Convergent thalamic and mesencephalic projections to the anterior medial cortex in the rat

Small microelectrophoretic deposits of horseradish‐peroxidase (HRP) were placed at various loci within the gray matter of the rat's anterior medial cortex. A comparison of the patterns of retrograde cell‐labeling charted in 26 such cases confirmed earlier findings in fiber‐degeneration studies according to which the respective thalamocortical projections of the mediodorsal (MD) and anteromedial nucleus (AM) overlap each other over a wide region of the anterior medial cortex. This region of thalamocortical convergence, extending from pregenual levels caudalward as far as the anterior border of the retrosplenial cortex, corresponds almost exactly to the cortical region from which locally deposited HRP was found to be transported so as to label cells in one or both of two mesencephalic cell groups: the ventral tegmental area (AVT) dorsal and lateral to the interpeduncular nucleus, and the medial one‐quarter of the pars compacta of the substantia nigra (SNC). The reported findings suggest that the corticipetal projections of AVT and SNC are organized so that AVT projects largely to the area infraradiata of the pregenual cortex which receives its major thalamocortical projection from MD, whereas SNC‐efferents are distributed to the area infraradiata of the supracallosal cortex which receives most of its thalamocortical afferents from AM; the two mesencephalocortical projections do, however, appear to overlap each other in a region of the medial cortex immediately posterior to the level of the genu corporis callosi. HRP injected into the medial cortex of five rats pre‐treated with 6‐hydroxydopamine, a drug that produces chemical lesions of catecholamine neurons, failed to label any cells in AVT and SNC but did label numerous cells in MD and AM. This finding suggests that the ventral tegmental cells projecting to the anterior medial cortex are exclusively dopamine neurons.