GABA release triggered by the activation of neuron-like non-NMDA receptors in cultured type 2 astrocytes is carrier-mediated

GABA, but Not Bestrophin-1, Is Localized in Astroglial Processes in the Mouse Hippocampus and the Cerebellum

GABA is proposed to act as a gliotransmitter in the brain. Differences in GABA release from astroglia are thought to underlie differences in tonic inhibition between the cerebellum and the CA1 hippocampus. Here we used quantitative immunogold cytochemistry to localize and compare the levels of GABA in astroglia in these brain regions. We found that the density of GABA immunogold particles was similar in delicate processes of Bergman glia in the cerebellum and astrocytes in the CA1 hippocampus. The astrocytic GABA release is proposed to be mediated by, among others, the Ca²⁺ activated Cl^- channel bestrophin-1. The bestrophin-1 antibodies did not show any significant bestrophin-1 signal in the brain of wt mice, nor in bestrophin-1 knockout mice. The bestrophin-1 signal was low both on Western blots and immunofluorescence laser scanning microscopic images. These results suggest that GABA is localized in astroglia, but in similar concentrations in the cerebellum and CA1 hippocampus, and thus cannot account for differences in tonic inhibition between these brain regions. Furthermore, our data seem to suggest that the GABA release from astroglia previously observed in the hippocampus and cerebellum occurs via mechanisms other than bestrophin-1.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

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.

Revised Ion/Substrate Coupling Stoichiometry of GABA Transporters

The purpose of this review is to highlight recent evidence in support of a 3 Na⁺: 1 Cl^-: 1 GABA coupling stoichiometry for plasma membrane GABA transporters (SLC6A1 , SLC6A11 , SLC6A12 , SLC6A13 ) and how the revised stoichiometry impacts our understanding of the contribution of GABA transporters to GABA homeostasis in synaptic and extrasynaptic regions in the brain under physiological and pathophysiological states. Recently, our laboratory probed the GABA transporter stoichiometry by analyzing the results of six independent measurements, which included the shifts in the thermodynamic transporter reversal potential caused by changes in the extracellular Na⁺, Cl^-, and GABA concentrations, as well as the ratio of charge flux to substrate flux for Na⁺, Cl^-, and GABA under voltage-clamp conditions. The shifts in the transporter reversal potential for a tenfold change in the external concentration of Na⁺, Cl^-, and GABA were 84 ± 4, 30 ± 1, and 29 ± 1 mV, respectively. Charge flux to substrate flux ratios were 0.7 ± 0.1 charges/Na⁺, 2.0 ± 0.2 charges/Cl^-, and 2.1 ± 0.1 charges/GABA. We then compared these experimental results with the predictions of 150 different transporter stoichiometry models, which included 1-5 Na⁺, 0-5 Cl^-, and 1-5 GABA per transport cycle. Only the 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry model correctly predicts the results of all six experimental measurements. Using the revised 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry, we propose that the GABA transporters mediate GABA uptake under most physiological conditions. Transporter-mediated GABA release likely takes place under pathophysiological or extreme physiological conditions.

Carrier-mediated GABA Release: Is There a Functional Role?

Neurotransmitters are normally released from neurons via calcium-dependent exocytosis of synaptic vesicles. However, after blockade of vesicular release by removal of calcium, or treatment with tetanus toxin, neurotransmitter release can still occur. In the case of GABA, nonvesicular release results from reversal of its uptake transporter, found on both neurons and glia. These GABA transporters are sodium-dependent and electrogenic, and therefore can be induced to operate in reverse by cell depolarization or by breakdown of the sodium gradient. Although demonstrated biochemically, less is known about whether this form of release occurs in vivo or whether it results in electrophysiological effects. Because conditions that favor reversal of the GABA transporter occur during high-frequency firing, nonvesicular GABA release may occur with excessive neuronal activity, such as during seizures. NEUROSCIENTIST 3:151-157, 1997

Astrocytic Actions on Extrasynaptic Neuronal Currents

In the last few decades, knowledge about astrocytic functions has significantly increased. It was demonstrated that astrocytes are not passive elements of the central nervous system (CNS), but active partners of neurons. There is a growing body of knowledge about the calcium excitability of astrocytes, the actions of different gliotransmitters and their release mechanisms, as well as the participation of astrocytes in the regulation of synaptic functions and their contribution to synaptic plasticity. However, astrocytic functions are even more complex than being a partner of the “tripartite synapse,” as they can influence extrasynaptic neuronal currents either by releasing substances or regulating ambient neurotransmitter levels. Several types of currents or changes of membrane potential with different kinetics and via different mechanisms can be elicited by astrocytic activity. Astrocyte-dependent phasic or tonic, inward or outward currents were described in several brain areas. Such currents, together with the synaptic actions of astrocytes, can contribute to neuromodulatory mechanisms, neurosensory and -secretory processes, cortical oscillatory activity, memory, and learning or overall neuronal excitability. This mini-review is an attempt to give a brief summary of astrocyte-dependent extrasynaptic neuronal currents and their possible functional significance.

