Ion regulation in the brain: implications for pathophysiology

The dual face of connexin-based astroglial Ca(2+) communication: a key player in brain physiology and a prime target in pathology

For decades, studies have been focusing on the neuronal abnormalities that accompany neurodegenerative disorders. Yet, glial cells are emerging as important players in numerous neurological diseases. Astrocytes, the main type of glia in the central nervous system , form extensive networks that physically and functionally connect neuronal synapses with cerebral blood vessels. Normal brain functioning strictly depends on highly specialized cellular cross-talk between these different partners to which Ca(2+), as a signaling ion, largely contributes. Altered intracellular Ca(2+) levels are associated with neurodegenerative disorders and play a crucial role in the glial responses to injury. Intracellular Ca(2+) increases in single astrocytes can be propagated toward neighboring cells as intercellular Ca(2+) waves, thereby recruiting a larger group of cells. Intercellular Ca(2+) wave propagation depends on two, parallel, connexin (Cx) channel-based mechanisms: i) the diffusion of inositol 1,4,5-trisphosphate through gap junction channels that directly connect the cytoplasm of neighboring cells, and ii) the release of paracrine messengers such as glutamate and ATP through hemichannels ('half of a gap junction channel'). This review gives an overview of the current knowledge on Cx-mediated Ca(2+) communication among astrocytes as well as between astrocytes and other brain cell types in physiology and pathology, with a focus on the processes of neurodegeneration and reactive gliosis. Research on Cx-mediated astroglial Ca(2+) communication may ultimately shed light on the development of targeted therapies for neurodegenerative disorders in which astrocytes participate. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Obstructive sleep apnea and cognitive impairment: addressing the blood-brain barrier

Increasing data support a connection between obstructive sleep apnea (OSA) and cognitive impairment but a causal link has yet to be established. Although neuronal loss has been linked to cognitive impairment, emerging theories propose that changes in synaptic plasticity can cause cognitive impairment. Studies demonstrate that disruption to the blood-brain barrier (BBB), which is uniquely structured to tightly maintain homeostasis inside the brain, leads to changes in the brain's microenvironment and affects synaptic plasticity. Cyclical intermittent hypoxia is a stressor that could disrupt the BBB via molecular responses already known to occur in either OSA patients or animal models of intermittent hypoxia. However, we do not yet know if or how intermittent hypoxia can cause cognitive impairment by mechanisms operating at the BBB. Therefore, we propose that initially, adaptive homeostatic responses at the BBB occur in response to increased oxygen and nutrient demand, specifically through regulation of influx and efflux BBB transporters that alter microvessel permeability. We further hypothesize that although these responses are initially adaptive, these changes in BBB transporters can have long-term consequences that disrupt the brain's microenvironment and alter synaptic plasticity leading to cognitive impairment.

CORTICAL EXCITABILITY AS A POTENTIAL CLINICAL MARKER OF EPILEPSY: A REVIEW OF THE CLINICAL APPLICATION OF TRANSCRANIAL MAGNETIC STIMULATION

Transcranial magnetic stimulation (TMS) can be used for safe, noninvasive probing of cortical excitability (CE). We review 50 studies that measured CE in people with epilepsy. Most showed cortical hyperexcitability, which can be corrected with anti-epileptic drug treatment. Several studies showed that decrease of CE after epilepsy surgery is predictive of good seizure outcome. CE is a potential biomarker for epilepsy. Clinical application may include outcome prediction of drug treatment and epilepsy surgery.

TRPC3 mediates hyperexcitability and epileptiform activity in immature cortex and experimental cortical dysplasia

Neuronal hyperexcitability plays an important role in epileptogenesis. Conditions of low extracellular calcium (Ca) or magnesium (Mg) can induce hyperexcitability and epileptiform activity with unclear mechanisms. Transient receptor potential canonical type 3 (TRPC3) channels play a pivotal role in neuronal excitability and are activated in low-Ca and/or low-Mg conditions to depolarize neurons. TRPC3 staining was highly enriched in immature, but very weak in mature, control cortex, whereas it was strong in dysplastic cortex at all ages. Depolarization and susceptibility to epileptiform activity increased with decreasing Ca and Mg. Combinations of low Ca and low Mg induced larger depolarization in pyramidal neurons and greater susceptibility to epileptiform activity in immature and dysplastic cortex than in mature and control cortex, respectively. Intracellular application of anti-TRPC3 antibody to block TRPC3 channels and bath application of the selective TRPC3 inhibitor Pyr3 greatly diminished depolarization in immature control and both immature and mature dysplastic cortex with strong TRPC3 expression. Epileptiform activity was initiated in low Ca and low Mg when synaptic activity was blocked, and Pyr3 completely suppressed this activity. In conclusion, TRPC3 primarily mediates low Ca- and low Mg-induced depolarization and epileptiform activity, and the enhanced expression of TRPC3 could make dysplastic and immature cortex more hyperexcitable and more susceptible to epileptiform activity.

Cerebrospinal Fluid Secretion by the Choroid Plexus

The choroid plexus epithelium is a cuboidal cell monolayer, which produces the majority of the cerebrospinal fluid. The concerted action of a variety of integral membrane proteins mediates the transepithelial movement of solutes and water across the epithelium. Secretion by the choroid plexus is characterized by an extremely high rate and by the unusual cellular polarization of well-known epithelial transport proteins. This review focuses on the specific ion and water transport by the choroid plexus cells, and then attempts to integrate the action of specific transport proteins to formulate a model of cerebrospinal fluid secretion. Significant emphasis is placed on the concept of isotonic fluid transport across epithelia, as there is still surprisingly little consensus on the basic biophysics of this phenomenon. The role of the choroid plexus in the regulation of fluid and electrolyte balance in the central nervous system is discussed, and choroid plexus dysfunctions are described in a very diverse set of clinical conditions such as aging, Alzheimer's disease, brain edema, neoplasms, and hydrocephalus. Although the choroid plexus may only have an indirect influence on the pathogenesis of these conditions, the ability to modify epithelial function may be an important component of future therapies.

