Metabotropic glutamate 1 receptor distribution and occupancy in the rat brain: a quantitative autoradiographic study using [ 3H]R214127

We used the selective metabotropic glutamate (mGlu) 1 receptor antagonist [ 3H]1-(3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-2-phenyl-1-ethanone ([ 3H]R214127) to investigate the distribution of mGlu1 receptor binding sites in rat brain. We found high mGlu1 receptor binding in the cerebellum, thalamus, dentate gyrus and medial central gray, moderate binding within the CA3 of the hippocampus and hypothalamus, and low mGlu1 receptor binding in the basal ganglia and cortex. The mGlu1 receptor is also present in variable degree in the dorsal lateral septal nucleus, amygdala, interpeduncular nucleus and median raphe nucleus. Additionally, we employed [ 3H]R214127 autoradiography as a means of investigating the occupancy of central mGlu1 receptors following in vivo administration of mGlu1 receptor antagonists that prevent binding of this radioligand. We found that the mGlu1 receptor antagonist (3aS,6aS)-6a-naphtalan-2-ylmethyl-5-methyliden-hexahydro-cyclopenta[c]furan-1-on (BAY 36-7620), administered subcutaneously (s.c.) at 10 mg/kg, only occupied about 30% of cerebellar and thalamic mGlu1 receptors. The mGlu1/5 receptor antagonist 2-quinoxaline-carboxamide- N-adamantan-1-yl (NPS 2390) exhibited a relatively high potency in occupying mGlu1 receptors in rat cerebellum (ED 50=0.75 mg/kg, s.c.) and thalamus (ED 50=0.63 mg/kg, s.c). In the future, this method can be employed to gain more insight into the in vivo profile and central activity of potential therapeutic agents that act upon the mGlu1 receptor.