GABA as a rising gliotransmitter

Gamma-amino butyric acid (GABA) is the major inhibitory neurotransmitter that is known to be synthesized and released from GABAergic neurons in the brain. However, recent studies have shown that not only neurons but also astrocytes contain a considerable amount of GABA that can be released and activate GABA receptors in neighboring neurons. These exciting new findings for glial GABA raise further interesting questions about the source of GABA, its mechanism of release and regulation and the functional role of glial GABA. In this review, we highlight recent studies that identify the presence and release of GABA in glial cells, we show several proposed potential pathways for accumulation and modulation of glial intracellular and extracellular GABA content, and finally we discuss functional roles for glial GABA in the brain.

Neuronal nicotinic receptors in sleep-related epilepsy: studies in integrative biology

Although Mendelian diseases are rare, when considered one by one, overall they constitute a significant social burden. Besides the medical aspects, they propose us one of the most general biological problems. Given the simplest physiological perturbation of an organism, that is, a single gene mutation, how do its effects percolate through the hierarchical biological levels to determine the pathogenesis? And how robust is the physiological system to this perturbation? To solve these problems, the study of genetic epilepsies caused by mutant ion channels presents special advantages, as it can exploit the full range of modern experimental methods. These allow to extend the functional analysis from single channels to whole brains. An instructive example is autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), which can be caused by mutations in neuronal nicotinic acetylcholine receptors. In vitro, such mutations often produce hyperfunctional receptors, at least in heterozygous condition. However, understanding how this leads to sleep-related frontal epilepsy is all but straightforward. Several available animal models are helping us to determine the effects of ADNFLE mutations on the mammalian brain. Because of the complexity of the cholinergic regulation in both developing and mature brains, several pathogenic mechanisms are possible, which also present different therapeutic implications.

GABA release by hippocampal astrocytes

Astrocytes can directly influence neuronal activity through the release of various transmitters acting on membrane receptors expressed by neurons. However, in contrast to glutamate and ATP for instance, the release of GABA (γ-amino-butyric acid) by astrocytes is still poorly documented. Here, we used whole-cell recordings in rat acute brain slices and electron microscopy to test whether hippocampal astrocytes release the inhibitory transmitter GABA. We observed that slow transient inhibitory currents due to the activation of GABA_A receptors occur spontaneously in principal neurons of the three main hippocampal fields (CA1, CA3, and dentate gyrus). These currents share characteristics with the slow NMDA receptor-mediated currents previously shown to result from astrocytic glutamate release: they occur in the absence of synaptic transmission and have variable kinetics and amplitudes as well as low frequencies. Osmotic pressure reduction, known to enhance transmitter release from astrocytes, similarly increased the frequency of non-synaptic GABA and glutamate currents. Simultaneous occurrence of slow inhibitory and excitatory currents was extremely rare. Yet, electron microscopy examination of immunostained hippocampal sections shows that about 80% of hippocampal astrocytes [positive for glial fibrillary acidic protein (GFAP)] were immunostained for GABA. Our results provide quantitative characteristics of the astrocyte-to-neuron GABAergic signaling. They also suggest that all principal neurons of the hippocampal network are under a dual, excitatory and inhibitory, influence of astrocytes. The relevance of the astrocytic release of GABA, and glutamate, on the physiopathology of the hippocampus remains to be established.

GABA Not Only a Neurotransmitter: Osmotic Regulation by GABA(A)R Signaling

Mature macroglia and almost all neural progenitor types express γ-aminobutyric (GABA) A receptors (GABA_ARs), whose activation by ambient or synaptic GABA, leads to influx or efflux of chloride (Cl⁻) depending on its electro-chemical gradient (E_Cl). Since the flux of Cl⁻ is indissolubly associated to that of osmotically obliged water, GABA_ARs regulate water movements by modulating ion gradients. In addition, since water movements also occur through specialized water channels and transporters, GABA_AR signaling could affect the movement of water by regulating the function of the channels and transporters involved, thereby affecting not only the direction of the water fluxes but also their dynamics. We will here review recent observations indicating that in neural cells GABA_AR-mediated osmotic regulation affects the cellular volume thereby activating multiple intracellular signaling mechanisms important for cell proliferation, maturation, and survival. In addition, we will discuss evidence that the osmotic regulation exerted by GABA may contribute to brain water homeostasis in physiological and in pathological conditions causing brain edema, in which the GABAergic transmission is often altered.

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.

GABA, a forgotten gliotransmitter

The amino acid gamma-aminobutiric acid (GABA) is a major inhibitory transmitter in the vertebrate central nervous system (CNS) where it can be released by neurons and by glial cells. Neuronal GABAergic signaling is well characterized: the mechanisms of GABA release, the receptors it targets and the functional consequences of their activation have been extensively studied. In contrast, the corresponding features of glial GABAergic signaling have attracted less attention. In this review, we first discuss evidence from the literature for GABA accumulation, production and release by glial cells. We then review the results of recent experiments that point toward functional roles of GABA as a "gliotransmitter".