How do astrocytes shape synaptic transmission? Insights from electrophysiology

A major breakthrough in neuroscience has been the realization in the last decades that the dogmatic view of astroglial cells as being merely fostering and buffering elements of the nervous system is simplistic. A wealth of investigations now shows that astrocytes actually participate in the control of synaptic transmission in an active manner. This was first hinted by the intimate contacts glial processes make with neurons, particularly at the synaptic level, and evidenced using electrophysiological and calcium imaging techniques. Calcium imaging has provided critical evidence demonstrating that astrocytic regulation of synaptic efficacy is not a passive phenomenon. However, given that cellular activation is not only represented by calcium signaling, it is also crucial to assess concomitant mechanisms. We and others have used electrophysiological techniques to simultaneously record neuronal and astrocytic activity, thus enabling the study of multiple ionic currents and in depth investigation of neuro-glial dialogues. In the current review, we focus on the input such approach has provided in the understanding of astrocyte-neuron interactions underlying control of synaptic efficacy.

Age dependency of trauma-induced neocortical epileptogenesis

Trauma and brain infection are the primary sources of acquired epilepsy, which can occur at any age and may account for a high incidence of epilepsy in developing countries. We have explored the hypothesis that penetrating cortical wounds cause deafferentation of the neocortex, which triggers homeostatic plasticity and lead to epileptogenesis (Houweling etal., 2005). In partial deafferentation experiments of adult cats, acute seizures occurred in most preparations and chronic seizures occurred weeks to months after the operation in 65% of the animals (Nita etal., 2006,2007; Nita and Timofeev, 2007). Similar deafferentation of young cats (age 8-12 months) led to some acute seizures, but we never observed chronic seizure activity even though there was enhanced slow-wave activity in the partially deafferented hemisphere during quiet wakefulness. This suggests that despite a major trauma, the homeostatic plasticity in young animals was able to restore normal levels of cortical excitability, but in fully adult cats the mechanisms underlying homeostatic plasticity may lead to an unstable cortical state. To test this hypothesis we made an undercut in the cortex of an elderly cat. After several weeks this animal developed seizure activity. These observations may lead to an intervention after brain trauma that prevents epileptogenesis from occurring in adults.

Connexin and pannexin hemichannels in brain glial cells: properties, pharmacology, and roles

Functional interaction between neurons and glia is an exciting field that has expanded tremendously during the past decade. Such partnership has multiple impacts on neuronal activity and survival. Indeed, numerous findings indicate that glial cells interact tightly with neurons in physiological as well as pathological situations. One typical feature of glial cells is their high expression level of gap junction protein subunits, named connexins (Cxs), thus the membrane channels they form may contribute to neuroglial interaction that impacts neuronal activity and survival. While the participation of gap junction channels in neuroglial interactions has been regularly reviewed in the past, the other channel function of Cxs, i.e., hemichannels located at the cell surface, has only recently received attention. Gap junction channels provide the basis for a unique direct cell-to-cell communication, whereas Cx hemichannels allow the exchange of ions and signaling molecules between the cytoplasm and the extracellular medium, thus supporting autocrine and paracrine communication through a process referred to as “gliotransmission,” as well as uptake and release of metabolites. More recently, another family of proteins, termed pannexins (Panxs), has been identified. These proteins share similar membrane topology but no sequence homology with Cxs. They form multimeric membrane channels with pharmacology somewhat overlapping with that of Cx hemichannels. Such duality has led to several controversies in the literature concerning the identification of the molecular channel constituents (Cxs versus Panxs) in glia. In the present review, we update and discuss the knowledge of Cx hemichannels and Panx channels in glia, their properties and pharmacology, as well as the understanding of their contribution to neuroglial interactions in brain health and disease.

131I-MIBG targeting of neuroblastoma cells is acutely enhanced by KCl stimulation through the calcium/calmodulin-dependent kinase pathway

The efficacy of (131)I-metaiodobenzylguanidine (MIBG) therapy relies on norepinephrine transporter (NET) function. The ionic make-up of the extracellular fluid critically controls neuronal cell activity and can also affect substrate transport. In this study, we explored the effect of treatment with elevated KCl concentration on MIBG uptake in SK-N-SH neuroblastoma cells. KCl stimulation caused a rapid increase of (131)I-MIBG uptake in a manner that was calcium-dependent and accompanied by activation of calcium/calmodulin-dependent protein kinase (CaMK)II. The effect was completely abolished by KN93, an inhibitor of CaMKI, II, and IV. STO609, a selective inhibitor of CaMK kinase required for activation of CaMKI and IV, but not CaMKII, only modestly attenuated the response. The KCl effect was also completely abrogated by ML7, a selective inhibitor of myosin light chain kinase (MLCK). This restricted form of CaMK activates myosin, which is required for vesicle trafficking. Saturation kinetic analysis revealed KCl stimulation to increase maximal transport velocity without affecting substrate affinity. In conclusion, KCl stimulation rapidly upregulates NET function through the CaMK pathway via activation of CaMKII and MLCK. These findings allow a better understanding of how NET function is acutely modulated by the ionic environment, which in turn may ultimately help improve the efficacy of (131)I-MIBG therapy.

Phasic oscillations of extracellular potassium (K(o)) in pregnant rat myometrium

K-sensitive microelectrodes were used to measure K(+) within the extracellular space (K(o)) of pregnant rat myometrium. Contractile activity was monitored by measuring either force or bioelectrical signals. Single and double-barreled electrodes were used. Double-barreled electrodes allowed monitoring of electrical activity 15 microns from the site of K(o) measurement. From double-barreled electrode experiments, the bioelectrical burst started first, and then K(o) began to rise 0.6 ± 0.1 seconds later. This delay indicates that K(+) leaves the cells in response to local electrical activity rather than vice versa. Four control experiments were performed to assess the influence of electrical artifacts caused by tissue motion on K(o) values. When observed, artifacts were negative and transient, and hence would result in an underestimation of K(o) rises. Artifacts were minimized when tissue motion was minimized by fixing the tissue at both ends. At 37°C, 7 single barreled experiments and 45 contractions were analyzed. Resting K(o) was within 1 mM of bath K(+) (5 mM) at the beginning and end of the experiments. K(o) rose during the contraction, fell after the completion of the contraction, and normalized before the next contraction began. Peak K(o) values observed during force production were 18.8 ± 5.9 mM, a value high enough to modulate tissue-level electrical activity. K(o) required 15.7 ± 2.8 seconds to normalize halfway (t50). Six experiments expressing 38 contractions were performed at 24°C. The contraction period was longer at 24°C. Values for peak K(o) (26.2 ± 9.9 mM) and t50 (29.8±16.2 sec) were both larger than at 37°C (p<0.0003 for both). The direct relationships between peak K(o), t50 and the contraction period, suggest elevations in K(o) may modulate contraction frequency. The myometrial interstitial space appears to be functionally important, and K(o) metabolism may participate in cell-cell interactions.

