外周输入依赖的嗅球颗粒细胞的突触结构可塑性

Peripheral Input-dependent Structural Plasticity of Granule Cells in The Mouse Olfactory Bulb

活动依赖的突触结构可塑性是学习和记忆的基础.哺乳动物,尤其是啮齿类动物,具有高度发达的嗅觉系统和惊人的气味学习和记忆能力.本研究以CNGA2敲除而导致外周输入缺失的小鼠为模型,研究嗅球内活动依赖的突触结构可塑性.利用特异性的突触前和突触后标记物,发现外周输入缺失减少了突触标记蛋白突触素(synaptophysin)和抑制性突触标记蛋白桥蛋白(gephyrin)在嗅球外网状层和颗粒细胞层中的表达;兴奋性突触标记蛋白囊泡谷氨酸转运蛋白1(VGluT1)的表达水平只在外网状层中有显著下降,而在颗粒细胞层中没有明显变化.进一步通过活体质粒电转标记嗅球颗粒细胞后发现,CNGA2敲除小鼠颗粒细胞上位于外网状层中的远端树突棘密度显著减小,而位于颗粒细胞层中的近端树突棘密度没有明显变化.这些结果表明颗粒细胞上的树-树突触具有对外周活动依赖的结构可塑性,而轴-树突触则无.

Activity-dependent synaptic structural plasticity underlies the leaming and memory. Mammals, especially the rodents, are very sensitive to odorants, and have considerable capability of odor leaming and memory. Here, the activity-dependent synaptic structural plasticity in the olfactory bulb （OB） of the CNGA2 knock-out transgenic mice （CNGA2 KO）, which is anosmic, was investigated. Using immunohistochemistry for specific presynaptic and postsynaptic markers, it was found that deficits of peripheral inputs induced significant decreases in the expression of synaptophysin, a general marker for synapses, and gephyrin, a marker for inhibitory synapses, in the external plexiform layer （EPL） and the granule cell layer （GCL）, but the vesicular glutamate transporters 1 （VGluT1） decreased only in EPL, not in GCL. Western-blots showed the decreases in the expression of gephyrin in the OB of CNGA2 KO mice, but not in the expression of the VGluT1. The results of immunohistochemistry and Westem blot revealed that the excitatory and inhibitory synapses may have changed after deficits of peripheral inputs. GCs were the main participants in the EPL and GCL in the OB. Dendritic spines are the postsynaptic sites of the majority of excitatory synapses in the mammalian central nervous system, and the morphology and dynamics of dendritic spines change in response to novel experiences and neuropathologies. In the OB, spines on mature GCs are recipients of glutamatergic synapses in the GCL and reciprocal synapses in the EPL. Almost all study related to structural plasticity of GCs concentrated on the adult-born GCs, but the number of new-bom granule cells in the OB is negligible compared with the number of preexisting GCs. In order to further reveal the quantitive changes ofglutamatergic synapses on GCs, in vivo adult brain plasmid electroporation to label mature GCs in the OB directly were adopted. Here, the spines in EPL were defined as distal spines and the spines in GCL as proximal spines. It was revealed that the density of spines on granule cells decreased significantly in EPL （distal spines） of CNGA2 KO mice, but did not change significantly in GCL （proximal spines）, as same as the result of optical density analysis in the VGluT 1 immunolabeling. These data suggest that the structural plasticity of the distal dendrodendritic synapses, rather than the proximate axon-dendritic synapses on granular ceils of the OB, are significantly affected by the peripheral olfactory inputs.