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Nitric oxide stimulates gamma-aminobutyric acid release and inhibits glycine release in retina

Nitric oxide (NO) modulates the uptake and/or release of neurotransmitters through a variety of cellular mechanisms. However, the pharmacological and biochemical processes underlying these neurochemical effects of NO often remain unclear. In our study, we used immunocytochemical methods to study the effects of NO, cyclic guanosine monophosphate (cGMP), and peroxynitrite on the uptake and release of gamma-aminobutyric acid (GABA) and glycine in the turtle retina. In addition, we examined the involvement of glutamate receptors, calcium, and the GABA transporter in this GABA uptake and release. We also tested for interactions between the GABAergic and glycinergic systems. In general, we show that NO stimulated GABA release and inhibited glycine release. The NO-stimulated GABA release involved calcium-dependent or calcium-independent synaptic release or reversal of the GABA transporter. Some effects of NO on GABA release involved glutamate, cGMP, or peroxynitrite. NO promoted glycine uptake and inhibited its release, and this inhibition of glycine release was influenced by GABAergic modulation. These findings indicate that NO modulates the levels of the inhibitory transmitters GABA and glycine through several specific biochemical mechanisms in different retinal cell types and layers. Thus it appears that some of the previously described reciprocal interactions between GABA and glycine in the retina function through specific NO signaling pathways.

Production of GABA by cultured hippocampal glial cells

Medium conditioned by cultured hippocampal glial contains an inhibitory factor that can hyperpolarize and suppress neuronal activity. Using biochemistry, electrophysiology, pharmacology, and mass spectrometry, we have identified the inhibitory factor as GABA (gamma-aminobutyric acid). Like GABA, the inhibitory factor increases chloride and potassium currents in neurons, which can be blocked by bicuculline. Mass spectrometry analysis of conditioned medium reveals peaks that are identical to that for GABA. Up to 500 micromolar GABA is found in conditioned medium from glial cultures. No GABA is found in conditioned medium from neuronal cultures. Hippocampal glia make much more GABA than cortical glia or glia from other brain regions. It is not clear how hippocampal glia synthesize GABA. Although they express GAD mRNA and adding glutamate to the culture medium increases the amount of GABA produced, other data suggest that glia do not use GAD to make GABA. Identifying the mechanism(s) by which GABA is produced by hippocampal glia would help clarify its role in modulating neuronal activity in the brain.

Glutamate-induced homocysteic acid release from astrocytes: possible implication in glia-neuron signaling

Glial cells synthesise neuroactive substances and release them upon neurotransmitter receptor activation. Homocysteic acid (HCA), an endogenous agonist for glutamatergic N-methyl-D-aspartate (NMDA) receptors, is predominantly localised in glial cells. We have previously demonstrated the release of HCA from mouse astrocytes in culture following activation of beta-adrenergic receptors. Moreover, a release of HCA has also been observed in vivo upon physiological stimulation of sensory afferents in the thalamus. Here we report the glutamate-induced release of HCA from astrocytes. The effect of glutamate was mediated by the activation of ionotropic (NMDA and non-NMDA) as well as by metabotropic receptors. In addition, the release of HCA was Ca(2+)- and Na(+)-dependent, and its mechanism involved the activation of the Na+/Ca(2+)-exchanger. Furthermore, we provide evidence for the presence of functional NMDA receptors on astrocytes, which are coupled to an intracellular Ca2+ increase via stimulation of the Na+/Ca(2+)-exchanger. Our data thus favour a participation of glial cells in excitatory neurotransmission and corroborate the role of HCA as a "gliotransmitter."

GAT-1 and reversible GABA transport in Bergmann glia in slices

Although glial GABA uptake and release have been studied in vitro, GABA transporters (GATs) have not been characterized in glia in slices. Whole cell patch-clamp recordings were obtained from Bergmann glia in rat cerebellar slices to characterize carrier-mediated GABA influx and efflux. GABA induced inward currents at -70 mV that could be pharmacologically separated into GABA(A) receptor and GAT currents. In the presence of GABA(A/B/C) receptor blockers, mean GABA-induced currents measured -48 pA at -70 mV, were inwardly rectifying between -70 and +50 mV, were inhibited by external Na(+) removal, and were diminished by reduction of external Cl(-). Nontransportable blockers of GAT-1 (SKF89976-A and NNC-711) and a transportable blocker of all the GAT subtypes (nipecotic acid) reversibly reduced GABA-induced transport currents by 68 and 100%, respectively. A blocker of BGT-1 (betaine) had no effect. SKF89976-A and NNC-711 also suppressed baseline inward currents that likely result from tonic GAT activation by background GABA. The substrate agonists, nipecotic acid and beta-alanine but not betaine, induced voltage- and Na(+)-dependent currents. With Na(+) and GABA inside the patch pipette or intracellular GABA perfusion during the recording, SKF89976-A blocked baseline outward currents that activated at -60 mV and increased with more depolarized potentials. This carrier-mediated GABA efflux induced a local accumulation of extracellular GABA detected by GABA(A) receptor activation on the recorded cell. Overall, these results indicate that Bergmann glia express GAT-1 that are activated by ambient GABA. In addition, GAT-1 in glia can work in reverse and release sufficient GABA to activate nearby GABA receptors.

AMPA receptor activation induces GABA release from neurons migrating tangentially in the intermediate zone of embryonic rat neocortex

In the intermediate zone of the embryonic rodent neocortex, neurons migrating tangentially from the basal ganglia express both functional amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and gamma-aminobutyric acid (GABA). To test the hypothesis of GABA release triggered by AMPA receptor activation, we used whole-hemisphere cultures prepared from rat embryos (day 15). We observed a marked decrease in the number of detectable GABA-positive cells in the intermediate zone after exposure to T-AMPA. This effect was blocked by coapplying GYKI 53655, an AMPA receptor antagonist. The decrease in GABA immunolabelling induced by T-AMPA did not require extracellular calcium. In contrast, it was abolished after sodium substitution by choline, or after coapplication of nipecotic acid, a GABA transporter inhibitor. Exposure to high potassium reduced the number of detectable GABA-positive cells. These results are compatible with carrier-mediated GABA release consecutive to sodium influx. GABA released from neurons migrating tangentially in the intermediate zone after AMPA receptor activation may influence neighbouring elements including radially migrating postmitotic neurons, proliferating progenitors and possibly the tangential cells themselves.

