Glutamate transporters regulate excitability in local networks in rat neocortex (original) (raw)
Related papers
Journal of Neurophysiology, 2004
It is well-known that excitatory synaptic transmission at the hippocampal CA3-CA1 synapse depends on the binding of released glutamate to ionotropic receptors. Here we report that during long-term application of Cs + (5 mM), stimulation of the Schaffer collateral -commisural pathway evokes an epileptic field potential (Cs-FP) in area CA1 of the rat hippocampal slice, which is resistant to antagonists of ionotropic glutamate and GABA A receptors. The Cs-FP was blocked by N-type but not L-type Ca 2+ -channel antagonists and was attenuated by adenosine (0.5 mM), as expected for a synaptically mediated response. These properties make the Cs-FP fundamentally different from other types of Cs + -induced epileptiform activity. Replacement of Cs + with antagonists of the hyperpolarization-activated non-selective cation current I h and inwardly rectifying potassium channels (K IR ), or partial inhibition of the Na + /K +pump did not cause Cs-FP-like potentials, which indicates that such actions of Cs + were not responsible for the Cs-FP. The effect of Cs + was partly mimicked by 4aminopyridine (4-AP, 2 mM), suggesting that an increase in transmitter release is involved. The group I metabotropic glutamate receptor (mGluR) agonist (RS)-3,5dihydroxyphenylglycine (DHPG) attenuated the Cs-FP. This effect was not, however, antagonized by group I mGluR antagonists. Selective and non-selective mGluR antagonists did not attenuate the Cs-FP. We conclude that long-term exposure to Cs + induces a state where excitatory synaptic transmission can exist between area CA3 and CA1 in the hippocampus, independent of ionotropic and metabotropic glutamate receptors and GABA A receptors.
Neuropharmacology, 2001
The regulation of γ-aminobutyric acid (GABA)-mediated spontaneous inhibitory synaptic transmission by endogenously released glutamate was studied in cultured rat hippocampal neurons. After 7 days in vitro (DIV), both spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents (sIPSCs) could be detected. After 15 DIV, most postsynaptic spontaneous currents occurred as sEPSC/sIPSC sequences when recorded at a holding voltage of Ϫ30 mV. In the presence of the glutamate α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subtype antagonist LY303070, both the frequency and amplitude of sIPSC were strongly and reversibly reduced. The N-methyl-d-aspartate (NMDA) receptor antagonist, 2amino-5-phosphonopentanoic acid (AP5), had no effect on sIPSC while cyclothiazide strongly increased sIPSC frequency. Under blockade of AMPA receptors, the kainate-and GluR5-selective kainate receptor agonists, (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid) (ATPA) and (S)-5-iodowillardiine (5IWill), induced a large enhancement of the frequency of smallamplitude sIPSC which was blocked by the non-NMDA receptor antagonist, 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX). All of these effects were sensitive to tetrodotoxin (TTX). In the presence of LY303070 and TTX, kainate could induce a small inward current while GluR5 agonists had no effect. In the presence of NMDA and AMPA receptor antagonists, the glutamate uptake inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC) could restore sIPSC. When NBQX was used as an AMPA antagonist, the stimulatory effect of t-PDC was blocked while the group I metabotropic glutamate agonist, 3,5-dihydroxyphenylglycine (DHPG), induced a strong enhancement of sIPSC. Therefore, both AMPA and kainate receptors can regulate inhibitory synaptic transmission in cultured hippocampal neurons, the former by tonic activation, the latter when the glutamate concentration is increased by impairing glutonate uptake.
