COUPLING OF METABOTROPIC GLUTAMATE RECEPTOR-2 AND RECEPTOR-4 TO G(ALPHA-15), G(ALPHA-16), AND CHIMERIC G(ALPHA-Q I) PROTEINS - CHARACTERIZATION OF NEW ANTAGONISTS/

Together with the calcium-sensing receptor, the metabotropic glutamate receptors (mGluRs) share no sequence homology with the other G protei n-coupled receptors (GPCRs) and therefore constitute a new family of r eceptors. Recently, it was reported that G(alpha 15) and G(alpha 16) s ubunits allow many GPCRs to activate phospholipase C (PLC). Furthermor e, the exchange of a few carboxyl-terminal residues of G(alpha q) by t hose of G(alpha i2) of G(alpha o) allows the resulting chimeric G(alph a) subunits (G(alpha qi) and G(alpha qo), respectively) to couple G(i) -coupled receptors to PLC. We report that mGluR2 and mGluR4, two recep tors negatively coupled to adenylyl cyclase, activate PLC when coexpre ssed with G(alpha 15), G(alpha qi), or G(alpha qo). This indicates tha t the carboxyl-terminal end of the G(alpha) subunit also plays an impo rtant role in the specific interaction between mGluRs and the G protei ns. In addition, the measurement of PLC activation by G(i)-coupled mGl uRs coexpressed with these G(alpha>) subunits constitutes an easy func tional assay for the pharmacological characterization of these recepto rs, The rank order of potency of antagonists was found to be (2S,3S,4S )-2-methyl-2-(carboxycyclopropyl)glycine approximate to (R,S)-alpha-me thyl-4-phosphonophenylglycine > (R,S)-alpha-methyl-4-sulfonophenylglyc ine > (R,S)-alpha-methyl-4-tetrazolylphenylglycine = (S)-2-amino-2-met hyl-4-phosphonobutyrate for mGluR2 and to be (R,S)-alpha-methyl-4-phos phonophenylglycine greater than or equal to (S)-2-amino-2-methyl-4-pho sphonobutyrate >> (R,S)-alpha-methyl-4-sulfonophenylglycine [(R,S)-alp ha-methyl-4-tetrazolylphenylglycine and (2S,3S,4S)-2-methyl-2-(carboxy cyclopropyl)glycine being inactive at 1 mM] for mGluR4. Using this fun ctional assay, (R,S)-alpha-methyl-4-phosphonophenylglycine was found t o have a similar K-B value for mGluR2 and mGluR4.