Diurnal modulation of the Na+/K+-ATPase and spontaneous firing in the rat retinorecipient clock neurons

Circadian Rhythms of the Blood-Brain Barrier and Drug Delivery

The blood-brain barrier (BBB) is a critical interface separating the central nervous system from the peripheral circulation, ensuring brain homeostasis and function. Recent research has unveiled a profound connection between the BBB and circadian rhythms, the endogenous oscillations synchronizing biological processes with the 24-hour light-dark cycle. This review explores the significance of circadian rhythms in the context of BBB functions, with an emphasis on substrate passage through the BBB. Our discussion includes efflux transporters and the molecular timing mechanisms that regulate their activities. A significant focus of this review is the potential implications of chronotherapy, leveraging our knowledge of circadian rhythms for improving drug delivery to the brain. Understanding the temporal changes in BBB can lead to optimized timing of drug administration, to enhance therapeutic efficacy for neurological disorders while reducing side effects. By elucidating the interplay between circadian rhythms and drug transport across the BBB, this review offers insights into innovative therapeutic interventions.

Afterhyperpolarization potential modulated by local [K+]o in K+ diffusion-restricted extracellular space in the central clock of suprachiasmatic nucleus

BACKGROUND

Intercellular coupling is essential for the suprachiasmatic nucleus (SCN) to serve as a coherent central clock. Synaptic release of neurotransmitters and neuropeptides is critical for synchronizing SCN neurons. However, intercellular coupling via non-synaptic mechanisms has also been demonstrated. In particular, the abundant perikaryal appositions with morphological specializations in the narrow extracellular space (ECS) may hinder molecular diffusion to allow for ion accumulation or depletion.

METHODS

The SCN neurons were recorded in the whole-cell current-clamp mode, with pipette filled with high (26 mM)-Na+ or low (6 mM)-Na+ solution.

RESULTS

Cells recorded with high-Na+ pipette solution could fire spontaneous action potentials (AP) with peak AHP more negative than the calculated value of K+ equilibrium potential (EK) and with peak AP more positive than calculated ENa. Cells recorded with low-Na+ pipette solution could also have peak AHP more negative than calculated EK. In contrast, the resting membrane potential (RMP) was always less negative to calculated EK. The distribution and the averaged amplitude of peak AHP, peak AP, or RMP was similar between cells recorded with high-Na+ and low-Na+ solution pipette. In a number of cells, the peak AHP could increase from more positive to become more negative than calculated EK spontaneously or after treatments to hyperpolarize the RMP. TTX blocked the Na+ -dependent APs and tetraethylammonium (TEA), but not Ba2+ or Cd2+, markedly reduced the peak AHP. Perforated-patch cells could also but rarely fire APs with peak AHP more negative than calculated EK.

CONCLUSION

The result of peak AHP negative to calculated EK indicates that local [K+]o sensed by the TEA-sensitive AHP K+ channels must be lower than bulk [K+]o, most likely due to K+ clearance from K+ diffusion-restricted ECS by the Na+/K+-ATPase. The K+ diffusion-restricted ECS may allow for K+-mediated ionic interactions among neurons to regulate SCN excitability.

Acarbose Mitigates Age-Dependent Alterations in Erythrocyte Membrane Transporters During Aging in Rats

Acarbose (ACA), a well-studied and effective inhibitor of α-amylase and α-glucosidase, is a postprandial-acting antidiabetic medicine. The membrane of the erythrocyte is an excellent tool for analyzing different physiological and biochemical activities since it experiences a range of metabolic alterations throughout aging. It is uncertain if ACA modulates erythrocyte membrane activities in an age-dependent manner. As a result, the current study was conducted to explore the influence of ACA on age-dependent deteriorated functions of transporters/exchangers, disrupted levels of various biomarkers such as lipid hydroperoxides (LHs), protein carbonyl (PCO), sialic acid (SA), total thiol (-SH), and erythrocyte membrane osmotic fragility. In addition to a concurrent increase in Na+/H+ exchanger activity and concentration of LH, PCO, and osmotic fragility, we also detected a considerable decrease in membrane-linked activities of Ca2+-ATPase (PMCA) and Na+/K+-ATPase (NKA), as well as concentrations of SA and -SH in old-aged rats. The aging-induced impairment of the activities of membrane-bound ATPases and the changed levels of redox biomarkers were shown to be effectively restored by ACA treatment.

Glutamate-Evoked Ca2+ Responses in the Rat Suprachiasmatic Nucleus: Involvement of Na+/K+-ATPase and Na+/Ca2+-Exchanger

Glutamate mediates photic entrainment of the central clock in the suprachiasmatic nucleus (SCN) by evoking intracellular Ca2+ signaling mechanisms. However, the detailed mechanisms of glutamate-evoked Ca2+ signals are not entirely clear. Here, we used a ratiometric Ca2+ and Na+ imaging technique to investigate glutamate-evoked Ca2+ responses. The comparison of Ca2+ responses to glutamate (100 μM) and high (20 mM) K+ solution indicated slower Ca2+ clearance, along with rebound Ca2+ suppression for glutamate-evoked Ca2+ transients. Increasing the length of exposure time in glutamate, but not in 20 mM K+, slowed Ca2+ clearance and increased rebound Ca2+ suppression, a result correlated with glutamate-induced Na+ loads. The rebound Ca2+ suppression was abolished by ouabain, monensin, Na+-free solution, or nimodipine, suggesting an origin of activated Na+/K+-ATPase (NKA) by glutamate-induced Na+ loads. Ouabain or Na+-free solution also slowed Ca2+ clearance, apparently by retarding Na+/Ca2+-exchanger (NCX)-mediated Ca2+ extrusion. Together, our results indicated the involvement of glutamate-induced Na+ loads, NKA, and NCX in shaping the Ca2+ response to glutamate. Nevertheless, in the absence of external Na+ (NMDG substituted), Ca2+ clearance was still slower for the Ca2+ response to glutamate than for 20 mM K+, suggesting participation of additional Ca2+ handlers to the slower Ca2+ clearance under this condition.

