Astrocyte Heterogeneity: Endogenous Amino Acid Levels and Release Evoked by Non-N-Methyl-D-Aspartate Receptor Agonists and by Potassium-Induced Swelling in Type-1 and Type-2 Astrocytes

STRUCTURAL AND FUNCTIONAL HETEROGENEITY OF ASTROCYTES IN THE BRAIN: ROLE IN NEURODEGENERATION AND NEUROINFLAMMATION

The review covers the current concepts on structural and functional heterogeneity of brain astrocytes that serve for numerous (patho)physiological processes in the central nervous system. Astrocytes from various subpopulations demonstrate different sensitivity to the action of pathogenic factors, varied behaviors in reactive processes and within the local immune response. Key functions of astrocytes like neurogenesis, neuron-astroglia metabolic coupling, glial control of local blood flow greatly depend on the origin and characteristics of astroglial cells. Changes at the initial stages of neurodegeneration or in neurodevelopmental disorders are associated with significant alterations in astroglial structural and functional properties, thus suggesting new approaches to therapeutic strategies implementing astroglia-expressing molecules and targets for effective

Monitoring neurochemical release from astrocytes using in vitro microdialysis coupled with high-speed capillary electrophoresis

We have developed a novel in vitro approach for monitoring fast neurochemical dynamics in model cell systems using microdialysis sampling coupled with high-speed capillary electrophoresis (CE). Cells from an immortalized astrocyte line (C8-D1A) were cultured in direct contact with the porous membrane of a microdialysis probe. Confocal microscopy was used to confirm cell viability and confluency over the microdialysis sampling region. Small molecules released from the astrocytes were efficiently sampled by the probe due to the direct contact with the membrane. Microdialysis sampling was coupled with online, high-speed CE allowing changes in the dialysate concentration of small molecule amine neurochemicals to be monitored with 20 s temporal resolution. Basal release of a number of important analytes was detected including glycine, taurine, D-serine, and glutamate. The ability of the in vitro microdialysis-CE instrument to monitor dynamic changes in analyte concentration was assessed by transferring a probe cultured with astrocytes from a solution containing artificial cerebrospinal fluid (aCSF) to a high K(+) solution (100 mM K(+)-aCSF). Upon stimulation, the observed concentration of a number of key neurochemicals increased dramatically including glycine (700%), taurine (185%), and serine (215%). Amino acids such as phenylalanine and valine, which are not known to respond to cellular swelling mechanisms, were unaffected by the K(+) stimulation.

Astroglial heterogeneity closely reflects the neuronal-defined anatomy of the adult murine CNS

Astroglia comprise an extremely morphologically diverse cell type that have crucial roles in neural development and function. Nonetheless, distinct regions of the CNS have traditionally been defined by the phenotypic characteristics and connectivity of neuros. In a complementary fashion, we present evidence that discrete regions of the adult CNS can be delineated based solely on the morphology, density and proliferation rates of astroglia. We used transgenic hGFAP-GFP mice in which robust expression of GFP in adult astroglia enables detailed morphological characterization of this diversely heterogeneous cell population with 3D confocal microscopy. By using three complementary methods for labeling adult astroglia (hGFAP-GFP expression, and GFAP and S100beta immunostaining), we find that there is a remarkably diverse, regionally stereotypical array of astroglial morphology throughout the CNS, and that discrete anatomical regions can be defined solely on the morphology of astroglia within that region. Second, we find that the density of astroglia varies dramatically across the CNS, and that astroglial density effectively delineates even the sub-regions of complex structures, such as the thalamus. We also find that regional astroglial density varies depending on how astroglia are labeled. To quantify and illustrate these broad differences in astroglial density, we generated an anatomical density atlas of the CNS. Third, the proliferation rate, or mitotic index, of astroglia in the adult CNS also effectively defines anatomical regions. These differences are present regardless of the astroglial-labeling method used. To supplement our atlas of astroglial density we generated an atlas of proliferation density for the adult CNS. Together, these studies demonstrate that the morphology, density and proliferation rate of astroglia can independently define the discrete cytoarchitecture of the adult mammalian CNS, and support the concept that regional astroglial heterogeneity reflects important molecular and functional differences between distinct classes of astroglia, much like the long-accepted heterogeneity of neuronal populations.

Neuronal release of D-serine: a physiological pathway controlling extracellular D-serine concentration

D-serine is thought to be a glia-derived transmitter that activates N-methyl D-aspartate receptors (NMDARs) in the brain. Here, we investigate the pathways for D-serine release using primary cultures, brain slices, and in vivo microdialysis. In contrast with the notion that D-serine is exclusively released from astrocytes, we found that D-serine is released by neuronal depolarization both in vitro and in vivo. Veratridine (50 microM) or depolarization by 40 mM KCl elicits a significant release of endogenous D-serine from primary neuronal cultures. Controls with astrocyte cultures indicate that glial cells are insensitive to veratridine, but release D-serine mainly by the opening of volume-regulated anion channels. In cortical slices perfused with veratridine, endogenous D-serine release is 10-fold higher than glutamate receptor-evoked release. Release of D-serine from slices does not require internal or external Ca(2+), suggesting a nonvesicular release mechanism. To confirm the neuronal origin of D-serine, we selectively loaded neurons in cortical slices with D-[(3)H]serine or applied D-alanine, which specifically releases D-serine from neurons. Depolarization with veratridine promotes D-serine release in vivo monitored by high temporal resolution microdialysis of the striatum. Our data indicate that the neuronal pool of D-serine plays a major role in D-serine dynamics, with implications for the regulation of NMDAR transmission.

Pharmacological "cross-inhibition" of connexin hemichannels and swelling activated anion channels

The study of ion channels has relied heavily on the use of pharmacological blocking agents. However, many of these agents have multiple effects, which may compromise interpretation of results when the affected mechanisms/pathways mediate similar functions. Volume regulated anion channels (VRAC) and connexin hemichannels can both mediate the release of glutamate and taurine, although these channels have distinct activation stimuli and hemichannels, but not VRAC, are permeable to Lucifer Yellow (LY). It has been reported that some anion channel blockers may inhibit connexin hemichannels. We further examined the effects of classic gap junction/hemichannel blockers and anion channel blockers on these channels. The typical VRAC blockers, NPPB, IAA-94, and tamoxifen blocked low divalent cation-induced glutamate and taurine release and LY loading, presumed due to hemichannel opening. The blocking action of these compounds on hemichannels was concentration dependent and fell within the same range where the drugs classically block VRACs. Conversely, carbenoxolone (CBX), the most widely used gap junction/hemichannel blocker, was an effective blocker of VRAC-mediated glutamate and taurine release, and blocked these channels at similar concentrations at which it blocked hemichannels. The CBX effect on VRACs was verified using astrocytes from connexin 43 knock out (Cx43 KO) animals. In these cells, the hypotonic induced amino acid flux was retained whereas the low divalent cation solution-induced flux was lost. These results extend our knowledge about "cross-inhibition" of VRACs and gap junctions/hemichannels by certain pharmacological agents. Given the overlap in function of these two types of channels, great care must be exerted in using pharmacological blockers to identify one channel from the other.

