Activity-dependent extracellular K+ accumulation in rat optic nerve: the role of glial and axonal Na+ pumps

The Dawn and Advancement of the Knowledge of the Genetics of Migraine

Background: Migraine is a prevalent episodic brain disorder known for recurrent attacks of unilateral headaches, accompanied by complaints of photophobia, phonophobia, nausea, and vomiting. Two main categories of migraine are migraine with aura (MA) and migraine without aura (MO). Main body: Early twin and population studies have shown a genetic basis for these disorders, and efforts have been invested since to discern the genes involved. Many techniques, including candidate-gene association studies, loci linkage studies, genome-wide association, and transcription studies, have been used for this goal. As a result, several genes were pinned with concurrent and conflicting data among studies. It is important to understand the evolution of techniques and their findings. Conclusions: This review provides a chronological understanding of the different techniques used from the dawn of migraine genetic investigations and the genes linked with the migraine subtypes.

Oligodendrocyte-axon metabolic coupling is mediated by extracellular K+ and maintains axonal health

The integrity of myelinated axons relies on homeostatic support from oligodendrocytes (OLs). To determine how OLs detect axonal spiking and how rapid axon-OL metabolic coupling is regulated in the white matter, we studied activity-dependent calcium (Ca2+) and metabolite fluxes in the mouse optic nerve. We show that fast axonal spiking triggers Ca2+ signaling and glycolysis in OLs. OLs detect axonal activity through increases in extracellular potassium (K+) concentrations and activation of Kir4.1 channels, thereby regulating metabolite supply to axons. Both pharmacological inhibition and OL-specific inactivation of Kir4.1 reduce the activity-induced axonal lactate surge. Mice lacking oligodendroglial Kir4.1 exhibit lower resting lactate levels and altered glucose metabolism in axons. These early deficits in axonal energy metabolism are associated with late-onset axonopathy. Our findings reveal that OLs detect fast axonal spiking through K+ signaling, making acute metabolic coupling possible and adjusting the axon-OL metabolic unit to promote axonal health.

Astrocytic Na+, K+ ATPases in physiology and pathophysiology

The Na+, K+ ATPases play a fundamental role in the homeostatic functions of astrocytes. After a brief historic prologue and discussion of the subunit composition and localization of the astrocytic Na+, K+ ATPases, the review focuses on the role of the astrocytic Na+, K+ pumps in extracellular K+ and glutamate homeostasis, intracellular Na+ and Ca2+ homeostasis and signaling, regulation of synaptic transmission and neurometabolic coupling between astrocytes and neurons. Loss-of-function mutations in the gene encoding the astrocytic α2 Na+, K+ ATPase cause a rare monogenic form of migraine with aura (familial hemiplegic migraine type 2). On the other hand, the α2 Na+, K+ ATPase is upregulated in spinal cord and brain samples from amyotrophic lateral sclerosis and Alzheimer disease patients, respectively. In the last part, the review focuses on i) the migraine relevant phenotypes shown by familial hemiplegic migraine type 2 knock-in mice with 50 % reduced expression of the astrocytic α2 Na+, K+ ATPase and the insights into the pathophysiology of migraine obtained from these genetic mouse models, and ii) the evidence that upregulation of the astrocytic α2 Na+, K+ ATPase in mouse models of amyotrophic lateral sclerosis and Alzheimer disease promotes neuroinflammation and contributes to progressive neurodegeneration.

Sodium homeostasis and signalling: The core and the hub of astrocyte function

Neuronal activity and neurochemical stimulation trigger spatio-temporal changes in the cytoplasmic concentration of Na+ ions in astrocytes. These changes constitute the substrate for Na+ signalling and are fundamental for astrocytic excitability. Astrocytic Na+ signals are generated by Na+ influx through neurotransmitter transporters, with primary contribution of glutamate transporters, and through cationic channels; whereas recovery from Na+ transients is mediated mainly by the plasmalemmal Na+/K+ ATPase. Astrocytic Na+ signals regulate the activity of plasmalemmal transporters critical for homeostatic function of astrocytes, thus providing real-time coordination between neuronal activity and astrocytic support.

Local extracellular K+ in cortex regulates norepinephrine levels, network state, and behavioral output

Extracellular potassium concentration ([K+]e) is known to increase as a function of arousal. [K+]e is also a potent modulator of transmitter release. Yet, it is not known whether [K+]e is involved in the neuromodulator release associated with behavioral transitions. We here show that manipulating [K+]e controls the local release of monoaminergic neuromodulators, including norepinephrine (NE), serotonin, and dopamine. Imposing a [K+]e increase is adequate to boost local NE levels, and conversely, lowering [K+]e can attenuate local NE. Electroencephalography analysis and behavioral assays revealed that manipulation of cortical [K+]e was sufficient to alter the sleep-wake cycle and behavior of mice. These observations point to the concept that NE levels in the cortex are not solely determined by subcortical release, but that local [K+]e dynamics have a strong impact on cortical NE. Thus, cortical [K+]e is an underappreciated regulator of behavioral transitions.

Behavioral and electrophysiological (ECoG) effects of haplophyllum robustum and TRPA1 antagonist in adult male wistar rats

This article aimed to investigate the effects of Haplophyllum robustum hydroalcoholic extract on animals' behavioral and electrocorticographic changes. This plant is mainly found in Turkey, Iran, and Central Asia, and is reported to have convulsive effects. In this article, we worked on the effects of its hydroalcoholic extract on electrocorticography (ECoG), along with changes induced by intracerebroventricular administration of GABAA antagonists. Furthermore, the effects of low doses of this extract on behavioral depression were examined. Four animal sets were used to compare ECoG in Wistar rats. A group of negative control, a group of positive control (PTZ), and two groups received an injection of plant extract (500 mg/kg, ip), with or without administration of Diazepam (5 mg/kg). Also, three sets were applied to compare receiving and not receiving intracerebroventricular (icv) injection of Transient receptor potential ankyrin 1 antagonist (HC-030031) (2 μg/kg) on plant-induced seizure delay and animal death. Two groups of control and a group with plant extract together with TRPA1 antagonist were administrated. Furthermore, in the present study, the forced swimming test (FST) was used as a model of depression. The behaviors of animals in three groups of negative control and positive control (Fluoxetine) and plant extract (200 mg/kg, ip) were compared. According to the ECoG, high doses of extract of plants led to seizures similar to PTZ, which were then reduced by diazepam injection. At this dose, injection of TRPA1 antagonist did not significantly delay the onset of seizures or the death of the animals. Further, a subconvulsive dose of hydroalcoholic plant extracts was equally effective in treating depression as Fluoxetine injections.

