Organization of retrosplenial cortical projections to the laterodorsal thalamic nucleus in the rat.

The organization of the projections from the retrosplenial cortex (areas 29a--d) to the laterodorsal thalamic nucleus (LD) was examined in the rat with axonal transport of the cholera toxin B subunit and biotinylated dextran amine. The results showed that an area of the retrosplenial cortex provides ipsilateral projections to a distinct part of the LD at the level of its rostral two-thirds. The projections originate from layer VI and, to a lesser extent, layer V cells of the retrosplenial cortex. Area 29a and area 29b project, respectively, to the dorsolateral and the dorsomedial part of the LD. Area 29c projects to the ventromedial two-thirds of the LD, in a topographic pattern such that the rostral part of area 29c projects more ventromedially within the projection field than the caudal part of area 29c. Area 29d projects to the ventrolateral two-thirds of the LD in a topographic pattern similar to area 29c; the rostral part of area 29d projects more ventromedially within the projection field than the caudal part. These precise topographic projections from the retrosplenial cortex to the LD may constitute part of the circuitry underlying spatial navigation and various memory and emotional functions.