Electrophysiological properties of glial cells: comparison of brain slices with primary cultures

Cell of all trades: oligodendrocyte precursor cells in synaptic, vascular, and immune function

Oligodendrocyte precursor cells (OPCs) are not merely a transitory progenitor cell type, but rather a distinct and heterogeneous population of glia with various functions in the developing and adult central nervous system. In this review, we discuss the fate and function of OPCs in the brain beyond their contribution to myelination. OPCs are electrically sensitive, form synapses with neurons, support blood-brain barrier integrity, and mediate neuroinflammation. We explore how sex and age may influence OPC activity, and we review how OPC dysfunction may play a primary role in numerous neurological and neuropsychiatric diseases. Finally, we highlight areas of future research.

Lamina-specific properties of spinal astrocytes

Astrocytes are indispensable for proper neuronal functioning. Given the diverse needs of neuronal circuits and the variety of tasks astrocytes perform, the perceived homogeneous nature of astrocytes has been questioned. In the spinal dorsal horn, complex neuronal circuitries regulate the integration of sensory information of different modalities. The dorsal horn is organized in a distinct laminar manner based on termination patterns of high‐ and low‐threshold afferent fibers and neuronal properties. Neurons in laminae I (L1) and II (L2) integrate potentially painful, nociceptive information, whereas neurons in lamina III (L3) and deeper laminae integrate innocuous, tactile information from the periphery. Sensory information is also integrated by an uncharacterized network of astrocytes. How these lamina‐specific characteristics of neuronal circuits of the dorsal horn are of functional importance for properties of astrocytes is currently unknown. We addressed if astrocytes in L1, L2, and L3 of the upper dorsal horn of mice are differentially equipped for the needs of neuronal circuits that process sensory information of different modalities. We found that astrocytes in L1 and L2 were characterized by a higher density, higher expression of GFAP, Cx43, and GLAST and a faster coupling speed than astrocytes located in L3. L1 astrocytes were more responsive to Kir4.1 blockade and had higher levels of AQP4 compared to L3 astrocytes. In contrast, basic membrane properties, network formation, and somatic intracellular calcium signaling were similar in L1–L3 astrocytes. Our data indicate that the properties of spinal astrocytes are fine‐tuned for the integration of nociceptive versus tactile information.

Physiology of Astroglia

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

Physiology of Astroglia

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

The role of glial-specific Kir4.1 in normal and pathological states of the CNS

Kir4.1 is an inwardly rectifying K(+) channel expressed exclusively in glial cells in the central nervous system. In glia, Kir4.1 is implicated in several functions including extracellular K(+) homeostasis, maintenance of astrocyte resting membrane potential, cell volume regulation, and facilitation of glutamate uptake. Knockout of Kir4.1 in rodent models leads to severe neurological deficits, including ataxia, seizures, sensorineural deafness, and early postnatal death. Accumulating evidence indicates that Kir4.1 plays an integral role in the central nervous system, prompting many laboratories to study the potential role that Kir4.1 plays in human disease. In this article, we review the growing evidence implicating Kir4.1 in a wide array of neurological disease. Recent literature suggests Kir4.1 dysfunction facilitates neuronal hyperexcitability and may contribute to epilepsy. Genetic screens demonstrate that mutations of KCNJ10, the gene encoding Kir4.1, causes SeSAME/EAST syndrome, which is characterized by early onset seizures, compromised verbal and motor skills, profound cognitive deficits, and salt-wasting. KCNJ10 has also been linked to developmental disorders including autism. Cerebral trauma, ischemia, and inflammation are all associated with decreased astrocytic Kir4.1 current amplitude and astrocytic dysfunction. Additionally, neurodegenerative diseases such as Alzheimer disease and amyotrophic lateral sclerosis demonstrate loss of Kir4.1. This is particularly exciting in the context of Huntington disease, another neurodegenerative disorder in which restoration of Kir4.1 ameliorated motor deficits, decreased medium spiny neuron hyperexcitability, and extended survival in mouse models. Understanding the expression and regulation of Kir4.1 will be critical in determining if this channel can be exploited for therapeutic benefit.

Maintaining the presynaptic glutamate supply for excitatory neurotransmission

Glutamate released from synapses during excitatory neurotransmission must be rapidly recycled to maintain neuronal communication. This review evaluates data from physiological experiments at hippocampal CA3 to CA1 synapses and the calyx of Held synapse in the brainstem to analyze quantitatively the rates of release and resupply of glutamate required to sustain neurotransmission. We calculate that, without efficient recycling, the presynaptic glutamate supply will be exhausted within about a minute of normal synaptic activity. We also discuss replenishment of the presynaptic pool by diffusion from the soma, direct uptake of glutamate back into the presynaptic terminal, and uptake of glutamate precursor molecules. Diffusion of glutamate from the soma is calculated to be fast enough to resupply presynaptic glutamate in the hippocampus but not at the calyx of Held. However, because the somatic cytoplasm will also quickly run out of glutamate and synapses can function continually even if the presynaptic axon is severed, mechanisms other than diffusion must be present to resupply glutamate for release. Direct presynaptic uptake of glutamate is not present at the calyx of Held but may play a role in glutamate recycling in the hippocampus. Alternatively, glutamine or tricarboxylic acid cycle intermediates released from glia can serve as a precursor for glutamate in synaptic terminals, and we calculate that the magnitude of presynaptic glutamine uptake is sufficient to supply enough glutamate to sustain neurotransmission. The nature of these mechanisms, their relative abundance, and the co-ordination between them remain areas of intensive investigation.

The expression of voltage-gated ca2+ channels in pituicytes and the up-regulation of L-type ca2+ channels during water deprivation

The primary components of the neurohypophysis are the neuroendocrine terminals that release vasopressin and oxytocin, and pituicytes, which are astrocytes that normally surround and envelop these terminals. Pituicytes regulate neurohormone release by secreting the inhibitory modulator taurine in an osmotically-regulated fashion and undergo a marked structural reorganisation in response to dehydration as well as during lactation and parturition. Because of these unique functions, and the possibility that Ca2+ influx could regulate their activity, we tested for the expression of voltage-gated Ca2+ channel alpha1 subunits in pituicytes both in situ and in primary culture. Colocalisation studies in neurohypophysial slices show that pituicytes (identified by their expression of the glial marker S100beta), are immunoreactive for antibodies directed against Ca2+ channel alpha1 subunits Ca(V)2.2 and Ca(V)2.3, which mediate N- and R-type Ca2+ currents, respectively. Pituicytes in primary culture express immunoreactivity for Ca(V)1.2, Ca(V)2.1, Ca(V)2.2, Ca(V)2.3 and Ca(V)3.1 (which mediate L-, P/Q-, N-, R- and T-type currents, respectively) and immunoblotting studies confirmed the expression of these Ca2+ channel alpha1 subunits. This increase in Ca2+ channel expression may occur only in pituicytes in culture, or may reflect an inherent capability of pituicytes to initiate the expression of multiple types of Ca2+ channels when stimulated to do so. We therefore performed immunohistochemistry studies on pituitaries obtained from rats that had been deprived of water for 24 h. Pituicytes in these preparations showed a significantly increased immunoreactivity to Ca(V)1.2, suggesting that expression of these channels is up-regulated during the adaptation to long-lasting dehydration. Our results suggest that Ca2+ channels may play important roles in pituicyte function, including a contribution to the adaptation that occurs in pituicytes when the need for hormone release is elevated.

