Glial ion channels

The effects of electrical stimulation on glial cell behaviour

Neural interface devices interact with the central nervous system (CNS) to substitute for some sort of functional deficit and improve quality of life for persons with disabilities. Design of safe, biocompatible neural interface devices is a fast-emerging field of neuroscience research. Development of invasive implant materials designed to directly interface with brain or spinal cord tissue has focussed on mitigation of glial scar reactivity toward the implant itself, but little exists in the literature that directly documents the effects of electrical stimulation on glial cells. In this review, a survey of studies documenting such effects has been compiled and categorized based on the various types of stimulation paradigms used and their observed effects on glia. A hybrid neuroscience cell biology-engineering perspective is offered to highlight considerations that must be made in both disciplines in the development of a safe implant. To advance knowledge on how electrical stimulation affects glia, we also suggest experiments elucidating electrochemical reactions that may occur as a result of electrical stimulation and how such reactions may affect glia. Designing a biocompatible stimulation paradigm should be a forefront consideration in the development of a device with improved safety and longevity.

Crosstalk between Neuron and Glial Cells in Oxidative Injury and Neuroprotection

To counteract oxidative stress and associated brain diseases, antioxidant systems rescue neuronal cells from oxidative stress by neutralizing reactive oxygen species and preserving gene regulation. It is necessary to understand the communication and interactions between brain cells, including neurons, astrocytes and microglia, to understand oxidative stress and antioxidant mechanisms. Here, the role of glia in the protection of neurons against oxidative injury and glia–neuron crosstalk to maintain antioxidant defense mechanisms and brain protection are reviewed. The first part of this review focuses on the role of glia in the morphological and physiological changes required for brain homeostasis under oxidative stress and antioxidant defense mechanisms. The second part focuses on the essential crosstalk between neurons and glia for redox balance in the brain for protection against oxidative stress.

Juvenile hormone receptor Met regulates sleep and neuronal morphology via glial-neuronal crosstalk

Juvenile hormone (JH) is one of the most important hormones in insects since it is essential for insect development. The mechanism by which JH affects the central nervous system still remains a mystery. In this study, we demonstrate that one of the JH receptors, Methoprene-tolerant (Met), is important for the control of neurite development and sleep behavior in Drosophila. With the identification of Met-expressing glial cells, the mechanism that Met negatively controls the mushroom body (MB) β lobes fusion and positively maintains pigment-dispersing factor sLNvs projection pruning has been established. Furthermore, despite the developmental effects, Met can also maintain nighttime sleep in a development-independent manner through the α/β lobe of MB. Combining analyses of neuronal morphology and entomological behavior, this study advances our understanding of how the JH receptor regulates the nervous system.

Astrocytes in the Ventromedial Hypothalamus Involve Chronic Stress-Induced Anxiety and Bone Loss in Mice

Chronic stress is one of the main risk factors of bone loss. While the neurons and neural circuits of the ventromedial hypothalamus (VMH) mediate bone loss induced by chronic stress, the detailed intrinsic mechanisms within the VMH nucleus still need to be explored. Astrocytes in brain regions play important roles in the regulation of metabolism and anxiety-like behavior through interactions with surrounding neurons. However, whether astrocytes in the VMH affect neuronal activity and therefore regulate chronic stress-induced anxiety and bone loss remain elusive. In this study, we found that VMH astrocytes were activated during chronic stress-induced anxiety and bone loss. Pharmacogenetic activation of the Gi and Gq pathways in VMH astrocytes reduced and increased the levels of anxiety and bone loss, respectively. Furthermore, activation of VMH astrocytes by optogenetics induced depolarization in neighboring steroidogenic factor-1 (SF-1) neurons, which was diminished by administration of N-methyl-D-aspartic acid (NMDA) receptor blocker but not by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blocker. These results suggest that there may be a functional "glial-neuron microcircuit" in VMH nuclei that mediates anxiety and bone loss induced by chronic stress. This study not only advances our understanding of glial cell function but also provides a potential intervention target for chronic stress-induced anxiety and bone loss therapy.

Drosophila ClC-a is required in glia of the stem cell niche for proper neurogenesis and wiring of neural circuits

Glial cells form part of the neural stem cell niche and express a wide variety of ion channels; however, the contribution of these channels to nervous system development is poorly understood. We explored the function of the Drosophila ClC‐a chloride channel, since its mammalian ortholog CLCN2 is expressed in glial cells, and defective channel function results in leukodystrophies, which in humans are accompanied by cognitive impairment. We found that ClC‐a was expressed in the niche in cortex glia, which are closely associated with neurogenic tissues. Characterization of loss‐of‐function ClC‐a mutants revealed that these animals had smaller brains and widespread wiring defects. We showed that ClC‐a is required in cortex glia for neurogenesis in neuroepithelia and neuroblasts, and identified defects in a neuroblast lineage that generates guidepost glial cells essential for photoreceptor axon guidance. We propose that glia‐mediated ionic homeostasis could nonautonomously affect neurogenesis, and consequently, the correct assembly of neural circuits.

Remembering Ben Barres

Ben Barres, who was at the heart of glial cell physiology for over 30 years, died aged 63 on December 27, 2017.[...]

Extracellular Electrophysiological Measurements of Cooperative Signals in Astrocytes Populations

Astrocytes are neuroglial cells that exhibit functional electrical properties sensitive to neuronal activity and capable of modulating neurotransmission. Thus, electrophysiological recordings of astroglial activity are very attractive to study the dynamics of glial signaling. This contribution reports on the use of ultra-sensitive planar electrodes combined with low noise and low frequency amplifiers that enable the detection of extracellular signals produced by primary cultures of astrocytes isolated from mouse cerebral cortex. Recorded activity is characterized by spontaneous bursts comprised of discrete signals with pronounced changes on the signal rate and amplitude. Weak and sporadic signals become synchronized and evolve with time to higher amplitude signals with a quasi-periodic behavior, revealing a cooperative signaling process. The methodology presented herewith enables the study of ionic fluctuations of population of cells, complementing the single cells observation by calcium imaging as well as by patch-clamp techniques.

Neuroprotective potential of astroglia

Astroglia are the homoeostatic cells of the central nervous system, which participate in all essential functions of the brain. Astrocytes support neuronal networks by handling water and ion fluxes, transmitter clearance, provision of antioxidants, and metabolic precursors and growth factors. The critical dependence of neurons on constant support from the astrocytes confers astrocytes with intrinsic neuroprotective properties. On the other hand, loss of astrocytic support or their pathological transformation compromises neuronal functionality and viability. Manipulating neuroprotective functions of astrocytes is thus an important strategy to enhance neuronal survival and improve outcomes in disease states. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Glutamate-Induced White Matter Injury: Excitotoxicity without Synapses

White matter of the brain and spinal cord is irreversibly damaged by ischemia and trauma. Recent evidence indicates that despite the absence of synaptic elements, excitotoxic mechanisms play an important role in the pathogenesis of white matter damage. Glial cells, including astrocytes and oligodendrocytes, possess non-NMDA glutamate receptors and are injured by excessive exposure to AMPA/kainate agonists. In addition, the myelin sheath itself appears to respond directly to glutamate stimulation via AMPA receptors, which may also lead to injury of this key constituent of myelinated axons. During white matter anoxia/ischemia or trauma, endogenous glutamate is released mainly from axoplasmic pools in a nonvesicular fashion through Na+-dependent glutamate transporters, stimulated to operate in the glutamate efflux mode by collapse of transmembrane ion gradients and depolarization. It appears that parallel mechanisms are triggered by injurious stimuli, involving reverse Na+-Ca2+ exchange and voltage-gated Ca2+ channels producing Ca2+ overload of the axon cylinder, whereas glutamate release with AMPA receptor overactivation causes Ca2+-dependent damage to the ensheathing myelin and sup-porting glia. The emerging complexity of white matter injury mechanisms requires a thorough understanding of the interrelated steps to optimize therapeutic design.

