Pharmacological and molecular characterization of the positive allosteric modulators of metabotropic glutamate receptor 2.

The metabotropic glutamate receptor 2 (mGlu ₂ ) plays an important role in the presynaptic control of glutamate release and several mGlu ₂ positive allosteric modulators (PAMs) have been under assessment for their potential as antipsychotics. The binding mode of mGlu ₂ PAMs is better characterized in functional terms while few data are available on the relationship between allosteric and orthosteric binding sites. Pharmacological studies characterizing binding and effects of two different chemical series of mGlu ₂ PAMs are therefore carried out here using the radiolabeled mGlu ₂ agonist ³ [H]-LY354740 and mGlu ₂ PAM ³ [H]-2,2,2-TEMPS. A multidimensional approach to the PAM mechanism of action shows that mGlu ₂ PAMs increase the affinity of ³ [H]-LY354740 for the orthosteric site of mGlu2 as well as the number of ³ [H]-LY354740 binding sites. ³ [H]-2,2,2-TEMPS binding is also enhanced by the presence of LY354740. New residues in the allosteric rat mGlu2 binding pocket are identified to be crucial for the PAMs ligand binding, among these Tyr ^3.40 and Asn ^5.46 . Also of remark, in the described experimental conditions S731A (Ser ^5.42 ) residue is important only for the mGlu ₂ PAM LY487379 and not for the compound PAM-1: an example of the structural differences among these mGlu ₂ PAMs. This study provides a summary of the information generated in the past decade on mGlu ₂ PAMs adding a detailed molecular investigation of PAM binding mode. Differences among mGlu ₂ PAM compounds are discussed as well as the mGlu2 regions interacting with mGlu ₂ PAM and NAM agents and residues driving mGlu2 PAM selectivity.
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