The NMDA (N-methyl-D-aspartate) receptor channel is important for synaptic plasticity, which is thought to underlie learning, memory and development. The NMDA receptor channel is formed by at least two members of the glutamate receptor (GluR) channel subunit families, the GluR epsilon (NR2) and GluR zeta (NR1) subunit families. The four epsilon subunits are distinct in distribution, properties and regulation. On the basis of the Mg2+ sensitivity and expression patterns, we have proposed that the epsilon 1 (NR2A) and epsilon 2 (NR2B) subunits play a role in synaptic plasticity. Here we show that targeted disruption of the mouse epsilon 1 subunit gene resulted in significant reduction of the NMDA receptor channel current and long-term potentiation at the hippocampal CA1 synapses. The mutant mice also showed a moderate deficiency in spatial learning. These results support the notion that the NMDA receptor channel-dependent synaptic plasticity is the cellular basis of certain forms of learning.

Receptors for excitatory amino acids in the mammalian central nervous system are classified into three major subtypes, ones which prefer N-methyl-D-aspartate (NMDA), quisqualate (QA), or kainate (KA) as type agonists respectively. These receptors are considered to mediate fast postsynaptic potentials by activating ion channels directly (ionotropic type). Recently it was reported that exposure of mammalian brain cells to glutamate (Glu) or its analogues causes enhanced hydrolysis of inositol phospholipids, but it is not clear whether the enhanced hydrolysis is the cause or effect of physiological responses. Membrane depolarization or Ca2+ influx, which can result from Glu receptor activation, can induce enhanced hydrolysis of inositol phospholipids. We have characterized the functional properties of two types of excitatory amino-acid responses, those activated by QA (or Glu) and those activated by KA, induced in Xenopus oocytes injected with rat-brain messenger RNA. We report evidence for a new type of Glu receptor, which prefers QA as agonist, and which directly activates inositol phospholipid metabolism through interaction with GTP-binding regulatory proteins (Gi or Go), leading to the formation of inositol 1,4,5-trisphosphate (InsP3) and mobilization of intracellular Ca2+. This QA/Glu reaction is inhibited by islet-activating protein (IAP, pertussis toxin), but was not blocked by Joro spider toxin (JSTX), a specific blocker of traditional ionotropic QA/Glu receptors.

Despite its implications for higher order functions of the brain, little is currently known about the molecular basis of left-right asymmetry of the brain. Here we report that synaptic distribution of N-methyl-D-aspartate (NMDA) receptor GluRepsilon2 (NR2B) subunits in the adult mouse hippocampus is asymmetrical between the left and right and between the apical and basal dendrites of single neurons. These asymmetrical allocations of epsilon2 subunits differentiate the properties of NMDA receptors and synaptic plasticity between the left and right hippocampus. These results provide a molecular basis for the structural and functional asymmetry of the mature brain.

SUMMARY1. Allosteric potentiation of the ionotropic quisqualate (iQA) receptor by a nootropic drug aniracetam (1 -p-anisoyl-2-pyrrolidinone) was investigated using Xenopus oocytes injected with rat brain mRNA and rat hippocampal slices.2. Aniracetam potentiates the iQA responses induced in Xenopus oocytes by rat brain mRNA in a reversible manner. This effect was observed above the concentrations of 0.1 mm. Kainate, N-methyl-D-aspartate and y-aminobutyric acid responses induced in the same oocytes were not affected.3. The specific potentiation of iQA responses was accompanied by an increase in the conductance change of iQA and a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses, but the affinity of receptors for agonist and the ion-selectivity of the channels (reversal potentials) were not changed. 4. Aniracetam reversibly potentiated the iQA responses recorded intracellularly from the pyramidal cells in the CAI region of rat hippocampal slices. The excitatory postsynaptic potentials (EPSPs) in Schaffer collateral-commissural-CAl synapses were also potentiated by aniracetam.5. Population EPSPs recorded in the mossy fibre-CA3 synapses as well as Schaffer-commissural synapses were also potentiated by aniracetam. The amplitudes of the potentiation were not changed by the formation of long-term potentiation.

