Molecular cloning and functional expression of glutamate receptor subunit genes

Solubilization and immunological identification of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the rat hippocampus

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been identified mostly as postsynaptic receptors mediating fast glutamatergic synaptic transmission. However, neurochemical studies based on the modulation of neurotransmitter release have suggested the existence of presynaptic AMPA receptors. We have used a recently described technique that allows a high-purity fractionation of the pre- and postsynaptic proteins of synaptic junctions to evaluate the distribution of the different AMPA receptor subunits in rat hippocampal synapses. Surprisingly, we found very high levels of GluR1- and GluR2/3-like immunoreactivity in the presynaptic fraction, but also in the postsynaptic and extrasynaptic fractions. GluR4-like immunoreactivity was much less abundant but was still detected, predominantly in the postsynaptic fraction. This methodology appears to be far more sensitive than the classical immunogold electron microscopy to determine the localization of synaptic receptors.

Development of chick retina cells in culture: cobalt entry through AMPA receptors and expression of GluR4 AMPA receptor subunit

The functionality of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors in chick embryo retina cells during development in vitro was studied by using Co(2+) uptake, and these data were correlated with the expression of the AMPA receptor subunit GluR4. We found that at 5 h in vitro only a small number of cells took up Co(2+) upon stimulation with 100 microM kainate or other AMPA receptor agonists, in the presence of cyclothiazide (CTZ), to inhibit desensitisation. The number of cells sensitive to kainate increased from 5 h in vitro to 3 days in vitro (DIV), and remained relatively constant until 14 DIV. When the cells were stimulated with (2S,4S)-4-methylglutamic acid (30 microM), a specific kainate receptor agonist, after inhibiting desensitisation with concanavalin A, we did not observe an increase in the number of cells responding, as compared to the control. The expression of the AMPA receptor subunit GluR4 during development was detected by immunofluorescence mainly at the perinuclear region of the cells, and the number of positive cells increased from 5 h in vitro to 7 DIV, and remained relatively constant until 14 DIV. The results suggest that AMPA receptors can be functionally active at early embryonic stages (5 h in vitro) in cultured retinal neurons, although in only a few cells, before synapse formation (E12). The localisation of GluR4 was well correlated with Co(2+) entry, since the strongest GluR4 immunoreactivity was found in the regions that showed the most intense labelling with Co(2+). Finally, we found that the expression levels of GluR4 at the neurites increased between 5 h in vitro and 7 DIV, near the period of synapse formation.

Thyroid hormone regulation of N-methyl-D-aspartic acid receptor subunit mRNA expression in adult brain

Thyroid hormone is an essential modulator of brain development, but little is known about its actions in the adult brain. Hypothyroidism is associated with gene expression changes in both central and peripheral nervous tissue. Functional consequences of adult-onset hypothyroidism include an inability to produce long-term potentiation in rat hippocampus and impaired learning and memory in both rats and man. Long-term potentiation is a form of learning that is dependent on functional N-methyl-d-aspartic acid (NMDA)-preferring ionotropic glutamate receptors. This work examines the expression of ionotropic glutamate receptor subunit mRNA following surgical thyroidectomy with or without thyroid hormone replacement. In situ hybridization histochemistry was used to determine the mRNA levels of the NMDA receptor subunits NR1, NR2A, NR2B, the AMPA receptor subunit GluR1, and the kainate receptor subunit KA2. Reducing circulating concentrations of thyroid hormone by surgical removal of the thyroid gland 2 weeks before sacrifice decreased the expression of NR1 mRNA exclusively in the hippocampus. Conversely, hyperthyroidism selectively reduced NR2B mRNA expression in the dorsal hippocampus. Altering thyroid hormone status had no effect on the expression of KA2 or GluR1 subunit mRNA. The regulation of expression of NR1 and NR2B mRNA by thyroid hormone is a novel mechanism for explaining the relationship between thyroid hormone and cognitive function.

Effects of single or repeated restraint stress on GluR1 and GluR2 flip and flop mRNA expression in the hippocampal formation

The mRNAs encoding the flip and flop isoforms of the glutamate receptor subunits GluR1 and 2 were detected and quantified by in situ hybridization in the hippocampal formation of rats following acute (6h) or chronic (6h daily for 21 days) restraint stress. The GluR1 flip mRNA was slightly reduced in CA1 after chronic stress and the GluR2 flip mRNA was increased in the dentate gyrus (DG), CA4, and CA3 after acute stress. There were no changes in the mRNA encoding the flop isoforms of either GluR1 or 2 in the hippocampus. In entorhinal cortex, the GluR1 flip mRNA was significantly increased after both acute and chronic stress, while the flop isoform increased only after chronic stress. The GluR2 flip mRNA was slightly increased after acute and chronic stress. However, no changes were found for the flop isoform of GluR2. These results suggest that different assembly of AMPA receptors subunits and isoforms may underlie, in a different way, the neuronal plastic changes induced by specific type and intensity of stressful stimuli.

GluR2-dependent properties of AMPA receptors determine the selective vulnerability of motor neurons to excitotoxicity

AMPA receptor-mediated excitotoxicity has been implicated in the selective motor neuron loss in amyotrophic lateral sclerosis. In some culture models, motor neurons have been shown to be selectively vulnerable to AMPA receptor agonists due to Ca(2+) influx through Ca(2+)-permeable AMPA receptors. Because the absence of GluR2 in AMPA receptors renders them highly permeable to Ca(2+) ions, it has been hypothesized that the selective vulnerability of motor neurons is due to their relative deficiency in GluR2. However, conflicting evidence exists about the in vitro and in vivo expression of GluR2 in motor neurons, both at the mRNA and at the protein level. In this study, we quantified electrophysiological properties of AMPA receptors, known to be dependent on the relative abundance of GluR2: sensitivity to external polyamines, rectification index, and relative Ca(2+) permeability. Cultured rat spinal cord motor neurons were compared with dorsal horn neurons (which are resistant to excitotoxicity) and with motor neurons that survived an excitotoxic insult. Motor neurons had a higher sensitivity to external polyamines, a lower rectification index, and a higher relative Ca(2+) permeability ratio than dorsal horn neurons. These findings confirm that motor neurons are relatively deficient in GluR2. The AMPA receptor properties correlated well with each other and with the selective vulnerability of motor neurons because motor neurons surviving an excitotoxic event had similar characteristics as dorsal horn neurons. These data indicate that the relative abundance of GluR2 in functional AMPA receptors may be a major determinant of the selective vulnerability of motor neurons to excitotoxicity in vitro.

GluR2-R4 AMPA subunit study in rat vestibular nuclei after unilateral labyrinthectomy: an in situ and immunohistochemical study

In the present investigation, we address the question of whether the expression of GluR2-R4 subunits mRNAs and GluR2 and GluR4 subunits protein of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptors are modulated in the vestibular nuclei following unilateral labyrinthectomy. Specific GluR2-R4 radioactive oligonucleotides were used to probe sections of rat vestibular nuclei according to in situ hybridization methods. The signal was detected by means of film or emulsion photography. GluR2 and GluR4 subunit expression were also measured in control and operated rats by use of specific monoclonal GluR2 and GluR4 antibodies. Animals were killed at different stages following the lesion: 1, 3 or 8 days for the in situ hybridization study and 4 and 8 days for the immunohistochemical study. In normal animals, several brainstem regions including the lateral, medial, superior and inferior vestibular nuclei expressed all the GluR2, GluR3 and GluR4 subunit mRNAs. Moreover, numerous vestibular nuclei neurons are endowed with AMPA receptors containing the GluR2 and the GluR4 subunits. In unilaterally labyrinthectomized rats, no asymmetry could be detected on autoradiographs between the two medial vestibular nuclei probed with the GluR2 and the GluR4 oligonucleotide probes regardless of the delay following the lesion. However, compared to control, a bilateral decrease (-22%) in GluR3 gene expression was observed in the medial vestibular nuclei 3 days after the lesion followed by a return to normal at day 8 post-lesion. No significant asymmetrical changes in the density of GluR2- and GluR4-immunopositive cells could be detected between the intact and deafferented sides in any part of the vestibular nuclear complex and at any times (day 4 or day 8) following the lesion. Our data show that the removal of glutamatergic vestibular input induced an absence of modulation of GluR2 and GluR4 gene and subunits expression. This demonstrates that GluR2 and GluR4 expression do not play a role in the recovery of the resting discharge of the deafferented medial vestibular nuclei neurons and consequently in the functional restoration of the static postural and oculomotor deficits. The functional role of the slight and bilateral GluR3 mRNA decrease in the vestibular nuclei remains to be elucidated.

