Oligodendroglial lineage cells express neuroligand receptors

Mitochondria support inositol 1,4,5-trisphosphate-mediated Ca2+ waves in cultured oligodendrocytes

We have examined the spatial and temporal nature of Ca2+ signals activated via the phosphoinositide pathway in oligodendrocytes and the cellular specializations underlying oligodendrocyte Ca2+ response characteristics. Cultured cortical oligodendrocytes were incubated with fluo 3 or fura 2, and digital video fluorescence microscopy was used to study the effect of methacholine on [Ca2+]i. Single peaks, oscillations, and steady-state plateau [Ca2+]i elevations were evoked by increasing agonist concentration. The peaks and oscillations were found to be Ca2+ wave fronts, which propagate via distinct amplification regions in the cell where the kinetics of Ca2+ release (amplitude and rate of rise of response) are elevated. Staining with 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolecarbocyanine++ + iodide (JC-1) and 3,3'-dihexyloxacarbocyanine iodide revealed that mitochondria are found in groups of three or more in oligodendrocyte processes and that the groups are distributed with considerable distance separating them. Cross-correlation analysis showed a high degree of correlation between sites where mitochondria are present and peaks in the amplitude and rate of rise of the Ca2+ response. Intramitochondrial Ca2+ concentration, measured using rhod 2, increased upon treatment with methacholine. Methacholine also evoked a rapid change in mitochondrial membrane potential as measured by the J-aggregate fluorescence of JC-1. Pretreatment with the mitochondrial inhibitors carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (1 microM, 2 min) or antimycin (2 microg/ml, 2 min) altered the methacholine-evoked Ca2+ response in most cells studied, responses being either markedly potentiated or inhibited. The results of this study demonstrate that stimulation of phosphoinositide-coupled muscarinic acetylcholinoceptors activates propagating Ca2+ wave fronts in oligodendrocytes and that the characteristics of these waves are dependent on mitochondrial location and function.

Functional N-methyl-D-aspartate receptors in O-2A glial precursor cells: a critical role in regulating polysialic acid-neural cell adhesion molecule expression and cell migration

The capacity for long-distance migration of the oligodendrocyte precursor cell, oligodendrocyte-type 2 astrocyte (O-2A), is essential for myelin formation. To study the molecular mechanisms that control this process, we used an in vitro migration assay that uses neurohypophysial explants. We provide evidence that O-2A cells in these preparations express functional N-methyl-D-aspartate (NMDA) receptors, most likely as homomeric complexes of the NR1 subunit. We show that NMDA evokes an increase in cytosolic Ca2+ that can be blocked by the NMDA receptor antagonist AP-5 and by Mg2+. Blocking the activity of these receptors dramatically diminished O-2A cell migration from explants. We also show that NMDA receptor activity is necessary for the expression by O-2A cells of the highly sialylated polysialic acid-neural cell adhesion molecule (PSA-NCAM) that is required for their migration. Thus, glutamate or glutamate receptor ligands may regulate O-2A cell migration by modulating expression of PSA-NCAM. These studies demonstrate how interactions between ionotropic receptors, intracellular signaling, and cell adhesion molecule expression influence cell surface properties, which in turn are critical determinants of cell migration.

Calcium signalling in glial cells

Glial cells respond to a variety of external stimuli such as neurotransmitters, hormones or even mechanical stress by generating complex changes in the cytoplasmic Ca2+ concentration. This Ca2+ signal is controlled by an interplay of different mechanisms including plasmalemmal and intracellular Ca2+ channels, Ca2+ transporters and cytoplasmic Ca2+ buffers. In astrocytes, the Ca2+ signal can travel as waves within the syncytium spreading via gap junctions which might be regarded as a possible means for interglial communication. Ca2+ signalling is also an important medium for neurone-glia interaction: neuronal activity can trigger Ca2+ signals in glial cells and, in turn, there is evidence that glial Ca2+ signals can elicit responses in neurones. While glial cells are not equipped with the proper channels to generate action potentials, Ca2+ signalling could be the instrument by which these cells integrate and propagate signals in the CNS.

