Modulation of synaptic efficacy and synaptic depression by glial cells at the frog neuromuscular junction

LNX1 is a perisynaptic Schwann cell specific E3 ubiquitin ligase that interacts with ErbB2

Non-myelinating perisynaptic Schwann cells wrap motor axon terminals and are required for both functional and structural integrity of the neuromuscular junction. Several lines of evidence indicate that fine-tuning of neuregulin-1/ErbB signaling is critical for maintaining perisynaptic Schwann cells at synapses and that this control may be achieved by the developmental downregulation of the ErbB2 receptor. Here, we identify a direct interaction between ErbB2 and LNX1, an E3 ubiquitin ligase that can target interacting proteins for degradation through ubiquitination. Immunostaining shows that LNX1 is specifically localized in perisynaptic Schwann cells but not in Schwann cells along the motor axon. Developmentally, levels of LNX1 protein are inversely correlated with the responsiveness of perisynaptic Schwann cells to neuregulin-1. Furthermore, the LNX1 staining disappears upon denervation, whereas ErbB2 reappears in Schwann cells after denervation. Taken together, these data suggest that LNX1 may play a role in regulating neuregulin-1/ErbB signaling in perisynaptic Schwann cells.

Glial calcium signaling and neuron–glia communication

The existence of bidirectional signaling between astrocytes and neurons has revealed an important active role of astrocytes in the physiology of the nervous system. As a consequence, there is a new concept of the synaptic physiology-"the tripartite synapse", where astrocytes exchange information with the pre- and postsynaptic elements and participate as dynamic regulatory elements in neurotransmission. The control of the Ca2+ excitability in astrocytes is a key element in this loop of information exchange. The ability of astrocytes to respond to neuronal activity and discriminate between the activity of different synapses, the modulation of the astrocytic cellular excitability by the synaptic activity, and the expression of cellular intrinsic properties indicate that astrocytes are endowed with cellular computational characteristics that process synaptic information. Therefore, we propose that astrocytes can be considered as cellular elements involved in the information processing by the nervous system.

Fluorescent proteins expressed in mouse transgenic lines mark subsets of glia, neurons, macrophages, and dendritic cells for vital examination

To enable vital observation of glia at the neuromuscular junction, transgenic mice were generated that express proteins of the green fluorescent protein family under control of transcriptional regulatory sequences of the human S100B gene. Terminal Schwann cells were imaged repetitively in living animals of one of the transgenic lines to show that, except for extension and retraction of short processes, the glial coverings of the adult neuromuscular synapse are stable. In other lines, subsets of Schwann cells were labeled. The distribution of label suggests that Schwann cells at individual synapses are clonally related, a finding with implications for how these cells might be sorted during postnatal development. Other labeling patterns, some present in unique lines, included astrocytes, microglia, and subsets of cerebellar Bergmann glia, spinal motor neurons, macrophages, and dendritic cells. We show that lines with labeled macrophages can be used to follow the accumulation of these cells at sites of injury.

Norepinephrine triggers release of glial ATP to increase postsynaptic efficacy

Glial cells actively participate in synaptic transmission. They clear molecules from the synaptic cleft, receive signals from neurons and, in turn, release molecules that can modulate signaling between neuronal elements. Whether glial-derived transmitters can contribute to enduring changes in postsynaptic efficacy, however, remains to be established. In rat hypothalamic paraventricular nucleus, we demonstrate an increase in the amplitude of miniature excitatory postsynaptic currents in response to norepinephrine that requires the release of ATP from glial cells. The increase in quantal efficacy, which likely results from an insertion of AMPA receptors, is secondary to the activation of P2X(7) receptors, an increase in postsynaptic calcium and the activation of phosphatidylinositol 3-kinase. The gliotransmitter ATP, therefore, contributes directly to the regulation of postsynaptic efficacy at glutamatergic synapses in the CNS.

Changes in the neuromuscular synapse induced by an antibody against gangliosides

In this study, we used a monoclonal IgM antibody from a patient with a pure motor chronic demyelinating polyneuropathy, which binds specifically to the complex gangliosides GM(2), GalNAc-GD(1a), and GalNAc-GM(1b), which appear to have a common epitope of -[GalNAcbeta1-4Gal(3-2alphaNeuAc)beta1]. This was done for the following reasons: (1) to localize these gangliosides in specific cellular components of the neuromuscular junction (NMJ), and (2) to describe the anti-ganglioside antibody-induced structural and functional changes in the NMJs to gain insight into the role of gangliosides in the synaptic function. Using immunofluorescence techniques, we found that these gangliosides are located only in the presynaptic component of the motor end-plates, both in nerve terminals and in Schwann cells. After 2 weeks of continued passive transfer of the IgM monoclonal antibody over the mouse levator auris longus muscle, electromyography showed an axonal or NMJ disorder. Morphology showed important nerve terminal growth and retraction changes. Using intracellular recording electrophysiology, we found neurotransmitter release alterations, including quantal content reduction and an immature expression of voltage-dependent calcium channels similar to what occurred during NMJ development and regeneration. These changes were complement independent. The results showed that these gangliosides were involved in the reciprocal Schwann cell-nerve terminal interactions, including structural stability and neurotransmission.

Dye coupling among satellite glial cells in mammalian dorsal root ganglia

Dorsal root ganglia (DRG) are key elements in sensory signaling under physiological and pathological conditions. Little is known about electrical coupling among cells in these ganglia. In this study, we injected the fluorescent dye Lucifer yellow (LY) into single cells to examine dye coupling in DRG. We found no dye coupling between neurons or between neurons and their attendant satellite glial cells (SGCs). In mouse DRG, we observed that in 26.2% of the cases SGCs that surround a given neuron were dye coupled. In only 3.2% of the cases SGCs that make envelopes around different neurons were coupled. The data from mouse ganglia were very similar to those from rat and guinea pig DRG. The results obtained by injection of the tracer biocytin were very similar to those observed with LY. The coupling incidence within the envelopes increased 3.1-fold by high extracellular pH (8.0), but coupling between envelopes was not affected. Acidic pH (6.8) reduced the coupling. High extracellular K+ (9.4 mM) increased the coupling 2.4-fold and 4.7-fold within and between envelopes, respectively. Low extracellular Ca2+ (0.5, 1.0 mM) partly reversed the effect of high K+ on coupling. The results showed that SGCs in mammalian sensory ganglia are connected by gap junctions. This coupling is very sensitive to changes in pH, and can therefore be modulated under various physiological and pathological conditions. The dependence of the coupling on extracellular K+ and Ca2+ suggests that the permeability of gap junctions can be altered by physiological and pharmacological stimuli.

