Level Of Neuronal Excitability Research Articles

WONOEP appraisal: The role of glial cells in focal malformations associated with early onset epilepsies.

Epilepsy represents a common neurological disorder in patients with developmental brain lesions, particularly in association with malformations of cortical development and low-grade glioneuronal tumors. In these diseases, genetic and molecular alterations in neurons are increasingly discovered that can trigger abnormalities in the neuronal network, leading to higher neuronal excitability levels. However, the mechanisms underlying epilepsy cannot rely solely on assessing the neuronal component. Growing evidence has revealed the high degree of complexity underlying epileptogenic processes, in which glial cells emerge as potential modulators of neuronal activity. Understanding the role of glial cells in developmental brain lesions such as malformations of cortical development and low-grade glioneuronal tumors is crucial due to the high degree of pharmacoresistance characteristic of these lesions. This has prompted research to investigate the role of glial and immune cells in epileptiform activity to find new therapeutic targets that could be used as combinatorial drug therapy. In a special session of the XVI Workshop of the Neurobiology of Epilepsy (WONOEP, Talloires, France, July 2022) organized by the Neurobiology Commission of the International League Against Epilepsy, we discussed the evidence exploring the genetic and molecular mechanisms of glial cells and immune response and their implications in the pathogenesis of neurodevelopmental pathologies associated with early life epilepsies.

Neuronal excitability modulates developmental time of Drosophila melanogaster

Developmental time is a fundamental life history trait that affects the reproductive success of animals. Developmental time is known to be regulated by many genes and environmental conditions, yet mechanistic understandings of how various cellular processes influence the developmental timing of an organism are lacking. The nervous system is known to control key processes that affect developmental time, including the release of hormones that signal transitions between developmental stages. Here we show that the excitability of neurons plays a crucial role in modulating developmental time. Genetic manipulation of neuronal excitability in Drosophila melanogaster alters developmental time, which is faster in animals with increased neuronal excitability. We find that selectively modulating the excitability of peptidergic neurons is sufficient to alter developmental time, suggesting the intriguing hypothesis that the impact of neuronal excitability on DT may be at least partially mediated by peptidergic regulation of hormone release. This effect of neuronal excitability on developmental time is seen during embryogenesis and later developmental stages. Observed phenotypic plasticity in the effect of genetically increasing neuronal excitability at different temperatures, a condition also known to modulate excitability, suggests there is an optimal level of neuronal excitability, in terms of shortening DT. Together, our data highlight a novel connection between neuronal excitability and developmental time, with broad implications related to organismal physiology and evolution.

Identifying the mechanism of action of the Kv7 channel opener, retigabine in the treatment of epilepsy.

Epilepsy is characterized by recurrent epileptic seizures caused by high levels of neuronal excitability in the brain. Voltage-sensitive K+ channels (Kv) of the Kv7 (KCNQ) family encoded by the KCNQ gene are involved in a wide range of cellular processes, i.e., KCNQ2 and KCNQ3 channels mediate M-currents to inhibit neuronal excitability and reduce transmitter release throughout the nervous system. Thus, as a positive allosteric modulator (or opener) of KCNQ channels, retigabine has been the only clinically approved anti-seizure medication that acts on the KCNQ channels. This review discusses the biochemical mechanisms about how retigabine acts on Kv7 channels, significance in neuronal pathophysiology, preclinical efficacy, and clinical stage of development. Additional efforts are being made to emphasize the possible benefits and drawbacks of retigabine compared to currently available medications for treatment-resistant epilepsy.

SIK3 suppresses neuronal hyperexcitability by regulating the glial capacity to buffer K+ and water.

Glial regulation of extracellular potassium (K+) helps to maintain appropriate levels of neuronal excitability. While channels and transporters mediating K+ and water transport are known, little is understood about upstream regulatory mechanisms controlling the glial capacity to buffer K+ and osmotically obliged water. Here we identify salt-inducible kinase 3 (SIK3) as the central node in a signal transduction pathway controlling glial K+ and water homeostasis in Drosophila Loss of SIK3 leads to dramatic extracellular fluid accumulation in nerves, neuronal hyperexcitability, and seizures. SIK3-dependent phenotypes are exacerbated by K+ stress. SIK3 promotes the cytosolic localization of HDAC4, thereby relieving inhibition of Mef2-dependent transcription of K+ and water transport molecules. This transcriptional program controls the glial capacity to regulate K+ and water homeostasis and modulate neuronal excitability. We identify HDAC4 as a candidate therapeutic target in this pathway, whose inhibition can enhance the K+ buffering capacity of glia, which may be useful in diseases of dysregulated K+ homeostasis and hyperexcitability.

