Distribution of terminal fields stained for zinc in the neocortex of the rat

Staining for zinc in terminal fields of the rat neocortex was studied by applying the sulphide/silver histochemical method. The stain was arranged in a distinct layered pattern. Two bands of heavy reaction were found in deep layer 1 plus layers 2-3 and in upper layer 5. A band of moderate-to-heavy reaction was found in layer 6. Three bands of lighter staining were found in upper layer 1, layer 4 and deep layer 5. The layers of reaction showed variations in width and intensity of staining from area to area. In the frontal and cingulate cortices and in the association cortices, the heavily stained bands were dominant over the narrow, inconspicuous, lightly stained bands. In contrast, in primary sensory regions (Par1, Oc1 and Te1), the lightly stained bands were wide and prominent. The sulphide/silver method gives a straightforward delimitation of many cortical areas and reveals a clear distinction between (A) allocortical and isocortical areas, and (B) primary sensory areas, secondary or association areas, and prefrontal plus motor areas. The presence of vivid bands of staining for zinc appears to be linked to areas with prominent pyramidal layers.