Efferent projections from the anterior thalamic nuclei to the cingulate cortex in the rat.

The organization of projections from the anterior thalamic nuclei to the cingulate cortex was analyzed in the rat by the anterograde transport of Phaseolus vulgaris-leucoagglutinin. The rostral part of the anteromedial nucleus projects to layers I, V and VI of the anterior cingulate areas 1 and 2, layers I and III of the ventral orbital area, layers I, V and VI of area 29D of the retrosplenial area, and layers I and V of the caudal part of the retrosplenial granular and agranular areas. In contrast, the caudal part of the anteromedial nucleus projects to layer V of the frontal area 2, and layers I and V of the rostral part of the retrosplenial granular and agranular areas. The interanteromedial nucleus projects to layers I, III and V of the frontal area 2, layer V of the agranular insular area, and layers I, V and VI of area 29D. The anteroventral nucleus projects to layers I and IV of the retrosplenial granular area, whereas the anterodorsal nucleus projects to layers I, III and IV of the same area. Projections from the anteroventral and anterodorsal nuclei were, furthermore, organized such that their ventral parts project to the rostral part of the retrosplenial granular area, whereas their dorsal parts project to the more caudal part. The results suggest that the anterior thalamic nuclei project to more widespread areas and laminae of the cingulate cortex than was previously assumed. The projections are organized such that the anteromedial and interanteromedial nuclei project to layer I and the deep layers of the anterior cingulate and retrosplenial cortex, whereas the anteroventral and anterodorsal nuclei project to the superficial layers of the retrosplenial cortex. These thalamocortical projections may play important roles in behavioral learning such as discriminative avoidance behavior.