Activity-dependent NMDA receptor-mediated activation of protein kinase B/Akt in cortical neuronal cultures

The serine/threonine protein kinase B (PKB)/Akt is a phosphoinositide 3-kinase (PI3K) effector that is thought to play an important roll in a wide variety of cellular events. The present study examined whether PKB activation in cortical neuronal cultures is coupled with synaptic activity. A 1-h incubation of neuronal cultures with tetrodotoxin (TTX), the PI3K inhibitor wortmannin, the NMDA receptor antagonist MK-801 or removal of extracellular calcium significantly reduced basal levels of phospho(Ser473)-PKB, indicating that activity-dependent glutamate release maintains PKB activation through an NMDA receptor-PI3K pathway. A 5-min exposure to NMDA (50 micro m) in the presence of TTX increased phospho-PKB back to levels observed in the absence of TTX. NMDA stimulation of phospho-PKB was blocked by wortmannin, the CaMKII inhibitor KN-93, MK-801, and removal of extracellular calcium. We have previously shown that NMDA receptors can bi-directionally regulate activation of extracellular-signal regulated kinase (ERK), and NMDA receptor stimulation of PKB in the present study appeared to mirror activation of ERK. These results suggest that in cultured cortical neurons, PKB activity is dynamically regulated by synaptic activity and is coupled to NMDA receptor activation. In addition, NMDA receptor activation of ERK and PKB may occur through overlapping signaling pathways that bifurcate at the level of Ras.     

Neuronal synchrony mediated by astrocytic glutamate through activation of extrasynaptic NMDA receptors

Fast excitatory neurotransmission is mediated by activation of synaptic ionotropic glutamate receptors. In hippocampal slices, we report that stimulation of Schaffer collaterals evokes in CA1 neurons delayed inward currents with slow kinetics, in addition to fast excitatory postsynaptic currents. Similar slow events also occur spontaneously, can still be observed when neuronal activity and synaptic glutamate release are blocked, and are found to be mediated by glutamate released from astrocytes acting preferentially on extrasynaptic NMDA receptors. The slow currents can be triggered by stimuli that evoke Ca2+ oscillations in astrocytes, including photolysis of caged Ca2+ in single astrocytes. As revealed by paired recording and Ca2+ imaging, a striking feature of this NMDA receptor response is that it occurs synchronously in multiple CA1 neurons. Our results reveal a distinct mechanism for neuronal excitation and synchrony and highlight a functional link between astrocytic glutamate and extrasynaptic NMDA receptors.     

Excitotoxicity induced by enhanced excitatory neurotransmission in cultured hippocampal pyramidal neurons.

Cultures of rat hippocampal pyramidal neurons were used to examine the roles of excitatory synaptic transmission, NMDA receptors, and elevated [Ca2+]i in the production of excitotoxicity. In integral of 70% of the cells observed, perfusion with Mg2(+)-free, glycine-supplemented medium induced large spontaneous fluctuations or maintained plateaus of [Ca2+]i. [Ca2+]i fluctuations could be blocked by tetrodotoxin, NMDA receptor antagonists, dihydropyridines, or compounds that inhibit synaptic transmission in the hippocampus, but not by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. When cells were treated with Mg2(+)-free, glycine-supplemented medium and examined 24 hr later, integral of 30% of the neurons were found to have died. Cell death could be inhibited by the same agents that reduced [Ca2+]i fluctuations. These results support a role for direct excitatory synaptic transmission, as opposed to the general release of glutamate, in excitotoxicity. A major role for synaptically activated NMDA receptors, rather than kainate/quisqualate receptors, is also indicated. Neuronal death may be produced by abnormal changes in neuronal [Ca2+]i.     