Calcium transients in astrocyte endfeet cause cerebrovascular constrictions

Cerebral blood flow (CBF) is coupled to neuronal activity and is imaged in vivo to map brain activation. CBF is also modified by afferent projection fibres that release vasoactive neurotransmitters in the perivascular region, principally on the astrocyte endfeet that outline cerebral blood vessels. However, the role of astrocytes in the regulation of cerebrovascular tone remains uncertain. Here we determine the impact of intracellular Ca(2+) concentrations ([Ca(2+)](i)) in astrocytes on the diameter of small arterioles by using two-photon Ca(2+) uncaging to increase [Ca(2+)](i). Vascular constrictions occurred when Ca(2+) waves evoked by uncaging propagated into the astrocyte endfeet and caused large increases in [Ca(2+)](i). The vasoactive neurotransmitter noradrenaline increased [Ca(2+)](i) in the astrocyte endfeet, the peak of which preceded the onset of arteriole constriction. Depressing increases in astrocyte [Ca(2+)](i) with BAPTA inhibited the vascular constrictions in noradrenaline. We find that constrictions induced in the cerebrovasculature by increased [Ca(2+)](i) in astrocyte endfeet are generated through the phospholipase A(2)-arachidonic acid pathway and 20-hydroxyeicosatetraenoic acid production. Vasoconstriction by astrocytes is a previously unknown mechanism for the regulation of CBF.

Phosphatase inhibitors remove the run-down of gamma-aminobutyric acid type A receptors in the human epileptic brain

The properties of gamma-aminobutyric acid (GABA) type A receptors (GABA(A) receptors) microtransplanted from the human epileptic brain to the plasma membrane of Xenopus oocytes were compared with those recorded directly from neurons, or glial cells, in human brains slices. Cell membranes isolated from brain specimens, surgically obtained from six patients afflicted with drug-resistant temporal lobe epilepsy (TLE) were injected into frog oocytes. Within a few hours, these oocytes acquired GABA(A) receptors that generated GABA currents with an unusual run-down, which was inhibited by orthovanadate and okadaic acid. In contrast, receptors derived from membranes of a nonepileptic hippocampal uncus, membranes from mouse brain, or recombinant rat alpha 1 beta 2 gamma 2-GABA receptors exhibited a much less pronounced GABA-current run-down. Moreover, the GABA(A) receptors of pyramidal neurons in temporal neocortex slices from the same six epileptic patients exhibited a stronger run-down than the receptors of rat pyramidal neurons. Interestingly, the GABA(A) receptors of neighboring glial cells remained substantially stable after repetitive activation. Therefore, the excessive GABA-current run-down observed in the membrane-injected oocytes recapitulates essentially what occurs in neurons, rather than in glial cells. Quantitative RT-PCR analyses from the same TLE neocortex specimens revealed that GABA(A)-receptor beta 1, beta 2, beta 3, and gamma 2 subunit mRNAs were significantly overexpressed (8- to 33-fold) compared with control autopsy tissues. Our results suggest that an abnormal GABA-receptor subunit transcription in the TLE brain leads to the expression of run-down-enhanced GABA(A) receptors. Blockage of phosphatases stabilizes the TLE GABA(A) receptors and strengthens GABAergic inhibition. It may be that this process can be targeted to develop new treatments for intractable epilepsy.

Expression of the green fluorescent protein in the oligodendrocyte lineage: a transgenic mouse for developmental and physiological studies

We generated a transgenic mouse expressing the enhanced green fluorescent protein (EGFP) under the control of the 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter. EGFP(+) cells were visualized in live tissue throughout embryonic and postnatal development. Immunohistochemical analysis in brain tissue and in sciatic nerve demonstrated that EGFP expression was restricted to cells of the oligodendrocyte and Schwann cell lineages. EGFP was also strongly expressed in "adult" oligodendrocyte progenitors (OPs) and in gray matter oligodendrocytes. Fluorescence-activated cell sorting allowed high-yield purification of EGFP(+) oligodendrocyte-lineage cells from transgenic brains. Electrophysiological patch clamp recordings of EGFP(+) cells in situ demonstrated that OP cells displayed large outward tetraethylammonium (TEA)-sensitive K(+) currents and very small inward currents, whereas mature oligodendrocytes were characterized by expression of large inward currents and small outward K(+) currents. The proliferation rate of EGFP(+) cells in developing white matter decreased with the age of the animals and was strongly inhibited by TEA. Oligodendrocyte development and physiology can be studied in live tissue of CNP-EGFP transgenic mice, which represent a source of pure EGFP(+) oligodendrocyte-lineage cells throughout development.

Type-2 astrocyte-like cells are more resistant than oligodendrocyte-like cells against non-N-methyl-D-aspartate glutamate receptor-mediated excitotoxicity

Glutamate causes excitotoxicity via non-N-methyl-D-aspartate (NMDA) glutamate receptors (GluR) in oligodendrocytes. Because both oligodendrocytes and type 2 astrocytes are differentiated from oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells, we investigated whether astrocytes are also vulnerable to non-NMDA GluR-mediated excitotoxicity. For these studies, oligodendrocyte-like cells (OLC) and type 2 astrocyte-like cells (2ALC) were derived from CG-4 cells, an immortalized rat O-2A progenitor cell line. About 50% of 2ALC were positive for glial fibrillary acidic protein and 90% were positive for A2B5, verifying that these cells have an type 2 astrocytic phenotype. A 24-hr exposure of OLC to 2 mM kainate, an activator of non-NMDA GluR, caused cell damage as shown by the release of lactate dehydrogenase. The extent of kainate-induced OLC damage was increased by cyclothiazide. In contrast, exposure of 2ALC to 2 mM kainate alone did not induce injury, though mild 2ALC injury was elicited by exposure to 2 mM kainate plus 100 microM cyclothiazide. Furthermore, we found that the kainate induced Ca(2+) uptake by 2ALC was 27.5% of that induced by kainate in OLC. Finally, both OLC and 2ALC expressed non-NMDA GluR subunit mRNAs, including GluR2, GluR3, GluR4, GluR6, GluR7, KA1, and KA2, but quantitative Western blot analysis revealed higher immunodetectable GluR2 and lower immunodetectable GluR3 and GluR4 in 2ALC than in OLC. Together, these results suggest that astrocytes are relatively resistant to non-NMDA GluR-mediated excitotoxicity because they have a higher expression of GluR2 and lower expression of GluR3 and GluR4.

