Journal of Neuroscience, Vol 15, 4712-4725, Copyright © 1995 by Society for Neuroscience
Analysis of synaptic distribution within single retinal axonal arbors after chronic NMDA treatment
L Yen, JT Sibley and M Constantine-Paton
Department of Biology, Yale University, New Haven, Connecticut 06520, USA.
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.