d-serine and serine racemase inhibition experiments in vivo

d-serine (250 mg kg⁻¹) dissolved in saline (n = 6, d-serine) or saline alone (n = 6, control) was administered in a single intraperitoneal (i.p.) injection of 0.3 ml. In another set of experiments, a single injection of 0.3 ml of saline (n = 6, controls) or saline containing 9 mg kg⁻¹ MET-phen (n = 12) was administered.

MET-phen and ET-phen have been used previously as d-serine racemase inhibitors: (1) in neonatal mice, 3 mg kg⁻¹ per day MET-phen i.p. injections for 3 days reduced the d-serine levels 24 h later to 86.4% of the control levels in the prefrontal cortex and 81.2% in the cerebellum⁵⁶; (2) in the tiger salamander retina in vitro, 10 µM ET-phen reduced the d-serine content by 50%, with no significant changes in the l-serine levels⁵⁷; (3) in primary cultured mouse spinal cord cells, 1 µM MET-phen reduced the serine racemase (l-ser β-elimination) activity to 64.5%⁵⁸; l-ser β-elimination is a secondary enzymatic reaction in which pyruvate is synthesized from l-serine. This reaction has a similar K _m value and the same enzymatic active site as the reaction for the isomerization of l-serine to d-serine⁵⁹. MET-phen and ET-phen have been used to inhibit granule cell migration along cerebellar Bergmann glia; the progression of this process depends on the d-serine levels⁶⁰. MET-phen and ET-phen demonstrated an IC₅₀ of 3 and 5 µM, respectively, in the reduction of granule cell migration; the reduction induced by 50 µM ET-phen was partially compensated by d-serine⁶⁰.