Circadian rhythm in intracellular Cl(-) activity of acutely dissociated neurons of suprachiasmatic nucleus

Circadian Rhythm Regulation by Pacemaker Neuron Chloride Oscillation in Flies

Circadian rhythms in physiology and behavior sync organisms to external environmental cycles. Here, circadian oscillation in intracellular chloride in central pacemaker neurons of the fly, Drosophila melanogaster, is reviewed. Intracellular chloride links SLC12 cation-coupled chloride transporter function with kinase signaling and the regulation of inwardly rectifying potassium channels.

Control of circadian rhythm on cortical excitability and synaptic plasticity

Living organisms navigate through a cyclic world: activity, feeding, social interactions are all organized along the periodic succession of night and day. At the cellular level, periodic activity is controlled by the molecular machinery driving the circadian regulation of cellular homeostasis. This mechanism adapts cell function to the external environment and its crucial importance is underlined by its robustness and redundancy. The cell autonomous clock regulates cell function by the circadian modulation of mTOR, a master controller of protein synthesis. Importantly, mTOR integrates the circadian modulation with synaptic activity and extracellular signals through a complex signaling network that includes the RAS-ERK pathway. The relationship between mTOR and the circadian clock is bidirectional, since mTOR can feedback on the cellular clock to shift the cycle to maintain the alignment with the environmental conditions. The mTOR and ERK pathways are crucial determinants of synaptic plasticity and function and thus it is not surprising that alterations of the circadian clock cause defective responses to environmental challenges, as witnessed by the bi-directional relationship between brain disorders and impaired circadian regulation. In physiological conditions, the feedback between the intrinsic clock and the mTOR pathway suggests that also synaptic plasticity should undergo circadian regulation.

Ion dynamics and the regulation of circadian cellular physiology

Circadian rhythms in physiology and behavior allow organisms to anticipate the daily environmental changes imposed by the rotation of our planet around its axis. Although these rhythms eventually manifest at the organismal level, a cellular basis for circadian rhythms has been demonstrated. Significant contributors to these cell-autonomous rhythms are daily cycles in gene expression and protein translation. However, recent data revealed cellular rhythms in other biological processes, including ionic currents, ion transport, and cytosolic ion abundance. Circadian rhythms in ion currents sustain circadian variation in action potential firing rate, which coordinates neuronal behavior and activity. Circadian regulation of metal ions abundance and dynamics is implicated in distinct cellular processes, from protein translation to membrane activity and osmotic homeostasis. In turn, studies showed that manipulating ion abundance affects the expression of core clock genes and proteins, suggestive of a close interplay. However, the relationship between gene expression cycles, ion dynamics, and cellular function is still poorly characterized. In this review, I will discuss the mechanisms that generate ion rhythms, the cellular functions they govern, and how they feed back to regulate the core clock machinery.

Intracellular Ion Control of WNK Signaling

The with no lysine (K) (WNK) kinases are an evolutionarily ancient group of kinases with atypical placement of the catalytic lysine and diverse physiological roles. Recent studies have shown that WNKs are directly regulated by chloride, potassium, and osmotic pressure. Here, we review the discovery of WNKs as chloride-sensitive kinases and discuss physiological contexts in which chloride regulation of WNKs has been demonstrated. These include the kidney, pancreatic duct, neurons, and inflammatory cells. We discuss the interdependent relationship of osmotic pressure and intracellular chloride in cell volume regulation. We review the recent demonstration of potassium regulation of WNKs and speculate on possible physiological roles. Finally, structural and mechanistic aspects of intracellular ion and osmotic pressure regulation of WNKs are discussed.

The opposing chloride cotransporters KCC and NKCC control locomotor activity in constant light and during long days

Cation chloride cotransporters (CCCs) regulate intracellular chloride ion concentration ([Cl-]i) within neurons, which can reverse the direction of the neuronal response to the neurotransmitter GABA.1 Na+ K+ Cl- (NKCC) and K+ Cl- (KCC) cotransporters transport Cl- into or out of the cell, respectively. When NKCC activity dominates, the resulting high [Cl-]i can lead to an excitatory and depolarizing response of the neuron upon GABAA receptor opening, while KCC dominance has the opposite effect.1 This inhibitory-to-excitatory GABA switch has been linked to seasonal adaption of circadian clock function to changing day length,2-4 and its dysregulation is associated with neurodevelopmental disorders such as epilepsy.5-8 In Drosophila melanogaster, constant light normally disrupts circadian clock function and leads to arrhythmic behavior.9 Here, we demonstrate a function for CCCs in regulating Drosophila locomotor activity and GABA responses in circadian clock neurons because alteration of CCC expression in circadian clock neurons elicits rhythmic behavior in constant light. We observed the same effects after downregulation of the Wnk and Fray kinases, which modulate CCC activity in a [Cl-]i-dependent manner. Patch-clamp recordings from the large LNv clock neurons show that downregulation of KCC results in a more positive GABA reversal potential, while KCC overexpression has the opposite effect. Finally, KCC and NKCC downregulation reduces or increases morning behavioral activity during long photoperiods, respectively. In summary, our results support a model in which the regulation of [Cl-]i by a KCC/NKCC/Wnk/Fray feedback loop determines the response of clock neurons to GABA, which is important for adjusting behavioral activity to constant light and long-day conditions.

