Synaptic NMDA receptor activity at resting membrane potentials

NMDA receptors (NMDARs) are crucial for glutamatergic synaptic signaling in the mammalian central nervous system. When activated by glutamate and glycine/D-serine, the NMDAR ion channel can open, but current flux is further regulated by voltage-dependent block conferred by extracellular Mg²⁺ ions. The unique biophysical property of ligand- and voltage-dependence positions NMDARs as synaptic coincidence detectors, controlling a major source of synaptic Ca²⁺ influx. We measured synaptic currents in layer 2/3 neurons after stimulation in layer 4 of somatosensory cortex and found measurable NMDAR currents at all voltages tested. This NMDAR current did not require concurrent AMPAR depolarization. In physiological ionic conditions, the NMDAR current response at negative potentials was enhanced relative to ionic conditions typically used in slice experiments. NMDAR activity was also seen in synaptic recordings from hippocampal CA1 neurons, indicating a general property of NMDAR signaling. Using a fluorescent Ca²⁺ indicator, we measured responses to stimulation in layer 4 at individual synaptic sites, and Ca²⁺ influx could be detected even with AMPARs blocked. In current clamp recordings, we found that resting membrane potential was hyperpolarized by ∼7 mV and AP firing threshold depolarized by ∼4 mV in traditional compared to physiological ionic concentrations, and that NMDARs contribute to EPSPs at resting membrane potentials. These measurements demonstrate that, even in the presence of extracellular Mg²⁺ and absence of postsynaptic depolarization, NMDARs contribute to synaptic currents and Ca²⁺ influx.