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 inj ections were placed in the lateral end of area Oc1 (area 17). Zinc pre sent in synaptic boutons precipitated as zinc selenide and was transpo rted retrogradely to the parent cell bodies. In addition to the ipsila teral labelling, contralateral retrogradely labelled somata were mainl y observed along the border between areas Oc1 and Oc2L (area 18a). Lab elled neurones were present in layer 2-3 and layer 6. In contrast to t racing studies with peroxidase, no labelled neurone was observed in la yers 4 and 5 except at the inner border of layer 5. This study reveals the chemical heterogeneity of callosal projections, which may be divi ded into zinc-rich and zinc-poor systems.