Plasma was shipped frozen to the Coyle laboratory and analyzed via high-performance liquid chromatography (HPLC) as described (31 (link)). Briefly, amino acids were extracted using 10% trichloroacetic acid, derivatized for detection using o-phthaldialdehyde (Alfa Aesar, Ward Hill, MA) and N-tert-butyloxycarbonyl-l-cysteine (Novabiochem, Gibbstown, NJ), and resolved using a Grace Alltima C18 column (3 µm; 150 × 4.6 mm) and a binary gradient of 25 mmol/L sodium acetate (pH 6.5) and acetonitrile. The gradient progressed from 10 to 40% acetonitrile over 40 min. d-serine concentrations (nmol/mL) were calculated by comparing an internal standard, l-homocysteic acid (l-HCA), against standard samples run at the beginning of each analysis using the following formula: (peak height of l-HCA in standard sample/peak height of d-serine in standard sample) × (peak height of d-serine in plasma sample/peak height of l-HCA in plasma sample) × (amount of l-HCA in plasma sample in μmol/amount of plasma in mL).