Varicosity-Schwann cell interactions mediated by ATP in the mouse vas deferens

Schwann cells, from a variety of sources, are known to possess P2Y purinergic metabotropic receptors. However, it is not known if Schwann cells associated with autonomic nerve terminals possess such receptors and if so whether these receptors are activated by the endogenous release of ATP from the nerve terminals. We show that such Schwann cells in the vas deferens give evoked calcium transients on nerve stimulation. These transients are mediated, at least in part, by the endogenous release of ATP, which acts on Schwann cell P2Y receptors to release calcium from within the cells. This work suggests the possibility that Schwann cells are active participants in the process of junctional transmission in the autonomic nervous system.

Glia–Neuron Interactions in Nervous System Function and Development

Nervous systems are generally composed of two cell types-neurons and glia. Early studies of neurons revealed that these cells can conduct electrical currents, immediately implying that they have roles in the relay of information throughout the nervous system. Roles for glia have, until recently, remained obscure. The importance of glia in regulating neuronal survival had been long recognized. However, this trophic support function has hampered attempts to address additional, more active functions of these cells in the nervous system. In this chapter, recent efforts to reveal some of these additional functions are described. Evidence supporting a role for glia in synaptic development and activity is presented, as well as experiments suggesting glial guidance of neuronal migration and process outgrowth. Roles for glia in influencing the electrical activity of neurons are also discussed. Finally, an exciting system is described for studying glial cells in the nematode C. elegans, in which recent studies suggest that glia are not required for neuronal viability.

Direct evidence for mutual interactions between perineuronal astrocytes and interneurons in the CA1 region of the rat hippocampus

Recent studies have demonstrated that astrocytes express a variety of ion channels and neurotransmitter receptors and can modulate the activity of neurons. Since a single astrocyte makes tight contacts with many neighboring neuronal cells, they can provide efficient and wide modulation of neuronal networks. Here, we provide direct evidence for mutual interactions between perineuronal astrocytes and interneurons in the stratum radiatum of the rat hippocampus. Direct depolarization of a perineuronal astrocyte suppressed the excitatory postsynaptic currents in an adjacent interneuron and increased the paired-pulse ratio, indicating that perineuronal astrocytes have a suppressive effect on presynaptic elements. Moreover, perineuronal astrocyte activation modulated the directly induced firing pattern of the interneuron, with initial facilitation and subsequent suppression. Conversely, direct firing of the interneuron depolarized the membrane potential and reduced the input resistance of the perineuronal astrocyte. These results directly demonstrate the existence of bidirectional interactions between neurons and perineuronal astrocytes.

Nitric oxide, cAMP and the biphasic muscarinic modulation of ACh release at the lizard neuromuscular junction

In this study, we characterized the pharmacology and physiology of the automodulation of ACh release at the lizard neuromuscular junction (NMJ). The activation of muscarinic ACh receptors generated a biphasic modulation of synaptic transmission. Muscarine-induced activation of M3 receptors (0-12 min) decreased release, whereas M1 activation (> 12 min) enhanced release. Both phases of the biphasic effect are dependent on nitric oxide. However, cAMP acting via protein kinase A is also necessary for the M1 effect. In summary, we present a novel biphasic role for muscarine and implicate M3 receptors in the inhibition and M1 receptors in the enhancement of transmitter releaseat the cholinergic lizard NMJ.

Synaptotagmin IV regulates glial glutamate release

Calcium-binding synaptotagmins (Syts) are membrane proteins that are conserved from nematode to human. Fifteen Syts (Syts I-XV) have been identified in mammalian species. Syt I has been well studied and is a candidate for the Ca(2+)-sensor that triggers evoked exocytosis underlying fast synaptic transmission. Whereas the functions of the other Syts are unclear, Syt IV is of particular interest because it is rapidly up-regulated after chronic depolarization or seizures, and because null mutations exhibit deficits in fine motor coordination and hippocampus-dependent memory. Screening Syts I-XIII, which are enriched in brain, we find that Syt IV is located in processes of astroglia in situ. Reduction of Syt IV in astrocytes by RNA interference decreases Ca(2+)-dependent glutamate release, a gliotransmission pathway that regulates synaptic transmission. Mutants of the C2B domain, the only putative Ca(2+)-binding domain in Syt IV, act in a dominant-negative fashion over Ca(2+)-regulated glial glutamate release, but not gliotransmission induced by changes in osmolarity. Because we find that Syt IV is expressed predominantly by astrocytes and is not in the presynaptic terminals of the hippocampus, and because Syt IV knockout mice exhibit hippocampal-based memory deficits, our data raise the intriguing possibility that Syt IV-mediated gliotransmission contributes to hippocampal-based memory.

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

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

Glutamatergic modulation of synaptic plasticity at a PNS vertebrate cholinergic synapse

The presence and the functionality of a glutamatergic regulation was studied at the frog neuromuscular junction (NMJ), a singly innervated cholinergic synapse. Bath application of glutamate reduced transmitter release without affecting nerve-evoked presynaptic Ca2+ entry and handling. (1S,3R)-aminocyclopentanedicarboxylic acid (ACPD), a metabotropic glutamate receptor (mGluR) agonist, mimicked the effects of glutamate while (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), a mGluR antagonist, blocked glutamate effects. MCPG had no effect on transmitter release evoked at low frequency (0.2 Hz) but significantly reduced synaptic depression (10 Hz, 80 s). This suggests that a frequency-dependent endogenous glutamatergic modulation is present at the frog NMJ and is mediated through mGluRs. Immunohistochemical labelling revealed the presence of mGluRs at the end plate area, primarily on muscle fibers. Functional glutamate uptake machinery was also found at the NMJ as blockade of glutamate transport by the inhibitor dl-threo-beta-benzyloxyaspartate (DL-TBOA) increased high frequency-induced depression, suggesting that the transporters system is used to eliminate glutamate from the extracellular space. Moreover, immunohistochemical labelling revealed that glutamate-aspartate transporters (GLASTs) are predominantly present on perisynaptic Schwann cells (PSCs). However, local application of glutamate on PSCs unreliability evoked small Ca2+ responses. Hence, these data suggest that functional glutamatergic interactions at a purely cholinergic synapse, shape synaptic efficacy and short-term plasticity in a frequency-dependent fashion.