Heterogeneity in the Membrane Current Pattern of Identified Glial Cells in the Hippocampal Slice

Glial cells, acutely isolated or in tissue culture, have previously been shown to express a variety of voltage-gated channels. To resolve the question whether such channels are also expressed by glial cells in their normal cellular environment, we have applied the patch-clamp technique to study glial cells in hippocampal slices of 10 - 12-day-old mice. Based on the membrane current pattern, we distinguished four glial cell types. One was characterized by passive, symmetrical K+ currents activated in depolarizing and hyperpolarizing directions. A second population showed a similar current pattern, but with a marked decay of the current during the 50-ms voltage jumps. In a third population, the decaying passive currents were superimposed with a delayed rectifier outward current and, in some cases, with a slow inward current activated by depolarization. The fourth population expressed delayed rectifying outward currents, an inward rectifier K+ current and fast inward currents activated by depolarization. To unequivocally identify the glial cells we combined electrophysiological and ultrastructural characterizations. Therefore, cells were filled with the fluorescent dye lucifer yellow during characterization of their membrane currents, the fluorescence of the dye was used to convert diaminobenzidine to an electron-dense material, and subsequently slices were inspected in the electron microscope. Recordings were obtained from cells in the stratum radiatum and were identified as glial by their size, the characteristic chromatin distribution, and the lack of synaptic membrane specializations.

Localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in mouse spinal cord

Prostaglandins are important mediators in spinal nociceptive processing. They are produced by cyclo-oxygenase isoforms, cyclo-oxygenase-1 and -2, which are both constitutively expressed in the central nervous system. The present immunohistochemical study details localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in lumbar spinal cord before and after induction of a painful paw inflammation in mice. Cyclo-oxygenase-1 immunoreactivity was found in glial cells of the dorsal and ventral horns, but not in neurons. In unstimulated mice, cyclo-oxygenase-2 immunoreactivity was found in motoneurons of the ventral horns and in lamina X, but not in dorsal horn neurons. After induction of a paw inflammation with zymosan, cyclo-oxygenase-2 immunoreactivity increased dramatically in dorsal horn neurons of laminae I-VI and X, paralleled by a significant increase in prostaglandin E(2) release from lumbar spinal cord. Cyclo-oxygenase-2 was co-localized with neuronal nitric oxide synthase immunoreactivity in several neurons in superficial laminae of the dorsal horns and in the area surrounding the central canal. Nitric oxide synthase was distributed in the cytoplasm and extended to processes of some neurons. In contrast, electron microscopy revealed that cyclo-oxygenase-2 immunoreactivity was restricted to the nuclear membrane and rough endoplasmic reticulum. It is shown in the present study that both cyclo-oxygenase isoforms are constitutively expressed in the spinal cord, cyclo-oxygenase-1 in glial cells of the dorsal and ventral horns and cyclo-oxygenase-2 in motoneurons. After induction of a hindpaw inflammation, several dorsal horn neurons express cyclo-oxygenase-2. Some of them are also positive for neuronal nitric oxide synthase, which is also induced following peripheral inflammation. Intracellularly, cyclo-oxygenase-2 is bound to the membranes of the nucleus and endoplasmic reticulum, whereas neuronal nitric oxide synthase is found in the cytoplasm.

Reciprocal communication systems between astrocytes and neurones

Over the past decade, a growing body of evidence has emerged on the existence in the brain of a close bidirectional communication system between neurones and astrocytes. This article reviews recent advances in understanding the rules governing these interactions and describes putative, novel functions attributable to astrocytes in neuronal transmission. Astrocytes can respond to the neurotransmitter released from active synaptic terminals, with cytosolic Ca(2+) oscillations whose frequency is under the dynamic control of neuronal activity. In response to these neuronal signals, astrocytes can signal back to neurones by releasing various neurone active compounds, such as the excitatory neurotransmitter glutamate. Interestingly, there is accumulating evidence that glutamate is released via a Ca(2+)-dependent mechanism which may share common properties with neurotransmitter exocytosis in neurones. This bidirectional communication system between neurones and astrocytes may lead to profound changes in neuronal excitability and synaptic transmission. While there clearly is an enormous amount of experimental and theoretical work yet to figure out, a coherent view is now emerging which incorporates the astrocyte, with the presynaptic terminal and the postsynaptic target neurone, as a possible third functional element of the synapse.

Gliotoxic action of glutamate on cultured astrocytes

Because of the well-documented importance of glutamate clearance by astrocytes in protecting neurons against excitotoxicity, it was interesting to examine whether L-glutamate exerts a toxic action on cultured astrocytes. Cell damage was evaluated by measuring activity of lactate dehydrogenase (LDH) released into the culture medium. Exposure of astrocyte cultures of the neonatal rat cerebral cortex to L-glutamate resulted in a concentration- and time-dependent increase in the release of LDH. L-Glutamate-induced gliotoxicity appeared to be mediated predominantly by the increase of oxidative stress because the reduced glutathione content and its effects were almost completely blocked by vitamin E and pyrrolidinedithiocarbamate. To support this notion further, the supplementation or depletion of intracellular reduced glutathione content attenuated or worsened L-glutamate toxicity, respectively. Activation of the glutamate transporter mimicked the action of L-glutamate on astrocytes. In addition, degrees of cell damage were not directly correlated to the levels of glutamate uptake. Moreover, the mechanism of this toxicity required energy and macromolecular synthesis. Taken together, brief exposure to L-glutamate resulted in glutamate uptake and cell swelling, whereas sustained exposure injured astrocytes via oxidative stress instead of the excitatory mechanism.

A subset of periglomerular neurons in the rat accessory olfactory bulb may be excited by GABA through a Na(+)-dependent mechanism

The periglomerular (PG) cells of the accessory olfactory bulb (AOB) are GABAergic interneurons which receive input from the vomeronasal sensory neurons and form dendrodendritic synapses with each other and with mitral cells. Their electrophysiological properties have not been investigated. We have developed a novel method of isolating PG cells from the AOB, and used the whole-cell patch and gramicidin-perforated patch clamp techniques to measure their basic electrophysiological characteristics and their response to GABA. PG cells were found to be excitable neurons with voltage-gated Na(+) and K(+) currents, though it was very difficult to get PG cells to fire an action potential. The voltage-gated Na(+) currents of PG cells activate at more positive potentials than those of typical CNS neurons. PG cells respond to GABA with currents in which GABA(A) receptors play a significant role. A subset ( approximately 40%) of PG cells respond to GABA with currents which have unusually high reversal potentials, indicating that GABA may be excitatory to these neurons. This phenomenon cannot be explained entirely by elevation of intracellular chloride concentrations, and is dependent on the presence of extracellular sodium.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K+]e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of 'arousal' of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K+]e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

Role of astrocytes in the clearance of excess extracellular potassium

The development of concepts describing potassium clearance mechanisms in the mammalian central nervous system in the last 35 years is reviewed. The pattern of excess potassium in the extracellular space is discussed as are the implications of these potassium levels for neuronal excitability. There is a systematic description of the available evidence for astrocytic involvement in situ. The three possible astrocytic potassium clearance mechanisms are introduced: spatial buffer mechanism; carrier-operated potassium chloride uptake as well as channel-operated potassium chloride uptake. The three mechanisms are compared and their compatibility is discussed. Evidence is now available showing that at least two of these if not all three mechanisms co-exist and complement each other. Finally, it is concluded that these potassium movements are not used as a signal system, only as a homeostatic feedback mechanisms. Such a genuine signal system involving glial elements exists--but it is based on calcium waves.