Melatonin stabilizes age-dependent alterations in erythrocyte membrane induced by 'Artificial Light at Night' in a chronodisrupted model of rat

Growing evidence has shown that Artificial light at night (ALAN) is one of the threatening risk factors which disrupt circadian homeodynamics of cellular processes. The chronobiological role of melatonin seems to represent an important aspect of its contribution to healthy aging. In the present study, we examined the age dependent effect of melatonin on erythrocyte membrane transporters and oxidative stress biomarkers against ALAN to understand the degree of photo-oxidative damage in chronodisrupted rat model. Young (3 months) and old (24 months) male Wistar rats were subdivided in the following four young groups (n = 4) ; (i) control (ii) melatonin (10 mg/kg) (iii) ALAN (500 lx) (iv) ALAN (500 lx) + melatonin (10 mg/kg) and four old groups (n = 4); (v) control (vi) melatonin (10 mg/kg) (vii) ALAN (500 lx) (viii) ALAN (500 lx) + melatonin (10 mg/kg) to the experimental conditions for 10 days. Our findings demonstrated that ALAN significantly enhanced erythrocyte membrane lipid hydroperoxides (LHPs), protein carbonyl (PCO) while reduced total thiol (T-SH), and sialic acid (SA) level with higher amplitude in old ALAN group is restored by exogenous supplementation of melatonin. Activity of membrane transporters, sodium potassium ATPase (NKA) and plasma membrane calcium ion ATPase (PMCA) is significantly reduced meanwhile sodium hydrogen exchanger (NHE) activity is enhanced under the influence of ALAN with higher extent in old groups is effectively ameliorated by melatonin treatment. Further melatonin reduced osmotic fragility of erythrocyte in both young and old rats. It has been concluded from results that ALAN provoked redox insult and disrupt transporters activity more prominently in erythrocyte membrane of aged groups. Exogenous supplementation of melatonin is one of the possible therapeutic approaches to reinforce circadian modulations against ALAN in aged populations.

Glycolytic metabolism and activation of Na+ pumping contribute to extracellular acidification in the central clock of the suprachiasmatic nucleus: Differential glucose sensitivity and utilization between oxidative and non-oxidative glycolytic pathways

BACKGROUND

The central clock of the suprachiasmatic nucleus (SCN) controls the metabolism of glucose and is sensitive to glucose shortage. However, it is only beginning to be understood how metabolic signals such as glucose availability regulate the SCN physiology. We previously showed that the ATP-sensitive K⁺ channel plays a glucose-sensing role in regulating SCN neuronal firing at times of glucose shortage. Nevertheless, it is unknown whether the energy-demanding Na⁺/K⁺-ATPase (NKA) is also sensitive to glucose availability. Furthermore, we recently showed that the metabolically active SCN constantly extrudes H⁺ to acidify extracellular pH (pHe). This study investigated whether the standing acidification is associated with Na⁺ pumping activity, energy metabolism, and glucose utilization, and whether glycolysis- and mitochondria-fueled NKAs may be differentially sensitive to glucose shortage.

METHODS

Double-barreled pH-selective microelectrodes were used to determine the pHe in the SCN in hypothalamic slices.

RESULTS

NKA inhibition with K⁺-free (0-K⁺) solution rapidly and reversibly alkalinized the pHe, an effect abolished by ouabain. Mitochondrial inhibition with cyanide acidified the pHe but did not inhibit 0-K⁺-induced alkalinization. Glycolytic inhibition with iodoacetate alkalinized the pHe, completely blocked cyanide-induced acidification, and nearly completely blocked 0-K⁺-induced alkalinization. The results indicate that glycolytic metabolism and activation of Na⁺ pumping contribute to the standing acidification. Glucoprivation also alkalinized the pHe, nearly completely eliminated cyanide-induced acidification, but only partially reduced 0-K⁺-induced alkalinization. In contrast, hypoglycemia preferentially and partially blocked cyanide-induced acidification. The result indicates sensitivity to glucose shortage for the mitochondria-associated oxidative glycolytic pathway.

CONCLUSION

Glycolytic metabolism and activation of glycolysis-fueled NKA Na⁺ pumping activity contribute to the standing acidification in the SCN. Furthermore, the oxidative and non-oxidative glycolytic pathways differ in their glucose sensitivity and utilization, with the oxidative glycolytic pathway susceptible to glucose shortage, and the non-oxidative glycolytic pathway able to maintain Na⁺ pumping even in glucoprivation.

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.