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.

The role of reactive species in epileptogenesis and influence of antiepileptic drug therapy on oxidative stress

Epilepsy is considered one of the most common neurological disorders. The focus of this review is the acquired form of epilepsy, with the development process consisting of three major phases, the acute injury phase, the latency epileptogenesis phase, and the phase of spontaneous recurrent seizures. Nowadays, an increasing attention is paid to the possible interrelationship between oxidative stress resulting in disturbance of physiological signalling roles of calcium and free radicals in neuronal cells and mitochondrial dysfunction, cell damage, and epilepsy. The positive stimulation of mitochondrial calcium signals by reactive oxygen species and increased reactive oxygen species generation resulting from increased mitochondrial calcium can lead to a positive feedback loop. We propose that calcium can pose both, physiological and pathological effects of mitochondrial function, which can lead in neuronal cell death and consequent epileptic seizures. Various antiepileptic drugs may impair the endogenous antioxidative ability to prevent oxidative stress. Therefore, some antiepileptic drugs, especially from the older generation, may trigger oxygen-dependent tissue injury. The prooxidative effects of these antiepileptic drugs might lead to enhancement of seizure activity, resulting in loss of their efficacy or apparent functional tolerance and undesired adverse effects. Additionally, various reactive metabolites of antiepileptic drugs are capable of covalent binding to macromolecules which may lead to deterioration of the epileptic seizures and systemic toxicity. Since neuronal loss seems to be one of the major neurobiological abnormalities in the epileptic brain, the ability of antioxidants to attenuate seizure generation and the accompanying changes in oxidative burden, further support an important role of antioxidants as having a putative antiepileptic potential.

Emerging role of glial cells in the control of body weight

Glia are the most abundant cell type in the brain and are indispensible for the normal execution of neuronal actions. They protect neurons from noxious insults and modulate synaptic transmission through affectation of synaptic inputs, release of glial transmitters and uptake of neurotransmitters from the synaptic cleft. They also transport nutrients and other circulating factors into the brain thus controlling the energy sources and signals reaching neurons. Moreover, glia express receptors for metabolic hormones, such as leptin and insulin, and can be activated in response to increased weight gain and dietary challenges. However, chronic glial activation can be detrimental to neurons, with hypothalamic astrocyte activation or gliosis suggested to be involved in the perpetuation of obesity and the onset of secondary complications. It is now accepted that glia may be a very important participant in metabolic control and a possible therapeutical target. Here we briefly review this rapidly advancing field.

Membrane properties of neuron-like cells generated from adult human bone-marrow-derived mesenchymal stem cells

Adult mesenchymal stem cells (MeSCs) isolated from human bone marrow are capable of generating neural stem cell (NSC)-like cells that can be subsequently differentiated into cells expressing molecular markers for neurons. Here we report that these neuron-like cells had functional properties similar to those of brain-derived neurons. Whole-cell patch-clamp recordings and calcium imaging experiments were performed on neuron-like cells differentiated from bone-marrow-derived NSC-like cells. The neuron-like cells were subjected to current pulses to determine if they were capable of generating depolarization-induced action potentials. We found that nearly all of the cells with neuron-like morphology exhibited active membrane properties in response to the depolarizing pulses. The most common response was a single spike-like event with an overshoot and brief afterhyperpolarization. Cells that did not generate overshooting spike-like events usually displayed rectifying current-voltage relationships. The prevalence of these active membrane properties in response to the depolarizing current pulses suggested that the human MeSCs (hMeSCs) were capable of converting to a neural lineage under defined culture conditions. The spike-like events were blocked by the voltage-gated sodium channel inhibitor lidocaine, but unaffected by another sodium channel inhibitor tetrodotoxin (TTX). In calcium imaging experiments, the neuron-like cells responded to potassium chloride depolarization and l-glutamate application with increases in the cytoplasmic calcium levels. Thus, the neuron-like cells appeared to express TTX-resistant voltage-gated sodium channels, voltage-gated calcium channels, and functional l-glutamate receptors. These results demonstrate that hMeSCs were capable of generating cells with characteristics typical of functional neurons that may prove useful for neuroreplacement therapies.

Altered functional properties of satellite glial cells in compressed spinal ganglia

The cell bodies of sensory neurons in the dorsal root ganglion (DRG) are enveloped by satellite glial cells (SGCs). In an animal model of intervertebral foraminal stenosis and low-back pain, a chronic compression of the DRG (CCD) increases the excitability of neuronal cell bodies in the compressed ganglion. The morphological and electrophysiological properties of SGCs were investigated in both CCD and uninjured, control lumbar DRGs. SGCs responded within 12 h of the onset of CCD as indicated by an increased expression of glial fibrillary acidic protein (GFAP) in the compressed DRG but to lesser extent in neighboring or contralateral DRGs. Within 1 week, coupling through gap junctions between SGCs was significantly enhanced in the compressed ganglion. Under whole-cell patch clamp recordings, inward and outward potassium currents, but not sodium currents, were detected in individual SGCs. SGCs enveloping differently sized neurons had similar electrophysiological properties. SGCs in the compressed vs. control DRG exhibited significantly reduced inwardly rectifying potassium currents (Kir), increased input resistances and positively shifted resting membrane potentials. The reduction in Kir was greater for nociceptive medium-sized neurons compared to non-nociceptive neurons. Kir currents of SGCs around spontaneously active neurons were significantly reduced 1 day after compression but recovered by 7 days. These data demonstrate rapid alterations in glial membrane currents and GFAP expression in close temporal association with the development of neuronal hyperexcitability in the CCD model of neuropathic pain. However, these alterations are not fully sustained and suggest other mechanisms for the maintenance of the hyperexcitable state.

Increase of intracellular Ca2+ by P2X and P2Y receptor-subtypes in cultured cortical astroglia of the rat

Astrocytes express purinergic receptors that are involved in glial-neuronal cell communication. Experiments were conducted to characterize the expression of functional P2X/P2Y nucleotide receptors in glial cells of mixed cortical cell cultures of the rat. The vast majority of these cells was immunopositive for glial fibrillary acidic protein (GFAP) and was considered therefore astrocyte-like; for the sake of simplicity they were termed "astroglia" throughout. Astroglia expressed predominantly P2X(4,6,7) as well as P2Y(1,2) receptor-subtypes. Less intensive immunostaining was also found for P2X(5) and P2Y(4,6,13,14) receptors. Pressure application of ATP and a range of agonists selective for certain P2X or P2Y receptor-subtypes caused a concentration-dependent increase of intracellular Ca(2+) ([Ca(2+)](i)). Of the agonists tested, only the P2X(1,3) receptor-selective alpha,beta-methylene ATP was ineffective. Experiments with Ca(2+)-free solution and cyclopiazonic acid, an inhibitor of the endoplasmic Ca(2+)-ATPase, indicated that the [Ca(2+)](i) response to most nucleotides, except for ATP and 2',3'-O-(benzoyl-4-benzoyl)-ATP, was due primarily to the release of Ca(2+) from intracellular stores. A Gprotein-mediated release of Ca(2+) is the typical signaling mechanism of various P2Y receptor-subtypes, whose presence was confirmed also by cross-desensitization experiments and by using selective antagonists. Thus, our results provide direct evidence that astroglia in mixed cortical cell cultures express functional P2Y (P2Y(1,2,6,14) and probably also P2Y(4)) receptors. Several unidentified P2X receptors, including P2X(7), may also be present, although they appear to only moderately participate in the regulation of [Ca(2+)](i). The rise of [Ca(2+)](i) is due in this case to the transmembrane flux of Ca(2+) via the P2X receptor-channel. In conclusion, P2Y rather than P2X receptor-subtypes are involved in modulating [Ca(2+)](i) of cultured astroglia and thereby may play an important role in cell-to-cell signaling.