Chloride oscillation in pacemaker neurons regulates circadian rhythms through a chloride-sensing WNK kinase signaling cascade

Central pacemaker neurons regulate circadian rhythms and undergo diurnal variation in electrical activity in mammals and flies.1,2 Circadian variation in the intracellular chloride concentration of mammalian pacemaker neurons has been proposed to influence the response to GABAergic neurotransmission through GABAA receptor chloride channels.3 However, results have been contradictory,4-9 and a recent study demonstrated circadian variation in pacemaker neuron chloride without an effect on GABA response.10 Therefore, whether and how intracellular chloride regulates circadian rhythms remains controversial. Here, we demonstrate a signaling role for intracellular chloride in the Drosophila small ventral lateral (sLNv) pacemaker neurons. In control flies, intracellular chloride increases in sLNvs over the course of the morning. Chloride transport through sodium-potassium-2-chloride (NKCC) and potassium-chloride (KCC) cotransporters is a major determinant of intracellular chloride concentrations.11Drosophila melanogaster with loss-of-function mutations in the NKCC encoded by Ncc69 have abnormally low intracellular chloride 6 h after lights on, loss of morning anticipation, and a prolonged circadian period. Loss of kcc, which is expected to increase intracellular chloride, suppresses the long-period phenotype of Ncc69 mutant flies. Activation of a chloride-inhibited kinase cascade, consisting of WNK (with no lysine [K]) kinase and its downstream substrate, Fray, is necessary and sufficient to prolong period length. Fray activation of an inwardly rectifying potassium channel, Irk1, is also required for the long-period phenotype. These results indicate that the NKCC-dependent rise in intracellular chloride in Drosophila sLNv pacemakers restrains WNK-Fray signaling and overactivation of an inwardly rectifying potassium channel to maintain normal circadian period length.

Modelling the functional roles of synaptic and extra-synaptic γ-aminobutyric acid receptor dynamics in circadian timekeeping

In the suprachiasmatic nucleus (SCN), γ-aminobutyric acid (GABA) is a primary neurotransmitter. GABA can signal through two types of GABAA receptor subunits, often referred to as synaptic GABAA (gamma subunit) and extra-synaptic GABAA (delta subunit). To test the functional roles of these distinct GABAA in regulating circadian rhythms, we developed a multicellular SCN model where we could separately compare the effects of manipulating GABA neurotransmitter or receptor dynamics. Our model predicted that blocking GABA signalling modestly increased synchrony among circadian cells, consistent with published SCN pharmacology. Conversely, the model predicted that lowering GABAA receptor density reduced firing rate, circadian cell fraction, amplitude and synchrony among individual neurons. When we tested these predictions, we found that the knockdown of delta GABAA reduced the amplitude and synchrony of clock gene expression among cells in SCN explants. The model further predicted that increasing gamma GABAA densities could enhance synchrony, as opposed to increasing delta GABAA densities. Overall, our model reveals how blocking GABAA receptors can modestly increase synchrony, while increasing the relative density of gamma over delta subunits can dramatically increase synchrony. We hypothesize that increased gamma GABAA density in the winter could underlie the tighter phase relationships among SCN cells.

Trace Elements, PPARs, and Metabolic Syndrome

Metabolic syndrome (MetS) is a constellation of metabolic derangements, including central obesity, insulin resistance, hypertension, glucose intolerance, and dyslipidemia. The pathogenesis of MetS has been intensively studied, and now many factors are recognized to contribute to the development of MetS. Among these, trace elements influence the structure of proteins, enzymes, and complex carbohydrates, and thus an imbalance in trace elements is an independent risk factor for MetS. The molecular link between trace elements and metabolic homeostasis has been established, and peroxisome proliferator-activated receptors (PPARs) have appeared as key regulators bridging these two elements. This is because on one hand, PPARs are actively involved in various metabolic processes, such as abdominal adiposity and insulin sensitivity, and on the other hand, PPARs sensitively respond to changes in trace elements. For example, an iron overload attenuates hepatic mRNA expression of Ppar-α; zinc supplementation is considered to recover the DNA-binding activity of PPAR-α, which is impaired in steatotic mouse liver; selenium administration downregulates mRNA expression of Ppar-γ, thereby improving lipid metabolism and oxidative status in the liver of high-fat diet (HFD)-fed mice. More importantly, PPARs' expression and activity are under the control of the circadian clock and show a robust 24 h rhythmicity, which might be the reasons for the side effects and the clinical limitations of trace elements targeting PPARs. Taken together, understanding the casual relationships among trace elements, PPARs' actions, and the pathogenesis of MetS is of great importance. Further studies are required to explore the chronopharmacological effects of trace elements on the diurnal oscillation of PPARs and the consequent development of MetS.

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.

