AMPA Receptor Subunit GluR1 Synaptic Expression/Traffick

越来越多的数据表明，谷氨酸受体的AMPA亚型在调节突触的短期和长期可塑性方面发挥着重要作用。此外，现在清楚的是，可塑性的调节“在很大程度上”是通过调节AMPA受体亚单位的运输，以及它们在突触中的插入和移除来实现的。值得注意的是，现在已知这种贩运在很大程度上依赖于AMPA受体亚单位的磷酸化，这三条主要信号通路是已知的情绪稳定剂的靶标？PKC、PKA和MAPK级联。鉴于越来越多的数据表明严重的情绪障碍可能与细胞可塑性障碍有关，我们进行了目前的一系列研究，以确定两种临床有效但结构高度不同的抗躁狂药物Li和VPA是否调节AMPA受体亚单位GluR1的突触表达。慢性给予锂或丙戊酸盐(在治疗相关浓度)后，大鼠海马区突触体内GluR1的水平分别降低了40%和20%。在培养的海马神经元中，锂和VPA均显著下调GluR1的表面表达，最大幅度为40%，且呈剂量和时间依赖关系。用抗N端GluR1抗体进行表面染色证实了这一结果。GluR1和突触素的双重免疫染色显示，慢性处理后锂盐和丙戊酸处理的神经元的GluR1阳性突触数量减少。然而，在体外和体内，锂和丙戊酸盐处理后，GluR1和突触素的总蛋白水平保持不变。锂和丙戊酸处理显著降低特定PKA位点(GluRp845)的磷酸化水平，分别为52%和33%。SP-cAMP处理逆转了锂和丙戊酸对GluR1s磷酸化的抑制作用，并将GluR1s带回表面，提示GluRp845的磷酸化参与了GluR1s表面衰减的机制。与之形成鲜明对比的是，已知会导致躁狂的药物，如丙咪嗪，会增加体内海马区GluR1的突触表达。这些研究表明，谷氨酸介导的突触可塑性的调节可能在情绪障碍的治疗中发挥作用，并增加了更直接影响突触GluR1的药物可能代表这种毁灭性疾病的新疗法的可能性。

A growing body of data is showing that the AMPA subtype of glutamate receptors play a major role in regulating short- and long-term forms of synaptic plasticity. Furthermore, it is now clear that regulation of plasticity occurs "in large part" by regulating the trafficking of AMPA receptor subunits, and their insertion and removal from the synapse. Notably, this trafficking is now known to be dependent , in large part, upon AMPA receptor subunit phosphorylation by 3 major signaling pathways known to be targets for mood stablilizers ? the PKC, PKA and MAPK cascades. In view of the growing body of data suggesting that severe mood disorders may be associated with impairments of cellular plasticity, we undertook the present series of studies to determine if two clinically effective, but structurally highly dissimilar antimanic agents, lithium & VPA regulate synaptic expression of AMPA receptor subunit GluR1. Administration of chronic lithium or valproate (at therapeutically relevant concentrations) reduced rat hippocampal synaptosomal levels of GluR1 after by 40% and 20%, respectively. In cultured hippocampal neurons, both lithium and VPA also significantly down-regulated the surface expression of GluR1 ~ 40% maximumally, in a dose and time-dependent manner. Surface staining with an anti-N terminal GluR1 antibody confirmed the result. Double-immunostaining of GluR1 and synaptotagmin showed that the numbers of GluR1 positive synapses of lithium and valproate-treated neurons were attenuated after chronic treatment. However, total protein levels of GluR1, and synaptotagmin remained unchanged after lithium and valproate treatment in vitro and in vivo. Phosphorylation of a specific PKA site (GluRp845) was significantly attenuated by lithium and valproate treatment by 52 and 33% respectively. Sp-cAMP treatment reversed the attenuation of phosphorylation by lithium and valproate and also brought GluR1s back to the surface, suggesting that phosphorylation of GluRp845 is involved in the mechanism of GluR1 surface attenuation. In striking contrast, drugs, which are known to induce mania, such as imipramine increase the synaptic expression of GluR1 in vivo in hippocampus. These studies suggest that regulation of glutamatergically mediated synaptic plasticity may play a role in the treatment of mood disorders, and raises the possibility that agents more directly affecting synaptic GluR1 may represent novel therapies for this devastating illness.