WHOLE-BRAIN MAPPING OF NUCLEUS REUNIENS POPULATION-SPECIFIC CONNECTIVITY

Attenuation of fear memories is essential for maintaining mental well-being, as its impairment can lead to maladaptive outcomes such as anxiety or post-traumatic stress disorders. The nucleus reuniens (NRe) of the thalamus plays a crucial role in fear memory attenuation by linking prefrontal and limbic structures. However, the precise neuronal circuitry underlying this function remains unclear, particularly regarding its population-level functional connectome. This project aims to address this gap by mapping the input–output organization of the NRe. To this end, we characterized the whole-brain connectome of two neuronal populations in the NRe, defined by their projection targets: the basolateral amygdala (BLA) or the medial prefrontal cortex (mPFC), two major NRe outputs implicated in fear memory attenuation. Within these populations, we mapped output collaterals using an intersectional viral strategy based on the injection of a retrogradely transported Cre virus into the postsynaptic structures (BLA, mPFC) combined with a Cre-dependent anterograde AAV virus injected into the NRe. For unbiased brain-wide quantification of axonal projections, we developed a pipeline for automatic axon segmentation integrated into the whole-brain mapping tool ABBA+BraiAn (Chiaruttini et al., 2025). This enabled a comparative description of brain-wide axon collateralization patterns of the two NRe neuronal populations. Subsequently, we employed TRIO rabies-based tracing to map whole-brain inputs to the same NRe populations, following the same analytical strategy. This analysis revealed significantly distinct input–output connectivity patterns for NRe→mPFC and NRe→BLA neurons, providing a comprehensive anatomical framework for future studies on how specific NRe circuits support adaptive forgetting.