Seizures and the neurosurgical intensive care unit

The cause of seizures in the neurosurgical intensive care unit (NICU) can be categorized as emanating from either a primary brain pathology or from physiologic derangements of critical care illness. Patients are typically treated with parenteral antiepileptic drugs. For early onset ICU seizures that are easily controlled, data support limited treatment. Late seizures have a more ominous risk for subsequent epilepsy and should be treated for extended periods of time or indefinitely. This review ends by examining the treatment algorithms for simple seizures and status epilepticus and the role newer antiepileptic use can play in the NICU.

The role of vasopressin V1A receptors in cytotoxic brain edema formation following brain injury

BACKGROUND

The hormone and neuropeptide arginine-vasopressin is designated to the maintenance of osmotic homoeostasis and blood pressure regulation. While experimental data show vasopressin V(1A) receptors to regulate aquaporin (AQP)4 water channel dependent brain water movement, the specific role in vasogenic and cytotoxic edema formation remains unclear. The present study was designed to quantify the V(1A) receptor mediated regional brain edema formation in two clinically relevant experimental models, brain injury combined with secondary insult and focal ischemia.

METHODS

Male Sprague-Dawley rats were randomly assigned to a continuous infusion of vehicle (1 % DMSO) or the selective non-peptide V(1A) antagonist SR49059 (83nM = 1 mg/kg) starting before controlled cortical impact (CCI) injury plus hypoxia and hypotension (HH, 30 min), or middle cerebral artery (MCA) occlusion (2 h + 2 h reperfusion).

RESULTS

A global analysis of brain water content by the wet/dry weight method allowed optimizing the SR49059 dosage, and demonstrated the down-regulation of brain AQP4 expression by immunoblotting. Microgravimetrical quantification in 64 one mm(3) samples per animal (n = 6 per group) from bregma +2.7 to -6.3 mm analysis demonstrated brain edema to be reduced at 4 h by SR49059 treatment in the injured and contralateral cortex following CCI + HH (p = 0.007, p < 0.001) and in the infarct area following MCA occlusion (p = 0.013, p = 0.002, p = 0.004).

CONCLUSIONS

Our findings demonstrate that an early cytotoxic brain edema component following brain injury plus secondary insult or focal ischemia results from a vasopressin V(1A) receptor mediated response, and occurs most likely through AQP4 up-regulation. The V(1A) antagonist SR49059 offers a new avenue in brain edema treatment and prompts further study into the role of vasopressin following brain injury.

Extracellular Ca2+ fluctuations in vivo affect afterhyperpolarization potential and modify firing patterns of neocortical neurons

Neocortical neurons can be classified in four major electrophysiological types according to their pattern of discharge: regular-spiking (RS), intrinsically-bursting (IB), fast-rhythmic-bursting (FRB), and fast-spiking (FS). Previously, we have shown that these firing patterns are not fixed and can change as a function of membrane potential and states of vigilance. Other studies have reported that extracellular calcium concentration ([Ca(2+)]o) fluctuates as a function of the phase of the cortical slow oscillation. In the present study we investigated how spontaneous and induced changes in [Ca(2+)]o affect the properties of action potentials (APs) and firing patterns in cortical neurons in vivo. Intracellular recordings were performed in cats anesthetized with ketamine-xylazine during spontaneous [Ca(2+)]o fluctuation and while changing [Ca(2+)]o with reverse microdialysis. When [Ca(2+)]o fluctuated spontaneously according to the phase of the slow oscillation, we found an increase of the firing threshold and a decrease of the afterhyperpolarization (AHP) amplitude during the depolarizing (active, up) phase of the slow oscillation and some neurons also changed their firing pattern as compared with the hyperpolarizing (silent, down) phase. Induced changes in [Ca(2+)]o significantly affected the AP properties in all neurons. The AHP amplitude was increased in high calcium conditions and decreased in low calcium conditions, in particular the earliest components. Modulation of spike AHP resulted in notable modulation of intrinsic firing pattern and some RS neurons revealed burst firing when [Ca(2+)]o was decreased. We also found an increase in AHP amplitude in high [Ca(2+)]o with in vitro preparation. We suggest that during spontaneous network oscillations in vivo, the dynamic changes of firing patterns depend partially on fluctuations of the [Ca(2+)]o.

Resonance in neocortical neurons and networks

Neocortical networks produce oscillations that often correspond to characteristic physiological or pathological patterns. However, the mechanisms underlying the generation of and the transitions between such oscillatory states remain poorly understood. In this study, we examined resonance in mouse layer V neocortical pyramidal neurons. To accomplish this, we employed standard electrophysiology to describe cellular resonance parameters. Bode plot analysis revealed a range of resonance magnitude values in layer V neurons and demonstrated that both magnitude and phase response characteristics of layer V neocortical pyramidal neurons are modulated by changes in the extracellular environment. Specifically, increased resonant frequencies and total inductive areas were observed at higher extracellular potassium concentrations and more hyperpolarised membrane potentials. Experiments using pharmacological agents suggested that current through hyperpolarization-activated cyclic nucleotide-gated channels (I(h) ) acts as the primary driver of resonance in these neurons, with other potassium currents, such as A-type potassium current and delayed-rectifier potassium current (Kv1.4 and Kv1.1, respectively), contributing auxiliary roles. The persistent sodium current was also shown to play a role in amplifying the magnitude of resonance without contributing significantly to the phase response. Although resonance effects in individual neurons are small, their properties embedded in large networks may significantly affect network behavior and may have potential implications for pathological processes.