Role of Intracellular Na+ in the Regulation of [Ca2+]i in the Rat Suprachiasmatic Nucleus Neurons

Transmembrane Ca²⁺ influx is essential to the proper functioning of the central clock in the suprachiasmatic nucleus (SCN). In the rat SCN neurons, the clearance of somatic Ca²⁺ following depolarization-induced Ca²⁺ transients involves Ca²⁺ extrusion via Na⁺/Ca²⁺ exchanger (NCX) and mitochondrial Ca²⁺ buffering. Here we show an important role of intracellular Na⁺ in the regulation of [Ca²⁺]_i in these neurons. The effect of Na⁺ loading on [Ca²⁺]_i was determined with the Na⁺ ionophore monensin and the cardiac glycoside ouabain to block Na⁺/K⁺-ATPase (NKA). Ratiometric Na⁺ and Ca²⁺ imaging was used to measure the change in [Na⁺]_i and [Ca²⁺]_i, and cell-attached recordings to investigate the effects of monensin and ouabain on spontaneous firing. Our results show that in spite of opposite effects on spontaneous firing and basal [Ca²⁺], both monensin and ouabain induced Na⁺ loading, and increased the peak amplitude, slowed the fast decay rate, and enhanced the slow decay phase of 20 mM K⁺-evoked Ca²⁺ transients. Furthermore, both ouabain and monensin preferentially enhanced nimodipine-insensitive Ca²⁺ transients. Together, our results indicate that in the SCN neurons the NKA plays an important role in regulating [Ca²⁺]_i, in particular, associated with nimodipine-insensitive Ca²⁺ channels.

The Na+/H+-Exchanger NHE1 Regulates Extra- and Intracellular pH and Nimodipine-sensitive [Ca2+]i in the Suprachiasmatic Nucleus

The central clock in the suprachiasmatic nucleus (SCN) has higher metabolic activity than extra-SCN areas in the anterior hypothalamus. Here we investigated whether the Na⁺/H⁺ exchanger (NHE) may regulate extracellular pH (pHe), intracellular pH (pHi) and [Ca²⁺]_i in the SCN. In hypothalamic slices bathed in HEPES-buffered solution a standing acidification of ~0.3 pH units was recorded with pH-sensitive microelectrodes in the SCN but not extra-SCN areas. The NHE blocker amiloride alkalinised the pHe. RT-PCR revealed mRNA for plasmalemmal-type NHE1, NHE4, and NHE5 isoforms, whereas the NHE1-specific antagonist cariporide alkalinised the pHe. Real-time PCR and western blotting failed to detect day-night variation in NHE1 mRNA and protein levels. Cariporide induced intracellular acidosis, increased basal [Ca²⁺]_i, and decreased depolarisation-induced Ca²⁺ rise, with the latter two effects being abolished with nimodipine blocking the L-type Ca²⁺ channels. Immunofluorescent staining revealed high levels of punctate colocalisation of NHE1 with serotonin transporter (SERT) or CaV1.2, as well as triple staining of NHE1, CaV1.2, and SERT or the presynaptic marker Bassoon. Our results indicate that NHE1 actively extrudes H⁺ to regulate pHi and nimodipine-sensitive [Ca²⁺]_i in the soma, and along with CaV1.2 may also regulate presynaptic Ca²⁺ levels and, perhaps at least serotonergic, neurotransmission in the SCN.

Diurnal variations of foveoschisis by optical coherence tomography in patients with RS1 X-linked juvenile retinoschisis

BACKGROUND

To evaluate diurnal variations in macular schisis cavities in patients with X-linked juvenile retinoschisis (XLRS) with pathogenic variants in the RS1 gene using spectral-domain optical coherence tomography (SD-OCT).

METHODS

Three consecutive patients with a clinical diagnosis of XLRS and pathogenic variants in the RS1, treated with carbonic anhydrase inhibitors (CAIs). Observational procedures: SD-OCT scans of the macula were acquired at 9 a.m., 1 p.m., and 4 p.m. within 24 h.

RESULTS

All patients demonstrated increased measures of central foveal thickness in the morning with gradual decrease through the day (9-43%). Major changes were observed between 9 a.m. and 1 p.m. in the central foveal thickness.

CONCLUSION

The central foveal thickness varies during daytime hours in patients with XLRS. This finding may explain the inconsistent and heterogeneous responses to treatment with CAIs and necessitate standardization of measurement times in treatment trials for XLRS as well as in the routine ophthalmic evaluation of these patients.

Differential regulation of nimodipine-sensitive and -insensitive Ca2+ influx by the Na+/Ca2+ exchanger and mitochondria in the rat suprachiasmatic nucleus neurons

Background

Transmembrane Ca²⁺ influx is critical for molecular rhythmicity, metabolic activity, and neuropeptide release in the central clock of the suprachiasmatic nucleus (SCN). We previously reported that both the Na⁺/Ca²⁺ exchanger (NCX) and mitochondria play a role in regulating intracellular Ca²⁺ homeostasis in the rat SCN neurons. Here we present evidence to show differential regulation by NCX and mitochondria of nimodipine-sensitive and -insensitive Ca²⁺ influx.

Methods

Ratiometric Ca²⁺ imaging was used to measure change in [Ca²⁺]_i and patch clamp recordings to study spontaneous firing, membrane potential, and voltage-dependent Ca²⁺ channels in neurons from reduced SCN slice preparations. Immunofluorescent staining was used to determine the distribution pattern of CaV1.2 and CaV1.3 and their colocalization with NCX1.