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BackgroundWe previously performed systematic association studies of glutamate receptor gene family members with schizophrenia, and found positive associations of polymorphisms in the GRM3 (a gene of metabotropic glutamate receptor 3: mGluR3) with the disorder. Physiological roles of GRM3 in brain functions and its functional roles in the pathogenesis of schizophrenia remain to be resolved.ResultsWe generated mGluR3 knockout (KO) mice and conducted comprehensive behavioral analyses. KO mice showed hyperactivity in the open field, light/dark transition, and 24-hour home cage monitoring tests, impaired reference memory for stressful events in the Porsolt forced swim test, impaired contextual memory in cued and contextual fear conditioning test, and impaired working memory in the T-Maze forced alternation task test. Hyperactivity and impaired working memory are known as endophenotypes of schizophrenia. We examined long-term synaptic plasticity by assessing long-term potentiation (LTP) in the CA1 region in the hippocampi of KO and wild-type (WT) mice. We observed no differences in the amplitude of LTP between the two genotypes, suggesting that mGluR3 is not essential for LTP in the CA1 region of the mouse hippocampus. As hyperactivity is typically associated with increased dopaminergic transmission, we performed in vivo microdialysis measurements of extracellular dopamine in the nucleus accumbens of KO and WT mice. We observed enhancements in the methamphetamine (MAP)-induced release of dopamine in KO mice.ConclusionsThese results demonstrate that a disturbance in the glutamate-dopamine interaction may be involved in the pathophysiology of schizophrenia-like behavior, such as hyperactivity in mGluR3 KO mice.

Input-dependent left-right asymmetry of NMDA receptor ⑀2 (NR2B) subunit allocation was discovered in hippocampal Schaffer collateral (Sch) and commissural fiber pyramidal cell synapses (Kawakami et al., 2003). To investigate whether this asymmetrical ⑀2 allocation is also related to the types of the postsynaptic cells, we compared postembedding immunogold labeling for ⑀2 in left and right Sch synapses on pyramidal cells and interneurons. To facilitate the detection of ⑀2 density difference, we used ⑀1 (NR2A) knock-out (KO) mice, which have a simplified NMDA receptor subunit composition.The labeling density for ⑀2 but not 1 (NR1) and subtype 2/3 glutamate receptor (GluR2/3) in Sch-CA1 pyramidal cell synapses was significantly different between the left and right hippocampus with opposite directions in strata oriens and radiatum; the left to right ratio of ⑀2 labeling density was 1:1.50 in stratum oriens and 1.44:1 in stratum radiatum. No significant difference, however, was detected in CA1 stratum radiatum between the left and right Sch-GluR4-positive (mostly parvalbumin-positive) and Sch-GluR4-negative interneuron synapses. Consistent with the anatomical asymmetry, the amplitude ratio of NMDA EPSCs to non-NMDA EPSCs in pyramidal cells was approximately two times larger in right than left stratum radiatum and vice versa in stratum oriens of ⑀1 KO mice. Moreover, the amplitude of long-term potentiation in the Sch-CA1 synapses of left stratum radiatum was significantly larger than that in the right corresponding synapses. These results indicate that the asymmetry of ⑀2 distribution is target cell specific, resulting in the left-right difference in NMDA receptor content and plasticity in Sch-CA1 pyramidal cell synapses in ⑀1 KO mice.

1. We have explored the effects of targeted disruption of the N-methyl-D-aspartate (NMDA) receptor el or e2 subunit gene on NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) and long-term potentiations (LTPs) at the two types of synapse in mouse hippocampal CA3 pyramidal neurons: those formed by the commissural/associational (C/A) and fimbrial (Fim) inputs. 2. Electrophysiological experiments were performed in hippocampal slices prepared from both wild-type and el-or e2-disrupted mice using extracellular and whole-cell patch recording techniques. To assess the el, e2 and ;1 subunit expression at cellular levels, we performed non-isotopic in situ hybridization with digoxigenin-labelled cRNA probes.3. We could record EPSCs in response to the stimulations to either of the C/A and Fim afferents from a single CA3 pyramidal neuron. The el, e2 and C1 subunits were expressed together in individual CA3 neurons. 4. The el subunit disruption selectively reduced NMDA EPSCs and LTP in the C/A-CA3 synapse without significantly affecting those in the Fim-CA3 synapse, whereas the e2 subunit mutation diminished NMDA EPSCs and LTP in the Fim-CA3 synapse with no appreciable functional modifications in the C/A-CA3 synapse. 5. These results suggest that NMDA receptors with different subunit compositions function within a single CA3 pyramidal cell in a synapse-selective manner.