Calcium signalling in mouse Bergmann glial cells mediated by alpha1-adrenoreceptors and H1 histamine receptors

The presence of adrenergic and histaminergic receptors in Bergmann glial cells from cerebellar slices from mice aged 20-25 days was determined using fura-2 Ca2+ microfluorimetry. To measure the cytoplasmic concentration of Ca2+ ([Ca2+]i), either individual cells were loaded with the Ca2+-sensitive probe fura-2 using the whole-cell patch-clamp technique or slices were incubated with a membrane permeable form of the dye (fura-2/AM) and the microfluorimetric system was focused on individual cells. The monoamines adrenalin and noradrenalin (0.1-10 microM) and histamine (10-100 microM) triggered a transient increase in [Ca2+]i. The involvement of the alpha1-adrenoreceptor was inferred from the observations that monoamine-triggered [Ca2+]i responses were locked by the selective alpha1-adreno-antagonist prazosin and were mimicked by the alpha1-adreno-agonist phenylephrine. The monoamine-induced [Ca2+]i signals were not affected by beta- and alpha2-adrenoreceptor antagonists (propranolol and yohimbine), and were not mimicked by beta- and alpha2-adrenoreceptor agonists (isoproterenol and clonidine). Histamine-induced [Ca2+]i responses demonstrated specific sensitivity to only H1 histamine receptor modulators. [Ca2+]i responses to monoamines and histamine did not require the presence of extracellular Ca2+ and they were blocked by preincubation of slices with thapsigargin (500 nM), indicating that the [Ca2+]i responses were recorded after application of aspartate, bradykinin, dopamine, GABA, glycine, oxytocin, serotonin, somatostatin, substance P, taurine or vasopressin. We conclude that cerebellar Bergmann glial cells are endowed with alpha1-adrenoreceptors and H1 histamine receptors which induce the generation of intracellular [Ca2+]i signals via activation of Ca2+ release from inositol-1,4,5-trisphosphate-sensitive intracellular stores.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

Glutamate induces c-fos proto-oncogene expression and inhibits proliferation in oligodendrocyte progenitors: receptor characterization

The effect of glutamate on c-fos expression in oligodendrocyte progenitors was investigated by Northern blot analysis. Glutamate caused rapid and transient induction. Both 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX), two competitive non-NMDA ionotropic receptor antagonists, reduced glutamate-induced c-fos expression, whereas the NMDA antagonist MK-801 was ineffective. In addition, the glutamate receptor agonists (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) and kainate strongly induced c-fos. However, the metabotropic receptor agonist trans-(+/-)-1-amino-(1S,3R)-cyclopentanedicarboxylic acid (trans-(+/-)-ACPD) did not increase c-fos mRNA level and the antagonist L-(+)-2-amino-3-phosphonopropionic acid did not block glutamate-induced c-fos mRNA. These findings indicate that c-fos induction in oligodendrocyte progenitors is mediated through the AMPA/kainate receptors, while NMDA and metabotropic receptor subtypes are not involved. Chelation of extracellular calcium by EDTA prevented glutamate-induced c-fos expression. Similarly, the protein kinase C inhibitor 1-(5-isoquinoline-sulphonyl)-2-methylpiperazine dihydrochloride (H7) and down-regulation of protein kinase C by prolonged exposure to phorbol-12-myristate 13-acetate blocked c-fos induction. These results suggest that induction of c-fos through AMPA/kainate receptors is dependent on extracellular calcium influx and involves downstream activation of phorbol ester-sensitive protein kinase C. The effect of glutamate on oligodendrocyte progenitor proliferation was assessed by [3H]thymidine incorporation. Glutamate and the agonists kainate and AMPA, but not trans-(+/-)-ACPD, caused a dose-dependent decrease in [3H]thymidine incorporation. All these pharmacological agents were not toxic to oligodendrocyte progenitors. CNQX reversed the inhibitory effects produced by glutamate and the various agonists. These results suggest that glutamate may modulate the growth and differentiation of oligodendrocytes in the central nervous system.