Results

Ratiometric Ca²⁺ imaging indicates that nimodipine (2 μM) blocked most of 20 (mM) K⁺-induced, but less so of 50 K⁺-induced, Ca²⁺ rise. The nimodipine-sensitive 50 K⁺-induced Ca²⁺ transient rose more rapidly but decayed similarly with the nimodipine-insensitive component, suggesting both components were extruded by NCX. Immunofluorescent stains showed the expression of both CaV1.2 and CaV1.3 and their colocalization with NCX1, whereas functional studies suggest that CaV1.2 mediated most of the nimodipine-sensitive Ca²⁺ rise but had insignificant effect on spontaneous firing. After normalization relative to the Ca²⁺-free solution, nimodipine reduced ~ 65% of basal Ca²⁺ influx, and TTX lowered it by ~ 35%, leaving ~ 25% basal Ca²⁺ influx in the combined presence of TTX and nimodipine. With the mitochondrial uncoupler carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) to inhibit mitochondrial Ca²⁺ uptake, 20 K⁺-induced Ca²⁺ transients became larger and slower, both in the absence and presence of nimodipine. FCCP markedly enhanced nimodipine-insensitive, but not nimodipine-sensitive, Ca²⁺ transients, suggesting that mitochondria preferentially buffer nimodipine-insensitive Ca²⁺ influx. Results from using CaV2 channel blockers further indicate that FCCP enhanced Ca²⁺ transients mediated by N-, P/Q-, and the blocker cocktail-insensitive Ca²⁺ channels.

Conclusions

The differential regulation of transmembrane Ca²⁺ influx by NCX and mitochondria suggests that Ca²⁺ entry via different sources may be regulated differently to play different roles in SCN physiology.

Circadian Plasticity in the Brain of Insects and Rodents

In both vertebrate and invertebrate brains, neurons, glial cells and synapses are plastic, which means that the physiology and structure of these components are modified in response to internal and external stimuli during development and in mature brains. The term plasticity has been introduced in the last century to describe experience-dependent changes in synapse strength and number. These changes result from local functional and morphological synapse modifications; however, these modifications also occur more commonly in pre- and postsynaptic neurons. As a result, neuron morphology and neuronal networks are constantly modified during the life of animals and humans in response to different stimuli. Nevertheless, it has been discovered in flies and mammals that the number of synapses and size and shape of neurons also oscillate during the day. In most cases, these rhythms are circadian since they are generated by endogenous circadian clocks; however, some rhythmic changes in neuron morphology and synapse number and structure are controlled directly by environmental cues or by both external cues and circadian clocks. When the circadian clock is involved in generating cyclic changes in the nervous system, this type of plasticity is called circadian plasticity. It seems to be important in processing sensory information, in learning and in memory. Disruption of the clock may affect major brain functions.

Sodium pump regulation of locomotor control circuits

Sodium pumps are ubiquitously expressed membrane proteins that extrude three Na⁺ ions in exchange for two K⁺ ions, using ATP as an energy source. Recent studies have illuminated additional, dynamic roles for sodium pumps in regulating the excitability of neuronal networks in an activity-dependent fashion. We review their role in a novel form of short-term memory within rhythmic locomotor networks. The data we review derives mainly from recent studies on Xenopus tadpoles and neonatal mice. The role and underlying mechanisms of pump action broadly match previously published data from an invertebrate, the Drosophila larva. We therefore propose a highly conserved mechanism by which sodium pump activity increases following a bout of locomotion. This results in an ultraslow afterhyperpolarization (usAHP) of the membrane potential that lasts around 1 min, but which only occurs in around half the network neurons. This usAHP in turn alters network excitability so that network output is reduced in a locomotor interval-dependent manner. The pumps therefore confer on spinal locomotor networks a temporary memory trace of recent network performance.

KATP Channels Mediate Differential Metabolic Responses to Glucose Shortage of the Dorsomedial and Ventrolateral Oscillators in the Central Clock

The suprachiasmatic nucleus (SCN) central clock comprises two coupled oscillators, with light entraining the retinorecipient vasoactive intestinal peptide (VIP)-positive ventrolateral oscillator, which then entrains the arginine vasopressin (AVP)-positive dorsomedial oscillator. While glucose availability is known to alter photic entrainment, it is unclear how the SCN neurones respond to metabolic regulation and whether the two oscillators respond differently. Here we show that the ATP-sensitive K⁺ (K_ATP) channel mediates differential responses to glucose shortage of the two oscillators. RT-PCR and electrophysiological results suggested the presence of Kir6.2/SUR1 K_ATP channels in the SCN neurones. Immunostaining revealed preferential distribution of Kir6.2 in the dorsomedial subregion and selective colocalization with AVP. Whole cell recordings with ATP-free pipette solution indicated larger tolbutamide-induced depolarisation and tolbutamide-sensitive conductance in dorsal SCN (dSCN) than ventral SCN (vSCN) neurones. Tolbutamide-sensitive conductance was low under perforated patch conditions but markedly enhanced by cyanide inhibition of mitochondrial respiration. Glucoprivation produced a larger steady-state inhibition in dSCN than vSCN neurones, and importantly hypoglycemia via opening K_ATP channels selectively inhibited the K_ATP-expressing neurones. Our results suggest that the AVP-SCN oscillator may act as a glucose sensor to respond to glucose shortage while sparing the VIP-SCN oscillator to remain in synch with external light-dark cycle.

