Pharmacology of the inhibitory glycine receptor: agonist and antagonist actions of amino acids and piperidine carboxylic acid compounds

To define structure-activity relations for ligands binding to the inhibitory glycine receptor (GlyR), the agonistic and antagonistic properties of alpha- and beta-amino acids were analyzed at the recombinant human alpha 1 GlyR expressed in Xenopus oocytes. The agonistic activity of alpha-amino acids exhibited a marked stereoselectivity and was highly susceptible to substitutions at the C alpha-atom. In contrast, alpha-amino acid antagonism was not enantiomer dependent and was influenced little by C alpha-atom substitutions. The beta-amino acids taurine, beta-aminobutyric acid (beta-ABA), and beta-aminoisobutyric acid (beta-AIBA) are partial agonists at the GlyR. Low concentrations of these compounds competitively inhibited glycine responses, whereas higher concentrations elicited a significant membrane current. Nipecotic acid, which contains a trans-beta-amino acid configuration, behaved as purely competitive GlyR antagonist. Our data are consistent with the existence of a common binding site for all amino acid agonists and antagonists, at which the functional consequences of binding depend on the particular conformation a given ligand adopts within the binding pocket. In the case of beta-amino acids, the trans conformation appears to mediate antagonistic receptor binding, and the cis conformation appears to mediate agonistic receptor binding. This led us to propose that the partial agonist activity of a given beta-amino acid is determined by the relative mole fractions of the respective cis/trans conformers.