The advent of endothermy is a defining feature of mammalian and avian evolution, achieved through continuous fine-tuned homeostatic regulation of body temperature and metabolism 1,2 . However, when challenged by food deprivation or harsh environmental conditions, many mammalian species initiate adaptive energy-conserving survival strategies, including torpor and hibernation, during which their body temperature decreases far below its homeostatic setpoint [3][4][5] . How homeothermic mammals initiate and regulate these extraordinary hypothermic states remains largely unknown. Here, we discover that entry into mouse torpor, a fasting-induced state with greatly decreased metabolic rate and body temperature as low as 20°C 6 , is regulated by neurons in the medial and lateral preoptic area of the hypothalamus. We show that re-stimulation of neurons activated during a previous bout of torpor is sufficient to initiate key features of torpor, even in animals that are not calorically restricted. Among these neurons we identify a population of glutamatergic Adcyap1+ cells whose activity accurately determines when animals naturally initiate and exit torpor, and whose inhibition disrupts the natural process of torpor entry, maintenance and arousal. Taken together, we discover a specific neuronal population in the mouse *

Enhancers are the primary DNA regulatory elements that confer cell type specificity of gene expression. Recent studies characterizing individual enhancers have revealed their potential to direct heterologous gene expression in a highly cell-type-specific manner. However, it has not yet been possible to systematically identify and test the function of enhancers for each of the many cell types in an organism. We have developed PESCA, a scalable and generalizable method that leverages ATAC- and single-cell RNA-sequencing protocols, to characterize cell-type-specific enhancers that should enable genetic access and perturbation of gene function across mammalian cell types. Focusing on the highly heterogeneous mammalian cerebral cortex, we apply PESCA to find enhancers and generate viral reagents capable of accessing and manipulating a subset of somatostatin-expressing cortical interneurons with high specificity. This study demonstrates the utility of this platform for developing new cell-type-specific viral reagents, with significant implications for both basic and translational research.

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