Astrocytes of the eye and optic nerve: heterogeneous populations with unique functions mediate axonal resilience and vulnerability to glaucoma

The role of glia, particularly astrocytes, in mediating the central nervous system's response to injury and neurodegenerative disease is an increasingly well studied topic. These cells perform myriad support functions under physiological conditions but undergo behavioral changes - collectively referred to as 'reactivity' - in response to the disruption of neuronal homeostasis from insults, including glaucoma. However, much remains unknown about how reactivity alters disease progression - both beneficially and detrimentally - and whether these changes can be therapeutically modulated to improve outcomes. Historically, the heterogeneity of astrocyte behavior has been insufficiently addressed under both physiological and pathological conditions, resulting in a fragmented and often contradictory understanding of their contributions to health and disease. Thanks to increased focus in recent years, we now know this heterogeneity encompasses both intrinsic variation in physiological function and insult-specific changes that vary between pathologies. Although previous studies demonstrate astrocytic alterations in glaucoma, both in human disease and animal models, generally these findings do not conclusively link astrocytes to causative roles in neuroprotection or degeneration, rather than a subsequent response. Efforts to bolster our understanding by drawing on knowledge of brain astrocytes has been constrained by the primacy in the literature of findings from peri-synaptic 'gray matter' astrocytes, whereas much early degeneration in glaucoma occurs in axonal regions populated by fibrous 'white matter' astrocytes. However, by focusing on findings from astrocytes of the anterior visual pathway - those of the retina, unmyelinated optic nerve head, and myelinated optic nerve regions - we aim to highlight aspects of their behavior that may contribute to axonal vulnerability and glaucoma progression, including roles in mitochondrial turnover and energy provisioning. Furthermore, we posit that astrocytes of the retina, optic nerve head and myelinated optic nerve, although sharing developmental origins and linked by a network of gap junctions, may be best understood as distinct populations residing in markedly different niches with accompanying functional specializations. A closer investigation of their behavioral repertoires may elucidate not only their role in glaucoma, but also mechanisms to induce protective behaviors that can impede the progressive axonal damage and retinal ganglion cell death that drive vision loss in this devastating condition.

Resilience of compound action potential peaks to high-frequency firing in the mouse optic nerve

Action potential conduction in axons triggers trans-membrane ion movements, where Na⁺ enters and K⁺ leaves axons, leading to disruptions in resting trans-membrane ion gradients that must be restored for optimal axon conduction, an energy dependent process. The higher the stimulus frequency, the greater the ion movements and the resulting energy demand. In the mouse optic nerve (MON), the stimulus evoked compound action potential (CAP) displays a triple peaked profile, consistent with subpopulations of axons classified by size producing the distinct peaks. The three CAP peaks show differential sensitivity to high-frequency firing, with the large axons, which contribute to the 1st peak, more resilient than the small axons, which produce the 3rd peak. Modeling studies predict frequency dependent intra-axonal Na⁺ accumulation at the nodes of Ranvier, sufficient to attenuate the triple peaked CAP. Short bursts of high-frequency stimulus evoke transient elevations in interstitial K⁺ ([K⁺ ]_o ), which peak at about 50 Hz. However, powerful astrocytic buffering limits the [K⁺ ]_o increase to levels insufficient to cause CAP attenuation. A post-stimulus [K⁺ ]_o undershoot below baseline coincides with a transient increase in the amplitudes of all three CAP peaks. The volume specific scaling relating energy expenditure to increasing axon size dictates that large axons are more resilient to high-frequency firing than small axons.

Activity-Dependent Fluctuations in Interstitial [K+]: Investigations Using Ion-Sensitive Microelectrodes

In the course of action potential firing, all axons and neurons release K⁺ from the intra- cellular compartment into the interstitial space to counteract the depolarizing effect of Na⁺ influx, which restores the resting membrane potential. This efflux of K⁺ from axons results in K⁺ accumulation in the interstitial space, causing depolarization of the K⁺ reversal potential (E_K), which can prevent subsequent action potentials. To ensure optimal neuronal function, the K⁺ is buffered by astrocytes, an energy-dependent process, which acts as a sink for interstitial K⁺, absorbing it at regions of high concentration and distributing it through the syncytium for release in distant regions. Pathological processes in which energy production is compromised, such as anoxia, ischemia, epilepsy and spreading depression, can lead to excessive interstitial K⁺ accumulation, disrupting sensitive trans-membrane ion gradients and attenuating neuronal activity. The changes that occur in interstitial [K⁺] resulting from both physiological and pathological processes can be monitored accurately in real time using K⁺-sensitive microelectrodes, an invaluable tool in electrophysiological studies.

Honey, I shrunk the extracellular space: Measurements and mechanisms of astrocyte swelling

Astrocyte volume fluctuation is a physiological phenomenon tied closely to the activation of neural circuits. Identification of underlying mechanisms has been challenging due in part to use of a wide range of experimental approaches that vary between research groups. Here, we first review the many methods that have been used to measure astrocyte volume changes directly or indirectly. While the field has recently shifted towards volume analysis using fluorescence microscopy to record cell volume changes directly, established metrics corresponding to extracellular space dynamics have also yielded valuable insights. We then turn to analysis of mechanisms of astrocyte swelling derived from many studies, with a focus on volume changes tied to increases in extracellular potassium concentration ([K+ ]o ). The diverse methods that have been utilized to generate the external [K+ ]o environment highlight multiple scenarios of astrocyte swelling mediated by different mechanisms. Classical potassium buffering theories are tempered by many recent studies that point to different swelling pathways optimized at particular [K+ ]o and that depend on local/transient versus more sustained increases in [K+ ]o .

Tailoring Materials for Epilepsy Imaging: From Biomarkers to Imaging Probes

Excising epileptic foci (EF) is the most efficient approach for treating drug-resistant epilepsy (DRE). However, owing to the vast heterogeneity of epilepsies, EF in one-third of patients cannot be accurately located, even after exhausting all current diagnostic strategies. Therefore, identifying biomarkers that truly represent the status of epilepsy and fabricating probes with high targeting specificity are prerequisites for identifying the "concealed" EF. However, no systematic summary of this topic has been published. Herein, the potential biomarkers of EF are first summarized and classified into three categories: functional, molecular, and structural aberrances during epileptogenesis, a procedure of nonepileptic brain biasing toward epileptic tissue. The materials used to fabricate these imaging probes and their performance in defining the EF in preclinical and clinical studies are highlighted. Finally, perspectives for developing the next generation of probes and their challenges in clinical translation are discussed. In general, this review can be helpful in guiding the development of imaging probes defining EF with improved accuracy and holds promise for increasing the number of DRE patients who are eligible for surgical intervention.

Molecular Mechanisms of Epilepsy: The Role of the Chloride Transporter KCC2

Epilepsy is a neurological disease characterized by abnormal or synchronous brain activity causing seizures, which may produce convulsions, minor physical signs, or a combination of symptoms. These disorders affect approximately 65 million people worldwide, from all ages and genders. Seizures apart, epileptic patients present a high risk to develop neuropsychological comorbidities such as cognitive deficits, emotional disturbance, and psychiatric disorders, which severely impair quality of life. Currently, the treatment for epilepsy includes the administration of drugs or surgery, but about 30% of the patients treated with antiepileptic drugs develop time-dependent pharmacoresistence. Therefore, further investigation about epilepsy and its causes is needed to find new pharmacological targets and innovative therapeutic strategies. Pharmacoresistance is associated to changes in neuronal plasticity and alterations of GABA_A receptor-mediated neurotransmission. The downregulation of GABA inhibitory activity may arise from a positive shift in GABA_A receptor reversal potential, due to an alteration in chloride homeostasis. In this paper, we review the contribution of K⁺-Cl^--cotransporter (KCC2) to the alterations in the Cl^- gradient observed in epileptic condition, and how these alterations are coupled to the increase in the excitability.