Activation of P2X(7) receptors induced [(3)H]GABA release from the RBA-2 type-2 astrocyte cell line through a Cl(-)/HCO(3)(-)-dependent mechanism

ATP is an important signaling molecule in the nervous system and it's signaling is mediated through the metabotropic P2Y and ionotropic P2X receptors. ATP is known to stimulate Ca(2+) influx and phospholipase D (PLD) activity in the type-2 astrocyte cell line, RBA-2; in this study, we show that the release of preloaded [(3)H]GABA from RBA-2 cells is mediated through the P2X(7) receptors. ATP and the ATP analogue 3'-O-(4-benoylbenoyl)-adenosine-5'-triphosphate (BzATP) both stimulated [(3)H]GABA release in a concentration dependent manner, while the nonselective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), the P2X(7)-sensitive antagonist oxidized ATP (oATP), and high extracellular Mg(2+) all inhibited the ATP-stimulated [(3)H]GABA release. The ATP-stimulated [(3)H]GABA release was not affected neither by removing extracellular Na(+) nor by changes in the intracellular or extracellular Ca(2+) concentration. The GABA transporter inhibitors nipecotic acid and beta-alanine also had no effect. The ATP-stimulated [(3)H]GABA release was blocked, however, when media Cl(-) was replaced with gluconate and when extracellular HCO(3)(-) was removed. The Cl(-) channel/exchanger blockers 4,4'-diisothiocyanatostilbene-2',2'-disulfonic acid (DIDS) and 4-acetamido-4'- isothiocyanatostilbene-2',2'-disulfonic acids (SITS), but not diphenylamine-2-carboxylic acid (DPC) and furosemide, blocked the ATP-stimulated [(3)H]GABA release. The anionic selectivity of the process was F(-) > Cl(-) > Br(-) which is the same as that reported for volume-sensitive Cl(-) conductance. Treating cells with phorbol-12-myristate 13-acetate (PMA), forskolin, dibutyryl-cAMP, PD98059, neomycin, and D609 all inhibited the ATP-stimulated [(3)H]GABA release. We concluded that in RBA-2 cells, ATP stimulates [(3)H]GABA release through the P2X(7) receptors via a Cl(-)/HCO(3)(-)-dependent mechanism that is regulated by PKC, PKA, MEK/ERK, and PLD.

GABA transaminase inhibition induces spontaneous and enhances depolarization-evoked GABA efflux via reversal of the GABA transporter

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant gamma-vinyl GABA (vigabatrin; 0.05-100 microm) resulted in a large leak current that was blocked by bicuculline (50 microm). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 microm). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA(A) receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 microm vigabatrin, at which time it reached an equivalent conductance of 9.0 +/- 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mm) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K(+)](o) from 3 to as little as 6 mm. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

Glial transporters for glutamate, glycine, and GABA: II. GABA transporters

The termination of chemical neurotransmission in the central nervous system (CNS) involves the rapid removal of neurotransmitter from synapses. This is fulfilled by specific transport systems in neurons and glia, including those for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain. Glial cells express the cloned Na(+)/Cl(-)-dependent, high-affinity GABA transporters (GATs) GAT1, GAT2, and GAT3, as well as the low-affinity transporter BGT1. In situ hybridization and immunocytochemistry have revealed that each transporter shows distinct regional distribution in the brain and the retina. The neuronal vs. glial localization of the different transporters is not clear-cut, and variations according to species, neighboring excitatory synapses, and developmental stage have been reported. The localization, stoichiometry, and regulation of glial GATs are outlined, and the participation of these structures in development, osmoregulation, and neuroprotection are discussed. A decrease in GABAergic neurotransmission has been implicated in the pathophysiology of several CNS disorders, particularly in epilepsy. Since drugs which selectively inhibit glial but not neuronal GABA uptake exert anticonvulsant activity, clearly the establishment of the molecular mechanisms controlling GATs in glial cells will be an aid in the chemical treatment of several CNS-related diseases.

Persistent activation of GABA(A) receptor/Cl(-) channels by astrocyte-derived GABA in cultured embryonic rat hippocampal neurons