Lipid composition modulates the intestine digestion rate and serum lipid status of different edible oils: a combination of in vitro and in vivo studies

The objective of the present study was to investigate the connections between lipid compositions and the digestion and absorption differences of different lipids. Five typical edible oils (palm oil, PO; leaf lard oil, LO; rapeseed oil, RO; sunflower oil, SO; linseed oil, LINO) were selected to conduct in vitro digestion experiments considering the lipid digestion extent and hydrolysis rate before analyzing the fatty acid composition and TAG profiles using GC and UHPLC-Q-TOF-MS/MS. Meanwhile, the postprandial lipid absorption status after gavage administration was examined in adult male Sprague-Dawley rats with respect to serum lipid profiles. The results showed that the maximum FFA release extent decreased in the order: PO > RO > LINO > SO > LO, and the FFA release apparent constants were PO > SO ≈ RO > LO ≈ LINO. This suggested that the fatty acid species and the location of fatty acids within TAG molecules could significantly affect the lipid digestion fates in the gastrointestinal tract, and short chain saturated fatty acids located at the Sn-1, 3 position could favor the lipid digestion process. PO and LO were both shown to be more likely to affect the serum TG levels and LDL-C : HDL-C ratio compared with RO, SO and LINO. Different fatty acids displayed different correlations with serum lipid profiles when examined by Pearson correlation analysis. This suggested that fatty acid composition and TAG profiles may influence first the digestion rate and then the serum lipid profiles. This further confirmed that lipid composition could modulate the digestion and absorption status under the gastrointestinal conditions. These findings may provide some basic understanding of the connections between lipid composition and their functional difference.

Training the Gut for Athletes

The gastrointestinal (GI) tract plays a critical role in delivering carbohydrate and fluid during prolonged exercise and can therefore be a major determinant of performance. The incidence of GI problems in athletes participating in endurance events is high, indicating that GI function is not always optimal in those conditions. A substantial body of evidence suggests that the GI system is highly adaptable. Gastric emptying as well as stomach comfort can be “trained” and perceptions of fullness decreased; some studies have suggested that nutrient-specific increases in gastric emptying may occur. Evidence also shows that diet has an impact on the capacity of the intestine to absorb nutrients. Again, the adaptations that occur appear to be nutrient specific. For example, a high-carbohydrate diet will increase the density of sodium-dependent glucose-1 (SGLT1) transporters in the intestine as well as the activity of the transporter, allowing greater carbohydrate absorption and oxidation during exercise. It is also likely that, when such adaptations occur, the chances of developing GI distress are smaller. Future studies should include more human studies and focus on a number of areas, including the most effective methods to induce gut adaptations and the timeline of adaptations. To develop effective strategies, a better understanding of the exact mechanisms underlying these adaptations is important. It is clear that “nutritional training” can improve gastric emptying and absorption and likely reduce the chances and/or severity of GI problems, thereby improving endurance performance as well as providing a better experience for the athlete. The gut is an important organ for endurance athletes and should be trained for the conditions in which it will be required to function.

Training the Gut for Athletes

The gastrointestinal (GI) tract plays a critical role in delivering carbohydrate and fluid during prolonged exercise and can therefore be a major determinant of performance. The incidence of GI problems in athletes participating in endurance events is high, indicating that GI function is not always optimal in those conditions. A substantial body of evidence suggests that the GI system is highly adaptable. Gastric emptying as well as stomach comfort can be "trained" and perceptions of fullness decreased; some studies have suggested that nutrient-specific increases in gastric emptying may occur. Evidence also shows that diet has an impact on the capacity of the intestine to absorb nutrients. Again, the adaptations that occur appear to be nutrient specific. For example, a high-carbohydrate diet will increase the density of sodium-dependent glucose-1 (SGLT1) transporters in the intestine as well as the activity of the transporter, allowing greater carbohydrate absorption and oxidation during exercise. It is also likely that, when such adaptations occur, the chances of developing GI distress are smaller. Future studies should include more human studies and focus on a number of areas, including the most effective methods to induce gut adaptations and the timeline of adaptations. To develop effective strategies, a better understanding of the exact mechanisms underlying these adaptations is important. It is clear that "nutritional training" can improve gastric emptying and absorption and likely reduce the chances and/or severity of GI problems, thereby improving endurance performance as well as providing a better experience for the athlete. The gut is an important organ for endurance athletes and should be trained for the conditions in which it will be required to function.

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.

GABAergic transmission to kisspeptin neurons is differentially regulated by time of day and estradiol in female mice

