PROJECT SUMMARY The thalamic reticular nucleus (TRN), the major source of thalamic inhibition, plays essential roles in sensory processing, arousal and cognition. Receiving inputs from cortical and subcortical regions, this structure is strategically positioned to influence thalamo-cortical interactions. During quiescence, the TRN participate in sleep rhythm generation, sleep stability and memory consolidation, while in active states, TRN neurons contribute to sensory filtering underlying attention. Perturbed TRN function may underlie behavioral deficits in disorders ranging from schizophrenia and autism to ADHD. Despite its importance, however, several key challenges have limited our ability to determine exactly how TRN circuitry contributes to various brain functions, a prerequisite for determining how it malfunctions in diseases and how its circuitry can be leveraged for diagnostic and therapeutic purposes. This proposal aims to address this critical gap in knowledge by capitalizing on a novel set of findings and tools that we generated. The TRN is a thin shell of GABAergic neurons surrounding thalamic projection nuclei. Within the TRN, neurons that have distinct structural and functional properties can be partially intermingled. This anatomical feature has been a major impediment for functional studies, since selective targeting of TRN neurons that share structural and functional properties with traditional methods is challenging. Using single cell RNAseq, we have recently discovered that TRN neurons can be dissociated into two major subtypes with distinct transcriptomic profiles, anatomical localizations, electrophysiological properties and thalamic connectivity. One group, located in the “core” region of the TRN and can be marked by the expression of the Spp1 gene, targets first-order sensory thalamic nuclei, and the other, located in the “shell” region of the TRN and marked by the expression of Ecel1 gene, targets higher- order ones. We have generated transgenic mice expressing Cre recombinase in each of these two populations individually. Here, we propose to use these new knowledge and genetic tools to answer fundamental questions about TRN structure-function organization as well as the contribution of this brain region to sensory processing, arousal and cognition.