D-cycloserine enhances short-delay, but not long-delay, conditioned taste aversion learning in rats

NMDA receptors have been implicated in conditioned taste aversion (CTA), a form of associative learning with the unique temporal characteristic of associating taste and toxic stimuli across very long delays. d-cycloserine (DCS), an NMDA receptor agonist, has been shown to enhance short-delay CTA learning. Here we examined the interaction of DCS with varying temporal parameters of CTA. DCS (15 mg/kg) administered prior to the pairing of 0.125% saccharin and LiCl (38 mM, 12 ml/kg) enhanced CTA when there was a short delay between the taste-toxin pairing (10 min), but not when there was a long delay (45 min). DCS activity remained at effective levels over the long delay, because DCS administered 60 min prior to a short-delay pairing enhanced CTA. The interaction of DCS with the delay between taste stimulus onset and LiCl injection was investigated by administering DCS and then 5 min access to saccharin 45 min prior to a short-delay pairing of saccharin and LiCl. DCS failed to enhance CTA in rats pre-exposed to saccharin, even with a short delay between the second saccharin exposure and LiCl injection. These results suggest that DCS enhancement of CTA is dependent on mechanisms underlying gustatory processing during long-delay taste-toxin associations.

In vitro and in vivo toxicity of type 2 ribosome-inactivating proteins lanceolin and stenodactylin on glial and neuronal cells

Lanceolin and stenodactylin, new type 2 ribosome-inactivating proteins (RIPs) from Adenia plants were recently isolated and their high cytotoxicity was described. Present experiments were performed to investigate the effect of these toxins on neural cells in culture and their in vivo retrograde transport and neurotoxicity in the central nervous system. The concentrations of lanceolin and stenodactylin inhibiting by 50% protein synthesis were in the 10(-11) and 10(-12) (cerebellar granule neurons), 10(-12) and 10(-13) (astrocytes), and 10(-13) (microglia) molar range, respectively. Both RIPs resulted toxic for glial cells in culture by MTT test, killing 50% of microglia, the most sensitive cell type, at concentrations around 10(-14)M. Stenodactylin was highly neurotoxic in vivo, when injected intracerebrally, and was retrogradely transported through axons projecting to the injected region. Stereotaxic injection of 1.3 ng toxin into the left dorsal hippocampus resulted in loss of cholinergic neurons in the ipsilateral medial septal nucleus, where cell bodies of neurons providing cholinergic input to the hippocampus are located. The retrograde transport of RIPs along neurons allows to perform experiments of target-selective lesioning, and can be exploited also to perform specific experiments of immunolesioning of selected neuronal populations.

Impairment of GABAergic neurotransmitter system in the amygdala of young rats after 4-week zinc deprivation

On the basis of the evidence that the excitability of hippocampal glutamatergic neurotransmitter system is enhanced by dietary zinc deficiency, the response of amygdalar neurotransmitter system was checked in young rats fed a zinc-deficient diet for 4 weeks. Extracellular zinc concentration in the amygdala, which was measured by the in vivo microdialysis, was almost the same as that in the hippocampus and decreased by zinc deficiency. Extracellular zinc concentration in the amygdala was increased both in the control and zinc-deficient rats by stimulation with 100mM KCl, suggesting that the increase in extracellular zinc in the amygdala, as well as that in the hippocampus, is linked with neuronal depolarization. In amygdalar extracellular fluid, the basal glutamate concentration was not significantly different between the control and zinc-deficient rats and was increased to almost the same extent between them by stimulation with 100mM KCl, unlike more increase in extracellular glutamate concentration in the hippocampus in zinc deficiency. On the other hand, the basal GABA concentration in the amygdalar extracellular fluid was significantly lower in zinc-deficient rats and was not increased both in the control and zinc-deficient rats by stimulation with 100mM KCl. These results suggest that GABAergic neurotransmitter system is critically impaired in the amygdala of young rats after 4-week zinc deprivation.

Effects of theanine, r-glutamylethylamide, on neurotransmitter release and its relationship with glutamic acid neurotransmission

t Theanine, r-glutamylethylamide, is one of the major amino acid components in green tea and many researchers have compared theanine's effects with glutamic acid because the chemical structure is similar. In the previous study, we demonstrated that theanine can pass brain-blood barrier and may play as an agonist or an antagonist of some receptors. In this study, we investigated the effects of theanine on neurotransmitter release in the rat brain striatum by in vivo brain microdialysis and examined whether theanine affected glutamate transporters by comparing it with a glutamate transporter blocker, L-trans-Pyrrolidine-2,4-dicarboxylic acid (L-trans-2,4-PDC). Because we investigated whether the effects of theanine is similar to L-trans-2,4-PDC on the brain neurotransmission, we measured dopamine release and some amino acids release which are known as excitatory or inhibitory neurotransmitters from neurons by theanine or L-trans-2,4-PDC perfusion into the rat brain striatum. L-trans-2,4-PDC or theanine perfusion into the brain striatum caused dopamine release from dopaminergic neurons. In addition, L-trans-2,4-PDC perfusion increased glutamic acid, aspartic acid and, whereas theanine perfusion prevented aspartic acid release and increased glycine release. These results suggested that the mechanism of dopamine release caused by theanine is different from glutamate transporter blockers or glutamic acid. Further, L-trans-2,4-PDC cause excitatory neurotransmission, whereas theanine may inhibit excitatory neurotransmission and cause inhibitory neurotransmission via glycine receptors.

Astrocytic glutamate release-induced transient depolarization and epileptiform discharges in hippocampal CA1 pyramidal neurons

A paroxysmal depolarization shift (PDS) has been suggested to be a hallmark for epileptic activity in partial-onset seizures. By monitoring membrane potentials and currents in pairs of pyramidal neurons and astrocytes with dual patch-clamp recording and exocytosis of vesicles from astrocytes with two-photon laser scanning microscopy in hippocampal slices, we found that infusion of inositol 1,4,5-trisphosphate (IP(3)) into astrocytes by patch pipettes induced astrocytic glutamate release that triggered a transient depolarization (TD) and epileptiform discharges in CA1 pyramidal neurons. The TD is due to a tetrodotoxin (TTX)-insensitive slowly decaying transient inward current (STC). Astrocytic glutamate release simultaneously triggers both the STC in pyramidal neurons and a transport current (TC) in astrocytes. The neuronal STC is mediated by ionotropic glutamate receptors leading to the TD and epileptiform discharges; while the astrocytic TC is a glutamate reuptake current resulting from transporting released glutamate into the patched astrocyte. Fusion of a large vesicle in astrocytes was immediately followed by an astrocytic TC, suggesting that the fused vesicle contains glutamate. Both fusion of large vesicles and astrocytic TCs were blocked by tetanus toxin (TeNT), suggesting that astrocytic glutamate release is via SNARE-dependent exocytosis of glutamate-containing vesicles. In the presence of TTX, the epileptogenic reagent, 4-AP, also induced similar neuronal STCs and astrocytic TCs, suggesting that astrocytic glutamate release may play an epileptogenic role in initiation of epileptic seizures under pathological conditions. Our study provides a novel mechanism, astrocytic release of glutamate, for seizure initiation.

Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer's disease

Extracellular amyloid plaques, intracellular neurofibrillary tangles, and loss of basal forebrain cholinergic neurons in the brains of Alzheimer's disease (AD) patients may be the end result of abnormalities in lipid metabolism and peroxidation that may be caused, or exacerbated, by beta-amyloid peptide (Abeta). Apolipoprotein E (apoE) is a major apolipoprotein in the brain, mediating the transport and clearance of lipids and Abeta. ApoE-dependent dendritic and synaptic regeneration may be less efficient with apoE4, and this may result in, or unmask, age-related neurodegenerative changes. The increased risk of AD associated with apoE4 may be modulated by diet, vascular risk factors, and genetic polymorphisms that affect the function of other transporter proteins and enzymes involved in brain lipid homeostasis. Diet and apoE lipoproteins influence membrane lipid raft composition and the properties of enzymes, transporter proteins, and receptors mediating Abeta production and degradation, tau phosphorylation, glutamate and glucose uptake, and neuronal signal transduction. The level and isoform of apoE may influence whether Abeta is likely to be metabolized or deposited. This review examines the current evidence for diet, lipid homeostasis, and apoE in the pathogenesis of AD. Effects on the cholinergic system and response to cholinesterase inhibitors by APOE allele carrier status are discussed briefly.

Glutamate receptor activation triggers a calcium-dependent and SNARE protein-dependent release of the gliotransmitter D-serine

The gliotransmitter D-serine is released upon (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate and metabotropic glutamate receptor stimulation, but the mechanisms involved are unknown. Here, by using a highly sensitive bioassay to continuously monitor extracellular D-serine levels, we have investigated the pathways used in its release. We reveal that D-serine release is inhibited by removal of extracellular calcium and augmented by increasing extracellular calcium or after treatment with the Ca(2+) ionophore A23187. Furthermore, release of the amino acid is considerably reduced after depletion of thapsigargin-sensitive intracellular Ca(2+) stores or chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate-acetoxymethyl ester. Interestingly, D-serine release also was markedly reduced by concanamycin A, a vacuolar-type H(+)-ATPase inhibitor, indicating a role for the vesicular proton gradient in the transmitter storage/release. In addition, agonist-evoked D-serine release was sensitive to tetanus neurotoxin. Finally, immunocytochemical and sucrose density gradient analysis revealed that a large fraction of D-serine colocalized with synaptobrevin/VAMP2, suggesting that it is stored in VAMP2-bearing vesicles. In summary, our study reveals the cellular mechanisms subserving D-serine release and highlights the importance of the glial cell exocytotic pathway in influencing CNS levels of extracellular D-serine.

Glycine taken up through GLYT1 and GLYT2 heterotransporters into glutamatergic axon terminals of mouse spinal cord elicits release of glutamate by homotransporter reversal and through anion channels

Glycine concentration-dependently elicited [3H]D-aspartate ([3H]D-ASP) release from superfused mouse spinal cord synaptosomes. Glycine effect was insensitive to strychnine or 5,7-dichlorokynurenic acid, but was prevented by the glycine transporter blocker glycyldodecylamide. Glycine also evoked release of endogenous glutamate, which was sensitive to glycyldodecylamide and abolished in low-Na+ medium. Experiments with purified synaptosomes and gliasomes show that the glycine-evoked [3H]D-ASP release largely originates from glutamatergic nerve terminals. The glycine-evoked [3H]D-ASP release was halved by NFPS, a selective blocker of GLYT1 transporters, or by Org 25543, a selective GLYT2 blocker, and almost abolished by a mixture of the two, suggesting that activation of GLYT1 and GLYT2 present on glutamatergic terminals triggers the release of [3H]D-ASP. Accordingly, confocal microscopy experiments show localization of GLYT1 and GLYT2 in purified synaptosomes immuno-stained for the vesicular glutamate transporter vGLUT1. The glycine effect was independent of extra- and intraterminal Ca2+ ions. It was partly inhibited by the glutamate transporter blocker DL-TBOA and largely prevented by the anion channel blockers niflumic acid and NPPB. To conclude, transporters for glycine (GLYT1 or/and GLYT2) and for glutamate coexist on the same spinal cord glutamatergic terminals. Activation of glycine heterotransporters elicits glutamate release partly by homotransporter reversal and largely through anion channels.

The N-methyl D-aspartate receptor glycine site and D-serine metabolism: an evolutionary perspective

The N-methyl D-aspartate (NMDA) type of glutamate receptor requires two distinct agonists to operate. Glycine is assumed to be the endogenous ligand for the NMDA receptor glycine site, but this notion has been challenged by the discovery of high levels of endogenous d-serine in the mammalian forebrain. I have outlined an evolutionary framework for the appearance of a glycine site in animals and the metabolic events leading to high levels of D-serine in brain. Sequence alignments of the glycine-binding regions, along with the scant experimental data available, suggest that the properties of invertebrate NMDA receptor glycine sites are probably different from those in vertebrates. The synthesis of D-serine in brain is due to a pyridoxal-5'-phosphate (B(6))-requiring serine racemase in glia. Although it remains unknown when serine racemase first evolved, data concerning the evolution of B(6) enzymes, along with the known occurrences of serine racemases in animals, point to D-serine synthesis arising around the divergence time of arthropods. D-Serine catabolism occurs via the ancient peroxisomal enzyme d-amino acid oxidase (DAO), whose ontogenetic expression in the hindbrain of mammals is delayed until the postnatal period and absent from the forebrain. The phylogeny of D-serine metabolism has relevance to our understanding of brain ontogeny, schizophrenia and neurotransmitter dynamics.

Release of amino acids by zinc in the hippocampus

Zinc exists in the synaptic vesicles of hippocampal mossy fibers in high concentrations. On the basis of inhibitory zinc action against glutamate release in the hippocampus, the role of zinc in release of several amino acids were studied in rat hippocampus by using in vivo microdialysis. When the hippocampal CA3 region was perfused with 10 microM ZnCl(2), the concentrations of glutamine, serine, arginine, aspartate, and glycine in the perfusate were significantly increased, whereas the concentrations of amino acids except for glycine were not increased by perfusion with 30 microM ZnCl(2). Chelation of endogenous zinc with 50 microM CaEDTA significantly decreased the concentrations of amino acids in the perfusate except for glycine. In the CA1 region, on the other hand, the concentrations of these five amino acids were not increased by perfusion with 10 microM ZnCl(2) and the concentrations of glutamine and glycine were decreased significantly. The present study suggests that zinc enhances release of glutamine, serine, arginine, and aspartate in the CA3 region and attenuates release of glutamine and glycine in the CA1 region. Zinc seems to modulate glutamatergic synapses multifunctionally in the hippocampus, because glutamine, serine, aspartate, and glycine are involved in synaptic neurotransmission.