Gonadotropin-releasing hormone (GnRH) secretion is regulated by estradiol feedback. This feedback switches from negative to positive in females; this switch depends on time of day in many species. Estradiol feedback is likely conveyed via afferents. Kisspeptin neurons of the arcuate nucleus and anteroventral-periventricular region (AVPV) may differentially regulate GnRH neurons during negative and positive feedback, respectively. We tested estradiol and time of day regulation of GABAergic transmission and postsynaptic response to GABA in these two populations using transgenic mice with GFP-identified kisspeptin neurons. Ovariectomized (OVX) mice treated or not with estradiol (E) were studied in the AM (negative feedback) or PM (positive feedback). GABAA receptor reversal potential was unaffected by time of day or estradiol. GABA depolarized the membrane potential of arcuate neurons from OVX+E mice; this response was blunted in cells from OVX mice. GABA hyperpolarized AVPV kisspeptin neurons, except in the OVX PM group in which GABA did not alter membrane potential attributable to a PM hyperpolarization of baseline membrane potential. In both kisspeptin neuron populations from OVX mice, the frequency of GABAergic spontaneous postsynaptic currents was increased in the PM; this increase was blunted by estradiol. In arcuate, but not AVPV, kisspeptin neurons, estradiol reduced miniature postsynaptic current amplitude independent of time of day. Using nonstationary fluctuation analysis and diazepam to manipulate GABAA receptor apparent affinity, the decrease in arcuate miniature postsynaptic current amplitude was attributed to decreased number of receptors bound by GABA. Time of day and estradiol feedback thus both target presynaptic and postsynaptic mechanisms to differentially regulate kisspeptin neurons via GABAergic transmission.

Emerging role of WNK1 in pathologic central nervous system signaling

WNK1 (with no lysine (K)) is a widely expressed serine/threonine protein kinase. The role of this kinase was first described in the kidney where it dynamically controls ion channels that regulate changes in cell volume. WNK1, through intermediates oxidative stress-responsive kinase-1 (OSR1) and STE20/SPS1-related proline/alanine-rich kinase (SPAK), phosphorylates the inwardly directed Na⁺-K+-Cl^---cotransporter 1 (NKCC1) and the outwardly directed K⁺-Cl^--cotransporter 2 (KCC2), activating and deactivating these channels, respectively. WNK1, NKCC1 and KCC2 are also expressed in the central nervous system (CNS). Growing evidence implicates WNK1 playing a critical role in pathologic nervous system signaling where changes in intracellular ion concentration in response to γ-aminobutyric-acid (GABA) can activate otherwise silent pathways. This review will focus on current research about WNK1, its downstream effectors and role in GABA signaling. Future perspectives include investigating WNK1 expression in the CNS after spinal cord injury (SCI), where altered neuronal signaling could underlie pathological states such as neuropathic pain (NP).

Circadian modulation of the Cl(-) equilibrium potential in the rat suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) constitute a circadian clock in mammals, where γ-amino-butyric acid (GABA) neurotransmission prevails and participates in different aspects of circadian regulation. Evidence suggests that GABA has an excitatory function in the SCN in addition to its typical inhibitory role. To examine this possibility further, we determined the equilibrium potential of GABAergic postsynaptic currents (E(GABA)) at different times of the day and in different regions of the SCN, using either perforated or whole cell patch clamp. Our results indicate that during the day most neurons in the dorsal SCN have an E(GABA) close to -30 mV while in the ventral SCN they have an E(GABA) close to -60 mV; this difference reverses during the night, in the dorsal SCN neurons have an E(GABA) of -60 mV and in the ventral SCN they have an E(GABA) of -30 mV. The depolarized equilibrium potential can be attributed to the activity of the Na(+)-K(+)-2Cl(-) (NKCC) cotransporter since the equilibrium potential becomes more negative following addition of the NKCC blocker bumetanide. Our results suggest an excitatory role for GABA in the SCN and further indicate both time (day versus night) and regional (dorsal versus ventral) modulation of E(GABA) in the SCN.

A sensitive membrane-targeted biosensor for monitoring changes in intracellular chloride in neuronal processes

Background

Regulation of chloride gradients is a major mechanism by which excitability is regulated in neurons. Disruption of these gradients is implicated in various diseases, including cystic fibrosis, neuropathic pain and epilepsy. Relatively few studies have addressed chloride regulation in neuronal processes because probes capable of detecting changes in small compartments over a physiological range are limited.

Methodology/Principal Findings

In this study, a palmitoylation sequence was added to a variant of the yellow fluorescent protein previously described as a sensitive chloride indicator (YFPQS) to target the protein to the plasma membrane (mbYFPQS) of cultured midbrain neurons. The reporter partitions to the cytoplasmic face of the cellular membranes, including the plasma membrane throughout the neurons and fluorescence is stable over 30–40 min of repeated excitation showing less than 10% decrease in mbYFPQS fluorescence compared to baseline. The mbYFPQS has similar chloride sensitivity (k₅₀ =  41 mM) but has a shifted pKa compared to the unpalmitoylated YFPQS variant (cytYFPQS) that remains in the cytoplasm when expressed in midbrain neurons. Changes in mbYFPQS fluorescence were induced by the GABA_A agonist muscimol and were similar in the soma and processes of the midbrain neurons. Amphetamine also increased mbYFPQS fluorescence in a subpopulation of cultured midbrain neurons that was reversed by the selective dopamine transporter (DAT) inhibitor, GBR12909, indicating that mbYFPQS is sensitive enough to detect endogenous DAT activity in midbrain dopamine (DA) neurons.

Conclusions/Significance

The mbYFPQS biosensor is a sensitive tool to study modulation of intracellular chloride levels in neuronal processes and is particularly advantageous for simultaneous whole-cell patch clamp and live-cell imaging experiments.

