NMDA Receptors in Layer 4 Spiny Stellate Cells of the Mouse Barrel Cortex Contain the NR2C Subunit

Abstract

In layer 4 of the somatosensory cortex, the glutamatergic synapses that interconnect spiny stellate (SpS) neurons, which are the major targets of thalamocortical input, differ from most other neocortical excitatory synapses in that they have an extremely large NMDA receptor (NMDAR)-mediated component that is relatively insensitive to voltage-dependent Mg 2ϩ blockade. We now report that this unique feature of the NMDA response reflects the distinctive subunit composition of the underlying receptors. We studied NMDARmediated miniature EPSCs (mEPSCs) and NMDA channel currents in tangential brain slices of mouse barrel cortex, which exclusively contain layer 4. NMDAR-mediated mEPSCs in SpS neurons were prominent at negative membrane potentials, and NMDA channels in outside-out patches excised from the somata of the same neurons had relatively low conductance and reduced susceptibility to Mg 2ϩ block. These are characteristic features of heteromeric NMDAR assemblies that contain the NR2C subunit. Some patches also contained NMDA channels with higher conductance and a greater sensitivity to Mg 2ϩ . In the neocortex of transgenic mice in which a ␤-galactosidase (lacZ) indicator gene was controlled by the NR2C promoter, the lacZ indicator was densely expressed in layer 4. In current-clamp recordings, blockade of NMDARs caused hyperpolarization and an increase in apparent input resistance. Our data demonstrate that the SpS neurons of layer 4 functionally express NR2C subunits; this is the likely explanation for their ability to generate large NMDAR-mediated EPSPs that are effective at resting potential, without previous depolarization.

Novel regional and developmental NMDA receptor expression patterns uncovered in NR2C subunit-β-galactosidase knock-in mice

“…Although lower sensitivity to magnesium block and low channel conductance are frequently used to identify NMDARs as NR2C-containing, these are not properties uniquely conferred by the NR2C subunit Sasaki et al, 2002). Therefore, knowing the precise patterns of NR2 and NR3 subunit expression is of major importance when considering if an NMDA channel is NR2C-containing, as recently reported for layer IV spiny stellate cells of the mouse barrel cortex (Binshtok et al, 2006).…”

Circuit-based framework for understanding neurotransmitter and risk gene interactions in schizophrenia

“…There is an enormous diversity of interneuron types [57]; it is thus important to note that large NMDAmediated EPSPs have been found in the parvalbumin-containing basket cells [54,58] that mediate the feedback inhibition ( Figure 1) discussed later in this review. It is also important to note that in addition to reducing the EPSP, NMDA antagonists hyperpolarize neurons by blocking the effect of ambient glutamate [59]; this would also make it more difficult to excite interneurons.…”

Sequential Generation of Two Distinct Synapse-Driven Network Patterns in Developing Neocortex

“…Interestingly, NMDAR activation has been critically implicated in several cortical oscillations at early developmental stages, including most pharmacologically induced patterns (Dupont et al, 2006;Demarque et al, 2004). A variation in the subunit composition of NMDARs in intracortical synapses (Kumar and Huguenard, 2003;Nevian and Sakmann, 2004;Pérez-Otaño et al, 2006), a weaker sensitivity to magnesium block near resting potential (Ben-Ari et al, 1988;Fleidervish et al, 1998;Binshtok et al, 2006;Cattani et al, 2007) or high glutamate affinity are features that should support their activation by ambient glutamate. Finally, cENOs and cGDPs are differentially affected by anoxic conditions since slowing down the perfusion rate and/or producing mild anoxic conditions blocks cGDPs and increases the frequency of cENOs.…”