Metabotropic glutamate receptors in the tripartite synapse as a target for new psychotropic drugs

Mental disorders, such as depression, anxiety and schizophrenia, has become a large medical and social problem recently. Studies performed in animal tests and early clinical investigations brought a new insight in the pharmacotherapy of these disorders. Latest investigations are focused mainly on the glutamatergic system, a main excitatory amino acid neurotransmitter in the brain. Evidence indicates that metabotropic glutamate receptors ligands have excellent antidepressant, anxiolytic and antipsychotic effects. Metabotopic glutamate receptors (mGlu) divaded into three groups (group I, II and III) are localized on nerve terminals, postsynaptic sites and glial cells and thus they can influence and modulate the action of glutamate on different levels in the synapse. Recent advances in the identification of selective and specific compounds (both ortho- and allosteric ligands), and the generation of transgenic animals enabled to have new insight into the pathophysiology and therapy of mood disorders. At present, the most potent seem to be negative allosteric modulators of the first group (mGlu1 and mGlu5), and positive allosteric modulators of the second (mGlu2 and mGlu3) and third (mGlu4/7/8) group of mGlu receptors.

Estradiol stimulates progesterone synthesis in hypothalamic astrocyte cultures

The brain synthesizes steroids de novo, especially progesterone. Recently estradiol has been shown to stimulate progesterone synthesis in the hypothalamus and enriched astrocyte cultures derived from neonatal cortex. Estradiol-induced hypothalamic progesterone has been implicated in the control of the LH surge. The present studies were undertaken to determine whether hypothalamic astrocytes derived from female neonatal or female postpubertal rats increased production of progesterone in response to an estradiol challenge. Estradiol induced progesterone synthesis in postpubertal astrocytes but not neonatal astrocytes. This estradiol action was blocked by the estrogen receptor antagonist ICI 182,780. Previously we had demonstrated that estradiol stimulates a rapid increase in free cytosolic Ca(2+) ([Ca(2+)](i)) spikes in neonatal cortical astrocytes acting through a membrane estrogen receptor. We now report that estradiol also rapidly increased [Ca(2+)](i) spikes in hypothalamic astrocytes. The membrane-impermeable estradiol-BSA construct also induced [Ca(2+)](i) spikes. Both estradiol-BSA and estradiol were blocked by ICI 182,780. Depleting intracellular Ca(2+) stores prevented the estradiol-induced increased [Ca(2+)](i) spikes, whereas removing extracellular Ca(2+) did not prevent estradiol-induced [Ca(2+)](i) spikes. Together these results indicate that estradiol acts through a membrane-associated receptor to release intracellular stores of Ca(2+). Thapsigargin, used to mimicked the intracellular release of Ca(2+) by estradiol, increased progesterone synthesis, suggesting that estradiol-induced progesterone synthesis involves increases in [Ca(2+)](i). Estradiol treatment did not change levels of steroid acute regulatory protein, P450 side chain cleavage, 3beta-hydroxysteroid dehydrogenase, and sterol carrier protein-2 mRNAs as measured by quantitative RT-PCR, suggesting that in vitro, estradiol regulation of progesterone synthesis in astrocytes does not depend on transcription of new steroidogenic proteins. The present results are consistent with our hypothesis that estrogen-positive feedback regulating the LH surge involves stimulating local progesterone synthesis by hypothalamic astrocytes.

Glucosamine-induced increase in Akt phosphorylation corresponds to increased endoplasmic reticulum stress in astroglial cells

Increased glucose flux through the hexosamine biosynthetic pathway (HBP) is known to affect the activity of a number of signal transduction pathways and lead to insulin resistance. Although widely studied in insulin responsive tissues, the effect of increased HBP activity on largely insulin unresponsive tissues, such as the brain, remains relatively unknown. Herein, we investigate the effects of increased HBP flux on Akt activation in a human astroglial cells line using glucosamine, a compound commonly used to mimic hyperglycemic conditions by increasing HBP flux. Cellular treatment with 8 mM glucosamine resulted in a 96.8% +/- 24.6 increase in Akt phosphorylation after 5 h of treatment that remained elevated throughout the 9-h time course. Glucosamine treatment also resulted in modest increases in global levels of the O-GlcNAc protein modification. Increasing O-GlcNAc levels using the O-GlcNAcase inhibitor streptozotocin (STZ) also increased Akt phosphorylation by 96.8% +/- 11.0 after only 3 h although for a shorter duration than glucosamine; however, the more potent O-GlcNAcase inhibitors O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) and 1,2-dideoxy-2'-propyl-alpha-D-glucopyranoso-[2,1-d]-Delta2'-thiazoline (NAGBT) failed to mimic the increases in phospho-Akt indicating that the Akt phosphorylation is not a result of increased O-GlcNAc protein modification. Further analysis indicated that this increased phosphorylation was also not due to increased osmotic stress and was not attenuated by N-acetylcysteine eliminating the potential role of oxidative stress in the observed phospho-Akt increases. Glucosamine treatment, but not STZ treatment, did correlate with a large increase in the expression of the endoplasmic reticulum (ER) stress marker GRP 78. Altogether, these results indicate that increased HBP flux in human astroglial cells results in a rapid, short-term phosphorylation of Akt that is likely a result of increased ER stress. The mechanism by which STZ increases Akt phosphorylation, however, remains unknown.

Distribution, characterization and clinical significance of microglia in glioneuronal tumours from patients with chronic intractable epilepsy

Cells of the microglia/macrophage lineage represent an important component of different brain tumours. However, there is little information about the microglia/macrophage cell system in glioneuronal tumours and its possible contribution to the high epileptogenecity of these lesions. In the present study, the distribution of cells of the microglia/macrophage lineage was studied by immunocytochemistry for CD68 and human leucocyte antigen (HLA)-DR in a group of glioneuronal tumours, including gangliogliomas (GG, n = 30), and dysembryoplastic neuroepithelial tumours (DNT, n = 17), from patients with chronic intractable epilepsy. A significant number of microglia/macrophage cells were observed in the large majority of glioneuronal tumours, both within the tumour and in the peritumoral region. Activated microglial cells positive for HLA-DR were localized around blood vessels and clustered around tumour neuronal cells. The density of activated microglial cells correlated with the duration of epilepsy, as well as with the frequency of seizures prior to surgical resection. These observations indicate that the presence of cells of the microglial/macrophage cell system is a feature of glioneuronal tumours and is functionally related to epilepsy, either directly in epileptogenesis or through activation following seizure activity.

