Brain-wide activation mapping at cellular resolution during learning and retrieval of aversive memory

The neuronal and molecular mechanisms underlying behavioral responses triggered by fear have received wide interest in the last few years, and various preclinical studies have addressed potential treatments for fear-related disorders. The central histaminergic system is an important modulator of memory related to adverse events and the integrity of the brain histamine system is necessary for the consolidation of this type of memory. Therefore, the use of antihistaminic drugs could be useful for the treatments of conditions such as obsessive-compulsive disorders, post-traumatic stress disorders and generalized anxiety. In this context, it is essential to understand the neuronal circuits involved in behavioral responses associated with adverse events. During my PhD, I developed a pipeline for mapping neuronal activation at micron resolution, combining transgenic approach, clearing protocol, high-resolution imaging, atlas registration, and automated 3D image analysis. The combination of high-resolution imaging and 3D analysis for processing sub-cellular information became the key point of this pipeline, enabling high performance. This pipeline was validated using a classical paradigm, as step-through passive inhibitory avoidance, to analyze neuronal activation patterns across the entire brain of male and female mice, at selected time points. This approach highlighted a strong sexual dimorphism, during fear learning and recall, which was not evident from the behavioral task. Further, it identified brain regions whose degree of activity correlated to specific behavioral features. Finally, micron-scale 3D resolution was exploited to investigate histaminergic subpopulations elicited by aversive events. The combination of behavioral, transgenic, optical and computational methods presented here represents an important tool to quantitatively characterize the neuronal pathways involved in fear memories.