Modulation of GABAergic transmission by endogenous glutamate in the rat supraoptic nucleus
European Journal of Neuroscience, 2003
The presence of group III metabotropic glutamate receptors on GABAergic terminals in the supraoptic nucleus suggests that the level of glutamate in the extracellular space may regulate synaptic strength at inhibitory synapses. To test this hypothesis we examined the consequences of increasing ambient glutamate on GABA-mediated synaptic activity in supraoptic neurons. The concentration of the excitatory amino acid in the extracellular space was increased pharmacologically by blocking glutamate transporters. Inhibition of the astrocyte-specific GLT-1 glutamate transporter led to a reversible decrease in evoked inhibitory postsynaptic current amplitude. This modulation had a presynaptic origin as revealed by analysis of paired-pulse ratio and miniature inhibitory currents. Furthermore, blocking group III metabotropic glutamate receptors with the specific antagonist MAP4 prevented the depression of GABAergic transmission induced by glutamate transporter blockade. Thus, presynaptic metabotropic glutamate receptors located on inhibitory terminals in the supraoptic nucleus appear to sense changes in ambient glutamate and modify GABA release accordingly. However, it seems that such changes need to reach a certain magnitude because the discrete deficit in glutamate clearance which occurs in the supraoptic nucleus of lactating rats is not sufficient to modulate GABA-mediated transmission. These results suggest that ambient glutamate contributes to the modulation of synaptic efficacy not only at glutamatergic synapses but also at inhibitory GABAergic synapses.
Neuroscience, 2000
The effects of group II and group III metabotropic glutamate receptor agonists on synaptic responses evoked by primary afferent stimulation in the dorsal horn, but mostly substantia gelatinosa, neurons were studied in the spinal cord slice preparation using conventional intracellular recording technique. Bath application of a potent metabotropic glutamate receptor 2-and 3selective agonist (2S,1 H R,2 H R,3 H R)-2-(2 H ,3 H-dicarboxycyclopropyl) glycine reversibly suppressed monosynaptic and polysynaptic excitatory postsynaptic potentials evoked by A primary afferent fibers stimulation, the effect likely mediated by mGlu3 receptor subtype. This suppressing effect of (2S,1 H R,2 H R,3 H R)-2-(2 H ,3 H-dicarboxycyclopropyl) glycine on primary afferent neurotransmission was dose dependent and reduced by (S)-a-ethylglutamate, a group II metabotropic glutamate receptor antagonist. (2S,1 H R,2 H R,3 H R)-2-(2 H ,3 H-dicarboxycyclopropyl) glycine suppressed excitatory postsynaptic potentials without inducing detectable changes of postsynaptic membrane potential and neuronal input resistance in dorsal horn neurons. The paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons was reduced by (2S,1 H R,2 H R,3 H R)-2-(2 H ,3 H-dicarboxycyclopropyl) glycine application, suggesting a presynaptic site of action. The selective group III metabotropic glutamate receptor agonist (S)-2amino-4-phosphonobutanoate also depressed A afferent fibers-evoked monosynaptic and polysynaptic excitatory postsynaptic potentials in a dose-dependent and reversible manner. The concentration-dependence of (S)-2-amino-4-phosphonobutanoatemediated depression was most consistent with activation of mGlu receptor subtypes 4 and 7. However, on the basis of anatomical distribution of mGlu 4 and 7 subtypes, it is also possible that the (S)-2-amino-4-phosphonobatanoate effect is due to interaction with mGlu 7 receptor alone. (RS)-a-cyclopropyl-4-phosphonophenylglycine a preferential antagonist at group III metabotropic glutamate receptors, completely reversed the depressant effects of (S)-2-amino-4-phosphonobutanoate on both monosynaptic and polysynaptic responses. (S)-2-amino-4-phosphonobutanoate reduced the paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons, but did not alter their postsynaptic membrane potential and input resistance. A clear facilitation of the (S)-2-amino-4-phosphonobutanoate-induced depression of monosynaptic and polysynaptic excitatory postsynaptic potentials in the absence of gamma-aminobutyric acid-subtype A receptor-and glycine-mediated synaptic inhibition was shown. Besides the depressant effect on excitatory synaptic transmission, inhibitory actions of group II and III metabotropic glutamate receptor agonists on the inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons were observed. These results suggest that group II and group III metabotropic glutamate receptors are expressed at primary afferent synapses in the dorsal horn region, and activation of the receptors suppresses synaptic transmission by an action on the presynaptic site.