L-serine-mediated release of apolipoprotein E and lipids from microglial cells

Apolipoprotein E (ApoE), one of the genetic risk factors for Alzheimer's disease, is considered to have a critical role in transporting lipids in the brain. In the present study, we investigated ApoE release in primary rat microglial cultures. Microglial cells released ApoE in response to L-Ser in culture medium, and ApoE-immunoreactivity was detected in granules in the cell periphery and in perinuclear structures. Immunocytochemical studies, immunoblotting, and reverse transcription-polymerase chain reaction (RT-PCR) results all supported the notion that microglial cells are the potential source of ApoE in the brain. L-Ser enhanced ApoE release in a concentration-dependent manner without upregulating ApoE mRNA expression. Astrocytes presumably enhanced production and release of ApoE by microglial cells through secretion of L-Ser. As revealed by gel chromatography, ApoE was secreted as a component of lipoproteins, and L-Ser enhanced release of cholesterol and triglycerides together with ApoE. Activation of microglial cells by lipopolysaccharides and serum resulted in an overall decrease of the ApoE release. These findings suggest that microglial cells are a significant source of lipoproteins containing ApoE in the brain under physiological conditions, and that L-Ser is an important mediator of the neuron-astrocyte-microglia network in the brain.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Evidence for astrocyte heterogeneity: a distinct subpopulation of protoplasmic-like glial cells is detected in transgenic mice expressing Lmo1-lacZ

The adult mammalian central nervous system (CNS) contains a large number of different cell types, which arise from the ventricular (VZ) and subventricular zones during embryonic development. In this study, we used a transgenic mouse expressing Lmo1-LacZ from a randomly inserted promoter/reporter gene construct to identify a glial subpopulation. LMO1 is an LIM domain-containing protein, thought to act in protein-protein interactions. We found first that in the adult transgenic CNS, beta-galactosidase (beta-gal) was expressed in a specific subpopulation of protoplasmic-like cells, which did not express detectable levels of glial fibrilary acidic protein unless a lesion was produced. Secondly, during development, beta-gal(+) cells were found arising from discrete regions of the VZ. Taken together, these results identify a subpopulation of protoplasmic glial cells in the adult CNS and suggest that they arise from a restricted VZ region during CNS development.

Functional hemichannels in astrocytes: a novel mechanism of glutamate release

Little is known about the expression and possible functions of unopposed gap junction hemichannels in the brain. Emerging evidence suggests that gap junction hemichannels can act as stand-alone functional channels in astrocytes. With immunocytochemistry, dye uptake, and HPLC measurements, we show that astrocytes in vitro express functional hemichannels that can mediate robust efflux of glutamate and aspartate. Functional hemichannels were confirmed by passage of extracellular lucifer yellow (LY) into astrocytes in nominal divalent cation-free solution (DCFS) and the ability to block this passage with gap junction blocking agents. Glutamate/aspartate release (or LY loading) in DCFS was blocked by multivalent cations (Ca2+, Ba2+, Sr2+, Mg2+, and La3+) and by gap junction blocking agents (carbenoxolone, octanol, heptanol, flufenamic acid, and 18alpha-glycyrrhetinic acid) with affinities close to those reported for blockade of gap junction intercellular communication. Glutamate efflux via hemichannels was also accompanied by greatly reduced glutamate uptake. Glutamate release in DCFS, however, was not significantly mediated by reversal of the glutamate transporter: release did not saturate and was not blocked by glutamate transporter blockers. Control experiments in DCFS precluded glutamate release by volume-sensitive anion channels, P2X7 purinergic receptor pores, or general purinergic receptor activation. Blocking intracellular Ca2+ mobilization by BAPTA-AM or thapsigargin did not inhibit glutamate release in DCFS. Divalent cation removal also induced glutamate release from intact CNS white matter (acutely isolated optic nerve) that was blocked by carbenoxolone, suggesting the existence of functional hemichannels in situ. Our results indicated that astrocyte hemichannels could influence CNS levels of extracellular glutamate with implications for normal and pathological brain function.

Release of glutamate and GABA in the hippocampus under zinc deficiency

Zinc homeostasis in the brain is affected by dietary zinc deficiency, and its alteration may cause brain dysfunctions. On the basis of the previous evidence that hippocampal zinc was responsive to 12-week zinc deprivation, responsiveness of hippocampal zinc to dietary zinc deficiency was examined in rats fed a zinc-deficient diet for 4 weeks. Zinc concentration in the hippocampus was not decreased by zinc deprivation for 4 weeks. However, Timm's stain was extensively attenuated in the brain of the zinc-deficient rats. In the brain of the zinc-deficient rats, moreover, zinc concentration in the hippocampal extracellular fluid was approximately 30% of that of control rats. These results demonstrate that vesicular zinc is responsive to dietary zinc and may decrease easily under zinc deficiency. Zinc concentration in the hippocampal extracellular fluid during stimulation with high K(+) was significantly increased even in zinc-deficient rats, although the increased levels of zinc were lower than the basal levels of zinc in control rats. The basal glutamate concentration in the hippocampal extracellular fluid was not significantly different between the control and zinc-deficient rats. However, glutamate concentration in the hippocampal extracellular fluid during stimulation with high K(+) was more increased in the zinc-deficient rats than in the control rats. Gamma aminobutyric acid (GABA) concentration in the hippocampal extracellular fluid during stimulation with high K(+) was increased in the control rats, but not in the zinc-deficient rats. The present study suggests that the excitability of hippocampal glutamatergic neurons is enhanced by dietary zinc deficiency.

Vesicular release mechanisms in astrocytic signalling

Astrocytes play an important role in chemical signalling, acting as receptive as well as secretory elements. They can express receptors for essentially all classical neurotransmitter substances and for a large variety of peptides. Recent evidence indicates that astrocytes are involved in the information processing within the nervous system. Astrocytes respond to various neurotransmitters with elevations in intracellular calcium which can either be long-duration Ca(2+) spikes or oscillations in Ca(2+) levels. Astrocytic excitation can be propagated to adjacent astrocytes in the form of Ca(2+) waves. Due to their intimate spatial relationship with synaptic contacts, astrocytes can directly respond to synaptically released messengers and communicate, via signalling substances, with neurons in a reciprocal manner. Cultured astrocytes and astroglioma cells express synaptic vesicle proteins and members of the synaptic SNARE complex. Astrocytes can release a variety of messenger substances via receptor-mediated mechanisms implicating their potential for regulated exocytosis and the participation of proteins of the SNARE complex.