Discovering novel phenotype-selective neurotrophic factors to treat neurodegenerative diseases

Astrocytes and neurons in the central nervous system (CNS) interact functionally to mediate processes as diverse as neuroprotection, neurogenesis and synaptogenesis. Moreover, the interaction can be homotypic, implying that astrocyte-derived secreted molecules affect their adjacent neurons optimally vs remote neurons. Astrocytes produce neurotrophic and extracellular matrix molecules that affect neuronal growth, development and survival, synaptic development, stabilization and functioning, and neurogenesis. This new knowledge offers the opportunity of developing astrocyte-derived, secreted proteins as a new class of therapeutics specifically to treat diseases of the CNS. However, primary astrocytes proliferate slowly in vitro, and when induced to immortalize by genetic manipulation, tend to lose their phenotype. These problems have limited the development of astrocytes as sources of potential drug candidates. We have successfully developed a method to induce spontaneous immortalization of astrocytes. Gene expression analysis, karyotyping and activity profiling data show that these spontaneously immortalized type-1 astrocyte cell lines retain the properties of their primary parents. The method is generic, such that cell lines can be prepared from any region of the CNS. To date, a library of 70 cell lines from four regions of the CNS: ventral mesencephalon, striatum, cerebral cortex and hippocampus, has been created. A phenotype-selective neurotrophic factor for dopaminergic neurons has been discovered from one of the cell lines (VMCL1). This mesencephalic astrocyte-derived neurotrophic factor (MANF) is a 20 kD, glycosylated, human secreted protein. Homologs of this protein have been identified in 16 other species including C. elegans. These new developments offer the opportunity of creating a library of astrocyte-derived molecules, and developing the ones with the best therapeutic indices for clinical use.

Fusion-related release of glutamate from astrocytes

Although cell culture studies have implicated the presence of vesicle proteins in mediating the release of glutamate from astrocytes, definitive proof requires the identification of the glutamate release mechanism and the localization of this mechanism in astrocytes at synaptic locales. In cultured murine astrocytes we show an array of vesicle proteins, including SNARE proteins, and vesicular glutamate transporters that are required to fill vesicles with glutamate. Using immunocytochemistry and single-cell multiplex reverse transcription-PCR we demonstrate the presence of these proteins and their transcripts within astrocytes freshly isolated from the hippocampus. Moreover, immunoelectron microscopy demonstrates the presence of VGLUT1 in processes of astrocytes of the hippocampus. To determine whether calcium-dependent glutamate release is mediated by exocytosis, we expressed the SNARE motif of synaptobrevin II to prevent the formation of SNARE complexes, which reduces glutamate release from astrocytes. To further determine whether vesicular exocytosis mediates calcium-dependent glutamate release from astrocytes, we performed whole cell capacitance measurements from individual astrocytes and demonstrate an increase in whole cell capacitance, coincident with glutamate release. Together, these data allow us to conclude that astrocytes in situ express vesicle proteins necessary for filling vesicles with the chemical transmitter glutamate and that astrocytes release glutamate through a vesicle- or fusion-related mechanism.

Glial cells maintain synaptic structure and function and promote development of the neuromuscular junction in vivo

To investigate the in vivo role of glial cells in synaptic function, maintenance, and development, we have developed an approach to selectively ablate perisynaptic Schwann cells (PSCs), the glial cells at the neuromuscular junction (NMJ), en masse from live frog muscles. In adults, following acute PSC ablation, synaptic structure and function were not altered. However, 1 week after PSC ablation, presynaptic function decreased by approximately half, while postsynaptic function was unchanged. Retraction of nerve terminals increased over 10-fold at PSC-ablated NMJs. Furthermore, nerve-evoked muscle twitch tension was reduced. In tadpoles, repeated in vivo observations revealed that PSC processes lead nerve terminal growth. In the absence of PSCs, growth and addition of synapses was dramatically reduced, and existing synapses underwent widespread retraction. Our findings provide in vivo evidence that glial cells maintain presynaptic structure and function at adult synapses and are vital for the growth and stability of developing synapses.

Glial modulation of synaptic transmission at the neuromuscular junction

The neuromuscular junction (NMJ) is a cholinergic synapse that controls muscle contraction. Glial cells, called perisynaptic Schwann cells, surround nerve terminals at the NMJ. Transmitter release induced by repetitive nerve stimulation, elicit a frequency-dependent activation of G-protein-coupled receptors on perisynaptic Schwann cells and the release of calcium from internal stores. In return, perisynaptic Schwann cells modulate synaptic activity during and following high-frequency stimulation through short-term plasticity. In the present review, we discuss evidence of glial involvement in the short-term plasticity at the NMJ and the potential impact of such modulation on synaptic efficacy.

The neurobiology of glia in the context of water and ion homeostasis

Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.

