Enantioselective inhibition of d ‐serine transport by ( S )‐ketamine

Background and Purpose Patients with major depressive disorder receiving racemic ketamine, ( R , S )‐ketamine, experience transient increases in Clinician‐Administered Dissociative States Scale scores and a coincident drop in plasma d ‐serine levels. The results suggest that ( R , S )‐ketamine produces an immediate, concentration‐dependent pharmacological effect on d ‐serine plasma concentrations. One potential source of this effect is ( R , S )‐ketamine‐induced inhibition of the transporter ASCT2 , which regulates intracellular d ‐serine concentrations. In this study, we tested this hypothesis by examining the effect of ( S )‐ and ( R )‐ketamine on ASCT2 ‐mediated transport of d ‐serine in PC ‐12 and 1321N1 cells and primary neuronal cells in culture. Experimental Approach Intracellular and extracellular d ‐serine levels were determined using capillary electrophoresis–laser‐induced fluorescence and liquid chromatography–mass spectrometry respectively. Expression of ASCT2 , A sc‐1 and serine racemase was determined utilizing W estern blotting. Key Results ( S )‐Ketamine produced a concentration‐dependent increase in intracellular d ‐serine and reduced extracellular d ‐serine accumulation. In contrast, ( R )‐ketamine decreased both intracellular and extracellular d ‐serine levels. The ASCT2 inhibitor, benzyl‐ d ‐serine ( BDS ), and ASCT2 gene knockdown mimicked the action of ( S )‐ketamine on d ‐serine in PC ‐12 cells, while the A sc‐1 agonist d ‐isoleucine reduced intracellular d ‐serine and increased extracellular d ‐serine accumulation. This response to d ‐isoleucine was not affected by BDS or ( S )‐ketamine. Primary cultures of rat neuronal cells expressed ASCT2 and were responsive to ( S )‐ketamine and BDS . ( S )‐ and ( R )‐ketamine increased the expression of monomeric serine racemase in all the cells studied, with ( S )‐ketamine having the greatest effect. Conclusions and Implications ( S )‐Ketamine decreased cellular export of d ‐serine via selective inhibition of ASCT2 , and this could represent a possible source of dissociative effects observed with ( R , S )‐ketamine.