The role of satellite glial cells in orofacial pain.

Some chronic pain conditions in the orofacial region are common, the mechanisms underlying which are unresolved. Satellite glial cells (SGCs) are the glial cells of the peripheral nervous system. In the sensory ganglia, each neuronal body is surrounded by SGCs forming distinct functional units. The unique structural organization enables SGCs to communicate with each other and with their enwrapped neurons via a variety of ways. There is a growing body of evidence that SGCs can influence the level of neuronal excitability and are involved in the development and/or maintenance of pain. The aim of this review was to summarize the latest advances made about the implication of SGCs in orofacial pain. It may offer new targets for the development of orofacial pain treatment.

INFLUENCE OF PRE-STIMULATION RHYTHMS ON PHOSPHENE PERCEPTION IN THE CONTEXT OF VISUAL HALLUCINATIONS

Background Complex visual hallucinations are a common symptom in Parkinson’s disease dementia (PDD). Cortical alpha rhythm is a measure of attention and visual perception in healthy controls, and is reported to dysfunction in PDD. Additionally, previous research has related the likelihood to experience visual hallucinations with levels of neuronal excitability in alpha band rhythms from the visual cortex. Methods A group of controls participants (n=17) and recently diagnosed PDD patients (n=26) where recruited for the study. Concurrent transcranial magnetic stimulation and electroencephalogram (TMS-EEG) was delivered to all participants and brain signals recorded. Using stereotactic navigation, the cortical region for optimal phosphene experience was determined within the visual cortex in an incremental power stimulation design, where TMS power was increased up to 75% of stimulator output. Only 11 PDD (mean age = 75.47 ± 5.4) and 9 control participants (mean age 73.82 ± 5.4) experienced phosphenes on 50% of trials (240 trials). Mixed effect analysis was implemented to investigate the effects of scalp region (anterior, posterior), pre-stimulus EEG power (within theta, low-alpha and high alpha bands), and participant group on phosphene experience. Results In the anterior region, there was a significant effect of phosphene or no-phosphene condition on the low-alpha relative power (F(1,18)=4.72, p=0.043, η2=0.208). Both groups showed decreased low-alpha power before phosphene experience and low-alpha power was lower in PDD participants when compared to controls. When separating the PDD hallucinators from the non-hallucinators, a significant effect of phosphene condition was found in high-alpha band (F(1,17)=7.79, p=0.013, η2=0.314). The posterior high-alpha band power was lower in the PDD hallucinator subgroup at both phosphene/no-phosphene conditions, but higher in the non-hallucinator subgroup when phosphenes were not experienced. Conclusions Decreased neural excitability at low-alpha band within the visual cortex is a driver of phosphene experience in both control and PDD patients. However, our data suggests that patients that normally experience complex visual hallucinations are in constant state of decreased alpha power that may elicit this symptom.

Phosphorylation of Tau protein correlates with changes in hippocampal theta oscillations and reduces hippocampal excitability in Alzheimer's model

Tau hyperphosphorylation at several sites, including those close to the microtubule domain region (MDr), is considered a key pathological event in the development of Alzheimer's disease (AD). Recent studies indicate that at the very early stage of this disease, increased phosphorylation in Tau's MDr domain correlates with reduced levels of neuronal excitability. Mechanistically, we show that pyramidal neurons and some parvalbumin-positive interneurons in 1-month-old triple-transgenic AD mice accumulate hyperphosphorylated Tau protein and that this accumulation correlates with changes in theta oscillations in hippocampal neurons. Pyramidal neurons from young triple-transgenic AD mice exhibited less spike accommodation and power increase in subthreshold membrane oscillations. Furthermore, triple-transgenic AD mice challenged with the potassium channel blocker 4-aminopyridine had reduced theta amplitude compared with 4-aminopyridine-treated control mice and, unlike these controls, displayed no seizure-like activity after this challenge. Collectively, our results provide new insights into AD pathogenesis and suggest that increases in Tau phosphorylation at the initial stages of the disease represent neuronal responses that compensate for brain circuit overexcitation.

