Afferent connections of the medial frontal cortex of the rat. A study using retrograde transport of fluorescent dyes. I. Thalamic afferents.

The present study of the medial frontal cortex of the rat was undertaken with two objectives. First, to compare the pattern of afferent thalamic neurons for each of the three subdivisions of the medial frontal cortex: the medial precentral (PrCm), dorsal anterior cingulate (ACd) and prelimbic (PL) areas. Second, to provide a firmer basis for anatomical comparisons of cortical regions between rat and monkey. Focal injections of retrogradely transported fluorescent tracers, true blue and diamidino yellow, were placed in different regions of the medial frontal cortex, to reveal the organization of afferent thalamic neurons. The PL area can be readily distinguished from PrCm and ACd areas because it receives afferents from a large number of neurons from both the medial and the lateral parts of the mediodorsal nucleus (MD) whereas only a few neurons, from the lateral MD exclusively, project to PrCm and ACd areas. Moreover, the paratenial and the paraventricular thalamic nuclei project only to the PL area, and the central medial nucleus projects mostly to the PL area. The ventrolateral nucleus projects only to the dorsal part of the medial frontal cortex. The rhomboid, reuniens, ventromedial, intralaminar, posterior and laterodorsal nuclei project to the whole medial frontal cortex. On the basis of these findings, the pattern of thalamic afferents to the PL area was compared to the pattern of thalamic afferents to cingulate and retrosplenial cortices in rat. The conclusion is that the PL area has a pattern of thalamic afferents which is different not only from those of PrCm and ACd areas but also from those of cingulate and retrosplenial cortices. On the basis of its rich innervation from the mediodorsal nucleus, the prelimbic area could very likely be a part of the prefrontal cortex of rat.