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はじめに
Dopamine- and cyclic AMP-regulated phosphoprotein, relative molecular mass 32,000 (DARPP-32)は,protein phosphataseのinhibitor Iのホモログであり,ドパミンの入力を受ける神経細胞に多く発現する1)。脳内では,尾状核,被殻,側坐核,嗅球などに多く認められるが,大脳皮質では前頭前野,帯状回などに多い2)。この前頭前野では,ドパミン,グルタミン酸,γ-aminobutyric acid(GABA)神経伝達系がネットワークを形成する。そのネットワークの機能調整にDARPP-32は重要な役割を担っていると考えられる。
前頭前野の機能障害を示す精神疾患に,統合失調症と双極性障害がある。この両疾患の病因はいまだ明らかではなく,内因性精神病と呼ばれている。最近のPET3,4)や剖検脳を用いた検討から,両疾患の前頭前野では,ドパミン,グルタミン酸,GABA神経伝達系の障害が報告されている5-9)。
したがって,統合失調症や双極性障害の前頭前野の障害とDARPP-32との関連について検討することは,極めて重要と考えられる。そこで本稿では,統合失調症や気分障害の病態とDARPP-32の変化との関連について概観する。
Abstract
Dopamine- and cyclic AMP-regulated phosphoprotein with a relative molecular weight of 32 kDa (DARPP-32) plays an important role in integrating information of about several neurotransmitters arriving at dopaminoceptive neurons. DARPP-32 is phosphorylated by dopamine D1 receptor at threonine 34 and converted to an inhibitor of protein phosphatase I. It facilitates the phosphorylation of several neurotransmitter receptors, including N-methyl-D-aspartic acid (NMDA)- and α-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)-type glutamate receptors and γ-aminobutyric acid (GABA)A receptors. In contrast, D2 receptor stimulation induces dephosphorylation of DARPP-32, which results in dephosphorylation of the glutamate and GABAA receptors. Thus, phosphorylation and dephosphorylation of DARPP-32 regulates the functions of neurotransmitter systems.
Recent studies from our laboratory and elsewhere have demonstrated that the amount of DARPP-32 in the dorsolateral prefrontal cortex (DLPFC) of subjects with schizophrenia is lower than that in the DLPFC of control subjects. Thus, it is plausible that DARPP-32 is associated with the concurrent alterations in dopamine, glutamate, and GABA neurotransmitter systems in subjects with schizophrenia. We have also found reduced levels of DARPP-32 in the DLPFC of subjects with bipolar disorder. Thus, it is important to elucidate the role of DARPP-32 in the pathophysiology of schizophrenia and bipolar disorder.
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