AMPA Neurotoxicity in Rat Cerebellar and Hippocampal Slices: Histological Evidence for Three Mechanisms

Excitatory amino acid-induced death of central neurons may be mediated by at least two receptor types, the so-called NMDA (N-methyl-d-aspartate) and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors. We have studied the neurodegenerative mechanisms set in motion by AMPA receptor activation using incubated slices of 8-day-old rat cerebellum and hippocampus. In both preparations, AMPA induced a pattern of degeneration that differed markedly from the one previously shown to be elicited by NMDA. In cerebellar slices, AMPA induced the degeneration of most Purkinje cells together with a population of Golgi cells; in hippocampal slices the neurons were affected in the order CA3 > CA1 > dentate granule cells. Three mechanisms could be discerned: an acute one in which neurons (e.g. cerebellar Golgi cells) underwent a rapid degeneration; a delayed one in which the neurons (Purkinje cells and hippocampal neurons) appeared to be only mildly affected immediately after a 30 min exposure but then underwent a protracted degeneration during the postincubation period (1.5 - 3 h); and finally a slow toxicity, which took place during long (2 h) exposures to AMPA (3 - 30 microM). Although Purkinje cells were vulnerable in both cases, the efficacy of AMPA was higher for the delayed mechanism than for the slow one. The pathology displayed by the acutely destroyed Golgi neurons was a classical oedematous necrosis, whereas most neurons vulnerable to the delayed and slow mechanisms displayed a 'dark cell degeneration', whose cytological features bore a close resemblance to those of neurons irreversibly damaged by ischaemia, hypoglycaemia or status epilepticus in vivo.

Mechanisms of AMPA Neurotoxicity in Rat Brain Slices

The mechanisms underlying the neurodegenerative effects of the glutamate receptor agonist, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate), were studied using brain slice preparations of young rat (8 - 9 days old) cerebellum and hippocampus. Rapid AMPA toxicity (exerted on some cerebellar interneurons) was inhibited by including the appropriate receptor blocker, CNQX (6-cyano-7-nitroquinoxaline-2,3-dione, 10 microM), in the exposing solution. The degeneration of other neurons, including Purkinje cells and hippocampal pyramidal neurons, persisted. It could, however, be largely prevented if CNQX was included for 1.5 h during the post-incubation period, suggesting that an enduring 'rebound' AMPA receptor activation was responsible for this delayed type of degeneration, not the exposure itself. In cerebellar slices, independent evidence for the occurrence, postexposure, of persisting AMPA receptor stimulation was obtained electrophysiologically. Omission of Ca2+ during the exposure period (and for 10 min beforehand) markedly reduced rapid AMPA toxicity but was ineffective in protecting most of the Purkinje cells. However, if the slices were previously starved of Ca2+ for 1 h, then most of these neurons survived, even if the ion was reinstated during the recovery period. Slow AMPA toxicity, which takes place during long (2 h) exposures, could be inhibited either by CNQX or by omission of Ca2+ (30 min preincubation). The results indicate that the rapid oedematous necrosis induced by AMPA, like that caused by N-methyl-d-aspartate and kainate, is likely to involve excessive influx of Ca2+. In contrast, the induction of the delayed mechanisms, as well as its 'expression' during the postincubation period, probably depends on intracellular Ca2+, rather than Ca2+ influx.

Dizocilpine infusion has a different effect in the development of morphine and cocaine sensitization: behavioral and neurochemical aspects

The stimulation of glutamate receptors plays a relevant role in the development of behavioral sensitization to psychostimulants, while less clear results have been obtained on their role in morphine sensitization. We addressed this issue by comparing the development of cocaine and morphine sensitization under a continuous s.c. infusion of the N-methyl-D-aspartate (NMDA) antagonist dizocilpine (0.1 mg/kg/24 h). Moreover, we studied the expression of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunits in discrete limbic areas of rats sensitized to morphine or cocaine with or without the concomitant dizocilpine infusion. It was observed that dizocilpine infusion did not prevent the development of morphine sensitization, while it prevented the development of tolerance to morphine-induced analgesia. Finally, morphine-sensitized animals did not present any modification in the subunit expression of glutamate receptors in the brain areas examined. In agreement with previous results, we found that dizocilpine infusion prevented the development of cocaine sensitization. Moreover, we observed that rats sensitized to cocaine presented a significant increase in the levels of GLUR1, NR1 and NR2B, in the nucleus accumbens, and of NR2B in the hippocampus compared to control animals. Such modifications were absent in rats administered cocaine under dizocilpine infusion. We conclude that: (i) morphine sensitization is a neuroadaptive phenomenon which does not appear to require NMDA receptor activity in order to develop; (ii) cocaine sensitization is clearly dependent on NMDA receptor activity, as dizocilpine infusion prevented the occurrence of glutamate receptors modifications as well as the development of sensitization.

AMPA glutamatergic receptor-immunoreactive subunits are expressed in lumbosacral neurons of the spinal cord and neurons of the dorsal root and pelvic ganglia controlling pelvic functions in the rat

Sacral preganglionic neurons innervate the pelvic organs via a relay in the major pelvic ganglion. Pudendal motoneurons innervate striated muscles and sphincters of the lower urinary, genital and digestive tracts. The activity of these spinal neurons is regulated by sensory afferents of visceral and somatic origins. Glutamate is released by sensory afferents in the spinal cord, and interacts with a variety of receptor subtypes. The aim of the present study was to investigated the presence of AMPA glutamate receptor subunits (GluR1-GluR4) in the neural network controlling the lower urogenital and digestive tracts of male rats. We performed double-immunohistochemistry directed against a neuronal tracer, the cholera toxin beta subunit (Ctbeta) and each of the four receptor subunits. GluR1, GluR2 and GluR3 subunits were present in many sacral preganglionic neurons retrogradely labelled with Ctbeta applied to the pelvic nerve, and in some dorsolateral and dorsomedian motoneurons retrogradely labelled with Ctbeta injected in ischiocavernosus and bulbospongiosus muscles. The four subunits were detected in postganglionic neurons of the major pelvic ganglion retrogradely labelled with Ctbeta injected in the corpus cavernosum, and in some somata of sensory afferents of the L6 dorsal root ganglion labelled with Ctbeta applied to the dorsal penile nerve or injected in corpus cavernosum. The results provide a detailed knowledge of the neural targets expressing the various AMPA receptor subunits and suggest that part of the neural network that controls pelvic organs, including sensory afferents and postganglionic neurons, is sensitive to glutamate through the whole family of AMPA subunits.

AMPA glutamate receptor-mediated calcium signaling is transiently enhanced during development of oligodendrocytes

Cells of the oligodendroglial lineage express Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-preferring glutamate receptors (AMPA-GluR) during development. Prolonged activation of their AMPA-GluR causes Ca2+ overload, resulting in excitotoxic death. Prior studies have shown that oligodendroglial progenitors and immature oligodendrocytes are susceptible to excitotoxicity, whereas mature oligodendrocytes are resistant. An unresolved issue has been why Ca2+-permeability of AMPA-GluR varies so markedly with oligodendroglial development, although the level of expression of edited GluR2, an AMPA-GluR subunit which blocks Ca2+ entry, is relatively constant. To address this question, we performed Ca2+ imaging, molecular and electrophysiological analyses using purified cultures of the rat oligodendroglial lineage. We demonstrate that transient up-regulation of expression of GluR3 and GluR4 subunits in oligodendroglial progenitors and immature oligodendrocytes results in the assembly by these cells, but not by oligodendroglial pre-progenitors or mature oligodendrocytes, of a population of AMPA-GluR which lack GluR2. This stage-specific up-regulation of edited GluR2-free, and hence Ca2+-permeable, AMPA-GluR explains the selective susceptibility to excitotoxicity of cells at these stages of oligodendroglial differentiation, and is likely to be important to these cells in the trans-synaptic Ca2+-signaling from glutamatergic neurons, which occurs in hippocampus

Selective and AMPA receptor-dependent astrocyte death following prolonged blockade of glutamate reuptake in rat cerebellar cultures

In this study we examined the effects of prolonged l-trans-pyrrolidine-2,4-dicarboxylate (PDC)-induced glutamate reuptake blockade on the viability of glial cells in cerebellar granule cell cultures. Immunofluorescence staining for the glial-specific intermediate filament protein, GFAP, revealed that the PDC- induced increase of extracellular glutamate concentration was accompanied by increased astrocyte death, while neurons and oligodendrocytes remained intact and viable. Astrocytic cell death was manifested as fragmentation of processes and cell bodies. The selective astrocyte death was completely prevented by the competitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist, NBQX (10 microM), whereas MK-801 (10 microM), a noncompetitive blocker of N-methyl-D-aspartate receptors, gave only partial protection. Double staining for GFAP and the AMPA receptor subunits GluR2/3 showed that astrocytes had much higher immunoreactivity for GluR2/3 than neurons or oligodendrocytes, suggesting that the number of AMPA receptors is likely to be higher on astrocytes. Furthermore, we employed real-time RT-PCR to measure GluR1-4 subunit mRNA expression in control and PDC-exposed cultures. Following treatment with PDC, GluR1 and GluR4 mRNAs were reduced by 40% and GluR3 was reduced by 70% relative to control levels. In contrast, GluR2 expression was not affected by the PDC treatment, indicating that GluR3 was the dominant type of AMPA receptor subunit expressed on astrocytes. Our results show that astrocytes appear to be more vulnerable than neurons or oligodendrocytes to a gradual increase in the extracellular glutamate concentration, suggesting that astrocytes may be critically involved in the pathophysiology of slowly developing chronic neurodegenerative disorders.