Whole cell patch-clamp recordings using Cl(-)-filled pipettes revealed more negative levels of baseline current and associated current variance in embryonic rat hippocampal neurons co-cultured on a monolayer of astrocytes than those cultured on poly-D-lysine. These effects were mimicked by culturing neurons on poly-D-lysine in astrocyte-conditioned medium (ACM). The baseline current and variance decreased immediately in all cells after either local perfusion with saline or exposure to bicuculline, an antagonist of GABA at GABA(A) receptor/Cl(-) channels. Baseline current and variance in all cells reached a nadir at approximately 0 mV, the calculated equilibrium potential for Cl(-). Perfusion of ACM rapidly induced a sustained current in neurons, which also reversed polarity at approximately 0 mV. Bicuculline attenuated or eliminated the ACM-induced current at a concentration that completely blocked micromolar GABA-induced current. Quantitative analyses of spontaneously occurring fluctuations superimposed on the ACM-induced current revealed estimated unitary properties of the underlying channel activity similar to those calculated for GABA's activation of GABA(A) receptor/Cl(-) channels. Bicuculline-sensitive synaptic-like transients, which reversed at approximately 0 mV, were also detected in neurons cultured in ACM, and these were immediately eliminated along with the negative baseline current and superimposed current fluctuations by perfusion. Furthermore bicuculline-sensitive synaptic-like transients were rapidly and reversibly triggered when ACM was acutely applied. ACM induced an increase in cytoplasmic Ca(2+) in cultured embryonic hippocampal neurons that was completely blocked by bicuculline and strychnine. We conclude that astrocytes release diffusible substances, most likely GABA, that persistently activate GABA(A) receptor/Cl(-) channels in co-cultured neurons.

Conditionally-immortalized astrocytic cell line expresses GAD and secretes GABA under tetracycline regulation

We have engineered conditionally-immortalized mouse astrocytes to express beta-galactosidase or GAD(65) in a tetracycline-controlled fashion. The engineered cell lines, BASlinbetagal and BASlin65, divide at 33 degrees C but cease division at 39 degrees C. We carried out morphological and biochemical analyses to further understand GABA production and release, and to determine the suitability of these cells for transplantation. Using the BASlinbetagal cell line, we showed a dramatic regulation of beta-galactosidase expression by tetracycline. The BASlin65 cell line showed functional GAD(65) enzymatic activity and GABA production, both of which were suppressed by growth in the presence of tetracycline. When cultured in the absence of tetracycline, BASlin65 cells have a total GABA content equal to or greater than other GABA-ergic cell lines. Immunofluorescence microscopy revealed that GAD(65) had a distinct perinuclear localization and punctate staining pattern. GABA, on the other hand, showed diffuse staining throughout the cytoplasm. BASlin65 cells not only synthesize GABA, they also release it into the extracellular environment. Their ability to produce and release significant amounts of GABA in a tetracycline-regulated manner makes BASlin65 cells a useful cellular model for the study of GABA production and release. Furthermore, their non-tumorigenicity makes them excellent candidates for transplantation into specific regions of the brain to provide a localized and regulatable source of GABA to the local neuronal circuitry.

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.

Role of group I metabotropic glutamate receptors in traumatic spinal cord white matter injury

Metabotropic glutamate receptors (mGluRs) participate in glutamate neural transmission, but their role in the pathophysiology of spinal cord injury (SCI) has not been explored. Accordingly, we examined the role of group I mGluRs, which are linked to phospholipase C, in mediating SCI using an in vitro model. A dorsal column segment was isolated from the spinal cord of adult rats, maintained in vitro, and injured by compression for 15 sec with a clip having a 2 g closing force. Under control conditions after SCI, the compound action potential (CAP) amplitude was reduced to 69.1 +/- 5.4% of baseline. Blockade of group I mGluR receptors with MCPG, 4CPG, or AIDA resulted in improved recovery of CAP amplitude (82.2 +/- 2.0%, 86.2 +/- 3.9%, and 86.0 +/- 2.5% of baseline, respectively). The group I/II agonist trans-ACPD and selective group I agonist DHPG exacerbated the posttraumatic reduction of CAP amplitude. The phospholipase C inhibitor U-73122 improved recovery of CAP amplitude after traumatic spinal cord axonal injury. Western blotting and immunocytochemistry demonstrated the presence of mGluR1alpha-immunopositive astrocytes and the absence of mGluR5 in spinal cord white matter. These studies are consistent with the hypothesis that activation of group I mGluR receptors after SCI exacerbates posttraumatic axonal injury through a phospholipase C dependent mechanism. The presence of mGluR1alpha labeling on astrocytes suggests a role for these cells in the pathophysiology of SCI. Additional studies in vivo, are required to further clarify the role of mGluRs in acute traumatic SCI.

Veratridine- and glutamate-induced release of [3H]-GABA from cultured chick retina cells: possible involvement of a GAT-1-like subtype of GABA transporter

Four subtypes of GABA carriers (GAT1-GAT4) that transport GABA in a sodium-dependent manner were identified so far. In this report, the sodium-dependent release of GABA was investigated in cultured chick retinal cells. Opening of voltage-sensitive sodium channels by veratridine or activation of non-NMDA glutamate receptors induced the release of GABA from cultured cells. The release of GABA was calcium-independent, but could be completely prevented by the substitution of sodium chloride by lithium or choline chloride in the extracellular medium, suggesting that GABA release could be triggered by multiple mechanisms that led to the flux of sodium into these cells. Pharmacological experiments revealed that, while GABA uptake was almost completely inhibited by the GAT-1 blockers NNC-711 (50 microM) or nipecotic acid (1 mM), the release of this amino acid was inhibited by NNC-711, but not by nipecotic acid. The incubation with beta-alanine (10 mM), a GAT-2/GAT-3 inhibitor, blocked 50% of GABA uptake but had no effect on the release. Our data suggest that sodium-dependent GABA release from cultured chick retina cells is mediated by a GAT-1 like transporter that shows some, but not all, the pharmacological properties of the GAT-1 carrier.