How Staying Negative Is Good for the (Adult) Brain: Maintaining Chloride Homeostasis and the GABA-Shift in Neurological Disorders

Excitatory-inhibitory (E-I) imbalance has been shown to contribute to the pathogenesis of a wide range of neurodevelopmental disorders including autism spectrum disorders, epilepsy, and schizophrenia. GABA neurotransmission, the principal inhibitory signal in the mature brain, is critically coupled to proper regulation of chloride homeostasis. During brain maturation, changes in the transport of chloride ions across neuronal cell membranes act to gradually change the majority of GABA signaling from excitatory to inhibitory for neuronal activation, and dysregulation of this GABA-shift likely contributes to multiple neurodevelopmental abnormalities that are associated with circuit dysfunction. Whilst traditionally viewed as a phenomenon which occurs during brain development, recent evidence suggests that this GABA-shift may also be involved in neuropsychiatric disorders due to the “dematuration” of affected neurons. In this review, we will discuss the cell signaling and regulatory mechanisms underlying the GABA-shift phenomenon in the context of the latest findings in the field, in particular the role of chloride cotransporters NKCC1 and KCC2, and furthermore how these regulatory processes are altered in neurodevelopmental and neuropsychiatric disorders. We will also explore the interactions between GABAergic interneurons and other cell types in the developing brain that may influence the GABA-shift. Finally, with a greater understanding of how the GABA-shift is altered in pathological conditions, we will briefly outline recent progress on targeting NKCC1 and KCC2 as a therapeutic strategy against neurodevelopmental and neuropsychiatric disorders associated with improper chloride homeostasis and GABA-shift abnormalities.

Changes in Astroglial K+ upon Brief Periods of Energy Deprivation in the Mouse Neocortex

Malfunction of astrocytic K⁺ regulation contributes to the breakdown of extracellular K⁺ homeostasis during ischemia and spreading depolarization events. Studying astroglial K⁺ changes is, however, hampered by a lack of suitable techniques. Here, we combined results from fluorescence imaging, ion-selective microelectrodes, and patch-clamp recordings in murine neocortical slices with the calculation of astrocytic [K⁺]. Brief chemical ischemia caused a reversible ATP reduction and a transient depolarization of astrocytes. Moreover, astrocytic [Na⁺] increased by 24 mM and extracellular [Na⁺] decreased. Extracellular [K⁺] increased, followed by an undershoot during recovery. Feeding these data into the Goldman-Hodgkin-Katz equation revealed a baseline astroglial [K⁺] of 146 mM, an initial K⁺ loss by 43 mM upon chemical ischemia, and a transient K⁺ overshoot of 16 mM during recovery. It also disclosed a biphasic mismatch in astrocytic Na⁺/K⁺ balance, which was initially ameliorated, but later aggravated by accompanying changes in pH and bicarbonate, respectively. Altogether, our study predicts a loss of K⁺ from astrocytes upon chemical ischemia followed by a net gain. The overshooting K⁺ uptake will promote low extracellular K⁺ during recovery, likely exerting a neuroprotective effect. The resulting late cation/anion imbalance requires additional efflux of cations and/or influx of anions, the latter eventually driving delayed astrocyte swelling.

The Nernst equation: using physico-chemical laws to steer novel experimental design

The application of physico-chemical principles has been routinely used to explain various physiological concepts. The Nernst equation is one example of this, used to predict the potential difference created by the transmembrane ion gradient resulting from uneven ion distribution within cellular compartments and the interstitial space. This relationship remains of fundamental importance to the understanding of electrical signaling in the brain, which relies on current flow across cell membranes. We describe four distinct occasions when the Nernst equation was ingeniously applied in experimental design to illuminate diverse cellular functions, from the dependence of the action potential on Na⁺ influx to K⁺ buffering in astrocytes. These examples are discussed with the aim of inspiring students to appreciate how the application of seemingly textbook-bound concepts can dictate novel experimental design across physiological disciplines.

The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention

BACKGROUND

When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD.

METHODS

In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na⁺/K⁺ pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate.

RESULTS

We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na⁺/K⁺ ATPase elicits SD. Elevated K⁺ or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling.

CONCLUSIONS

Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.

Role of Na+/K+-ATPase in ischemic stroke: in-depth perspectives from physiology to pharmacology

Na⁺/K⁺-ATPase (NKA) is a large transmembrane protein expressed in all cells. It is well studied for its ion exchanging function, which is indispensable for the maintenance of electrochemical gradients across the plasma membrane and herein neuronal excitability. The widely recognized pump function of NKA closely depends on its unique structure features and conformational changes upon binding of specific ions. Various Na⁺-dependent secondary transport systems are rigorously controlled by the ionic gradients generated by NKA and are essential for multiple physiological processes. In addition, roles of NKA as a signal transducer have also been unveiled nowadays. Plethora of signaling cascades are defined including Src-Ras-MAPK signaling, IP3R-mediated calcium oscillation, inflammation, and autophagy though most underlying mechanisms remain elusive. Ischemic stroke occurs when the blood flow carrying nutrients and oxygen into the brain is disrupted by blood clots, which is manifested by excitotoxicity, oxidative stress, inflammation, etc. The protective effect of NKA against ischemic stress is emerging gradually with the application of specific NKA inhibitor. However, NKA-related research is limited due to the opposite effects caused by NKA inhibitor at lower doses. The present review focuses on the recent progression involving different aspects about NKA in cellular homeostasis to present an in-depth understanding of this unique protein. Moreover, essential roles of NKA in ischemic pathology are discussed to provide a platform and bright future for the improvement in clinical research on ischemic stroke.

Transient Receptor Potential Ankyrin-1 (TRPA1) Block Protects against Loss of White Matter Function during Ischaemia in the Mouse Optic Nerve

Oligodendrocytes produce myelin, which provides insulation to axons and speeds up neuronal transmission. In ischaemic conditions, myelin is damaged, resulting in mental and physical disabilities. Recent evidence suggests that oligodendrocyte damage during ischaemia can be mediated by Transient Receptor Potential Ankyrin-1 (TRPA1), whose activation raises intracellular Ca²⁺ concentrations and damages compact myelin. Here, we show that TRPA1 is constitutively active in oligodendrocytes and the optic nerve, as the specific TRPA1 antagonist, A-967079, decreases basal oligodendrocyte Ca²⁺ concentrations and increases the size of the compound action potential (CAP). Conversely, TRPA1 agonists reduce the size of the optic nerve CAP in an A-967079-sensitive manner. These results indicate that glial TRPA1 regulates neuronal excitability in the white matter under physiological as well as pathological conditions. Importantly, we find that inhibition of TRPA1 prevents loss of CAPs during oxygen and glucose deprivation (OGD) and improves the recovery. TRPA1 block was effective when applied before, during, or after OGD, indicating that the TRPA1-mediated damage is occurring during both ischaemia and recovery, but importantly, that therapeutic intervention is possible after the ischaemic insult. These results indicate that TRPA1 has an important role in the brain, and that its block may be effective in treating many white matter diseases.

Neuromodulation of Astrocytic K+ Clearance

Potassium homeostasis is fundamental for brain function. Therefore, effective removal of excessive K+ from the synaptic cleft during neuronal activity is paramount. Astrocytes play a key role in K+ clearance from the extracellular milieu using various mechanisms, including uptake via Kir channels and the Na⁺-K⁺ ATPase, and spatial buffering through the astrocytic gap-junction coupled network. Recently we showed that alterations in the concentrations of extracellular potassium ([K⁺]_o) or impairments of the astrocytic clearance mechanism affect the resonance and oscillatory behavior of both the individual and networks of neurons. These results indicate that astrocytes have the potential to modulate neuronal network activity, however, the cellular effectors that may affect the astrocytic K+ clearance process are still unknown. In this study, we have investigated the impact of neuromodulators, which are known to mediate changes in network oscillatory behavior, on the astrocytic clearance process. Our results suggest that while some neuromodulators (5-HT; NA) might affect astrocytic spatial buffering via gap-junctions, others (DA; Histamine) primarily affect the uptake mechanism via Kir channels. These results suggest that neuromodulators can affect network oscillatory activity through parallel activation of both neurons and astrocytes, establishing a synergistic mechanism to maximize the synchronous network activity.

