Synaptic and Extrasynaptic NMDA Receptors Are Gated by Different Endogenous Coagonists

N-methyl-d-aspartate receptors (NMDARs) are located in neuronal cell membranes at synaptic and extrasynaptic locations, where they are believed to mediate distinct physiological and pathological processes. Activation of NMDARs requires glutamate and a coagonist whose nature and impact on NMDAR physiology remain elusive. We report that synaptic and extrasynaptic NMDARs are gated by different endogenous coagonists, d-serine and glycine, respectively. The regionalized availability of the coagonists matches the preferential affinity of synaptic NMDARs for d-serine and extrasynaptic NMDARs for glycine. Furthermore, glycine and d-serine inhibit NMDAR surface trafficking in a subunit-dependent manner, which is likely to influence NMDARs subcellular location. Taking advantage of this coagonist segregation, we demonstrate that long-term potentiation and NMDA-induced neurotoxicity rely on synaptic NMDARs only. Conversely, long-term depression requires both synaptic and extrasynaptic receptors. Our observations provide key insights into the operating mode of NMDARs, emphasizing functional distinctions between synaptic and extrasynaptic NMDARs in brain physiology. [Display omitted] ► Synaptic and extrasynaptic NMDARs are gated by d-serine and glycine, respectively ► d-serine and glycine differentially impact NMDAR membrane diffusion ► Synaptic NMDARs mediate LTP and excitotoxicity ► Both synaptic and extrasynaptic receptors are required for LTD induction NMDA receptors present in the synapse are gated by different amino acids than those located outside of the synapse in neuronal membranes. This differential gating enabled dissection of distinct functional roles for each pool of receptors and led to the surprising finding that glutamate toxicity and long-term potentiation are mediated solely by synaptic NMDARs