Neuropathic pain activates the endogenous kappa opioid system in mouse spinal cord and induces opioid receptor tolerance

Release of endogenous dynorphin opioids within the spinal cord after partial sciatic nerve ligation (pSNL) is known to contribute to the neuropathic pain processes. Using a phosphoselective antibody [kappa opioid receptor (KOR-P)] able to detect the serine 369 phosphorylated form of the KOR, we determined possible sites of dynorphin action within the spinal cord after pSNL. KOR-P immunoreactivity (IR) was markedly increased in the L4-L5 spinal dorsal horn of wild-type C57BL/6 mice (7-21 d) after lesion, but not in mice pretreated with the KOR antagonist nor-binaltorphimine (norBNI). In addition, knock-out mice lacking prodynorphin, KOR, or G-protein receptor kinase 3 (GRK3) did not show significant increases in KOR-P IR after pSNL. KOR-P IR was colocalized in both GABAergic neurons and GFAP-positive astrocytes in both ipsilateral and contralateral spinal dorsal horn. Consistent with sustained opioid release, KOR knock-out mice developed significantly increased tactile allodynia and thermal hyperalgesia in both the early (first week) and late (third week) interval after lesion. Similarly, mice pretreated with norBNI showed enhanced hyperalgesia and allodynia during the 3 weeks after pSNL. Because sustained activation of opioid receptors might induce tolerance, we measured the antinociceptive effect of the kappa agonist U50,488 using radiant heat applied to the ipsilateral hindpaw, and we found that agonist potency was significantly decreased 7 d after pSNL. In contrast, neither prodynorphin nor GRK3 knock-out mice showed U50,488 tolerance after pSNL. These findings suggest that pSNL induced a sustained release of endogenous prodynorphin-derived opioid peptides that activated an anti-nociceptive KOR system in mouse spinal cord. Thus, endogenous dynorphin had both pronociceptive and antinociceptive actions after nerve injury and induced GRK3-mediated opioid tolerance.

Epileptogenesis in the Dysplastic Brain: A Revival of Familiar Themes

Brain malformations are now widely recognized in many forms of epilepsy. To investigate how malformed brain regions participate in the generation of seizure activity researchers have focused on animal models. Here we describe recent advances in this field.

Dynamic signaling between astrocytes and neurons

Astrocytes, a sub-type of glia in the central nervous system, are dynamic signaling elements that integrate neuronal inputs, exhibit calcium excitability, and can modulate neighboring neurons. Neuronal activity can lead to neurotransmitter-evoked activation of astrocytic receptors, which mobilizes their internal calcium. Elevations in astrocytic calcium in turn trigger the release of chemical transmitters from astrocytes, which can cause sustained modulatory actions on neighboring neurons. Astrocytes, and perisynaptic Schwann cells, by virtue of their intimate association with synapses, are strategically positioned to regulate synaptic transmission. This capability, that has now been demonstrated in several studies, raises the untested possibility that astrocytes are an integral element of the circuitry for synaptic plasticity. Because the highest ratio of glia-to-neurons is found at the top of the phylogenetic tree in the human brain, these recent demonstrations of dynamic bi-directional signaling between astrocytes and neurons leave us with the question as to whether astrocytes are key regulatory elements of higher cortical functions.

Ionotropic and metabotropic glutamate receptor protein expression in glioneuronal tumours from patients with intractable epilepsy

Glioneuronal tumours are an increasingly recognized cause of chronic pharmaco-resistant epilepsy. In the present study the immunocytochemical expression of various glutamate receptor (GluR) subtypes was investigated in 41 gangliogliomas (GG) and 16 dysembryoplastic neuroepithelial tumours (DNT) from patients with intractable epilepsy. Immunocytochemistry with antibodies specific for ionotropic NR1, NR2A/B (NMDA) GluR1, GluR2 (AMPA), GluR5-7 (kainate), and metabotropic mGluR1, mGluR2-3, mGluR5, mGluR7a subtypes demonstrated in both GG and DNT the presence of an highly differentiated neuronal population, containing subunits from each receptor class. More than 50% of tumours contained a high percentage of neuronal cells immunolabelled for NMDA, AMPA and kainate receptor subunits. A high percentage of neurones showed strong expression of NR2A-B, which co-localized with NR1. Group I mGluRs (mGluR1 and mGluR5) were highly represented in the neuronal component of the tumours. Immunolabelling for several GluRs was also present in the glial component. Increased expression of mGluR2-3, mGluR5 and GluR5-7 was observed in reactive astrocytes in the perilesional zone compared to normal cortex. The neurochemical profile of glioneuronal tumours, with high expression of specific GluR subtypes, supports the central role of glutamatergic transmission in the mechanisms underlying the intrinsic and high epileptogenicity of these lesions.

Neuronal signals regulate neurotrophin expression in oligodendrocytes of the basal forebrain

Previous studies suggest that oligodendrocytes express trophic molecules, including neurotrophins. These molecules have been shown to influence nearby neurons. To determine whether neuronal signals may, in turn, affect oligodendrocyte-derived trophins, we examined regulation of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) mRNA expression in cultured oligodendrocytes of the basal forebrain. Neuronal signals had distinct effects on individual neurotrophins. KCl elicited increases in BDNF mRNA, but did not affect expression of NGF or NT-3. The cholinergic agonist, carbachol, increased expression of NGF, but did not affect expression of BDNF or NT-3. Glutamate elicited a decrease in BDNF, but did not affect expression of NGF or NT-3. This glutamate effect is not due to toxicity, since the number of total cells was unchanged, while the number of mature myelin basic protein positive (MBP+) cells increased. Our observations suggest that individual neuronal signals distinctly influence the trophic function of oligodendrocytes.

A novel barium-sensitive calcium influx into rat astrocytes at low external potassium

Cultured rat cerebellar astrocytes, loaded with the Ca2+-sensitive fluorescent dyes Fura-2 or Fluo-3, responded with cytoplasmic Ca2+ transients, when the external K+ concentration was reduced from 5 mM to below 1 mM. Ca2+ transients were generated after changing to a saline containing 0.2 mM K+ in 82% of the cells (n =303) with a delay of up to 4 min. Cultured rat cortical neurones, which responded in high-K+ saline (50 mM) with Ca2+ transients, showed no Ca2+ responses in low K+ (n =22). In acute rat hippocampal brain slices, presumed glial cells responded with Ca2+ transients in low K+ similar to astrocytes in culture (88%, n =17). The Ca2+ transients were observed both in somatic and dendritic regions of cultured astrocytes, as examined with confocal laser scanning microscopy. Patch-clamped astrocytes hyperpolarized in 0.2 mM K+ from an average resting potential of -65 +/- 4 mV to -98 +/- 20 mV (n =15). The Ca2+ transients in low K+ were suppressed in Ca2+-free saline, buffered with 0.5 mM EGTA, but not after depletion of intracellular Ca2+ stores by thapsigargin, cyclopiazonic acid or by Ruthenium Red, indicating that they were due to Ca2+ influx into the cells, and not caused by intracellular Ca2+ release. The addition of different divalent cations revealed that Ba2+, but not Ni2+, Cd2+, Sr2+ or Mg2+, reversibly blocked the Ca2+ transients in low K+. There was a significant reduction of the Ca2+ responses at micromolar Ba2+ concentrations (Ki = 3.8 microM). The application of different K+ channel blockers, tetraethylammonium, dequalinium, tolbutamide, clotrimazole, or quinidine had no effect on the Ca2+ responses. Removal of external Na+, or intracellular acidification by the addition of 40 mM propionate to the saline, had also no influence on the generation of the Ca2+ transients. The results suggest that reducing the external K+ concentration elicits a Ca2+ influx into rat astrocytes which is highly sensitive to Ba2+. It is discussed that this Ca2+ influx might occur through K+ inward rectifier channels, which become Ca2+-permeable when the extracellular K+ concentration decreases to 1 mM or below.