Gastric emptying and short-term digestibility of thermally oxidized sunflower oil used for frying in fasted and nonfasted rats

Four-hour in vivo digestibility of sunflower oil used in frying was tested in fasted and nonfasted rats. For three consecutive days, 12 male Wistar rats received 1 g of unused oil (controls, C), while 12 received 1 g of used oil (test group, T). On the night of day 3, 6 rats from each group were fasted (FC, FT) while the other 6 animals from each group had free access to food (NFC, NFT). On day 4, FC and NFC received 2 g of unused oil, while FT and NFT received 2 g of used oil. Luminal gastric and intestinal fats were studied by column and HPSE chromatography after endogenous corrections. Gastric emptying in FT was significantly slower than in NFT and FC. The luminal gastric fat profile differed from that of the oils administered, suggesting that nonoxidized triacylglycerols passed quickly into the intestines. All glyceridic compounds present in the luminal intestinal fat were affected by oil type (at least P < 0.01). Oil digestibility value order was FT < NFT < FC < NFC. FT and NFT presented lower (P < 0.001) triacylglycerol polymer and dimer digestibilities than NFC and FC. In conclusion, oil type determined luminal intestinal fat compounds and their digestibility more than nutritional status.

A multiscale model to investigate circadian rhythmicity of pacemaker neurons in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is a multicellular system that drives daily rhythms in mammalian behavior and physiology. Although the gene regulatory network that produces daily oscillations within individual neurons is well characterized, less is known about the electrophysiology of the SCN cells and how firing rate correlates with circadian gene expression. We developed a firing rate code model to incorporate known electrophysiological properties of SCN pacemaker cells, including circadian dependent changes in membrane voltage and ion conductances. Calcium dynamics were included in the model as the putative link between electrical firing and gene expression. Individual ion currents exhibited oscillatory patterns matching experimental data both in current levels and phase relationships. VIP and GABA neurotransmitters, which encode synaptic signals across the SCN, were found to play critical roles in daily oscillations of membrane excitability and gene expression. Blocking various mechanisms of intracellular calcium accumulation by simulated pharmacological agents (nimodipine, IP3- and ryanodine-blockers) reproduced experimentally observed trends in firing rate dynamics and core-clock gene transcription. The intracellular calcium concentration was shown to regulate diverse circadian processes such as firing frequency, gene expression and system periodicity. The model predicted a direct relationship between firing frequency and gene expression amplitudes, demonstrated the importance of intracellular pathways for single cell behavior and provided a novel multiscale framework which captured characteristics of the SCN at both the electrophysiological and gene regulatory levels.

Circadian rhythms are ∼24 hour cycles in biochemical, physiological and behavioral processes observed in a diverse range of organisms including Cyanobacteria, Neurospora, Drosophila, mice and humans. In mammals, the dominant circadian pacemaker that drives daily rhythms is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is composed of a highly connected network of ∼20,000 neurons. Within each individual SCN neuron core clock genes and proteins interact through intertwined regulatory loops to generate circadian oscillations on the molecular level. These neurons express daily rhythmicity in their firing frequency and other electrophysiological properties. The mechanisms by which the core clock produces synchronized rhythms in neural firing and gene expression are postulated to involve intracellular calcium, a second messenger that regulates many cellular processes. The interaction between the various clock components however remains unknown. In this paper, we present a single cell model that incorporates the circadian gene regulatory pathway, cellular electrophysiological properties, and cytosolic calcium dynamics. Our results suggest a possible system architecture that accounts for the robustness of the circadian clock at the single cell level. Our simulations predict a dual role for intracellular pathways instigated by intracellular calcium and VIP: maintaining the periodicity and amplitude of the core clock genes as well as the firing frequency oscillations.

Cell-type specific distribution of chloride transporters in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is a circadian oscillator and biological clock. Cell-to-cell communication is important for synchronization among SCN neuronal oscillators and the great majority of SCN neurons use GABA as a neurotransmitter, the principal inhibitory neurotransmitter in the adult CNS. Acting via the ionotropic GABA(A) receptor, a chloride ion channel, GABA typically evokes inhibitory responses in neurons via Cl(-) influx. Within the SCN GABA evokes both inhibitory and excitatory responses although the mechanism underlying GABA-evoked excitation in the SCN is unknown. GABA-evoked depolarization in immature neurons in several regions of the brain is a function of intracellular chloride concentration, regulated largely by the cation-chloride cotransporters NKCC1 (sodium/potassium/chloride cotransporter for chloride entry) and KCC1-4 (potassium/chloride cotransporters for chloride egress). It is well established that changes in the expression of the cation-chloride cotransporters through development determines the polarity of the response to GABA. To understand the mechanisms underlying GABA-evoked excitation in the SCN, we examined the SCN expression of cation-chloride cotransporters. Previously we reported that the K(+)/Cl(-) cotransporter KCC2, a neuron-specific chloride extruder conferring GABA's more typical inhibitory effects, is expressed exclusively in vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) neurons in the SCN. Here we report that the K(+)/Cl(-) cotransporter isoforms KCC4 and KCC3 are expressed solely in vasopressin (VP) neurons in the rat SCN whereas KCC1 is expressed in VIP neurons, similar to KCC2. NKCC1 is expressed in VIP, GRP and VP neurons in the SCN as is WNK3, a chloride-sensitive neuron-specific with no serine-threonine kinase which modulates intracellular chloride concentration via opposing actions on NKCC and KCC cotransporters. The heterogeneous distribution of cation-chloride cotransporters in the SCN suggests that Cl(-) levels are differentially regulated within VIP/GRP and VP neurons. We suggest that GABA's excitatory action is more likely to be evoked in VP neurons that express KCC4.