Na+-K+-ATPase functions in the developing hippocampus: regional differences in CA1 and CA3 neuronal excitability and role in epileptiform network bursting

The Na⁺-K⁺-ATPase (Na⁺-K⁺ pump) is essential for setting resting membrane potential and restoring transmembrane Na⁺ and K⁺ gradients after neuronal firing, yet its roles in developing neurons are not well understood. This study examined the contribution of the Na⁺-K⁺ pump to resting membrane potential and membrane excitability of developing CA1 and CA3 neurons and its role in maintaining synchronous network bursting. Experiments were conducted in postnatal day (P)9 to P13 rat hippocampal slices using whole cell patch-clamp and extracellular field-potential recordings. Blockade of the Na⁺-K⁺ pump with strophanthidin caused marked depolarization (23.1 mV) in CA3 neurons but only a modest depolarization (3.3 mV) in CA1 neurons. Regarding other membrane properties, strophanthidin differentially altered the voltage-current responses, input resistance, action-potential threshold and amplitude, rheobase, and input-output relationship in CA3 vs. CA1 neurons. At the network level, strophanthidin stopped synchronous epileptiform bursting in CA3 induced by 0 Mg²⁺ and 4-aminopyridine. Furthermore, dual whole cell recordings revealed that strophanthidin disrupted the synchrony of CA3 neuronal firing. Finally, strophanthidin reduced spontaneous excitatory postsynaptic current (sEPSC) bursts (i.e., synchronous transmitter release) and transformed them into individual sEPSC events (i.e., nonsynchronous transmitter release). These data suggest that the Na⁺-K⁺ pump plays a more profound role in membrane excitability in developing CA3 neurons than in CA1 neurons and that the pump is essential for the maintenance of synchronous network bursting in CA3. Compromised Na⁺-K⁺ pump function leads to cessation of ongoing synchronous network activity, by desynchronizing neuronal firing and neurotransmitter release in the CA3 synaptic network. These findings have implications for the regulation of network excitability and seizure generation in the developing brain.NEW & NOTEWORTHY Despite the extensive literature showing the importance of the Na⁺-K⁺ pump in various neuronal functions, its roles in the developing brain are not well understood. This study reveals that the Na⁺-K⁺ pump differentially regulates the excitability of CA3 and CA1 neurons in the developing hippocampus, and the pump activity is crucial for maintaining network activity. Compromised Na⁺-K⁺ pump activity desynchronizes neuronal firing and transmitter release, leading to cessation of ongoing epileptiform network bursting.

The α2 Na+/K+-ATPase isoform mediates LPS-induced neuroinflammation

Na⁺/K⁺-ATPase is a transmembrane ion pump that is essential for the maintenance of ion gradients and regulation of multiple cellular functions. Na⁺/K⁺-ATPase has been associated with nuclear factor kappa B (NFκB) signalling, a signal associated with lipopolysaccharides (LPSs)-induced immune response in connection with activated Toll-like receptor 4 (TLR4) signalling. However, the contribution of Na⁺/K⁺-ATPase to regulating inflammatory responses remains elusive. We report that mice haploinsufficient for the astrocyte-enriched α₂Na⁺/K⁺-ATPase isoform (α₂^+/G301R mice) have a reduced proinflammatory response to LPS, accompanied by a reduced hypothermic reaction compared to wild type litter mates. Following intraperitoneal injection of LPS, gene expressions of Tnf-α, Il-1β, and Il-6 was reduced in the hypothalamus and hippocampus from α₂^+/G301R mice compared to α₂^+/+ littermates. The α₂^+/G301R mice experienced increased expression of the gene encoding an antioxidant enzyme, NRF2, in hippocampal astrocytes. Our findings indicate that α₂Na⁺/K⁺-ATPase haploinsufficiency negatively modulates LPS-induced immune responses, highlighting a rational pharmacological target for reducing LPS-induced inflammation.

Heterogeneity of Astrocytic and Neuronal GLT-1 at Cortical Excitatory Synapses, as Revealed by its Colocalization With Na+/K+-ATPase α Isoforms

GLT-1, the major glutamate transporter, is expressed at perisynaptic astrocytic processes (PAP) and axon terminals (AxT). GLT-1 is coupled to Na+/K+-ATPase (NKA) α1-3 isoforms, whose subcellular distribution and spatial organization in relationship to GLT-1 are largely unknown. Using several microscopy techniques, we showed that at excitatory synapses α1 and α3 are exclusively neuronal (mainly in dendrites and in some AxT), while α2 is predominantly astrocytic. GLT-1 displayed a differential colocalization with α1-3. GLT-1/α2 and GLT-1/α3 colocalization was higher in GLT-1 positive puncta partially (for GLT-1/α2) or almost totally (for GLT-1/α3) overlapping with VGLUT1 positive terminals than in nonoverlapping ones. GLT-1 colocalized with α2 at PAP, and with α1 and α3 at AxT. GLT-1 and α2 gold particles were ∼1.5-2 times closer than GLT-1/α1 and GLT-1/α3 particles. GLT-1/α2 complexes (edge to edge interdistance of gold particles ≤50 nm) concentrated at the perisynaptic region of PAP membranes, whereas neuronal GLT-1/α1 and GLT-1/α3 complexes were fewer and more uniformly distributed in AxT. These data unveil different composition of GLT-1 and α subunits complexes in the glial and neuronal domains of excitatory synapses. The spatial organization of GLT-1/α1-3 complexes suggests that GLT-1/NKA interaction is more efficient in astrocytes than in neurons, further supporting the dominant role of astrocytic GLT-1 in glutamate homeostasis.

Clearance of activity-evoked K+ transients and associated glia cell swelling occur independently of AQP4: A study with an isoform-selective AQP4 inhibitor

The mammalian brain consists of 80% water, which is continuously shifted between different compartments and cellular structures by mechanisms that are, to a large extent, unresolved. Aquaporin 4 (AQP4) is abundantly expressed in glia and ependymal cells of the mammalian brain and has been proposed to act as a gatekeeper for brain water dynamics, predominantly based on studies utilizing AQP4-deficient mice. However, these mice have a range of secondary effects due to the gene deletion. An efficient and selective AQP4 inhibitor has thus been sorely needed to validate the results obtained in the AQP4^-/- mice to quantify the contribution of AQP4 to brain fluid dynamics. In AQP4-expressing Xenopus laevis oocytes monitored by a high-resolution volume recording system, we here demonstrate that the compound TGN-020 is such a selective AQP4 inhibitor. TGN-020 targets the tested species of AQP4 with an IC₅₀ of ~3.5 μM, but displays no inhibitory effect on the other AQPs (AQP1-AQP9). With this tool, we employed rat hippocampal slices and ion-sensitive microelectrodes to determine the role of AQP4 in glia cell swelling following neuronal activity. TGN-020-mediated inhibition of AQP4 did not prevent stimulus-induced extracellular space shrinkage, nor did it slow clearance of the activity-evoked K⁺ transient. These data, obtained with a verified isoform-selective AQP4 inhibitor, indicate that AQP4 is not required for the astrocytic contribution to the K⁺ clearance or the associated extracellular space shrinkage.