Muscarinic autoreceptors related with calcium channels in the strong and weak inputs at polyinnervated developing rat neuromuscular junctions

Using intracellular recording, we studied how several muscarinic antagonists affected the evoked endplate potentials in singly and dually innervated endplates of the levator auris longus muscle from 3 to 6-day-old rats. In dually innervated fibers, a second endplate potential (EPP) may appear after the first one when we increase the stimulation intensity. The lowest and highest EPP amplitudes are designated "small-EPP" and "large-EPP," respectively. In singly innervated endplates and large-EPP, we found an inhibition of acetylcholine release by M1-receptor antagonists pirenzepine and MT-7 (more than 30%) and M2-receptor antagonists methoctramine and AF-DX 116 (more than 40%). The small-EPP was also inhibited by both M2-receptor antagonists methoctramine (approximately 70%) and AF-DX 116 (approximately 40%). However, the small-EPP was enhanced by M1-receptor antagonists pirenzepine (approximately 90%) and MT-7 (approximately 50%). The M4-receptor selective antagonists tropicamide and MT-3 can also increase the small-EPP amplitude (75% and 120%, respectively). We observed a graded change from a multichannel involvement (P/Q- N- and L-type voltage-dependent calcium channels) of all muscarinic responses (M1-, M2- and M4-mediated) in the small-EPP to the single channel (P/Q-type) involvement of the M1 and M2 responses in the singly innervated endplates. This indicates the existence of a progressive calcium channels shutoff in parallel with the specialization of the adult type P/Q channel. In conclusion, muscarinic autoreceptors can directly modulate large-EPP generating ending potentiation, and small-EPP generating ending depression through their association with the calcium channels during development.

Acetylcholine binding protein (AChBP): a secreted glial protein that provides a high-resolution model for the extracellular domain of pentameric ligand-gated ion channels

Acetylcholine binding protein (AChBP) has recently been identified from molluskan glial cells. Glial cells secrete it into cholinergic synapses, where it plays a role in modulating synaptic transmission. This novel mechanism resembles glia-dependent modulation of glutamate synapses, with several key differences. AChBP is a homolog of the ligand binding domain of the pentameric ligand-gated ion-channels. The crystal structure of AChBP provides the first high-resolution structure for this family of Cys-loop receptors. Nicotinic acetylcholine receptors and related ion-channels such as GABAA, serotonin 5HT3, and glycine can be interpreted in the light of the 2.7 A AChBP structure. The structural template provides critical details of the binding site and helps create models for toxin binding, mutational effects, and molecular gating.

Signaling at the gliovascular interface

Advances in fluorescent calcium indicating dyes over the past decade have identified calcium signaling as the tool by which astrocytes communicate among themselves and with neighboring neurons. Studies of astrocyte-neuron interactions have shown that calcium signaling is a potent modulator of the strength of both excitatory and inhibitory synapses. The concept that astrocytes possess a mechanism for rapid cell communication has not been incorporated, however, into the supportive functions of astrocytes. Because many of the classical tasks of astrocytes are linked to the blood-brain barrier, we have here examined the expression of proteins required for calcium signaling in their vascular end-foot processes. The gap junction protein, Cx43, was expressed intensively around the vessels interconnecting astrocytic end-foot processes. These gap junctions permitted diffusion of Lucifer yellow, specifically along the path of glial end feet apposed to the vessel wall. The purinergic receptors, P2Y(2) and P2Y(4), were also strongly expressed at the gliovascular interface and colocalized with GFAP around larger vessels in cortex. Multiphoton imaging of freshly prepared brain slices loaded with Fluo-4/AM revealed that ATP mobilized cytosolic calcium in astrocytic end feet, whereas electrical stimulation triggered calcium waves propagating along the vessel wall. Brain endothelial cells and pericytes were physically separated from astrocytes by the basal lamina and responded only weakly to ATP. These observations identify astrocytic end-foot processes plastered at the vessel wall as a center for purinergic signaling. It is speculated that calcium signaling may play a role in astrocytic functions related to the blood-brain barrier, including blood flow regulation, metabolic trafficking, and water homeostasis.

IP3 receptors and associated Ca2+ signals localize to satellite cells and to components of the neuromuscular junction in skeletal muscle

Recently, we described an inositol 1,4,5-trisphosphate (IP3) signaling system in cultured rodent skeletal muscle, triggered by high K+ and affecting gene transcription (Powell et al., 2001). Now, in a study of adult rodent skeletal muscle, using immunocytology and confocal microscopy, we have found a high level of IP3 receptor (IP3R) staining in satellite cells, which have been shown recently to contribute to nuclei in adult fibers after muscle exercise. These IP3R staining cells are positively identified as satellite cells by their position, morphology and staining with satellite-cell-specific antibodies such as desmin and neural cell adhesion molecule. IP3Rs are also localized to postsynaptic components of the neuromuscular junction (NMJ), in areas surrounding the nuclei of the motor end plate, and in perisynaptic Schwann cells, and localized close to nicotinic acetylcholine receptors of the endplate gutters. Ca2+ imaging experiments show calcium release at the motor endplate upon K+ depolarization precisely in these IP3R-rich regions. We suggest that electrical activity stimulates IP3-associated Ca2+ signals that may be involved in gene regulation in satellite cells and in elements of the NMJ, contributing both to muscle fiber growth and stabilization of the NMJ.

Inositol derivatives modulate spontaneous transmitter release at the frog neuromuscular junction

One of the consequences of G-protein-coupled receptor activation is stimulation of phosphoinositol metabolism, leading to the generation of IP3 and its metabolites 1,3,4,5-tetrakisphosphate (IP4) and inositol 1,2,3,4,5,6-hexakisphosphate (IP6). Previous reports indicate that high inositol polyphosphates (IP4 and IP6) are involved in clathrin-coated vesicular recycling. In this study, we examined the effects of IP4 and IP6 on spontaneous transmitter release in the form of miniature endplate potentials (MEPP) and on enhanced vesicular recycling by high K+ at frog motor nerve endings. In resting conditions, IP4 and IP6 delivered intracellularly via liposomes, caused concentration-dependent increases in MEPP frequency and amplitude. Pretreatment with the protein kinase A (PKA) inhibitor H-89 or KT 5720 reduced the IP4-mediated MEPP frequency increase by 60% and abolished the IP6-mediated MEPP frequency increases as well as the enhancement in MEPP amplitude. Pretreatment with antibodies against phosphatidylinositol 3-kinase (PI 3-K), enzyme also associated with clathrin-coated vesicular recycling, did not alter the IP4 and IP6-mediated MEPP frequency increases, but reduced the MEPP amplitude increase by 50%. In our previous reports, IP3, but not other second messengers releasing Ca2+ from internal Ca2+ stores, is able to enhance the MEPP amplitude. In order to dissociate the effect of Ca2+ release vs. metabolism to IP4 and IP6, we evaluated the effects of 3-deoxy-3-fluoro-inositol 1,4,5-trisphosphate (3F-IP3), which is not converted to IP4 or IP6. 3F-IP3 produced an increase then decrease in MEPP frequency and a decrease in MEPP amplitude. In elevated vesicle recycling induced by high K+-Ringer solution, IP4 and IP6 have similar effects, except decreasing MEPP frequency at a higher concentration (10(-4) M). We conclude that (1) high inositol polyphosphates may represent a link between IP3 and cAMP pathways; (2) the IP3-induced increase of MEPP amplitude is likely to be due to its high inositol metabolites; (3) PI 3-K is not involved in the IP4 and IP6-mediated MEPP frequency increases, but may be involved in MEPP size.

