DISTINCT PRE- AND POSTSYNAPTIC NMDA RECEPTOR POOLS GOVERN DIVERSE FORMS OF CORTICAL PLASTICITY

NMDA receptors (NMDARs) are classically considered postsynaptic coincidence detectors. However, accumulating evidence for controversial presynaptic NMDAR signalling challenges this view. We therefore combined immuno-EM, quadruple patch clamp, and sparse NMDAR deletion to resolve how distinct NMDAR pools contribute to diverse forms of cortical plasticity.
Immuno-EM revealed expression of GluN1, GluN2A and GluN2B subunits both pre- and postsynaptically in V1 layer-5 excitatory synapses of P21 C57BL/6 mice, with GluN2B dominating presynaptically (p < 0.001). Next, we developed a sparse genetic NMDAR deletion model via neonatal injection of AAV-eSYN-iCre into NR1^fl/fl mice. MNI-NMDA uncaging confirmed successful deletion (p < 0.001).
Using quadruple patch in P10-20 acute slices, we tested pyramidal cell (PC) pairs with NMDARs deleted (Del) either pre- or postsynaptically, versus wildtype (WT). AP5 reduced EPSP amplitude in WT→WT (p < 0.001) and WT→Del pairs (p < 0.001), but not Del→WT pairs (p = 0.46), indicating that pre- but not postsynaptic NMDARs regulate release. Similarly, timing-dependent long-term depression (tLTD) remained intact in WT→Del pairs (p < 0.01), unlike Del→WT pairs (p = 0.29). In contrast, tLTP was abolished in WT→Del pairs (p = 0.24), whereas Del→WT pairs exhibited robust tLTP (p < 0.05). Taken together, our observations reveal a surprising double dissociation in spike-timing-dependent plasticity. Moreover, we found that presynaptic NMDARs regulate spontaneous release, while postsynaptic NMDARs shape PC morphology and dendritic spine density.
In conclusion, NMDAR signalling is not monolithic but follows compartment-specific rules that give rise to distinct forms of cortical plasticity.