Glioblastoma (GBM) and tumor-related epilepsy (TRE) are intimately linked and devastating neurological disorders lacking effective therapies despite decades of promising pre-clinical and clinical research. TRE is reported in 40-60% of all GBM patients, often as the presenting symptom, and therefore new treatments would be highly significant, not only for slowing tumor progression but also improving seizure-free quality of life. In recent years, multiple lines of evidence have shown that malignant GBM cells can utilize numerous pathways of interaction to drive peritumoral neural tissue into hyperactive states, thereby facilitating their own proliferation and setting in motion a vicious feedback loop resulting in runaway disease progression in most patients. Specifically, excess glutamate seems to be involved in many cellular neuron-glia interactions including, increased synaptogenesis and hyperexcitability, and we now have evidence for specific genes expressed in our genetic murine tumor model that drive malignant processes. However, we do not know how these processes unfold over time in a native, immunocompetent mammalian tumor model, and how potential therapeutic windows could be exploited to intervene and slow tumor growth. I propose to dissect the mechanisms of tumor-induced glutamate dysregulation and, vice versa, tumor regulation by neural activity using, for the first time, chronic in vivo cellular and macroscale imaging in a CRISPR/Cas9 genetic model of GBM. We recently published the first widefield-calcium imaging study of this genetic GBM model and characterized spatial and temporal profiles of seizures and spreading depolarization waves. We will use these techniques, as well as cellular resolution 2-photon imaging, simultaneous EEG and behavioral monitoring to address the following questions:1) How does GBM cause the degradation of glutamate homeostasis and calcium activity over time? Using chronic widefield imaging of genetically expressed glutamate and calcium activity indicators, we will follow neural activity and malignant glia invasion in vivo to determine spatial and temporal dynamics of hyperexcitability and GBM growth. 2) How do these dysregulation dynamics change when we perturb the genetic tumor driver composition? We will add genes encoding glypicans 3 and 6, recently identified as synaptogenic proteins secreted by astrocytes, to our CRISPR/Cas9 construct. 3) How do different methods of controlling neural hyperexcitability affect GBM growth? We will determine which targets are more amenable to effective intervention than others by studying the effect of NMDA- and AMPA receptor blockers over time in vivo. In addition, we will assess the contribution of non-synaptic glutamate release using mice with a genetic deletion of the xCT cystine-glutamate astrocytic antiporter. Finally, we will directly silence local peritumoral neurons using inhibitory DREADD constructs and measure subsequent GBM deceleration.