Structure-Activity Relationship of NMDA Receptor Ligands and Their Activities on the ERK Activation through Metabotropic Signaling Pathway

The N-methyl-D-aspartate receptor (NMDA-R) subunit GluN2B is abundantly expressed in brain regions critical for synaptic plasticity and cognitive processes. This study investigated the structure-activity relationships (SAR) of NMDA-R ligands using GluN2B as a molecular target. Thirty potential NMDA-R antagonists were categorized into two structural classes: 1-(1-phenylcyclohexyl) amines (series A) and α-amino-2-phenylcyclohexanone derivatives (series B). In series A compounds, the phenyl ring and R1 substituents were positioned at the carbon center of the cyclohexyl ring, with R2 substituents at the para- or meta-positions of the phenyl ring. SAR analysis revealed optimal binding affinity when R1 was carbonyl (C=O) and R2 was 4-methoxy (4-OMe). Series B compounds featured a cyclohexanone scaffold with NH-R1 at the α-position and a phenyl ring bearing R2 substituents at ortho-, meta-, or para-positions. Maximum binding affinity was achieved with R1 as hydrogen (H) and R2 as hydroxyl (OH). Compounds were assessed for GluN2B-mediated ERK activation to evaluate potential metabotropic signaling properties. Approximately 50% of the compounds demonstrated ERK activation through a non-ionotropic signaling cascade involving Src, phosphatidylinositol 3-kinase, and protein kinase C. This study elucidated key structural determinants for NMDA-R binding and characterized a novel metabotropic signaling pathway. Notably, our findings suggest that compounds acting as antagonists at the ionotropic site may simultaneously function as agonists through non-ionotropic mechanisms.