Peri-infarct blood-brain barrier dysfunction facilitates induction of spreading depolarization associated with epileptiform discharges

Recent studies showed that spreading depolarizations (SDs) occurs abundantly in patients following ischemic stroke and experimental evidence suggests that SDs recruit tissue at risk into necrosis. We hypothesized that BBB opening with consequent alterations of the extracellular electrolyte composition and extravasation of albumin facilitates generation of SDs since albumin mediates an astrocyte transcriptional response with consequent disturbance of potassium and glutamate homeostasis. Here we show extravasation of Evans blue-albumin complex into the hippocampus following cortical photothrombotic stroke in the neighboring neocortex. Using extracellular field potential recordings and exposure to serum electrolytes we observed spontaneous SDs in 80% of hippocampal slices obtained from rats 24 h after cortical photothrombosis. Hippocampal exposure to albumin for 24 h through intraventricular application together with serum electrolytes lowered the threshold for the induction of SDs in most slices irrespective of the pathway of stimulation. Exposing acute slices from naive animals to albumin led also to a reduced SD threshold. In albumin-exposed slices the onset of SDs was usually associated with larger stimulus-induced accumulation of extracellular potassium, and preceded by epileptiform activity, which was also observed during the recovery phase of SDs. Application of ifenprodil (3 μM), an NMDA-receptor type 2 B antagonist, blocked stimulus dependent epileptiform discharges and generation of SDs in slices from animals treated with albumin in-vivo. We suggest that BBB opening facilitates the induction of peri-infarct SDs through impaired homeostasis of K+.

Sustained NMDA receptor activation by spreading depolarizations can initiate excitotoxic injury in metabolically compromised neurons

Spreading depolarizations (SDs) are slowly propagating waves of near-complete neuronal and glial depolarization. SDs have been recorded in patients with brain injury, and the incidence of SD significantly correlates with outcome severity. Although it is well accepted that the ionic dyshomeostasis of SD presents a severe metabolic burden, there is currently limited understanding of SD-induced injury processes at a cellular level. In the current study we characterized events accompanying SD in the hippocampal CA1 region of murine brain slices, using whole-cell recordings and single-cell Ca(2+) imaging. We identified an excitatory phase that persisted for approximately 2 min following SD onset, and accompanied with delayed dendritic ionic dyshomeostasis. The excitatory phase coincided with a significant increase in presynaptic glutamate release, evidenced by a transient increase in spontaneous EPSC frequency and paired-pulse depression of evoked EPSCs. Activation of NMDA receptors (NMDARs) during this late excitatory phase contributed to the duration of individual neuronal depolarizations and delayed recovery of extracellular slow potential changes. Selectively targeting the NMDAR activation following SD onset (by delayed pressure application of a competitive NMDAR antagonist) significantly decreased the duration of cellular depolarizations. Recovery of dendritic Ca(2+) elevations following SD were also sensitive to delayed NMDA antagonist application. Partial inhibition of neuronal energy metabolism converted SD into an irrecoverable event with persistent Ca(2+) overload and membrane compromise. Delayed NMDAR block was sufficient to prevent these acute injurious events in metabolically compromised neurons. These results identify a significant contribution of a late component of SD that could underlie neuronal injury in pathological circumstances.

Elevated potassium provides an ionic mechanism for deep brain stimulation in the hemiparkinsonian rat

The mechanism of high-frequency stimulation used in deep brain stimulation (DBS) for Parkinson's disease (PD) has not been completely elucidated. Previously, high-frequency stimulation of the rat entopeduncular nucleus, a basal ganglia output nucleus, elicited an increase in [K(+)](e) to 18 mm, in vitro. In this study, we assessed whether elevated K(+) can elicit DBS-like therapeutic effects in hemiparkinsonian rats by employing the limb-use asymmetry test and the self-adjusting stepping test. We then identified how these effects were meditated with in-vivo and in-vitro electrophysiology. Forelimb akinesia improved in hemiparkinsonian rats undergoing both tests after 20 mm KCl injection into the substantia nigra pars reticulata (SNr) or the subthalamic nucleus. In the SNr, neuronal spiking activity decreased from 38.2 ± 1.2 to 14.6 ± 1.6 Hz and attenuated SNr beta-frequency (12-30 Hz) oscillations after K(+) treatment. These oscillations are commonly associated with akinesia/bradykinesia in patients with PD and animal models of PD. Pressure ejection of 20 mm KCl onto SNr neurons in vitro caused a depolarisation block and sustained quiescence of SNr activity. In conclusion, our data showed that elevated K(+) injection into the hemiparkinsonian rat SNr improved forelimb akinesia, which coincided with a decrease in SNr neuronal spiking activity and desynchronised activity in SNr beta frequency, and subsequently an overall increase in ventral medial thalamic neuronal activity. Moreover, these findings also suggest that elevated K(+) may provide an ionic mechanism that can contribute to the therapeutic effects of DBS for the motor treatment of advanced PD.

Comorbidity and metabolic context are crucial factors determining neurological sequelae of hypoglycaemia

AIM

To determine risk factors for neurological sequelae following hypoglycemia.

METHOD

We analysed the neurological outcome in 164 patients (mean age 10y 10mo, SD 5.9) following hypoglycemia due to three diseases with various metabolic contexts, different ages at onset, and combinations with comorbidity (fever/infection, hypoxia/ischemia): glycogen storage disease type I (GSDI) (21 patients, mean age at first hypoglycemic episode 3.8mo, SD 3.5); fatty acid β-oxidation defects (FAOD) (29 patients, mean age at first hypoglycemic episode 14.8mo, SD 12.6); and hyperinsulinism (HIns) (114 patients, mean age at first hypoglycemic episode 2.3mo, SD 4.7).

RESULTS

Risk factors of poor neurological outcome were aetiology (p<0.006), comorbidity (p<0.001), and prolonged convulsions (p<0.001). Ordinal logistic regression showed that comorbidity (p<0.001) and status epilepticus (p=0.002) were the main determinants of sequelae. Asymptomatic hypoglycemia did not lead to sequelae, whatever the aetiology. Age was not correlated to sequelae, whatever the aetiology. The highest prevalence of hypoglycemic sequelae was found in FAOD and HIns combined with comorbidity, the lowest in GSDI (p<0.001) in which hypoglycemia is often asymptomatic, associated with increased plasma lactate, and rarely combined with comorbidity.

