Neuropeptidergic Modulation and Neural Pathways in Contextual Fear Conditioning and Extinction: Insights from the Retrosplenial Cortex and Nucleus Incertus

Abstract

This work examines the mechanisms underlying contextual fear conditioning and the role of neural networks in memory processing. We assessed how different procedural factors influence the acquisition and extinction of contextual fear in rats, revealing that spaced sessions of conditioning enhance extinction, while minimal shocks produce context-specific fear. More intense conditioning, however, leads to fear generalization across contexts, which can be extinguished similarly to the conditioned context. Additionally, we explored the role of the the neuropeptide relaxin-3 (RLN3) signaling via the RXFP3 receptor in the retrosplenial cortex (RSC) in contextual fear conditioning. Modulation of RXFP3 in the RSC delayed fear extinction without affecting acquisition, suggesting that RLN3/RXFP3 signaling enhances memory strength, making extinction more resistant. Anatomical studies confirmed a strong presence of RLN3-immunoreactive fibers in the RSC, particularly co-localizing with inhibitory circuits. Finally, we investigated the connectivity between the RSC and other brain regions, particularly the bidirectional projections between the RSC, prefrontal cortex, and nucleus incertus. These connections were confirmed using anterograde and retrograde tracers, highlighting a broader cortical and subcortical network that involves key regions such as the prelimbic and orbitofrontal cortices, the medial septum habenula, interpeduncular, and the supramammillary nucleus. All together, these findings provide a more integrated understanding of the circuitry involved in contextual fear and memory processing, emphasizing the long-loop interactions between the retrosplenial cortex, prefrontal areas, and subcortical structures in regulating fear acquisition and extinction processes.