Organization of projections of rat retrosplenial cortex to the anterior thalamic nuclei.

The organization of the projections from the retrosplenial cortex (Brodmann's area 29) to the anterior thalamic nuclei was examined in the rat with retrograde transport of the cholera toxin B subunit and anterograde transport of biotinylated dextran amine. Areas 29a and 29b project mainly ipsilaterally to the rostral two-thirds of the anteroventral nucleus, with area 29a projecting more rostrodorsally than area 29b. Area 29c projects bilaterally to the ventromedial part of the anteroventral nucleus. The projections from area 29c are organized in a topographic pattern such that the rostral area 29c projects to the caudoventral part of the anteroventral nucleus, whereas the caudal area 29c projects to the more rostrodorsal parts. Caudal area 29d projects mainly ipsilaterally to the rostrodorsal part of the anteromedial nucleus, and the rostral and dorsal parts of the anteroventral nucleus, whereas rostral area 29d projects bilaterally to the caudodorsal part of the anteromedial nucleus and the caudolateral part of the anteroventral nucleus. All the areas of the retrosplenial cortex provide sparse projections, mainly ipsilateral, to the anterodorsal nucleus, with a crude topographic pattern such that the rostrocaudal axis of the retrosplenial cortex corresponds to the caudorostral axis of the anterodorsal nucleus. The results indicate that each area of the retrosplenial cortex has a distinct projection field within the anterior thalamic nuclei. This suggests that each of these projections transmits distinct information that is important for complex memory and learning functions, e.g. discriminative avoidance learning and spatial memory.