INTERPRETATION

Hypoglycemia is severely deleterious for the brain in the context of fever/infection and/or hypoxia/ischemia, and status epilepticus. The metabolic context providing alternative fuels may improve neurological outcome.

Ammonia induces aquaporin-4 rearrangement in the plasma membrane of cultured astrocytes

Aquaporin-4 (AQP4) is a water channel protein mainly located in the astroglial plasma membrane, the precise function of which in the brain edema that accompanies hepatic encephalopathy (HE) is unclear. Since ammonia is the main pathogenic agent in HE, its effect on AQP4 expression and distribution in confluent primary astroglial cultures was examined via their exposure to ammonium chloride (1, 3 and 5 mM) for 5 and 10 days. Ammonia induced the general inhibition of AQP4 mRNA synthesis except in the 1 mM/5 day treatment. However, the AQP4 protein content measured was dependent on the method of analysis; an apparent increase was recorded in treated cells in in-cell Western assays, while an apparent reduction was seen with the classic Western blot method, perhaps due to differences in AQP4 aggregation. Ammonia might therefore induce the formation of insoluble AQP4 aggregates in the astroglial plasma membrane. The finding of AQP4 in the pellet of classic Western blot samples, plus data obtained via confocal microscopy, atomic force microscopy (using immunolabeled cells with gold nanoparticles) and scanning electron microscopy, all corroborate this hypothesis. The effect of ammonia on AQP4 seems not to be due to any osmotic effect; identical osmotic stress induced by glutamine and salt had no significant effect on the AQP4 content. AQP4 functional analysis (subjecting astrocytes to a hypo-osmotic medium and using flow cytometry to measure cell size) demonstrated a smaller water influx in ammonia-treated astrocytes suggesting that AQP4 aggregates are representative of an inactive status; however, more confirmatory studies are required to fully understand the functional status of AQP4 aggregates. The present results suggest that ammonia affects AQP4 expression and distribution, and that astrocytes change their expression of AQP4 mRNA as well as the aggregation status of the ensuing protein depending on the ammonia concentration and duration of exposure.

Curious and contradictory roles of glial connexins and pannexins in epilepsy

Glia play an under-recognized role in epilepsy. This review examines the involvement of glial connexins (Cxs) and pannexins (Panxs), proteins which form gap junctions and membrane hemichannels (connexins) and hemichannels (pannexins), in epilepsy. These proteins, particularly glial Cx43, have been shown to be upregulated in epileptic brain tissue. In a cobalt model of in vitro seizures, seizures increased Panxs1 and 2 and Cx43 expression, and remarkably reorganized the interrelationships between their mRNA levels (transcriptome) which then became statistically significant. Gap junctions are highly implicated in synchronous seizure activity. Blocking gap junctional communication (GJC) is often anticonvulsant, and assumed to be due to blocking gap junctionally-medicated electrotonic coupling between neurons. However, in organotypic hippocampal slice cultures, connexin43 specific peptides, which attenuate GJC possibly by blocking connexon docking, diminished spontaneous seizures. Glia have many functions including extracellular potassium redistribution, in part via gap junctions, which if blocked, can be seizuregenic. Glial gap junctions are critical for the delivery of nutrients to neurons, which if interrupted, can depress seizure activity. Other functions of glia possibly related to epileptogenesis are mentioned including anatomic reorganization in chronic seizure models greatly increasing the overlapping domains of glial processes, changes in neurotransmitter re-uptake, and possible glial generation of currents and fields during seizure activity. Finally there is recent evidence for Cx43 hemichannels and Panx1 channels in glial membranes which could play a role in brain damage and seizure activity. Although glial Cxs and Panxs are increasingly recognized as contributing to fundamental mechanisms of epilepsy, the data are often contradictory and controversial, requiring much more research. This article is part of a Special Issue entitled Electrical Synapses.

Extracellular fluid conductivity analysis by dielectric spectroscopy for in vitro determination of cortical tissue vitality

Brain tissue is extremely metabolically active in part due to its need to constantly maintain a precise extracellular ionic environment. Under pathological conditions, unhealthy cortical tissue loses its ability to maintain this precise environment and there is a net efflux of charged particles from the cells. Typically, this ionic efflux is measured using ion-selective microelectrodes, which measure a single ionic species at a time. In this paper, we have used a bio-sensing method, dielectric spectroscopy (DS), which allows for the simultaneous measurement of the net efflux of all charged particles from cells by measuring extracellular conductivity. We exposed cortical brain slices from the mouse to different solutions that mimic various pathological states such as hypokalemia, hyperkalemia and ischemia (via oxygen-glucose deprivation). We have found that the changes in conductivity of the extracellular solutions were proportional to the severity of the pathological insult experienced by the brain tissue. Thus, DS allows for the measurement of changes in extracellular conductivity with enough sensitivity to monitor the health of brain tissue in vitro.

Extracellular K⁺ and astrocyte signaling via connexin and pannexin channels

Astrocytes utilize two major pathways to achieve long distance intercellular communication. One pathway involves direct gap junction mediated signal transmission and the other consists of release of ATP through pannexin channels and excitation of purinergic receptors on nearby cells. Elevated extracellular potassium to levels occurring around hyperactive neurons affects both gap junction and pannexin1 channels. The action on Cx43 gap junctions is to increase intercellular coupling for a period that long outlasts the stimulus. This long term increase in coupling, termed "LINC", is mediated through calcium and calmodulin dependent activation of calmodulin dependent kinase (CaMK). Pannexin1 can be activated by elevations in extracellular potassium through a mechanism that is quite different. In this case, potassium shifts activation potentials to more physiological range, thereby allowing channel opening at resting or slightly depolarized potentials. Enhanced activity of both these channel types by elevations in extracellular potassium of the magnitude occurring during periods of high neuronal activity likely has profound effects on intercellular signaling among astrocytes in the nervous system.