New roles for astrocytes: Regulation of synaptic transmission

Abstract Although glia often envelop synapses, they have traditionally been viewed as passive participants in synaptic function. Recent evidence has demonstrated, however, that there is a dynamic two-way communication between glia and neurons at the synapse. Neurotransmitters released from presynaptic neurons evoke Ca2+ concentration increases in adjacent glia. Activated glia, in turn, release transmitters, including glutamate and ATP. These gliotransmitters feed back onto the presynaptic terminal either to enhance or to depress further release of neurotransmitter. Transmitters released from glia can also directly stimulate postsynaptic neurons, producing either excitatory or inhibitory responses. Based on these new findings, glia should be considered an active partner at the synapse, dynamically regulating synaptic transmission.

Glial Cells and Neurotransmission

Glial cells throughout the nervous system are closely associated with synapses. Accompanying these anatomical couplings are intriguing functional interactions, including the capacity of certain glial cells to respond to and modulate neurotransmission. Glial cells can also help establish, maintain, and reconstitute synapses. In this review, we discuss evidence indicating that glial cells make important contributions to synaptic function.

Astrocyte-mediated control of cerebral microcirculation

Characterization of astrocyte Ca2+ dynamics has been a topic of considerable emphasis for more than a decade. Only recently, however, has the physiological significance of astrocyte Ca2+ signaling started to become clear. Several studies have shown that astrocyte Ca2+ levels become elevated in response to neuronal input and that this, in turn, influences synaptic activity. A novel function of astrocyte Ca2+ signaling has been described by Zonta et al., whereby neuron-induced astrocyte Ca2+ elevations can lead to secretion of vasodilatory substances from perivascular astrocyte endfeet, resulting in improved local blood flow. This finding represents a breakthrough in our knowledge both of astrocyte function and of the mechanism of activity-dependent cerebral blood flow regulation.

Perisynaptic Schwann cells at the neuromuscular junction: nerve- and activity-dependent contributions to synaptic efficacy, plasticity, and reinnervation

Glial cells are increasingly recognized for their important contributions to CNS and PNS synaptic function. Perisynaptic Schwann cells, which are glial cells at the neuromuscular junction, have proven to be an exceptionally useful model for studying these roles. Recent studies have shown that they detect and reciprocally modulate synaptic efficacy in an activity-dependent manner in the short term. In addition, perisynaptic Schwann cells guide reinnervating nerve sprouts after deinnervation, and many important parameters of this are dependent on synapse activity. Thus, it is hypothesized that perisynaptic Schwann cells are key integrators in a continuum of synaptic efficacy, stability, and plasticity at the neuromuscular junction, which is important for maintaining and restoring synaptic efficacy.

Modulation of glutamatergic transmission by bergmann glial cells in rat cerebellum in situ

We obtained patch-clamp recordings from neuron-glial cell pairs in cerebellar brain slices to examine the contribution of glutamate (Glu) uptake by Bergmann glial cells to shaping excitatory postsynaptic currents (EPSCs) at the parallel fiber to Purkinje cell synapse. We show that electrical stimulation of parallel fibers not only activates EPSCs in Purkinje cells but also activates inward currents in antigenically identified Bergmann glial cells that invest Purkinje cell synapse with their processes. The inward current is partially due to 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX)- and 2-amino-5-phosphonopentanoic acid (AP5)-sensitive ionotropic Glu receptors, but >/=70% of the current was mediated by D,L-threo-beta-hydroxyaspartate (THA)-sensitive Glu transporters. Glu inward currents were completely and reversibly inhibited by depolarization of Bergmann glial cells to positive membrane potentials allowing biophysical inhibition of Glu uptake into a single glial cell. Inhibition of Glu transport into Bergmann glial cells by voltage-clamping the cell to depolarized potentials caused a reversible increase in spontaneous EPSC frequency in the Purkinje cell. This increase could also be achieved by pharmacological inhibition of Glu transport with the Glu transport inhibitor THA, suggesting that inhibition of Glu uptake into Bergmann glial cells is responsible for the modulation of postsynaptic EPSCs. THA modulation of spontaneous EPSCs could only be observed in the absence of TTX, suggesting primarily a presynaptic effect. Taken together these data suggest that glial Glu uptake can profoundly affect excitatory transmission in the cerebellum, most likely by regulating presynaptic glutamate release.

Glial glutamate transporters mediate a functional metabolic crosstalk between neurons and astrocytes in the mouse developing cortex

Neuron-glia interactions are essential for synaptic function, and glial glutamate (re)uptake plays a key role at glutamatergic synapses. In knockout mice, for either glial glutamate transporters, GLAST or GLT-1, a classical metabolic response to synaptic activation (i.e., enhancement of glucose utilization) is decreased at an early functional stage in the somatosensory barrel cortex following activation of whiskers. Investigation in vitro demonstrates that glial glutamate transport represents a critical step for triggering enhanced glucose utilization, but also lactate release from astrocytes through a mechanism involving changes in intracellular Na(+) concentration. These data suggest that a metabolic crosstalk takes place between neurons and astrocytes in the developing cortex, which would be regulated by synaptic activity and mediated by glial glutamate transporters.

