Group I metabotropic glutamate receptors (mGluRs) have diverse functions in some fundamental neuronal processes, including modulation of synaptic plasticity and dysregulation of these receptors could lead to various neuropsychiatric disorders. Trafficking of group I mGluRs plays critical roles in controlling the precise spatio-temporal localization and activity of these receptors, both of which contribute to proper downstream signaling. Using “molecular replacement” approach in hippocampal neurons derived from mice of both sexes, we demonstrate a critical role for the post-synaptic density protein Norbin in regulating the ligand-induced internalization of group I mGluRs. We show that Norbin associates with protein kinase A (PKA) through its N-terminus and anchors mGluR5 through its C-terminus, both of which are necessary for the ligand-mediated endocytosis of mGluR5, a member of the group I mGluR family. A point mutation (A687G) at the C-terminus of Norbin inhibits the binding of Norbin to mGluR5 and block...