Spatial Coupling Tunes NMDA Receptor Responses via Ca 2+ Diffusion

In the CNS, NMDA receptors generate large and highly regulated Ca signals, which are critical for synaptic development and plasticity. They are highly clustered at postsynaptic sites and along dendritic arbors, but whether this spatial arrangement affects their output is unknown. Synaptic NMDA receptor currents are subject to Ca -dependent inactivation (CDI), a type of activity-dependent inhibition that requires intracellular Ca and calmodulin (CaM). We asked whether Ca influx through a single NMDA receptor influences the activity of nearby NMDA receptors, as a possible coupling mechanism. Using cell-attached unitary current recordings from GluN1-2a/GluN2A receptors expressed in human HEK293 cells and from NMDA receptors native to hippocampal neurons from male and female rats, we recorded unitary currents from multichannel patches and used a coupled Markov model to determine the extent of signal coupling (κ). In the absence of extracellular Ca , we observed no cooperativity (κ < 0.1), whereas in 1.8 mm external Ca , both recombinant and native channels showed substantial negative cooperativity (κ = 0.27). Intracellular Ca chelation or overexpression of a Ca -insensitive CaM mutant, reduced coupling, which is consistent with CDI representing the coupling mechanism. In contrast, cooperativity increased substantially (κ = 0.68) when overexpressing the postsynaptic scaffolding protein PSD-95, which increased receptor clustering. Together, these new results demonstrate that NMDA receptor currents are negatively coupled through CDI, and the degree of coupling can be tuned by the distance between receptors. Therefore, channel clustering can influence the activity-dependent reduction in NMDA receptor currents. At central synapses, NMDA receptors are a major class of excitatory glutamate-gated channels and a source of activity-dependent Ca influx. In turn, fluxed Ca ions bind to calmodulin-primed receptors and reduce further entry, through an autoinhibitory mechanism known as Ca -dependent inactivation (CDI). Here, we show that the diffusion of fluxed Ca between active channels situated within submicroscopic distances amplified receptor inactivation. Thus, calmodulin-mediated gating modulation, an evolutionarily conserved regulatory mechanism, endows synapses with sensitivity to both the temporal sequence and spatial distribution of Ca signals. Perturbations in this mechanism, which coordinates the activity of NMDA receptors within a cluster, may cause signaling al