Expression of voltage-activated chloride currents in acute slices of human gliomas

Using whole-cell patch-clamp recordings, we identified a novel voltage-activated chloride current that was selectively expressed in glioma cells from 23 patient biopsies. Chloride currents were identified in 64% of glioma cells studied in acute slices of nine patient biopsies. These derived from gliomas of various pathological grades. In addition, 98% of cells acutely isolated or in short-term culture from 23 patients diagnosed with gliomas showed chloride current expression. These currents, which we termed glioma chloride currents activated at potentials >45 mV, showed pronounced outward rectification, and were sensitive to bath application of the presumed Cl- channel specific peptide chlorotoxin (approximately 600 nM) derived from Leiurus scorpion venom. Interestingly, low grade tumours (e.g., pilocytic astrocytomas), containing more differentiated, astrocyte-like cells showed expression of glioma chloride currents in concert with voltage-activated sodium and potassium currents also seen in normal astrocytes. By contrast, high grade tumours (e.g., glioblastoma multiforme) expressed almost exclusively chloride currents, suggesting a gradual loss of Na+ currents and gain of Cl- currents with increasing pathological tumour grade. To expand on the observation that these chloride currents are glioma-specific, we introduced experimental tumours in scid mice by intracranial injection of D54MG glioma cells and subsequently recorded from tumour cells and adjacent normal glial cells in acute slices. We consistently observed expression of chlorotoxin-sensitive chloride channels in implanted glioma cells, but without evidence for expression of chloride channels in surrounding "normal" host glial cells, suggesting that these chloride channels are probably a glioma-specific feature. Finding of this novel glioma specific Cl- channel in gliomas in situ and it's selective binding of chlorotoxin may provide a way to identify or target glioma cells in the future.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Intracellular calcium transients and potassium current oscillations evoked by glutamate in cultured rat astrocytes

Glutamate responses in cultured rat astrocytes from cerebella of neonatal rats were investigated using the perforated-patch configuration to record membrane currents without rundown of intracellular messenger cascades, and microfluorometric measurements to measure the intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) with fura-2 AM and 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein acetoxy methylester respectively. In the perforated-patch mode, glutamate evoked single or multiple outward current transients in 82% of the cells, which disappeared when the recording technique was converted into a conventional whole-cell mode. The outward current transients were accompanied by [Ca2+]i transients, whereas pHi fell monophasically, without any sign of oscillation. Pharmacological analysis of the glutamate-induced responses indicated that ionotropic receptor activation evoked an inward current but no outward current transients, and metabotropic receptor activation (of the mGluR1/5 type) elicited outward current transients but no inward current. The outward current transients were reduced in frequency, or even abolished, after depletion of the intracellular Ca2+-stores by the Ca2+-ATPase inhibitor cyclopiaconic acid (10 microM). They reversed near -85 mV and were reduced by tetraethylammonium (10 mM), suggesting that they were caused by K+ channel activation. It is concluded that glutamate evoked these K+ outward current transients by oscillatory Ca2+ release mediated by mGluR activation. The corresponding membrane potential waves across the astroglial syncytium could provide spatial and temporal dynamics to the glial K+ uptake capacity and other voltage-dependent processes.

Blockade of spreading depression in chronic epileptic rats: reversion by diazepam

Following pilocarpine-induced status epilepticus, rats become chronically epileptic showing 2-3 spontaneous recurrent seizures per week. The aim of this work was to verify the characteristics of spreading depression (SD) in these chronic epileptic rats (n = 16). SD was evoked in one point of the frontal cortex by topical application of KCl solution at 20 min intervals, and recordings were made in two points over the parietal cortex (a 'near' point and a 'remote' one, about 3 and 8 mm posterior to the stimulating region, respectively). In all control animals (n = 10), KCl stimulation elicited SD which in 100% of the cases propagated regularly to the two recording points. In the chronic epileptic rats only about 50% of the KCl applications were effective in eliciting SD, detected at the 'near' recording point. Of these, only 3% propagated to the 'remote' recording point. In eight of the above epileptic rats, diazepam (5-10 mg/kg, i.v.) was injected after 1-2 h of recording, when SD incidence in response to KCl stimulation has been established. After diazepam, the incidence of SDs propagating regularly to the 'near' and to the 'remote' recording points increased significantly, (132 and 53 SDs, respectively, out of 139 KCl stimulations), as compared with the incidence in the pretreatment period (33 and 1, respectively, out of 63 stimulations; P < 0.005). These data indicate an impairment in the susceptibility to cortical SD in chronic epileptic rats suggesting modifications in cortical excitability in the pilocarpine model of epilepsy. They also indicate a facilitatory effect of diazepam on SD, confirming previous observations in non-epileptic rats.

Expression of kainate-selective glutamate receptor subunits in glial cells of the adult bovine white matter

It is now well established that glial cells, like neurons, express a large variety of non-NMDA glutamate receptors. However, knowledge about the subunits forming the receptors and the types of glial cells which express them is incomplete. The aim of this study was to determine the presence and distribution of kainate-preferring glutamate receptor subunits in glial cells in the white matter of adult cattle. To this end, reverse transcription and polymerase chain reaction (RT-PCR) analysis of mRNA extracted from the corpus callosum, fornix and optic nerve of brains of adult cattle was carried out using specific primers that allowed selective amplification of the kainate transcripts for all five subunits were detected. Restriction digestion of the amplified products confirmed the specificity of the PCR amplifications and showed that the extent of Q/R editing in the GluR6 subunit was lower than that described in the rat whole brain. Immunohistochemical experiments revealed that GluR5-7 and KA2 subunits appeared in approximately 50% of astrocytes and 40% of oligodendrocytes. In astrocytes the receptor subunits were located throughout the cell body and processes, whereas in oligodendrocytes they were mostly restricted to the soma. Double-labelling studies demonstrated that in the white matter areas studied GluR5-7 and KA2 subunits are coexpressed in the majority of macroglial cell endowed with kainate receptors. In addition, astrocytes, but not oligodendrocytes, expressing kainate-selective receptors were found to possess the GluR1 subunit, an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit which has previously been found in this macroglial cell type. Taken together, these results provide evidence for the presence and molecular diversity of kainate-selective receptor subunits in glial cells of the adult white matter and for their coexpression with AMPA receptor subunits. The widespread distribution of kainate receptors in glial cells of the white matter suggests that these receptors may be important in axon-glia and/or glia-to-glia signalling.