Going the Extra (Synaptic) Mile: Excitotoxicity as the Road Toward Neurodegenerative Diseases

Excitotoxicity is a phenomenon that describes the toxic actions of excitatory neurotransmitters, primarily glutamate, where the exacerbated or prolonged activation of glutamate receptors starts a cascade of neurotoxicity that ultimately leads to the loss of neuronal function and cell death. In this process, the shift between normal physiological function and excitotoxicity is largely controlled by astrocytes since they can control the levels of glutamate on the synaptic cleft. This control is achieved through glutamate clearance from the synaptic cleft and its underlying recycling through the glutamate-glutamine cycle. The molecular mechanism that triggers excitotoxicity involves alterations in glutamate and calcium metabolism, dysfunction of glutamate transporters, and malfunction of glutamate receptors, particularly N-methyl-D-aspartic acid receptors (NMDAR). On the other hand, excitotoxicity can be regarded as a consequence of other cellular phenomena, such as mitochondrial dysfunction, physical neuronal damage, and oxidative stress. Regardless, it is known that the excessive activation of NMDAR results in the sustained influx of calcium into neurons and leads to several deleterious consequences, including mitochondrial dysfunction, reactive oxygen species (ROS) overproduction, impairment of calcium buffering, the release of pro-apoptotic factors, among others, that inevitably contribute to neuronal loss. A large body of evidence implicates NMDAR-mediated excitotoxicity as a central mechanism in the pathogenesis of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and epilepsy. In this review article, we explore different causes and consequences of excitotoxicity, discuss the involvement of NMDAR-mediated excitotoxicity and its downstream effects on several neurodegenerative disorders, and identify possible strategies to study new aspects of these diseases that may lead to the discovery of new therapeutic approaches. With the understanding that excitotoxicity is a common denominator in neurodegenerative diseases and other disorders, a new perspective on therapy can be considered, where the targets are not specific symptoms, but the underlying cellular phenomena of the disease.

Molecular mechanisms of K+ clearance and extracellular space shrinkage-Glia cells as the stars

Neuronal signaling in the central nervous system (CNS) associates with release of K⁺ into the extracellular space resulting in transient increases in [K⁺ ]_o . This elevated K⁺ is swiftly removed, in part, via uptake by neighboring glia cells. This process occurs in parallel to the [K⁺ ]_o elevation and glia cells thus act as K⁺ sinks during the neuronal activity, while releasing it at the termination of the pulse. The molecular transport mechanisms governing this glial K⁺ absorption remain a point of debate. Passive distribution of K⁺ via Kir4.1-mediated spatial buffering of K⁺ has become a favorite within the glial field, although evidence for a quantitatively significant contribution from this ion channel to K⁺ clearance from the extracellular space is sparse. The Na⁺ /K⁺ -ATPase, but not the Na⁺ /K⁺ /Cl^- cotransporter, NKCC1, shapes the activity-evoked K⁺ transient. The different isoform combinations of the Na⁺ /K⁺ -ATPase expressed in glia cells and neurons display different kinetic characteristics and are thereby distinctly geared toward their temporal and quantitative contribution to K⁺ clearance. The glia cell swelling occurring with the K⁺ transient was long assumed to be directly associated with K⁺ uptake and/or AQP4, although accumulating evidence suggests that they are not. Rather, activation of bicarbonate- and lactate transporters appear to lead to glial cell swelling via the activity-evoked alkaline transient, K⁺ -mediated glial depolarization, and metabolic demand. This review covers evidence, or lack thereof, accumulated over the last half century on the molecular mechanisms supporting activity-evoked K⁺ and extracellular space dynamics.

Serotonin, norepinephrine, and acetylcholine differentially affect astrocytic potassium clearance to modulate somatosensory signaling in male mice

Changes in extracellular potassium ([K⁺ ]_e ) modulate neuronal networks via changes in membrane potential, voltage-gated channel activity, and alteration to transmission at the synapse. Given the limited extracellular space in the central nervous system, potassium clearance is crucial. As activity-induced potassium transients are rapidly managed by astrocytic Kir4.1 and astrocyte-specific Na⁺ /K⁺ -ATPase, any neurotransmitter/neuromodulator that can regulate their function may have indirect influence on network activity. Neuromodulators differentially affect cortical/thalamic networks to align sensory processing with differing behavioral states. Given serotonin (5HT), norepinephrine (NE), and acetylcholine (ACh) differentially affect spike frequency adaptation and signal fidelity ("signal-to-noise") in somatosensory cortex, we hypothesize that [K⁺ ]_e may be differentially regulated by the different neuromodulators to exert their individual effects on network function. This study aimed to compare effects of individually applied 5HT, NE, and ACh on regulating [K⁺ ]_e in connection to effects on cortical-evoked response amplitude and adaptation in male mice. Using extracellular field and K⁺ ion-selective recordings of somatosensory stimulation, we found that differential effects of 5HT, NE, and ACh on [K⁺ ]_e regulation mirrored differential effects on amplitude and adaptation. 5HT effects on transient K⁺ recovery, adaptation, and field post-synaptic potential amplitude were disrupted by barium (200 µM), whereas NE and ACh effects were disrupted by ouabain (1 µM) or iodoacetate (100 µM). Considering the impact [K⁺ ]_e can have on many network functions; it seems highly efficient that neuromodulators regulate [K⁺ ]_e to exert their many effects. This study provides functional significance for astrocyte-mediated buffering of [K⁺ ]_e in neuromodulator-mediated shaping of cortical network activity.

Astrocytic modulation of potassium under seizures

The contribution of an impaired astrocytic K⁺ regulation system to epileptic neuronal hyperexcitability has been increasingly recognized in the last decade. A defective K⁺ regulation leads to an elevated extracellular K⁺ concentration ([K⁺]_o). When [K⁺]_o reaches peaks of 10-12 mM, it is strongly associated with seizure initiation during hypersynchronous neuronal activities. On the other hand, reactive astrocytes during a seizure attack restrict influx of K⁺ across the membrane both passively and actively. In addition to decreased K⁺ buffering, aberrant Ca²⁺ signaling and declined glutamate transport have also been observed in astrogliosis in epileptic specimens, precipitating an increased neuronal discharge and induction of seizures. This review aims to provide an overview of experimental findings that implicated astrocytic modulation of extracellular K⁺ in the mechanism of epileptogenesis.

Astrocyte-Selective Volume Increase in Elevated Extracellular Potassium Conditions Is Mediated by the Na+/K+ ATPase and Occurs Independently of Aquaporin 4

Astrocytes and neurons have been shown to swell across a variety of different conditions, including increases in extracellular potassium concentration (^[K+]o). The mechanisms involved in the coupling of K+ influx to water movement into cells leading to cell swelling are not well understood and remain controversial. Here, we set out to determine the effects of ^[K+]o on rapid volume responses of hippocampal CA1 pyramidal neurons and stratum radiatum astrocytes using real-time confocal volume imaging. First, we found that elevating [K+]o within a physiological range (to 6.5 mM and 10.5 mM from a baseline of 2.5 mM), and even up to pathological levels (26 mM), produced dose-dependent increases in astrocyte volume, with absolutely no effect on neuronal volume. In the absence of compensating for addition of KCl by removal of an equal amount of NaCl, neurons actually shrank in ^[K+]o, while astrocytes continued to exhibit rapid volume increases. Astrocyte swelling in ^[K+]o was not dependent on neuronal firing, aquaporin 4, the inwardly rectifying potassium channel Kir 4.1, the sodium bicarbonate cotransporter NBCe1, , or the electroneutral cotransporter, sodium-potassium-chloride cotransporter type 1 (NKCC1), but was significantly attenuated in 1 mM barium chloride (BaCl2) and by the Na+/K+ ATPase inhibitor ouabain. Effects of 1 mM BaCl2 and ouabain applied together were not additive and, together with reports that BaCl2 can inhibit the NKA at high concentrations, suggests a prominent role for the astrocyte NKA in rapid astrocyte volume increases occurring in ^[K+]o. These findings carry important implications for understanding mechanisms of cellular edema, regulation of the brain extracellular space, and brain tissue excitability.