Amplification of calcium signals at dendritic spines provides a method for CNS quantal analysis

It has been proposed that the small volume of a dendritic spine can amplify Ca2+ signals during synaptic transmission. Accordingly, we have performed calculations to determine whether the activation of N-methyl-D-aspartate (NMDA) type glutamate receptors during synaptic transmission results in significant elevation in intracellular Ca2+ levels, permitting optical detection of synaptic signals within a single spine. Simple calculations suggest that the opening of even a single NMDA receptor would result in the influx of approximately 310 000 Ca2+ ions into the small volume of a spine, producing changes in Ca2+ levels that are readily detectable using high affinity Ca2+ indicators such as fura-2 or fluo-3. Using fluorescent Ca2+ indicators, we have imaged local Ca2+ transients mediated by NMDA receptors in spines and dendritic shafts attributed to spontaneous miniature synaptic activity. Detailed analysis of these quantal events suggests that the current triggering these transients is attributed to the activation of <10 NMDA receptors. The frequency of these miniature synaptic Ca2+ transients is not randomly distributed across synapses, as some synapses can display a >10-fold higher frequency of transients than others. As expected for events mediated by NMDA receptors, miniature synaptic Ca2+ transients were suppressed by extracellular Mg2+ at negative membrane potentials; however, the Mg2+ block could be removed by depolarization.     

N-methyl-d-Aspartic Acid Differentially Regulates Extracellular Dopamine, GABA, and Glutamate Levels in the Dorsolateral Neostriatum of the Halothane-Anesthetized Rat: An In Vivo Microdialysis Study

Abstract: The effects of local perfusion with the glutamate receptor agonist NMDA and the noncompetitive NMDA receptor antagonist dizolcipine (MK-801) on extracellular dopamine (DA), GABA, and glutamate (Glu) levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane-anesthetized rat. In addition, the sensitivity of both the basal and NMDA-induced increases in levels of these neurotransmitter substances to perfusion with tetrodotoxin (TTX; 10^?5 M) and a low Ca²⁺ concentration (0.1 mM) was studied. The results show that the local perfusion (10 min) with both the 10^?3 and 10^?4 M dose of NMDA increased striatal DA and GABA outflow, whereas only the (10^?3 M) dose of NMDA was associated with a small and delayed increase in extracellular Glu levels. The NMDA-induced effects were dose-dependently counteracted by simultaneous perfusion with MK-801 (10^?6 and 10^?5 M). Both the basal and NMDA (10^?3 M)-induced increase in extracellular striatal DA content was reduced in the presence of TTX and a low Ca²⁺ concentration, whereas both basal and NMDA-stimulated GABA levels were unaffected by these treatments. Both the basal and NMDA-stimulated Glu levels were enhanced following TTX treatment, whereas perfusion with a low Ca²⁺ concentration reduced basal Glu levels and enhanced and prolonged the NMDA-induced stimulation. These data support the view that NMDA receptor stimulation plays a role in the regulation of extracellular DA, GABA, and Glu levels in the dorsolateral neostriatum and provide evidence for a differential effect of NMDA receptor stimulation on these three striatal neurotransmitter systems, possibly reflecting direct and indirect actions mediated via striatal NMDA receptors.     

Regulation of synaptic strength by protein phosphatase 1.

We investigated the role of postsynaptic protein phosphatase 1 (PP1) in regulating synaptic strength by loading CA1 pyramidal cells either with peptides that disrupt PP1 binding to synaptic targeting proteins or with active PP1. The peptides blocked synaptically evoked LTD but had no effect on basal synaptic currents mediated by either AMPA or NMDA receptors. They did, however, cause an increase in synaptic strength following the induction of LTD. Similarly, PP1 had no effect on basal synaptic strength but enhanced LTD. In cultured neurons, synaptic activation of NMDA receptors increased the proportion of PP1 localized to synapses. These results suggest that PP1 does not significantly regulate basal synaptic strength. Appropriate NMDA receptor activation, however, allows PP1 to gain access to synaptic substrates and be recruited to synapses where its activity is necessary for sustaining LTD.     

Single synaptic events evoke NMDA receptor-mediated release of calcium from internal stores in hippocampal dendritic spines

We have used confocal microscopy to monitor synaptically evoked Ca2+ transients in the dendritic spines of hippocampal pyramidal cells. Individual spines respond to single afferent stimuli (<0.1 Hz) with Ca2+ transients or failures, reflecting the probability of transmitter release at the activated synapse. Both AMPA and NMDA glutamate receptor antagonists block the synaptically evoked Ca2+ transients; the block by AMPA antagonists is relieved by low Mg2+. The Ca2+ transients are mainly due to the release of calcium from internal stores, since they are abolished by antagonists of calcium-induced calcium release (CICR); CICR antagonists, however, do not depress spine Ca2+ transients generated by backpropagating action potentials. These results have implications for synaptic plasticity, since they show that synaptic stimulation can activate NMDA receptors, evoking substantial Ca2+ release from the internal stores in spines without inducing long-term potentiation (LTP) or depression (LTD).     

