Molecular Mechanisms for Activity-dependent Elimination of Supernumerary Excitatory Synapses in Developing Cerebellum

小脑浦肯野细胞突触的攀升纤维是研究突触消除的细胞和分子机制的一个很好的模型，通过它可以细化发育早期形成的多余连接。在啮齿动物出生后的早期，大多数浦肯野细胞被多个攀爬纤维支配。然后，多余的攀爬纤维被清除，直到大约出生后第21天，攀爬纤维与浦肯野细胞之间达到一对一的关系(P21)。这种关系在一生中一直保持着。这一过程已被证明依赖于涉及NMDA受体的神经活动(Rabacchi等人，1991年)。在本研究中，我们发现持续局部应用河豚毒素或NMDA受体拮抗剂MK-801在发育中的小鼠小脑中可在约40%的浦肯野细胞中产生持续性的多条爬行纤维神经。我们还证明，阻断Nmda受体介导的…神经活动小脑在P15-P16期间较多，但在此期间之前和之后均未引起持续性多条爬行纤维神经支配。以及运动不协调。阻断NMDA受体并未引起小脑形态和基本突触特性的明显改变。我们的结果表明，依赖NMDA受体的爬行纤维突触在这一关键时期得以消除，其破坏会导致小脑功能的持续性损害。利用基因缺失技术，我们已经证明了代谢型谷氨酸受体亚型L(MGluR1)、α亚单位Gq(GαQ)、磷脂酶Cβ4(β4)和蛋白激酶Cγ(PKCγ)参与的信号转导是在出生后第3周攀升纤维突触消除所必需的，这与本研究揭示的临界期相吻合。我们假设沿着苔藓纤维-颗粒细胞-平行纤维通路的神经活动激活了浦肯野细胞中mGluR1和随后的级联，这是消除多余攀爬纤维所必需的。较少

The climbing fiber to Purkinje cell synapse in the cerebellum has been a good model to study cellular and molecular mechanisms of synapse elimination by which redundant connections formed earlier during development are refined. In early postnatal days of rodents' life, most Purkinje cells are innervated by multiple climbing fibers. Then, elimination of supernumerary climbing fibers occurs until the one-to-one relations between climbing fibers and Purkinje cells are attained at approximately postnatal day 21 (P21). This relationship is maintained throughout life. This process has been shown to depend on neural activity involving NMDA receptors (Rabacchi et al., 1991). In the present study, we found that continuous and local application of tetrodotoxin or an NMDA receptor antagonist, MK-801 to developing mouse cerebella resulted in persistent multiple climbing fiber-innervation in about 40% of Purkinje cells. We also demonstrated that blockade of NMDA receptor-mediated neural activity in … More the cerebellum during P15-P16, but not before nor after this period, caused persistent multiple climbing fiber innervation. as well as motor discoordination. The NMDA receptor blockade did not cause apparent change in cerebellar morphology and basic synaptic properties. Our results suggest that the NMDA receptor-dependent climbing fiber synapse elimination is achieved during this critical period, and its disruption leads to persistent impairment of cerebellar function. By using gene deletion technique in mice, we have demonstrated previously that the signal transduction involving metabotropic glutamate receptor subtype l (mGluR1), the α subunit Gq (Gαq), phospholipase Cβ4 (PLCβ4) and protein kinase Cγ(PKCγ) is required for climbing fiber synapse elimination during the third postnatal week that coincide with the critical period revealed in the present study. We assume that neural activity along the mossy fiber-granule cell-parallel fiber pathway activates mGluR1 and the following cascade in Purkinje cells that is required for elimination of supernumerary climbing fibers. Less