GRM5 and Alzheimer disease: Of the 8 types of mGluRs, mGluR1 and mGluR5 levels have been shown to be decreased in cerebral cortex and hippocampus of AD patients and AD transgenic murine models (Albasanz et al., 2005; Fang et al., 2017; Bie et al., 2019; Srivastava et al., 2020).