Excitatory amino acid receptor-channels in Purkinje cells in thin cerebellar slices.

Glutamate receptors of the N-methyl-D-aspartate (NMDA) and non-NMDA type serve different functions during excitatory synaptic transmission. Although many central neurons bear both types of receptor, the evidence concerning the sensitivity of cerebellar Purkinje cells to NMDA is contradictory. To investigate the receptor types present in Purkinje cells, we have used whole-cell and outside-out patch-clamp methods to record from cells in thin cerebellar slices from young rats. At a holding potential of -70 mV (in nominally Mg(2+)-free medium, with added glycine) NMDA caused a whole-cell current response which consisted of a dramatic increase in the frequency of synaptic currents. In the presence of tetrodotoxin (TTX) and the gamma-aminobutyric acidA (GABAA) receptor antagonist bicuculline, spontaneous synaptic currents and responses to NMDA were inhibited. In a proportion of cells a small polysynaptic response to NMDA persisted, which was further reduced by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). The non-NMDA glutamate receptor agonists kainate (KA), quisqualate (QA) and s-alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (s-AMPA), evoked large inward currents due to the direct activation of receptors in Purkinje cells. NMDA applied to excised membrane patches failed to evoke any single-channel currents, whereas s-AMPA and QA caused small inward currents accompanied by marked increases in current noise. Spectral analysis of the s-AMPA noise in patches gave an estimated mean channel conductance of approximately 4 pS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neuron–astrocyte interactions in the medial nucleus of the trapezoid body

The calyx of Held (CoH) synapse serves as a model system to analyze basic mechanisms of synaptic transmission. Astrocyte processes are part of the synaptic structure and contact both pre- and postsynaptic membranes. In the medial nucleus of the trapezoid body (MNTB), midline stimulation evoked a current response that was not mediated by glutamate receptors or glutamate uptake, despite the fact that astrocytes express functional receptors and transporters. However, astrocytes showed spontaneous Ca²⁺ responses and neuronal slow inward currents (nSICs) were recorded in the postsynaptic principal neurons (PPNs) of the MNTB. These currents were correlated with astrocytic Ca²⁺ activity because dialysis of astrocytes with BAPTA abolished nSICs. Moreover, the frequency of these currents was increased when Ca²⁺ responses in astrocytes were elicited. NMDA antagonists selectively blocked nSICs while D-serine degradation significantly reduced NMDA-mediated currents. In contrast to previous studies in the hippocampus, these NMDA-mediated currents were rarely synchronized.     

A persistent postsynaptic modification mediates long-term potentiation in the hippocampus

Long-term potentiation (LTP) is a long-lasting enhancement of synaptic transmission that can be induced by brief repetitive stimulation of excitatory pathways in the hippocampus. One of the most controversial points is whether the process underlying the enhanced synaptic transmission occurs pre- or postsynaptically. To examine this question, we have taken advantage of the novel physiological properties of excitatory synaptic transmission in the CA1 region of the hippocampus. Synaptically released glutamate activates both NMDA and non-NMDA receptors on pyramidal cells, resulting in an excitatory postsynaptic potential (EPSP) with two distinct components. A selective increase in the non-NMDA component of the EPSP was observed with LTP. This result suggests that the enhancement of synaptic transmission during LTP is caused by an increased sensitivity of the postsynaptic neuron to synaptically released glutamate.     

A long-term depression of AMPA currents in cultured cerebellar Purkinje neurons.

Cerebellar long-term depression (LTD) is a model of synaptic plasticity in which conjunctive stimulation of parallel fiber and climbing fiber inputs to a Purkinje neuron induces a persistent depression of the parallel fiber-Purkinje neuron synapse. We report that an analogous phenomenon may be elicited in the cultured mouse Purkinje neuron when iontophoretic glutamate application and depolarization of the Purkinje neurons are substituted for parallel fiber and climbing fiber stimulation, respectively. The induction of LTD in these cerebellar cultures requires activation of both ionotropic (AMPA) and metabotropic quisqualate receptors, together with depolarization in the presence of external Ca2+. This postsynaptic alteration is manifest as a depression of glutamate or AMPA currents, but not aspartate or NMDA currents. These results strengthen the contention that the expression of cerebellar LTD is at least in part postsynaptic and provide evidence that activation of both ionotropic and metabotropic quisqualate receptors are necessary for LTD induction.     

