Postsynaptic and Presynaptic NMDARs Have Distinct Roles in Visual Circuit Development.

To study contributions of N-methyl-D-aspartate receptors (NMDARs) in presynaptic and postsynaptic neurons of the developing visual system, we microinject antisense Morpholino oligonucleotide (MO) against GluN1 into one cell of two-cell-stage Xenopus laevis embryos. The resulting bilateral segregation of MO induces postsynaptic NMDAR (postNMDAR) knockdown in tectal neurons on one side and presynaptic NMDAR (preNMDAR) knockdown in ganglion cells projecting to the other side. PostNMDAR knockdown reduces evoked NMDAR- and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated retinotectal currents. Although the frequency of spontaneous synaptic events is increased, the probability of evoked release is reduced. PreNMDAR knockdown results in larger evoked and unitary synaptic responses. Structurally, postNMDAR and preNMDAR knockdown produce complementary effects. Axonal arbor complexity is reduced by preNMDAR-MO and increased by postNMDAR-MO, whereas tectal dendritic arbors exhibit the inverse. The current study illustrates distinct roles for pre- and postNMDARs in circuit development and reveals extensive transsynaptic regulation of form and function.