Carrier-mediated GABA release activates GABA receptors on hippocampal neurons

gamma-Aminobutyric acid (GABA) transporters are electrogenic and sodium-dependent and can operate in reverse when cells are depolarized or when there is reversal of the inward sodium gradient. However, the functional relevance of this phenomenon is unclear. We have examined whether depolarization induced by a physiologically relevant increase in extracellular [K+] leads to sufficient amounts of carrier-mediated GABA release to activate GABAA receptors on neurons. Patch-clamp recordings were made from rat hippocampal neurons in culture with solutions designed to isolate chloride currents in the recorded neuron. Pressure microejection was used to increase extracellular [K+] from 3 to 12 mM. After blockade of vesicular GABA release by removal of extracellular calcium, this stimulus induced a large conductance increase in hippocampal neurons [18.9 +/- 6.8 (SD) nS; n = 16]. This was blocked by the GABAA receptor antagonists picrotoxin and bicuculline and had a reversal potential that followed the Nernst potential for chloride, indicating that it was mediated by GABAA receptor activation. Similar responses occurred after block of vesicular neurotransmitter release by tetanus toxin. GABAA receptors also were activated when an increase in extracellular [K+] (from 3 to 13 mM) was combined with a reduction in extracellular [Na+] or when cells were exposed to a decrease in extracellular [Na+] alone. These results indicate that depolarization and/or reversal of the Na+ gradient activated GABA receptors via release of GABA from neighboring cells. We found that the GABA transporter antagonists 1-(4, 4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF89976A; 20-100 microM) and 1-(2-([(diphenylmethylene)amino]oxy)ethyl) -1, 2, 5, 6 - tetrahydro - 3 - pyridine - carboxylic acid hydrochloride (NO-711; 10 microM) both decreased the responses, indicating that the release of GABA resulted from reversal of the GABA transporter. We propose that carrier-mediated GABA release occurs in vivo during high-frequency neuronal firing and seizures, and dynamically modulates inhibitory tone.

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

Glutamate-modulated production of GABA in immortalized astrocytes transduced by a glutamic acid decarboxylase-expressing retrovirus

Replication-defective Moloney murine leukemia virus expressing the GAD67 gene under the control of the GFAP promoter was produced using selected clones of a fibroblast-packaging cell line. A spontaneously immortalized astrocyte cell line was infected with this virus and cellular clones expressing GAD67 selected. Astrocyte and fibroblast clones expressed functional GAD (detected by glutamic acid decarboxylation), but only fibroblasts were able to also produce GABA in the extracellular medium. When exposed to 200 microM glutamate, despite an observed difference in the rates of glutamate accumulation in control and GAD67-expressing astrocytes, similar proportions of glutamate taken up were detected. In GAD67-expressing astrocytes, the glutamate was mainly converted into GABA, suggesting GAD transgene activity to be dominant over other glutamate metabolic pathways, such as glutamine synthetase and glutamate dehydrogenase. Moreover, rapid GABA release into the cell medium was also observed, suggesting the involvement of reverse GABA transporters. The use of the GFAP promoter might be able to take advantage of its activation in response to factors inducing reactive gliosis observed in pathological insults. GAD67-expressing astrocytes might therefore be used for future grafting in pathological situations in which an excess of glutamate results in neuronal dysfunction or cell death.

Glutamate-induced release of the nitric oxide precursor, arginine, from glial cells

Arginine, the nitric oxide precursor, is predominantly localized in glial cells, whereas the constitutive nitric oxide synthase is mainly found in neurons. Therefore, a transfer of arginine from glial cells to neurons is needed to replenish the neuronal precursor pool. This is further supported by the finding that arginine is released upon selective pathway stimulation both in vitro and in vivo. We investigated the mechanism underlying this glial-neuronal interaction by analysing the effect of glutamate receptor agonists on the extracellular [3H]arginine level in cerebellar and cortical slices and in cultures of either cortical astroglial cells or neurons. We present data indicating that arginine is released from cerebellar and cortical slices and astroglial cell cultures upon activation of ionotropic non-NMDA glutamate receptors. Glutamate had no effect on the extracellular [3H]arginine level in neuronal cultures. Moreover, the effect of glutamate in cerebellar slices was tetrodotoxin-insensitive, and the calcium ionophore A23187 evoked the release of [3H]arginine from astroglial cell cultures. Thus, nitric oxide synthesis and nitric oxide transmission may be based on the glial-neuronal transfer of arginine which is induced by activation of excitatory amino acid receptors on glial cells.

Intracellular calcium oscillations in astrocytes: a highly plastic, bidirectional form of communication between neurons and astrocytes in situ

The spatial-temporal characteristics of intracellular calcium ([Ca2+]i) changes elicited in neurons and astrocytes by various types of stimuli were investigated by means of confocal fluorescent microscopy in acute rat brain slices loaded with the Ca2+ indicator indo-1. Neurons and astrocytes from the visual cortex and CA1 hippocampal region were identified in situ on the basis of their morphological, electrophysiological, and pharmacological features. We show here that stimulation of neuronal afferents triggered periodic [Ca2+]i oscillations in astrocytes. The frequency of these oscillations was under a dynamic control by neuronal activity as it changed according to the pattern of stimulation. After repetitive episodes of neuronal stimulation as well as repetitive stimulation with a metabotropic glutamate receptor agonist, astrocytes displayed a long-lasting increase in [Ca2+]i oscillation frequency. Oscillating astrocytes were accompanied by repetitive [Ca2+]i elevations in adjacent neurons, most likely because of the release of glutamate via a tetanus toxin-resistant process. These results reveal that [Ca2+]i oscillations in astrocytes represent a highly plastic signaling system that underlies the reciprocal communication between neurons and astrocytes.