Lactate in the brain: from metabolic end-product to signalling molecule.

Lactate in the brain has long been associated with ischaemia; however, more recent evidence shows that it can be found there under physiological conditions. In the brain, lactate is formed predominantly in astrocytes from glucose or glycogen in response to neuronal activity signals. Thus, neurons and astrocytes show tight metabolic coupling. Lactate is transferred from astrocytes to neurons to match the neuronal energetic needs, and to provide signals that modulate neuronal functions, including excitability, plasticity and memory consolidation. In addition, lactate affects several homeostatic functions. Overall, lactate ensures adequate energy supply, modulates neuronal excitability levels and regulates adaptive functions in order to set the 'homeostatic tone' of the nervous system.

Altering neuronal excitability to preserve network connectivity in a computational model of Alzheimer's disease.

Neuronal hyperactivity and hyperexcitability of the cerebral cortex and hippocampal region is an increasingly observed phenomenon in preclinical Alzheimer’s disease (AD). In later stages, oscillatory slowing and loss of functional connectivity are ubiquitous. Recent evidence suggests that neuronal dynamics have a prominent role in AD pathophysiology, making it a potentially interesting therapeutic target. However, although neuronal activity can be manipulated by various (non-)pharmacological means, intervening in a highly integrated system that depends on complex dynamics can produce counterintuitive and adverse effects. Computational dynamic network modeling may serve as a virtual test ground for developing effective interventions. To explore this approach, a previously introduced large-scale neural mass network with human brain topology was used to simulate the temporal evolution of AD-like, activity-dependent network degeneration. In addition, six defense strategies that either enhanced or diminished neuronal excitability were tested against the degeneration process, targeting excitatory and inhibitory neurons combined or separately. Outcome measures described oscillatory, connectivity and topological features of the damaged networks. Over time, the various interventions produced diverse large-scale network effects. Contrary to our hypothesis, the most successful strategy was a selective stimulation of all excitatory neurons in the network; it substantially prolonged the preservation of network integrity. The results of this study imply that functional network damage due to pathological neuronal activity can be opposed by targeted adjustment of neuronal excitability levels. The present approach may help to explore therapeutic effects aimed at preserving or restoring neuronal network integrity and contribute to better-informed intervention choices in future clinical trials in AD.

S33 Network intervention modelling in Alzheimer’s disease

Objective This presentation aims to provide a personal perspective on how brain connectivity research may get closer to obtaining a clinical role. I will argue that network intervention modeling, which unites the strengths of network analysis and computational modeling, is a great candidate for this purpose, as it can offer an attractive test environment in which positive and negative influences on network integrity can be explored, with the ultimate aim to find effective countermeasures against neurodegenerative network damage. Recent evidence suggests that neuronal hyperactivity has a prominent role in AD pathophysiology, making it a potentially interesting therapeutic target. However, although neuronal activity can be manipulated by various (non-)pharmacological means, intervening in a highly integrated system that depends on complex dynamics can produce counterintuitive or adverse effects. Dynamic network modeling may serve as a virtual test ground for developing effective interventions. Methods To explore this approach, a large-scale neural mass model with human brain topology was used to simulate the temporal evolution of AD-like, activity-dependent network degeneration. In addition, defense strategies that enhance or diminish neuronal excitability were tested against the degeneration process, targeting excitatory and inhibitory neurons combined or separately. Outcome measures consisted of various quantitative markers describing oscillatory, connectivity and topological features of the network. Results Over time, the various interventions produced diverse large-scale network effects. Contrary to our hypothesis, the most successful strategy was a selective stimulation of all excitatory neurons in the network; it substantially prolonged the preservation of network integrity. Conclusion The results of this study imply that functional network damage due to pathological neuronal activity in AD can be opposed by adjusting neuronal excitability levels. Significance The present approach may help to explore and explain therapeutic effects aimed at preserving or restoring neuronal network integrity, and thereby contribute to making better-informed intervention choices in future clinical trials.