Iron mediates N-methyl-D-aspartate receptor-dependent stimulation of calcium-induced pathways and hippocampal synaptic plasticity

Iron deficiency hinders hippocampus-dependent learning processes and impairs cognitive performance, but current knowledge on the molecular mechanisms underlying the unique role of iron in neuronal function is sparse. Here, we investigated the participation of iron on calcium signal generation and ERK1/2 stimulation induced by the glutamate agonist N-methyl-D-aspartate (NMDA), and the effects of iron addition/chelation on hippocampal basal synaptic transmission and long-term potentiation (LTP). Addition of NMDA to primary hippocampal cultures elicited persistent calcium signals that required functional NMDA receptors and were independent of calcium influx through L-type calcium channels or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors; NMDA also promoted ERK1/2 phosphorylation and nuclear translocation. Iron chelation with desferrioxamine or inhibition of ryanodine receptor (RyR)-mediated calcium release with ryanodine-reduced calcium signal duration and prevented NMDA-induced ERK1/2 activation. Iron addition to hippocampal neurons readily increased the intracellular labile iron pool and stimulated reactive oxygen species production; the antioxidant N-acetylcysteine or the hydroxyl radical trapper MCI-186 prevented these responses. Iron addition to primary hippocampal cultures kept in calcium-free medium elicited calcium signals and stimulated ERK1/2 phosphorylation; RyR inhibition abolished these effects. Iron chelation decreased basal synaptic transmission in hippocampal slices, inhibited iron-induced synaptic stimulation, and impaired sustained LTP in hippocampal CA1 neurons induced by strong stimulation. In contrast, iron addition facilitated sustained LTP induction after suboptimal tetanic stimulation. Together, these results suggest that hippocampal neurons require iron to generate RyR-mediated calcium signals after NMDA receptor stimulation, which in turn promotes ERK1/2 activation, an essential step of sustained LTP.     

Excitatory Amino Acid Receptors Coupled to the Nitric Oxide/Cyclic GMP Pathway in Rat Cerebellum During Development

The coupling of excitatory amino acid receptors to the formation of nitric oxide (NO) from arginine during the postnatal development of rat cerebellum was assayed in slice preparations by measuring cyclic GMP accumulation. In the immature tissue, N-methyl-D-aspartate (NMDA) and glutamate were highly efficacious agonists, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and quisqualate evoked only small responses. The effect of glutamate at all concentrations tested (up to 10 mM) was abolished by the NMDA antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801). In adult slices, AMPA and quisqualate were much more effective and their effects were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist for ionotropic non-NMDA receptors, whereas the apparent efficacy of NMDA was greatly reduced. The major changes took place between 8 and 14 days postnatum and, in the case of NMDA, part of the loss of sensitivity appeared to reflect a decline in the ambient levels of glycine with age. Moreover, a component of the response to glutamate in the adult was resistant to MK-801. Cyclic GMP accumulations induced by NMDA and non-NMDA agonists alike were Ca(2+)-dependent and could be antagonized by competitive NO synthase inhibitors in an arginine-sensitive manner, indicating that they are all mediated by NO formation. With one of the inhibitors, L-NG-nitroarginine, a highly potent component (IC50 = 6 nM) evident in slices from rats of up to 8 days old was lost during maturation, indicating that there may be a NO synthase isoform which is prominent only in the immature tissue. Cyclic GMP levels in adult slices under "basal" conditions were reduced markedly by blocking NMDA receptors, by inhibiting action potentials with tetrodotoxin, or by NO synthase inhibition, suggesting that the endogenous transmitter released during spontaneous synaptic activity acts mainly through NMDA receptors to trigger NO formation.     

