrained locomotor activity rhythms and free-running rhythms under constant darkness and also exhibited normal reentrainment to 6-hr LD shifts and phase delays with single light pulses. Thus, present results demonstrated no significant NR2C contribution to circadian oscillation and photic entrainment, even though expression of NR2C mRNA was highly observed in the SCN. On the other hand, the period of the free-running activity rhythm in NR2A-/- mice but not NR2C-/- mice was slightly longer than that in wild-type mice in spite of low expression of NR2A in the SCN. Furthermore, reentrainment to an LD advance in NR2A-/- mice was slower under low-intensity light conditions. Thus, we suggest that NR2A plays a role in determining the behavioral state that affects the circadian rhythm. In order to elucidate the role of NR2A and NR2C in the SCN, we examined NMDA-induced Ca(2+) elevations in the SCN of mutant mice using a Ca(2+) imaging method. A partial reduction in Ca(2+) elevation was observed in both NR2A-/- and NR2C-/- mice when high concentrations (100 or 300 microM) of NMDA were applied. The present results suggest that NR2A plays a weak role in oscillation or entrainment of the biological clock, and that NR2C does not participate in the functions of circadian oscillation and light entrainment.