Action potential initiation in a two-compartment model of pyramidal neuron mediated by dendritic Ca2+ spike

Dendritic Ca2+ spike endows cortical pyramidal cell with powerful ability of synaptic integration, which is critical for neuronal computation. Here we propose a two-compartment conductance-based model to investigate how the Ca2+ activity of apical dendrite participates in the action potential (AP) initiation to affect the firing properties of pyramidal neurons. We have shown that the apical input with sufficient intensity triggers a dendritic Ca2+ spike, which significantly boosts dendritic inputs as it propagates to soma. Such event instantaneously shifts the limit cycle attractor of the neuron and results in a burst of APs, which makes its firing rate reach a plateau steady-state level. Delivering current to two chambers simultaneously increases the level of neuronal excitability and decreases the threshold of input-output relation. Here the back-propagating APs facilitate the initiation of dendritic Ca2+ spike and evoke BAC firing. These findings indicate that the proposed model is capable of reproducing in vitro experimental observations. By determining spike initiating dynamics, we have provided a fundamental link between dendritic Ca2+ spike and output APs, which could contribute to mechanically interpreting how dendritic Ca2+ activity participates in the simple computations of pyramidal neuron.

Characterization of 4-Aminopyridine-induced network plasticity in rat cortical and human iPSC-derived neuronal cultures

Event Abstract Back to Event Characterization of 4-Aminopyridine-induced network plasticity in human iPSC-derived neuronal cultures Gaye Tanriöver1* and Katja Nieweg1 1 Phillips University, Institute of Pharmacology and Clinical Pharmacy, Germany Motivation Homeostatic network plasticity is a key regulatory mechanism for fine-tuning network activity. 4-aminopyridine (4-AP), which is commonly used to model epilepsy in vitro and in vivo has recently been described to trigger homeostatic plasticity in cultures of mouse hippocampal neurons [1]. The inhibition of several types of K+ channels (D-type and A-type K+ currents and a fraction of delayed rectifier currents) by 4-AP leads to prolonged action potentials and increased neurotransmitter release [2]. This induces hyperactivity, which is counteracted by mechanisms leading to decreased neuronal excitability. Human induced pluripotent stem cell (hiPSC)-derived neurons have been proposed to be a highly valuable in vitro model to be used in neurotoxicology, drug screening and regenerative medicine. Since spontaneous synchronous network activity develops in hiPSC-derived neuronal networks, our aim was to study the capacity of hiPSC-derived neuronal networks to exhibit homeostatic plasticity in response to 4-AP. Material and Methods hiPSCs were differentiated to cortical neurons as described previously [3]. Fifty thousand cells were plated after enrichment of neurons by immunopurification. After long-term (2-3 months) culture of the neurons for maturation, networks were transferred on the PO and laminin-coated 60-electrode multi-electrode arrays (Multichannel Systems, 200 µm electrode distance, 30µm electrode diameter) (MEAs) for recording. Calcium imaging was performed using Fura-2-AM ratiometric dye to identify single cell activity. Cells were loaded with 3µM Fura-2 AM for 20 min at 37°C in HEPES buffer. Fluorescence intensities at the two wavelengths (F340 and F380) were recorded separately and combined (fluorescence ratio: r=F340/F380) after background subtraction. Cells were imaged before and during 4-AP treatment as well as after removal of 4-AP. Neuronal networks with spontaneous network activity were incubated with 4-AP (100 µM) in order to trigger hyperactivity and measured during 4 days of 4-AP treatment as well as post-washout to monitor homeostatic plasticity. Results We modified our previously published [3] protocol for immunopanning of human iPSC derived neurons in order to permit differentiation of astrocytes from persisting neural precursor cells. Thus, we were able to generate mature, spontaneously active cortical like astrocyte-neuronal networks, shown by the presence of synchronous calcium transients around two months after immunoisolation. Analysis of network maturation was performed by culturing on 60-electrode MEAs. Single spikes and spike trains could be detected long before the occurrence of synchronous network bursts, which appeared two to three months after immunoisolation. To test, whether 4-AP triggered hyperactivity can induce homeostatic plasticity in human iPSC derived neuronal networks as it has been described for mouse hippocampal neurons [1], we treated human neurons of different maturational stages with 100µM 4-AP for 2-4 days and analyzed spontaneous network activity by MEA and calcium imaging. Mature, bursting human networks responded to 4-AP treatment with increased spiking and bursting rates (Fig. 1). Within the first 24h of treatment, mechanisms of homeostatic plasticity lead to downregulation of the activity close to baseline levels. One day after 4-AP removal, we observed that the overall network activity was strongly reduced even below baseline levels. The activity slowly recovered during the following days. Calcium imaging, which permits analyzing the activity of single cells within a network revealed similar results in mature human networks. However, immature human cultures without synchronous bursting activity did not respond to 4-AP and thus homeostatic plasticity could not be triggered in these cultures. Discussion 4-AP treatment of dissociated mouse hippocampal neurons cultured on MEAs was described, as a model to study homeostatic network plasticity by Pozzi et al. [1]. They showed that 4-AP induced hyperactivity lead to downregulation of neuronal excitability in mouse cultures. In our study, we applied this model to hiPSC derived neuronal networks and could show, that the maturational status of the network is a crucial parameter. Only spontaneously bursting cultures were able to respond in the expected manner. Whether downregulation of overall network activity occurs at the level of neuronal excitability or by synaptic downscaling will be analyzed by patch clamp recording. Conclusion In our study, we could show that mature hiPSC-derived neuronal networks cultured on MEAs are a valuable model to investigate mechanisms of homeostatic network plasticity in vitro. Since homeostatic plasticity is impaired in many psychiatric disorders and also at early stages of neurodegenerative diseases, a combination of patient derived iPSCs and the 4-AP plasticity model could serve as an innovative platform for drug screening.