Astrocytes are endogenous regulators of basal transmission at central synapses

Basal synaptic transmission involves the release of neurotransmitters at individual synapses in response to a single action potential. Recent discoveries show that astrocytes modulate the activity of neuronal networks upon sustained and intense synaptic activity. However, their ability to regulate basal synaptic transmission remains ill defined and controversial. Here, we show that astrocytes in the hippocampal CA1 region detect synaptic activity induced by single-synaptic stimulation. Astrocyte activation occurs at functional compartments found along astrocytic processes and involves metabotropic glutamate subtype 5 receptors. In response, astrocytes increase basal synaptic transmission, as revealed by the blockade of their activity with a Ca(2+) chelator. Astrocytic modulation of basal synaptic transmission is mediated by the release of purines and the activation of presynaptic A(2A) receptors by adenosine. Our work uncovers an essential role for astrocytes in the regulation of elementary synaptic communication and provides insight into fundamental aspects of brain function.     

Glutamate release from astrocytes as a non-synaptic mechanism for neuronal synchronization in the hippocampus.

Synchronization of activity of anatomically distributed groups of neurons represents a fundamental event in the processing of information in the brain. While this phenomenon is believed to result from dynamic interactions within the neuronal circuitry, how exactly populations of neurons become synchronized remains largely to be clarified. We propose that astrocytes are directly involved in the generation of neuronal synchrony in the hippocampus. By using a combination of experimental approaches in hippocampal slice preparations, including patch-clamp recordings and confocal microscopy calcium imaging, we studied the effect on CA1 pyramidal neurons of glutamate released from astrocytes upon various stimuli that trigger Ca2+ elevations in these glial cells, including Schaffer collateral stimulation. We found that astrocytic glutamate evokes synchronous, slow inward currents (SICs) and Ca2+ elevations in CA1 pyramidal neurons by acting preferentially, if not exclusively, on extrasynaptic NMDA receptors. Due to desensitization, AMPA receptors were not activated by astrocytic glutamate unless cyclothiazide was present. In the virtual absence of extracellular Mg2+, glutamate released from astrocytes was found to evoke, in paired recordings, highly synchronous SICs from two CA1 pyramidal neurons and, in Ca2+ imaging experiments, Ca2+ elevations that occurred synchronously in domains composed of 2-12 CA1 neurons. In the presence of extracellular Mg2+ (1 mM), synchronous SICs in two neurons as well as synchronous Ca2+ elevations in neuronal domains were still observed, although with a reduced frequency. Our results reveal a functional link between astrocytic glutamate and extrasynaptic NMDA receptors that contributes to the overall dynamics of neuronal synchrony. Our observations also raise a series of questions on possible roles of this astrocyte-to-neuron signaling in pathological changes in the hippocampus such as excitotoxic neuronal damage or the generation of epileptiform activity.     

Effect of ambient extracellular glutamate on <Emphasis Type="Italic">Drosophila</Emphasis> glutamate receptor trafficking and function

Measurements suggest that the hemolymph glutamate concentrations in Drosophila are relatively high. This raises the possibility that extracellular glutamate could be an important regulator of glutamatergic transmission in vivo. Using voltage clamp electrophysiology, we found that synaptic currents in D. melanogaster larval neuromuscular junctions are reduced by extracellular glutamate (EC50: ~0.4 mM), such that only 10–30% of receptors were functionally available in 1 mM extracellular glutamate. The kinetics of synaptic currents were also slowed in a dose-dependent fashion (EC50: ~1 mM), consistent with the idea that extracellular glutamate preferentially removes the fastest-desensitizing receptors from the functional pool. Prolonged exposure (several hours) to extracellular glutamate also triggers loss of glutamate receptor immunoreactivity from neuromuscular junctions. To determine whether this receptor loss requires that glutamate bind directly to the lost receptors, we examined glutamate-dependent loss of receptor immunoreactivity in larvae with glutamate receptor ligand binding mutations. Our results suggest that glutamate-dependent receptor loss requires binding of glutamate directly to the lost receptors. To determine whether lost receptor protein is degraded or merely redistributed, we used immunoblots. Results suggest that glutamate receptor protein is redistributed, but not degraded, after prolonged exposure to high extracellular glutamate. K. Chen and H. Augustin contributed equally to this work.     