Cellular Links between Neuronal Activity and Energy Homeostasis

Neuronal activity, astrocytic responses to this activity, and energy homeostasis are linked together during baseline, conscious conditions, and short-term rapid activation (as occurs with sensory or motor function). Nervous system energy homeostasis also varies during long-term physiological conditions (i.e., development and aging) and with adaptation to pathological conditions, such as ischemia or low glucose. Neuronal activation requires increased metabolism (i.e., ATP generation) which leads initially to substrate depletion, induction of a variety of signals for enhanced astrocytic function, and increased local blood flow and substrate delivery. Energy generation (particularly in mitochondria) and use during ATP hydrolysis also lead to considerable heat generation. The local increases in blood flow noted following neuronal activation can both enhance local substrate delivery but also provides a heat sink to help cool the brain and removal of waste by-products. In this review we highlight the interactions between short-term neuronal activity and energy metabolism with an emphasis on signals and factors regulating astrocyte function and substrate supply.

Kir4.1 channels mediate a depolarization of hippocampal astrocytes under hyperammonemic conditions in situ

Increased ammonium (NH(4) (+) ) concentration in the brain is the prime candidate responsible for hepatic encephalopathy (HE), a serious neurological disorder caused by liver failure and characterized by disturbed glutamatergic neurotransmission and impaired glial function. We investigated the mechanisms of NH(4) (+) -induced depolarization of astrocytes in mouse hippocampal slices using whole-cell patch-clamp and potassium-selective microelectrodes. At postnatal days (P) 18-21, perfusion with 5 mM NH(4) (+) evoked a transient increase in the extracellular potassium concentration ([K(+) ](o) ) by about 1 mM. Astrocytes depolarized by on average 8 mV and then slowly repolarized to a plateau depolarization of 6 mV, which was maintained during NH(4) (+) perfusion. In voltage-clamped astrocytes, NH(4) (+) induced an inward current and a reduction in membrane resistance. Amplitudes of [K(+) ](o) transients and astrocyte depolarization/inward currents increased from P3-4 to P18-21. Perfusion with 100 μM Ba(2+) did not alter [K(+) ](o) transients but strongly reduced both astrocyte depolarization and inward currents. NH(4) (+) -induced depolarization and inward currents were also virtually absent in slices from Kir4.1 -/- mice, while [K(+) ](o) transients were unaltered. Blocking Na(+) /K(+) -ATPase with ouabain caused an immediate and complex increase in [K(+) ](o) . Taken together, our results are in agreement with the hypothesis that reduced uptake of K(+) by the Na(+) , K(+) -ATPase in the presence of NH(4) (+) disturbs the extracellular K(+) homeostasis. Furthermore, astrocytes depolarize in response to the increase in [K(+) ](o) and by influx of NH(4) (+) through Kir4.1 channels. The depolarization reduces the astrocytes' capacity for channel-mediated flux of K(+) and for uptake of glutamate and might hereby contribute to the pathology of HE.

Imaging mass spectrometry evaluation of the effects of various irrigation fluids in a rat model of postoperative cerebral edema

BACKGROUND

Using imaging mass spectrometry (IMS), we investigated the cerebral protective effect of an artificial cerebrospinal fluid (CSF), ARTCEREB (Artcereb, Otsuka Pharmaceutical Factory, Inc., Tokushima, Japan), as an irrigation and perfusion solution for neurosurgical procedures in a rat craniotomy model.

METHODS

Wounds created in the rat cerebral cortex were continuously irrigated with Artcereb, normal saline, or lactated Ringer's solution at a steady rate for 4 hours, after which brain tissue was collected. Brain slices were prepared and analyzed using IMS.

RESULTS

In tissue surrounding the injury, the signal intensity for Na adduct ions to phosphatidylcholine was high and that for K adduct ions to phosphatidylcholine was low. This is thought to reflect the level of water retention in brain cells and to be a change accompanying edema. The signal intensity with Na adduct ions to phosphatidylcholine was significantly lower in the Artcereb group than in the physiological saline or lactated Ringer's solution groups.

CONCLUSIONS

IMS analysis in a rat craniotomy model indicated that the level of water retention in brain cells, calculated from the signal intensity of Na-adducted phosphatidylcholine around the wound area, was lowest in the Artcereb group, suggesting that artificial CSF that has similar composition and properties to human CSF can minimize edema in the brain surrounding the surgical wound.

The impact of the glial spatial buffering on the K(+) Nernst potential

Astrocytes play a critical role in CNS metabolism, regulation of volume and ion homeostasis of the interstitial space. Of special relevance is their clearance of K(+) that is released by active neurons into the extracellular space. Mathematical analysis of a modified Nernst equation for the electrochemical equilibrium of neuronal plasma membranes, suggests that K(+) uptake by glial cells is not only relevant during neuronal activity but also has a non-neglectable impact on the basic electrical membrane properties, specifically the resting membrane potential, of neurons and might be clinically valuable as a factor in the genetics and epigenetics of the epilepsy and tuberous sclerosis complex.

Treatment of seizures in the neurologic intensive care unit

Seizures occur more often in the neurologic intensive care unit (NICU) than in general or other specialty ICUs, in part because of the patient population, but also due to the enhanced neurologic monitoring undertaken in such units. Especially important for the detection of seizures is the use of specialty trained personnel and the use of continuous electroencephalographic monitoring. The etiology of seizures often can be categorized either by primary brain pathology, at macro- or microscopic level, or by physiologic derangements of critical care illness, such as toxic or metabolic abnormalities. Particular etiologies at risk for seizures include hemorrhagic stroke and traumatic brain injury. The use of prophylactic antiepileptic drug administration remains controversial. If seizures occur, patients are typically treated with parenteral antiepileptic drugs. The duration of treatment is unclear in most situations, but data support limited treatment for early-onset ICU seizures that are easily controlled, with treatment not extending beyond a few weeks or a month. Late seizures, which occur more than 2 weeks after the insult, have a more ominous correlative risk for subsequent epilepsy and should be treated for extended periods of time or indefinitely. Electrolyte and glucose abnormalities, when corrected, usually lead to seizure control. This review concludes by examining the treatment algorithms for simple seizures and status epilepticus and the role newer antiepileptic use can play in the NICU.

