Development of an anatomical technique for visualizing the mode of climbing fiber innervation in Purkinje cells and its application to mutant mice lacking GluRδ2 and Ca(v)2.1.

In the adult cerebellum, a single climbing fiber (CF) innervates proximal dendrites of Purkinje cells (PCs). This monoinnervation is established by the developmental elimination of surplus CFs through homosynaptic competition among multiply innervating CFs and heterosynaptic competition between CFs and parallel fibers, i.e., granule cell axons innervating distal PC dendrites. Although the developmental process of CF monoinnervation and defects in it in mutant and experimental animal models have been extensively studied by electrophysiological techniques, for quite some time this subject was poorly understood from a morphological perspective due to a lack of neuroanatomical methods that could distinguish CFs with different neuronal origins. Soon after the identification of type 2 vesicular glutamate transporter (VGluT2) that selectively detects CF terminals in the molecular layer, we developed a novel method of combined anterograde tracer labeling and VGluT2 immunohistochemistry. This method enables us to identify the mode (mono vs. multiple) of CF innervation and the site of multiple innervation. Since then, we have applied this method to various kinds of gene-manipulated mice manifesting ataxia and other cerebellar phenotypes. In this review, we summarize experimental procedures for the combined tracer/VGluT2 labeling method, and then introduce what we have learned by applying this method in studies on the role of GluRδ2 and Ca(v)2.1 in CF monoinnervation. This method has provided informative anatomical correlates to electrophysiological data and vice versa, and will extend our knowledge of the molecular and cellular mechanisms for the development, plasticity, degeneration, and repair of the CF-PC projection system.