Acutely isolated astrocytes as models to probe astrocyte functions

Neuroscientists have become increasingly aware and accepting of the concept that astrocytes likely have many important functions in the CNS. One limitation in establishing these functions is the usual problem of what constitutes suitable experimental approaches. A major experimental step for functional studies of astrocytes has been the widespread use of primary astrocyte cultures, an approach that Leif Hertz pioneered. However, it is now becoming clear that, building on this work, an experimental paradigm shift is now needed. Namely, to increasingly study preparations corresponding to in situ conditions, such as slices. An alternative experimental system where the cells have some of the technical advantages of primary astrocyte cultures is freshly isolated astrocytes. Recent experiments from our laboratory have shown metabotropic glutamate receptor expression by such cells. Examples are given of how functional receptor studies and channel activity measured by patch clamp electrophysiology can be combined with single cell RT-PCR to define further the receptor or channel type are described to illustrate the uses of such preparations.

Ion channel expression by astrocytes in situ: comparison of different CNS regions

Patch-clamp recordings were obtained in brain slices from 283 rat astrocytes. The expression of voltage-activated whole-cell currents was compared in four different CNS regions (hippocampus, cerebral cortex, spinal cord, and cerebellum). Our data show that CNS astrocytes do not show significant regional differences in their ion channel complement. With the exception of cerebellar Bergmann glial cells, essentially all astrocytes express a combination of delayed rectifying outward K(+) currents, transient A-type K(+) currents, and small Na(+) currents. Developmentally, an increasing percentage of astrocytes and Bergmann glial cells express inwardly rectifying K(+) currents. We did not observe cells that were passive, i.e., lacking voltage-activated currents. A few cells that appeared "passive" in initial recordings showed voltage-activated K(+) currents after off-line leak subtraction. The heterogeneity observed in the ion channel complement was found to be identical when cell-to-cell variations observed within a given CNS region and between various CNS regions were compared, suggesting a common and fairly stereotypical complement of ion channels in CNS astrocytes. Ion channel expression in Bergmann glial cells differed from that of all other CNS regions studied. These cells typically showed very low input resistances attributable to a significant time- and voltage-independent resting K(+) conductance. However, as with electrophysiologically "passive"-appearing astrocytes, Bergmann glial cells showed expression of delayed rectifying K(+) currents after off-line leak subtraction. Inwardly rectifying K(+) currents were observed in Bergmann glial cells after postnatal day 17. Collectively, our data suggest that all astrocytes contain voltage-gated ion channels that display a common pattern of expression during development.

Purification and analysis of in vivo-differentiated oligodendrocytes expressing the green fluorescent protein

A complete understanding of the molecular mechanisms involved in the formation and repair of the central nervous system myelin sheath requires an unambiguous identification and isolation of in vivo-differentiated myelin-forming cells. In order to develop a novel tool for the analysis of in vivo-differentiated oligodendrocytes, we generated transgenic mice expressing a red-shifted variant of the green fluorescent protein under the control of the proteolipid protein promoter. We demonstrate here that green fluorescent protein-derived fluorescence in the central nervous system of 9-day- to 7-week-old mice is restricted to mature oligodendrocytes, as determined by its spatiotemporal appearance and by both immunocytochemical and electrophysiological criteria. Green fluorescent protein-positive oligodendrocytes could easily be visualized in live and fixed tissue. Furthermore, we show that this convenient and reliable identification now allows detailed physiological analyses of differentiated oligodendrocytes in situ. In addition, we developed a novel tissue culture system for in vivo-differentiated oligodendrocytes. Initial data using this system indicate that, for oligodendrocytes isolated after differentiation in vivo, as yet unidentified factors secreted by astrocytes are necessary for survival and/or reappearance of a mature phenotype in culture.

Differential inhibition of glial K(+) currents by 4-AP

Spinal cord astrocytes express four biophysically and pharmacologically distinct voltage-activated potassium (K(+)) channel types. The K(+) channel blocker 4-aminopyridine (4-AP) exhibited differential and concentration-dependent block of all of these currents. Specifically, 100 microM 4-AP selectively inhibited a slowly inactivating outward current (K(SI)) that was insensitive to dendrototoxin (< or = 10 microM) and that activated at -50 mV. At 2 mM, 4-AP inhibited fast-inactivating, low-threshold (-70 mV) A-type currents (K(A)) and sustained, TEA-sensitive noninactivating delayed-rectifier-type currents (K(DR)). At an even higher concentration (8 mM), 4-AP additionally blocked inwardly rectifying, Cs(+)- and Ba(2+)-sensitive K(+) currents (K(IR)). Current injection into current-clamped astrocytes in culture or in acute spinal cord slices induced an overshooting voltage response reminiscent of slow neuronal action potentials. Increasing concentrations of 4-AP selectively modulated different phases in the repolarization of these glial spikes, suggesting that all four K(+) currents serve different roles in stabilization and repolarization of the astrocytic membrane potential. Our data suggest that 4-AP is an useful, dose-dependent inhibitor of all four astrocytic K(+) channels. We show that the slowly inactivating astrocytic K(+) currents, which had not been described as separate current entities in astrocytes, contribute to the resting K(+) conductance and may thus be involved in K(+) homeostatic functions of astrocytes. The high sensitivity of these currents to micromolar 4-AP suggests that application of 4-AP to inhibit neuronal A-currents or to induce epileptiform discharges in brain slices also may influence astrocytic K(+) buffering.

Loss of IA expression and increased excitability in postnatal rat Cajal-Retzius cells

Although an important secretory function of Cajal-Retzius (CR) cells has been discovered recently, the precise electrical status of these cells among other layer I neurons in particular and in cortical function in general is still unclear. In this paper, early postnatal CR cells from rat neocortex were found to express an inactivating K current whose molecular substrate is likely to be the Kv1.4 channel. Both electrophysiological and immunocytochemical experiments revealed that expression of this A-type current is down-regulated in vivo and virtually disappears by the end of the second postnatal week. At this time, CR cells have become capable of evoked repetitive firing, and their action potentials are larger and faster, yet these electrical properties still appear incompatible with a role in cortical network function, as inferred from comparisons with other cortical neurons. Also at this time, a large proportion of CR cells display spontaneous spiking activity, which suggests the possibility of additional roles for these cells. We conclude that the loss of A channels along with an increase in Na channel density shape the changes in excitability of postnatal CR cells, in terms of both the patterns of evoked firing and the emergence of spontaneous spiking.

Tripartite synapses: glia, the unacknowledged partner

According to the classical view of the nervous system, the numerically superior glial cells have inferior roles in that they provide an ideal environment for neuronal-cell function. However, there is a wave of new information suggesting that glia are intimately involved in the active control of neuronal activity and synaptic neurotransmission. Recent evidence shows that glia respond to neuronal activity with an elevation of their internal Ca2+ concentration, which triggers the release of chemical transmitters from glia themselves and, in turn, causes feedback regulation of neuronal activity and synaptic strength. In view of these new insights, this article suggests that perisynaptic Schwann cells and synaptically associated astrocytes should be viewed as integral modulatory elements of tripartite synapses.