Efficacy and Time Course of Theta Burst Stimulation in Healthy Humans.

In the past decade research has shown that continuous (cTBS) and intermittent theta burst stimulation (iTBS) alter neuronal excitability levels in the primary motor cortex. Quantitatively review the magnitude and time course on cortical excitability of cTBS and iTBS. Sixty-four TBS studies published between January 2005 and October 2014 were retrieved from the scientific search engine PubMED and included for analyses. The main inclusion criteria involved stimulation of the primary motor cortex in healthy volunteers with no motor practice prior to intervention and motor evoked potentials as primary outcome measure. ITBS applied for 190s significantly increases cortical excitability up to 60min with a mean maximum potentiation of 35.54±3.32%. CTBS applied for 40s decreases cortical excitability up to 50min with a mean maximum depression of-22.81±2.86%, while cTBS applied for 20s decreases cortical excitability (mean maximum-27.84±4.15%) for 20min. The present findings offer normative insights into the magnitude and time course of TBS-induced changes in cortical excitability levels.

Electrical control of epilepsy.

Epilepsy afflicts approximately 1-2% of the world's population. The mainstay therapy for treating the chronic recurrent seizures that are emblematic of epilepsy are drugs that manipulate levels of neuronal excitability in the brain. However, approximately one-third of all epilepsy patients get little to no clinical relief from this therapeutic regimen. The use of electrical stimulation in many forms to treat drug-refractory epilepsy has grown markedly over the past few decades, with some devices and protocols being increasingly used as standard clinical treatment. This article seeks to review the fundamental modes of applying electrical stimulation-from the noninvasive to the nominally invasive to deep brain stimulation-for the control of seizures in epileptic patients. Therapeutic practices from the commonly deployed clinically to the experimental are discussed to provide an overview of the innovative neural engineering approaches being explored to treat this difficult disease.

Timing control by redundant inhibitory neuronal circuits

Rhythms and timing control of sequential activity in the brain is fundamental to cognition and behavior. Although experimental and theoretical studies support the understanding that neuronal circuits are intrinsically capable of generating different time intervals, the dynamical origin of the phenomenon of functionally dependent timing control is still unclear. Here, we consider a new mechanism that is related to the multi-neuronal cooperative dynamics in inhibitory brain motifs consisting of a few clusters. It is shown that redundancy and diversity of neurons within each cluster enhances the sensitivity of the timing control with the level of neuronal excitation of the whole network. The generality of the mechanism is shown to work on two different neuronal models: a conductance-based model and a map-based model.