Astrocytes selectively enhance N-type calcium current in hippocampal neurons

Astrocytes influence neuronal development, synapse formation, and synaptic transmission, partly through affecting neuronal calcium signals. In order to elucidate the extent to which astrocytes modulate neuronal voltage-gated calcium currents, we performed a whole-cell patch clamp analysis of neurons in astrocyte-deplete and astrocyte-enriched conditions. We demonstrate that hippocampal neurons in an astrocyte-enriched environment show augmentation of voltage-gated calcium current at 1-3 days in vitro. Further study in pairs of adjacent neurons showed that the augmentation in calcium current was dependent on direct contact with the astrocyte. Pharmacological analysis demonstrated the augmentation is selective for the N-type calcium current, although immunochemical labeling of the alpha1(B) subunit of the N-type calcium channel was unchanged. These findings show that astrocytes regulate neuronal voltage-gated calcium currents in a contact-dependent manner. The specificity of the effect for the N-type calcium current at early days in culture has special significance regarding the role of astrocytes in hippocampal synaptogenesis.

Storage and release of ATP from astrocytes in culture

ATP is released from astrocytes and is involved in the propagation of calcium waves among them. Neuronal ATP secretion is quantal and calcium-dependent, but it has been suggested that ATP release from astrocytes may not be vesicular. Here we report that, besides the described basal ATP release facilitated by exposure to calcium-free medium, astrocytes release purine under conditions of elevated calcium. The evoked release was not affected by the gap-junction blockers anandamide and flufenamic acid, thus excluding purine efflux through connexin hemichannels. Sucrose-gradient analysis revealed that a fraction of ATP is stored in secretory granules, where it is accumulated down an electrochemical proton gradient sensitive to the v-ATPase inhibitor bafilomycin A(1). ATP release was partially sensitive to tetanus neurotoxin, whereas glutamate release from the same intoxicated astrocytes was almost completely impaired. Finally, the activation of metabotropic glutamate receptors, which strongly evokes glutamate release, was only slightly effective in promoting purine secretion. These data indicate that astrocytes concentrate ATP in granules and may release it via a regulated secretion pathway. They also suggest that ATP-storing vesicles may be distinct from glutamate-containing vesicles, thus opening up the possibility that their exocytosis is regulated differently.

Neuron-to-astrocyte signaling is central to the dynamic control of brain microcirculation

The cellular mechanisms underlying functional hyperemia--the coupling of neuronal activation to cerebral blood vessel responses--are not yet known. Here we show in rat cortical slices that the dilation of arterioles triggered by neuronal activity is dependent on glutamate-mediated [Ca(2+)](i) oscillations in astrocytes. Inhibition of these Ca(2+) responses resulted in the impairment of activity-dependent vasodilation, whereas selective activation--by patch pipette--of single astrocytes that were in contact with arterioles triggered vessel relaxation. We also found that a cyclooxygenase product is centrally involved in this astrocyte-mediated control of arterioles. In vivo blockade of glutamate-mediated [Ca(2+)](i) elevations in astrocytes reduced the blood flow increase in the somatosensory cortex during contralateral forepaw stimulation. Taken together, our findings show that neuron-to-astrocyte signaling is a key mechanism in functional hyperemia.

Sexual differentiation of astrocyte morphology in the developing rat preoptic area

The preoptic area is an important brain region controlling sex-typic behaviour and physiology, and astrocytes of this region are responsive to steroids perinatally. Utilizing glial fibrillary acidic protein immunocytochemistry, the morphology of astrocytes in the preoptic area of male and female rat pups was examined on the day of birth and on postnatal day 3. As early as the day of birth, astrocytes of the male preoptic area exhibit both significantly greater primary process length and number of primary processes, and these differences remain at postnatal day 3. Application of exogenous steroid to females suggested that gonadal steroids, in particular oestradiol, mediate the sex difference. Pups received 100 micro g of steroid on the day of birth and again on postnatal day 1, and astrocyte morphology was assessed on postnatal day 3. Both oestradiol and testosterone induced significant changes in process length and number compared to vehicle-treated controls. Astrocytes of oestradiol-treated females did not differ on PN3 from those of PN3-untreated males. Exposure to the nonaromatizable steroid, dihydrotestosterone, had no effect on any attribute of astrocyte morphology. This suggests the effects induced by testosterone are mediated by oestradiol following local aromatization of the steroid, and not through direct activation of the androgen receptor. Astrocytes are important in synapse formation and efficacy, and we hypothesize a role for astrocyte complexity and differentiation in the establishment of synaptic patterning.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

New insights into neuron-glia communication

Two-way communication between neurons and nonneural cells called glia is essential for axonal conduction, synaptic transmission, and information processing and thus is required for normal functioning of the nervous system during development and throughout adult life. The signals between neurons and glia include ion fluxes, neurotransmitters, cell adhesion molecules, and specialized signaling molecules released from synaptic and nonsynaptic regions of the neuron. In contrast to the serial flow of information along chains of neurons, glia communicate with other glial cells through intracellular waves of calcium and via intercellular diffusion of chemical messengers. By releasing neurotransmitters and other extracellular signaling molecules, glia can affect neuronal excitability and synaptic transmission and perhaps coordinate activity across networks of neurons.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Characterization of a synaptiform transmission between a neuron and a glial cell in the leech central nervous system

The cross-talk between neurons and glial cells is receiving increased attention because of its potential role in information processing in nervous systems. Stimulation of a single identifiable neuron, the neurosecretory Leydig interneuron in segmental ganglia of the leech Hirudo medicinalis, which modulates specific behaviors in the leech, evokes membrane hyperpolarization directly in the giant glial cell (Schmidt and Deitmer. Eur J Neurosci 11:3125-3133, 1999). We have studied the neuron-to-glia signal transmission in the voltage-clamped giant glial cell to determine whether this interaction exhibits properties of a chemical synapse. The glial response had a mean latency of 4.9 s and was dependent on the action potential frequency; the glial cell responded to as few as five Leydig neuron action potentials in 50% of the trials. The glial current was sustained for minutes during repetitive Leydig neuron activity without any sign of desensitization. The current was sensitive to tetraethylammonium, and its reversal potential of -78 mV shifted with the external K+ concentration. The glial response increased with the duration of the neuronal action potentials and was sensitive to the external Ca2+/Mg2+ concentration ratio. The results suggest that Leydig neuron activity leads to a Ca2+-dependent release of transmitter from the neuronal dendrites, evoking an K+ outward current in the giant glial cell, implying a synapse-like transmission between a neuron and a glial cell.

