Summary: Scientists have mapped two distinct neural circuits within the retrosplenial cortex (RSC) that play separate but complementary roles in spatial navigation and memory. One pathway, projecting to the secondary motor cortex (M2), translates spatial information into action. The other, projecting to the anterodorsal thalamus (AD), supports the storage and recall of location-specific memories.
Using precise viral tracing and manipulation techniques, the research team showed that disrupting these circuits produces different behavioral deficits: interfering with M2-projecting RSC neurons impairs the ability to link places with actions as well as object-location memory, while inhibiting AD-projecting neurons selectively weakens memory for object locations. These findings clarify how discrete RSC circuits contribute to spatial cognition and provide anatomical targets for studying neurodegenerative disorders such as Alzheimer’s disease.
Key Facts:
- Two distinct RSC pathways were identified: an M2-projecting circuit involved in converting spatial information into action, and an AD-projecting circuit involved in location-specific memory.
- Selective inhibition of M2-projecting neurons impaired both place-action associations and object-location memory; inhibition of AD-projecting neurons selectively reduced object-location memory.
- Characterizing these projection-specific circuits helps link RSC microcircuits to cognitive functions and may inform targeted approaches for treating disorders that affect spatial memory, including Alzheimer’s disease.
Source: UC Irvine
Overview
A team led by researchers at the University of California, Irvine mapped projection-specific cell populations within the retrosplenial cortex and demonstrated how those pathways differentially contribute to spatial cognition. The study, published in Molecular Psychiatry, combines anatomical tracing with chemogenetic manipulation to compare the inputs and outputs of RSC neurons that project either to the secondary motor cortex (M2) or to the anterodorsal thalamus (AD).
The RSC is connected broadly across cortical and subcortical regions and has been implicated in navigation, memory, and in the pathology of neuropsychiatric and neurodegenerative disorders. By isolating projection-defined subpopulations within the RSC, the researchers revealed semi-independent circuits that differ in both their afferent input patterns and their behavioral contributions.
Lead and co-corresponding author Xiangmin Xu, UC Irvine Chancellor’s Professor of anatomy and neurobiology and director of the Center for Neural Circuit Mapping, explained that these results establish a detailed anatomical foundation for future studies. “By showing how specific RSC circuits support distinct aspects of spatial cognition, we can better understand how disorders like Alzheimer’s disease affect particular cell types and pathways,” Xu said.
Anatomical mapping revealed significant differences in input sources: M2-projecting RSC neurons received comparatively stronger input from regions such as the dorsal subiculum, AD, lateral dorsal and lateral posterior thalamic nuclei, and somatosensory cortex. In contrast, AD-projecting RSC neurons received relatively greater input from the anterior cingulate cortex and medial septum. Although both projection-defined populations share some collateral targets, their direct outputs are distinct—each principally targeting either M2 or AD.
Behaviorally, the team used chemogenetic inhibition to test the functional roles of each pathway. Inhibiting M2-projecting RSC neurons disrupted object-location memory and the ability to associate specific places with actions. Inhibiting AD-projecting neurons impaired object-location memory but did not affect place-action associations, indicating a more selective role for the RSC→AD pathway in memory encoding or retrieval.
These projection-specific differences suggest the RSC is organized into parallel circuits that process spatial information in complementary ways: one circuit linking spatial representations to motor plans and actions, and another specializing in encoding and recalling the spatial context of objects.
Xu and colleagues are extending this work to map additional RSC pathways and to examine how different neuronal subtypes within these circuits contribute to navigation and memory. Their long-term goal is to assemble a detailed map of the brain’s spatial-processing network—an effort that could reveal cellular and circuit-level targets for interventions in cognitive disorders.
Other contributors to the study include Xiaoxiao Lin, Ali Ghafuri, Xiaojun Chen, Musab Kazmi, and co-corresponding author Douglas A. Nitz from UC San Diego. The research was supported by the National Institutes of Health under grants NS078434, MH120020, and U01AG076791.
About this research
Author: Patricia Harriman
Source: UC Irvine
Contact: Patricia Harriman – UC Irvine
Image credit: Neuroscience News
Original research: Open access. “Projection-specific circuits of retrosplenial cortex with differential contributions to spatial cognition” by Xiangmin Xu et al., published in Molecular Psychiatry.
Abstract (condensed)
The retrosplenial cortex (RSC) maintains reciprocal connections with diverse cortical and subcortical regions but the organization and behavioral roles of projection-defined RSC subpopulations were not fully understood. Using retrograde and anterograde viral tracers along with monosynaptic rabies labeling, the study quantitatively compared input and output distributions of RSC neurons projecting to M2 versus AD. M2- and AD-projecting neurons overlap in some collateral targets but are distinct in their primary projections. Afferent patterns differ between these populations, with stronger dorsal subiculum, AD, lateral dorsal/lateral posterior thalamus, and somatosensory inputs to M2-projecting neurons and greater anterior cingulate and medial septum inputs to AD-projecting neurons. Chemogenetic inhibition demonstrated that M2-projecting neurons contribute to both object-location memory and place-action association, whereas the RSC→AD pathway selectively affects object-location memory. The results indicate semi-independent RSC circuits that differ anatomically and functionally and that contribute differentially to spatial cognition.