Thalamic projections to retrosplenial cortex in the rat.

The topographic relationships between anterior thalamic neurons and their terminal projection fields in the retrosplenial cortex of the rat were characterized by experiments with the fluorescent dye retrograde labeling technique. The results demonstrate that the anterodorsal (DAD) and anteroventral (AV) nuclei project heavily to retrosplenial granular cortex (Rg) and to a lesser extent to retrosplenial agranular cortex (Rag). In contrast, the anteromedial (AM) and lateral dorsal (LD) nuclei project heavily to Rag and more lightly to Rg. Irrespective of terminal field in Rg or Rag, the neuronal cell bodies in AD and AV are organized topographically so that the neurons in the caudal part of each nucleus project to rostral retrosplenial cortex and the neurons in the rostral portion of each nucleus project to the caudal retrosplenial cortex. Further, the ventromedial AD and AV neurons project to rostral retrosplenial cortex, whereas dorsolateral neurons in both nuclei project to caudal retrosplenial cortex. LD neurons display a different topographic organization. The neurons in the medioventral part of LD project primarily to the rostral retrosplenial cortex, and the neurons in lateral LD project to the caudal retrosplenial cortex. This latter projection to the caudal retrosplenial cortex is also contributed to by neurons residing in the mediodorsal part of caudal LD. The neurons in AM that project to the retrosplenial cortex display less segregation than the AV, AD, or LD neurons. In all experiments, a number of neurons in the dorsal ventro-anterolateral nucleus were labeled by retrosplenial injections. The largest number of cells in this nucleus were labeled after Rag injections, and these were topographically organized such that the neurons projecting to the rostral Rag were located immediately deep to the internal medullary lamina, and the neurons projecting to the caudal Rag were more ventrally located. Very few thalamic neurons have axon collaterals to different areas of the retrosplenial cortex as shown by double labeling experiments. Together, these results demonstrate a highly organized thalamic projection to the retrosplenial cortex.