Role of Na⁺/Ca²⁺ exchanger in Ca²⁺ homeostasis in rat suprachiasmatic nucleus neurons

Intracellular Ca(2+) is critical to the central clock of the suprachiasmatic nucleus (SCN). However, the role of Na(+)/Ca(2+) exchanger (NCX) in intracellular Ca(2+) concentration ([Ca(2+)]i) homeostasis in the SCN is unknown. Here we show that NCX is an important mechanism for somatic Ca(2+) clearance in SCN neurons. In control conditions Na(+)-free solution lowered [Ca(2+)]i by inhibiting TTX-sensitive as well as nimodipine-sensitive Ca(2+) influx. With use of the Na(+) ionophore monensin to raise intracellular Na(+) concentration ([Na(+)]i), Na(+)-free solution provoked rapid Ca(2+) uptake via reverse NCX. The peak amplitude of 0 Na(+)-induced [Ca(2+)]i increase was larger during the day than at night, with no difference between dorsal and ventral SCN neurons. Ca(2+) extrusion via forward NCX was studied by determining the effect of Na(+) removal on Ca(2+) clearance after high-K(+)-induced Ca(2+) loads. The clearance of Ca(2+) proceeded with two exponential decay phases, with the fast decay having total signal amplitude of ∼85% and a time constant of ∼7 s. Na(+)-free solution slowed the fast decay rate threefold, whereas mitochondrial protonophore prolonged mostly the slow decay. In contrast, blockade of plasmalemmal and sarco(endo)plasmic reticulum Ca(2+) pumps had little effect on the kinetics of Ca(2+) clearance. RT-PCR indicated the expression of NCX1 and NCX2 mRNAs. Immunohistochemical staining showed the presence of NCX1 immunoreactivity in the whole SCN but restricted distribution of NCX2 immunoreactivity in the ventrolateral SCN. Together our results demonstrate an important role of NCX, most likely NCX1, as well as mitochondrial Ca(2+) uptake in clearing somatic Ca(2+) after depolarization-induced Ca(2+) influx in SCN neurons.

Circadian regulation of the Na+/K+-ATPase alpha subunit in the visual system is mediated by the pacemaker and by retina photoreceptors in Drosophila melanogaster

We investigated the diurnal oscillation in abundance of the catalytic α subunit of the sodium/potassium pump (ATPα) in the brain of Drosophila melanogaster. This rhythm is bimodal and is particularly robust in the glia cells of the lamina, the first optic neuropil. We observed loss of ATPα cycling in lamina glia in behaviourally arrhythmic per⁰¹ and tim⁰¹ mutants and in flies overexpressing the pro-apoptotic gene hid in the PDF-positive clock neurons. Moreover, the rhythm of ATPα abundance was altered in cry⁰¹ and Pdf⁰ mutants, in flies with a weakened clock mechanism in retina photoreceptor cells and in those subject to downregulation of the neuropeptide ITP by RNAi. This complex, rhythmic regulation of the α subunit suggests that the sodium/potassium pump may be a key target of the circadian pacemaker to impose daily control on brain activities, such as rhythmic changes in neuronal plasticity, which are best observed in the visual system.

Intracellular Na+ and metabolic modulation of Na/K pump and excitability in the rat suprachiasmatic nucleus neurons

Na/K pump activity and metabolic rate are both higher during the day in the suprachiasmatic nucleus (SCN) that houses the circadian clock. Here we investigated the role of intracellular Na+ and energy metabolism in regulating Na/K pump activity and neuronal excitability. Removal of extracellular K+ to block the Na/K pump excited SCN neurons to fire at higher rates and return to normal K+ to reactivate the pump produced rebound hyperpolarization to inhibit firing. In the presence of tetrodotoxin to block the action potentials, both zero K+-induced depolarization and rebound hyperpolarization were blocked by the cardiac glycoside strophanthidin. Ratiometric Na+ imaging with a Na+-sensitive fluorescent dye indicated saturating accumulation of intracellular Na+ in response to pump blockade with zero K+. The Na+ ionophore monensin also induced Na+ loading and hyperpolarized the membrane potential, with the hyperpolarizing effect of monensin abolished in zero Na+ or by pump blockade. Conversely, Na+ depletion with Na+-free pipette solution depolarized membrane potential but retained residual Na/K pump activity. Cyanide inhibition of oxidative phosphorylation blocked the Na/K pump to depolarize resting potential and increase spontaneous firing in most cells, and to raise intracellular Na+ levels in all cells. Nonetheless, the Na/K pump was incompletely blocked by cyanide but completely blocked by iodoacetate to inhibit glycolysis, indicating the involvement of both oxidative phosphorylation and glycolysis in fueling the Na/K pump. Together, the results indicate the importance of intracellular Na+ and energy metabolism in regulating Na/K pump activity as well as neuronal excitability in the SCN neurons.

