Stereoselectivity for the (R)‐Enantiomer of HA‐966 (l‐Hydroxy‐3‐Aminopyrrolidone‐2) at the Glycine Site of the N‐Methyl‐d‐Aspartate Receptor Complex

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Abstract
HA-966 (1-hydroxy-3-aminopyrrolidone-2) is an antagonist at the glycine allosteric site of the N-methyl-D-aspartate receptor ionophore complex. Unlike presently known glycine antagonists, HA-966 is chiral. We report stereoselectivity for the (R)-enantiomer at the glycine antagonist site. In [3H]glycine binding, the (R)-enantiomer has an IC50 of 4.1 .+-. 0.6 .mu.M. The racemic mixture has an IC50 of 11.2 .+-. 0.5 .mu.M, whereas (S)-HA-966 has an IC50 greater than 900 .mu.M. In glycine-stimulated [3H]1-[1-(2-thienyl)cyclohexyl]piperidine binding, the (R)-enantiomer inhibits with an IC50 of 121 .+-. 61 .mu.M, whereas the racemic mixture has an IC50 of 216 .+-. 113 .mu.M and (S)-HA-966 is inactive. The inhibition by (R)-HA-966 can be prevented by the addition of glycine. (R)-HA-966 and racemic HA-966, but not (S)-HA-966, also prevent N-methyl-D-aspartate cytotoxicity in cortical cultures. The (R)-enantiomer and, less potently, the (S)-enantiomer inhibit N-methyl-D-aspartate-evoked [3H]norepinephrine release from rat hippocampal slices (IC50 values of about 0.3 mM and 1.6 mM, respectively), but only the inhibition by (R)-HA-966 is reversed by added glycine. In glutamate-evoked contractions of the guinea pig ileum, (R)-HA-966 causes a glycine-reversible inhibition (IC50 of about 150 .mu.M), whereas (S)-HA-966 is much less potent (IC50 of greater than 1 mM). These results demonstrate stereoselectivity of the glycine antagonist site of the N-methyl-D-aspartate receptor complex in a variety of tissues and assays. The stereoselectivity also confirms the specificity of N-methyl-D-aspartate receptors in glutamate-evoked contractions of the guinea pig ileum, and supports their similarity to central N-methyl-D-aspartate receptors.

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