A novel P2-purinoceptor expressed by a subpopulation of astrocytes from the dorsal spinal cord of the rat

1. Astrocytes from the dorsal spinal cord express P2-purinoceptors which, when stimulated, produce a rise in the intracellular level of free Ca2+ ([Ca2+]i). Previously we have found that the P2Y class of receptor is expressed by nearly all astrocytes from the dorsal horn. To determine whether other metabotropic P2-purinoceptor classes are also present, in this study we investigated the effects of UTP. 2. Application of UTP (1-500 microM, 5-20 s) produced a transient rise in [Ca2+]i in a subpopulation of astrocytes. The magnitude of the peak increase in [Ca2+]i was dependent upon UTP concentration and the EC50 was found to be 5.2 +/- 0.2 microM. Ca2+ responses were maximum at 100 microM UTP. 3. The rise in [Ca2+]i in response to UTP was not affected by removal of extracellular Ca2+. On the other hand, application of the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, abolished responses to UTP. These findings indicate that UTP stimulates the release of Ca2+ from a thapsigargin-sensitive intracellular pool. 4. The Ca2+ response to UTP was unaffected by treatment with pertussis toxin, suggesting that UTP responses may be mediated via a pertussis toxin-insensitive G protein. 5. While all cells tested (n = 52) responded to the P2Y-purinoceptor agonist, 2-methylthio-ATP, only a subpopulation of astrocytes (n = 67/93) was responsive to UTP. The presence of UTP-sensitive and UTP-insensitive cells requires the existence of two discrete types of receptor. One receptor, expressed by UTP-insensitive cells, appears to be activated selectively by 2-methylthio-ATP. 6. To investigate whether UTP and 2-methylthio-ATP activate a common type of receptor in UTP-responsive cells, a cross-desensitization strategy was used. Desensitization with prolonged exposure to a high concentration of 2-methylthio-ATP failed to affect responses to UTP and vice versa, indicating that receptors activated by UTP are distinct from those activated by 2-methylthio-ATP. 7. The P2-purinoceptor antagonist, suramin (100 microM), blocked Ca2+ responses to UTP and to 2-methylthio-ATP. 8. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), has been reported to block responses mediated by P2X- and P2Y-purinoceptors in other systems and therefore we investigated its effects on responses to 2-methylthio-ATP and to UTP. PPADS was found to block Ca2+ responses to 2-methylthio-ATP in a concentration-dependent manner with an IC50 of 0.92 +/- 0.1 microM. PPADS also blocked UTP-evoked responses and the IC50 was 7.2 +/- 1.9 microM. At a concentration of 10 microM, PPADS produced a rightward shift in the dose-response curve for UTP and did not affect the maximum response. 9. Calcium responses evoked by the muscarinic agonist, carbachol, were unaffected either by suramin (100 microM) or by PPADS (50 microM). 10. The present results indicate the presence of a novel class of metabotropic P2U-purinoceptor in dorsal spinal astrocytes. In contrast to P2Y-purinoceptors, the P2U-purinoceptor is expressed only by a subpopulation of astrocytes and its sensitivity to suramin and PPADS distinguish this receptor from P2U-purinoceptors found in other tissues.

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

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

Calcium signaling in cultured rat oligodendrocytes

The syntax of neuronal-glial or axonal-glial interaction is frequently communicated through transient changes in internal calcium (Cai). We examined mechanisms for Cai signaling and intercellular propagation of Cai responses in cultured oligodendrocytes (OLGs) derived from adult spinal cord (SC), postnatal day 21 (P21) SC, and P21 brain. We found that (1) cultured OLGs exhibited a heterogeneous response to norepinephrine, carbachol, ATP, histamine, and glutamate; (2) receptor-mediated Cai increases were derived from both Ca2+ influx and intracellular Ca2+ release; (3) the percentage of responders to neuroligands varied as a function of cell origin; (4) cultured OLGs exhibited a thapsigargin-sensitive, but not a caffeine-sensitive, intracellular Ca2+ pool; and (5) gap junctional contacts between OLGs permitted limited intercellular propagation of mechanically stimulated Cai responses. Receptor-mediated Cai signaling appears to occur not only in cultured OLGs but also in acutely dissociated OLGs. The heterogeneity in Cai responses as a function of cell origin may reflect the existence of OLG subsets or differences in the maturation stage of OLGs.

