Cellular determinants of odor discrimination behaviour in rodents

我们的感官是大脑了解世界的窗口，也是科学家了解大脑的窗口。通过研究关于外部世界的信息是如何被神经元网络处理的，我们旨在阐明大脑是如何执行各种任务的。许多研究需要侵入性方法来观察或干扰系统，并评估随后的功能变化。由于显而易见的原因，这类研究必须使用动物模型，最值得注意的是，老鼠是一种流行的哺乳动物模型系统，允许进行基因修改。为了研究小鼠的感觉加工，重点关注对选择的动物最重要的感觉是有益的。老鼠是夜行动物，因此我选择的模型系统是嗅觉系统——不仅因为老鼠容易自然地在气味和食物奖励之间形成直接联系，还因为与其他感官相比，嗅觉系统的组织方式相当简单：鼻子里的受体细胞投射到一个叫做嗅球的大脑区域，在那里它们主要与一组神经元连接，这些神经元又直接投射到大脑的各个区域，涉及记忆形成、恐惧处理、决策等各种各样的话题。因此，嗅球似乎是处理特定气味信息的主要区域。在嗅球中，被激活的神经元倾向于抑制由大量被称为“颗粒细胞”（GCs）的小细胞介导的其他神经元。许多人推测这些连接对于气味处理很重要，例如，通过增强相似气味之间的差异：强烈激活的神经元抑制弱激活的神经元，从而进一步降低其活性并增加初始差异。尽管这种特殊的模型受到许多人的激烈争议，但大多数人都同意，这种抑制连接对于塑造OB中气味的表现方式至关重要。在我的项目中，我将通过改变这些抑制连接，测量变化并评估其行为后果来直接验证这一假设。这个项目的主要挑战是将修改限制在这些连接上。大多数经典的基因修饰都是消除或修饰整个身体中的一个基因。然而，我们的方法是将病毒直接注射到嗅球中。这种病毒会变得无害（也就是说，它可以感染细胞，但不能在细胞中生长），但会产生“分子剪刀”。它会在病毒感染的所有细胞中切除一个特定的基因。这是通过使用一种特殊的老鼠品种来实现的，这种老鼠经过基因改造，在我们的目标基因两侧包含“切割位点”，这些位点可以被分子剪刀特异性地识别出来。改变嗅球中重要的交流基因，我们可以验证早期嗅觉系统的抑制回路对气味处理至关重要的假设。在第一步，我们必须证明我们的具体修改确实改变了沟通。为了达到这个目的，我们将记录嗅球中单个神经元的活动，并测量它们受到的抑制量。为了研究我们的改变是否也会影响行为，我们将精确地（以毫秒精度）测量老鼠区分两种气味所需的时间。这个时间在动物之间是高度可复制的，这使我们能够测量与仅限于嗅球中gc的修饰相关的微小变化。通过分析嗅球回路中抑制改变对行为的影响，我们将朝着理解气味在嗅觉系统中是如何表征和处理的方向迈出一步。因此，我们的研究将使我们有机会通过啮齿动物的嗅觉来窥视大脑的运作。

Our senses are our brains' window to the world - they are also the scientists' window to understanding the brain. By investigating how information about the external world is processed by networks of neurones, we aim to elucidate how the brain performs its variety of tasks. Many lines of research require invasive methods to observe or disturb the system and asses subsequent alterations in function. For obvious reasons, such lines of research have to use animal models, most notably the mouse as a popular mammalian model system allowing genetic modifications. In order to investigate sensory processing in mice, it is beneficial to focus on senses of prime importance to the animal of choice. Mice are nocturnal animals, thus my model system of choice is the olfactory system - not only because mice form direct associations between smells and food rewards easily and naturally, but also because in contrast to other senses the olfactory system is organized in a rather simple way: Receptor cells in the nose project to a brain area called olfactory bulb where they connect mainly with one group of neurones that in turn directly project to various areas of the brain involved in topics so diverse as memory formation, fear processing, decision making etc. Thus, the olfactory bulb seems to be the prime area for the processing of smell-specific information. In the olfactory bulb, activated neurones tend to inhibit other neurones mediated by a large number of small cells called 'granule cells' (GCs). Many people have speculated that these connections are important for odour processing, e.g. by enhancing the differences between similar odours: Strongly activated neurones inhibit weakly activated ones, thus further reducing their activity and increasing the initial difference. Though this particular model is hotly disputed by many, most people agree that such inhibitory connections are of crucial importance in shaping the way odours are represented in the OB. In my project I will directly test this hypothesis by altering these inhibitory connections, measuring the alterations and assessing their behavioural consequence. A prime challenge in this project is to restrict the modification to only these connections. Most classical genetic modifications eliminate or modify one gene in the entire body. Our approach, however, is to inject a virus directly into the olfactory bulb. This virus will be rendered harmless (that is, it can infect cells but not grow in them) but produce a 'molecular pair of scissors'. It will cut out one specific gene in all cells the virus infects. This is made possible by the use of a specific strain of mice, genetically modified to contain 'cutting sites' on both sides of our target gene that are specifically recognised by the molecular scissors. Altering genes important in communication in the olfactory bulb we can test the hypothesis that the inhibitory circuitry of the early olfactory system is crucial for odour processing. In a first step, we have to demonstrate that our specific modifications have indeed changed communication. To achieve this we will record the activity of single neurones in the olfactory bulb and measure the amount of inhibition they receive. In order to investigate whether our modifications also influence behaviour we will accurately (with millisecond precision) measure the time it takes a mouse to discriminate two odours. This time is highly reproducible between animals allowing us to measure the small changes associated with modifications restricted to only GCs in the olfactory bulb. By analyzing the effects of altered inhibition in the circuitry of the olfactory bulb on behaviour we will make one step towards understanding how odours are represented and processed in the olfactory system. Thus, our research will give us the chance to glance into the brains' workings through the rodents' sense of smell.