Modulating NMDA Receptor Function with D-Amino Acid Oxidase Inhibitors: Understanding Functional Activity in...

Deficits in N-methyl-D-aspartate receptor (NMDAR) function are increasingly linked to persistent negative symptoms and cognitive deficits in schizophrenia. Accordingly, clinical studies have been targeting the modulatory site of the NMDA receptor, based on the decreased function of NMDA receptor, to see whether increasing NMDA function can potentially help treat the negative and cognitive deficits seen in the disease. Glycine and D-serine are endogenous ligands to the NMDA modulatory site, but since high doses are needed to affect brain levels, related compounds are being developed, for example glycine transport (GlyT) inhibitors to potentially elevate brain glycine or targeting enzymes, such as D-amino acid oxidase (DAAO) to slow the breakdown and increase the brain level of D-serine. In the present study we further evaluated the effect of DAAO inhibitors 5-chloro-benzo[d]isoxazol-3-ol (CBIO) and sodium benzoate (NaB) in a phencyclidine (PCP) rodent mouse model to see if the inhibitors affect PCP-induced locomotor activity, alter brain D-serine level, and thereby potentially enhance D-serine responses. D-Serine dose-dependently reduced the PCP-induced locomotor activity at doses above 1000 mg/kg. Acute CBIO (30 mg/kg) did not affect PCP-induced locomotor activity, but appeared to reduce locomotor activity when given with D-serine (600 mg/kg); a dose that by itself did not have an effect. However, the effect was also present when the vehicle (Trappsol(®)) was tested with D-serine, suggesting that the reduction in locomotor activity was not related to DAAO inhibition, but possibly reflected enhanced bioavailability of D-serine across the blood brain barrier related to the vehicle. With this acute dose of CBIO, D-serine level in brain and plasma were not increased. Another weaker DAAO inhibitor NaB (400 mg/kg), and NaB plus D-serine also significantly reduced PCP-induced locomotor activity, but without affecting plasma or brain D-serine level, arguing against a DAAO-mediated effect. However, NaB reduced plasma L-serine and based on reports that NaB also elevates various plasma metabolites, for example aminoisobutyric acid (AIB), a potential effect via the System A amino acid carrier may be involved in the regulation of synaptic glycine level to modulate NMDAR function needs to be investigated. Acute ascorbic acid (300 mg/kg) also inhibited PCP-induced locomotor activity, which was further attenuated in the presence of D-serine (600 mg/kg). Ascorbic acid may have an action at the dopamine membrane carrier and/or altering redox mechanisms that modulate NMDARs, but this needs to be further investigated. The findings support an effect of D-serine on PCP-induced hyperactivity. They also offer suggestions on an interaction of NaB via an unknown mechanism, other than DAAO inhibition, perhaps through metabolomic changes, and find unexpected synergy between D-serine and ascorbic acid that supports combined NMDA glycine- and redox-site intervention. Although mechanisms of these specific agents need to be determined, overall it supports continued glutamatergic drug development.