Analysis of synaptic distribution within single retinal axonal arbors after chronic NMDA treatment

Activation of the NMDA subtype of glutamate receptor has been implicated in structural synaptic plasticity in many developing sensory systems. In the frog retinotectal system, chronic exposure of the optic tectum to NMDA, which decreases the effectiveness of NMDA receptors (Debski et al., 1991), results in the pruning of the branches of retinal terminal arbors (Cline and Constantine-Paton, 1990). However, it is difficult from these studies to relate the involvement of NMDA receptors to changes in synapse distribution. In this study, we have developed an EM sampling procedure to quantitatively compare the amount and the distribution of synaptic contact within single retinal arbors. We report that within each retinal arbor, synaptic contact gradually increases from the main branches to the end branches of the arbor. Chronic NMDA treatment, however, significantly reduces the total amount of synaptic contact within each arbor. This reduced synaptic contact appears to be due to the pruning of the end branches, and the synapses these branches bear. The results are consistent with the hypothesis that NMDA receptor is an integral part of the mechanism that stabilizes coactive synapses, and that maintenance of an axonal branch requires a minimum density of synapses that are correlated with converging neighbors.