Dendroarchitecture and lateral inhibition in thalamic barreloids

Thalamic cells that relay vibrissa information to barrel cortex are clustered within whisker-related modules termed barreloids. Each barreloid receives input from one principal whisker and inhibitory inputs from reticular thalamic neurons with receptive fields that correspond to that same whisker. Although the proximal dendrites of relay cells are confined to their home barreloid, distal dendrites often extend into surrounding barreloids representing adjacent whiskers on the mystacial pad. It was proposed that this arrangement provides a substrate for a mechanism of lateral inhibition that operates remotely on extrabarreloid dendrites. In the present study, we identified adjacent whiskers that suppressed activity below background levels in barreloid cells, and we used a double-labeling protocol to relate the efficacy of inhibition to the dendroarchitecture of the cells. Significant suppression of background discharges was produced by 92% of adjacent whiskers within rows, by 48% of adjacent whiskers within arcs, but was never observed after deflection of nonadjacent whiskers. The magnitude of lateral inhibition increases linearly as the cumulated length of dendrites increases in the barreloid representing an adjacent whisker (R2 = 0.86; p < 0.0001). As distance between cell bodies and the border of an adjacent barreloid increases, dendritic length in that adjacent barreloid diminishes and so does inhibition. Considering time differences between the arrival of principal and adjacent whisker inputs in barreloids, our data suggest that inhibition operating distally on dendrites acts as a spatial filter that primarily suppresses adjacent whisker inputs and so contributes to enhance edge detection.