7月1日, 中科院上海生科院神经所郭爱克研究组在《PNAS》杂志在线发表了题为《Transformation of odor selectivity from projection neurons to single mushroom body neurons mapped with dual-color calcium imaging》的研究文章。该工作引入了一种新的双色钙成像方法，并利用这种方法对果蝇中单个蘑菇体神经元对嗅觉刺激的反应特性是如何由前级的投射神经元转换而来进行了探讨。
在绝大多数情况下，单个神经元从许多突触前神经元接收输入，并将这些输入信号整合为单一的输出信号。所以，单个神经元对外界刺激的反应特性在很大程度上是由它所接收的突触前神经元的反应特性所转换而来的。如果能够将一个神经元所接收的突触前神经元的反应特性逐一测量出来，并和这个神经元自身的反应特性进行比较，就可以对这个神经元所参与的环路计算得到清晰的了解。然而，由于神经环路的结构极端复杂，迄今为止尚没有一种有效的策略能够将某一神经元的突触前神经元逐一找出，并进行详细的功能研究。
为了解决这个问题，郭爱克研究组引入了一种基于双色钙成像的新策略，并将其应用于果蝇的蘑菇体神经元（KC）上。作者们用绿色钙指示蛋白G-CaMP标出单个KC，同时用红色钙指示蛋白R-GECO标出许多前级的投射神经元（PN）；通过结构上的追踪，就可以分辨出单个KC从哪些PN轴突末梢接收输入，并用功能成像将这个KC和它所接收的PN轴突末梢对气味的反应逐一测量出来。
使用这种策略，作者们发现单个KC对气味的反应选择性在很大程度上可以从它所接收的PN轴突末梢的气味反应进行预测。只要将这些PN轴突末梢的气味反应进行线性相加，并对所得到的结果和预先设定的阈值进行比较，就可以较为精确的预测单个KC对哪些气味起反应。这说明KC对气味的反应选择性主要是由其接收的突触前PN的反应特性所决定的。即使某一气味不能激活KC的输出，也经常可以在该KC的树突上观察到空间上局限于突触后位点的钙反应。这种局部钙反应很可能对应于单个PN轴突末梢的激活在KC中所引起的阈下兴奋性突触后电位。单个局部钙反应的大小和其相应的突触前PN轴突末梢的钙反应大小经常具有线性相关性，使作者们得以对单个突触位点进行突触强度的测量。另外，单个KC所接收的PN轴突末梢的数目和这些PN-KC突触的平均强度具有反相关的关系。
这项工作所引入的策略对果蝇蘑菇体中的信息传递和整合的过程提供了一种全新的视角，并一定程度上揭示了蘑菇体神经元气味编码特性的来源。在未来，这种策略有可能应用于其它多种环路，以揭示更加复杂的神经元反应特性是如何产生的。（生物谷Bioon.com）
生物谷推荐英文摘要：
Pnas 10.1073/pnas.1305857110
Transformation of odor selectivity from projection neurons to single mushroom body neurons mapped with dual-color calcium imaging
Hao Lia, Yiming Li, Zhengchang Lei, Kaiyu Wanga, and Aike Guo
Although the response properties of most neurons are, to a large extent, determined by the presynaptic inputs that they receive, comprehensive functional characterization of the presynaptic inputs of a single neuron remains elusive. Toward this goal, we introduce a dual-color calcium imaging approach that simultaneously monitors the responses of a single postsynaptic neuron together with its presynaptic axon terminal inputs in vivo. As a model system, we applied the strategy to the feed-forward connections from the projection neurons (PNs) to the Kenyon cells (KCs) in the mushroom body of Drosophila and functionally mapped essentially all PN inputs for some of the KCs. We found that the output of single KCs could be well predicted by a linear summation of the PN input signals, indicating that excitatory PN inputs play the major role in generating odor-selective responses in KCs. When odors failed to activate KC output, local calcium transients restricted to individual postsynaptic sites could be observed in the KC dendrites. The response amplitudes of the local transients often correlated linearly with the presynaptic response amplitudes, allowing direct assay of the strength of single synaptic sites. Furthermore, we found a scaling relationship between the total number of PN terminals that a single KC received and the average synaptic strength of these PN-KC synapses. Our strategy provides a unique perspective on the process of information transmission and integration in a model neural circuit and may be broadly applicable for the study of the origin of neuronal response properties.