Ionic dynamics mediate spontaneous termination of seizures and postictal depression state

Epileptic seizures are characterized by periods of recurrent, highly synchronized activity that spontaneously terminates, followed by postictal state when neuronal activity is generally depressed. The mechanisms for spontaneous seizure termination and postictal depression remain poorly understood. Using a realistic computational model, we demonstrate that termination of seizure and postictal depression state may be mediated by dynamics of the intracellular and extracellular ion concentrations. Spontaneous termination was linked to progressive increase of intracellular sodium concentration mediated by activation of sodium channels during highly active epileptic state. In contrast, an increase of intracellular chloride concentration extended seizure duration making possible long-lasting epileptic activity characterized by multiple transitions between tonic and clonic states. After seizure termination, the extracellular potassium was reduced below baseline, resulting in postictal depression. Our study suggests that the coupled dynamics of sodium, potassium, and chloride ions play a critical role in the development and termination of seizures. Findings from this study could help identify novel therapeutics for seizure disorder.

Kir4.1 K+ channels are regulated by external cations

The inwardly rectifying potassium channel (Kir), Kir4.1 mediates spatial K(+)-buffering in the CNS. In this process the channel is potentially exposed to a large range of extracellular K(+) concentrations ([K(+)]o). We found that Kir4.1 is regulated by K(+)o. Increased [K(+)]o leads to a slow (mins) increase in the whole-cell currents of Xenopus oocytes expressing Kir4.1. Conversely, removing K(+) from the bath solution results in a slow decrease of the currents. This regulation is not coupled to the pHi-sensitive gate of the channel, nor does it require the presence of K67, a residue necessary for K(+)o-dependent regulation of Kir1.1. The voltage-dependent blockers Cs(+) and Ba(2+) substitute for K(+) and prevent deactivation of the channel in the absence of K(+)o. Cs(+) blocks and regulates the channel with similar affinity, consistent with the regulatory sites being in the selectivity-filter of the channel. Although both Rb(+) and NH4(+) permeate Kir4.1, only Rb(+) is able to regulate the channel. We conclude that Kir4.1 is regulated by ions interacting with specific sites in the selectivity filter. Using a kinetic model of the permeation process we show the plausibility of the channel's sensing the extracellular ionic environment through changes in the selectivity occupancy pattern, and that it is feasible for an ion with the selectivity properties of NH4(+) to permeate the channel without inducing these changes.

Maintenance and regulation of extracellular volume and the ion environment in Drosophila larval nerves

In mammals and insects, paracellular blood barriers isolate the nervous system from the rest of the animal. Glia and accessory cells of the nervous system use pumps, channels, cotransporters, and exchangers collectively to maintain the extracellular ion environment and osmotic balance in the nervous system. At present, the molecular mechanisms that regulate this process remain unclear. In humans, loss of extracellular ion and volume regulation in the nervous system poses serious health threats. Drosophila is a model genetic organism with a proven track record for uncovering molecular mechanisms relevant to human health and disease. Here, we review what is known about extracellular ion and volume regulation in larval abdominal nerves, present some new data about the impact of neural activity on the extracellular environment, and relate the findings to mammalian systems. Homologies have been found at the level of morphology, physiology, molecular mechanisms, and mutant phenotypes. The Fray-Ncc69 module regulates extracellular volume in larval nerves. Genetic rescue experiments with the mammalian orthologs prove this module has a direct correlate in humans. This and other molecular homologies, together with the similar physiological needs, suggest that uncovering the molecular mechanisms of ion and volume regulation in larval nerves will likely provide significant insights into this process in mammalian systems.

Drosophila glia use a conserved cotransporter mechanism to regulate extracellular volume

The nervous system is protected by blood barriers that use multiple systems to control extracellular solute composition, osmotic pressure, and fluid volume. In the human nervous system, misregulation of the extracellular volume poses serious health threats. Here, we show that the glial cells that form the Drosophila blood-nerve barrier have a conserved molecular mechanism that regulates extracellular volume: the Serine/Threonine kinase Fray, which we previously showed is an ortholog of mammalian PASK/SPAK; and the Na-K-Cl cotransporter Ncc69, which we show is an ortholog of human NKCC1. In mammals, PASK/SPAK binds to NKCC1 and regulates its activity. In Drosophila, larvae mutant for Ncc69 develop a peripheral neuropathy, where fluid accumulates between glia and axons. The accumulation of fluid has no detectable impact on action potential conduction, suggesting that the role of Ncc69 is to maintain volume or osmotic homeostasis. Drosophila Ncc69 has kinetics similar to human NKCC1, and NKCC1 can rescue Ncc69, suggesting that they function in a conserved physiological mechanism. We show that fray and Ncc69 are coexpressed in nerve glia, interact in a yeast-two-hybrid assay, and have an essentially identical bulging nerve phenotype. We propose that normally functioning nerves generate extracellular solutes that are removed by Ncc69 under the control of Fray. This mechanism may perform a similar role in humans, given that NKCC1 is expressed at the blood-brain barrier.

Synaptic impairment induced by paroxysmal ionic conditions in neocortex

PURPOSE

Seizures are associated with a reduction in extracellular Ca²(+) concentration ([Ca²(+) ](o) ) and an increase in extracellular K(+) concentration ([K(+) ](o) ). The long-range synchrony observed between distant electrodes during seizures is weak. We hypothesized that changes in extracellular ionic conditions during seizures are sufficient to alter synaptic neuronal responses and synchrony in the neocortex.

METHODS

  We obtained in vivo and in vitro electrophysiologic recordings combined with microstimulation from cat/rat neocortical neurons during seizures and seizure-like ionic conditions. In vitro the [K(+) ](o) was 2.8, 6.25, 8.0, and 12 mm and the [Ca²(+) ](o) was 1.2 and 0.6 mm.

KEY FINDINGS

During seizures recorded in vivo, we observed abolition of evoked synaptic responses. In vitro, the membrane potential of both regular-spiking and fast-spiking neurons was depolarized in high [K(+) ](o) conditions and hyperpolarized in high [Ca²(+) ](o) conditions. During high [K(+) ](o) conditions, changes in [Ca²(+) ](o) did not affect membrane potential. The synaptic responsiveness of both regular-spiking and fast-spiking neurons was reduced during seizure-like ionic conditions. A reduction in [Ca²(+) ](o) to 0.6 mm increased failure rates but did not abolish responses. However, an increase in [K(+) ](o) to 12 mm abolished postsynaptic responses, which depended on a blockade in axonal spike propagation.

SIGNIFICANCE

We conclude that concomitant changes in [K(+) ](o) and [Ca²(+) ](o) observed during seizures contribute largely to the alterations of synaptic neuronal responses and to the decrease in long-range synchrony during neocortical seizures.