Localization and age-dependent expression of the inward rectifier K+ channel subunit Kir 5.1 in a mammalian reproductive system

Kir 5.1 is a member of the inward rectifier potassium channel superfamily which does not form functional channels when expressed by itself in Xenopus laevis oocytes. rt-PCR reveals high levels of Kir 5.1 mRNA expression in testis but the function of this channel remains unknown. To determine the cell-specific expression of this channel in the testis we raised a polyclonal antibody against an external epitope of Kir 5.1 and tested its specificity in Xenopus oocytes expressing several cloned Kir subunits. Strong immunoreactivity for Kir 5.1 was found in seminiferous tubules of rat testis and, particularly, in spermatogonia, primary and secondary spermatocytes, spermatids and in the head and body of spermatozoa. The intensity of Kir 5.1 immunofluorescence, quantified using laser scanning microscopy, increased with age at every stage in the development of sperm from spermatogonia and reached a peak in 60-day-old rats. In contrast, the immunofluorescence decreased in 90-day-old animals and was detected mostly in spermatozoa. The results demonstrate that Kir 5.1 expression in the testis is localised to cells involved in spermatogenesis, showing a temporal pattern of expression during sexual maturity.

Developmental study of Müller cells in the rat retina using a new monoclonal antibody, RT10F7

We produced the monoclonal antibody RT10F7, characterized its antigenic specificity and expression in the adult and developing retina, in cultured retinal cells and in other parts of the central nervous system. In metabolically-labelled retinal cultures RT10F7 immunoprecipitated a protein of approximately 36,000 mol. wt. In the adult, RT10F7 stained endfeet of Müller cells in the ganglion cell layer, four horizontal bands in the inner plexiform layer, and radial fibres in the outer plexiform layer which terminated at the outer limiting membrane. In the inner nuclear layer, most somata were underlined by Müller processes that wrapped around them, but some cell bodies were immunoreactive for RT10F7 in the cytoplasm. During development, postnatal day 21 was the first age at which the adult pattern of immunoreactivity was present, although a fourth band in the inner plexiform layer was less clear than for the adult. By 14 and eight days after birth, the pattern of RT10F7 immunoreactivity approximated that of the adult; however, only three bands and one band were present, respectively, in the inner plexiform layer. At earlier ages, postnatal days 4, 1 and embryonic ages 19 and 15, the monoclonal antibody stained Müller cell endfeet and radial fibres, from the inner plexiform layer through the neuroblastic layer to the outer limiting membrane. At these ages, the immunoreactivity was more prominent at the level of Müller cell endfeet. The monoclonal antibody stained glia in preparations of dissociated retinal cells maintained in culture but not astrocytes or oligodendrocytes from optic nerve cultures. In brain sections, tanycytes exhibited RT10F7 immunoreactivity. The monoclonal antibody RT10F7 recognized a specific cell type in the retina, the Müller cell. In the adult and developing retina, RT10F7 recognized an antigen that is present primarily in Müller cell processes. This feature allowed us to follow the maturation of the Müller cell and correlate it with developmental events in the retina. RT10F7 is a specific marker for Müller cells in vivo and in vitro and may be useful for studies of function of Müller cells after ablation or after injuries that are known to activate Müller cells.

Regulation of K+ channels by cell contact in a cloned folliculo-stellate cell (TtT/GF)

Membrane currents in a folliculo-stellate cell line (TtT/GF) were examined using the whole-cell clamp technique. The cultured cells were morphologically categorized as isolated spread cells, isolated round cells, contact spread cells, and contact round cells. A distinct outward current was observed in isolated spread cells, contact round cells, and contact spread cells, whereas the outward current detected in isolated round cells was barely perceivable. The reversal potentials of the outward tail current obeyed the Nernst equation, indicating that the outward current was carried by K+ ions. The activation and deactivation processes of the K+ current could be fitted by single exponential curves. 4-aminopyridine, tetraethylammonium, and Ba2+ inhibited the K+ current. The concentrations for half-maximal inhibition of these agents to block the K+ current were 0.2 mM, 0.8 mM, and 8 mM, respectively. The biophysical and pharmacological characteristics of the K+ current in TtT/GF cells were similar to those of the K+ current in glial and Schwann cells. According to the results of the present study, it was concluded that TtT/GF cells possessed outward K+ channels, characteristics of which were similar to those of the K+ channels in glial and Schwann cells, and that the amplitude of the K+ current in TtT/GF cells seemed to be regulated by the condition of the cell contact.

Voltage-gated Na+ channels in glia: properties and possible functions

Glial cells are nervous-system cells that have classically been considered to be inexcitable. Despite their lack of electrical excitability, they can express voltage-activated Na+ channels with properties similar to the Na+ channels used by excitable cells to generate action potentials. The functional role that these voltage-activated Na+ channels play in glia is unclear. Three functions have been proposed: (1) glial cells might synthesize Na+ channels and donate them to adjacent neurons, thereby reducing the biosynthetic load of neurons; (2) Na+ channels might endow glial cells with the ability to sense electric activity of neighboring neurons, and might thus play a role in neuro-glial communication; and (3) Na+ influx through voltage-gated Na+ channels could be important to fuel the glial (Na+,K+)-ATPase, thereby facilitating and possibly modulating K+ uptake from the extracellular space.

A weakly inward rectifying potassium channel of the salmon brain. Glutamate 179 in the second transmembrane domain is insufficient for strong rectification

A cDNA encoding for a weakly inward rectifying K+ channel (sWIRK: salmon weakly inward rectifying K+ channel) was isolated from the masu salmon brain by expression cloning. The sWIRK channel exhibited the highest similarity with members of the ROMK1 subfamily, BIR10/KAB-2 (70% amino acid identity) and ROMK1 (46%). An ATP binding motif which is characteristic of this subfamily was also conserved. The sWIRK RNA was detected in the brain, but not in the heart, kidney, skeletal muscle, liver, testis, and ovary. In the brain, the expression was observed in the ependymoglial cells on the surface of the ventricles as well as in the small perineuronal glia-like cells in the midbrain and the medulla. When compared with the strong inward rectifier IRK1 channel, the sWIRK channel showed a much weaker inward rectification property, and the activation kinetics upon hyperpolarization was slower and less voltage-dependent. The slope conductance of the single channel inward current was 37 pS (140 mM K+o), and outward current channel events were also observed. The weak rectification of sWIRK is significant in that it has a negatively charged residue (glutamate) in the M2 region which is reported to cause strong inward rectification. By introducing a point mutation to remove this negative charge (glutamine), the sWIRK E179Q mutant channel lost its inward rectification property completely, and the single channel property (45 pS; 140 mM K+o) was ohmic up to highly depolarized potential, even in the presence of the physiological cytoplasmic blockers such as Mg2+ and polyamines.