Region‐specific expression of a water channel protein, aquaporin 4, on brain astrocytes

Cellular activities within the brain display regional specificity and a neuronal and glia interdependence. Components characterizing the regional specificity of neurons have been identified. However, characterization of the astrocyte remains in question. To identify region specific features of astrocytes, we have characterized the molecular phenotype of cells derived from regions with different levels of neuronal excitability, the cortex and striatum. Astrocytes were identified in cryostat sections of adult rat brain by rapid immunostaining for glial fibrillary acidic protein (GFAP), and individual cells were collected from each region by using laser microdissection (LMD). Total RNA was isolated and subjected to DNA microarray analysis. At least eight genes showed a differential expression level. Among them, aquaporin 4 (AQP4), a water channel protein, was expressed at higher levels within the cortex compared with the striatum, as confirmed by immunohistochemistry. Primary cultured astrocytes isolated from rat cortex or striatum also showed a differential expression of AQP4. These data may reflect unique properties of astrocytes across different brain regions. However, they may also reflect the interactive demands of neurons with different activity levels. Further examination of the heterogeneous astrocyte populations within each region will lend additional support to the regional specificity of neuronal functions and neuronal-glial interactions.

Enhanced Sensitivity to Ethanol-Induced Inhibition of LTP in CA1 Pyramidal Neurons of Socially Isolated C57BL/6J Mice: Role of Neurosteroids

Ethanol (EtOH) induced impairment of long-term potentiation (LTP) in the rat hippocampus is prevented by the 5α-reductase inhibitor finasteride, suggesting that this effect of EtOH is dependent on the increased local release of neurosteroids such as 3α,5α-THP that promote GABA-mediated transmission. Given that social isolation (SI) in rodents is associated with altered plasma and brain levels of such neurosteroids as well as with an enhanced neurosteroidogenic action of EtOH, we examined whether the inhibitory effect of EtOH on LTP at CA3–CA1 hippocampal excitatory synapses is altered in C57BL/6J mice subjected to SI for 6 weeks in comparison with group-housed (GH) animals. Extracellular recording of field excitatory postsynaptic potentials (fEPSPs) as well as patch-clamp analysis were performed in hippocampal slices prepared from both SI and GH mice. Consistent with previous observations, recording of fEPSPs revealed that the extent of LTP induced in the CA1 region of SI mice was significantly reduced compared with that in GH animals. EtOH (40 mM) inhibited LTP in slices from SI mice but not in those from GH mice, and this effect of EtOH was abolished by co-application of 1 μM finasteride. Current-clamp analysis of CA1 pyramidal neurons revealed a decrease in action potential (AP) frequency and an increase in the intensity of injected current required to evoke the first AP in SI mice compared with GH mice, indicative of a decrease in neuronal excitability associated with SI. Together, our data suggest that SI results in reduced levels of neuronal excitability and synaptic plasticity in the hippocampus. Furthermore, the increased sensitivity to the neurosteroidogenic effect of EtOH associated with SI likely accounts for the greater inhibitory effect of EtOH on LTP in SI mice. The increase in EtOH sensitivity induced by SI may be important for the changes in the effects of EtOH on anxiety and on learning and memory associated with the prolonged stress attributable to SI.

Differential influence of non-synaptic mechanisms in two in vitro models of epileptic field bursts