Exocytotic release of alanine from cultured cerebellar neurons

Many neurons release a variety of amino acids in response to depolarizing stimuli. Although some of these amino acids, namely, glutamate, aspartate, and gamma-aminobutyric acid (GABA), have been qualified as neurotransmitters, functional roles of the other amino acids including alanine remain obscure. We investigated the mechanism and the origin of alanine release from cultured rat cerebellar cells. High-K(+)-induced depolarization produced a considerable amount (139+/-8 pmol/2 min/dish) of alanine release, comparable to that of glutamate (103+/-7 pmol/2 min/dish). Other depolarizing agents including veratridine or 4-aminopyridine also induced alanine release, suggesting that the major source is excitable neurons, rather than non-excitable glial cells. Depolarization-evoked alanine release was suppressed in the absence of extracellular Ca(2+), and was almost abolished by treating the cells with botulinum type B neurotoxin (BoNT/B), indicating that alanine is released by Ca(2+)-dependent exocytosis of vesicle-associated membrane protein-2 (VAMP-2)-containing vesicles. The properties of alanine release were different from those of glutamate and GABA in several aspects: (a) Depolarization-dependent alanine release appeared as early as 7 days in vitro, much earlier than that of GABA. (b) Fifty microM kainate, which causes selective cell death of GABAergic neurons in the culture, only partially reduced alanine release, whereas it had no effect on glutamate release. (c) Alanine release was not affected by phorbol ester, which enhanced glutamate and GABA release in a kinase-dependent manner. We therefore conclude that alanine release occurs via exocytosis of a pool of synaptic vesicles distinct from those containing glutamate or GABA.

Activation of spinal group I metabotropic glutamate receptors in rats evokes local glutamate release and spontaneous nociceptive behaviors: effects of 2-methyl-6-(phenylethynyl)-pyridine pretreatment

Intrathecal (i.t.) administration of the group I metabotropic glutamate receptor (mGluR) agonist (RS)-3,5-dihydroxyphenylglycine ((RS)-3,5-DHPG) to rats produces an immediate display of spontaneous nociceptive behaviors (SNBs) persisting for up to 10 h after injection (NeuroReport 7 (1996) 2743). The mechanisms underlying these behavioral effects are not entirely understood but may include enhanced release of glutamate within the dorsal horn of the spinal cord. The current experiments used microdialysis in awake moving animals to test: (1), whether i.t. (S)-3,5-DHPG increases the local release of glutamate at doses that also induce SNBs; and (2), whether the effects on glutamate release (as well as SNBs) can be blocked by pretreatment with the mGluR5 selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP). Male Sprague-Dawley rats were implanted with a microdialysis probe inserted into the i.t. space of the spinal cord (J. Neurosci. Methods 62 (1995) 43) and then tested under i.t. drug conditions (0.01, 0.1 and 1 mM (S)-3,5-DHPG) following a 2-3 day recovery period. As predicted, local application of (S)-3,5-DHPG via the microdialysis probe increased the release of glutamate in a dose-dependent manner. Significant SNBs were also noted in the 0.1 and 1 mM groups in a manner paralleling the onset and duration of the glutamate response. Pretreatment with MPEP (55 mg/kg, intraperitoneally) blocked glutamate release to the 0.1 mM dose of (S)-3,5-DHPG, and also decreased the proportion of animals displaying SNBs in this dose group. No effects of MPEP were seen against the higher dose of (S)-3,5-DHPG (1 mM). These results suggest that stimulation of spinal mGluR5 leads to glutamate release within the spinal cord, a response that may in part account for the nociceptive behaviors evoked by i.t. (S)-3,5-DHPG.

Effect of glycine on synaptic transmission at the third order giant synapse of the squids Alloteuthis subulata and Loligo vulgaris

Intracellular microelectrode recordings were made from presynaptic and postsynaptic regions of the third order giant synapses of the squids Alloteuthis subulata and Loligo vulgaris. Synaptically generated postsynaptic action potential trains, and excitatory postsynaptic potentials (EPSPs) were reversibly decreased by glycine, beta - alanine or taurine while presynaptic action potentials (APs) were unaltered. Glycine was effective in the presence of strychnine (30-50 microM), NMDA (500 microM), AP-5 (50 microM), CPP (100 microM), or MK 801 (which also had no effect on normal synaptic transmission). The glycine effect was reduced reversibly by D-tubocurarine (100 microM) and blocked by reducing extracellular chloride by 50% with propionate. Excitatory postsynaptic currents (EPSCs) were decreased by glycine addition without altering resting membrane conductance. We postulate that glycine or a glycine like substance provides an excitatory postsynaptic input during synaptic stimulation. Bath addition of glycine desensitises these receptors and decreases the amplitude of the EPSPs and EPSCs. Modulation of this synaptic input may provide an effective mechanism to suppress or potentiate synaptic transmission in the squid giant synapse.

Spatial buffering during slow and paroxysmal sleep oscillations in cortical networks of glial cells in vivo

The ability of neuroglia to buffer local increases of extracellular K(+) has been known from in vitro studies. This property may confer on these cells an active role in the modulation and spreading of cortical oscillatory activities. We addressed the question of the spatial buffering in vivo by performing single and double intraglial recordings, together with measures of the extracellular K(+) and Ca(2+) concentrations ([K(+)](out) and [Ca(2+)](out)) in the cerebral cortex of cats under ketamine and xylazine anesthesia during patterns of slow sleep oscillations and spike-wave seizures. In addition, we estimated the fluctuations of intraglial K(+) concentrations ([K(+)](in)). Measurements obtained during the slow oscillation indicated that glial cells phasically take up part of the extracellular K(+) extruded by neurons during the depolarizing phase of the slow oscillation. During this condition, the redistribution of K(+) appeared to be local. Large steady increases of [K(+)](out) and phasic potassium accumulations were measured during spike-wave seizures. In this condition, [K(+)](in) rose before [K(+)](out) if the glial cells were located at some distance from the epileptic focus, suggesting faster K(+) diffusion through the interglial syncytium. The simultaneously recorded [Ca(2+)](out) dropped steadily during the seizures to levels incompatible with efficient synaptic transmission, but also displayed periodic oscillations, in phase with the intraseizure spike-wave complexes. In view of this fact, and considering the capability of K(+) to modulate neuronal excitability both at the presynaptic and postsynaptic levels, we suggest that the K(+) long-range spatial buffering operated by glia is a parallel synchronizing and/or spreading mechanism during paroxysmal oscillations.