An intrinsic distinction in neuromuscular junction assembly and maintenance in different skeletal muscles

We analyzed the formation of neuromuscular junctions (NMJs) in individual muscles of the mouse embryo. Skeletal muscles can be assigned to one of two distinct classes of muscles, termed "Fast Synapsing" (FaSyn) and "Delayed Synapsing" (DeSyn) muscles, which differ significantly with respect to the initial focal clustering of postsynaptic AChRs, the timing of presynaptic maturation, and the maintenance of NMJs in young adult mice. Differences between classes were intrinsic to the muscles and manifested in the absence of innervation or agrin. Paralysis or denervation of young adult muscles resulted in disassembly of AChR clusters on DeSyn muscles, whereas those on FaSyn muscles were preserved. Our results show that postsynaptic differentiation processes intrinsic to FaSyn and DeSyn muscles influence the formation of NMJs during development and their maintenance in the adult.

Testosterone regulates terminal Schwann cell number and junctional size during developmental synapse elimination

Previous work has shown that exposure to exogenous testosterone during synapse elimination permanently stabilizes synapses that would normally be lost in the androgen-sensitive levator ani (LA) muscle, indicating that testosterone is a potent stabilizing factor for developing LA synapses. Terminal Schwann cells (TSCs), which cap the neuromuscular junction, have also been implicated in the control of synaptic stability and may play a decisive role in the selective stabilization of synapses during synapse elimination. In this study, we begin to investigate the possible role of TSCs in the effect of testosterone on synapse elimination by determining whether testosterone influences their number. As the number of TSCs generally correlates with the size of endplates, we also measured endplate size. Male rats were castrated or sham gonadectomized at postnatal day (P) 7 and given capsules containing either testosterone or nothing. Three weeks later (P27-28), LA neuromuscular junctions (NMJs) were stained using immune and nonimmune markers. As expected, testosterone treatment during synapse elimination maintained synapses that would ordinarily be eliminated. In addition, we find that the size of LA endplates and the number of TSCs per LA junction were also increased by the testosterone treatment. However, testosterone significantly increased the number of TSCs on both singly and multiply innervated fibers, indicating that the effect of testosterone on the number of TSCs is not specific to its effect on synapse maintenance. Rather the testosterone-induced increase in the number of TSCs appears related to the size of LA NMJs, a relationship that has precedence.

Synaptically released acetylcholine evokes Ca2+ elevations in astrocytes in hippocampal slices

Recent results have demonstrated the existence of bidirectional communication between glial cells and neurons. We investigated in brain slices whether rat hippocampal astrocytes respond to acetylcholine synaptically released by an extrinsic pathway. We stimulated the stratum oriens/alveus, which contains cholinergic afferents from the septum and diagonal band of Broca, and recorded whole-cell membrane currents and intracellular Ca2+ levels of astrocytes located in the hippocampal stratum oriens. Nerve-fiber stimulation evoked a long-lasting inward current and increased the Ca2+ levels in astrocytes. Both astrocytic responses were abolished by tetrodotoxin or Cd2+ and were increased by 4-aminopyridine, indicating that the responses were attributable to synaptically released neurotransmitter. The inward current was inhibited by glutamate transporter antagonists, indicating that it was attributable to the electrogenic glutamate transporter activity. The synaptically evoked intracellular Ca2+ elevations were not affected by glutamate receptor antagonists but were abolished by atropine, indicating that they were mediated by muscarinic cholinergic receptors. Thapsigargin prevented the Ca2+ elevation but did not modify the inward current, indicating that the Ca2+ signal was attributable to intracellular Ca2+ mobilization. These results indicate that hippocampal astrocytes respond to acetylcholine released by synaptic terminals. The synaptically released acetylcholine acts on muscarinic receptors, mobilizing Ca2+ from the intracellular stores. Different regions in the recorded astrocytes showed independent stimulus-induced Ca2+ variations, suggesting the existence of subcellular domains in the astrocytic responses evoked by the synaptic cholinergic activity. Therefore, our results show the existence of cholinergic neuron-astrocyte signaling and suggest that astrocytes are a target of axonal inputs from different brain areas.

Communication between astrocytes and neurons: a complex language

In recent years, accumulating evidence suggests the existence of bidirectional communication between astrocytes and neurons, indicating an important active role of astrocytes in the physiology of the nervous system. As a consequence of this evidence, a new concept of the synaptic physiology--"the tripartite synapse"--has been proposed, in which the synapse is formed by three functional elements, i.e. the pre- and postsynaptic elements and the surrounding astrocytes. In the present article we review and discuss the current knowledge on the cellular mechanisms and physiological properties of this communication that displays highly complex characteristics. We are beginning to realize that the communication between astrocytes and neurons uses a quite complex language.

Inhibition of skeletal muscle nicotinic receptors by the atypical antipsychotic clozapine

We have previously observed that certain atypical antipsychotic drugs reduce the amplitude and duration of miniature end-plate currents (EPCs) at the frog neuromuscular junction (Effects of atypical antipsychotics on vertebrate neuromuscular transmission, Nguyen, Q.-T., Yang, J., Miledi, R. Neuropharmacology 42, 2002, 670-676), therefore suggesting that these drugs act on nicotinic acetylcholine receptors. In this study we examined the effects of the atypical antipsychotic clozapine on nicotinic receptors of frog neuromuscular end-plates or in Xenopus oocytes expressing the alpha(1)beta(1)gamma delta mouse skeletal muscle nicotinic receptor. At neuromuscular junctions, postsynaptic currents were reduced by micromolar concentrations of clozapine. This compound also acted presynaptically by increasing the quantal content of EPCs of muscles without noticeably affecting paired-pulse facilitation. In oocytes, clozapine inhibited alpha(1)beta(1)gamma delta receptors with an IC(50) of 10 microM and a Hill coefficient of 1. Blockage of alpha(1)beta(1)gamma delta receptors by clozapine bears several hallmarks of open-channel blockers, including faster response decays, strong voltage dependence of the block, large rebound currents upon wash, and reduction of peak responses even at saturating concentrations of acetylcholine. However, clozapine increased the EC(50) for acetylcholine and its blocking effect was enhanced by preincubation. These results suggest that clozapine antagonizes muscle nicotinic receptors by blocking open channels, and possibly also by another mechanism which still remains to be investigated.