Activation of P2-purinoreceptors triggered Ca2+ release from InsP3-sensitive internal stores in mammalian oligodendrocytes

1. The subcellular characteristics of an ATP-induced elevation of the cytoplasmic free calcium concentration ([Ca2+]i) were studied in cultured cells of the oligodendrocyte lineage obtained from mouse cortex and rabbit retina, as well as in oligodendrocytes from mouse corpus callosum slices, using laser scanning confocal microfluorometry. 2. With the stage- and lineage-specific antibodies O4 and O10, three developmental stages within the oligodendrocyte lineage were distinguished prior to Ca2+ recording. 3. Bath application of 1-100 microM ATP induced a transient increase of [Ca2+]i in late precursors and oligodendrocytes but not in early glial precursor cells from retinal and cortical cultures and from corpus callosum slices. This effect of ATP was observed in Ca(2+)-free extracellular solution, suggesting that the ATP-mediated elevation of [Ca2+]i is due to a Ca2+ liberation from intracellular stores. 4. In both late precursors and oligodendrocytes from retina, the amplitude of ATP-induced [Ca2+]i transients was significantly higher in processes as compared with the soma; in cortical cultures such an uneven response was only observed in oligodendrocytes, while in immature cells responses in soma and processes were of similar amplitude. 5. The rank order of potency for the purine and pyrimidine nucleotides was UTP > or = ATP > ADP >> AMP = adenosine = Me-ATP for retinal oligodendrocytes, and ADP > or = ATP >> UTP = AMP = adenosine = Me-ATP for cortical oligodendrocytes. The response to ATP and related nucleotides was blocked by suramin, indicating the involvement of a P2-purinoreceptor in the ATP-mediated [Ca2+]i response. 6. ATP-induced elevation of the cytosolic Ca2+ concentration was inhibited by incubating cells with thapsigargin (10 microM) and by intracellular administration of heparin (1 microM). These findings indicate that ATP triggers a release of Ca2+ ions from InsP3-sensitive internal stores. 7. The ATP receptors may play a role in neuron-glial signal transfer; ATP is released as neurotransmitter, but also under pathological conditions from damaged cells.

Identification of G protein-regulated inwardly rectifying K+ channels in rat brain oligodendrocytes

Rat brain oligodendroglia in culture express a dominant inwardly rectifying K+ current IKIR which can be inhibited through G. proteins by the activation of glial G protein-coupled receptors. Electrophysiologically we have isolated in these cells several conductances of K+ inward rectifiers (KIR) between 12 and 175 pS. Experiments on the single channel level with preloading or bath-application of GTP gamma S revealed the selective suppression of an 18 pS and maybe other KIR conductances, possibly via a direct membrane-delimited mechanism. mRNA amplification from single oligodendrocytes together with polymerase chain reaction resulted in the isolation of IRK-type K+ channels which may correspond to the channel species negatively controlled by G proteins.

