Evidence for two distinct thalamocortical circuits in retrosplenial cortex

• Main Authors: Aggleton, J.P (Author), Cheng, H.Y (Author), Jeffery, K.J (Author), Lomi, E. (Author), Mathiasen, M.L (Author), Mitchell, A.S (Author), Zhang, N. (Author)
• Format: Article
• Language: English
• Published: Academic Press Inc. 2021
• Subjects: anatomy and histology; animal; animal cell; animal experiment; animal tissue; Animals; Anterior thalamus; anteroventral thalamus; Article; brain cortex; brain electrophysiology; brain nerve cell; cell subpopulation; Cerebral Cortex; Chronic rodent electrophysiology; cingulate gyrus; cognition; connectome; controlled study; electroencephalography; Electroencephalography; Gyrus Cinguli; Head direction cells; hippocampus; male; Male; nerve tract; Neural Pathways; Neuroanatomy; nonhuman; oscillation; physiology; rat; Rats; retrosplenial cortex; Retrosplenial cortex; Spatial memory; spike; thalamocortical tract; thalamus; Thalamus; thalamus nucleus; theta rhythm; Theta Rhythm; Theta-modulation
• Online Access: View Fulltext in Publisher

 Retrosplenial cortex (RSC) lies at the interface between sensory and cognitive 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 thalamic connectivity, as a step towards understanding their functions. The gRSC is more closely connected to the hippocampal formation, 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 test for differential inputs to RSC from the anteroventral thalamus (AV). We found that gRSC and dRSC differ in their afferents from two AV subfields: dorsomedial (AVDM) and ventrolateral (AVVL). Specifically: (1) as a whole AV projects more strongly to gRSC; (2) AVVL targets both gRSC and dRSC, while AVDM provides a selective projection to gRSC, (3) the gRSC projection is layer-specific: AVDM targets specifically gRSC superficial layers. These same AV projections are topographically organized with ventral AV neurons innervating rostral RSC and dorsal AV neurons innervating caudal RSC. 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. © 2021 The Authors