Schwann cells express active agrin and enhance aggregation of acetylcholine receptors on muscle fibers

To explore novel roles of glial cells in synaptic function and formation, we examined the expression of agrin in frog Schwann cells and tested their role in the aggregation of acetylcholine receptors (AChRs). Using reverse transcription-PCR, we found that Schwann cells along nerve fibers in tadpoles expressed only the inactive agrin isoform B0 but began to also express active agrin isoforms B11 and B19 at approximately metamorphosis. During nerve regeneration in the adult, the expression of these active agrin isoforms in Schwann cells was upregulated, including the appearance of the most potent isoform, B8. This upregulation was induced by regenerating axons but not by nerve injury per se. In muscle cultures, the presence of adult Schwann cells enhanced the number and the total area of AChR aggregates 2.2- and 4.5-fold, respectively, and this enhancement was eliminated by heparin treatment. Furthermore, adult Schwann cells in culture expressed active agrin isoforms and produced agrin protein. Using a novel technique to selectively ablate perisynaptic Schwann cells (PSCs) at the neuromuscular junction, we found that PSCs also expressed active agrin isoforms B11 and B19, and these active isoforms were upregulated, including the appearance of B8, during reinnervation. Observation in vivo showed that extrajunctional AChR aggregates were associated with PSC sprouts after nerve injury and subsequent reinnervation. These results suggest that, contrary to the prevailing view that only neurons express active agrin, glial cells also express active agrin and play a role in the aggregation of AChRs both in vitro and in vivo.

Glutamate on demand: astrocytes as a ready source

The past decade of studies has changed our view of the integrative capacities and roles of glia. A picture is emerging in which neurons and astrocytes, a subtype of glial cell, are in a continuous regulatory dialogue. Initial studies demonstrated that chemical transmitters, which are released from neurons, induce elevations of astrocytic calcium. Furthermore, stimulation of neuronal afferents at modest frequencies induces a calcium response in astrocytes that is graded with stimulation frequency. The consequence of this astrocytic calcium response is now beginning to be appreciated in that changes in calcium level can induce the release of the chemical transmitter glutamate from this nonneuronal cell. During the past few years, it has been shown that by releasing glutamate, astrocytes can regulate synaptic transmission and contribute to certain forms of synaptic plasticity. The roles played in information processing by this glial feedback loop remain to be determined. However, it is likely that the results of these recent studies will signal a new way of thinking about the nervous system, in which the glial cell comes to the forefront of our attention.

Peripheral glia direct axon guidance across the CNS/PNS transition zone

CNS glia have integral roles in directing axon migration of both vertebrates and insects. In contrast, very little is known about the roles of PNS glia in axonal pathfinding. In vertebrates and Drosophila, anatomical evidence shows that peripheral glia prefigure the transition zones through which axons migrate into and out of the CNS. Therefore, peripheral glia could guide axons at the transition zone. We used the Drosophila model system to test this hypothesis by ablating peripheral glia early in embryonic neurodevelopment via targeted overexpression of cell death genes grim and ced-3. The effects of peripheral glial loss on sensory and motor neuron development were analyzed. Motor axons initially exit the CNS in abnormal patterns in the absence of peripheral glia. However, they must use other cues within the periphery to find their correct target muscles since early pathfinding errors are largely overcome. When peripheral glia are lost, sensory axons show disrupted migration as they travel centrally. This is not a result of motor neuron defects, as determined by motor/sensory double-labeling experiments. We conclude that peripheral glia prefigure the CNS/PNS transition zone and guide axons as they traverse this region.

Differential mechanisms of Ca2+ responses in glial cells evoked by exogenous and endogenous glutamate in rat hippocampus

The mechanisms of Ca2+ responses evoked in hippocampal glial cells in situ, by local application of glutamate and by synaptic activation, were studied in slices from juvenile rats using the membrane permeant fluorescent Ca2+ indicator fluo-3AM and confocal microscopy. Ca2+ responses induced by local application of glutamate were unaffected by the sodium channel blocker tetrodotoxin and were therefore due to direct actions on glial cells. Glutamate-evoked responses were significantly reduced by the L-type Ca2+ channel blocker nimodipine, the group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), and the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)2-amino-5-phosphonopentanoic acid (APV). However, glutamate-induced Ca2+ responses were not significantly reduced by the non-NMDA receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). These results indicate that local application of glutamate increases intracellular Ca2+ levels in glial cells via the activation of L-type Ca2+ channels, NMDA receptors, and metabotropic glutamate receptors. Brief (1 s) tetanization of Schaffer collaterals produced increases in intracellular Ca2+ levels in glial cells that were dependent on the frequency of stimulation (> or =50 Hz) and on synaptic transmission (abolished by tetrodotoxin). These Ca2+ responses were also antagonized by the L-type Ca2+ channel blocker nimodipine and the metabotropic glutamate receptor antagonist MCPG. However, the non-NMDA receptor antagonist CNQX significantly reduced the Schaffer collateral-evoked Ca2+ responses, while the NMDA antagonist APV did not. Thus, these synaptically mediated Ca2+ responses in glial cells involve the activation of L-type Ca2+ channels, group I/II metabotropic glutamate receptors, and non-NMDA receptors. These findings indicate that increases in intracellular Ca2+ levels induced in glial cells by local glutamate application and by synaptic activity share similar mechanisms (activation of L-type Ca2+ channels and group I/II metabotropic glutamate receptors) but also have distinct components (NMDA vs. non-NMDA receptor activation, respectively). Therefore, neuron-glia interactions in rat hippocampus in situ involve multiple, complex Ca2+-mediated processes that may not be mimicked by local glutamate application.