Resident Calmodulin Primes NMDA Receptors for Ca²⁺-Dependent Inactivation

N-methyl-d-aspartate (NMDA) receptors are glutamate- and glycine-gated channels that flux Na⁺ and Ca²⁺ into postsynaptic neurons during synaptic transmission. The resulting intracellular Ca²⁺ transient is essential to physiological and pathological processes related to synaptic development, plasticity, and apoptosis. It also engages calmodulin (CaM) to reduce subsequent NMDA receptor activity in a process known as Ca²⁺-dependent inactivation (CDI). Here, we used whole-cell electrophysiology to measure CDI and computational modeling to dissect the sequence of events that underlies it. With these approaches, we estimate that CaM senses NMDA receptor Ca²⁺ influx at ∼9 nm from the channel pore. Further, when we controlled the frequency of Ca²⁺ influx through individual channels, we found that a kinetic model where apoCaM associates with channels before their activation best predicts the measured CDI. These results provide, to our knowledge, novel functional evidence for CaM preassociation to NMDA receptors in living cells. This particular mechanism for autoinhibitory feedback reveals strategies and challenges for Ca²⁺ regulation in neurons during physiological synaptic activity and disease.