Project Summary Oxytocin is a neuropeptide hormone produced in the paraventricular (PVN) and supraoptic nuclei of the hypothalamus that is implicated in physiological processes such as birth and lactation, as well as in the expression of social behaviors. To date, many studies have linked brain oxytocin with particular social behaviors, yet the neural pathways by which external social information is primarily conveyed to oxytocin neurons to modulate their neural activity and regulate social behavior have been understudied. My preliminary data indicates that many brain regions implicated in sensory processing and social behavior send monosynaptic inputs to the PVN, including the posterior intralaminar (PIL) complex of the thalamus. I further show that many PIL-PVN projections are glutamatergic, that the PIL inputs directly onto oxytocin cells specifically, and that the PIL is activated during social interaction. Previous studies have shown that glutamate regulates oxytocin neural activity in vivo in lactating female rodents and in vitro in slices obtained from adult males and neonates. However, the functional relevance of glutamate-oxytocin circuits in social behavior remains unknown. Emerging research indicates that dysfunction in glutamatergic signaling leads to impairments in synaptic transmission causing deficits in social behavior, which is a core symptom of autism spectrum disorder. Additionally, many of the identified risk genes for autism spectrum disorder are key components of glutamatergic synapses, including SHANK3. Therefore, I hypothesize that glutamatergic inputs to the PVN, especially those originating from brain regions that are implicated in the processing of social stimuli, are essential for regulating the activity of oxytocin neurons during social behavior and that Shank3 mutations disturb this activity. In this proposal, I will address my hypothesis by characterizing the identity of inputs to oxytocin neurons in the PVN and manipulating them, using viral and chemogenetic tools, to assess their role in social behavior. I will also use Miniscope technology to perform in vivo calcium imaging of oxytocin neurons to capture neural activity during social behavior in freely moving wild-type and Shank3-deficient mice. Furthermore, using this technology in combination with chemogenetic tools, I will dissect the contribution of specific glutamatergic-PVN circuits in modulating the activity of oxytocin neurons. This study will provide knowledge about the functional role of glutamatergic inputs to the PVN in regulating social behavior and oxytocin neural activity. Additionally, this study will examine the specific effect of Shank3 mutations on the oxytocin system, which could inform potential targets for treatment.