Non-synaptic interactions are known to promote epileptiform activity through mechanisms that have primarily been studied in one particular in vitro model (low Ca 2+ model). Here we characterize another non-synaptic model, where ictal-like field bursts are induced in the CA1 area of rat hippocampal slices by exposure to Cs + (4–5 mM) together with blockers of fast chemical synaptic transmission, and compare it with the low Ca 2+ model. The Cs-induced field bursts were blocked by 1 µM tetrodotoxin, but persisted in the presence of 200 µM Cd 2+ or 300 µM Ni 2+. Hyperosmotic condition (addition of 30 mM sucrose), reduced burst amplitude, but, unlike field bursts induced by 0 mM Ca 2+/5.25 mM K +, sucrose had no effect on frequency or duration. Intracellular alkalinization–acidification sequence induced by NH 4Cl potentiated and blocked, respectively, the field bursts. Octanol (100–250 µM) blocked all activity in most experiments. A quantitative comparison of three gap junction antagonists (carbenoxolone (100 µM), quinidine (100–250 µM), and endothelin-3 (1–2 µM)) indicated that gap junction communication is implicated in both models. However, endothelin-3 had selective effect on the low Ca 2+-induced field burst. The data suggest that extracellular space-dependent processes, including field effects, significantly contribute to ongoing field burst activity, whereas initiation of a field burst can occur with or without the aid of such interactions, depending on the level of neuronal excitability. Gap junctions seem to have a general role in initiating field bursts. However, the contribution to this effect from neuronal versus glial connexin types differs in the two epileptic models studied.

Molecular Mechanisms Underlying Membrane Potential-Mediated Regulation of Neuronal K2P2.1 Channels

The activity of background K2P channels adjusts the resting membrane potential to enable plasticity of excitable cells. Here we have studied the regulation of neuronal human K2P2.1 (KCNK2, TREK-1) channel activity by resting membrane potential. When heterologously expressed in Xenopus laevis oocytes, K2P2.1 currents gradually increased several fold at hyperpolarizing potentials and declined several fold at depolarizing potentials, with a midpoint potential of −60 mV. As K2P channels are not equipped with an integral voltage sensor, we sought extrinsic cellular components that could convert changes in the membrane electrical field to cellular activity that would indirectly modify K2P2.1 currents. K2P2.1 voltage sensitivity was found not to be mediated by the activity of either voltage activated calcium channels, the Xenopus voltage sensitive proton channel (Xl-Hv) or the Xenopus voltage sensor-containing phosphatase (Xl-VSP). On the other hand, we report that membrane depolarization activated the Gq protein-coupled receptor pathway, in the apparent absence of ligand, resulting in phosphatidylinositol-4,5-bisphosphate (PIP2) depletion through the action of phospholipase C. Our results suggest a novel mechanism in which an indirect pathway confers membrane potential regulation onto channels that are not intrinsically voltage-sensitive to enhance regulation of neuronal excitability levels. The ability of these proteins to operate without any external ligand enhances plasticity at the single cell level, independent of higher regulatory pathways at the tissue or even the organism levels.

Plasticity of the α4βδ GABAA receptor

The GABAR [GABA(A) (gamma-aminobutyric acid type A) receptor], which mediates most inhibition in the brain, is regulated homoeostatically to maintain an optimal level of neuronal excitability. In particular, the alpha(4)betadelta subtype of the GABAR plays a pivotal role in this regulation. This receptor, which is expressed extrasynaptically on the dendrites, normally has low expression in the brain, but displays a remarkable degree of plasticity. It can also be a sensitive target for endogenous neurosteroids such as THP (3alpha-hydroxy-5[alpha]beta-pregnan-20-one (allo-pregnanolone); a neurosteroid and positive modulator of the GABAR), which is released during stress, although the effect of the steroid is polarity-dependent, such that it increases inward current, but decreases outward current, at alpha(4)beta(2)delta GABAR. Expression of alpha(4)beta(2)delta GABAR in CA1 hippocampus is also tightly regulated by fluctuating levels of neurosteroids, as seen at the onset of puberty. Declining levels of inhibition resulting from the decrease in THP at puberty are compensated for by an increase in alpha(4)betadelta GABAR along the apical dendrites of CA1 hippocampal pyramidal cells, which reduces neuronal excitability by decreasing the input resistance. However, excessive decrease of neuronal function is averted when THP levels rise, as would occur during stress, because this steroid decreases the outward GABAergic tonic current via inhibition of alpha(4)beta(2)delta GABAR, thereby restoring measures of neuronal excitability to pre-pubertal levels. Thus the homoeostatic regulation of alpha(4)betadelta GABAR expression plays an important role in maintaining ambient levels of neuronal excitability at puberty.