The structure and function of glutamate receptor ion channels

As in the case of many ligand-gated ion channels, the biochemical and electrophysiological properties of the ionotropic glutamate receptors have been studied extensively. Nevertheless, we still do not understand the molecular mechanisms that harness the free energy of agonist binding, first to drive channel opening, and then to allow the channel to close (desensitize) even though agonist remains bound. Recent crystallographic analyses of the ligand-binding domains of these receptors have identified conformational changes associated with agonist binding, yielding a working hypothesis of channel function. This opens the way to determining how the domains and subunits are assembled into an oligomeric channel, how the domains are connected, how the channel is formed, and where it is located relative to the ligand-binding domains, all of which govern the processes of channel activation and desensitization.

Correlation of the expression of kainate receptor subtypes to responses evoked in cultured cortical and spinal cord neurones

Responses of cultured rat cortical and spinal cord neurones to kainate (KA) have been related to the expression of KA receptor subunits revealed by single-cell reverse transcription polymerase chain reaction (RT-PCR). Semi-rapid application of KA evoked non-desensitizing responses with EC(50) values of 82 microM for cortical and 67 microM for spinal cord neurones. In the presence of concanavalin A, GYKI 53655 (100 microM) reduced responses of both types of neurone to KA by about 80% to leave a KA receptor-mediated response with an EC(50) of 4 microM on spinal cord neurones and 27 microM (P<0.001) on cortical neurones. Ultra-fast application of KA to outside-out patches of cortical neurones evoked a non-decaying response which was reduced by about 30% by GYKI 53655 to reveal a transient response that desensitized by 92.5% with a time constant (tau(des)) of 26.2 ms. Responses of spinal cord patches decayed by 47.8%. GYKI 53655 reduced the peak response by 8.3% and the residual response desensitized by 75.8%, with a tau(des) of 17.3 ms, all values being significantly smaller than for cortical neurones. Single-cell RT-PCR showed relative abundances of mRNAs for the KA receptors, GluR5, GluR6 and GluR7, of 12, 33 and 54% for cortical neurones and 38, 10 and 54% for spinal cord neurones, respectively. The relative abundances of KA1 and KA2 were 12 and 88% for cortical neurones, and 19 and 79% for spinal cord neurones, respectively. The most likely expression patterns of functional KA receptors is GluR6/KA2 for cortical neurones and GluR5/KA2 for spinal cord neurones.

Expression of AMPA/kainate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The activity and the subunit expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the AMPA/kainate receptors was monitored by following the changes in the intracellular free calcium concentration ([Ca(2+)](i)), in response to AMPA, kainate or to L-glutamate, and the expression of the receptor subunits GluR1, GluR2/3, GluR4 and GluR6/7 was determined in the retinospheroids and in chick retinas by immunodetection using polyclonal antibodies. The changes in [Ca(2+)](i) in response to 400 microM kainate increased from 5h in vitro to 3 days, and remained constant until day 14, whereas the [Ca(2+)](i) in response to 500 microM L-glutamate or 400 microM AMPA increased from 5h in vitro to 3 days, and thereafter decreased slightly until day 14. The [Ca(2+)](i) responses to kainate are mainly due to AMPA receptor stimulation, since the signals were abolished by LY303070, the AMPA receptor antagonist, and were not affected by MK-801, the NMDA receptor antagonist. In retinospheroids, the levels of expression of GluR1 subunit increased from 5h in vitro until day 7, then decreased until day 14. The levels of expression of GluR2/3 and GluR4 subunits increased from 5h in vitro until day 10, and remained constant until day 14. The levels of kainate receptor subunits GluR6/7 increased from 5h in vitro until day 3, and thereafter decreased slightly until day 14. In the retinas, the expression of GluR1 and GluR6/7 subunits increased from day 8 until day 15, and then decreased until day 22 (post-natal 1). The subunits GluR2/3 and GluR4 increased from day 8 until day 18, and remained constant until day 22. The results suggest that AMPA/kainate receptors are expressed at early embryonic stages, although at low levels and before synapse formation (E12). However, the AMPA receptors are not completely functional at the first stage studied since they do not respond to the agonist AMPA. Also, the patterns of AMPA/kainate receptor subunit expression in retinospheroids of chick embryo retina cells cultured in vitro and in retina cells developing in the embryo (in vivo) were similar, indicating that the AMPA/kainate receptor subunits expression in these primary cultures mimics their expression in the developing chick retina.

Phosphorylation of GluR4 AMPA-type glutamate receptor subunit by protein kinase C in cultured retina amacrine neurons

We have previously reported that the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors is potentiated by protein kinase C (PKC) in cultured chick retina amacrine neurons, and that constitutive PKC activity is necessary for basal AMPA receptor activity (Carvalho et al., 1998). In this study, we evaluated the phosphorylation of the GluR4 subunit, which is very abundant in cultured amacrine neurons, to correlate it with the effects of PKC on AMPA receptor activity in these cells. 32P-labelling of GluR4 increased upon AMPA receptor stimulation or cell treatment with phorbol 12-myristate 13-acetate (PMA) before stimulating with kainate. By contrast, phosphorylation of GluR4 was not changed when PKC was inhibited by treating the cells with the selective PKC inhibitor GF 109203X before stimulation with kainate. We conclude that GluR4 is phosphorylated upon PKC activation and/or stimulation of AMPA receptors in cultured amacrine cells. Additionally, AMPA receptor activation with kainate in cultured chick amacrine cells leads to translocation of conventional and novel PKC isoforms to the cell membrane, suggesting that PKC could be activated upon AMPA receptor stimulation in these cells.

The aniracetam metabolite 2-pyrrolidinone induces a long-term enhancement in AMPA receptor responses via a CaMKII pathway

The present study was conducted to assess the effect of aniracetam and its metabolites, such as 2-pyrrolidinone, p-anisic acid, and anisamide butyrate, on the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, heteromerically formed of GluR1,2 (GluR1 and GluR2), GluR1,3 (GluR1 and GluR3), and GluR1,2,3 (GluR1, GluR2, and GluR3), expressed in Xenopus oocytes. 2-Pyrrolidinone potentiated kainate-evoked currents through GluR1,2,3 channels in a bell-shaped dose-dependent manner at concentrations ranged from 1 nM to 300 microM, with a maximal effect at 100 microM. The potentiation was long-lasting, reaching approximately 180% of basal levels 60 min after 5-min treatment with 2-pyrrolidinone at 100 microM. 2-Pyrrolidinone (100 microM) potentiated GluR1,3 channel currents as observed in GluR1,2,3, but instead it depressed GluR1,2 currents. Aniracetam and p-anisic acid potentiated GluR1,2,3 channel currents, but to a lesser extent, each about 130 and 103% of basal levels 60 min after treatment at 100 microM. In contrast, anisamide butyrate had no potentiating effect on the currents. Potentiation of GluR1,2,3 channel currents obtained with 2-pyrrolidinone was inhibited by KN-93, a selective inhibitor of calcium/calmodulin-dependent protein kinase (CaMKII), while it was not affected by GF109203X, a selective inhibitor of protein kinase C or H-89, a selective inhibitor of cAMP-dependent protein kinase. The results of the present study suggest that 2-pyrrolidinone persistently enhances activity of the Ca2+-permeable AMPA receptors, GluR1,3 and GluR1,2,3, by interacting with CaMKII.

Control of kinetic properties of GluR2 flop AMPA-type channels: impact of R/G nuclear editing

The GluR2 flop subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors greatly determines calcium permeability and kinetic properties of heteromeric AMPA subunit assemblies. Post-transcriptional editing of this subunit at the Q/R/N site controls calcium permeability whereas editing at the R/G site is involved in the regulation of biophysical properties. We used patch-clamp techniques with ultrafast solution exchange to examine the kinetics of recombinant human homomeric GluR2 flop channels transiently expressed in HEK293 cells [edited at the R/G site and Q/R/N site (GR), and unedited (RN) and edited (GN) at the R/G site both with asparagine (N) at the Q/R/N site]. The time constant of desensitization after application of 10 mm glutamate was 1.38 +/- 0.05 ms (n = 10), 5.53 +/- 0.57 ms (n = 7) and 1.33 +/- 0.06 ms (n = 12) for the GluR2 flop GR, RN and GN channels, respectively. The time constant of resensitization was 75 ms for the GluR2 flop RN and 30 ms for the GN channels. The dose-dependence of the peak current amplitude, kinetics of activation and deactivation, and peak open probability did not differ between RN and GN channels. The study shows that desensitization and resensitization kinetics of homomeric GluR2 flop channels are controlled by a single amino acid exchange (glycine by arginine) at the R/G site. Quantitative analysis by computer simulation using a circular kinetic scheme allows the prediction of the main experimental results.

