Presynaptic NMDA receptors: dynamics and distribution in developing axons in vitro and in vivo

During cortical development, NMDA receptors (NMDARs) facilitate presynaptic terminal formation, enhance neurotransmitter release, and are required in presynaptic neurons for spike timing-dependent LTD. However, the extent to which NMDARs are found within cortical presynaptic terminals has remained controversial, and the sub-synaptic localization and dynamics of axonal NMDARs are unknown. Here, using live confocal imaging and biochemical purification of presynaptic membranes, we provide strong evidence that NMDARs localize to presynaptic terminals in vitro and in vivo, in a developmentally regulated manner. NR1 and NR2B subunits are found within the active zone membrane, where they could respond to synaptic glutamate release. Surprisingly, NR1 also appears in glutamatergic and GABAergic synaptic vesicles. During synaptogenesis, NR1 is mobile throughout axons - including growth cones and filopodia, structures that are involved in synaptogenesis. Upon synaptogenic contact, NMDA receptors are quickly recruited to terminals by neuroligin-1/neurexin signaling. Unlike dendrites, the trafficking and distribution of axonal NR1 are insensitive to activity changes, including NMDA exposure, local glutamate uncaging or action potential blockade. These results support the idea that presynaptic NMDARs play an early role in presynaptic development.