Lack of NMDA receptor subtype selectivity for hippocampal long-term potentiation

Abstract

NMDA receptor (NMDAR) 2A (NR2A)- and NR2B-type NMDARs coexist in synapses of CA1 pyramidal cells. Recent studies using pharmacological blockade of NMDAR subtypes proposed that the NR2A type is responsible for inducing long-term potentiation (LTP), whereas the NR2B type induces long-term depression (LTD). This contrasts with the finding in genetically modified mice that NR2B-type NMDARs induce LTP when NR2A signaling is absent or impaired, although compensatory mechanisms might have contributed to this result. We therefore assessed the contribution of the two NMDAR subtypes to LTP in mouse hippocampal slices by different induction protocols and in the presence of NMDAR antagonists, including the NR2A-type blocker NVP-AAM077, for which an optimal concentration for subtype selectivity was determined on recombinant and native NMDARs. Partial blockade of NMDA EPSCs by 40%, either by preferentially antagonizing NR2A- or NR2B-type NMDARs or by the nonselective antagonist D-AP-5, did not impair LTP, demonstrating that hippocampal LTP induction can be generated by either NMDAR subtype.

Figure 1.

Effects of NVP-AAM077 on recombinant and synaptic NMDARs and on LTP elicited by tetanization. A, Example traces of glutamate-evoked currents in HEK293 cells expressing NR1/NR2A or NR1/NR2B receptors in the absence (black) and presence (gray) of NVP. Summary of results using different NVP concentrations (gray; ^*p < 0.0005) or 10 μm CP-101,606 (striped) is shown. B, NMDA EPSC traces recorded in slices from P28 NR2A knock-out mice in 10 μm bicuculline, 5 μm NBQX, and 10 μm glycine at -40 and +40 mV in the absence (black) and presence (gray) of 50 nm NVP. Summary of results using different NVP concentrations (gray; ^*p = 0.007 and ^**p = 0.0004) is shown. C, Single tetanization (100 Hz; 1 s) induced LTP in hippocampal slices from P28 wild-type mice (open circles) in the presence of 50 and 400 nm NVP (^*p < 0.001 and ^*p = 0.003, filled circles and diamonds, respectively). Repeated tetanizations (4 times with 5 min intervals) in the presence of 50 nm NVP (filled squares) increased LTP to levels obtained without the blocker (open squares). A representative control pathway is shown (open triangles). The arrow indicates the time of both the first and the fourth tetanization. Data are mean ± SEM.

Figure 2.

Comparison of the effects of different NMDAR antagonists on synaptic NMDA EPSCs and on LTP elicited by LFS pairing. A, EPSCs were evoked at -70 mV in 10 μm BMI and 10 μm glycine in slices from P28 wild-type mice. LFS pairing (bar) increased EPSCs of the test (open circles) but not of the control pathway (open triangles). Representative EPSCs for test are labeled with 1 and 2 and, for control pathway, with a and b. B, NMDA EPSC traces recorded in slices from P28 wild-type mice at -40 and +40 mV in the absence (black) and presence (gray) of 50 nm NVP. Summary of results at +40 mV using NVP (gray; ^*p < 0.0001), CP (hatched), or d-AP-5 (cross-hatched) is shown. C, Changes in EPSC amplitude after LFS pairing in the absence (white) and presence (B) of different NMDAR antagonists (^*p = 0.002). Data are mean ± SEM.