L-Glutamate (Glu) activates at least eight different G protein-coupled receptors known as metabotropic glutamate (mGlu) receptors, which mostly act as regulators of synaptic transmission. These receptors consist of two domains: an extracellular domain in which agonists bind and a transmembrane heptahelix region involved in G protein activation. Although new mGlu receptor agonists and antagonists have been described, few are selective for a single mGlu subtype. Here, we have examined the effects of a novel compound, BAY36-7620 [(3aS,6aS)- 6a-Naphtalen-2-ylmethyl-5-methyliden-hexahydro-cyclopental[c]furan-1-on], on mGlu receptors (mGlu1-8), transiently expressed in human embryonic kidney 293 cells. BAY36-7620 is a potent (IC(50) = 0.16 microM) and selective antagonist at mGlu1 receptors and inhibits >60% of mGlu1a receptor constitutive activity (IC(50) = 0.38 microM). BAY36-7620 is therefore the first described mGlu1 receptor inverse agonist. To address the mechanism of action of BAY36-7620, Glu dose-response curves were performed in the presence of increasing concentrations of BAY36-7620. The results show that BAY36-7620 largely decreases the maximal effect of Glu. Moreover, BAY36-7620 did not displace the [(3)H]quisqualate binding from the Glu-binding pocket, further indicating that BAY36-7620 is a noncompetitive mGlu1 antagonist. Studies of chimeric receptors containing regions of mGlu1 and regions of DmGluA, mGlu2, or mGlu5, revealed that the transmembrane region of mGlu1 is necessary for activity of BAY36-7620. Transmembrane helices 4 to 7 are shown to play a critical role in the selectivity of BAY36-7620. This specific site of action of BAY36-7620 differs from that of competitive antagonists and indicates that the transmembrane region plays a pivotal role in the agonist-independent activity of this receptor. BAY36-7620 will be useful to further delineate the functional importance of the mGlu1 receptor, including its putative agonist-independent activity.