Topographic organization and collateralization of the projections of the anterior and laterodorsal thalamic nuclei to cingulate areas 24 and 29 in the rat

Fast blue and Rhodamine microspheres were used to analyze the topography and collateralization of the limbic nuclei of the thalamus to 4 defined areas of the cingulate gyrus (anterior 24, posterior 24, anterior 29, and posterior 29). The anterodorsal nucleus and the anteroventral nucleus project to posterior area 24 and all of area 29. The anteromedial nucleus projects generally to both areas 24 and 29, but principally to anterior area 24. The laterodorsal nucleus projects only to area 29. The anterior and posterior portions of the thalamic nuclei topographically project to the anterior and posterior regions of the cingulate gyrus, respectively. Neurons in the medial region of the anterior nuclei project to the anterior area of the cingulate gyrus while neurons located in the lateral region project to posterior areas. Collaterals (9–13%) from the anteromedial nucleus project diffusely to areas 24 and 29, while collaterals from other nuclei project primarily to areas adjacent to each other. Modulation of limbic system functions probably occurs via these projection fibers. Furthermore, the idea appears to be reinforced that at least a dual and coupled function exists for these direct thalamocingulate connections.