Glial potassium channels activated by neuronal firing or intracellular cyclic AMP in Helix

1. Cell-attached and whole cell patch clamp experiments were performed on satellite glial cells adhering to the cell body of neurones in situ within the nervous system of the snail Helix pomatia. The underlying neurone was under current or voltage-clamp control. 2. Neuronal firing induced a delayed (20-30 s) persistent (3-4 min) increase in the opening probability of glial K+ channels. The channels were also activated by perfusing the ganglion with a depolarizing high-K+ saline, except when the underlying neurone was prevented from depolarizing under voltage-clamp conditions. 3. Two K(+)-selective channels were detected in the glial membrane. The channel responding to neuronal firing was present in 95% of the patches (n = 393). It had a unitary conductance of 56 pS, a Na+ :K+ permeability ratio < 0.02 and displayed slight inward rectification in symmetrical [K+] conditions. It was sensitive to TEA, Ba2+ and Cs+. The following results refer to this channel as studied in the cell-attached configuration. 4. The glial K+ channel was activated by bath application of the membrane-permeant cyclic AMP derivatives 8-bromo-cAMP and dibutyryl-cAMP, the adenylyl cyclase activator forskolin and the diesterase inhibitors IBMX, theophylline and caffeine. It was insensitive to cyclic GMP activators and to conditions that might alter the intracellular [Ca2+] (ionomycin, low-Ca2+ saline and Ca2+ channel blockers). 5. The forskolin-induced changes in channel behaviour (open and closed time distributions, burst duration, short and long gaps within bursts) could be accounted for by a four-state model (3 closed states, 1 open state) by simply changing one of the six rate parameters. 6. The present results suggest that the signal sent by an active neurone to satellite glial cells is confined to the glial cells round that neurone. The effect of this signal on the class of glial K+ channels studied can be mimicked by an increase in glial cAMP concentration. The subsequent delayed opening of the glial K+ channels does not appear to play a role in siphoning the excess K+ released by active neurones. It is hypothesized that the cAMP-gated glial K+ channels may be involved in the control of glial cell proliferation.

Identified glial cells in the early postnatal mouse hippocampus display different types of Ca2+ currents

Based on their typical pattern of membrane currents, four populations of glial cells could be identified in thin brain slices of the postnatal hippocampus. In the present study, we applied the patch-clamp technique to glial cells in the hippocampal CA1 region, which are characterized by a complex pattern of different Na+ and K+ currents ("complex" cells). These cells were identified as non-neuronal cells, most likely astrocytes, by their glutamine synthetase immunoreactivity. Two types of glial Ca2+ currents could be identified that differed in their kinetics and pharmacological properties. A low-voltage activated (LVA), fast inactivating component was activated at membrane potentials positive to -60 mV and reached maximum current amplitudes at about -20 mV. This current was sensitive to amiloride and thus displayed properties of neuronal LVA currents. The threshold potential of the second Ca2+ current component was at about -40 mV, and peak currents were observed at 0 mV. In contrast to the LVA component, the inactivation of these high-voltage activated (HVA) currents slowed down with increasing depolarizations. This current was sensitive to low concentrations of Cd2+ but was not affected by amiloride. A small fraction of the HVA currents was sensitive to nifedipine, and omega-conotoxin GVIA (omega-CgTx) was also found to reduce the glial HVA component. The study provides electrophysiological and pharmacological characterization of different types of Ca2+ currents in gray matter glial cells in situ.

Excitatory amino acid receptors in glia: different subtypes for distinct functions?

It is now well established that expression of voltage- and ligand-gated ionic channels, as well as G protein-coupled receptors, is not a property unique to neurons, but is also shared by macroglial cells (astrocytes and oligodendrocytes). These glial cells can receive a variety of signals from neurons at different stages of their development. Activation of membrane receptors may affect glial cell activity, proliferation, maturation, and survival through a complex cascade of intracellular events leading to long-term changes in glial cell phenotype and functional organization. Here we review the experimental evidence for glutamate receptor expression in glial cells in culture and in situ, and the molecular and functional properties of these receptors. We also describe some experimental models that identify possible functions of glutamate receptors in glia. Now that the existence of glutamate receptors in glia has been unambiguously demonstrated, future research will have to 1) determine which receptor subtypes are expressed in macroglial cells in vivo; 2) analyze, in adequate experimental models, the short- and long-term changes produced by glutamate receptor activation in glia; and 3) establish whether these receptors play a role in neuron-glia communication in the brain.

Two distinct inwardly rectifying conductances are expressed in long term dibutyryl-cyclic-AMP treated rat cultured cortical astrocytes

Long term incubation (1-3 weeks) with 250 microM dibutyryl-cyclic-AMP (dBcAMP) of pure cultured cortical astrocytes from newborn rats leads to the expression of voltage-dependent, inward-rectifying potassium (K+) and chloride (Cl-) currents which are lacking in shortly treated (4-24 h) and in control cultured astrocytes. Both conductances are already activated at the holding potential of -60 mV and are distinguishable for their gating kinetics and pharmacological sensitivity. K+ currents have a fast activation kinetic and show a time- and voltage-dependent inactivation at potentials negative to -120 mV. The conductive property of the K+ currents increases upon elevation of the extracellular K+ concentration ([K+]o) and they are reversibly blocked by extracellular 0.1 mM barium ions (Ba2+). Cl- currents are activated only at negative membrane potentials; they display a slow activation kinetic, no time-dependent inactivation and are not affected by 0.1 mM Ba2+. In individual astrocyte the K+ and Cl- conductances can be expressed singularly or in combination. The results indicate that the expression of these two conductances is controlled by a cAMP-dependent molecular signalling, presumably by regulating a late gene activation. Thus, the strengthening of this signalling would contribute to promote the maturation of less differentiated astrocytes in culture, implicating the expression of K+ and Cl- membrane conductances which may operate together in the regulation of [K+]o homeostasis via the mechanism of the local accumulation.

The astrocyte response to gamma-aminobutyric acid attenuates with age in the rat optic nerve

There is increasing evidence that glial cells respond to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and astrocytes have been shown to possess GABAA receptors both in vivo and in vitro. A recent study by Sakatani et al. (Proc. R. Soc. Lond. B247, 155 (1992)) demonstrated the transient expression of functional GABAA receptors in the developing rat optic nerve, but axonal and glial components of the response were not distinguished. To help address this problem, we have determined the electrophysiological response to GABA in astrocytes of the isolated intact optic nerves from neonatal rats, identified morphologically following intracellular injection of horseradish peroxidase. Astrocytes responded to GABA by a GABAA receptor-mediated depolarization which attenuated gradually during post-natal development; astrocytes in 21-day-old nerves were not observed to respond to GABA. The results indicate the transient presence of functional GABAA receptors in developing rat optic nerve astrocytes in situ, and we speculate upon a role for GABA in glial signalling and the organization of axonglial interrelations during development.

Potassium-dependent calcium influx in acutely isolated hippocampal astrocytes

Potassium depolarization can increase the intracellular ionized calcium concentration ([Ca2+]i) of cultured astrocytes, but it is not known if astrocytes that have matured in the intact CNS also exhibit voltage-dependent [Ca2+]i signalling. To address this issue, fluorometric measurements of [Ca2+]i were obtained from astrocytes acutely isolated from young adult rat hippocampus. In control artificial cerebrospinal fluid containing 5 mM [K+]o, average resting [Ca2+]i was 195 nM. Elevation of [K+]o to 50 mM caused [Ca2+]i to increase 150 nM to 1 microM above resting levels. The threshold [K+]o necessary to evoke an elevation in [Ca2+]i was 20-25 mM, and the magnitude of the [Ca2+]i signal grew progressively with increasing [K+]o (up to 50 mM). These [Ca2+]i increases were blocked completely by removal of external Ca2+, and markedly suppressed by the calcium channel blockers verapamil (30 microM and greater) and Co2+ (1 mM). Neither reversal of Na(+)-Ca2+ exchange, nor Ca(2+)-activated Ca2+ release, nor Ca2+ influx through stretch-activated channels contributed to the [Ca2+]i increase. These results suggest that [K+]o-evoked [Ca2+]i signals are mediated by influx through voltage-gated calcium channels. In contrast to results from cultured astrocytes and acutely isolated neurons, these [Ca2+]i increases were insensitive to dihydropyridine compounds. We conclude that increases in interstitial [K+], observed in situ during several pathological conditions, trigger voltage-dependent [Ca2+]i signals in astroglial cells. This may constitute an important form of neuron-to-glial communication.

