Anti-memories through compartmentalised activity in a single neuron in a Drosophila memory centre

大多数神经元通过自我再生信号进行交流。它们在神经元的输入部分接收并汇总信号，如果这些输入超过一定的阈值，它们就会向神经元的输出部分发送一个自我再生的电脉冲，输出部分释放化学物质向其他神经元发出信号。但在许多神经元中，活动是“分隔的”：激活一个部分的神经元，使其只在该部分释放输出信号，而不是其他部分，因为电活动不容易传播到神经元的其他部分。他们为什么这么做？在大多数情况下，我们不知道：很少有例子可以清楚地理解区隔神经元活动的行为功能。我们通过研究果蝇的嗅觉记忆来填补这一空白。苍蝇学会避开惩罚气味，接近奖励气味。这些记忆被果蝇大脑中一种叫做“APL”的神经元所抑制，这种神经元会抑制储存记忆的神经元——凯尼恩细胞（KCs）。解释并不像“APL抑制记忆是因为APL抑制KCs”那么简单。相反，我们认为APL抑制学习是因为它的活动是分开的。这是因为对气味的行为反应是由两种相反类型的神经元之间的平衡控制的，这使得苍蝇要么接近气味，要么避开气味。这些“接近”和“回避”的神经元被称为“蘑菇体输出神经元”（MBONs），它们是由气味激活的KCs激活的。气味+惩罚训练削弱了KCs与接近（但不是逃避）mbcs的联系，因此回避占主导地位，苍蝇避开惩罚气味。同样，气味+奖励会削弱KC->回避连接。因此，学习行为是由MBON信号的平衡决定的，而不是KCs的总输出。因此，APL只有在减轻气味诱发的接近和回避MBONs之间的不平衡时才能抑制记忆。要做到这一点，它必须比其他的更强烈地抑制一些KC->MBON连接：例如，通过抑制KC->回避连接而不是KC->接近连接来抑制惩罚记忆。为了实现这一目标：(1)APL活性（及其抑制输出）必须分区化。这将允许单个神经元APL不同地抑制KC->途径和KC->回避连接，因为它们位于不同的空间“区域”。(2) APL在接近区和回避区具有不同的活性。APL的活性由KCs控制，因此如果KC->APL连接与KC->MBON连接在同一区域通过相同的学习方式进行修改，则会发生这种情况：例如，气味+惩罚选择性地削弱“接近”区域的KC->APL连接。在这种情况下，由于APL局部抑制KCs，学习后，APL在回避区比在接近区更强烈地抑制KCs（从而抑制MBONs）。这将减轻由气味+惩罚（逃避大于接近）引起的MBON活动的不平衡。因此，如果APL活动是区分开的，学习可以同时诱导支持记忆的突触修饰（KC->MBON连接）和反对记忆的突触修饰（局部KC->APL连接）。我们称后者为“反记忆”，因为它们是一种主动的变化，充当记忆的“镜像”，而不是被动的衰退。这种反记忆可能会限制记忆的形成或表达，并将为分区活动提供第一个明确的认知功能。我们的初步数据已经证实了上面的一些预测。我们将用脑成像来测试其余的部分：我们将采用一种工程蛋白质，当神经元活跃时，它会发光，把这种蛋白质放在KCs、APL或MBONs中，当苍蝇闻到气味时，对感兴趣的神经元的整个体积进行成像。我们将在训练前后，或在控制神经回路小区域的活动时进行这项研究，我们将分析APL不同部分的活动差异。

Most neurons communicate through self-regenerating signals. They take in and sum up signals in the input part of the neuron, and if these inputs surpass a certain threshold, they send a self-regenerating electrical impulse to the output part of the neuron, which releases chemicals to signal to other neurons. But in many neurons, activity is 'compartmentalised': activating the neuron in one part makes it release output signals only in that one part, not other parts, because electrical activity does not spread readily to other parts of the neuron. Why do they do this? In most cases, we don't know: there are few examples where the behavioural function of compartmentalised neuronal activity is clearly understood.We address this gap by studying olfactory memory in fruit flies. Flies learn to avoid punished odours and approach rewarded odours. These memories are suppressed by a neuron in the fly brain called 'APL', which inhibits memory-storing neurons called Kenyon cells (KCs). The explanation isn't as simple as 'APL inhibits memories because APL inhibits KCs.' Rather, we propose that APL suppresses learning because its activity is compartmentalised.This is because behavioural responses to odours are controlled by the balance between two opposing types of neurons, which make the fly either approach or avoid the odour. These 'approach' and 'avoidance' neurons, called 'mushroom body output neurons' (MBONs), are activated by KCs, which are activated by odours. Odour+punishment training weakens connections from KCs onto approach (but not avoidance) MBONs, so that avoidance dominates and flies avoid the punished odour. Similarly, odour+reward weakens KC->avoidance connections. Thus, learned behaviour is determined by the balance in MBON signalling, not the total output of KCs.Therefore, APL can only suppress memory if it lessens the imbalance between odour-evoked activity in approach and avoidance MBONs. To do this, it must inhibit some KC->MBON connections more strongly than others: e.g., suppress punishment memories by inhibiting KC->avoidance connections more than KC->approach connections. To achieve this:(1) APL activity (and thus its inhibitory output) must be compartmentalised. This would allow the single neuron APL to differentially inhibit KC->approach and KC->avoidance connections because they are in different spatial 'zones'.(2) APL must have different activity in approach vs. avoidance zones. APL's activity is controlled by KCs, so this would occur if KC->APL connections are modified by learning in the same way as KC->MBON connections in the same zone: e.g., odour+punishment selectively weakens KC->APL connections in the 'approach' zone. In this scenario, because APL inhibits KCs locally, after learning APL would inhibit KCs (and thereby MBONs) more strongly in the avoidance zone than in the approach zone. This would lessen the imbalance in MBON activity induced by odour+punishment (avoidance greater than approach).Thus, if APL activity is compartmentalised, learning could simultaneously induce synaptic modifications that support memory (KC->MBON connections) and modifications that oppose memory (local KC->APL connections). We call the latter 'anti-memories' because they are an active change that acts as a 'mirror opposite' to memory, rather than passive decay. Such anti-memories might gate memory formation or expression, and would provide the first clear cognitive function for compartmentalised activity. Our preliminary data has already confirmed some of the predictions above. We will test the rest using brain imaging: we will take an engineered protein that lights up when neurons are active, put the protein in KCs, APL or MBONs, and image the whole volume of the neurons of interest while the fly smells odours. We will do this before and after training, or while manipulating activity in small areas of the neural circuit, and we will analyse how activity differs in different parts of APL.