Journal of Neurochemistry, 2003
In this study we have tested the effects of a wide range of metabotropic glutamate receptor ligands on (i) depolarisationevoked efflux of pre-accumulated D-[ 3 H]aspartic acid (D-[ 3 H]asp) from rapidly superfused rat cerebrocortical minislices, and (ii) Na +-dependent uptake of D-[ 3 H]asp into cerebrocortical tissue. Transient elevations in extracellular K + produced concentration-dependent increases in D-[ 3 H]asp efflux. A submaximally effective concentration (50 mM) was used in all subsequent experiments. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; EC 50 17.8 lM], the group I mGlu-selective agonist (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG; EC 50 0.5 lM] and the mGlu5 receptor subtype-selective agonist (RS)-2-chloro-5-hydroxyphenylglycine [(RS)-CHPG; EC 50 7.3 lM] all concentration-dependently potentiated high K +-evoked D-[ 3 H]asp efflux in the absence of effects on basal outflow of radiolabel. At concentrations selective for mGlu1 receptors, the antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid [(RS)-AIDA; 10-300 lM]; (+)-2methyl-4-carboxyphenylglycine [LY367385; 1-100 lM] and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylate ethyl ester [CPCCOEt, 1-30 lM] all failed to inhibit responses to (S)-3,5-DHPG. However, the broad-spectrum mGlu receptor antagonist (S)-a-methyl-4-carboxyphenylglycine [(S)-MCPG; IC 50 88.5 lM] together with the recently described mGlu5-selective antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP; IC 50 0.6 lM), 6-methyl-2-(phenyl-azo)-3-pyridinol (SIB-1757; IC 50 4.4 lM) and (E)-2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893; IC 50 3.1 lM), at mGlu5-selective concentrations, all powerfully and concentration-dependently inhibited (S)-3,5-DHPG-evoked responses. Two selective excitatory amino acid (EAA) uptake inhibitors, L-trans-2,4pyrrolidine dicarboxylate (L-trans-2,4-PDC; IC 50 229 lM) and DL-threo-b-benzyloxyaspartate (DL-TBOA; IC 50 665 lM) both inhibited the Na +-dependent uptake of D-[ 3 H]asp into cerebrocortical minislices. Importantly, none of the mGlu ligands utilized in the present study significantly inhibited D-[ 3 H]asp uptake at concentrations shown to potentiate K +-evoked efflux. These data demonstrate for the first time that mGlu5 ligands modulate extracellular EAA concentrations by a direct effect on mGlu5-type autoreceptors on EAA nerve terminals as they evoke clear changes in EAA release in the absence of any effects on EAA uptake. Selective mGlu5 receptor antagonists that show high potency and good central bioavailability may provide novel classes of neuroprotective agents for the treatment of brain disorders associated with abnormal EAAergic neurotransmission.
Glutamate receptors of the kainate type and synaptic transmission
Trends in Neurosciences, 1997
V I E W P O I N T I N THE MAMMALIAN BRAIN, excitatory transmission is mediated by the amino acid glutamate at most synaptic contacts. Glutamate interacts with postsynaptic receptors forming cationic channels with different ion permeabilities, conductances, activationdeactivation kinetics and pharmacological properties. In 1989, the expression cloning of a cDNA coding for a subunit of the glutamate receptor 1 represented a breakthrough in the study of these receptors. The subsequent isolation of cDNAs for different subunits of the ionotropic glutamate receptors demonstrated that they belong to a common gene family (for reviews, see Refs 2,3). Further studies have provided insight into the functional and structural diversity of NMDA and non-NMDA type of glutamate-receptor channels 3 , indicating that all glutamate-receptor channels might be assembled from structurally homologous subunits. Patch-clamp recordings from CNS neurones revealed that native non-NMDA receptors are similar to the AMPA-type recombinant receptors expressed from cDNA clones 4,5 , but have failed so far to detect receptor-channels with properties of the kainate type. Although functional receptors with a higher affinity for kainate than previously observed were described in peripheral neurones some time ago 6-8 , in CNS neurones such receptors have remained elusive for a long time. Indeed, rapidly desensitizing responses to kainate have only recently been demonstrated in cultured hippocampal neurones 9 and glia 10 , indicating that functional receptors of the kainate type are expressed by native brain cells.