Voltage-dependent ion channels in glial cells

Glial cells, although non-excitable, express a wealth of voltage-activated ion channels that are typically characteristic of excitable cells. Since these channels are also observed in acutely isolated cells and in brain slices, they have to be considered functional in the intact brain. Numerous studies over the past 10 years have yielded detailed characterizations of glial channels permitting comparison of their properties to those of their neuronal counterparts. While for the most part such comparisons have demonstrated a high degree of similarity, they also provide evidence for the expression of some uniquely glial ion channels. An increasing number of studies indicate that the expression of "glial" channels is influenced by the cells' microenvironment. For example, the presence of neurons can induce or inhibit (depending on the preparation and type of channel studied) the expression of glial ion channels. Like ion channels in excitable cells, glial channels can be functionally regulated by activation of second-messenger pathways, allowing for short-term modulation of their membrane properties. Although the extent to which most of the characterized ion channels are involved in glial function is presently unclear, a growing body of data suggests that certain channels play an active role in glial function. Thus inwardly rectifying K+ channels in concert with delayed rectifying K+ channels are thought to be involved in the removal and redistribution of excess K+ in the brain, a process referred to as "spatial buffering". Glial K+ channels may also be crucial in modulating glial proliferation. Cl- channels and stretch-activated cation channels are believed to be involved in volume regulation. Na+ channels appear to be important in fueling the glial Na+/K(+)-pump, and Ca2+ channels are likely involved in numerous cellular events in which intracellular Ca2+ is a critical second messenger.

Excitatory amino acid receptors in glial progenitor cells: molecular and functional properties

We have analyzed the molecular and biophysical properties of glutamate-gated channels in cells of the oligodendrocyte lineage, using both the CG-4 primary cell line (Louis et al: J. Neurosci. Res. 31:193-204, 1992a) and oligodendrocyte progenitors purified from the rat cerebral cortex. CG-4 progenitor cells, as well as primary progenitors, were stained with a specific anti-GABA antibody. In whole-cell patch-clamp recordings, rapid perfusion of the agonists L-glutamate, kainate, and AMPA produced rapidly desensitizing currents in CG-4 cells. NMDA was ineffective. Both rapidly desensitizing and steady-state components of responses to kainate were inhibited by the kainate/AMPA receptor antagonist CNQX. Northern blot analysis of total mRNA isolated from CG-4 cells revealed co-expression of both AMPA- and kainate-preferring glutamate receptor subunits. The activation of glutamate receptors in CG-4 cells caused a rapid and transient elevation of mRNAs for the immediate early gene NGFI-A.

Cockroach glial cell cultures: morphological development and voltage-gated potassium channels

Insect glial cells derived from the embryonic brain of Periplaneta americana cockroaches have been kept in primary culture conditions for up to 3 weeks. Under the culture conditions used, the glial cells differentiated and formed a complex cellular network on the floor of the culture vessels from which glial-glial and glial-neuronal contacts could be seen. Single-channel currents from cell-attached glial membrane patches were recorded using the gigaseal technique. Depolarisation of membrane patches activated outward currents, which were abolished in the presence of extracellular 50 mM tetraethylammonium or 5 mM 4-aminopyridine, and were insensitive to 1 microM tetrodotoxin, 10 microM picrotoxin and 2 mM Cd2+. The amplitude of the outward currents increased linearly with depolarisation, and amplitude histograms obtained at several pipette potentials could be reasonably fitted with a single Gaussian corresponding to a single channel type with a slope conductance of 37 +/- 11 pS. The extrapolated equilibrium potential of the outward current was about 5 +/- 10 mV positive to the resting potential and both the channel open time constant and relative open time probability were sensitive to membrane potential, increasing markedly with depolarisation. The results presented in this paper show the presence of a cadmium-insensitive, voltage-dependent outward potassium channel in insect glial cells in vitro.

Response of astrocytes to gamma-aminobutyric acid in the neonatal rat optic nerve

The electrophysiological response to gamma-aminobutyric acid (GABA) was determined in astrocytes of the isolated intact optic nerves of rats aged 8 to 12 days old, identified morphologically following intracellular injection of horseradish peroxidase. At this age, astrocytes had a mean (+/- S.E.M.) resting membrane potential of -62.25 +/- 1.9 mV (n = 32), and responded to GABA by a depolarization characterized by an initial peak of mean 9.1 +/- 0.6 mV, which was not sustained and fell to a plateau level. The effect of GABA was mimicked by the GABAA-receptor agonist muscimol, but not by the GABAB-receptor agonist baclofen, and was reduced in the presence of the GABAA-receptor antagonist bicuculline. Astrocytes responded to 10 mM [K+]o by a single phase depolarization of 16 +/- 2 mV (n = 5). It is concluded that GABA acts directly on astrocytes and its effect is not mediated by K+ released by axons. This study indicates the presence of functional GABAA receptors in neonatal rat optic nerve astrocytes in situ. The results suggest immature astrocytes may be the source of the GABA response described in two recent studies on the rat whole optic nerve preparation. The astrocyte response to GABA may be important in axon-glial signalling during development.

Impaired neuronal function induced by the immune mediator leukotriene B4

Inflammatory cerebral processes, mediated by immunologically active substances or invading of macrophages are frequently associated with neuronal dysfunction. This study describes the effects of leukotriene B4 on membrane potential, membrane resistance and potassium currents of cultured cortical neurons from the embryonic rat. Leukotriene B4 (1 microM) did not depolarize cortical neurons but induced a reversible reduction of voltage-dependent potassium outward currents (IK) in a subpopulation of these cells (35%). The results suggest that, in comparison to astrocytes, cortical neurons lack receptors for LTB4 or its intracellular activation pathway. Immune mediators, such as leukotrienes, may contribute to neuronal dysfunction during inflammatory diseases by affecting neuronal membrane currents.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Depolarization of cultured astrocytes by leukotriene B4. Evidence for the induction of a K+ conductance inhibitor

Since astrocytes have been shown to participate in intracerebral immunological processes we investigated the effect of the immune mediators, leukotrienes (LT) B4, LTC4 and LTD4 on membrane properties of cultured astrocytes from neonatal rat brain. When LTB4 was added to the bath solution the membrane potential slowly decreased from -96 mV to -38 mV. While LTB4 at a concentration of 500 nM was ineffective, depolarization occurred when concentrations of 750 nM and above were used. The depolarizing effect was specific for LTB4, since LTC4 and LTD4, other arachidonic acid derivates, failed to depolarize astrocytes at even higher concentrations (1 microM). When the K+ conductance blocker, Ba2+ (2 mM), was added to the bath solution, astrocytes depolarized to the same degree but no further depolarization was achieved when LTB4 was added. Bath application of Co2+ (1 mM), in order to reduce putative Ca2+ inward currents or reduced internal chloride concentration, did not alter the LTB4-induced depolarization, thus arguing against additional Ca(2+)- or Cl(-)-dependent depolarizing effects. The LTB4-induced depolarization could be markedly reduced, however, by preincubation of the cells with cycloheximide (2 microM), which blocks translation and thereby protein synthesis. Cycloheximide alone had no effect on the membrane potential. These data indicate that, in astrocytes, LTB4 stimulates the synthesis of a protein, which, in turn, inhibits K+ conductances. This effect could impair glial, as well as neuronal, functions during CNS diseases accompanied by immunological processes.