5-Hydroxytryptamine2A receptors on cultured rat Schwann cells

Intracellular calcium responses of cultured rat Schwann cells to 5-hydroxytryptamine (5-HT) were examined using the calcium indicator dye fluo-3. Consistent changes in [Ca2+]i were observed with bath application of 5-HT and the basis of these responses was characterized. Application of 5-HT elicited a transient increase in intracellular calcium in a subpopulation of cultured Schwann cells. In many responding cells, the response recurred at approximately regular intervals following the initial transient. In some cases, these oscillations lasted for hours following removal of 5-HT from the bath. The increase in intracellular calcium evoked by 5-HT still occurred in the absence of extracellular calcium, suggesting that 5-HT induces calcium release from intracellular stores. Consistent with this hypothesis, the response to 5-HT was prevented by depletion of inositol trisphosphate-sensitive intracellular calcium stores with thapsigargin. Bath application of caffeine, known to activate Ca2+ release from ryanodine receptor-mediated stores, did not elicit an increase in [Ca2+]i. These results also suggested that 5-HT acted by stimulating a member of the 5-HT2 receptor family since this family employs inositol trisphosphate as a second messenger. In agreement with this interpretation, it was found that the 5-HT-induced intracellular calcium transients could be reversibly blocked by both ketanserin and spiperone, suggesting that the transients are mediated by 5-HT2A receptors. Additional support for this conclusion was obtained by immunocytochemistry using an anti-idiotypic antibody that recognizes a subset of 5-HT receptors.

Inhibitory interactions between two inward rectifier K+ channel subunits mediated by the transmembrane domains

Inwardly rectifying K+ channel subunits may form homomeric or heteromeric channels with distinct functional properties. Hyperpolarizing commands delivered to Xenopus oocytes expressing homomeric Kir 4.1 channels evoke inwardly rectifying K+ currents which activate rapidly and undergo a pronounced decay at more hyperpolarized potentials. In addition, Kir 4.1 subunits form heteromeric channels when coexpressed with several other inward rectifier subunits. However, coexpression of Kir 4.1 with Kir 3.4 causes an inhibition of the Kir 4.1 current. We have investigated this inhibitory effect and show that it is mediated by interactions between the predicted transmembrane domains of the two subunit classes. Other subunits within the Kir 3.0 family also exhibit this inhibitory effect which can be used to define subgroups of the inward rectifier family. Further, the mechanism of inhibition is likely due to the formation of an "inviable complex" which becomes degraded, rather than by formation of stable nonconductive heteromeric channels. These results provide insight into the assembly and regulation of inwardly rectifying K+ channels and the domains which define their interactions.

Hypomyelination alters K+ channel expression in mouse mutants shiverer and Trembler

Voltage-gated K+ channels are localized to juxtaparanodal regions of myelinated axons. To begin to understand the role of normal compact myelin in this localization, we examined mKv1.1 and mKv1.2 expression in the dysmyelinating mouse mutants shiverer and Trembler. In neonatal wild-type and shiverer mice, the focal localization of both proteins in axon fiber tracts is similar, suggesting that cues other than mature myelin can direct initial K+ channel localization in shiverer mutants. In contrast, K+ channel localization is altered in hypomyelinated axonal fiber tracts of adult mutants, suggesting that abnormal myelination leads to channel redistribution. In shiverer adult, K+ channel expression is up-regulated in both axons and glia, as revealed by immunocytochemistry, RNase protection, and in situ hybridization studies. This up-regulation of K+ channels in hypomyelinated axon tracts may reflect a compensatory reorganization of ionic currents, allowing impulse conduction to occur in these dysmyelinating mouse mutants.

A novel ATP-dependent inward rectifier potassium channel expressed predominantly in glial cells

We have isolated a novel inward rectifier K+ channel predominantly expressed in glial cells of the central nervous system. Its amino acid sequence exhibited 53% identity with ROMK1 and approximately 40% identity with other inward rectifier K+ channels. Xenopus oocytes injected with cRNA derived from this clone expressed a K+ current, which showed classical inward rectifier K+ channel characteristics. Intracellular Mg.ATP was required to sustain channel activity in excised membrane patches, which is consistent with a Walker type-A ATP-binding domain on this clone. We designate this new clone as KAB-2 (the second type of inward rectifying K+ channel with an ATP-binding domain). In situ hybridization showed KAB-2 mRNA to be expressed predominantly in glial cells of the cerebellum and forebrain. This is the first description of the cloning of a glial cell inward rectifier potassium channel, which may be responsible for K+ buffering action of glial cells in the brain.

Sodium channel distribution in a spider mechanosensory organ

A site-directed antibody was used immunocytochemically to measure the distribution of sodium channels in the tissues of a spider mechanoreceptor organ. The VS-3 slit sense organ contains 7-8 pairs of bipolar sensory neurons; these neurons are representative of a wide range of arthropod mechanoreceptors. Sensory transduction is thought to occur at the tips of the dendrites and to cause action potentials that are regeneratively conducted to the cell bodies, although it has not been possible to confirm this by direct intracellular recordings from the dendrites. Wholemount preparations were labelled by immunofluorescence and thin sections were immunogold labelled, using an antibody to the highly conserved SP19 sequence of the voltage-activated sodium channel. Labelling for sodium channels was found in the neurons and in their surrounding glial cells. Both cytoplasm and membranes were labelled, but immunogold particles were clearly aligned along cell membranes, indicating that the majority of labelling represented membrane-bound sodium channels. Channel density in the dendrites was similar to the axons and higher than in the cell bodies, supporting the idea of active conduction in the sensory dendrites. Labelling in glial cell membranes was indistinguishable from the neighboring neurons, suggesting a significant role for sodium channels in the functions of these supporting cells.

Contributions of the C-terminal domain to gating properties of inward rectifier potassium channels

Two inward rectifier potassium channels, the G protein-dependent GIRK1 and the G protein-independent BIR10, display large differences in rectification and macroscopic kinetics. A chimeric channel was constructed in which the putative intracellular carboxy-terminal domain of the G protein-dependent channel replaced the corresponding domain of the G protein-independent channel. The chimeric channel conducted potassium ions without the requirement of activated G proteins, yet displayed activation and deactivation kinetics and rectification properties similar to those of the G protein-dependent channel. The results demonstrate that structural elements in the C-terminus can independently control gating but not G protein signal transduction. The voltage dependence, time course, and kinetics of gating suggest a mechanism in which the pore may be occluded by reversible interactions with charged residues in the C-terminus.

Localization of mRNAs of voltage-dependent Ca(2+)-channels: four subtypes of alpha 1- and beta-subunits in developing and mature rat brain

The heterogeneous gene expression for four subtypes of alpha 1 (A,B,C,D)- and beta (beta 1,beta 2,beta 3,beta 4)-subunits of voltage-dependent calcium channels was demonstrated in developing and adult rat brain by in situ hybridization histochemistry. In the adult rat brain the gene expression for A- and B-subtypes was predominant in the cerebellar cortex and hippocampal neuronal layers, with the A-subtype expressed most intensely in the Purkinje cells, while the expression for C- and D-subtypes was predominant in the olfactory mitral and granule cells and the dentate granule cells. The expression of beta 1-mRNA was prominent in the olfactory mitral cells and dentate granule cells whereas that of beta 2-mRNA was evident in the hippocampal neuronal layers and cerebellar Purkinje cells. The expression of beta 3-mRNA was prominent in the olfactory mitral and internal granule cells and medial habenula, whereas that of beta 4-mRNA in the olfactory mitral cells and cerebellar Purkinje and granule cells. Comparison between the expression patterns for individual alpha- and beta-subunits suggests that the beta 4-subunit contributes to P-type channel, whereas the beta 1- and beta 3-subunits contribute respectively to D- and C-subtypes of L-type channels, although dissociation in the expression patterns were also noted in several brain regions. In addition to neuronal populations, the gene expression for the C-subtype of L-type channel was detected at substantial level in glial cells. In developing brains, the genes for the all subtypes of alpha 1- and beta-subunits were expressed in the mantle zones, but not the ventricular zones, of the entire neuraxis and the expression was more or less attenuated during early postnatal periods in most of the brain regions except for the olfactory bulb, hippocampus and cerebellar cortex, suggesting that the Ca(2+)-channels are intimately involved in the neuronal differentiation.