The expression of dominant-negative subunits selectively suppresses neuronal AMPA and kainate receptors

Glutamate-gated ion channels are widely expressed in neurons where they serve a host of cellular functions. An appealing, but yet unexplored, way to delineate the functions of particular glutamate receptor subtypes is to direct the expression of dominant-negative and gain-of-function mutant subunits. We tested the ability of two dominant-negative subunits, an alpha-amino-3-hydroxy-5-methyl-isoxazolproprionic acid receptor subunit and a kainate receptor subunit, to silence recombinant and neuronal glutamate receptors. Co-expression studies in non-neuronal cells indicated that the inclusion of a single mutant subunit was sufficient to silence the receptor. When expressed in cerebellar granule cells, the dominant-negative subunits silenced native channels in a subtype-specific fashion. Immunocytochemical staining of control and transfected neurons, as well as studies with a gain-of-function glutamate receptor-1 mutant, indicated that the mutant subunits were expressed at levels roughly equal to the total abundance of related native subunits, and both dominant-negatives suppressed native channel expression 60-65% when tested 24 h post-transfection. If co-assembly of the mutant subunits with related native subunits is combinatorial, this level of suppression gives receptor half-lives of approximately 20 h.

N -Acetylaspartylglutamate acts as an agonist upon homomeric NMDA receptor (NMDAR1) expressed in Xenopus oocytes

The electrophysiological effects of N-acetylaspartylglutamate (NAAG), an endogenous peptide restrictively distributed in the central nervous system, were studied using Xenopus oocytes injected with RNAs transcribed from cloned glutamate receptor cDNAs. NAAG induced an inward current, dose dependently, in oocytes injected with RNA for an N-methyl-D-aspartate receptor subunit (NMDAR1). In contrast, the oocytes injected with RNAs for AMPA-selective glutamate receptors (GluR1, GluR3, GluR1+GluR2 and GluR2+GluR3) scarcely responded to NAAG, and the oocytes injected with RNA for kainate receptor (GluR6) did not respond to NAAG. The half-maximal response (ED50) value of NAAG on expressed NMDAR1 was 185 microM, which shows that NAAG is about 115-times less potent than L-glutamate (Glu), the ED50 of which value was 1.6 microM. The maximal current amplitude induced by NAAG was about 70% of that by Glu. NAAG-induced current in NMDAR1-injected oocytes was potentiated by glycine, dose-dependently antagonized by DL-2-amino-5-phosphonovaleric acid, and blocked by magnesium ions in a voltage-dependent fashion. These results suggest that NAAG is one of the endogenous agonists selective for NMDAR1.

Changes in AMPA receptor binding and subunit messenger RNA expression in hippocampus and cortex in the pentylenetetrazole-induced 'kindling' model of epilepsy

'Kindling' is a phenomenon of epileptogenesis, which has been widely used as an experimental model of temporal lobe epilepsy. In the present study, we have examined the contribution of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptors and their subunits (GluR-A, -B, -C and -D) to the acquisition and maintenance of the kindled state in the pentylenetetrazole (PTZ)-induced 'kindling' mouse model, by using quantitative autoradiography and in situ hybridization. Region-specific increases in [3H]AMPA binding were seen in kindled animals in the CA3 region of hippocampus and in the temporal cortex 1 week after the last PTZ injection. At the same time, a significant decrease in the level of transcripts encoding the GluR-B and -C subunits was detected in the hippocampal CA1 region and dentate gyrus, suggestive of a higher proportion of Ca(2+)-permeable AMPA receptors in these neurons. These changes did not persist 1 month after establishment of kindling, indicating a transient role of AMPA receptors in the acquisition of the kindled state. At 1 month after the last PTZ injection, an upregulation in [3H]AMPA binding appeared in the motor cortex and the basal ganglia of kindled animals, which is consistent with electrophysiological data showing hyperexcitability in the cortex of the PTZ-kindled animals at that time. Interestingly, an increase in mRNA for the GluR-B subunit appeared in the outer layers of motor and somatosensory cortices of the kindled animals 1 month after acquisition of the kindled state, possibly as part of a gene-regulated, compensatory mechanism against seizure susceptibility, since this change should give rise to a higher proportion of Ca(2+)-impermeable AMPA receptors. These results support the evidence of a transient role of hippocampal AMPA receptors in the acquisition of the 'kindling' phenomenon and they also suggest an involvement of AMPA receptors in the maintenance of kindled state at least in two brain areas, cortex and basal ganglia.

N-methyl-D-aspartate-induced alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor down-regulation involves interaction of the carboxyl terminus of GluR2/3 with Pick1. Ligand-binding studies using Sindbis vectors carrying AMPA receptor decoys

The dynamics of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)-type glutamate receptors, as represented by their exocytosis, endocytosis and cytoskeletal linkage, has often been implicated in N-methyl-d-aspartate (NMDA)-dependent synaptic plasticity. To explore the molecular mechanisms underlying the AMPA receptor dynamics, cultured hippocampal neurons were stimulated with 100 microm NMDA, and the biochemical and pharmacological changes in the ligand binding activity of AMPA receptor complexes and its subunits, GluR1 and GluR2/3, were investigated. The NMDA treatment reduced the total amount of bound [(3)H]AMPA on the surface of the neurons but not in their total membrane fraction. This process was mimicked by a protein kinase C activator, phorbol ester, but blocked by an inhibitor of the same kinase, calphostin C. The NMDA-induced down-regulation of the ligand binding activity was also reflected by the decreased AMPA-triggered channel activity as well as by the cells' reduced immunoreactivity for GluR1. In parallel, the NMDA treatment markedly altered the interaction between the AMPA receptor subunits and their associating molecule(s); the association of PDZ molecules, including Pick1, with GluR2/3 was enhanced in a protein-kinase-C-dependent manner. Viral expression vectors carrying GluR1 and GluR2 C-terminal decoys, both fused to enhanced green fluorescent protein, were transfected into hippocampal neurons to disrupt their interactions. The overexpression of the C-terminal decoy for GluR2 specifically and significantly blocked the NMDA-triggered reduction in [(3)H]AMPA binding, whereas that for GluR1 had no effects. Co-immunoprecipitation using anti-Pick1 antibodies revealed that the overexpressed GluR2 C-terminal decoy indeed prevented Pick1 from interacting with the endogenous GluR2/3. Therefore, these observations suggest that the NMDA-induced down-regulation of the functional AMPA receptors involves the interaction between GluR2/3 subunits and Pick1.

LY293558, an AMPA glutamate receptor antagonist, prevents and reverses levodopa-induced motor alterations in Parkinsonian rats

To evaluate the possible involvement of glutamate AMPA receptor-mediated mechanisms in levodopa-induced motor fluctuations, we investigated the effects of LY293558, a competitive AMPA receptor antagonist, on levodopa-induced motor alterations in rats with unilateral 6-OHDA lesion. Acute and chronic administration of LY293558 was studied to evaluate the possible reversion or prevention of these levodopa effects. In the first set of experiments, rats were treated with levodopa (25 mg/kg with benserazide, twice daily, i.p.) for 22 days and on day 23 LY293558 (5 mg/kg, i.p.) was administered immediately before levodopa. In the second set of experiments, rats were treated daily for 22 days with levodopa and LY293558 (5 mg/kg, twice daily, i.p.). In the third set of experiments, the effect of LY293558 (5 mg/kg, i.p.) administration on selective dopamine D-1 (SKF38393, 1.5 mg/kg, s.c.) and D-2 agonist (quinpirole, 0.1 mg/kg, i.p.)-induced rotational behavior after daily levodopa treatment was studied. The duration of the rotational behavior induced by chronic levodopa decreased by 30% after 22 days. Acute administration of LY293558 on day 23 reversed this effect. The group of animals that were chronically treated with levodopa and LY293558 did not show the decrease in this motor response duration. Chronic levodopa treatment attenuated the rotational response to the D-1 agonist SKF38393 and increased the response to the D-2 agonist quinpirole. LY293558 did not reverse the effect of levodopa on rotational behavior induced by the D-1 agonist but significantly reduced the rotational response to the D-2 agonist in levodopa-treated animals by 40%. Our results demonstrate that an AMPA receptor antagonist reverses and prevents levodopa-induced motor alterations in parkinsonian rats and that this effect on motor fluctuations induced by chronic levodopa is probably due to a modulation of the indirect output pathway of the basal ganglia.

