Schizophrenia is believed to involve altered activation of dopamine receptors, and support for this hypothesis comes from the antipsychotic effect of antagonists of the dopamine D2 receptor (D2R). D2R is expressed most highly in the striatum, but most of the recent positron emission tomography (PET) studies have failed to show any change in D2R densities in the striatum of schizophrenics, raising the possibility that other receptors may also be involved. In particular, the dopamine D1 receptor (D1R), which is highly expressed in the prefrontal cortex, has been implicated in the control of working memory, and working memory dysfunction is a prominent feature of schizophrenia. We have therefore used PET to examine the distribution of D1R and D2R in brains of drug-naive or drug-free schizophrenic patients. Although no differences were observed in the striatum relative to control subjects, binding of radioligand to D1R was reduced in the prefrontal cortex of schizophrenics. This reduction was related to the severity of the negative symptoms (for instance, emotional withdrawal) and to poor performance in the Wisconsin Card Sorting Test. We propose that dysfunction of D1R signalling in the prefrontal cortex may contribute to the negative symptoms and cognitive deficits seen in schizophrenia.

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Patients with methylenetetrahydrofolate reductase (MTHFR) deficiency often show psychiatric manifestations. Since a common variant of the MTHFR gene, T677(Ala), responsible for the thermolabile MTHFR with less than 50% specific MTHFR activity, has been reported, we examined whether the T677 allele is associated with psychiatric disorders in an unrelated Japanese population consisting of 297 schizophrenics, 32 patients with major depression, 40 patients with bipolar disorder, and 419 controls. The genotype homozygous for the T677 allele was significantly frequently observed in schizophrenics with an odds ratio of 1.9 (P = 0.0006), and in patients with major depression with an odds ratio of 2.8 (P = 0.005). Our data suggest associations of the MTHFR gene variant with schizophrenia and depression in the Japanese.

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