Organization of intrinsic connections of the retrosplenial cortex in the rat.

The retrosplenial cortex consists of areas 29a-d, each of which has different connections with other cortical and subcortical regions. Although these areas also make complex interconnections that constitute part of a neural circuit subserving various functions, such as spatial memory and navigation, the details of such interconnections have not been studied comprehensively. In the study reported here, we investigated the organization of associational and commissural connections of areas 29a-d within the retrosplenial cortex in the rat, using the retrograde tracer cholera toxin B subunit and anterograde tracer biotinylated dextran amine. The results demonstrated that each of these areas has a distinct set of interconnections within the retrosplenial cortex. Each area interconnects strongly along the transverse axis of the retrosplenial cortex: area 29a, area 29b, caudal area 29c, and caudal area 29d connect with each other, and rostral area 29c and rostral area 29d connect with each other. In the longitudinal direction, rostral-to-caudal projections from rostral areas 29c and 29d to areas 29a and 29b and caudal areas 29c and 29d are strong, whereas reciprocal caudal-to-rostral projections are relatively weak. Although most of the intrinsic connections are homotopical, contralateral connections are weaker and less extensive than ipsilateral connections. These findings suggest that each retrosplenial area may not only process specific information somewhat independently but that it may also integrate and transmit such information through intrinsic connections to other areas in order to achieve retrosplenial cortical functions, such as spatial memory and learning.