CALLOSAL NEURONS GIVE RISE TO ZINC-RICH BOUTONS IN THE RAT VISUAL-CORTEX

SODIUM selenide was used as a retrograde tracer to assess the callosal origin of zinc-rich boutons in the neocortex of the rat. Selenide injections were placed in the lateral end of area Oc1 (area 17). Zinc present in synaptic boutons precipitated as zinc selenide and was transported retrogradely to the parent cell bodies. In addition to the ipsilateral labelling, contralateral retrogradely labelled somata were mainly observed along the border between areas Oc1 and Oc2L (area 18a). Labelled neurones were present in layer 2-3 and layer 6. In contrast to tracing studies with peroxidase, no labelled neurone was observed in layers 4 and 5 except at the inner border of layer 5. This study reveals the chemical heterogeneity of callosal projections, which may be divided into zinc-rich and zinc-poor systems.