The goal of the proposed research is to discover new mechanisms underlying the development of hyperactive neuronal networks in the brain. Hyperactivity of neuronal networks due to the loss or impaired function of inhibitory neurons can lead to neural dysfunctions and seizures. Both impaired inhibition and glial dysfunctions have been linked to several neurodevelopmental disorders including autism. Autism is an increasingly prevalent neurodevelopmental disorder that affects approximately 1 in 59 children in the United States. We hypothesize that ephrin- B/EphB receptor signaling controls the development of inhibitory networks by regulating the interactions between pyramidal neurons and parvalbumin-expressing (PV) cells that are critical for normal development. Premise of the study is supported by our new and unexpected discovery that implicated astrocytic ephrin-B1 and Ephrin-B/EphB receptor signaling in the development of connections between inhibitory PV cells and excitatory pyramidal cells in the hippocampus. Our preliminary findings also show that loss of astrocytic ephrin-B1 increases susceptibility to seizures and reduces sociability in mice. As genetic studies have linked de novo variants in gene encoding EphB2 receptor with autism spectrum disorders (EPHB2 gene is identified as a strong candidate with score 2 in SAFARI database), this study may also contribute to our understanding of the pathophysiological mechanisms of these brain disorders. We will test our hypothesis in three specific aims: Aim 1 will determine if EphB receptors expressed in PV and SOM cells negatively regulate inhibition of CA1 pyramidal neurons by examining the effects of PV- and SOM-specific ablation of EphB2 and EphB1 on the inhibitory synapse development in the hippocampus during critical developmental period using biochemical, immunohistochemical and optogenetic approaches. Aim 2 will determine the mechanism of EphB signaling in regulating inhibitory synapse formation by establishing the role of the neuronal ephrin-B/EphB signaling in inhibitory synapse formation. Aim 3 will test if astrocytic ephrin-B1 positively regulates inhibitory synapse formation on pyramidal cells through the displacement of EphB receptors from PV boutons. The proposed study will further our understanding of the mechanisms which lead to neurodevelopmental disorders and will allow us to discover novel interventions for treating these disorders through targeting EphB receptor signaling and astrocytes during specific developmental period.