Glutamate receptor genes as candidates for schizophrenia susceptibility

Influence of GRIA1, GRIA2 and GRIA4 polymorphisms on diagnosis and response to antipsychotic treatment in patients with schizophrenia

The present study is aimed at exploring whether some single nucleotide polymorphisms (SNPs) within GRIA1, GRIA2 and GRIA4 could be associated with schizophrenia and whether they could predict clinical outcomes in Korean in-patients treated with antipsychotics. One hundred forty five patients with MD, 221 in-patients with schizophrenia and 170 psychiatrically healthy controls were genotyped for 17 SNPs within GRIA1, GRIA2 and GRIA4. Baseline and final clinical measures, including the Positive and Negative Symptoms Scale (PANSS), were recorded. No significant association was found with the diagnosis of schizophrenia. We observed an association between rs3813296 genotype and improvement on PANSS negative scores. Our findings provide no evidence for an association between SNPs within GRIA1, GRIA2 and GRIA4 under investigation and schizophrenia susceptibility, although rs3813296 (GRIA2) could be associated with improvement on PANSS negative scores. However, taking into account the several limitations of our study, further research is needed to draw more definitive conclusions.

Comprehensive behavioural study of GluR4 knockout mice: implication in cognitive function

Fast excitatory transmission in the mammalian central nervous system is mediated by AMPA-type glutamate receptors. The tetrameric AMPA receptor complexes are composed of four subunits, GluR1-4. The GluR4 subunit is highly expressed in the cerebellum and the early postnatal hippocampus and is thought to be involved in synaptic plasticity and the development of functional neural circuitry through the recruitment of other AMPA receptor subunits. Previously, we reported an association of the human GluR4 gene (GRIA4) with schizophrenia. To examine the role of the GluR4 subunit in the higher brain function, we generated GluR4 knockout mice and conducted electrophysiological and behavioural analyses. The mutant mice showed normal long-term potentiation (LTP) in the CA1 region of the hippocampus. The GluR4 knockout mice showed mildly improved spatial working memory in the T-maze test. Although the retention of spatial reference memory was intact in the mutant mice, the acquisition of spatial reference memory was impaired in the Barnes circular maze test. The GluR4 knockout mice showed impaired prepulse inhibition. These results suggest the involvement of the GluR4 subunit in cognitive function.

Study on GRIA2, GRIA3 and GRIA4 genes highlights a positive association between schizophrenia and GRIA3 in female patients

Impairment of glutamatergic neurotransmission is one of the major hypotheses proposed to explain the neurobiology of schizophrenia. Therefore, the genes involved in the glutamate neurotransmitter system could be considered potential candidate genes for schizophrenia susceptibility. A systematic study on alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor genes has been carried out and the results obtained from the analysis on GRIA2, GRIA3 and GRIA4 are reported. No evidence of association with schizophrenia was found for the GRIA2 and GRIA4 genes; strong evidence of association with schizophrenia was found for GRIA3. This X-linked gene showed a different behavior in the two genders; a positive association with schizophrenia was observed among females but not in males. Female carriers of rs1034428 A allele were found to have a 2.19-fold higher risk of developing schizophrenia compared to non-carriers and 3.28-fold higher risk for developing a non-paranoid phenotype. The analysis at the haplotype level showed that susceptibility to schizophrenia was associated with the specific haplotype rs989638-rs1034428-rs2227098 CAC (P = 0.0008). We conclude that, of the three AMPA genes analyzed here, only GRIA3 seems to be involved in the pathogenesis of schizophrenia, but only in females.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.