Changing strengths of learned associations: neuronal ensemble mechanism

学习和记住有关生物意义重大经验和感官刺激的习得联系的变化含义的能力对生存至关重要。例如，动物对预测自然奖励(如食物)的可获得性的外部线索做出反应。然而，如果同样的线索无法预测它们的可用性，他们将学会抑制对这些线索的反应。几十年来，神经科学的一个主要目标一直是揭示大脑中关于这些联系的表征的准确位置和生理性质。最近的研究表明，少数稀疏分布的、被激活的神经元在形成关于显著经验和相关环境线索的习得联系的心理表征方面起到了作用。虽然已经确定这些组合在调节这些联系方面起到了因果作用，但我们并不完全了解它们形成的机制。此外，一旦学习到的联想的原始含义发生变化，比如当线索不再预测食物的可获得性时，这些神经元会发生什么，我们知之甚少。在神经元集合中，一个可能发生这些变化以编码这些关联意义的位置是突触，这是神经元交流的关键位置。人们认为，突触的持久变化类似于神经元的重新布线，是形成新的联想表征的基础。然而，由于识别和记录激活神经元的技术挑战，这种重新连接过程的确切性质，特别是关于神经元集合如何建立和维持这些联系，在很大程度上尚不清楚。这项资助的目的是阐明在神经元集合中，在突触和局部电路水平上，调节自然奖赏和相关环境线索之间习得联系强度的机制。我们将通过使用联想学习的动物模型来实现这一点，在该模型中，小鼠学习如何将音调提示与蔗糖传递联系起来。一旦这种关联形成，音调提示就会引起对蔗糖输送容器的条件性接近反应。相反，当动物知道这一线索不能预测蔗糖的输送时，这种反应在一种被称为“消亡学习”的过程中被抑制。人们普遍认为，对于这种类型的学习，最初的联想记忆并没有被抹去，而是被抑制了，因为暴露在蔗糖中可以重新触发接近反应。在神经元水平上，蔗糖预测信号会引起前额叶皮质中少数神经元的激活。这一大脑区域对动机行为很重要，比如确保食物来源，这些被激活的神经元被认为编码关于蔗糖及其相关线索的信息。我们将使用转基因小鼠选择性地记录这些神经元，在这些转基因小鼠中，这些激活的神经元可以被识别和生理特征。我们将研究它们突触的结构变化，以揭示神经元连接的变化，在小鼠了解到该线索预测或不再预测蔗糖输送后。我们还将研究一个乐团成员与另一个乐团成员沟通能力的变化，这一过程可能会促进整个乐团代表的激活。最后，我们将通过沉默这些神经元的活动并测试这种对条件性接近反应的影响来表征这些神经元的行为角色，以及小鼠是否可以重新学习原始的条件反射任务。理解这些神经机制将有助于揭示健康认知功能所必需的基本联想学习过程。此外，它将揭示与行为相关的神经元上突触的特定变化，并可能揭示新的治疗靶点。

The ability to learn and remember the changing meanings about learned associations of biologically significant experiences and sensory stimuli is crucial for survival. For example, animals respond to external cues that predict the availability of natural rewards (e.g. food). However, they will learn to inhibit responding to these cues if the same cues fail to predict their availability. For many decades, a major goal of neuroscience has been to reveal the precise location and physiological nature of representations about these associations in the brain. Recent studies implicate the role of a minority of sparsely distributed, activated neurons called 'neuronal ensembles' in forming mental representations about learned associations about salient experiences and associated environmental cues. Although it is established that these ensembles play a causal role in mediating these associations, we do not fully understand the mechanisms that are involved in their formation. Also, little is known about what happens to these neurons once the original meaning of the learned associations change, such as when a cue no longer predicts food availability. A likely site where these changes occur to encode the meaning of these associations at neuronal ensembles is at the synapse, the key sites of neuronal communication. It is thought that enduring changes at the synapse akin to 'neuronal rewiring' underlie the formation of new associative representations. However, the precise nature of this rewiring process, especially with regards to how neuronal ensembles establish and maintain these associations, is largely unknown due to the technical challenges in identifying and recording from activated neurons. The aim of this grant is to elucidate the mechanisms that modulate the strength of learned associations between natural rewards and associated environmental cues at the synaptic and local circuit level in neuronal ensembles. We will accomplish this by using an animal model of associative learning, where a mouse learns how to associate a tone cue with sucrose delivery. Once this association forms, a tone cue elicits a conditioned approach response towards the sucrose delivery receptacle. Conversely, this response is inhibited following a procedure called 'extinction learning' when the animal learns that the cue does not predict sucrose delivery. It is widely believed for this type of learning that the original associative memory is not erased but rather suppressed, since exposure to sucrose can re-trigger the approach response. At the neuronal level, a sucrose predictive cue elicits activation of a minority of neurons in the prefrontal cortex. This brain area is important for motivated behaviours, such as securing food sources, and these activated neurons are believed to encode information about sucrose and its associated cues. We will selectively record from these neurons using transgenic mice in which these activated neurons can be identified and physiologically characterised. We will examine structural alterations at their synapses to reveal the changes in neuronal wiring, after mice have learned that the cue predicts or no longer predicts sucrose delivery. We will also examine changes in 'connectivity' or the ability of one ensemble member to communicate with another, a process that may facilitate activation of the entire ensemble representation. Finally, we will characterize the behavioural role of these neurons that undergo rewiring by silencing their activity and testing this effect on conditioned approach responses, and whether the mice can relearn the original conditioning task.Understanding these neuronal mechanisms will shed light on basic associative learning processes that are integral to healthy cognitive functioning. Moreover, it will reveal specific alterations at synapses on behaviourally relevant neurons and could potentially reveal new therapeutic targets.