WNK protein kinases modulate cellular Cl- flux by altering the phosphorylation state of the Na-K-Cl and K-Cl cotransporters

Precise control of cellular Cl(-) transport is necessary for many fundamental physiological processes. For example, the intracellular concentration of Cl(-), fine-tuned through the coordinated action of cellular Cl(-) influx and efflux mechanisms, determines whether a neuron's response to GABA is excitatory or inhibitory. In epithelia, synchrony between apical and basolateral Cl(-) flux, and transcellular and paracellular Cl(-) transport, is necessary for efficient transepithelial Cl(-) reabsorption or secretion. In cells throughout the body, coordination of Cl(-) entry and exit mechanisms help defend against changes in cell volume. The Na-K-Cl and K-Cl cotransporters of the SLC12 gene family are important molecular determinants of Cl(-) entry and exit, respectively, in these systems. The WNK serine-threonine kinase family, members of which are mutated in an inherited form of human hypertension, are components of a signaling pathway that coordinates Cl(-) influx and efflux through SLC12 cotransporters to dynamically regulate intracellular Cl(-) activity.

WNK3 modulates transport of Cl- in and out of cells: implications for control of cell volume and neuronal excitability

The regulation of Cl(-) transport into and out of cells plays a critical role in the maintenance of intracellular volume and the excitability of GABA responsive neurons. The molecular determinants of these seemingly diverse processes are related ion cotransporters: Cl(-) influx is mediated by the Na-K-2Cl cotransporter NKCC1 and Cl(-) efflux via K-Cl cotransporters, KCC1 or KCC2. A Cl(-)/volume-sensitive kinase has been proposed to coordinately regulate these activities via altered phosphorylation of the transporters; phosphorylation activates NKCC1 while inhibiting KCCs, and dephosphorylation has the opposite effects. We show that WNK3, a member of the WNK family of serine-threonine kinases, colocalizes with NKCC1 and KCC1/2 in diverse Cl(-)-transporting epithelia and in neurons expressing ionotropic GABA(A) receptors in the hippocampus, cerebellum, cerebral cortex, and reticular activating system. By expression studies in Xenopus oocytes, we show that kinase-active WNK3 increases Cl(-) influx via NKCC1, and that it inhibits Cl(-) exit through KCC1 and KCC2; kinase-inactive WNK3 has the opposite effects. WNK3's effects are imparted via altered phosphorylation and surface expression of its downstream targets and bypass the normal requirement of altered tonicity for activation of these transporters. Together, these data indicate that WNK3 can modulate the level of intracellular Cl(-) via opposing actions on entry and exit pathways. They suggest that WNK3 is part of the Cl(-)/volume-sensing mechanism necessary for the maintenance of cell volume during osmotic stress and the dynamic modulation of GABA neurotransmission.

The metabolism of structured triacylglycerols

The triacylglycerol (TAG) structure in addition to the overall fatty acid profile is of importance when considering the nutritional effect of a dietary fat. This review aims at summarizing our current knowledge of the digestion, absorption, uptake, and transport of structured TAGs, with particular emphasis on the following aspects: gastric emptying, specificity of pancreatic lipase, lymphatic transport and clearance of chylomicrons, effects of lipid structure on tissue lipid compositions and the fecal loss of fats. So an overview will be provided for how the structure and fatty acid composition of TAGs affect their absorption and the distribution of the fatty acids in the body following digestion and absorption.

A GABAergic Mechanism Is Necessary for Coupling Dissociable Ventral and Dorsal Regional Oscillators within the Circadian Clock

BACKGROUND

Circadian rhythms in mammalian behavior, physiology, and biochemistry are controlled by the central clock of the suprachiasmatic nucleus (SCN). The clock is synchronized to environmental light-dark cycles via the retino-hypothalamic tract, which terminates predominantly in the ventral SCN of the rat. In order to understand synchronization of the clock to the external light-dark cycle, we performed ex vivo recordings of spontaneous impulse activity in SCN slices of the rat.

RESULTS

We observed bimodal patterns of spontaneous impulse activity in the dorsal and ventral SCN after a 6 hr delay of the light schedule. Bisection of the SCN slice revealed a separate fast-resetting oscillator in the ventral SCN and a distinct slow-resetting oscillator in the dorsal SCN. Continuous application of the GABA(A) antagonist bicuculline yielded similar results as cut slices. Short application of bicuculline at different phases of the circadian cycle increased the electrical discharge rate in the ventral SCN but, unexpectedly, decreased activity in the dorsal SCN.

