d-Serine and Serine Racemase are Localized to Neurons in the Adult Mouse and Human Forebrain

d-Serine, a co-agonist at the NMDA receptor (NMDAR), is synthesized from l-serine by the enzyme serine racemase (SR), which is heavily expressed in the forebrain. Although SR was originally reported to be localized exclusively to astrocytes, recent conditional knock out results demonstrate that little SR is expressed in forebrain astrocytes. As a consequence, the cellular location of its product, d-serine, in the brain is also uncertain. Immunocytochemistry now indicates that SR is expressed primarily in forebrain glutamatergic neurons with the remainder in GABAergic interneurons. We utilized SR deficient (SR–/–) mice, which have