ROLE OF NMDA RECEPTORS IN THE PROPAGATION OF EXCITATION IN RAT VISUAL-CORTEX AS STUDIED BY OPTICAL IMAGING

To examine the role of the N-methpl-D-aspartate (NMDA) type of glutama te receptors in the propagation of information in visual cortex, optic al imaging with high spatial and temporal resolution of neuronal activ ity was used in cortical slices of rats. Single-shock stimulation of t he white matter elicited a vertical propagation of excitation toward t he cortical surface simultaneously with a horizontal spread of excitat ion in lower layers. The horizontal spread in upper layers occurred su bsequent to the vertical spread reaching these layers. The results fro m perfusion of Ca2+-free medium and application of an antagonist of no n-NMDA receptors indicated that this intracortical propagation of sign als is due mostly, if not exclusively, to the postsynaptic excitation of cortical neurons. Blockade of NMDA receptors attenuated the rising and peak phases of the upper horizontal spread, but did not affect tho se of the lower horizontal or vertical propagation of excitation. Perf usion with Mg2+-free solution enhanced the upper horizontal spread, bu t in most cases did not significantly change the spread of excitation in the other pathways. These results indicate that NMDA receptors are involved in the flow of information in the upper layers of visual cort ex, and further suggest that this propagation of activity is mediated mainly by horizontal connections intrinsic to the upper layers. (C) 19 98 Elsevier Science Ireland Ltd. All rights reserved.