Sequential changes in AMPA and NMDA protein levels during Fe(3+)-induced epileptogenesis

Seizure susceptibility is related to enhanced glutamatergic excitatory synaptic transmission with alterations in the expressions of ionotropic glutamate receptors. We wondered if levels of AMPA and NMDA receptor subunits changed following epileptogenesis induced by amygdalar FeCl(3) injection. We used Western blots to measure levels of subunits in the ipsilateral and contralateral hippocampus at various times after FeCl(3) injection into the amygdaloid body. With acute seizures, at +5 days after the injection, levels of GluR1, NMDAR1, and NMDAR2 were markedly increased in both hippocampi, with quantities at least 2-4 times baseline. By +15 and +30 days after injection, when chronic spontaneous seizures were occurring, the levels of GluR2 were increased, while GluR1 and NMDAR1&2A/B were decreased. Increased NMDAR1&2A/B levels at +5 days are consistent with the occurrence of upregulation of NMDA receptor production in the early stages of epileptogenesis. Since GluR2 suppresses glutamate receptor-mediated Ca(2+)-influx, increased expression of GluR2 with development of chronic, recurrent seizures may be a compensatory effect during epileptogenesis from neural responses to propagated seizures.

GluR2 AMPA receptor subunit expression in motoneurons at low and high risk for degeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder that results in selective degeneration of most, but not all, groups of motoneurons. The greater susceptibility of vulnerable motoneurons to glutamate excitotoxicity and neurodegeneration has been hypothesized to result from their lower expression of the GluR2 AMPA receptor subunit under control conditions, which renders these receptors permeable to calcium. To address the question of whether there is differential expression of the GluR2 subunit in motoneurons, we compared in normal adult rats expression of GluR2 mRNA and protein within two cranial motor nuclei that are either resistant (III; oculomotor nucleus) or vulnerable (XII; hypoglossal nucleus) to degeneration in ALS. RT-PCR analysis of tissue punched from III and XII motor nuclei detected mRNA for all AMPA subunits (GluR1-R4). In situ hybridization demonstrated no significant difference in GluR2 mRNA expression between III and XII nuclei. Immunohistochemical examination of GluR2 (and GluR4) protein levels demonstrated a similar pattern of the subunit expression in both motor nuclei. This equivalent expression of GluR2 mRNA and protein in motoneurons that differ in their vulnerability to degeneration in ALS suggests that reduced expression of GluR2 is not a factor predisposing motoneurons to degeneration.

Straight-chain alcohols exhibit a cutoff in potency for the inhibition of recombinant glutamate receptor subunits

The effects of n-alcohols (methanol to 1-decanol) on kainate-activated AMPA receptor subunit GluR1 and GluR3 ion currents were studied in Xenopus oocytes using the two-electrode voltage-clamp recording technique. For short-chain alcohols from methanol to 1-hexanol, potency for inhibition of GluR1 and GluR3 receptor-mediated current increased in proportion to the chain length or hydrophobicity of the alcohol. The IC(50) values of these alcohols for GluR1 were: methanol, 702 mM; ethanol, 170 mM; 1-propanol, 69 mM; 1-butanol, 20 mM; 1-pentanol, 17 mM; and 1-hexanol, 10 mM. For GluR3, IC(50) values were: methanol, 712 mM; ethanol, 238 mM; 1-propanol, 50 mM; 1-butanol, 32 mM; 1-pentanol, 13 mM; and 1-hexanol, 7 mM. For long-chain alcohols, 1-heptanol was less potent than 1-hexanol (estimated IC(50): 19 mM for GluR1 and 18 mM for GluR3), 1-octanol had little effect only on GluR3, and 1-nonanol and 1-decanol did not significantly inhibit both GluR1 and GluR3 responses. The observations indicate that straight-chain n-alcohols exhibit a cutoff in their potency for inhibition of the function of non-NMDA glutamate receptor subunits, GluR1 and GluR3. The cutoff in potency of n-alcohols for inhibition of non-NMDA glutamate receptor function is consistent with the interpretation that alcohols affect the function of these receptor-channels by interacting with an alcohol binding site of specific dimensions on the receptor protein.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Absolute quantification of AMPA receptor subunit mRNAs in single hippocampal neurons

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit (GluR1-4) mRNAs expressed by single neurons in rat hippocampal cultures were quantified by single-cell RT-PCR using an internal standard RNA after whole-cell patch-clamp recording. The internal standard RNA, derived from GluR2 with a single nucleotide substitution, was reverse-transcribed and PCR-amplified with the same efficiency as GluR1-4 mRNAs. The mean mRNA numbers harvested in vitro from pyramidal-like neurons on day 9 were 1150 +/- 324 molecules of GluR1, 1080 +/- 273 molecules of GluR2, 100 +/- 20 molecules of GluR3, and 50 +/- 10 molecules of GluR4 (mean +/- SEM, n = 12). In a non-pyramidal neuronal population that expresses AMPA receptors characterized by high Ca(2+) permeability, the numbers of GluR1, GluR3 and GluR4 mRNA molecules harvested per cell were 354 +/- 64, 25 +/- 17 and 168 +/- 36, respectively (n = 8). The GluR2 mRNA was not detected in this cell type. The calculated ratio of AMPAR mRNA molecules per total mRNA molecules was 1/240 in pyramidal-like neurons (1/500 for GluR2), being in the range obtained with total RNA from rat forebrain and cerebellum (1/170 and 1/380, respectively). Finally, our results indicated that the proportion of GluR1-4 mRNA located in neurites reached approximately 60% in pyramidal-like neurons. However, we found no evidence of preferential subcellular distribution of a given subunit.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Binding of an AMPA receptor potentiator ([3H]LY395153) to native and recombinant AMPA receptors

LY395153 is a member of a newly described class of arylpropylsulfonamide AMPA receptor potentiators. Here, we characterize and compare [(3)H]LY395153 binding to native AMPA receptors from rat cerebral cortex and recombinant human GluR4(flip) receptors expressed in HEK293 cells. L-Glutamate and AMPA increased [(3)H]LY395153 binding to both native and recombinant AMPA receptors in a concentration dependent and stereoselective manner; this effect of AMPA receptor agonists reflects an apparent increase in ligand affinity. In the presence of L-glutamate (500 microM), [(3)H]LY395153 binding is saturable; the affinity of this radioligand is slightly, albeit statistically significantly higher at human GluR4(flip) (K(d)=55.6+/-5.3nM) than rat cortical receptors (K(d)=110+/-15.1nM). NBQX competitively inhibited L-glutamate-induced increases in [(3)H]LY395153 binding in both native and recombinant receptors, whilst LY303070 (the active isomer of GYKI53655) noncompetitively inhibited this effect in native, but not recombinant receptors. The prototypic AMPA receptor potentiator cyclothiazide competitively inhibited [(3)H]LY395153 binding with a potency (K(i) approximately 7 microM) comparable to EC(50) values reported in electrophysiological studies. In contrast, the structurally unrelated AMPA receptor potentiator CX 516 did not inhibit [(3)H]LY395153 binding at concentrations of up to 600 microM. Further, at concentrations reported to facilitate AMPA receptor desensitization, thiocyanate acts as a competitive inhibitor of [(3)H]LY395153 binding. [(3)H]LY395153 binding was unaffected by a variety of structurally (and mechanistically) diverse compounds tested at a concentration of 10 microM. These data indicate [(3)H]LY395153 is a useful probe for labeling a unique modulatory site on both native and recombinant AMPA receptors.