L-Quisqualic acid transport into hippocampal neurons by a cystine-sensitive carrier is required for the induction of quisqualate sensitization

A brief exposure of hippocampal slices to L-quisqualic acid sensitizes CA1 pyramidal neurons 30-250-fold to depolarization by two classes of excitatory amino acid analogues: (1) those whose depolarizing effects are rapidly terminated following washout, e.g. L-2-amino-4-phosphonobutanoic acid (L-AP4) and L-2-amino-6-phosphonohexanoic acid (L-AP6) and (2) those whose depolarizing effects persist following washout, e.g. L-aspartate-beta-hydroxamate (L-AbetaH). This process has been termed quisqualate sensitization. In this study we directly examine the role of amino acid transport systems in the induction of quisqualate sensitization. We report that L-quisqualate is a low-affinity substrate (K(M)=0.54 mM) for a high capacity (V(max)=0.9 nmol (mg protein)(-1) min(-1)) Na(+)-dependent transport system(s) and a high-affinity substrate (K(M)=0.033 mM) for a low-capacity (V(max)=0.051 nmol (mg protein)(-1) min(-1)) transporter with properties similar to the cystine/glutamate exchange carrier, System x(c-). We present evidence that suggests that System x(c-) participates in quisqualate sensitization. First, simultaneous application of L-quisqualate and inhibitors of System x(c-), but not inhibitors of Na(+)-dependent glutamate transporters, prevents the subsequent sensitization of hippocampal neurons to phosphonates or L-AbetaH. Second, L-quisqualic acid only sensitizes hippocampal neurons to other substrates of System x(c-), including cystine. Third, immunocytochemical analysis of L-quisqualate uptake demonstrates that only inhibitors of System x(c-) inhibit the highly concentrative uptake of L-quisqualate into a widely dispersed group of GABAergic hippocampal interneurons. We conclude that quisqualate sensitization is a direct consequence of the unique interaction of various excitatory amino acids, namely L-quisqualate, cystine, and phosphonates, with the exchange carrier, System x(c-). Therefore, the results of this study have important implications for the mechanism by which L-quisqualate, and other substrates of this transporter which are also excitatory amino acid agonists (such as glutamate and beta-N-oxalyl-L-alpha,beta-diaminopropionic acid, beta-L-ODAP) may trigger neurotoxicity.

(1R,3S)-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD) reduces intracellular glutamate levels in astrocytes

(+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) is an equimolar mixture of two enantiomers: (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) and 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD). t-ACPD and SR-ACPD have been commonly used as agonists for metabotropic glutamate receptors (mGluR). Here we demonstrated that RS-ACPD, but not SR-ACPD, is transported into astrocytes with a K(m) of 6.51 +/- 2.38 mM and V(max) of 22.8 +/- 3.4 nmol/mg/min. This low-affinity transport is Na(+)-dependent and is competitively blocked by glutamate or other substrates for the glutamate transporter. RS-ACPD therefore is probably taken up by the glutamate transporter. Prolonged incubation with high levels of RS-ACPD (> 500 microM) induced significant swelling of astrocytes. At lower concentrations (100 microM), RS-ACPD reduced intracellular glutamate content ([Glu](i)) by > 50% without obvious morphological changes. The reduction in [Glu](i) was accompanied by an increase in [glutamine](i). The RS-ACPD's effect on [Glu](i) required glutamine and high levels of phosphate, suggesting that RS-ACPD inhibited phosphate-activated glutaminase (PAG). These data suggest that astrocytic PAG is actively involved in determining the equilibrium between intracellular glutamate and glutamine. By reducing [Glu](i), RS-ACPD reduces the amount of glutamate available for release.

Acidic amino acid transport characteristics of a newly developed conditionally immortalized rat type 2 astrocyte cell line (TR-AST)

To characterize acidic amino acid transport in type 2 astrocytes, we established conditionally immortalized rat astrocyte cell lines (TR-AST) from newly developed transgenic rats harboring temperature-sensitive SV40 large T-antigen gene. TR-AST exhibited positive immunostaining for anti-GFAP antibody and A2B5 antibody, characteristics associated with type 2 astrocytes, and expressed glutamine synthetase. Acidic amino acid transporters, GLT-1 and system xc-, which consists of xCT and 4F2hc, were expressed in all TR-ASTs by RT-PCR. On the other hand, GLAST expression was found in TR-AST3 and 5. The characteristics of [3H]L-glutamic acid (L-Glu) uptake by TR-AST5 include an Na+-dependent and Na+-independent manner, concentration-dependence, and inhibition by L-aspartic acid (L-Asp) and D-aspartic acid (D-Asp). The corresponding Michaelis-Menten constants for the Na+-dependent and Na+-independent process were 36.3 microM and 155 microM, respectively. [3H]L-Asp and [3H]D-Asp uptake by TR-AST5 had an Na+-dependent and Na+-independent manner. This study demonstrated that GLT-1, system xc-, and GLAST were expressed in TR-AST, which has the characteristics of type 2 astrocytes and is able to transport acidic amino acids.

L-Serine regulates the activities of microglial cells that express very low level of 3-phosphoglycerate dehydrogenase, an enzyme for L-Serine biosynthesis

Microglia are well known to become activated during various kinds of neuropathological events. The factors that are responsible for the activation, however, are not fully determined. In the present study, L-Ser was shown to enhance production of nitric oxide (NO), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF alpha) by lipopolysaccharide (LPS)-stimulated cultured rat microglial cells. L-Ser, however, did not enhance the expression of mRNAs encoding inducible NO synthase, IL-6 and TNF alpha. On the other hand, astrocytes did not depend on L-Ser for release of IL-6 and TNF alpha. The expression of an enzyme 3-phosphoglycerate dehydrogenase (3PGDH), which is essential for L-Ser biosynthesis from a glycolytic intermediate 3-phosphoglycerate, was investigated. As revealed by Western blotting and immunocytochemical staining, 3PGDH-protein expression in vitro was the highest in astrocytes, intermediate in neurons and the lowest in microglial cells. Semiquantitative RT-PCR showed that microglial cells expressed 3PGDH-mRNA at a lower level than astrocytes. In frozen sections from rat forebrain, only astrocytes were immunoreactive for 3PGDH. The present study suggested that L-Ser is able to modulate microglial function mainly at the translation level because microglial cells cannot synthesize sufficient amount of L-Ser due to the scarce expression of 3PGDH.

Improvement of the viability of cultured rat neurons by the non-essential amino acids L-serine and glycine that upregulates expression of the anti-apoptotic gene product Bcl-w

The non-essential amino acids L-serine (Ser) and glycine (Gly) have recently been shown to exhibit specific actions in the nervous system. In the present study, L-Ser and Gly promoted the survival of cultured rat cerebrocortical neurons in a concentration-dependent manner as revealed by Alamar blue assay and microtubule-associated protein-2 (MAP2) immunoblotting. The maximum effects of the amino acids were detected at the concentrations of 30-100 microM. L-Ser was more effective than Gly. D-Ser failed to promote neuronal survival. L-Ser and Gly upregulated expression of the anti-apoptotic gene product Bcl-w, while they did not affect the expression of Bcl-xL. The promotion of neuronal survival by L-Ser and Gly may be, at least in part, attributable to the upregulated Bcl-w.

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.