CONCLUSIONS

GABA transmits phase information between the ventral and dorsal SCN oscillators. GABA can act excitatory in the dorsal SCN and inhibits neurons in the ventral SCN. We hypothesize that this difference results in asymmetrical interregional coupling within the SCN, with a stronger phase-shifting effect of the ventral on the dorsal SCN than vice versa. A model is proposed that focuses on this asymmetry and on the role of GABA in phase regulation.

Persistent subthreshold voltage-dependent cation single channels in suprachiasmatic nucleus neurons

The hypothalamic suprachiasmatic nucleus (SCN) contains the primary circadian pacemaker in mammals, and transmits circadian signals by diurnal modulation of neuronal firing frequency. The ionic mechanisms underlying the circadian regulation of firing frequency are unknown, but may involve changes in membrane potential and voltage-gated ion channels. Here we describe novel tetrodotoxin- and nifedipine-resistant subthreshold, voltage-dependent cation (SVC) channels that are active at resting potential of SCN neurons and increase their open probability (P(o)) with membrane depolarization. The increased P(o) reflects changes in the kinetics of the slow component of the channel closed-time, but not the channel open-time or fast closed-time. This study provides a background for investigation of the possible role of SVC channels in regulation of circadian oscillations of membrane excitability in SCN neurons.

Membranes, Ions, and Clocks: Testing the Njus–Sulzman–Hastings Model of the Circadian Oscillator

Current circadian clock models based on interlocking autoregulatory transcriptional?translational negative feedback loops have arisen out of an explosion of molecular genetic data obtained over the last decade (for review, see Stanewsky, 2003; Young and Kay, 2001). An earlier model of circadian oscillation was based on feedback interactions between membrane ion transport systems and ion concentration gradients (Njus et al., 1974, 1976). This membrane model was posited as a more plausible alternative at the time to the even earlier "chronon" model, which was based on autoregulatory genetic feedback loops (Ehret and Trucco, 1967). The membrane model has been tested in a number of experimental systems by pharmacologically manipulating either ionic gradients across the plasma membrane or ion transport systems, but with inconsistent results. In the meantime, the scope and explanatory power of the genetic models overshadowed inquiries into the role of membrane ion fluxes in clock function. However, several recently developed techniques described in this article have provided a new glimpse into the essential role that membrane ion fluxes play in the mechanism of the core circadian oscillator and indicate that a complete understanding of the clock must include both genetic and membrane-based feedback loops.

Chloride accumulation in mammalian olfactory sensory neurons

The generation of an excitatory receptor current in mammalian olfactory sensory neurons (OSNs) involves the sequential activation of two distinct types of ion channels: cAMP-gated Ca(2+)-permeable cation channels and Ca(2+)-gated Cl(-) channels, which conduct a depolarizing Cl(-) efflux. This unusual transduction mechanism requires an outward-directed driving force for Cl(-), established by active accumulation of Cl(-) within the lumen of the sensory cilia. We used two-photon fluorescence lifetime imaging microscopy of the Cl(-)-sensitive dye 6-methoxy-quinolyl acetoethyl ester to measure the intracellular Cl(-) concentration in dendritic knobs of OSNs from mice and rats. We found a uniform intracellular Cl(-) concentration in the range of 40-50 mm, which is indicative of active Cl(-) accumulation. Functional assays and PCR experiments revealed that NKCC1-mediated Cl(-) uptake through the apical membrane counteracts Cl(-) depletion in the sensory cilia, and thus maintains the responsiveness of OSNs to odor stimulation. To permit Cl(-) accumulation, OSNs avoid the "chloride switch": they do not express KCC2, the main Cl(-) extrusion cotransporter operating in neurons of the adult CNS. Cl(-) accumulation provides OSNs with the driving force for the depolarizing Cl(-) current that is the basis of the low-noise receptor current in these neurons.

Circadian rhythm in inhibitory synaptic transmission in the mouse suprachiasmatic nucleus

It is widely accepted that most suprachiasmatic nucleus (SCN) neurons express the neurotransmitter GABA and are likely to use this neurotransmitter to regulate excitability within the SCN. To evaluate the possibility that inhibitory synaptic transmission varies with a circadian rhythm within the mouse SCN, we used whole cell patch-clamp recording in an acute brain slice preparation to record GABA-mediated spontaneous inhibitory postsynaptic currents (sIPSCs). We found that the sIPSC frequency in the dorsal SCN (dSCN) exhibited a TTX-sensitive daily rhythm that peaked during the late day and early night in mice held in a light:dark cycle. We next evaluated whether vasoactive intestinal peptide (VIP) was responsible for the observed rhythm in IPSC frequency. Pretreatment of SCN slices with VPAC(1)/VPAC(2)- or VPAC(2)-specific receptor antagonists prevented the increase in sIPSC frequency in the dSCN. The rhythm in sIPSC frequency was absent in VIP/peptide histidine isoleucine (PHI)-deficient mice. Finally, we were able to detect a rhythm in the frequency of inhibitory synaptic transmission in mice held in constant darkness that was also dependent on VIP and the VPAC(2) receptor. Overall, these data demonstrate that there is a circadian rhythm in GABAergic transmission in the dorsal region of the mouse SCN and that the VIP is required for expression of this rhythm.