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor-mediated stimulation of glutamate and GABA release in the rat striatum in vivo: a dual-label microdialysis study

The existence of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate autoreceptors on glutamate nerve terminals in vitro has recently been demonstrated using synaptosomal and brain slice preparations. In the present study we have used a modification of a rapid dual-label intracerebral microdialysis method, previously developed by Young and co-workers(80,81) for the study of presynaptic mechanisms of neurotransmitter release, to investigate whether presynaptic AMPA receptors also play a role in the control of striatal glutamate release in vivo. For comparative purposes, the action of locally applied AMPA on striatal GABA release in vivo was also monitored. Local application of AMPA (0.01-100 microM), by reverse dialysis, into the striatum resulted in concentration-dependent increases in the Ca(2+)-dependent efflux of both [3H]L-glutamate and [14C]GABA. Maximum responses reached 142.0+/-6.5% and 166.8+/-7.7% of basal efflux for [3H]L-glutamate and [14C]GABA, respectively. No marked behavioural changes were observed at any dose of the agonist. Unexpectedly, the AMPA-evoked responses were not potentiated by the AMPA receptor desensitization inhibitors cyclothiazide (10-100microM) or aniracetam (1mM). Consistent with this finding, AMPA-stimulated [3H]L-glutamate and [14C]GABA efflux were significantly attenuated by co-perfusion with the selective, competitive AMPA receptor antagonist 6-nitro-7-sulphamoylbenzo(F)quinoxaline-2,3-dione (100microM) but not 1-(aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine (100microM), a non-competitive AMPA receptor antagonist known to interact with the cyclothiazide site to control AMPA receptor function. The broad spectrum ionotropic glutamate receptor antagonist, kynurenic acid (100-1000microM) also markedly inhibited the AMPA-evoked responses in the striatum in vivo. None of the antagonists, when given alone, influenced basal efflux of [3H]L-glutamate suggesting a lack of tonic regulatory control of glutamate release via presynaptic AMPA-type autoreceptors in the rat striatum. These results demonstrate the presence of presynaptic AMPA receptors, of a novel cyclothiazide- and aniracetam-insensitive subtype, on presynaptic nerve terminals in the rat striatum in vivo, acting to enhance glutamate and GABA release. Our data support the concept of AMPA receptor heterogeneity in vivo, a finding which may facilitate the development of novel, more selective drugs for the treatment of a range of neurological disorders associated with abnormal cerebral glutamate release. The pharmacological profile of these novel presynaptic receptors is currently under investigation.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Theta-frequency facilitation of AMPA receptor-mediated synaptic currents in the principal cells of the medial septum

Recent evidence suggests that Ca(2+)-permeable AMPA receptors display rapid, short-lasting current facilitation. In this study, we investigated the properties of AMPA receptor-mediated synaptic currents in medial septal neurons of the rat in an in vitro slice preparation. Immunocytochemistry with a selective antibody to the GluR2 subunit revealed that both choline acetyltransferase-containing and parvalbumin-containing neurons of the medial septum express no detectable GluR2 subunit immunoreactivity. We used whole cell voltage-clamp recordings to measure synaptically evoked AMPA receptor-mediated currents from medial septal neurons following stimulation of midline afferents. The GYKI 52466 (50 microM)- and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) (20 microM)-sensitive AMPA receptor-mediated component of the synaptic response was isolated by blocking GABA(A)- and N-methyl-D-aspartate receptor-mediated currents with 30 microM bicuculline and 100 microM 2-amino-5-phosphonovaleric acid, respectively. In some cases, patched cells were filled with Lucifer yellow (0.1%) and imaged using 2-photon laser scanning microscopy. AMPA receptor-mediated currents that were observed in large medial septal neurons (20--30 microm) displayed rectification. These currents were sensitive to external application of philanthotoxin-343 (PhTx-343, 50 microM), a potent, high-affinity antagonist of Ca(2+)-permeable, GluR2-lacking AMPA receptors. Rectifying AMPA receptor-mediated currents also displayed a rapid increase in amplitude when evoked five times at low frequency such as 6 Hz. In contrast to currents observed in large medial septal neurons, AMPA-receptor mediated currents evoked in the remaining small (8--11 microm) neurons were nonrectifying and displayed rapid synaptic depression when stimulated five times at 6 Hz. The currents evoked in these cells were unaffected by external application of PhTx-343 and were therefore GluR2-containing AMPA receptors. The results of the present study demonstrate that the principal projection neurons of the medial septum contain PhTx-343-sensitive, GluR2-lacking AMPA receptors that display rapid current facilitation when stimulated at low frequencies.

Characteristics of AMPA receptor-mediated responses of cultured cortical and spinal cord neurones and their correlation to the expression of glutamate receptor subunits, GluR1-4

Electrophysiological recordings have been used to characterize responses mediated by AMPA receptors expressed by cultured rat cortical and spinal cord neurones. The EC(50) values for AMPA were 17 and 11 microM, respectively. Responses of cortical neurones to AMPA were inhibited competitively by NBQX (pK(i)=6.6). Lower concentrations of NBQX (< or =1 microM) also potentiated the plateau responses of spinal cord neurones to AMPA, which could be attributed to a depression of desensitization to AMPA. GYKI 52466 inhibited responses of spinal cord neurones to AMPA to about twice the extent of responses of cortical neurones. Blockade of AMPA receptor desensitization by cyclothiazide (CTZ) potentiated responses of spinal cord neurones (6.8 fold) significantly more than responses of cortical neurones (4.8 fold). Responses of cortical neurones to KA were potentiated 3.5 fold by CTZ, while responses of spinal cord neurones were unaffected. Ultra-fast applications of AMPA to outside-out patches showed responses of spinal cord neurones desensitized by 97.5% and exhibit marked inward rectification, whereas cortical neurones desensitized by 91% and exhibited slight outward rectification. The time constants of deactivation and desensitization were about twice as fast in spinal cord than cortical neurones. In cortical neurones, single-cell RT - PCR showed GluR2 and GluR1 accounted for 91% of all subunits and were expressed together in 67% of neurones, predominantly as the flip variants (78%). GluR2 was detected alone in 24% of neurones. GluR3 and GluR4 were present in only 14 and 29% of neurones, respectively. For spinal cord neurones, GluR4(o) was detected in 81% of neurones, whereas predominantly flop versions of GluR1, 2 and 3 were detected in 38, 13 and 13% of neurones, respectively. These expression patterns are related to the respective pharmacological and mechanistic properties.

Synaptic localization of ionotropic glutamate receptors in the rat substantia nigra

Glutamatergic neurotransmission in the substantia nigra pars compacta and pars reticulata is mediated through N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxaline propionic acid/kainate (AMPA) type receptors as well as other glutamate receptors and is critical for basal ganglia functioning. A major glutamatergic input to the substantia nigra originates in the subthalamic nucleus, and the long-lasting stimulation of the dopaminergic cells of the substantia nigra pars compacta by the subthalamic neurons has been implicated in the pathophysiology of Parkinson's disease. The objectives of the present study were to determine the subcellular and subsynaptic localization of subunits of the N-methyl-D-aspartate and AMPA receptors in the substantia nigra, and also to determine whether co-localization of N-methyl-D-aspartate and AMPA receptor subunits occur at individual synapses. To achieve this, pre-embedding and post-embedding immunocytochemistry was applied to sections of substantia nigra using antibodies that recognize the NR1 and NR2A/B subunits of the N-methyl-D-aspartate receptor, and GluR2/3 subunits of the AMPA receptor. In both regions of the substantia nigra, immunolabelling for each of the subunits was observed in numerous perikarya and proximal dendrites. At the subcellular level, silver-intensified immunogold particles localizing N-methyl-D-aspartate and AMPA receptor subunits were most commonly present within dendrites where they were associated with a variety of intracellular organelles and with the internal surface of the plasma membrane. Post-embedding immunogold labelling revealed immunoparticles labelling for NR1, NR2A/B and GluR2/3 to be enriched at asymmetric synaptic specializations, although a large proportion of asymmetric synapses were immunonegative. Double immunolabelling revealed, in addition to single-labelled synapses, the co-localization of subunits of the N-methyl-D-aspartate receptor and subunits of the AMPA receptor at individual asymmetric synapses. Similarly, double immunolabelling also revealed the co-localization of the NRl and NR2A/B subunits of the N-methyl-D-aspartate receptor at individual asymmetric synapses. Labelling for NR1 and GluR2/3 was, on average, relatively evenly distributed across the width of the synapse with a gradual reduction towards the periphery when analysed in single sections. In summary, the present results demonstrate that AMPA and N-methyl-D-aspartate receptors are selectively localized at a subpopulation of asymmetric synapses in the substantia nigra pars compacta and reticulata and that the two receptor types, at least partially co-localize at individual synapses. It is concluded that glutamatergic transmission in the substantia nigra pars compacta and pars reticulata occurs primarily at asymmetric synapses and, at least in part, is mediated by both N-methyl-D-aspartate and AMPA receptors.

Overexpression of the AtGluR2 gene encoding an Arabidopsis homolog of mammalian glutamate receptors impairs calcium utilization and sensitivity to ionic stress in transgenic plants

We have identified a homolog of the mammalian ionotropic glutamate receptor genes in Arabidopsis thaliana (AtGluR2). This gene was found to alter Ca2+ utilization when overexpressed in A. thaliana. These transgenic plants displayed symptoms of Ca2+ deficiency, including browning and death of the shoot apex, necrosis of leaf tips, and deformation of leaves. Supplementation with Ca2+ alleviated these phenotypes. Overall levels of Ca2+ in tissues of control plants were not significantly different from those of transgenic plants, suggesting that overexpression of the AtGluR2 gene did not affect Ca2+ uptake. However, the relative growth yield as a function of Ca2+ levels revealed that the critical deficiency content of Ca2+ in transgenic plants was three times higher than that of control plants. The transgenic plants also exhibited hypersensitivity to Na+ and K+ ionic stresses. The ion hypersensitivity was ameliorated by supplementation with Ca2+. The results showed that overexpression of the AtGluR2 gene caused reduced efficiency of Ca2+ utilization in the transgenic plants. The promoter of the AtGluR2 gene was active in vascular tissues, particularly in cells adjacent to the conducting vessels. This suggests that AtGluR2 encodes a functional channel that unloads Ca2+ from the xylem vessels. The results together suggest that appropriate expression of the AtGluR2 protein may play critical roles in Ca2+ nutrition by controlling the ion allocation among different Ca2+ sinks both during normal development and during adaptation to ionic stresses.