Molecular biology of glycinergic neurotransmission

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem of vertebrates. Glycine is accumulated into synaptic vesicles by a proton-coupled transport system and released to the synaptic cleft after depolarization of the presynaptic terminal. The inhibitory action of glycine is mediated by pentameric glycine receptors (GlyR) that belong to the ligand-gated ion channel superfamily. The synaptic action of glycine is terminated by two sodium- and chloride-coupled transporters, GLYT1 and GLYT2, located in the glial plasma membrane and in the presynaptic terminals, respectively. Dysfunction of inhibitory glycinergic neurotransmission is associated with several forms of inherited mammalian myoclonus. In addition, glycine could participate in excitatory neurotransmission by modulating the activity of the NMDA subtype of glutamate receptor. In this article, we discuss recent progress in our understanding of the molecular mechanisms that underlie the physiology and pathology of glycinergic neurotransmission.

Long-term potentiation of glial synaptic currents in cerebellar culture

Glial cells in the brain express neurotransmitter receptors and can respond appropriately to application of exogenous neurotransmitters such as glutamate. However, activation of receptors by endogenous, synaptically released transmitter has been difficult to demonstrate directly. Using cell-pair recording in cerebellar cultures from embryonic mouse, it is shown that activation of a cerebellar granule neuron can give rise to a rapid inward current in an adjacent glial cell. This current is mediated by activation of Ca2+-permeable AMPA/kainate receptors and is largely independent of glutamate reuptake or gap junctional coupling. Furthermore, prolonged stimulation of the granule neuron at 4 Hz can give rise to long-term potentiation (LTP) of the glial synaptic current that has similar properties to LTP of granule neuron-Purkinje neuron synaptic transmission--its induction is independent of postsynaptic depolarization, postsynaptic Ca2+ influx, or glutamate receptor activation but requires presynaptic Ca2+ influx. These findings suggest a model in which cerebellar LTP is both induced and expressed presynaptically and therefore may be detected by either neuronal or glial postsynaptic cells.

Role of NMDA and non-NMDA ionotropic glutamate receptors in traumatic spinal cord axonal injury

We examined the role of glutamatergic mechanisms in acute injury to rat spinal cord white matter. Compound action potentials (CAPs) were recorded from isolated dorsal column segments in vitro. Under control conditions (Ringer's solution), the CAPs decreased to 71.4 +/- 2.0% of preinjury values after compression injury with a clip exerting a closing force of 2 g. The combination of the NMDA receptor blocker APV (50 microM) and the AMPA/kainate (KA) receptor blocker CNQX (10 microM) resulted in significantly improved recovery of CAP amplitude postinjury; however, the NMDA receptor antagonist APV alone did not enhance postinjury recovery, and infusion of NMDA (10 microM) did not affect recovery of the CAPs. In contrast, the AMPA/KA receptor blockers NBQX (10 microM) or CNQX (10 microM) significantly enhanced the recovery of CAP amplitude postinjury. The agonists AMPA (100 microM) or KA (100 microM) resulted in significant attenuation of CAP amplitude postinjury. Coapplication of AMPA/KA plus NBQX and CNQX was also associated with improved functional recovery. After incubation with AMPA and KA, Co(2+)-positive glia were visualized in spinal cord white matter. Similar results were seen after compressive injury but not in control cords. Immunohistochemistry and Western blot analysis demonstrated AMPA (GluR4)- and KA (GluR6/7 and KA2)-positive astrocytes in spinal cord white matter. In summary, non-NMDA ionotropic glutamate receptors seem to be involved in the pathophysiology of traumatic spinal cord injury. The presence of AMPA (GluR4) and KA (GluR6/7 and KA2) receptors on periaxonal astrocytes suggests a role for these cells in glutamatergic white matter injury.

Glial-neuronal interactions in non-synaptic areas of the brain: studies in the optic nerve

Optic nerves, like other CNS tracts, consist of axons closely apposed across narrow extracellular clefts to the cell bodies and processes of glial cells. Despite the anatomical simplicity of these pathways and the absence of synapses, a surprising range of interactions occurs between axons and glial cells mediated by changes in the chemical composition of the extracellular fluid produced by glial or neuronal stimulation. Some of the interactions are relatively brief, resulting from alterations in extracellular ions such as K+ or H+, or alterations of small molecules like glutamate or ATP. Other interactions involve much longer time periods and presumably larger signaling molecules, like peptides or proteins. These play a role not only in the development of axonal pathways but also in the processes of degeneration and regeneration that follow brain injury or disease.

