Analysis of agonist and antagonist activities of phenylglycine derivatives for different cloned metabotropic glutamate receptor subtypes

Abstract

The metabotropic glutamate receptors (mGluRs) consist of at least seven different subtypes and are coupled to intracellular signal transduction via G proteins. However, the lack of specific antagonists for the mGluRs limited the precise characterization of the role of the individual mGluRs. In this study, we investigated the agonist and antagonist activities of a series of phenylglycine derivatives for the mGluRs by examining their effects on the signal transduction of representative mGluR1, mGluR2, and mGluR4 subtypes expressed individually in Chinese hamster ovary cells. The phenylglycine derivatives examined included (S)- and (R)-forms of 3-hydroxyphenylglycine (3HPG), 4-carboxy-phenylglycine (4CPG). 4-carboxy-3-hydroxyphenylglycine (4C3HPG), 3-carboxy-4-hydroxyphenylglycine (3C4HPG), and (+)- and (-)-α- methyl-4-carboxyphenylglycine (αM4CPG). Among these 10 compounds, (S)-3HPG acted as an agonist for mGluR1, while (S)-4C3HPG, (S)-3C4HPG, and (S)-4CPG served as effective agonists for mGluR2. The rank order of agonist potencies for mGluR2 was L-glutamate > (S)-4C3HPG > (S)-3C4HPG > (S)-4CPG. No other phenylglycine derivatives showed any definite agonist activity on either mGluR1 or mGluR2. Among the phenylglycine derivatives with no mGluR1 agonist activity, (S)-4C3HPG, (S)-3C4HPG, (S)-4CPG, and (+)-αM4CPG effectively antagonized the action of L-glutamate on mGluR1. The rank order of antagonist potencies was (S)-4C3HPG ≥ (S)-4CPG ≥ (+)-αM4CPG > (S)-3C4HPG. The Schild plot analysis indicated that (RS)-4C3HPG, (S)-4CPG, and (+)-αM4CPG all act as competitive antagonists for mGluR1 with pA2 values of 4.38, 4.46, and 4.38, respectively. Furthermore, (+)-αM4CPG was an effective competitive antagonist for mGluR2 with a pA2 value of 4.29. None of the 10 phenylglycine derivatives showed any agonist or antagonist activity for mGluR4. This investigation demonstrated that (S)-4C3HPG, (S)-4CPG, and (S)-3C4HPG act both as agonists for mGluR2 and as antagonists for mGluR1, and more importantly, that (+)-αM4CPG serves purely as an antagonist for mGluR1 and mGluR2. The phenylglycine derivatives will be useful for investigating the functions of the mGluR family.