Sensory hypersensitivity and abnormal sensory processing contribute significantly to behavioral problems associated with Fragile X Syndrome (FXS). Sensory hypersensitivity and audiogenic seizures in the FXS mouse model, Fmr1 knockout (KO) mice, suggest a hyperexcitability of sensory circuits. We have discovered a robust hyperexcitability, as well as changes in specific excitatory and inhibitory synaptic connections, in neocortical circuits in the Fmr1 KO mice. Our work has also revealed a novel molecular mechanism underlying hyperexcitability - impaired scaffolding among Homer proteins and resulting enhanced metabotropic glutamate receptor (mGluR5) signaling. We hypothesize that, in addition to enhancing mGluR5 function, impaired Homer scaffolding causes a displacement of the endocannabinoid (eCB) synaptic signaling resulting in differentially altered eCB-dependent plasticity of excitatory and inhibitory synapses in the Fmr1 KO. Evidence also indicates that ERK activity is involved in this altered plasticity. New data find that increased CamKlla phosphorylation of Homer causes the loss of scaffolding. As part of the FXS center, we propose to examine the role that these biochemical mechanisms and 3 synaptic pathways play in hyperexcitability of the auditory cortex in the Fmr1 KO mouse. Coordination of experiments are planned to link the alterations we find in the auditory neocortex with deficits in auditory sensory processing in Fmr1 KO mice (P2) and FXS patients (P3). In, Aim 1, we examine the developmental and circuit mechanisms underlying circuit hyperexcitability. Based on existing candidate targets for potential FXS treatment, we also examine the acute effects of clinically-approved, potential therapeutics on neocortical hyperexcitability. In Aim 2, we determine if mGluR5-eCB synaptic plasticity is dysregulated at 3 different neocortical synaptic pathways with known changes in the Fmr1 KO that could underlie hyperexcitability. In Aim 3, we determine if enhanced CaMKlla phosphorylation of Homer causes circuit hyperexcitibility and dysregulated mGluR5-eCB plasticity of specific synaptic circuits. In Aim 4, and in collaboration with P2, we examine the role of altered matrix-metalloproteinase (MMP9) signaling in hyperexcitability and disrupted Homer scaffolds