TETRODOTOXIN ANALYSIS OF DIRECT CORTICAL RESPONSES

Excitation of the negative wave type of direct cortical response (N wave DCR) is reversibly blocked by placing a small volume of 10-5 to 10-6 g/ml tetrodotoxin on the stimulated site. A similar low concentration of tetrodotoxin placed over the line of a molecular layer preparation made by cutting all layers up to but not including the first, reversibly blocked transmission of the N wave DCR. Tetrodotoxin placed on the responding site produces either a block or a reversal of the response, a surface positive potential. This reversed potential was shown by laminar recording to represent a source to sinks present lower down in the intermediate cortical layers, at 300-1000 μ. The interpretation of these results is that axons passing laterally in the cortex synapse on upper apical dendrites and, to a lesser extent, also synapse deeper; e.g., on shafts of apical dendrites, basilar dendrites and somas in the intermediate layers. With the upper apical dendrites blocked by tetrodotoxin, these sinks in the intermediate levels of the cortex are responsible for the upper dendritic membranes acting as sources. © 1968.