Involvement of Na+/K+ pump in fine modulation of bursting activity of the snail Br neuron by 10 mT static magnetic field

The spontaneously active Br neuron from the brain-subesophageal ganglion complex of the garden snail Helix pomatia rhythmically generates regular bursts of action potentials with quiescent intervals accompanied by slow oscillations of membrane potential. We examined the involvement of the Na(+)/K(+) pump in modulating its bursting activity by applying a static magnetic field. Whole snail brains and Br neuron were exposed to the 10-mT static magnetic field for 15 min. Biochemical data showed that Na(+)/K(+)-ATPase activity increased almost twofold after exposure of snail brains to the static magnetic field. Similarly, (31)P NMR data revealed a trend of increasing ATP consumption and increase in intracellular pH mediated by the Na(+)/H(+) exchanger in snail brains exposed to the static magnetic field. Importantly, current clamp recordings from the Br neuron confirmed the increase in activity of the Na(+)/K(+) pump after exposure to the static magnetic field, as the magnitude of ouabain's effect measured on the membrane resting potential, action potential, and interspike interval duration was higher in neurons exposed to the magnetic field. Metabolic pathways through which the magnetic field influenced the Na(+)/K(+) pump could involve phosphorylation and dephosphorylation, as blocking these processes abolished the effect of the static magnetic field.

Linking neural activity and molecular oscillations in the SCN

Neurons in the suprachiasmatic nucleus (SCN) function as part of a central timing circuit that drives daily changes in our behaviour and underlying physiology. A hallmark feature of SCN neuronal populations is that they are mostly electrically silent during the night, start to fire action potentials near dawn and then continue to generate action potentials with a slow and steady pace all day long. Sets of currents are responsible for this daily rhythm, with the strongest evidence for persistent Na(+) currents, L-type Ca(2+) currents, hyperpolarization-activated currents (I(H)), large-conductance Ca(2+) activated K(+) (BK) currents and fast delayed rectifier (FDR) K(+) currents. These rhythms in electrical activity are crucial for the function of the circadian timing system, including the expression of clock genes, and decline with ageing and disease. This article reviews our current understanding of the ionic and molecular mechanisms that drive the rhythmic firing patterns in the SCN.

Sodium MRI in a rat migraine model and a NEURON simulation study support a role for sodium in migraine

INTRODUCTION

Increased lumbar cerebrospinal fluid (CSF) sodium has been reported during migraine. We used ultra-high field MRI to investigate cranial sodium in a rat migraine model, and simulated the effects of extracellular sodium on neuronal excitability.

METHODS

Behavioral changes in the nitroglycerin (NTG) rat migraine model were determined from von Frey hair withdrawal response and photography. Central sensitization was measured by counting cFos-immunoreactive cells in the trigeminal nucleus caudalis (TNC). Sodium was quantified in vivo by ultra-high field sodium MRI at 21 Tesla. Effects of extracellular sodium on neuronal excitability were modeled using NEURON software.

RESULTS

NTG decreased von Frey withdrawal threshold (p=0.0003), decreased eyelid vertical height:width ratio (p<0.0001), increased TNC cFos stain (p<0.0001), and increased sodium between 7.5 and 17% in brain, intracranial CSF, and vitreous humor (p<0.05). Simulated neurons exposed to higher sodium have more frequent and earlier spontaneous action potentials, and corresponding earlier sodium and potassium currents.

CONCLUSIONS

In the rat migraine model, sodium rises to levels that increase neuronal excitability. We propose that rising sodium in CSF surrounding trigeminal nociceptors increases their excitability and causes pain and that rising sodium in vitreous humor increases retinal neuronal excitability and causes photosensitivity.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

Cerebrospinal fluid sodium rhythms

Background

Cerebrospinal fluid (CSF) sodium levels have been reported to rise during episodic migraine. Since migraine frequently starts in early morning or late afternoon, we hypothesized that natural sodium chronobiology may predispose susceptible persons when extracellular CSF sodium increases. Since no mammalian brain sodium rhythms are known, we designed a study of healthy humans to test if cation rhythms exist in CSF.

Methods

Lumbar CSF was collected every ten minutes at 0.1 mL/min for 24 h from six healthy participants. CSF sodium and potassium concentrations were measured by ion chromatography, total protein by fluorescent spectrometry, and osmolarity by freezing point depression. We analyzed cation and protein distributions over the 24 h period and spectral and permutation tests to identify significant rhythms. We applied the False Discovery Rate method to adjust significance levels for multiple tests and Spearman correlations to compare sodium fluctuations with potassium, protein, and osmolarity.

Results

The distribution of sodium varied much more than potassium, and there were statistically significant rhythms at 12 and 1.65 h periods. Curve fitting to the average time course of the mean sodium of all six subjects revealed the lowest sodium levels at 03.20 h and highest at 08.00 h, a second nadir at 09.50 h and a second peak at 18.10 h. Sodium levels were not correlated with potassium or protein concentration, or with osmolarity.

Conclusion

These CSF rhythms are the first reports of sodium chronobiology in the human nervous system. The results are consistent with our hypothesis that rising levels of extracellular sodium may contribute to the timing of migraine onset. The physiological importance of sodium in the nervous system suggests that these rhythms may have additional repercussions on ultradian functions.

