Factors regulating serine racemase and d-amino acid oxidase expression in the mouse striatum.

d-Serine plays an important role in modulating N-methyl-d-aspartate receptor (NMDAR) neurotransmission in the mammalian brain by binding to the receptor's glycine modulatory site (GMS). The cytosolic enzyme serine racemase (SR) converts L-serine to d-serine, while the peroxisomal enzyme d-amino acid oxidase (DAAO) catalyzes the breakdown of d-serine. Although it is important to understand how the activities of SR and DAAO regulate d-serine levels, very little is known about the mechanisms that regulate the expression of SR and DAAO. In this study, we investigated whether the different centrally active drugs affect the expression of SR and DAAO in adult mouse brain. We found that the NMDAR antagonist, MK801, and cocaine, psychotropic drugs that both augment glutamate release, reduce the expression of SR and DAAO. This regulation is brain region selective, and in the case of cocaine, is reversed in part byα-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX). However, d-serine and antipsychotics do not regulate SR and DAAO protein levels. In a genetic model of SR disruption, we found that DAAO expression was unaltered in SR conditional knockout mice, in which tissue d-serine content remains fairly stable despite marked reduction in SR expression. This study reveals a new mechanism by which AMPAR activity could regulate NMDAR function via d-serine availability.