Heterogeneity of Astrocytes in Grey and White Matter

Astrocytes are a diverse and heterogeneous type of glial cells. The major task of grey and white matter areas in the brain are computation of information at neuronal synapses and propagation of action potentials along axons, respectively, resulting in diverse demands for astrocytes. Adapting their function to the requirements in the local environment, astrocytes differ in morphology, gene expression, metabolism, and many other properties. Here we review the differential properties of protoplasmic astrocytes of grey matter and fibrous astrocytes located in white matter in respect to glutamate and energy metabolism, to their function at the blood-brain interface and to coupling via gap junctions. Finally, we discuss how this astrocytic heterogeneity might contribute to the different susceptibility of grey and white matter to ischemic insults.

Limited contribution of astroglial gap junction coupling to buffering of extracellular K+ in CA1 stratum radiatum

Astrocytes form large networks, in which individual cells are connected via gap junctions. It is thought that this astroglial gap junction coupling contributes to the buffering of extracellular K⁺ increases. However, it is largely unknown how the control of extracellular K⁺ by astroglial gap junction coupling depends on the underlying activity patterns and on the magnitude of extracellular K⁺ increases. We explored this dependency in acute hippocampal slices (CA1, stratum radiatum) by direct K⁺ -sensitive microelectrode recordings and acute pharmacological inhibition of gap junctions. K⁺ transients evoked by synaptic and axonal activity were largely unaffected by acute astroglial uncoupling in slices obtained from young and adult rats. Iontophoretic K⁺ -application enabled us to generate K⁺ gradients with defined spatial properties and magnitude. By varying the K⁺ -iontophoresis position and protocol, we found that acute pharmacological uncoupling increases the amplitude of K⁺ transients once their initial amplitude exceeded ~10 mM. Our experiments demonstrate that the contribution of gap junction coupling to buffering of extracellular K⁺ gradients is limited to large and localized K⁺ increases.

Cortex-wide Changes in Extracellular Potassium Ions Parallel Brain State Transitions in Awake Behaving Mice

Brain state fluctuations modulate sensory processing, but the factors governing state-dependent neural activity remain unclear. Here, we tracked the dynamics of cortical extracellular K+ concentrations ([K+]o) during awake state transitions and manipulated [K+]o in slices, during visual processing, and during skilled motor execution. When mice transitioned from quiescence to locomotion, [K+]o increased by 0.6-1.0 mM in all cortical areas analyzed, and this preceded locomotion by 1 s. Emulating the state-dependent [K+]o increase in cortical slices caused neuronal depolarization and enhanced input-output transformation. In vivo, locomotion increased the gain of visually evoked responses in layer 2/3 of visual cortex; this effect was recreated by imposing a [K+]o increase. Elevating [K+]o in the motor cortex increased movement-induced neuronal spiking in layer 5 and improved motor performance. Thus, [K+]o increases in a cortex-wide state-dependent manner, and this [K+]o increase affects both sensory and motor processing through the dynamic modulation of neural activity.

Glial and neuronal expression of the Inward Rectifying Potassium Channel Kir7.1 in the adult mouse brain

Inward Rectifying Potassium channels (Kir) are a large family of ion channels that play key roles in ion homeostasis and neuronal excitability. The most recently described Kir subtype is Kir7.1, which is known as a K⁺ transporting subtype. Earlier studies localised Kir7.1 to subpopulations of neurones in the brain. However, the pattern of Kir7.1 expression across the brain has not previously been examined. Here, we have determined neuronal and glial expression of Kir7.1 in the adult mouse brain, using immunohistochemistry and transgenic mouse lines expressing reporters specific for astrocytes [glial fibrillary acidic protein‐enhanced green fluorescent protein (GFAP‐EGFP], myelinating oligodendrocytes (PLP‐DsRed), oligodendrocyte progenitor cells (OPC, Pdgfra‐creER^T2/Rosa26‐YFP double‐transgenic mice) and all oligodendrocyte lineage cells (SOX10‐EGFP). The results demonstrate significant neuronal Kir7.1 immunostaining in the cortex, hippocampus, cerebellum and pons, as well as the striatum and hypothalamus. In addition, astrocytes are shown to be immunopositive for Kir7.1 throughout grey and white matter, with dense immunostaining on cell somata, primary processes and perivascular end‐feet. Immunostaining for Kir7.1 was observed in oligodendrocytes, myelin and OPCs throughout the brain, although immunostaining was heterogeneous. Neuronal and glial expression of Kir7.1 is confirmed using neurone‐glial cortical cultures and optic nerve glial cultures. Notably, Kir7.1 have been shown to regulate the excitability of thalamic neurones and our results indicate this may be a widespread function of Kir7.1 in neurones throughout the brain. Moreover, based on the function of Kir7.1 in multiple transporting epithelia, Kir7.1 are likely to play an equivalent role in the primary glial function of K⁺ homeostasis. Our results indicate Kir7.1 are far more pervasive in the brain than previously recognised and have potential importance in regulating neuronal and glial function.

Adrenergic receptor antagonism induces neuroprotection and facilitates recovery from acute ischemic stroke

Spontaneous waves of cortical spreading depolarization (CSD) are induced in the setting of acute focal ischemia. CSD is linked to a sharp increase of extracellular K⁺ that induces a long-lasting suppression of neural activity. Furthermore, CSD induces secondary irreversible damage in the ischemic brain, suggesting that K⁺ homeostasis might constitute a therapeutic strategy in ischemic stroke. Here we report that adrenergic receptor (AdR) antagonism accelerates normalization of extracellular K⁺, resulting in faster recovery of neural activity after photothrombotic stroke. Remarkably, systemic adrenergic blockade before or after stroke facilitated functional motor recovery and reduced infarct volume, paralleling the preservation of the water channel aquaporin-4 in astrocytes. Our observations suggest that AdR blockers promote cerebrospinal fluid exchange and rapid extracellular K⁺ clearance, representing a potent potential intervention for acute stroke.

Bursting at the Seams: Molecular Mechanisms Mediating Astrocyte Swelling

Brain swelling is one of the most robust predictors of outcome following brain injury, including ischemic, traumatic, hemorrhagic, metabolic or other injury. Depending on the specific type of insult, brain swelling can arise from the combined space-occupying effects of extravasated blood, extracellular edema fluid, cellular swelling, vascular engorgement and hydrocephalus. Of these, arguably the least well appreciated is cellular swelling. Here, we explore current knowledge regarding swelling of astrocytes, the most abundant cell type in the brain, and the one most likely to contribute to pathological brain swelling. We review the major molecular mechanisms identified to date that contribute to or mitigate astrocyte swelling via ion transport, and we touch upon the implications of astrocyte swelling in health and disease.

Developmental maturation of activity-induced K+ and pH transients and the associated extracellular space dynamics in the rat hippocampus

KEY POINTS

Neuronal activity induces fluctuation in extracellular space volume, [K+ ]o and pHo , the management of which influences neuronal function The neighbour astrocytes buffer the K+ and pH and swell during the process, causing shrinkage of the extracellular space In the present study, we report the developmental rise of the homeostatic control of the extracellular space dynamics, for which regulation becomes tighter with maturation and thus is proposed to ensure efficient synaptic transmission in the mature animals The extracellular space dynamics of volume, [K+ ]o and pHo evolve independently with developmental maturation and, although all of them are inextricably tied to neuronal activity, they do not couple directly.