Cholinergic modulation of neuronal excitability in the rat suprachiasmatic nucleus

The central cholinergic system regulates both the circadian clock and sleep-wake cycle and may participate in the feedback control of vigilance states on neural excitability in the suprachiasmatic nucleus (SCN) that houses the circadian clock. Here we investigate the mechanisms for cholinergic modulation of SCN neuron excitability. Cell-attached recordings indicate that the nonspecific cholinergic agonist carbachol (CCh) inhibited 55% and excited 21% SCN neurons, leaving 24% nonresponsive. Similar response proportions were produced by two muscarinic receptor [muscarinic acetylcholine receptor (mAChR)] agonists, muscarine and McN-A-343 (M1/4 agonist), but not by two nicotinic receptor (nAChR) agonists, nicotine and choline (alpha7-nAChR agonist), which, however, produced similar response proportions. Whole cell and perforated-patch recordings indicate that CCh inhibition of firing was mediated by membrane hyperpolarization due to activation of background K(+) currents, which were sensitive to submillimolar concentrations of Ba(2+) and to millimolar concentrations of TEA. RT-PCR analysis demonstrated the presence of mRNA for M1 to M5 mAChRs in SCN. The CCh-induced hyperpolarization and activation of background K(+) currents were blocked by M4 antagonists and to a lesser degree by M1 antagonists but were insensitive to the antagonists for M2 or M3, suggesting the involvement of M4 and M1 mAChRs in mediating CCh inhibition of firing. CCh enhancement of firing was mediated by membrane depolarization, as a result of postsynaptic inhibition of background K(+) currents. The multiple actions of cholinergic modulation via multiple receptors and ion channels may allow acetylcholine to finely control SCN neuron excitability in different physiological settings.

Circadian regulation of ion channels and their functions

Ion channels are the gatekeepers to neuronal excitability. Retinal neurons of vertebrates and invertebrates, neurons of the suprachiasmatic nucleus (SCN) of vertebrates, and pinealocytes of non-mammalian vertebrates display daily rhythms in their activities. The interlocking transcription-translation feedback loops with specific post-translational modulations within individual cells form the molecular clock, the basic mechanism that maintains the autonomic approximately 24-h rhythm. The molecular clock regulates downstream output signaling pathways that further modulate activities of various ion channels. Ultimately, it is the circadian regulation of ion channel properties that govern excitability and behavior output of these neurons. In this review, we focus on the recent development of research in circadian neurobiology mainly from 1980 forward. We will emphasize the circadian regulation of various ion channels, including cGMP-gated cation channels, various voltage-gated calcium and potassium channels, Na(+)/K(+)-ATPase, and a long-opening cation channel. The cellular mechanisms underlying the circadian regulation of these ion channels and their functions in various tissues and organisms will also be discussed. Despite the magnitude of chronobiological studies in recent years, the circadian regulation of ion channels still remains largely unexplored. Through more investigation and understanding of the circadian regulation of ion channels, the future development of therapeutic strategies for the treatment of sleep disorders, cardiovascular diseases, and other illnesses linked to circadian misalignment will benefit.

Cyclical expression of Na+/K+-ATPase in the visual system of Drosophila melanogaster

In the first (lamina) and second (medulla) optic neuropils of Drosophila melanogaster, sodium pump subunit expression changes during the day and night, controlled by a circadian clock. We examined alpha-subunit expression from the intensity of immunolabeling. For the beta-subunit, encoded by Nervana 2 (Nrv2), we used Nrv2-GAL4 to drive expression of GFP, and measured the resultant fluorescence in whole heads and specific optic lobe cells. All optic neuropils express the alpha-subunit, highest at the beginning of night in both lamina and medulla in day/night condition and the oscillation was maintained in constant darkness. This rhythm was lacking in the clock arrhythmic per(0) mutant. GFP driven by Nrv2 was mostly detected in glial cells, mainly in the medulla. There, GFP expression occurs in medulla neuropil glia (MNGl), which express the clock gene per, and which closely contact the terminals of clock neurons immunoreactive to pigment dispersing factor. GFP fluorescence exhibited circadian oscillation in whole heads from Nrv2-GAL4+UAS-S65T-GFP flies, although significant GFP oscillations were lacking in MNGl, as they were for both subunit mRNAs in whole-head homogenates. In the dissected brain tissues, however, the mRNA of the alpha-subunit showed a robust daily rhythm in concentration changes while changes in the beta-subunit mRNA were weaker and not statistically significant. Thus in the brain, the genes for the sodium pump subunits, at least the one encoding the alpha-subunit, seem to be clock-controlled and the abundance of their corresponding proteins mirrors daily changes in mRNA, showing cyclical accumulation in cells.

Acid-sensing ion channels in neurones of the rat suprachiasmatic nucleus

We used reduced slice reparations to study ASIC-like currents in the rat central clock suprachiasmatic nucleus (SCN). In reduced SCN preparations, a drop of extracellular pH evoked a desensitizing inward current to excite SCN neurones to fire at higher rates. Under voltage-clamped conditions, all SCN neurones responded to a 5 s pH step to 6.4 with an inward current that decayed with an average time constant of 1.2 s to 10% of the peak at the end of step. The current was blocked by amiloride with an IC(50) of 14 microm and was carried mainly by Na(+), suggesting an origin of ASIC-like channels. The SCN neurones were sensitive to neutral pH, with 94% of cells responding to pH 7.0 with an inward current. The study of sensitivity to pH between 7.0 and 4.4 revealed a two-component dose-dependent H(+) activation in most SCN neurones, with the first component (85% in amplitude) having a pH(50) of 6.6, and the second (15%) a pH(50) of 5. The ASIC-like currents were potentiated by lactate and low Ca(2+), but were inhibited by Zn(2+). RT-PCR analysis demonstrated the presence of mRNA for ASIC1a, 2a, 2b, and 3 in SCN. Compared to other central neurones, the unique presence of ASIC3 along with ASIC1a in SCN neurones may contribute to the high pH sensitivity and unusual inhibition by Zn(2+). The high pH sensitivity suggests that the SCN neurones are susceptive to extracellular acidification of physiological origins and that the ASIC current might play a role in regulating SCN excitability.

