Connections of the retrosplenial granular a cortex in the rat.

Although the retrosplenial granular a cortex (Rga) is situated in a critical position between the hippocampal formation and the neocortex, few studies have examined its connections. The present experiments use both retrograde and anterograde tracing techniques to characterize the afferent and efferent connections of Rga. Cortical projections to Rga originate in the ipsilateral area infraradiata, the retrosplenial agranular and granular b cortices, the ventral subiculum, and the contralateral Rga. Subcortical projections originate in the claustrum, the diagonal band of Broca, the thalamus, the midbrain raphe nuclei, and the locus coeruleus. The thalamic projections to Rga originate mainly in the anterodorsal (AD) and laterodorsal (LD) nuclei with sparse projections arising in the anteroventral (AV) and reuniens nuclei. Each projection to Rga terminates in distinct layers of the cortex. The thalamic projection from AD terminates primarily in layers I, III, and IV of Rga, whereas the axons arising from the LD nucleus have a dense terminal plexus only in layer 1. The projections arising from the subiculum end predominantly in layer II, whereas the postsubiculum projects to layers I and III-V. Axons from the contralateral Rga form a dense terminal plexus in layers IV and V, with a smaller number of terminals in layers I and VI. Rga projects ipsilaterally to the AV and LD nuclei of the thalamus and to the anterior cingulate, retrosplenial agranular,a and postsubicular cortices. Contralaterally it projects to the retrosplenial agranular and Rga cortices. Rga projections to the thalamus terminate ipsilaterally in the dorsal part of LD and bilaterally in AV. Together, these data suggest that Rga integrates thalamic with limbic information.