Extracellular calcium modulates actions of orthosteric and allosteric ligands on metabotropic glutamate receptor 1α

Metabotropic glutamate receptor 1α (mGluR1α), a member of the family C G protein-coupled receptors, is emerging as a potential drug target for various disorders, including chronic neuronal degenerative diseases. In addition to being activated by glutamate, mGluR1α is also modulated by extracellular Ca(2+). However, the underlying mechanism is unknown. Moreover, it has long been challenging to develop receptor-specific agonists due to homologies within the mGluR family, and the Ca(2+)-binding site(s) on mGluR1α may provide an opportunity for receptor-selective targeting by therapeutics. In the present study, we show that our previously predicted Ca(2+)-binding site in the hinge region of mGluR1α is adjacent to the site where orthosteric agonists and antagonists bind on the extracellular domain of the receptor. Moreover, we found that extracellular Ca(2+) enhanced mGluR1α-mediated intracellular Ca(2+) responses evoked by the orthosteric agonist l-quisqualate. Conversely, extracellular Ca(2+) diminished the inhibitory effect of the mGluR1α orthosteric antagonist (S)-α-methyl-4-carboxyphenylglycine. In addition, selective positive (Ro 67-4853) and negative (7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester) allosteric modulators of mGluR1α potentiated and inhibited responses to extracellular Ca(2+), respectively, in a manner similar to their effects on the response of mGluR1α to glutamate. Mutations at residues predicted to be involved in Ca(2+) binding, including E325I, had significant effects on the modulation of responses to the orthosteric agonist l-quisqualate and the allosteric modulator Ro 67-4853 by extracellular Ca(2+). These studies reveal that binding of extracellular Ca(2+) to the predicted Ca(2+)-binding site in the extracellular domain of mGluR1α modulates not only glutamate-evoked signaling but also the actions of both orthosteric ligands and allosteric modulators on mGluR1α.