Regulation of dendritic excitability by activity-dependent trafficking of the A-type K+ channel subunit Kv4.2 in hippocampal neurons

Voltage-gated A-type K+ channel Kv4.2 subunits are highly expressed in the dendrites of hippocampal CA1 neurons. However, little is known about the subcellular distribution and trafficking of Kv4.2-containing channels. Here we provide evidence for activity-dependent trafficking of Kv4.2 in hippocampal spines and dendrites. Live imaging and electrophysiological recordings showed that Kv4.2 internalization is induced rapidly upon glutamate receptor stimulation. Kv4.2 internalization was clathrin mediated and required NMDA receptor activation and Ca2+ influx. In dissociated hippocampal neurons, mEPSC amplitude depended on functional Kv4.2 expression level and was enhanced by stimuli that induced Kv4.2 internalization. Long-term potentiation (LTP) induced by brief glycine application resulted in synaptic insertion of GluR1-containing AMPA receptors along with Kv4.2 internalization. We also found evidence of Kv4.2 internalization upon synaptically evoked LTP in CA1 neurons of hippocampal slice cultures. These results present an additional mechanism for synaptic integration and plasticity through the activity-dependent regulation of Kv4.2 channel surface expression.     

Co-stimulation of mGluR5 and N-methyl-D-aspartate receptors is required for potentiation of excitatory synaptic transmission in hippocampal neurons

In the central nervous system, excitatory synaptic transmission is mediated by the neurotransmitter glutamate and its receptors. Interestingly, stimulation of group I metabotropic glutamate receptors (mGluRs) can either enhance or depress synaptic transmission at CA1 hippocampal synapses. Here we report that co-activation of mGluR5, a member of the group I mGluR family, and N-methyl-d-aspartate receptors (NMDARs) potentiates NMDAR currents and induces a long lasting enhancement of excitatory synaptic transmission in primary cultured hippocampal neurons. Unexpectedly, activation of mGluR5 alone fails to enhance evoked NMDAR currents and synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) AMPAR currents. The observed potentiation requires an mGluR5-induced, inositol 1,4,5-trisphosphate receptor-mediated mobilization of intracellular Ca2+, which acts in concert with a protein kinase C, calcium-activated tyrosine kinase cascade to induce a long lasting enhancement of NMDAR and AMPAR currents.     

Excitatory synaptic currents in Purkinje cells

The N-methyl-D-aspartate (NMDA) and non-NMDA classes of glutamate receptor combine in many regions of the central nervous system to form a dual-component excitatory postsynaptic current. Non-NMDA receptors mediate synaptic transmission at the resting potential, whereas NMDA receptors contribute during periods of postsynaptic depolarization and play a role in the generation of long-term synaptic potentiation. To investigate the receptor types underlying excitatory synaptic transmission in the cerebellum, we have recorded excitatory postsynaptic currents (EPSCS), by using whole-cell techniques, from Purkinje cells in adult rat cerebellar slices. Stimulation in the white matter or granule-cell layer resulted in an all-or-none synaptic current as a result of climbing-fibre activation. Stimulation in the molecular layer caused a graded synaptic current, as expected for activation of parallel fibres. When the parallel fibres were stimulated twice at an interval of 40 ms, the second EPSC was facilitated; similar paired-pulse stimulation of the climbing fibre resulted in a depression of the second EPSC. Both parallel-fibre and climbing-fibre responses exhibited linear current-voltage relations. At a holding potential of -40 mV or in the nominal absence of Mg2+ these synaptic responses were unaffected by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV), but were blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). NMDA applied to the bath failed to evoke an inward current, whereas aspartate or glutamate induced a substantial current; this current was, however, largely reduced by CNQX, indicating that non-NMDA receptors mediate this response. These results indicate that both types of excitatory input to adult Purkinje cells are mediated exclusively by glutamate receptors of the non-NMDA type, and that these cells entirely lack NMDA receptors.     

Astrocyte-induced modulation of synaptic transmission

The idea that astrocytes simply provide structural and trophic support to neurons has been challenged by recent evidence demonstrating that astrocytes exhibit a form of excitability and communication based on intracellular Ca2+ variations and intercellular Ca2+ waves, which can be initiated by neuronal activity. These astrocyte Ca2+ variations have now been shown to induce glutamate-dependent Ca2+ elevations and slow inward currents in neurons. More recently, it has been demonstrated that synaptic transmission between cultured hippocampal neurons can be directly modulated by astrocytes. We have reported that astrocyte stimulation can increase the frequency of miniature synaptic currents. Furthermore, we also have demonstrated that an elevation in the intracellular Ca2+ in astrocytes induces a reduction in both excitatory and inhibitory evoked synaptic transmission through the activation of selective presynaptic metabotropic glutamate receptors.