Response of oligodendrocytes to glutamate and γ-aminobutyric acid in the intact mouse optic nerve

The electrophysiological response to glutamate and gamma-aminobutyric acid (GABA) is determined in oligodendrocytes of the isolated intact mouse optic nerve, identified by their characteristic morphology following iontophoretic injection with horseradish peroxidase (HRP). In this study, mature myelin-forming oligodendrocytes are shown for the first time to respond to glutamate and GABA in situ, by a 2-3 mV depolarization. Morphologically homogeneous oligodendrocytes exhibit a heterogeneous response to glutamate and GABA; some cells respond to both excitatory amino acids, whereas others respond to one but not the other, and some oligodendrocytes do not respond to either. Oligodendrocytes uniformly depolarize in elevated [K+]o and it is concluded that the effect of glutamate and GABA is not mediated by an increase in [K+]o released from axons or astrocytes. The oligodendrocyte response to amino acids may be important in axon-to-oligodendrocyte signalling at the nodes of Ranvier.

Co-cultures of microglia and astrocytes from kainic acid-lesioned adult rat hippocampus: Effects of glutamate

The long-standing question concerning the direct actions of glutamate on the membrane potential of astroglial cells in the central nervous system was addressed using the in vitro kainic acid-lesioned hippocampal slice preparation and primary cell co-cultures of astrocytes and microglia derived from such lesions. The ultrastructure of the lesioned hippocampus was examined to aid in the identification of the cells appearing in culture. In culture, microglia appeared as flat cells, less than 1 micron in thickness at the edge of the cell, but thicker (about 5 microns) near the nucleus. The cytoplasm was packed with granular inclusions. Microglia appeared in two morphological forms, amoeboid and ramified. The amoeboid form was characterized by a cell body with a single process, and was always observed 1 day after starting the cell culture. Such cells became less frequent after 1 week in culture. The ramified form appeared as a rounded cell, devoid of processes, and were frequently observed in older cultures (greater than 1 week). Microglia did not round up after exposure to dibutyrylcyclic adenosine monophosphate (cAMP), and did not stain for glial fibrillary acidic protein (GFAP). An ultrastructural examination of the lesion demonstrated that microglia were present and that they contained many cytoplasmic granules similar to lipofuscin-containing granules. No filaments were observed in the cytoplasm of microglia. By contrast, the cytoplasm of astrocytes in culture had far fewer granules, rounded up to dibutyryl-cAMP, exhibited multiple processes, and stained for GFAP. In slices, astrocytes had no lipofuscin-containing granules, but numerous cytoplasmic filaments were present.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-circuiting effects of K+ currents on electrical responses of type-1-like astrocytes from mouse cerebral cortex

The membrane potential and membrane input resistance of cortical astrocytes from newborn mice were recorded with and without exposure to 1 mM barium. Barium treatment drastically decreased the membrane response to 0 and 35 mM K+. It also revealed an electrogenic component of the Na+,K(+)-ATPase as evident by a biphasic depolarization as a response to ouabain, which was monophasic without barium presence. Untreated mouse astrocytes reacted with small monophasic depolarizations to GABA and glutamate exposure. Barium-treated astrocytes exhibited additional transient responses to both transmitters, similar to those responses of rat astrocytes as found in the literature. The transmitter responses were not changed by exposure to uptake blockers for both transmitter substances. Thus, this electrophysiological study confirms earlier studies with radioactive K+ fluxes in showing that astrocytes derived from mouse brain are capable of short-circuiting electrogenic components and transmitter responses. This extreme high K+ permeability resembles the one reported for endfeet of retinal Muller cells and dissociated astrocytes from optic nerve.

Neurotransmitter-mediated changes in the electrophysiological properties of pituicytes

Abstract Intracellular recordings were obtained from pituicytes in the neural lobe of the isolated rat pituitary. Like other glia, pituicytes lacked action potentials in response to depolarizing current injection, but they tended to have more positive resting membrane potentials and higher input resistances than astrocytes in other preparations. Dye-coupling typical of astrocytes was also demonstrated amongst pituicytes, and their morphologies were similar to those of pituicytes stained for glial fibrillary acidic protein. Action potentials, anode-break spikes or barium spikes were not observed in pituicytes, even under conditions that maximized the elicitation of Ca(2+)-dependent responses. This suggests that pituicytes either have no or a very low density of Ca(2+) channels or Ca(2+) currents that are too small to generate action potentials. Dynorphin A (1-13), a kappa-opioid agonist, produced long-lasting increases in pituicyte input resistance with no significant changes in resting membrane potential. Dynorphin's action was concentration-dependent and was blocked by the opioid antagonist naloxone. This is consistent with previous reports demonstrating kappa-opioid receptors on pituicytes in the neurohypophysis. The beta-adrenergic agonist isoproterenol (100 muM) reversed the increases in pituicyte input resistance produced by opioid application, with no significant changes in resting membrane potential. The fact that pituicytes responded to neurotransmitters suggests a functional link between pituicytes and neurosecretory nerve fibres.

Modulation of intracellular Ca++ in cultured astrocytes by influx through voltage-activated Ca++ channels

Fura-2 and indo-1 fluorescence measurements were used to examine intracellular Ca++ concentration ([Ca++]i) and its modulation by voltage-activated influx in murine cortical astrocytes in primary cell culture. Extracellular K+ was increased from 5 to 50 mM to depolarize cells to determine if Ca++ influx through voltage activated Ca++ channels could alter [Ca++]i. In confluent 4 to 6 weeks in vitro astrocyte cultures 50 mM K+ increased [Ca++]i 3-4-fold (from 150 nM up to 550 nM); this increase was blocked by nifedipine and enhanced by BayK 8644 indicating that influx was through L-type channels. However, in 1 to 2 weeks in vitro astrocyte cultures, high K+ reduced [Ca++]i. L-type channels were apparently present in these cells because high K+ in combination with BayK 8644 increased [Ca++]i. Following pretreatment of 1 to 2 weeks in vitro astrocytes with dibutyryl cAMP (dbcAMP) high K+ increased [Ca++]i in the absence of BayK 8644 indicating enhanced activity of Ca++ channels in agreement with previous voltage-clamp studies. Ca++ influx through voltage-activated channels in cultured cortical astrocytes can substantially increase [Ca++]i and these channels can be dynamically modulated by dihydropyridines. Immature astrocytes may express 'silent' or inactive Ca++ channels or have a much lower number of channels.

