Potentiation of the NMDA receptor in the treatment of schizophrenia: focused on the glycine site.

N-methyl-D-aspartate receptor (NMDAR) hypo-function theory of schizophrenia proposes that impairment in NMDAR function be associated with the pathophysiology of schizophrenia and suggests that enhancement of the receptor function may produce efficacy for schizophrenia. Consistent with this theory, for the last decade, clinical trials have demonstrated that the enhancement of NMDAR function by potentiating the glycine site of the receptor is efficacious in the treatment of schizophrenia. Full agonists of the glycine site, glycine and D-serine and a glycine transporter-1 inhibitor, sarcosine, added to antipsychotic drugs, have been shown to be effective in the treatment of negative symptoms and possibly cognitive symptoms without significantly affecting the positive symptoms of schizophrenia. A partial agonist of the glycine site, D-cycloserine, added to antipsychotic drugs, can be effective for the negative symptoms at the therapeutic doses. However, these drugs have not shown clinical efficacy when added to clozapine, suggesting that the interactions of clozapine and the glycine site potentiators may be different from those of other antipsychotic drugs and the potentiators. This article suggests that the glycine site potentiators may produce efficacy for negative and cognitive symptoms by blocking apoptosis-like neuropathological processes in patients with chronic schizophrenia and thereby can deter progressive deterioration of the disorder. This article proposes a polypharmacy of glycine site potentiators augmented with antipsychotic drugs to control positive and negative symptoms in a synergistic manner and block deterioration in schizophrenia. Since the NMDAR complex consists of multiple sites modulating receptor functions, the efficacy of glycine site potentiators for schizophrenia suggests the possibility that manipulation of other modulating sites of the NMDAR can also be efficacious in the treatment of schizophrenia.