Glutamate is the main excitatory neurotransmitter in the vertebrate nervous system and regulates a number of cellular signaling pathways. In addition to the ionotropic glutamate receptors of the NMDA, AMPA and kainate type, eight metabotropic receptors (mGluR1 - 8) have been described so far (1). This receptor family can be subdivided into three groups (2): Group I receptors (mGluR1, mGluR5) are activated by dihydroxyphenylglycine (DHPG) (3), group II receptors (mGluR2, mGluR3) by dicarboxycyclopropylglycine (DCG-IV), and group III receptors (mGluR4, mGluR6, mGluR7, mGluR8) by aminophosphonobutyrate (L-AP4) (4).