Extrasynaptic NR2B and NR2D subunits of NMDA receptors shape ‘superslow’ afterburst EPSC in rat hippocampus

Abstract

In conditions of facilitated synaptic release, CA3/CA1 synapses generate anomalously slow NMDA receptor-mediated EPSCs (EPSC(NMDA)). Such a time course has been attributed to the cooperation of synapses through glutamate spillover. Imitating a natural pattern of activity, we have applied short bursts (2-7 stimuli) of high-frequency stimulation and observed a spike-to-spike slow-down of the EPSC(NMDA) kinetics, which accompanied synaptic facilitation. It was found that the early component of the EPSC(NMDA) and the burst-induced late component of the EPSC(NMDA) have distinct pharmacological properties. The competitive NMDA antagonist R-(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (D-CPP), which has higher affinity to NR2A than to NR2B subunits and lowest affinity at NR2D subunits, significantly slowed down the decay rate of the afterburst EPSC while leaving the kinetics of the control current unaffected. In contrast, ifenprodil, a highly selective NR2B antagonist, and [+/-]-cis-1-[phenanthren-2yl-carbonyl]piperazine-2,3-dicarboxylic acid (PPDA), a competitive antagonist that is moderately selective for NR2D subunits, more strongly inhibited the late component of the afterburst EPSC(NMDA). The receptors formed by NR2B and (especially) NR2D subunits are known to have higher agonist sensitivity and much slower deactivation kinetics than NR2A-containing receptors. Furthermore, NR2B is preferentially and NR2D is exclusively located on extrasynaptic membranes. As the density of active synapses increases, the confluence of released glutamate makes EPSC decay much longer by activating more extrasynaptic NR2B- and NR2D-subunit-containing receptors. Long-term potentiation (LTP) induced by successive rounds of burst stimulation is accompanied by a long-term increase in the contribution of extrasynaptic receptors in the afterburst EPSC(NMDA.)

Acquisition of Fear Extinction Requires Activation of NR2B-Containing NMDA Receptors in the Lateral Amygdala

“…First, it is possible that the NR2B subunit of the NMDAR plays a special role in extinction learning that is not detectable by CPP because of its slightly higher affinity (B10-fold) for NR2A than for NR2B subunits, compared with the strong selective affinity (B400-fold) of ifenprodil for NR2B (Lozovaya et al, 2004). A second possible explanation is that CPP may have masked the role of NMDARs in extinction by producing acute behavioral effects, such as a disruption of overall behavioral responsivity (Starr and Starr, 1994), which interfered with behavioral measurements during the extinction training session.…”

Hallucinogen-Induced UP States in the Brain Slice of Rat Prefrontal Cortex: Role of Glutamate Spillover and NR2B-NMDA Receptors

“…Since recent studies in the adult rat have suggested that extrasynaptic glutamate transmission is mediated disproportionately by NMDA receptors that contain the NR2B subunit (Lozovaya et al, 2004;Scimemi et al, 2004), we assessed the ability of the selective NR2B antagonists Ro25-6981 (Fischer et al, 1997) and ifenprodil (Williams, 1993) to suppress DOI-enhanced late EPSCs. As shown in Figure 4, the late EPSC was markedly suppressed by either Ro25-6981 (500 nM, 10 min) or ifenprodil (3 mM, 10 min).…”

“…A growing body of literature suggests that activation of high-affinity extrasynaptic NMDA receptors by glutamatergic spillover may play a significant role in the brain function (Kullmann et al, 1996;Barbour and Hausser, 1997;Rusakov and Kullmann, 1998;Scimemi et al, 2004;Lozovaya et al, 2004). According to this view, the activation of any one synapse would not only activate its immediate postsynaptic partner, but may alsoFdepending on conditionsFbe able to activate high-affinity NMDA receptors at adjacent spines or presynaptic terminals.…”

Target-Cell-Specific Left-Right Asymmetry of NMDA Receptor Content in Schaffer Collateral Synapses in ϵ1/NR2A Knock-Out Mice

“…Although immunogold particles for GluR4 were mostly concentrated in postsynaptic membrane specialization on the interneuron dendrites, ⑀4 may have a nonsynaptic localization in the GluR4-positive interneurons. NMDA receptors composed of 1/⑀4 have distinctively slow deactivation kinetics, low single-channel conductance, and low sensitivity to voltage-dependent Mg 2ϩ block (Monyer et al, 1994;Wyllie et al, 1998;Misra et al, 2000b) and are suggested to be localized to extrasynaptic membrane in several cell types in the cerebellum and hippocampus (Misra et al, 2000a,b;Brickley et al, 2003;Lozovaya et al, 2004). It is thus unlikely that ⑀4 composes NMDA receptors as a major subunit in the GluR4-positive interneuron synapses.…”