Schwann cell ATP-mediated calcium increases in vitro and in situ are dependent on contact with neurons

Schwann cells freshly isolated from the sciatic nerves of neonatal rats respond to exogenously applied ATP with a rapid increase in cytosolic calcium. This increase in [Ca2+]i is mediated by a P2Y-purinergic pathway (Lyons et al.: J. Neurochem. 63:552-560, 1994) and was measured using the calcium indicator dye, fura-2/AM, and a video-enhanced calcium imaging system. The ability to respond to ATP with increases in intracellular calcium is lost over a period of several days in culture; this loss can be prevented or reversed by application of cAMP analogs in a defined medium. We now demonstrate that the direct contact of Schwann cells with neurons also induces and stabilizes this ATP responsiveness. The induction of ATP responsiveness was observed among all Schwann cells contacting neurites, including those forming myelin, and regardless of whether the source of neurons was dorsal root ganglion neurons or superior cervical ganglion neurons. Approximately 85% of Schwann cells responded to ATP over the time studied (72 d in coculture). Addition of axolemma to Schwann cell cultures did not induce ATP responsiveness. We also examined the ATP responsiveness of Schwann cells in situ (excised nerves) using laser-scanning confocal microscopy and the calcium indicator dye, fluo-3/AM. Schwann cells in intact sciatic nerve segments isolated from neonatal and 16-day-old rats exhibited ATP-mediated [Ca2+]i increases. We conclude that neuronal contact is necessary for the expression of the ATP-mediated calcium responses in Schwann cells and that these responses are independent of myelin formation or maintenance.

Fast inhibition of inwardly rectifying K+ channels by multiple neurotransmitter receptors in oligodendroglia

An essential function of myelinating oligodendroglia in the mammalian central nervous system is the regulation of extracellular potassium levels by means of a prominent inwardly rectifying K+ current. Cardiac and neuronal K+ inward rectifiers are either activated by hyperpolarizing voltages or controlled by neurotransmitters through the action of receptor-activated G proteins. Neuromodulation of inward rectifiers has not previously been considered as a way to regulate oligodendrocyte function. Here we report the expression of serotonin, somatostatin and muscarinic acetylcholine G protein-coupled receptors in rat brain oligodendrocytes. Activation of these receptors leads to pertussis toxin-sensitive inhibition of inwardly rectifying K+ channels within < 1 s. By contrast, in the heart and in neurons, similar pathways activate an inwardly rectifying conductance. Thus, transmitter-mediated blockade of inward rectifiers appears to be an oligodendrocyte-specific variation of a common motif for convergent signalling pathways. In vivo, expression of this mechanism, which may be dependent on neuron-glia signalling, may have a regulatory role in K+ homeostasis during neuron activity in the central nervous system.

AMPA/kainate receptor activation in murine oligodendrocyte precursor cells leads to activation of a cation conductance, calcium influx and blockade of delayed rectifying K+ channels

Studies during the last few years have shown that glial cells can express a large repertoire of neurotransmitter receptors. In this study, we have characterized the properties of a glutamate receptor in oligodendrocytes and their precursor cells from cultures of mouse brain, using the patch-clamp technique to measure ligand-activated currents and a fura-2 imaging system to determine changes in free cytosolic Ca2+ concentration ([Ca2+]i). The precursor cells were identified by their characteristic morphology and their voltage-gated currents as described previously [Sontheimer H. et al. (1989) Neuron 2, 1135-1145]. The ligands kainate, domoate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), as well as L-glutamate but not trans-1-amino-1,3-cyclopentanedicarboxylate elicited inward currents at a holding potential of -70 mV and the antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the glutamate- and kainate-induced response reversibly, indicating the expression of an AMPA/kainate-type glutamate receptor. The response is due to the activation of a cationic conductance as revealed by analysing the reversal potential of the kainate-activated current. Receptor activation is accompanied by two additional responses: (i) an increase in [Ca2+]i mediated by depolarization and a subsequent activation of voltage-gated Ca2+ channels and (ii) a transient blockade of a delayed rectifying K+ current, but not of the A-type K+ current. The blockade of the K+ current was not due to the increase in [Ca2+]i since it was also observed in Ca(2+)-free bathing solution when no increase in [Ca2+]i was detectable after exposure to kainate. In contrast to precursor cells, oligodendrocytes responded weakly or not at all to glutamate or related ligands. We conclude that glutamate activates a complex pattern of physiological events in the glial precursor cells, which may play a role during the differentiation process of these cells.