Thermodynamic regulation of NKCC1-mediated Cl- cotransport underlies plasticity of GABA(A) signaling in neonatal neurons

In the adult brain, chloride (Cl-) influx through GABA(A) receptors is an important mechanism of synaptic inhibition. However, under a variety of circumstances, including acquired epilepsy, neuropathic pain, after trains of action potentials or trauma, and during normal early brain development, GABA(A) receptor activation excites neurons by gating Cl- efflux because the intracellular Cl- concentration (Cl(i)) is elevated. These findings require an inducible, active mechanism of chloride accumulation. We used gramicidin-perforated patch recordings to characterize Cl- transport via NKCC1, the principal neuronal Cl- accumulator, in neonatal CA1 pyramidal neurons. NKCC1 activity was required to maintain elevated Cl(i) such that GABA(A) receptor activation was depolarizing. Kinetic analysis of NKCC1 revealed reversible transmembrane Cl- transport characterized by a large maximum velocity (vmax) and high affinity (Km), so that NKCC1 transport was limited only by the net electrochemical driving force for Na+, K+, and Cl-. At the steady-state Cl(i), NKCC1 was at thermodynamic equilibrium, and there was no evidence of net Cl- transport. Trains of action potentials that have been previously shown to induce persistent changes in neuronal E(Cl) (reversal potential for Cl-) did not alter vmax or Km of NKCC1. Rather, action potentials shifted the thermodynamic set point, the steady-state Cl(i) at which there was no net NKCC1-mediated Cl- transport. The persistent increase in Cl(i) required intact alpha2/alpha3 Na+-K+-ATPase activity, indicating that trains of action potentials reset the thermodynamic equilibrium for NKCC1 transport by lowering Na(i). Activity-induced changes in Na+-K+-ATPase activity comprise a novel mechanism for persistent alterations in synaptic signaling mediated by GABA.

Electrophysiology of the suprachiasmatic circadian clock

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

Effects of sodium pump activity on spontaneous firing in neurons of the rat suprachiasmatic nucleus

Cell-attached and whole cell recording techniques were used to study the effects of electrogenic sodium pump on the excitability of rat suprachiasmatic nucleus (SCN) neurons. Blocking the sodium pump with the cardiac steroid strophanthidin or zero K+ increased the spontaneous firing of SCN neurons to different degrees with different recording modes, whereas turning the sodium pump into a nonselective cation channel with the marine toxin palytoxin invariably increased the spontaneous firing to the point of total blockade. Current-clamp recordings indicated that strophanthidin increased the rate of membrane depolarization and reduced the peak afterhyperpolarization potential (AHP), whereas zero K+ also increased the rate of depolarization, but enhanced the peak AHP. The dual effect of zero K+ was reflected by the biphasic time course of voltage responses to zero K+: an inhibitory phase with enhanced peak AHP and slower firing, followed by a delayed excitatory phase with faster rate of membrane depolarization and faster firing. In the presence of strophanthidin to block the sodium pump, zero K+ consistently decreased firing by enhancing the peak AHP. Repetitive applications of K+ -free solution gradually turned the biphasic inhibitory-followed-by-excitatory voltage response into a monophasic inhibitory response in cells recorded with the whole cell (but not the cell-attached) mode, suggesting rundown of sodium pump activity. Taken together, the results suggest that spontaneous firing of SCN neurons is regulated by sodium pump activity as well as the AHP, and that sodium pump activity is modulated by intracellular soluble substances subject to rundown under the whole cell conditions.

Hyperphagia and obesity in Na,K-ATPase alpha2 subunit-defective mice

OBJECTIVE

The Na,K-ATPase alpha2 subunit gene (Atp1a2) is expressed in the brain, skeletal muscles, heart, and adipocytes. Specific function of the alpha2 subunit, such as involvement in differentiation and function of adipocytes, has not been addressed. The aim of this study was to examine whether Atp1a2-defective heterozygous mice show obesity and reveal the mechanisms underlying the obesity.

RESEARCH METHODS AND PROCEDURES

We measured the differentiation and glucose uptake function of in vitro-differentiated adipocytes derived from embryonic fibroblasts of Atp1a2-defective mice. Food intake, body temperature, metabolic rate, and spontaneous activity and mRNA levels of neuropeptide genes were compared between the heterozygous and wild-type adult mice.

RESULTS

Atp1a2 heterozygous female mice developed obesity after middle age. The time course of in vitro adipocyte differentiation of embryonic fibroblasts isolated from wild type, heterozygous, and homozygous mice was not different, glucose and Rb uptake activities of the in vitro-differentiated adipocytes were not altered, and the effects of insulin on glucose uptake and those of monensin and ouabain on Rb uptake were similar among the genotypes. However, food intake in the light phase was significantly greater in the heterozygous mice than the wild type in the 24-hour dark-light cycle, whereas it was similar under constant-light condition. Body temperature, metabolic rate at rest, and spontaneous motor activity of the heterozygous mice were similar to those of the wild type. Orexin mRNA level was lower in heterozygous than wild-type mice.

DISCUSSION

The Na,K-ATPase alpha2 subunit is not involved in the differentiation or in glucose and Rb uptake function of in vitro-differentiated adipocytes. Hyperphagia is the likely primary cause of obesity in Atp1a2 heterozygous mice.