Fritschy Jean-Marc, Panzanelli Patrizia, Kralic Jason E, Vogt Kaspar E, Sassoè-Pognetto Marco
Institute of Pharmacology and Toxicology, University of Zurich, CH-8057 Zurich, Switzerland.
J Neurosci. 2006 Mar 22;26(12):3245-55. doi: 10.1523/JNEUROSCI.5118-05.2006.
Synapse formation and maintenance require extensive transsynaptic interactions involving multiple signal transduction pathways. In the cerebellum, Purkinje cells (PCs) receive GABAergic, axo-dendritic synapses from stellate cells and axo-somatic synapses from basket cells, both with GABAA receptors containing the alpha1 subunit. Here, we investigated the effects of a targeted deletion of the alpha1 subunit gene on GABAergic synaptogenesis in PCs, using electrophysiology and immunoelectron microscopy. Whole-cell patch-clamp recordings in acute slices revealed that PCs from alpha1(0/0) mice lack spontaneous and evoked IPSCs, demonstrating that assembly of functional GABAA receptors requires the alpha1 subunit. Ultrastructurally, stellate cell synapses on PC dendrites were reduced by 75%, whereas basket cell synapses on the soma were not affected, despite the lack of GABAA-mediated synaptic transmission. Most strikingly, GABAergic terminals were retained in the molecular layer of adult alpha1(0/0) mice and formed heterologous synapses with PC spines characterized by a well differentiated asymmetric postsynaptic density. These synapses lacked presynaptic glutamatergic markers and postsynaptic AMPA-type glutamate receptors but contained delta2-glutamate receptors. During postnatal development, initial steps of GABAergic synapse formation were qualitatively normal, and heterologous synapses appeared in parallel with maturation of dendritic spines. These results suggest that synapse formation in the cerebellum is governed by neurotransmitter-independent mechanisms. However, in the absence of GABAA-mediated transmission, GABAergic terminals in the molecular layer apparently become responsive to synaptogenic signals from PC spines and form stable heterologous synapses. In contrast, maintenance of axo-somatic GABAergic synapses does not depend on functional GABAA receptors, suggesting differential regulation in distinct subcellular compartments.
突触的形成和维持需要广泛的跨突触相互作用，涉及多个信号转导途径。在小脑中，浦肯野细胞（PCs）接收来自星状细胞的GABA能轴突-树突突触和来自篮状细胞的轴突-胞体突触，两者都含有α1亚基的GABAA受体。在这里，我们使用电生理学和免疫电子显微镜研究了α1亚基基因靶向缺失对PCs中GABA能突触发生的影响。急性脑片的全细胞膜片钳记录显示，α1(0/0)小鼠的PCs缺乏自发和诱发的抑制性突触后电流（IPSCs），表明功能性GABAA受体的组装需要α1亚基。超微结构上，PC树突上的星状细胞突触减少了75%，而胞体上的篮状细胞突触不受影响，尽管缺乏GABAA介导的突触传递。最引人注目的是，GABA能终末保留在成年α1(0/0)小鼠的分子层中，并与PC棘形成异源突触，其特征是具有高度分化的不对称突触后致密物。这些突触缺乏突触前谷氨酸能标记物和突触后AMPA型谷氨酸受体，但含有δ2-谷氨酸受体。在出生后发育过程中，GABA能突触形成的初始步骤在质量上是正常的，异源突触与树突棘的成熟同时出现。这些结果表明，小脑突触的形成受神经递质非依赖性机制的控制。然而，在没有GABAA介导的传递的情况下，分子层中的GABA能终末显然对来自PC棘的突触发生信号有反应，并形成稳定的异源突触。相比之下，轴突-胞体GABA能突触的维持不依赖于功能性GABAA受体，这表明在不同的亚细胞区室中存在差异调节。