Potassium channels in crustacean glial cells

Unitary currents through single ion channels in the glial cells, which ensheath the abdominal stretch receptor neurons of the crayfish, were characterized with respect to their basic kinetic properties. In cell-attached and excised patches two types of Ca(++)-independent K+ channels were observed with slope conductances of 57 pS and 96 pS in symmetrical K+ solution. The 57 pS K+ channel was weakly voltage-dependent with a slope of the Po vs. membrane potential relationship of +95 mV for an e-fold change in Po. In addition to the main conductance level, the channel displayed conductance levels of 80 and 109 pS. In excised patches, channel activity of this "subconductance" K+ channel showed "rundown" that could be prevented with 2 mM ATP-Mg on the cytoplasmic side of the membrane. The 96 pS K+ channel was strongly voltage-dependent with a slope of +12 mV for an e-fold change in Po. Averaged single-channel currents elicited by voltage jumps proved the channel to be of the delayed rectifying type. Channel activity persisted in excised patches with minimal salt solution and in virtually Ca(++)-free saline. Because of its dependence on intracellular ATP-Mg, the subconductance K+ channel is discussed as a target of modulation by transmitters or peptides via phosphorylation of the channel.

Glutaminergic responses of neuropile glial cells and Retzius neurones in the leech central nervous system

The effects of glutaminergic agonists on neuropile glial cells and on Retzius neurones in the central nervous system (CNS) of the leech, Hirudo medicinalis were investigated using double-barrelled ion-sensitive microelectrodes. In both types of cells, bath-application of L-glutamate (Glu), kainate (Ka) and quisqualate (Qui) elicited substantial membrane depolarizations which were accompanied by increases of the intracellular Na+ activity aiNa and by concomitant decrease of the intracellular K+ activity aiK. In the glial cells, these alterations of aiNa and aiK were preceded by a transient decrease of aiNa and an increase of aiK upon administration of Ka and Qui. In both glial cells and neurones, N-methyl-D-aspartate (NMDA) did not affect Em, aiK and aiNa. As found for Ka, the neuronal as well as the glial responses to glutaminergic agonists persisted during inhibition of synaptic transmission in high Mg2+, low Ca2+ solutions. The results indicate that leech neuropile glial cells have a Ka/Qui-preferring non-NMDA glutamate receptor similar to that in the Retzius neurones.

Neuronal modulation of calcium channel activity in cultured rat astrocytes

The patch-clamp technique was used to study whether cocultivation of neurons and astrocytes modulates the expression of calcium channel activity in astrocytes. Whole-cell patch-clamp recordings from rat brain astrocytes cocultured with rat embryonic neurons revealed two types of voltage-dependent inward currents carried by Ca2+ and blocked by either Cd2+ or Co2+ that otherwise were not detected in purified astrocytes. This expression of calcium channel activity in astrocytes was neuron dependent and was not observed when astrocytes were cocultured with purified oligodendrocytes.

Electrophysiological properties of reactive glial cells in the kainate-lesioned hippocampal slice

Kainate-lesioned hippocampal slices provide an excellent system for examining the electrophysiological properties of non-cultured glial cells without interfering signals from surrounding neurons. Intracellular recordings in the intensely gliotic CA3 region indicated that the resting membrane potential and input resistance of these reactive glial cells were similar to those reported for non-reactive astrocytes. Dye-coupling typical of astrocytes was also demonstrated amongst these cells, and was considerably reduced by cytoplasmic acidification. Occasionally these cells demonstrated spontaneous, rhythmic oscillations of membrane potential associated with large changes in whole cell input resistance. The action potentials reported in cultured astrocytes were not observed in reactive glial cells, even under conditions that maximize the observation of Ca2(+)-dependent responses. This suggests that reactive glial cells in this preparation have either no voltage-activated Ca2+ channels or a very low density of such channels. These cells also lack a bicarbonate conductance, but they do appear to have an apamin-sensitive conductance, possibly a Ca2(+)-activated gK.

Carrier-mediated Cl- transport in cultured mouse oligodendrocytes

We studied the steady state and the regulation of intracellular Cl- activity (aCl-i) and the mechanisms of KCl uptake in cultured oligodendrocytes from mouse spinal cord using Cl(-)-selective microelectrodes. The majority of oligodendrocytes actively accumulated Cl- above passive distribution (2-3 mM), few cells showed a passive Cl- distribution. To identify the carriers mediating Cl- uptake, oligodendrocytes were maintained in a solution with low extracellular Cl- concentration ([Cl-]o) which resulted in a rapid decrease in aCl-i. The recovery of aCl-i above its passive distribution in normal [Cl-]o was blocked in the absence of Na+ or in the presence of furosemide and of bumetanide, which has been reported to inhibit Na+/K+/Cl- cotransport. We therefore conclude that Cl- uptake is primarily due to the activity of a Na+K+/Cl- transport system. Cl- uptake above passive distribution was not affected in HCO3(-)-free solution or in the presence of SITS and DIDS, indicating that Cl-/HCO3- exchange is not involved in Cl- uptake by oligodendrocytes. Elevation of [K+]o induced an increase in aCl-i and, as shown earlier, intracellular K+ activity. This K+-induced Cl- uptake was not blocked by bumetanide, furosemide, SITS, or DIDS, suggesting that under conditions of raised [K+]o the combined uptake of K+ and Cl- is not mediated by a carrier, but can be explained by the entry through channels driven by Donnan forces.

gamma-Aminobutyric acid-induced ion movements in the guinea pig hippocampal slice

gamma-Aminobutyric acid (GABA)-induced regional changes of extracellular Cl, K and Na concentration ([Cl]o, [K]o, [Na]o), as well as of the extracellular space were measured with ion-sensitive microelectrodes in guinea pig hippocampal slices. Microdrop application of GABA to the pyramidal cell layer of CA3 or CA1 induced a decrease of [Cl]o, while application to the dendritic layer of CA3 or CA1 induced an increase of [Cl]o in addition. All changes of [Cl]o persisted in the presence of TTX and were blocked by bath-applied bicuculline. The GABA-induced decrease of [Cl]o was reduced by bicuculline application to the pyramidal cell layer. The increase of [Cl]o was blocked by bicuculline application to the dendritic layer. Additionally, GABA induced an increase of [K]o and decreases/increases of [Na]o. Changes of [Cl]o, [K]o and [Na]o together were approximately electroneutral. [Cl]o increases were exaggerated and [Cl]o decreases partly masked by shrinkage of the extracellular space after GABA application. Changing [K] in the superfusate transiently changed GABA-induced [Cl]o movements in a way predicted from a change in driving force due to the effect of [K] on membrane potential. Then a partial recovery followed towards the original [Cl]o change. We conclude that inward and outward Cl transports maintain [Cl]i below equilibrium in CA3 and CA1 pyramidal somata and above equilibrium in CA3 and CA1 dendrites. The significance of this Cl-distribution for hippocampal inhibition is discussed.