Functional downregulation of GluR2 in piriform cortex of kindled animals

We have previously shown kindling-induced downregulation of the AMPA receptor GluR2 subunit in piriform cortex, as measured by Western blotting. In the present studies, we performed whole-cell patch clamp analysis of AMPA receptor-mediated currents from kindled and control animals to determine if the downregulation observed previously had any functional significance. These experiments were done in the absence and presence of N-hydroxyphenylpropanoyl spermine (HPPS), a polyamine that blocks currents through AMPA receptors lacking GluR2. We report that AMPA receptor-mediated currents recorded from piriform cortex layer II pyramidal cells in slices from animals kindled to 10 fully generalized seizures were blocked by HPPS. In contrast, application of HPPS had no effect on current amplitude in control animals, or in animals that had not been fully kindled. Western blotting revealed that decreases in GluR2 were seen in animals that had experienced at least one fully generalized seizure, but were not observed at earlier stages of kindling development. The increased polyamine sensitivity of AMPA receptor-mediated currents in kindled animals is consistent with the hypothesis that kindling induces formation of AMPA receptors that lack GluR2 in piriform cortex pyramidal cells. It has been demonstrated that polyamine sensitivity is directly correlated with the calcium permeability of the AMPA receptor, suggesting that kindling results in the formation of AMPA receptors that are calcium-permeable. Increases in intracellular calcium through these receptors could act as a second messenger and play a role in the initiation of long-term changes that contribute to the pathogenesis of kindling-induced epilepsy.

The Na+/Ca2+ exchanger NCX1 has oppositely oriented reentrant loop domains that contain conserved aspartic acids whose mutation alters its apparent Ca2+ affinity

We examined the membrane topology and functional importance of residues in regions of the Na(+)/Ca(2+) exchanger NCX1 encompassing the conserved internal alpha repeats by substituted cysteine scanning analysis and kinetic analysis of site-directed mutants. The results suggest that both the alpha-1 repeat and a region encompassing the alpha-2 repeat and its immediately C-terminal segment contain reentrant loop domains, each oriented in an opposite direction with respect to the membrane. We found that single or multiple mutations of six residues including Asn-125 and conserved aspartates Asp-130, Asp-825, and Asp-829 in the alpha repeat reentrant domains reduce the apparent affinity of the exchanger for extracellular Ca(2+) by up to 6-fold. In contrast, the triple cysteine mutation D130C/D825C/D829C did not influence the current-voltage (I-V) relationship of the exchange current. Cysteine accessibility scanning with different thiol modifiers suggested that N125C, D130C, and D825C may be located in a restricted aqueous space in the membrane accessible only to ions when examined with external probes, although N125C and D825C were previously shown to be internally accessible during exchange reaction. The results suggest that these reentrant domains in the alpha repeats may participate in the formation of the ion transport pathway in the exchanger with some of the aspartates possibly lining it or located close to it.

Age-related loss of the AMPA receptor subunits GluR2/3 in the human nucleus basalis of Meynert

Magnocellular cholinergic neurons in the basal forebrain have long been recognized as vulnerable to the pathology of Alzheimer's disease. Despite numerous anatomical, pharmacological, behavioral, and physiological investigations of these neurons the cellular mechanism that underlines their selective vulnerability remains unclear. As part of an ongoing investigation into the molecular mechanism(s) underlying neuronal vulnerability in Alzheimer's disease and normal aging, we employed immunocytochemical techniques and examined the cellular localization of the alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionate (AMPA) glutamate receptor subunits GluR1 and GluR2/3 in the basal forebrain of eight nondemented elderly human subjects (66-102 years). For each case we observed GluR1-positive magnocellular cells darkly labeled within all main divisions of the basal forebrain (Ch1-Ch4). Double-labeling immunohistochemical techniques confirmed that the overwhelming majority (94%) of these neurons were also positive for the p75NGFr antibody, thus substantiating the cholinergic nature of these neurons. In contrast, GluR2/3 immunolabeling upon magnocellular neurons was relatively faint or nonexistent. The latter observations were most apparent in cases of advanced age and in the posterior part of the nucleus basalis of Meynert (NBM) (i.e., Ch4). In contrast, in adjacent structures (e.g., globus pallidus), a number of robustly labeled GluR2/3-positive cells were observed. In addition to the eight elderly subjects, we examined GluR1 and GluR2/3 immunostaining in the NBM of five younger cases, 5, 33, 36, 47, and 48 years of age. Although practical considerations limited our observations to the Ch4 region, we observed both GluR1 and GluR2/3 labeling upon NBM neurons in this latter region. On average, the distribution of labeled cells and intensity of immunoreaction were comparable between GluR1 and GluR2/3. The presence of GluR2/3- and GluR1-labeled neurons in the Ch4 region of younger cases but primarily GluR1 in cases of advanced age suggests an age-related decrease in GluR2/3. Functionally, the loss of GluR2 from the AMPA receptor complex results in ion channels highly permeable to Ca(2+). These alterations in cation permeability of the AMPA receptor together with the occurrence of a number of other intrinsic and extrinsic events (i.e., decrease Ca(2+)-binding protein) likely contribute to the vulnerability of these neurons in aging and in AD.

Autoimmunity to munc-18 in Rasmussen's encephalitis

Rasmussen's encephalitis (RE) is a rare disease of the central nervous system characterized by severe epileptic seizures, progressive degeneration of a single cerebral hemisphere, and autoimmunity directed against glutamate receptor subunit, GluR3. We report here the identification of high-titer autoantibodies directed against munc-18 in the serum of a single patient with RE previously shown to have anti-GluR3 antibodies. Munc-18 is an intracellular protein residing in presynaptic terminals, which is required for secretion of neurotransmitters. These findings are consistent with the possibility of intermolecular epitope spreading between GluR3, a postsynaptic cell surface protein, and munc-18, a presynaptic intracellular protein. Immune attack on these two proteins, which participate at distinct steps of synaptic transmission, could act in an additive or synergistic manner to impair synaptic function and lead to seizures and neuronal death.

Glutamate-mediated transmission, alcohol, and alcoholism

Glutamate-mediated neurotransmission may be involved in the range of adaptive changes in brain which occur after ethanol administration in laboratory animals, and in chronic alcoholism in human cases. Excitatory amino acid transmission is modulated by a complex system of receptors and other effectors, the efficacy of which can be profoundly affected by altered gene or protein expression. Local variations in receptor composition may underlie intrinsic regional variations in susceptibility to pathological change. Equally, ethanol use and abuse may bring about alterations in receptor subunit expression as the essence of the adaptive response. Such considerations may underlie the regional localization characteristic of the pathogenesis of alcoholic brain damage, or they may form part of the homeostatic change that constitutes the neural substrate for alcohol dependence.

Presynaptic nicotinic acetylcholine receptors as a functional target of nefiracetam in inducing a long-lasting facilitation of hippocampal neurotransmission

Nefiracetam (1-10 microM), a nootropic (or cognition-enhancing) agent, persistently potentiated currents through Torpedo acetylcholine (ACh) receptors expressed in Xenopus oocytes as a result of interacting with a protein kinase C pathway and the ensuing protein kinase C phosphorylation of the receptors. A similar effect was found in neuronal nicotinic ACh receptors (alpha4beta2 and alpha7). In contrast, the other nootropic agents such as piracetam and aniracetam had no potentiating action on the receptors. A sustained enhancement in the activity of nicotinic ACh receptors induced by nefiracetam caused a marked increase in the glutamate release, leading to a long-term potentiation-like facilitation of hippocampal synaptic transmissions. One of the consistent neuropathologic features of the Alzheimer brain is a loss of nicotinic ACh receptors. This fact, together with the results of our study, raises the possibility that the loss of nicotinic ACh receptors may be a key factor in the decline of cognitive function observed in Alzheimer disease and that agents targeting neuronal nicotinic ACh receptors like nefiracetam could, therefore, be of great therapeutic importance.

