Zinc-rich neurones in the rat visual cortex give rise to two laminar segregated systems of connections.

Zinc-rich synaptic boutons in the neocortex arise from the neocortex itself. However, the precise organisation of these circuits is not known. Therefore, the laminar and areal pattern of zinc-rich cortico-cortical connections between visual areas was studied by retrograde tracing using intracerebral injections of sodium selenite. This tracer was injected in supragranular and infragranular layers in various cortical visual areas in order to precipitate zinc in the synaptic boutons, which was retrogradely transported to neuronal somata. Supragranular injections led to retrogradely labelled neurones in layer II-III, ipsilaterally and contralaterally. Neurones often appeared in groups or clusters. Infragranular injections labelled neurones in layers II-III, VI and, to a lesser extent, in layer V, both ipsilaterally and contralaterally. Neurones in layer VI formed a wide continuous band. Concerning the connections between visual (=occipital) areas, injections in occipital area 2, lateral part (Oc2L), rendered the largest number of retrogradely labelled neurones, which were located in occipital area 1 (Oc1), occipital area 2, medial part (Oc2M) and outside the visual cortex. Callosal zinc-rich projections were dense in the homotopic area but sparse in Oc1 and temporal cortex. Injections in Oc1 rendered moderate numbers of labelled neurones in occipital areas, in both hemispheres. Injections in Oc2M labelled moderate numbers of neurones in occipital areas in both hemispheres and in the frontal and cingulate cortices. These results indicate that zinc-rich cortico-cortical connections are organised into two segregated systems arising from either supragranular or infragranular neurones. In addition, in the visual cortex, zinc-rich systems appear to converge on Oc2L. Zinc-rich connections appear as an extensive, highly organised association system.