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The Journal of Neurophysiology Vol. 86 No. 3 September 2001, pp. 1179-1194
Copyright ©2001 by the American Physiological Society
Graduate Program in Neurobiology and Behavior, Department of Biology, Georgia State University, Atlanta, Georgia 30303
Huang, L. and
S. L. Pallas.
NMDA Antagonists in the Superior Colliculus
Prevent Developmental Plasticity But Not Visual Transmission or
Map Compression. J. Neurophysiol. 86: 1179-1194, 2001. Partial ablation of the superior colliculus (SC) at birth in hamsters
compresses the retinocollicular map, increasing the amount of visual
field represented at each SC location. Receptive field sizes of single
SC neurons are maintained, however, preserving receptive field
properties in the prelesion condition. The mechanism that allows single
SC neurons to restrict the number of convergent retinal inputs and thus
compensate for induced brain damage is unknown. In this study, we
examined the role of N-methyl-D-aspartate (NMDA)
receptors in controlling retinocollicular convergence. We found that
chronic 2-amino-5-phosphonovaleric acid (APV) blockade of NMDA
receptors from birth in normal hamsters resulted in enlarged single-unit receptive fields in SC neurons from normal maps and further
enlargement in lesioned animals with compressed maps. The effect was
linearly related to lesion size. These results suggest that NMDA
receptors are necessary to control afferent/target convergence in the
normal SC and to compensate for excess retinal afferents in lesioned
animals. Despite the alteration in receptive field size in the
APV-treated animals, a complete visual map was present in both normal
and lesioned hamsters. Visual responsiveness in the treated SC was
normal; thus the loss of compensatory plasticity was not due to reduced
visual responsiveness. Our results argue that NMDA receptors are
essential for map refinement, construction of receptive fields, and
compensation for damage but not overall map compression. The results
are consistent with a role for the NMDA receptor as a coincidence
detector with a threshold, providing visual neurons with the ability to
calculate the amount of visual space represented by competing retinal
inputs through the absolute amount of coincidence in their firing
patterns. This mechanism of population matching is likely to be of
general importance during nervous system development.
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