Stress control and human nutrition

Stress is a pervasive factor in everyday life that critically affects development and functioning. Severe and prolonged stress exposure impairs homeostatic mechanisms, particularly associated with the onset of depressive illness. Brain food is aimed at preventing as well as treating a growing number of stress-related mental disorders. Some topics on the association of stress and nutrition is reviewed. (1) An increased activity of serotonergic neurons in the brain is an established consequence of stress. An increase in brain tryptophan levels on the order of that produced by eating a carbohydrate-rich/protein-poor meal causes parallel increases in the amounts of serotonin released into synapses. (2) Eating is thought to be suppressed during stress, due to anorectic effects of corticotrophin releasing hormone, and increased during recovery from stress, due to appetite stimulating effects of residual cortisol. (3) A strong inverse association between coffee intake and risk of suicide. (4) Night eating syndrome has been found to occur during periods of stress and is associated with poor results at attempts to lose weight and disturbances in the hypothalamic-pituitary-adrenal axis. (5) Dietary antioxidants present in fruits and vegetables may improve cognitive function. Therefore, it is concluded that the establishment of functional foods that correctly regulate stress response must be firmly based upon scientific knowledge and legal regulation.

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.

Night eating syndrome: effects of brief relaxation training on stress, mood, hunger, and eating patterns

BACKGROUND

Night eating syndrome (NES) is characterized by a lack of appetite in the morning, consumption of 50% or more of daily food intake after 6:00 p.m., and difficulty falling and/or staying asleep. It has been associated with stress and with poor results at attempts to lose weight.

OBJECTIVE

The purpose of this study was to determine whether a relaxation intervention (Abbreviated Progressive Muscle Relaxation Therapy, APRT) that has been shown to significantly reduce stress levels in normal, healthy adults would also benefit an NES sample.

RESEARCH METHODS AND PROCEDURES

A total of 20 adults with NES were randomly assigned to either a relaxation training (APRT) or a Control (quietly sitting for the same amount of time) group, and all subjects attended two laboratory sessions 1 week apart. Pre- and postsession indices of stress, anxiety, relaxation, and salivary cortisol were obtained, as well as Day 1 and Day 8 indices of mood. Food diaries and hunger ratings were also obtained.

RESULTS

The results indicated that 20 min of a muscle relaxation exercise significantly reduced stress, anxiety, and salivary cortisol immediately postsession. After practicing these exercises daily for a week, subjects exhibited lowered stress, anxiety, fatigue, anger, and depression on Day 8. APRT was also associated with significantly higher a.m. and lower p.m. ratings of hunger, and a trend of both more breakfast and less night-time eating.

DISCUSSION

These data support the role of stress and anxiety in NES and suggest that practicing relaxation may be an important component of treatment for this condition.

Stimulation of Na+/Mg2+ antiport in rat erythrocytes by intracellular Cl-

Mg(2+) efflux from rat erythrocytes was measured in NaCl, NaNO(3), NaSCN and Na gluconate medium. Substitution of extracellular and intracellular Cl(-) with the permeant anions NO(3)(-) and SCN(-) reduced Mg(2+) efflux via Na(+)/Mg(2+) antiport. After substitution of extracellular Cl(-) with the non-permeant anion gluconate, Mg(2+) efflux was not significantly reduced. In Na gluconate medium, an influence of the changed membrane potential and intracellular pH on Mg(2+) efflux could be excluded. The results indicate the existence of Cl(-)-independent Na(+)/Mg(2+) antiport and of Na(+)/Mg(2+) antiport stimulated by intracellular Cl(-). Intracellular Cl(-), as determined by means of (36)Cl(-), was found to stimulate Na(+)/Mg(2+) antiport through a cooperative effect according to a sigmoidal kinetics. The Hill coefficient for intracellular Cl(-) amounted to 1.4-1.8, indicating that two intracellular Cl(-) may be simultaneously active. With respect to specificity, Cl(-) was most effective, followed by Br(-), J(-), and F(-). Stimulation of Na(+)/Mg(2+) antiport by intracellular Cl(-) together with intracellular Mg(2+) may play a role during deoxygenation of erythrocytes and in essential hypertension.

Cation-chloride co-transporters in neuronal communication, development and trauma

Electrical signaling in neurons is based on the operation of plasmalemmal ion pumps and carriers that establish transmembrane ion gradients, and on the operation of ion channels that generate current and voltage responses by dissipating these gradients. Although both voltage- and ligand-gated channels are being extensively studied, the central role of ion pumps and carriers is largely ignored in current neuroscience. Such an information gap is particularly evident with regard to neuronal Cl- regulation, despite its immense importance in the generation of inhibitory synaptic responses by GABA- and glycine-gated anion channels. The cation-chloride co-transporters (CCCs) have been identified as important regulators of neuronal Cl- concentration, and recent work indicates that CCCs play a key role in shaping GABA- and glycine-mediated signaling, influencing not only fast cell-to-cell communication but also various aspects of neuronal development, plasticity and trauma.