Updated project metadata. Dysregulated protein synthesis is a core pathogenic mechanism in Fragile X Syndrome (FX). The mGluR Theory of FX predicts that pathological synaptic changes arise from the excessive translation of mRNAs downstream of mGlu1/5 activation. Here, we use a combination of CA1 pyramidal neuron-specific Translating Ribosome Affinity Purification and RNA-seq (TRAP-seq) and proteomics to identify the overtranslating mRNAs supporting exaggerated mGlu1/5 -induced long-term synaptic depression (mGluR-LTD) in the FX mouse model (Fmr1-/y). Our results identify a robust translation of ribosomal proteins (RPs) upon mGlu1/5 stimulation that coincides with a reduced translation of long mRNAs encoding synaptic proteins. These changes are mimicked and occluded in Fmr1-/y neurons. In addition, proteomics analyses identify increased degradation of multiple proteins at hippocampal synapses, many of which are underexpressed at steady-state.