Spontaneous epileptiform activity in rat neocortex after controlled cortical impact injury

A hallmark of severe traumatic brain injury (TBI) is the development of post-traumatic epilepsy (PTE). However, the mechanisms underlying PTE remain poorly understood. In this study, we used a controlled cortical impact (CCI) model in rats to examine post-traumatic changes in neocortical excitability. Neocortical slices were prepared from rats at 7-9 days (week 1) and 14-16 days (week 2) after CCI injury. By week 2, we observed a substantial gray matter lesion with a cavity that extended to the hippocampal structure. Fluoro-Jade B staining of slices revealed active neuronal degeneration during weeks 1 and 2. Intracellular and extracellular recordings obtained from layer V revealed evoked and spontaneous epileptiform discharges in neocortices of CCI-injured rats. At week 1, intracellular recordings from pyramidal cells revealed evoked epileptiform firing that was synchronized with population events recorded extracellularly, suggestive of increased excitability. This activity was characterized by bursts of action potentials that were followed by recurrent, repetitive after-discharges. At week 2, both spontaneous and evoked epileptiform firing were recorded in slices from injured rats. The evoked discharges resembled those observed at week 1, but with longer burst durations. Spontaneous activity included prolonged, ictal-like discharges lasting up to 8-10 sec, and briefer interictal-like burst events (<1 sec). These results indicate that during the first 2 weeks following severe CCI injury, there is a progressive development of neocortical hyperexcitability that ultimately leads to spontaneous epileptiform firing, suggesting a rapid epileptogenic process.

K(+) and Ca²(+) dependence of inspiratory-related rhythm in novel "calibrated" mouse brainstem slices

Recently developed transversal newborn rat brainstem slices with "calibrated" rostrocaudal margins unraveled novel features of rhythmogenic inspiratory active pre-Bötzinger complex (preBötC) neural networks (Ballanyi and Ruangkittisakul, 2009). For example, slice rhythm in physiological (3 mM) superfusate K(+) is depressed by modestly raised Ca²(+) and restored by raised K(+). Correspondingly, we generated here calibrated preBötC slices from commonly used newborn C57BL/6 mice in which rostrocaudal extents of respiratory marker structures, e.g., the inferior olive, turned out to be smaller than in newborn rats. Slices of 400-600 μm thickness with likely centered preBötC kernel ("m-preBötC slices") generated rhythm in 3 mM K(+) and 1mM Ca(2+) for several hours although its rate decreased to < 5 bursts/min after >1 h. Rhythm was stable at 8-12 bursts/min in 6-7 mM K(+), depressed by 2 mM Ca²(+), and restored by 9 mM K(+). Our findings provide the basis for future structure-function analyses of the mouse preBötC, whose activity depends critically on a "Ca(+)/K(+) antagonism" as in rats.

Windows to cell function and dysfunction: signatures written in the boundary layers

The medium surrounding cells either in culture or in tissues contains a chemical mix varying with cell state. As solutes move in and out of the cytoplasmic compartment they set up characteristic signatures in the cellular boundary layers. These layers are complex physical and chemical environments the profiles of which reflect cell physiology and provide conduits for intercellular messaging. Here we review some of the most relevant characteristics of the extracellular/intercellular space. Our initial focus is primarily on cultured cells but we extend our consideration to the far more complex environment of tissues, and discuss how chemical signatures in the boundary layer can or may affect cell function. Critical to the entire essay are the methods used, or being developed, to monitor chemical profiles in the boundary layers. We review recent developments in ultramicro electrochemical sensors and tailored optical reporters suitable for the task in hand.

Network bistability mediates spontaneous transitions between normal and pathological brain states

Little is known about how cortical networks support the emergence of remarkably different activity patterns. Physiological activity interspersed with epochs of pathological hyperactivity in the epileptic brain represents a clinically relevant yet poorly understood case of such rich dynamic repertoire. Using a realistic computational model, we demonstrate that physiological sparse and pathological tonic-clonic activity may coexist in the same cortical network for identical afferent input level. Transient perturbations in the afferent input were sufficient to switch the network between these two stable states. The effectiveness of the potassium regulatory apparatus determined the stability of the physiological state and the threshold for seizure initiation. Our findings contrast with the common notions of (1) pathological brain activity representing dynamic instabilities and (2) necessary adjustments of experimental conditions to elicit different network states. Rather, we propose that the rich dynamic repertoire of cortical networks may be based on multistabilities intrinsic to the network.

A silk platform that enables electrophysiology and targeted drug delivery in brain astroglial cells

Astroglial cell survival and ion channel activity are relevant molecular targets for the mechanistic study of neural cell interactions with biomaterials and/or electronic interfaces. Astrogliosis is the most typical reaction to in vivo brain implants and needs to be avoided by developing biomaterials that preserve astroglial cell physiological function. This cellular phenomenon is characterized by a proliferative state and altered expression of astroglial potassium (K(+)) channels. Silk is a natural polymer with potential for new biomedical applications due to its ability to support in vitro growth and differentiation of many cell types. We report on silk interactions with cultured neocortical astroglial cells. Astrocytes survival is similar when plated on silk-coated glass and on poly-D-lysine (PDL), a well known polyionic substrate used to promote astroglial cell adhesion to glass surfaces. Comparative analyses of whole-cell patch-clamp experiments reveal that silk- and PDL-coated cells display depolarized resting membrane potentials (-40 mV), very high input resistance, and low specific conductance, with values similar to those of undifferentiated glial cells. Analysis of K(+) channel conductance reveals that silk-astrocytes express large outwardly delayed rectifying K(+) current (K(DR)). The magnitude of K(DR) in PDL- and silk-coated astrocytes is similar, indicating that silk does not alter the resting K(+) current. We also demonstrate that guanosine- (GUO) embedded silk enables the direct modulation of astroglial K(+) conductance in vitro. Astrocytes plated on GUO-embedded silk are more hyperpolarized and express inward rectifying K(+) conductance (K(ir)). The K(+) inward current increases and this is paralleled by upregulation and membrane polarization of K(ir)4.1 protein signal. Collectively these results indicate that silk is a suitable biomaterial platform for the in vitro studies of astroglial ion channel responses and related physiology.