Polarity of processes with Golgi apparatus in a subpopulation of type I astrocytes

The Golgi apparatus-complex (GA), is a key organelle involved in several posttranslational modifications of polypeptides destined for lysosomes, plasma membranes and secretion. As reported from this laboratory, certain astrocytes in rat brain contain cisternae of the GA not only in perikarya, but also in processes. In order to further investigate which type of astrocytes contain GA in processes we conducted the present study using primary cultures of rat astrocytes and organelle specific antibodies against the GA and the rough endoplasmic reticulum (RER). While the perikarya of all cells contained elements of the GA, only a single process of a subset of type I astrocytes, negative to antibodies A2B5 and HNK-1, contained GA. In contrast, elements of the RER were found within perikarya and all processes. In order to confirm that the immunostained structures in processes indeed represent the GA, we exposed cultures to Brefeldin A (BFA), a secretion blocker which disperses the GA and redistributes it to the RER. We observed that BFA disrupted the GA of both perikarya and processes. However, astrocytes were resistant to prolonged incubations with BFA, while a similar treatment killed cultured fibroblasts and PC-12 cells. Furthermore, in astrocytes exposed to BFA for several days, the delicate network of glial fibrillary acidic protein (GFAP), was replaced by large perinuclear masses of the protein. These observations demonstrate that a subset of type I astrocytes have a single process with elements of the GA. We suggest that this specialization of the GA may be related to yet unrecognized secretory or protein processing functions of these cells. The resistance of astrocytes to BFA and the striking changes in their cytoskeleton induced by the drug, may contribute to studies on the mechanism(s) of action of BFA.

The expression of rat brain voltage-sensitive Na+ channel mRNAs in astrocytes

Astrocytes from various regions of CNS have been shown to express voltage-activated Na+ currents. To date, three distinct subtypes (I, II and III) of Na+ channels have been cloned from rat brain. We have applied a combined technique of reverse transcription and polymerase chain reaction (RT-PCR) to examine the expression of rat brain Na+ channels in rat astrocytes in vivo and in vitro. Five PCR primer sets were used to amplify coding or 3' non-coding regions of subtype I, II, and III Na+ channels. We were able to amplify all three of these rat brain Na+ channel subtypes from rat optic nerve, which does not have neuronal cell bodies but does contain astrocytes known to express voltage-sensitive Na+ channels. In studies on cultured spinal cord astrocytes, we were also able to amplify all three subtypes of rat brain Na+ channel mRNAs. In control experiments, RT-PCR was performed on RNAs prepared from several rat tissues, including brain, skeletal muscle, and liver. Rat brain was shown to express the three Na+ channel subtypes as expected. In rat skeletal muscle, subtype I and III Na+ channel mRNAs, but not subtype II, were amplified. In rat liver, Na+ channel messages were not detectable. The present study provides the first direct evidence that astrocytes in vivo and in vitro express rat brain voltage-sensitive Na+ channel mRNAs, which have been considered as mainly neuronal-type Na+ channel messages.

Properties of single calcium-activated potassium channels of large conductance in rat hippocampal neurons in culture

Patch-clamp recordings were made on rat hippocampal neurons maintained in culture. In cell-attached and excised inside-out and outside-out patches a large single-channel current was observed. This channel had a conductance of 220 and 100 pS in 140 mM [K+]i/140 mM [K+]o and 140 mM [K+]i/3 mM [K+]o respectively. From the reversal potential the channel was highly selective for K+, the PK+/PNa+ ratio being 50/1. Channel activity was voltage-dependent, the open probability at 100 nM [Ca2+]i increasing by e-fold for a 22 mV depolarization. It was also dependent on [Ca2+]i at both resting and depolarized membrane potentials. Channel open states were best described by the sum of two exponentials with time constants that increased as the membrane potential became more positive. Channel activity was sensitive to both external (500 microM) and internal (5 mM) tetraethylammonium chloride. These data are consistent with the properties of maxi-K+ channels described in other preparations, and further suggest a role for maxi-channel activity in regulating neuronal excitability at the resting membrane potential. Channel activity was not altered by 8-chlorophenyl thio cAMP, concanavalin A, pH reduction or neuraminidase. In two of five patches lemakalim (BRL 38227) increased channel activity. Internal ruthenium red (10 microM) blocked the channel by shortening the duration of both open states. This change in channel gating was distinct from the 'mode switching' seen in two patches, where a channel switched spontaneously from normal activity typified by two open states to a mode where only short openings were represented.

Excitatory amino acid-induced phosphoinositide hydrolysis in Müller glia

The presence of excitatory amino acid (EAA) receptors coupled to phosphoinositide metabolism in primary cultures of Müller (glial) cells from the chick retina was established. The order of potency of analogues for stimulating [3H]inositol phosphate (IP) accumulation was quisqualate (QA) > L-glutamate (L-Glu) = kainate (KA) > N-methyl-D-aspartate (NMDA) > L-aspartate (L-Asp) with EC50 in the range of 1-100 microM. 1-Aminocyclopentane-1,3-dicarboxylate (trans-ACPD), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), 2-amino-3-phosphonopropionate (AP3), and 2-amino-4-phosphonobutyrate (AP4) showed no effect either on basal concentration or on stimulated accumulation of [3H]IPs. The effect of EAA was potently inhibited by the ionotropic NMDA receptor antagonists 2-amino-5-phosphonopentanoate (AP5), 3-[(RS)-2-carboxy-piperazin-4-yl)]-propyl-1-phosphonate (CPP), and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-10- imine (MK-801); L-Glu antagonists at non-NMDA receptors, the quinoxalines NBQX and DNQX, inhibited weakly the response to L-Glu, KA, and NMDA, and more potently that to QA. The translocation of protein kinase C was also stimulated by EAA with the same pharmacological profile, and was partially inhibited by kynurenate (KYN). L-Glu and KA induced 45Ca2+ influx, which was decreased by KYN and CNQX. EAA-induced [3H]IPs accumulation was decreased by verapamil but not by nifedipine, and slightly diminished by dantrolene. Results demonstrate that EAA-induced phosphoinositide hydrolysis in Müller cells shows pharmacological differences with that in astrocytes and neuronal cells and could be triggered by a different mechanism.

Primary structure and functional expression of a mouse inward rectifier potassium channel

A complementary DNA encoding an inward rectifier K+ channel (IRK1) was isolated from a mouse macrophage cell line by expression cloning. This channel conducts inward K+ current below the K+ equilibrium potential but passes little outward K+ current. The IRK1 channel contains only two putative transmembrane segments per subunit and corresponds to the inner core structure of voltage-gated K+ channels. The IRK1 channel and an ATP-regulated K+ channel show extensive sequence similarity and constitute a new superfamily.