Evidence for two distinct thalamocortical circuits in retrosplenial cortex
Retrosplenial cortex (RSC) lies at the interface between perceptual and memory networks in the brain and mediates between these, although it is not yet known how. It has two distinct subregions, granular (gRSC) and dysgranular (dRSC). The present study investigated how these subregions differ with respect to their electrophysiology and connections, as a step towards understanding their functions. gRSC is more closely connected to the hippocampal system, in which theta-band local field potential oscillations are prominent. We therefore compared theta-rhythmic single-unit activity between the two RSC subregions and found, mostly in gRSC, a subpopulation of non-directional cells with spiking activity strongly entrained by theta oscillations, suggesting a stronger coupling of gRSC to the hippocampal system. We then used retrograde tracers to examine whether differences in neural coding between RSC subregions might reflect differential inputs from the anterior thalamus, which is a prominent source of RSC afferents. We found that gRSC and dRSC differ in their afferents from two AV subfields: dorsomedial (AVDM) and ventrolateral (AVVL). AVVL targets both gRSC and dRSC, while AVDM provides a selective projection to gRSC. These combined results suggest the existence of two distinct but interacting RSC subcircuits: one connecting AVDM to gRSC that may comprise part of the cognitive hippocampal system, and the other connecting AVVL to both RSC regions that may link hippocampal and perceptual regions. We suggest that these subcircuits are distinct to allow for differential weighting during integration of converging sensory and cognitive computations: an integration that may take place in thalamus, RSC or both. HighlightsO_LIThe two retrosplenial cortex subregions, gRSC and dRSC, differ in their temporal firing characteristics and relation to theta oscillations. C_LIO_LIThere are differential afferents from the anteroventral thalamic nucleus to gRSC and dRSC, with the dorsomedial subnucleus projecting selectively to gRSC. C_LIO_LIThe anteroventral thalamus-retrosplenial cortex circuitry thus comprises two functionally and anatomically distinct but connected circuits, differentially connected to the hippocampal system, that may support the integration of cognitive and perceptual information. C_LI