Mechanisms for activation and antagonism of an AMPA-sensitive glutamate receptor: crystal structures of the GluR2 ligand binding core

Crystal structures of the GluR2 ligand binding core (S1S2) have been determined in the apo state and in the presence of the antagonist DNQX, the partial agonist kainate, and the full agonists AMPA and glutamate. The domains of the S1S2 ligand binding core are expanded in the apo state and contract upon ligand binding with the extent of domain separation decreasing in the order of apo > DNQX > kainate > glutamate approximately equal to AMPA. These results suggest that agonist-induced domain closure gates the transmembrane channel and the extent of receptor activation depends upon the degree of domain closure. AMPA and glutamate also promote a 180 degrees flip of a trans peptide bond in the ligand binding site. The crystal packing of the ligand binding cores suggests modes for subunit-subunit contact in the intact receptor and mechanisms by which allosteric effectors modulate receptor activity.

Gene expression of receptors and enzymes involved in GABAergic and glutamatergic neurotransmission in the CNS of rats behaviourally dependent on ethanol

The steady state levels of the messenger RNA (mRNA) of eight GABA(A) receptor subunits, five glutamate receptor subunits and seven enzymes involved in the synthesis of glutamate and GABA were measured in eight regions of rat brain in a recently developed animal model of 'behavioural dependence' on ethanol. 'Behavioural dependence' including loss of control was induced by offering the rats the choice between ethanol and water over a 9-month period (Group A). This group was compared with a group given the choice between ethanol and water for only 2 months (not yet 'behaviourally dependent', Group B), a group forced to consume ethanol as sole fluid over a 9-month period (also not 'behaviourally dependent', Group C) and ethanol-naive control rats (Group D). All groups were sacrificed 1 month after the ethanol was withdrawn. The mRNA concentrations of all eight GABA receptor subunits, four out of the five subunits of different glutamate receptors and those of seven enzymes involved in GABA and glutamate production were reduced almost exclusively in the parieto-occipital cortex in Groups A and B, but not Group C. These data suggest that the synthesis of glutamate and GABA and the activities of their respective neurons are selectively impaired in the parieto-occipital cortex in the groups having consumed ethanol in a free-choice design, in which its rewarding properties can better take effect than after forced administration. As the parieto-occipital cortex is believed to contain emotional memory structures, it may be hypothesized that the glutamatergic and GABAergic neuronal systems in this area are involved in the development of memory for reward from ethanol. However, they are not specifically associated with 'behavioural dependence'.

Postnatal developmental changes in AMPA and NMDA receptors in the rat vestibular nuclei

Changes in the expression of the AMPA receptor subunits GluR1-4 and of the NMDA receptor subunits NR1, NR2A-D were investigated in the developing rat medial and lateral vestibular nuclei. Analyses were performed using nonradioactive in situ hybridization and immunoblotting with subunit-specific antibodies. During the postnatal development, glutamatergic receptor subunits were differentially expressed in the vestibular nuclei. The level of expression of GluR1, GluR4 and NR1 subunits was higher in the developing brain as compared to the adult. We observed a gradual increase in GluR2/3, NR2A, NR2B and NR2C levels of expression in the medial and lateral vestibular nuclei during the first 3 weeks of postnatal development. In situ hybridization results were consistent with immunoblot analyses. The differential expression of AMPA and NMDA receptor subunits in immature vestibular neurons is consistent with changes in glutamate receptor properties. This may be related to the postsynaptic regulation of receptor subunits associated with the synaptic plasticity of the vestibular neuron connections during specific sequences of postnatal development.

The selective AMPA receptor antagonist GYKI 53784 blocks action potential generation and excitotoxicity in the guinea pig cochlea

The role of AMPA receptors in cochlear synaptic transmission and excitotoxicity was investigated by comparing the actions of a selective AMPA antagonist GYKI 53784 (LY303070) with additional AMPA/kainate antagonists, GYKI 52466 and DNQX, and the NMDA antagonist, D-AP5, in several electrophysiological, neurotoxicological and histochemical tests. GYKI 53784 had the same potency as DNQX and was 10 times more potent than GYKI 52466 in reducing auditory nerve activity. The NMDA antagonist D-AP5 had no effect on auditory nerve activity. When single-fiber activity was blocked with GYKI 53784, the effects of AMPA or kainate were also antagonized. GYKI 53784 completely blocked excitotoxicity (i.e. destruction of the afferent nerve endings) induced by AMPA and kainate. The histochemical detection of Co(2+) uptake was used to study Ca(2+) influx within the primary auditory nerve cells. Application of AMPA induced no significant Co(2+) uptake into the cells, suggesting that these receptors normally have a very low permeability to Ca(2+). Application of kainate induced significant Co(2+) uptake that was blocked by the AMPA receptor antagonist GYKI 53784 suggesting that kainate stimulated Ca(2+) entry through AMPA receptor channels. Results suggest that AMPA-preferring receptors are functionally located at the sensory cell-afferent synapse whereas NMDA and kainate receptors are not.

Depression of long term potentiation in gerbil hippocampus following postischemic hypothermia

To investigate the mechanism of chronic cell death following postischemic hypothermia, the change of N-methyl-D-aspartate receptor (NMDAR) were examined by immunohistochemistry of NMDAR1 and long-term potentiation (LTP) in the CA1 subfield of the gerbil hippocampus. At 1 week following postischemic hypothermia (32 degrees Cx4 h), all CA1 neurons survived; however, immunoreactivity of NMDAR1 increased in neuronal perikarya whereas decreased in dendrites in the CA1 neurons. The abnormality was still observed in remaining CA1 neurons at 1 month after hypothermia. LTP was also significantly depressed at 1 week after hypothermia. These results suggest that some abnormalities in the glutamate receptor may be caused by ischemia; such abnormality would persist in spite of hypothermia treatment, resulting in the depression of LTP.

A high GluR1 : GluR2 expression ratio is correlated with expression of Ca2+-binding proteins in rat forebrain neurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazle propionic acid (AMPA) receptors are ubiquitously expressed; however, their subtypes and abundance vary from region to region. We classified the neurons in various forebrain regions (hippocampus, striatum, amygdala, piriform cortex and somatosensory cortex) into six types: [R1+/R2+], [R1-/R2+], [R1+/R2-], [R1-/R2-], [R1++/R2+] and [R1++/R2-], and analysed the expression of Ca2+-binding proteins, such as parvalbumin and calbindin-D28k, using a triple-staining method. The neurons showing a high GluR1 : GluR2 expression ratio, [R1+/R2-], [R1++/R2+] and [R1++/R2-] neurons, comprised 13-30% of the total neuronal population. In addition, the expression of Ca2+-binding proteins was mainly observed in these three types of neurons. The results suggest that Ca2+-binding protein-positive neurons express Ca2+-permeable AMPA receptors, because the Ca2+-permeability of AMPA receptors is enhanced by the relative scarcity of the GluR2 subunit. To directly test the possibility that Ca2+-binding protein-positive neurons express Ca2+-permeable AMPA receptors, we performed Ca2+-imaging experiments in cultured cortical neurons. Ca2+ influx through AMPA receptors was measured selectively by addition of AMPA together with cyclothiazide in the presence of blockers of other Ca2+ influx routes. More than half of the calbindin-D28k-positive neurons showed a large increase in the intracellular Ca2+ concentration ([Ca2+]i), whilst most of the calbindin-D28k-undetectable neurons exhibited only a slight rise in [Ca2+]i after AMPA addition. These results suggest that the expression of calbindin-D28k is related to the expression of Ca2+-permeable AMPA receptors.

Nefiracetam facilitates hippocampal neurotransmission by a mechanism independent of the piracetam and aniracetam action

Nefiracetam, a nootropic (cognition-enhancing) agent, facilitated neurotransmission in the dentate gyrus of rat hippocampal slices in a dose-dependent manner at concentrations ranged from 1 nM to 1 microM, being evident at 60-min washing-out of the drug. The facilitatory action was blocked by the nicotinic acetylcholine (ACh) receptor antagonists, alpha-bungarotoxin and mecamylamine. A similar facilitation was induced by the other nootropic agents, piracetam and aniracetam, but the facilitation was not inhibited by nicotinic ACh receptor antagonists and it did not occlude the potentiation induced by nefiracetam. In the Xenopus oocyte expression systems, nefiracetam potentiated currents through a variety of neuronal nicotinic ACh receptors (alpha 3beta 2, alpha 3beta 4, alpha 4 beta 2, alpha 4 beta 4, and alpha 7) to a different extent. In contrast, neither piracetam nor aniracetam had any potentiating action on alpha 7 receptor currents. While aniracetam delayed the decay time of currents through the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor, GluR1, -2, -3, expressed in oocytes, nefiracetam or piracetam had no effect on the currents. Nefiracetam, thus, appears to facilitate hippocampal neurotransmission by functionally targeting nicotinic ACh receptors, independently of the action of piracetam and aniracetam.