Rat oligodendrocytes express muscarinic receptors coupled to phosphoinositide hydrolysis and adenylyl cyclase

Muscarinic receptors expressed by rat oligodendrocyte primary cultures were examined by measuring changes in second messengers following exposure to carbachol, an acetylcholine analog, and by polymerase chain reaction. Inositol phosphate levels were measured in [3H]myo-inositol-labelled young oligodendrocyte cultures following stimulation with carbachol. Atropine, a specific muscarinic antagonist, prevented the carbachol-induced accumulation of inositol phosphates. The formation of inositol trisphosphate was concentration- and time-dependent, with the peak at 100 microM carbachol and 10 min. Carbachol increased intracellular calcium levels, which were dependent both on the mobilization of intracellular stores and influx of extracellular calcium. In initial experiments with more selective antagonists, the mobilization of intracellular calcium was preferentially inhibited by pirenzepine, a selective M1 antagonist, but not methoctramine, a selective M2 antagonist, suggesting M1 muscarinic receptor involvement. A role for protein kinase C in the regulation of carbachol-stimulated inositol phosphate formation and intracellular calcium mobilization was demonstrated, as acute pretreatment with phorbol-12,13-myristate acetate abolished the formation of both second messengers. Pretreatment with 100 microM carbachol abolished the 40% increase in the cyclic AMP accumulation stimulated by isoproterenol, a specific beta-adrenergic agonist. In turn, the inhibition was alleviated by pretreatment with atropine, suggesting muscarinic receptor involvement. Polymerase chain reaction carried out with specific m1 and m2 muscarinic receptor oligonucleotide primers, confirmed that these cells express, at least, the two muscarinic receptor subtypes. Without excluding the expression of other subtypes, these results suggest that developing oligodendrocytes express m1 (M1) and m2 (M2) muscarinic receptors capable of mediating phosphoinositide hydrolysis, mobilization of intracellular calcium and the attenuation of beta-adrenergic stimulation of cyclic AMP formation.

Histamine and prostanoid receptors on glial cells

Glial cells in vitro express at least two types (H1 and H2) of histamine receptors and three types (EP, FP, and TP) of prostanoid receptors. The receptors expressed by glial cells differ according to the cell type and source in the brain. Furthermore primary astrocytes of same type derived from the same brain region are composed of heterogeneous subpopulations expressing different subsets of receptors. Fura-2 based Ca2+ microscopy revealed that astrocyte processes are important sites for histamine-induced Ca2+ signalling. Histamine and prostanoid receptors on glial cells may play important roles in the actions of histamine and prostanoids in the central nervous system.

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

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

Neurotransmitter-mediated signaling between axons and glial cells

Neurotransmitter-mediated signaling is not restricted to the synaptic regions of the nervous system but also takes places along fiber tracts lacking vesicular means of releasing neuroactive substances. The first demonstration for dynamic signaling of this type came in the early 1970s from studies by Villegas and co-workers in squid axons and their satellite Schwann cells. In this invertebrate system, glutamate has been identified as the mediator of this signaling in being first released from the active axons thus setting off a series of cascades, leading to a cholinergic activation of the Schwann cell membrane. Recent evidence suggests that receptor-mediated signaling also exists between glial cells and axons in vertebrates. In the frog optic nerve, axonal activity facilitated the activity of glial ion channels. In the neonatal rat optic nerve, electrical activity of axons triggered oscillations in intracellular calcium in a subset of glial cells. These observations have been postulated to reflect receptor-mediated signaling, including a mechanism in which glutamate is released from axons via the reversal of a transporter and induces intracellular calcium spiking in glial cells via metabotropic glutamate receptors. The efficacy of "axon-to-glia" transmission may, like that in "neuron-to-neuron" transmission, be modulated by co-release of multiple neuroactive substances. One possibility is that adenosine, which is known to be released from fiber tracts, can modulate glutamate signaling in white matter by modulating the periaxonal glutamate concentration through an effect on the glial glutamate uptake system.