PROJECT SUMMARY Studies in humans suggest that Neurexin1α (Nrxn1α), a presynaptically expressed organizer of synaptic structure, is a key genetic risk factor for multiple neuropsychiatric diseases, all of which exhibit impairments in goal-directed processing. Given the pernicious effects of goal-directed dysfunction on daily quality of life, a better understanding of the underlying neurobiology – both at the molecular and neural circuit level - is strongly warranted. Our prior work has developed quantitative behavioral and in vitro electrophysiological approaches while demonstrating that mice with mutations in Nrxn1α are an excellent system to study alterations in neural circuit activity driving impaired goal-directed behavior. Efficient goal-directed behavior relies on reinforcement, whereby outcomes shape future actions, as well as flexibility, the ability to adapt to changes in contingencies or context. Both processes are thought to occur in part within the striatum, where excitatory inputs representing choice-relevant information interact with neuromodulators, such as dopamine (DA) and acetylcholine (ACh), which signal information about rewards, surprise, and uncertainty from the external world. Nrxn1 is broadly expressed in these cortico-striatal-thalamic circuits including the presynaptic terminals of striatal-targeting cortical and thalamic neurons, as well as within striatal cholinergic interneurons and striatal-targeting dopaminergic projections from the midbrain. Here we examine how Nrxn1 functions within midbrain dopamine neurons and striatal cholinergic interneurons, the major sources of striatal dopamine and acetylcholine, respectively. We employ cell type specific Nrxn1 deletion together with acute slice recordings to mechanistically address how Nrxn1 contributes to the striatal release of these key neuromodulators. In parallel, we use in vivo imaging of novel sensors for dopamine and acetylcholine to investigate how these modulators are altered during value-based choice tasks in both circuit-specific and brain-wide Nrxn1 KOs. We will correlate abnormal neuromodulatory signals with abnormalities in choice behavior, focusing on reinforcement processes and choice flexibility. To probe the functional relevance of altered neuromodulatory signals for behavior, we optogenetically impose abnormal ACh and DA signals in WT mice and observe impacts on choice selection and flexibility. Finally, we examine how Nrxn1 mutations impact striatal processing during selection of actions via in vivo recordings in wildtype and Nrxn1 KO mice. The proposed work will inform us of how mutations in Nrxn1 alter striatal cholinergic and dopaminergic signals and striatal processing of goal-directed actions, while providing a framework to understand the diversity of goal-directed dysfunction seen across neuropsychiatric disorders.