Effect of ionotropic glutamate receptors antagonists on the modifications in extracellular glutamate and aspartate levels during picrotoxin seizures: a microdialysis study in freely moving rats

Our previous studies have shown a local decrease in glutamate and aspartate levels during seizures, induced by picrotoxin microdialysis in the hippocampus of chronic freely moving rats. In this paper, we study the effect of continuous hippocampal microperfusion of the NMDA, AMPA and kainate glutamate receptor inhibitors 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801); 6,7-dinitroquinoxaline-2,3-dione (DNQX), and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). We also examine the action of L(-)-threo-3-hydroxyaspartic acid (THA), a glutamate and aspartate reuptake blocker, on the modification of extracellular glutamate and aspartate levels induced by picrotoxin, using the microdialysis method in freely moving rats. We found that changes in extracellular hippocampal concentrations in both amino acids are prevented by NMDA, AMPA and kainate receptor inhibitors. Seizures elicited under DNQX also induce a transient increase in aspartate extracellular levels coincident with seizure time. L(-)-threo-3-hydroxyaspartic acid increased the basal extracellular concentrations of both amino acids, but did not prevent the seizure-related decrease. Our results suggest that glutamate, the major neurotransmitter at the synaptic level, may also play an important role in non-synaptic transmission during seizures.

Neuronal and glial membrane potentials during sleep and paroxysmal oscillations in the neocortex

This study investigated the fluctuations in the membrane potential of cortical neurons and glial cells during the slow sleep oscillation and spike-wave (SW) seizures. We performed dual neuron-glia intracellular recordings together with multisite field potential recordings from cortical suprasylvian association areas 5 and 7 of cats under ketamine-xylazine anesthesia. Electrical stimuli applied to the cortex elicited responses consisting of a biphasic depolarization in glial cells, which was associated with an EPSP-IPSP sequence in neurons. During the slow (<1 Hz) oscillation, extracellular measurements of the potassium concentration revealed periodic increases with an amplitude of 1-2 mm, similar in shape to glial activities. We suggest that, through their uptake mechanisms, glia cells modulate the neuronal excitability and contribute to the pacing of the slow oscillation. The slow oscillation often evolved into SW paroxysms, mimicking sleep-triggered seizures. This transition was associated with increased coupling between the depolarizing events in neurons and glial cells. During seizures, the glial membrane potential displayed phasic negative events related to the onset of the paroxysmal depolarizing shifts in neurons. These events were not voltage dependent and increased their incidence and amplitude with the development of the seizure. It is suggested that the intraglial transient negativities represent field reflections of synchronized neuronal potentials. We propose that the mechanisms underlying the neuron-glia communication include, besides the traditional neurotransmitter- and ion-mediated pathways, field effects crossing their membranes as a function of the state of the cortical network.

Neuroglial responses to elevated glutamate in the medial basal hypothalamus of the infant mouse

Elevated plasma glutamate can cause selective loss of neurons in the brains of infant mice. The arcuate nucleus-median eminence region exhibits the greatest sensitivity to glutamate while it undergoes developmental maturation during early postnatal life. To investigate glutamate-induced cellular responses, groups of nursing 7-d-old mice (n = 31-93) were given single subcutaneous injections of 0.1-0.5 mg monosodium glutamate (MSG)/g body wt or an equivalent volume (30-50 microL) of water vehicle (n = 93). Injection of 0.2 mg MSG/g body wt produced a 16-fold rise in plasma glutamate after 15 min (2.10 vs. 0. 122 mmol/L control) and was the lowest harmful dose tested. It not only induced injury of small bilateral groups of medial basal hypothalamic neurons at 5 h postinjection, but also enhanced their expression of the N-methyl-D-aspartate (NMDA)R1 glutamate receptor subunit. Higher dosages of 0.3-0.5 mg MSG/g body wt yielded dose-related increases in NMDAR1 staining intensity and larger numbers of damaged neurons within the ventromedial arcuate nucleus. Administration of the live-cell nuclear stain bis-benzimide (0.95 micromol/L) indicated that MSG accessed the entire brain (n = 20) and methylene blue (1.0 g/L) permeated extracellular spaces by 15 min postinjection (n = 19), before cell death was evident (0.75 mmol/L propidium iodide) from co-injected MSG. Immunostaining, which mimicked that for glial fibrillary acidic protein, suggested that glutamate was retained in tanycytes. We conclude that elevated plasma glutamate induces glutamate receptor expression during selective injury of ventromedial arcuate neurons and propose that by sequestering glutamate, tanycytes may amplify local concentrations and promote neuronal damage in infant mice.

Ionic changes accompanying astrocytic intercellular calcium waves triggered by mechanical cell damaging stimulation

Mechanically poking or damaging a single cell within a confluent astrocyte culture produces the so-called intercellular calcium (Ca(2+)) waves, that is, cell-to-cell propagating changes of intracellular free Ca(2+). We were interested whether intercellular Ca(2+) waves are also associated with changes in other intra- or extracellular ions. To that purpose, we investigated spatiotemporal changes of intracellular Ca(2+) (Ca(i)2+), sodium (Na(i)+) and protons (H(i)+) in primary cultures of rat cortical astrocytes using microfluorescence imaging with fura-2, SBFI and BCECF, respectively; changes of extracellular potassium (K(e)+) were monitored with K(+)-sensitive microelectrodes. Mechanical damage to a single cell by stimulation with a piezo-electrically driven micropipette initiated intercellular Ca(2+) waves that propagated to about 160 microm away from the stimulation point. Na(i)(+) increases could be detected in cells located 2-3 cell diameters from the stimulated cell, acidification was observed 1-2 cell diameters away and Ke(+) increases were measured up to 75 microm away. Kinetic analysis suggests that the Na(i)(+) and H(i)(+) changes occur after, and thus secondary to the Ca(i)(2+) changes. In contrast, K(e)(+) changes occurred very fast, even before the Ca(i)(2+) changes, but their propagation speed was too fast to implicate them as a trigger of Ca(i)(2+) changes. As Na(i)(+) is an important regulator of glycolysis in astrocytes, we hypothesize that astrocytic Na(i)(+) changes in cells located remotely from a damaged cell might be a signal that activates glycolysis thereby producing more lactate that is transferred to the neurons and increases their energy potential to survive the inflicted damage.

Neurochemical and cellular reorganization of the spinal cord in a murine model of bone cancer pain

The cancer-related event that is most disruptive to the cancer patient's quality of life is pain. To begin to define the mechanisms that give rise to cancer pain, we examined the neurochemical changes that occur in the spinal cord and associated dorsal root ganglia in a murine model of bone cancer. Twenty-one days after intramedullary injection of osteolytic sarcoma cells into the femur, there was extensive bone destruction and invasion of the tumor into the periosteum, similar to that found in patients with osteolytic bone cancer. In the spinal cord, ipsilateral to the cancerous bone, there was a massive astrocyte hypertrophy without neuronal loss, an expression of dynorphin and c-Fos protein in neurons in the deep laminae of the dorsal horn. Additionally, normally non-noxious palpation of the bone with cancer induced behaviors indicative of pain, the internalization of the substance P receptor, and c-Fos expression in lamina I neurons. The alterations in the neurochemistry of the spinal cord and the sensitization of primary afferents were positively correlated with the extent of bone destruction and the growth of the tumor. This "neurochemical signature" of bone cancer pain appears unique when compared to changes that occur in persistent inflammatory or neuropathic pain states. Understanding the mechanisms by which the cancer cells induce this neurochemical reorganization may provide insight into peripheral factors that drive spinal cord plasticity and in the development of more effective treatments for cancer pain.

