Phenotype-driven screening reveals a causal role for the cortex in pupil control

Phenotype-driven forward screening offers a powerful strategy to discover neuronal substrates underlying physical and behavioral traits without prior anatomical or functional assumptions. Historically, such approaches have identified many important neuronal circuits using invertebrate model organisms, but application to mammals has remained limited by the lack of appropriate strategies. Here we quantitatively profiled 56 neurological phenotypic features across more than 200 adult mice when neurodevelopmentally classified neurons were activated or inhibited by chemogenetics. This screen yielded a wide spectrum of robust neurological phenotypes. Upon analyzing these phenotypes computationally, we narrowed down to the hypothesis that activation of cortical neurons enlarges pupil size. Experimental evidence using optogenetics and in utero electroporation indicated that this hypothesis is true. Our results provide proof of principle that phenotype-driven forward approach in mammals is a powerful alternative as a laboratory approach to uncover brain substrates, and offers a general framework for systematically mapping neural circuits that regulate physical and behavioral phenotypes.