Metabotropic glutamate receptors (mGluRs), heptahelical transmembrane receptors coupled to G-proteins, contribute to the regulation of neuronal excitability and synaptic transmission1. Genes coding for eight mGluRs (denoted mGluR1-8) with different splice variants have been identified and classified into three groups based on sequence homology, pharmacology, and signaling pathways. Group I mGluRs (mGluR1 and mGluR5) are primarily located postsynaptically. Group II (mGluR2 and mGluR3) are present both postsynaptically and presynaptically outside the release site. Group III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8) couple to G