ABSTRACT

Neuronal activity in the mammalian central nervous system associates with transient extracellular space (ECS) dynamics involving elevated K+ and pH and shrinkage of the ECS. These ECS properties affect membrane potentials, neurotransmitter concentrations and protein function and are thus anticipated to be under tight regulatory control. It remains unresolved to what extent these ECS dynamics are developmentally regulated as synaptic precision arises and whether they are directly or indirectly coupled. To resolve the development of homeostatic control of [K+ ]o , pH, and ECS and their interaction, we utilized ion-sensitive microelectrodes in electrically stimulated rat hippocampal slices from rats of different developmental stages (postnatal days 3-28). With the employed stimulation paradigm, the stimulus-evoked peak [K+ ]o and pHo transients were stable across age groups, until normalized to neuronal activity (field potential amplitude), in which case the K+ and pH shifted significantly more in the younger animals. By contrast, ECS dynamics increased with age until normalized to the field potential, and thus correlated with neuronal activity. With age, the animals not only managed the peak [K+ ]o better, but also displayed swifter post-stimulus removal of [K+ ]o , in correlation with the increased expression of the α1-3 isoforms of the Na+ /K+ -ATPase, and a swifter return of ECS volume. The different ECS dynamics approached a near-identical temporal pattern in the more mature animals. In conclusion, although these phenomena are inextricably tied to neuronal activity, our data suggest that they do not couple directly.

Astrocyte-Neuron Networks: A Multilane Highway of Signaling for Homeostatic Brain Function

Research on glial cells over the past 30 years has confirmed the critical role of astrocytes in pathophysiological brain states. However, most of our knowledge about astrocyte physiology and of the interactions between astrocytes and neurons is based on the premises that astrocytes constitute a homogeneous cell type, without considering the particular properties of the circuits or brain nuclei in which the astrocytes are located. Therefore, we argue that more-sophisticated experiments are required to elucidate the specific features of astrocytes in different brain regions, and even within different layers of a particular circuit. Thus, in addition to considering the diverse mechanisms used by astrocytes to communicate with neurons and synaptic partners, it is necessary to take into account the cellular heterogeneity that likely contributes to the outcomes of astrocyte-neuron signaling. In this review article, we briefly summarize the current data regarding the anatomical, molecular and functional properties of astrocyte-neuron communication, as well as the heterogeneity within this communication.

Norepinephrine stimulates glycogenolysis in astrocytes to fuel neurons with lactate

The mechanism of rapid energy supply to the brain, especially to accommodate the heightened metabolic activity of excited states, is not well-understood. We explored the role of glycogen as a fuel source for neuromodulation using the noradrenergic stimulation of glia in a computational model of the neural-glial-vasculature ensemble (NGV). The detection of norepinephrine (NE) by the astrocyte and the coupled cAMP signal are rapid and largely insensitive to the distance of the locus coeruleus projection release sites from the glia, implying a diminished impact for volume transmission in high affinity receptor transduction systems. Glucosyl-conjugated units liberated from glial glycogen by NE-elicited cAMP second messenger transduction winds sequentially through the glycolytic cascade, generating robust increases in NADH and ATP before pyruvate is finally transformed into lactate. This astrocytic lactate is rapidly exported by monocarboxylate transporters to the associated neuron, demonstrating that the astrocyte-to-neuron lactate shuttle activated by glycogenolysis is a likely fuel source for neuromodulation and enhanced neural activity. Altogether, the energy supply for both astrocytes and neurons can be supplied rapidly by glycogenolysis upon neuromodulatory stimulus.

Although efficient compared to computers, the human brain utilizes energy at 10-fold the rate of other organs by mass. How the brain is supplied with sufficient on-demand energy to support its activity in the absence of neuronal storage capacity remains unknown. Neurons are not capable of meeting their own energy requirements, instead energy supply in the brain is managed by an oligocellular cartel composed of neurons, glia and the local vasculature (NGV), wherein glia can provide the ergogenic metabolite lactate to the neuron in a process called the astrocyte-to-neuron shuttle (ANLS). The only means of energy storage in the brain is glycogen, a polymerized form of glucose that is localized largely to astrocytes, but its exact role and conditions of use are not clear. In this computational model we show that neuromodulatory stimulation by norepinephrine induces astrocytes to recover glucosyl subunits from glycogen for use in a glycolytic process that favors the production of lactate. The ATP and NADH produced support metabolism in the astrocyte while the lactate is exported to feed the neuron. Thus, rapid energy demands by both neurons and glia in a stimulated brain can be met by glycogen mobilization.

Oligodendrocytes control potassium accumulation in white matter and seizure susceptibility

The inwardly rectifying K⁺ channel K_ir4.1 is broadly expressed by CNS glia and deficits in K_ir4.1 lead to seizures and myelin vacuolization. However, the role of oligodendrocyte K_ir4.1 channels in controlling myelination and K⁺ clearance in white matter has not been defined. Here, we show that selective deletion of K_ir4.1 from oligodendrocyte progenitors (OPCs) or mature oligodendrocytes did not impair their development or disrupt the structure of myelin. However, mice lacking oligodendrocyte K_ir4.1 channels exhibited profound functional impairments, including slower clearance of extracellular K⁺ and delayed recovery of axons from repetitive stimulation in white matter, as well as spontaneous seizures, a lower seizure threshold, and activity-dependent motor deficits. These results indicate that K_ir4.1 channels in oligodendrocytes play an important role in extracellular K⁺ homeostasis in white matter, and that selective loss of this channel from oligodendrocytes is sufficient to impair K⁺ clearance and promote seizures.

MeCP2 Deficiency Leads to Loss of Glial Kir4.1

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder usually caused by mutations in methyl-CpG-binding protein 2 (MeCP2). RTT is typified by apparently normal development until 6-18 mo of age, when motor and communicative skills regress and hand stereotypies, autonomic symptoms, and seizures present. Restoration of MeCP2 function selectively to astrocytes reversed several deficits in a murine model of RTT, but the mechanism of this rescue is unknown. Astrocytes carry out many essential functions required for normal brain functioning, including extracellular K⁺ buffering. Kir4.1, an inwardly rectifying K⁺ channel, is largely responsible for the channel-mediated K⁺ regulation by astrocytes. Loss-of-function mutations in Kir4.1 in human patients result in a severe neurodevelopmental disorder termed EAST or SESAME syndrome. Here, we evaluated astrocytic Kir4.1 expression in a murine model of Rett syndrome. We demonstrate by chromatin immunoprecipitation analysis that Kir4.1 is a direct molecular target of MeCP2. Astrocytes from Mecp2-deficient mice express significantly less Kir4.1 mRNA and protein, which translates into a >50% deficiency in Ba²⁺-sensitive Kir4.1-mediated currents, and impaired extracellular potassium dynamics. By examining astrocytes in isolation, we demonstrate that loss of Kir4.1 is cell autonomous. Assessment through postnatal development revealed that Kir4.1 expression in Mecp2-deficient animals never reaches adult, wild-type levels, consistent with a neurodevelopmental disorder. These are the first data implicating a direct MeCP2 molecular target in astrocytes and provide novel mechanistic insight explaining a potential mechanism by which astrocytic dysfunction may contribute to RTT.