Effects of GABA uptake inhibitors on posthypoxic myoclonus in rats

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. Previous results showed that dysfunction of central GABAergic neurotransmission may contribute to the disease. In current studies, effects of GABA uptake inhibitors, guvacine hydrochloride (1,2,5,6-tetrahydro-3-pyridine carboxylic acid hydrochloride) and (+/-)-cis-4-hydroxynipecotic acid ([+/-]-cis-4-hydroxy-3-piperidine carboxylic acid), in the pathophysiology of posthypoxic myoclonus were investigated. Administration of guvacine (1 or 10 mg/kg, IP) or nipecotic acid (0.5 or 5 mg/kg, IP) significantly attenuated myoclonus scores of the animals. Tolerance to antimyoclonus effects of these two compounds did not develop after chronic administration (twice a day for 14 days) of guvacine (10 mg/kg, IP) or nipecotic acid (5 mg/kg, IP). On the other hand, tolerance was noticed with clonazepam (2.5 mg/kg, IP twice a day for 7 days). The results indicate that guvacine or nipecotic acid may be used in combination with (at reduced doses) or as alternatives to clonazepam to treat patients with the disease so as to reduce tolerance phenomenon usually associated with clonazepam.

Neurotransmitters. NMDA receptors: do glia hold the key?

NMDA receptor stimulation requires binding of a 'co-agonist' and the neurotransmitter glutamate at separate sites. Are ligands for the co-agonist site liberated by glia, following activation of glial glutamate receptors?

Hippocampal GABA transporter function in temporal-lobe epilepsy

Electrophysiological studies of human temporal-lobe epilepsy suggest that a loss of hippocampal GABA-mediated inhibition may underlie the neuronal hyperexcitability. However, GABA (gamma-aminobutyric acid)-containing cells are preserved and GABA receptors are maintained in the surviving hippocampal neurons. Diminished GABA release may therefore mediate the loss of inhibition. Here we show that, in the human brain, potassium-stimulated release of GABA was increased, and glutamate-induced, calcium-independent release of GABA was markedly decreased, in epileptogenic hippocampi, in contrast with contralateral, non-epileptogenic hippocampi. The glutamate-induced GABA release in vivo was transporter-mediated in rats. Furthermore, in amygdala-kindled rats, a model for human epilepsy, a decrease in glutamate-induced GABA release was associated with a 48% decrease in the number of GABA transporters. These data suggest that temporal-lobe epilepsy is characterized in part by a loss of glutamate-stimulated GABA release that is secondary to a reduction in the number of GABA transporters.

Expression and regulation of kainate and AMPA receptors in uncommitted and committed neural progenitors

Here we review experimental evidence of non-NMDA glutamate receptor expression in the embryonic central nervous system. AMPA- and kainate-preferring glutamate receptor subunit mRNA transcripts are detected in embryonic neurons, glia and neural progenitors. Functional assays demonstrate that in some cell subpopulations ionotropic glutamate receptors are expressed by progenitors before synapse formation and terminal differentiation, and may be present before lineage determination is specified. The activation of these receptors triggers induction of immediate early gene transcription in progenitor cells. The cloning and transcriptional analysis of upstream regulatory regions of glutamate receptor genes governing their temporal and tissue-specific expression are also discussed.

Release mechanisms of [125I]meta-iodobenzylguanidine in neuroblastoma cells: evidence of a carrier-mediated efflux

[131I]metaiodobenzylguanidine ([131I]MIBG) is selectively taken up and stored by tumours derived from the neural crest, and is used for diagnosis and treatment of neuroblastoma (NB). The antitumoral effect of [131I]MIBG is closely related to the intracellular level of the radiopharmaceutical compound, which is dependent on uptake and storage/release mechanisms. While MIBG uptake is well characterised, storage and release mechanisms are still controversial. In order to better characterise [125I]MIBG release mechanisms, we studied the basal and stimulated efflux of [125I]MIBG in the human NB cell line, SH-SY5Y, preloaded with 0.1 microM [125I]MIBG for 1 h. We found that [125I]MIBG basal efflux is highly temperature-dependent, that [125I]MIBG release, induced by cell depolarisation with high potassium, is mainly calcium-independent, and induced by exchange with cold MIBG or noradrenaline, inversion of the sodium gradient across the cell membrane by veratridine by substitution of sodium chloride with equimolar concentration of lithium chloride. The exposure of NB cells to imipramine, an Uptake-1 inhibitor, also produces a net stimulatory effect on [125I]MIBG release. However, when used in association with other releasing stimuli, such as higher levels of intracellular sodium or external agonists, imipramine abolishes the consequent increase of [125I]MIBG release. Our findings suggest that stimulated [125I]MIBG release is mediated by a carrier, most probably the uptake carrier working in a reverse mode, while a minimal fraction of [125I]MIBG is released by an exocytotic mechanism.

Type-2 astrocytes have much greater susceptibility to heat stress than type-1 astrocytes

The present investigations were undertaken to examine the susceptibility of type-2 astrocytes to elevated temperature. Type-2 astrocytes are much more easily injured by temperature elevation than type-1 astrocytes. This may be related to cellular redox potential. Type-1 astrocytes have a greater cytosolic NAD redox potential (i.e., higher NADH:NAD levels) than type-2 astrocytes as evidenced by a 9-fold higher ratio of lactate to pyruvate released into the medium by type-1 astrocytes than type-2 astrocytes. Heat stress causes the induction of hsp-72 in both type-2 and type-1 astrocytes; however, hsp-72 protein expression is retained for a longer period of time by the type-2 astrocyte. A possible basis for the greater sensitivity of type-2 astrocytes to stress may be a poorer ability to scavenge free radicals. This differential sensitivity of one neural cell type relative to another to elevated temperature may be of significance in understanding the effects of hyperthermia on the developing brain.