Compromised glutamate transport in human glioma cells: reduction-mislocalization of sodium-dependent glutamate transporters and enhanced activity of cystine-glutamate exchange

Elevated levels of extracellular glutamate ([Glu](o)) can induce seizures and cause excitotoxic neuronal cell death. This is normally prevented by astrocytic glutamate uptake. Neoplastic transformation of human astrocytes causes malignant gliomas, which are often associated with seizures and neuronal necrosis. Here, we show that Na(+)-dependent glutamate uptake in glioma cell lines derived from human tumors (STTG-1, D-54MG, D-65MG, U-373MG, U-251MG, U-138MG, and CH-235MG) is up to 100-fold lower than in astrocytes. Immunohistochemistry and subcellular fractionation show very low expression levels of the astrocytic glutamate transporter GLT-1 but normal expression levels of another glial glutamate transporter, GLAST. However, in glioma cells, essentially all GLAST protein was found in cell nuclei rather than the plasma membrane. Similarly, brain tissues from glioblastoma patients also display reduction of GLT-1 and mislocalization of GLAST. In glioma cell lines, over 50% of glutamate transport was Na(+)-independent and mediated by a cystine-glutamate exchanger (system x(c)(-)). Extracellular L-cystine dose-dependently induced glutamate release from glioma cells. Glutamate release was enhanced by extracellular glutamine and inhibited by (S)-4-carboxyphenylglycine, which blocked cystine-glutamate exchange. These data suggest that the unusual release of glutamate from glioma cells is caused by reduction-mislocalization of Na(+)-dependent glutamate transporters in conjunction with upregulation of cystine-glutamate exchange. The resulting glutamate release from glioma cells may contribute to tumor-associated necrosis and possibly to seizures in peritumoral brain tissue.

Taurine, glutamate and GABA modulate the outgrowth from goldfish retinal explants and its concentrations are affected by the crush of the optic nerve

The amino acid taurine plays an important trophic role during development and regeneration of the central nervous system. Other amino acid systems, such as those for glutamate and gamma-aminobutyric acid (GABA), are modified during the same physiological and pathological processes. After crushing the optic nerve, goldfish retinal explants were plated in the absence and in the presence of different amino acids and amino acid receptor agonists. The length and the density of the neurites were measured at 5 days in culture. Taurine increased the length and the density of neurites. Glutamate and glycine increased them at low concentration, but were inhibitors at higher concentration. The combination of N-methyl-D-aspartate (NMDA) and glycine produced a greater inhibitory effect than NMDA alone. NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) added simultaneously with taurine impaired the stimulatory effect of the latter. GABA stimulated the emission of neurites in a concentration dependent manner. Hypotaurine also elevated the length of neurites, but cysteinsesulfinic acid did not produce a significant effect. The concentrations of taurine, glutamate and GABA were determined by HPLC with fluorescent detection in the retina of goldfish at various days post-crushing the optic nerve. The levels of taurine were significantly increased at 48h after the crush, and were elevated up to 20 days. Glutamate level decreased after the lesion of the optic nerve and was still low at 20 days. GABA concentration was not significantly different from the control. The interaction of these amino acids during the regenerative period, especially the balance between taurine and glutamate, may be a determinant in restoring vision after the crush.

The release of amino acids from rat neostriatum and substantia nigra in vivo: a dual microdialysis probe analysis

It has previously been demonstrated, in dual probe microdialysis studies, that stimulation of the neostriatum with kainic acid causes the release of GABA both locally within the neostriatum and distally in the substantia nigra, observations that are consistent with the known anatomy of the basal ganglia. The object of the present study was to further examine the characteristics of GABA release and to determine whether taurine, which has been proposed to be present in striatonigral neurons, has similar characteristics of release, and to examine the release of excitatory amino acids under the same conditions. To this end, dual probe microdialysis studies were carried out on freely-moving rats. The application of kainic acid to neostriatum enhanced the release of GABA, taurine, aspartate and glutamate locally in the neostriatum and distally in the substantia nigra. The distal release of each amino acid in the substantia nigra was sensitive to the administration of 6,7-dinitroquinoxaline-2,3-dione and tetrodotoxin to the neostriatum. Similarly the local release of GABA, aspartate and glutamate but not taurine was sensitive to the intrastriatal application of 6,7-dinitroquinoxaline-2,3-dione or tetrodotoxin. It is concluded that the release of taurine from the substantia nigra has similar characteristics to that of GABA and may be released from the terminals of striatonigral neurons following the stimulation of their cell bodies in the neostriatum. The release of taurine in the neostriatum however, is likely to be mediated mainly by different mechanisms and not related to neuronal activity. The release of excitatory amino acids is likely to involve indirect effects in the neostriatum and polysynaptic pathways in the substantia nigra.

Glial cells potentiate kainate-induced neuronal death in a motoneuron-enriched spinal coculture system

AMPA/kainate receptor-mediated excitotoxicity is believed to play a pathogenic role in amyotrophic lateral sclerosis. To further characterize the mechanisms involved in AMPA/kainate receptor-mediated motoneuron injury, we investigated the influence of spinal glial cells on kainate-induced motoneuron death in vitro. A motoneuron-enriched neuronal population was obtained from embryonic mouse spinal cord by metrizamide density centrifugation. This population was cultured either on a pre-established glial feeder layer of ventral spinal origin (coculture) or in glia-free conditions (monoculture). Glial feeder layers significantly enhanced basal survival of neurons, and supported neuronal differentiation as judged by neuronal morphology and expression of the motoneuron markers peripherin and SMI-32. Neuronal vulnerability to kainate was two- to three-fold higher in coculture than in monoculture, and increased significantly with time in coculture. The effects of glial feeder layers on neuronal basal survival, differentiation and kainate vulnerability were not mimicked by conditioned medium from glial cells. The increase in neuronal kainate vulnerability with time in coculture was associated with a marked rise in the proportion of cocultured neurons possessing Ca2+-permeable AMPA/kainate receptors, as determined by kainate-activated Co2+-uptake. Neurons in monoculture were unstained by kainate-activated Co2+-uptake. Neurons were immunoreactive to specific antibodies against the AMPA receptor subunits GluR1 and GluR2 both in monoculture and coculture. This study indicates that motoneuron differentiation in coculture is associated with increased vulnerability to kainate and increased expression of Ca2+-permeable AMPA/kainate receptors. In this paradigm glial cells support basal survival and differentiation of neurons, but potentiate kainate-induced neuronal death.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

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.