It's All about Timing: The Involvement of Kir4.1 Channel Regulation in Acute Ischemic Stroke Pathology

An acute ischemic stroke is characterized by the presence of a blood clot that limits blood flow to the brain resulting in subsequent neuronal loss. Acute stroke threatens neuronal survival, which relies heavily upon proper function of astrocytes. Neurons are more susceptible to cell death when an astrocyte is unable to carry out its normal functions in supporting the neuron in the area affected by the stroke (Rossi et al., 2007; Takano et al., 2009). For example, under normal conditions, astrocytes initially swell in response to changes in extracellular osmotic pressure and then reduce their regulatory volume in response to volume-activated potassium (K+) and chloride channels (Vella et al., 2015). This astroglial swelling may be overwhelmed, under ischemic conditions, due to the increased levels of glutamate and extracellular K+ (Lai et al., 2014; Vella et al., 2015). The increase in extracellular K+ contributes to neuronal damage and loss through the initiation of harmful secondary cascades (Nwaobi et al., 2016). Reducing the amount of extracellular K+ could, in theory, limit or prevent neuronal damage and loss resulting in an improved prognosis for individuals following ischemic stroke. Kir4.1, an inwardly rectifying K+ channel, has demonstrated an ability to regulate the rapid reuptake of this ion to return the cell to basal levels allowing it to fire again in rapid transmission (Sibille et al., 2015). Despite growing interest in this area, the underlying mechanism suggesting that neuroprotection could occur through modification of the Kir4.1 channel's activity has yet to be described. The purpose of this review is to examine the current literature and propose potential underlying mechanisms involving Kir4.1, specially the mammalian target of rapamycin (mTOR) and/or autophagic pathways, in the pathogenesis of ischemic stroke. The hope is that this review will instigate further investigation of Kir4.1 as a modulator of stroke pathology.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Effects of L-theanine on anxiety-like behavior, cerebrospinal fluid amino acid profile, and hippocampal activity in Wistar Kyoto rats

RATIONALE AND OBJECTIVES

The amino acid L-theanine (N-ethyl-L-glutamine) has historically been considered a relaxing agent. In the present study, we examined the effects of repeated L-theanine administration on behavior, levels of amino acids in the cerebrospinal fluid (CSF), and hippocampal activity in Wistar Kyoto (WKY) rats, an animal model of anxiety and depressive disorders.

METHODS

Behavioral tests were performed after 7-10 days of L-theanine (0.4 mg kg^-1 day^-1) or saline administration, followed by CSF sampling for high-performance liquid chromatography (HPLC) analysis. An independent set of animals was subjected to [¹⁸F]fluorodeoxyglucose positron emission tomography (PET) scanning after the same dose of L-theanine or saline administration for 7 days.

RESULTS

In the elevated plus maze test, the time spent in the open arms was significantly longer in the L-theanine group than in the saline group (P = 0.035). In addition, significantly lower CSF glutamate (P = 0.039) and higher methionine (P = 0.024) concentrations were observed in the L-theanine group than in the saline group. A significant increase in the standard uptake value ratio was observed in the hippocampus/cerebellum of the L-theanine group (P < 0.001).

CONCLUSIONS

These results suggest that L-theanine enhances hippocampal activity and exerts anxiolytic effects, which may be mediated by changes in glutamate and methionine levels in the brain. Further study is required to more fully elucidate the mechanisms underlying the effects of L-theanine.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Hypoxia Stress Modifies Na+/K+-ATPase, H+/K+-ATPase, [Formula: see text], and nkaα1 Isoform Expression in the Brain of Immune-Challenged Air-Breathing Fish

Fishes are equipped to sense stressful stimuli and are able to respond to environmental stressor such as hypoxia with varying pattern of stress response. The functional attributes of brain to hypoxia stress in relation to ion transport and its interaction during immune challenge have not yet delineated in fish. We, therefore, explored the pattern of ion transporter functions and messenger RNA (mRNA) expression of α1-subunit isoforms of Na⁺/K⁺-ATPase (NKA) in the brain segments, namely, prosencephalon (PC), mesencephalon (MC), and metencephalon (MeC) in an obligate air-breathing fish exposed either to hypoxia stress (30 minutes forced immersion in water) or challenged with zymosan treatment (25-200 ng g⁻¹ for 24 hours) or both. Zymosan that produced nonspecific immune responses evoked differential regulation of NKA, H⁺/K⁺-ATPase (HKA), and Na+/NH4+-ATPase (NNA) in the varied brain segments. On the contrary, hypoxia stress that demanded activation of NKA in PC and MeC showed a reversed NKA activity pattern in MeC of immune-challenged fish. A compromised HKA and NNA regulation during hypoxia stress was found in immune-challenged fish, indicating the role of these brain ion transporters to hypoxia stress and immune challenges. The differential mRNA expression of α1-subunit isoforms of NKA, nkaα1a, nkaα1b, and nkaα1c, in hypoxia-stressed brain showed a shift in its expression pattern during hypoxia stress-immune interaction in PC and MC. Evidence is thus presented for the first time that ion transporters such as HKA and NNA along with NKA act as functional brain markers which respond differentially to both hypoxia stress and immune challenges. Taken together, the data further provide evidence for a differential Na⁺, K⁺, H⁺, and NH4+ ion signaling that exists in brain neuronal clusters during hypoxia stress-immune interaction as a result of modified regulations of NKA, HKA, and NNA transporter functions and nkaα1 isoform regulation.

The α2β2 isoform combination dominates the astrocytic Na+ /K+ -ATPase activity and is rendered nonfunctional by the α2.G301R familial hemiplegic migraine type 2-associated mutation

Synaptic activity results in transient elevations in extracellular K+ , clearance of which is critical for sustained function of the nervous system. The K+ clearance is, in part, accomplished by the neighboring astrocytes by mechanisms involving the Na+ /K+ -ATPase. The Na+ /K+ -ATPase consists of an α and a β subunit, each with several isoforms present in the central nervous system, of which the α2β2 and α2β1 isoform combinations are kinetically geared for astrocytic K+ clearance. While transcript analysis data designate α2β2 as predominantly astrocytic, the relative quantitative protein distribution and isoform pairing remain unknown. As cultured astrocytes altered their isoform expression in vitro, we isolated a pure astrocytic fraction from rat brain by a novel immunomagnetic separation approach in order to determine the expression levels of α and β isoforms by immunoblotting. In order to compare the abundance of isoforms in astrocytic samples, semi-quantification was carried out with polyhistidine-tagged Na+ /K+ -ATPase subunit isoforms expressed in Xenopus laevis oocytes as standards to obtain an efficiency factor for each antibody. Proximity ligation assay illustrated that α2 paired efficiently with both β1 and β2 and the semi-quantification of the astrocytic fraction indicated that the astrocytic Na+ /K+ -ATPase is dominated by α2, paired with β1 or β2 (in a 1:9 ratio). We demonstrate that while the familial hemiplegic migraine-associated α2.G301R mutant was not functionally expressed at the plasma membrane in a heterologous expression system, α2+/G301R mice displayed normal protein levels of α2 and glutamate transporters and that the one functional allele suffices to manage the general K+ dynamics.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

Astrocytes Provide Metabolic Support for Neuronal Synaptic Function in Response to Extracellular K. Neurochem Res 2017;42:2588-2594. [PMID: 28664400 DOI: 10.1007/s11064-017-2315-8]

It is an honour to have this opportunity write an article in recognition of the immense contributions of Bruce Ransom to the field of glial research. For me (BAM) personally there are many highlights both as a colleague and a friend that come to mind when I reflect on the many years that I have known Bruce. My own entry into the glial field was inspired by the early work by Ransom and his lab showing the sensitivity of astrocytes to neuronal activity. During my PhD and postdoctoral research I read these early papers and was inspired to ask the question when I first set up my independent lab in 1983: what if astrocytes also express some of the multitude of ion channels or transmitter receptors that were beginning to be described in neurons? Could they modify neuronal excitability during seizures or behaviour? As it turned out this was not only true but glial-neuronal interactions continues to be a growing and exciting field that I am still working in. I first met Bruce at the 1984 Society for Neuroscience meeting in Anaheim at my poster describing voltage gated calcium channels in astrocytes in cell culture. That was the start of a great friendship and years of discussions and collaborations. This review describes recent work from my lab led by Hyun Beom Choi that followed and was inspired by the groundbreaking studies by Bruce on electrophysiological and pH recordings from astrocytes and on glycogen mobilization in astrocytes to protect white matter axons.