The mGlu receptors are G-protein-coupled protein receptors (GPCRs) located at the neuronal synapse, extrasynaptically as well as on glial cells (Fig. 2). These receptors are divided into three groups. Group I mGluRs (mGlu1 and mGlu5) are predominately postsynaptic and engage in slow excitatory neurotransmission; Group II mGluRs (mGlu2 and mGlu3) are predominately presynaptic, with some postsynaptic and glial localization (Fig. 2) where they engage in slow inhibitory neurotransmission; and Group III mGluRs (mGlu4, mGlu6, mGlu7, and mGlu8) are generally restricted to the presynaptic terminal and, like Group II mGluRs, engage in slow inhibitory neurotransmission. The mesocorticolimbic and associated reward circuitry express high levels of mGlu1, mGlu2, mGlu3, and/or mGlu5, notably in the Acb, caudate nucleus, cortex, lateral septum, dorsal striatum, amygdala, and hippocampus.39–43