Glutamate metabotropic receptors as targets for drug therapy in epilepsy
European Journal of Pharmacology, 2003
Metabotropic glutamate (mGlu) receptors have multiple actions on neuronal excitability through G-protein-linked modifications of enzymes and ion channels. They act presynaptically to modify glutamatergic and g-aminobutyric acid (GABA)-ergic transmission and can contribute to long-term changes in synaptic function. The recent identification of subtype-selective agonists and antagonists has permitted evaluation of mGlu receptors as potential targets in the treatment of epilepsy. Agonists acting on group I mGlu receptors (mGlu 1 and mGlu 5 ) are convulsant. Antagonists acting on mGlu 1 or mGlu 5 receptors are anticonvulsant against 3,5-dihydroxyphenylglycine (DHPG)-induced seizures and in mouse models of generalized motor seizures and absence seizures. The competitive, phenylglycine mGlu 1/5 receptor antagonists generally require intracerebroventricular administration for potent anticonvulsant efficacy but noncompetitive antagonists, e.g., (3aS,6aS)-6a-naphthalen-2-ylmethyl-5-methyliden-hexahydrocyclopenta[c]furan-1-on (BAY36-7620), 2-methyl-6-(phenylethynyl)pyridine hydrochloride MPEP), and 2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893) block generalized seizures with systemic administration. Agonists acting on group II mGlu receptors (mGlu 2 , mGlu 3 ) to reduce glutamate release are anticonvulsant, e.g., 2R,4R-aminopyrrolidine-2,4dicarboxylate [(2R,4R)-APDC], (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), and (À)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268). The classical agonists acting on group III mGlu receptors such as L-(+)-2-amino-4-phosphonobutyric acid, and L-serine-O-phosphate are acutely proconvulsant with some anticonvulsant activity. The more recently identified agonists (R,S)-4phosphonophenylglycine [(R,S)-PPG] and (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG] and (1S,3R,4S)-1-aminocyclopentane-1,2,4tricarboxylic acid [ACPT-1] are all anticonvulsant without proconvulsant effects. Studies in animal models of kindling reveal some efficacy of mGlu receptor ligands against fully kindled limbic seizures.
Neuroscience, 1996
The effects of a recently synthesized benzoyl-piperidine drug that enhances currents mediated by ct-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors were tested on monosynaptic and polysynaptic responses in hippocampal slices of the rat. Stimulation of perforant path inputs to the dentate gyrus evoked extracellular responses in field CA1 that had latencies and laminar profiles indicating that they were relayed through the trisynaptic intrahippocampal circuit. Under control conditions, trisynaptic field excitatory postsynaptic potentials did not show larger paired-pulse facilitation than monosynaptic responses and failed to exhibit frequency facilitation. Low concentrations of picrotoxin greatly enhanced trisynaptic responses and, under these conditions, frequency facilitation was obtained. Benzoyl-piperidine-12 (250 laM) had a threefold greater effect on the amplitude of trisynaptic responses than on monosynaptic field excitatory postsynaptic potentials, indicating that the drug's effect is amplified across the successive stages of a polysynaptic circuit. The AMPA receptor modulator did not change the frequency characteristics ofmonosynaptic potentials and had only a modest influence on those of the trisynaptic response. The effect of benzoyl-piperidine-12 on trisynaptic responses was significantly greater when GABAergic inhibition was partially blocked with picrotoxin; the GABA blocker did not alter the effects of benzoyl-piperidine-12 on monosynaptic responses. These results indicate that centrally active AMPA receptor modulators are likely to have a greater influence on brain operations involving long chains of connections than on those mediated by simple reflex-like circuits, and will vary markedly in their effects depending upon the excitability of local interneurons.