Fletcher Max L, Masurkar Arjun V, Xing Junling, Imamura Fumiaki, Xiong Wenhui, Nagayama Shin, Mutoh Hiroki, Greer Charles A, Knöpfel Thomas, Chen Wei R
Department of Neurobiology, Yale University School of Medicine, New Haven, Connecticut, USA.
J Neurophysiol. 2009 Aug;102(2):817-30. doi: 10.1152/jn.00020.2009. Epub 2009 May 27.
Olfactory glomeruli are the loci where the first odor-representation map emerges. The glomerular layer comprises exquisite local synaptic circuits for the processing of olfactory coding patterns immediately after their emergence. To understand how an odor map is transferred from afferent terminals to postsynaptic dendrites, it is essential to directly monitor the odor-evoked glomerular postsynaptic activity patterns. Here we report the use of a transgenic mouse expressing a Ca(2+)-sensitive green fluorescence protein (GCaMP2) under a Kv3.1 potassium-channel promoter. Immunostaining revealed that GCaMP2 was specifically expressed in mitral and tufted cells and a subpopulation of juxtaglomerular cells but not in olfactory nerve terminals. Both in vitro and in vivo imaging combined with glutamate receptor pharmacology confirmed that odor maps reported by GCaMP2 were of a postsynaptic origin. These mice thus provided an unprecedented opportunity to analyze the spatial activity pattern reflecting purely postsynaptic olfactory codes. The odor-evoked GCaMP2 signal had both focal and diffuse spatial components. The focalized hot spots corresponded to individually activated glomeruli. In GCaMP2-reported postsynaptic odor maps, different odorants activated distinct but overlapping sets of glomeruli. Increasing odor concentration increased both individual glomerular response amplitude and the total number of activated glomeruli. Furthermore, the GCaMP2 response displayed a fast time course that enabled us to analyze the temporal dynamics of odor maps over consecutive sniff cycles. In summary, with cell-specific targeting of a genetically encoded Ca(2+) indicator, we have successfully isolated and characterized an intermediate level of odor representation between olfactory nerve input and principal mitral/tufted cell output.
嗅小球是首个气味表征图谱出现的位点。小球层包含精巧的局部突触回路，用于在嗅觉编码模式出现后立即对其进行处理。为了理解气味图谱是如何从传入终端传递到突触后树突的，直接监测气味诱发的小球突触后活动模式至关重要。在此，我们报告了一种转基因小鼠的应用，该小鼠在Kv3.1钾通道启动子的控制下表达一种钙敏感绿色荧光蛋白（GCaMP2）。免疫染色显示，GCaMP2特异性表达于僧帽细胞、簇状细胞以及一部分近小球细胞中，但在嗅神经终端中不表达。体外和体内成像结合谷氨酸受体药理学证实，GCaMP2报告的气味图谱源自突触后。因此，这些小鼠为分析反映纯粹突触后嗅觉编码的空间活动模式提供了前所未有的机会。气味诱发的GCaMP2信号具有局部和扩散的空间成分。局部化的热点对应于单个激活的小球。在GCaMP2报告突触后气味图谱中，不同的气味剂激活不同但重叠的小球集合。增加气味浓度会增加单个小球的反应幅度以及激活小球的总数。此外，GCaMP2反应呈现出快速的时间进程，这使我们能够分析连续吸气周期中气味图谱的时间动态。总之，通过对基因编码钙指示剂进行细胞特异性靶向，我们成功分离并表征了嗅觉神经输入和主要僧帽/簇状细胞输出之间的中间水平气味表征。
