An autoradiographic examination of corticocortical and subcortical projections of the mediodorsal‐projection (prefrontal) cortex in the rat

Abstract

The efferent connections of the cortex projected upon by the mediodorsal thalamic nucleus (MD‐projection cortex) have been re‐examined autoradiographically in the rat following the microelectrophoretic injection of 3 H‐proline‐leucine into different parts of the medial and sulcal MD‐projection cortex. Contrary to previous negative findings, the present experiments revealed a system of extensive corticocortical projections and indicated that different areas of the MD‐projection cortex have distinctive patterns in both their corticocortical and subcortical projections. Thus, cell of Brodmann's area 32 send axons to the retrosplenial cortex, area 29d, the peri‐ and entorhinal cortices, and the presubiculum. Both supragenual and more posterior regions of area 24 project to the retrosplenial cortex and area 29d, but only the posterior portion projects additionally to the entorhinal area and presubiculum. The cortical targets of axons from the sulcal MD‐projection cortex are mainly the anterior part of the piriform cortex and, for the posterolateral part of the sulcal cortex (insular area), the retrosplenial area, lateral entorhinal area, and presubiculum. While the medial and sulcal divisions of the MD‐projection cortex project upon one another, the medial‐to‐sulcal projection is in general denser than its reciprocal. Earlier findings of projections to subcortical structures affiliated with the limbic system such as midline thalamic nuclei, hypothalamus, and paramedian mesencephalic tegmentum are confirmed, and appear to originate primarily in area 32 and the insular part of the sulcal cortex. The corticothalamic projection to MD in general terms reciprocates the organization seen in the thalamocortical projection from the various subnuclei within MD. Previously undocumented projections are demonstrated mainly from area 32 of the medial cortex and the insular part of the sulcal cortex to the lateral and basal amygdaloid nuclei, the medial part of the lateral septal nucleus, the nucleus accumbens, and the deep layers of the olfactory tubercle; the medial part of the lateral habenular nucleus receives a projection from areas 32 and 24. Projections to the pretectal area and superior colliculus appear to originate from all parts of the medial MD‐projection cortex, but are markedly denser when the posterior part of area 24 is injected. The distribution of this corticotectal projection shows a highly characteristic configuration in which areas of high grain concentration surround areas of lower grain concentration.