In neurons, membrane depolarization leads to the expression of immediate early gene transcription factors (IEG-TFs), including NPAS4, that regulate programs of gene expression associated with plasticity. IEG-TFs are widely used as tools to identify task-relevant neurons in vivo, yet it is unclear if these proteins are induced in response to changes in the action potential (AP) output or synaptic inputs (EPSPs) to the neuron. Even less is known about whether APs and EPSPs can lead to distinct patterns of gene regulation and cellular phenotypes. This information “transfer function” is an essential component of how neurons monitor and regulate their own activity. In the specific case of NPAS4, an IEG-TF that regulates excitatory-inhibitory (E-I) balance, studying this transfer function will provide valuable insight into the mechanisms underlying neurodevelopmental and psychiatric disorders that stem from dysregulation of E-I balance. We have developed an acute hippocampal slice preparation from the mouse that allows us to independently evoke APs or EPSPs, from defined populations of inputs, within the context of an intact circuit. We propose investigating the activity requirements for NPAS4 expression and the divergent genomic and synaptic regulation that follows from each type of stimulus. We have used this approach to demonstrate that APs and EPSPs lead to NPAS4 expression with distinct spatio-temporal profiles and have extensive preliminary results characterizing the unexpected underlying mechanisms. Using the methods developed for this proposal, in combination with electrophysiology, optical, and sequencing techniques, we are poised to determine how APs and EPSPs differentially impact activity-dependent gene regulation and synapse function. This proposal is a significant departure from how IEG-TFs are typically studied. The execution of these aims will yield important new insights into the mechanics of activity-dependent gene regulation in neurons and how this biology is disrupted in disorders of the brain such as Autism Spectrum Disorders and schizophrenia.