SAG and Alzheimer disease: However, the role of GRKs may be a pivotal one in AD pathology, as GRK-mediated desensitization, in the absence of phosphorylation and arrestin binding, has been reported for metabotropic glutamate receptor 1 (mGluR1), the gamma-aminobutyric acid B receptors [96], and in regulation of metabotropic glutamate receptor 5 function and expression [97].