How does plasticity in target interneurons influence functional recovery during naturally occurring neuronal regeneration?

在哺乳动物的嗅觉系统中，鼻子中的嗅觉神经元(OSNs)将轴突发送到它们的中央目标--嗅球(OB)。在那里，感觉处理的初始阶段在被称为肾小球的模块单元中控制信息传输到OB的输出神经元，二尖瓣/簇状细胞(M/TCS)。在OSN因损伤、感染或中毒损伤而整体死亡后，整个OSN种群可以恢复，其轴突可以生长到大脑中，以重建嗅觉功能。使用一种可靠、可重复性和选择性的方法来诱导单剂量的嗅觉毒素他巴唑(MMZ)的这种破坏和再生，我们已经确定了这种解剖和功能恢复的关键里程碑。然而，在这个描述良好的、完全自然发生的成功神经元再生的哺乳动物模型中，我们目前几乎不知道可塑性在靶电路中所起的作用。OB是大脑中一个非常可塑性的部分，我们最近的工作发现，即使在短暂的感觉剥夺之后，OB细胞类型的特定可塑性也会发生。这涉及OB肾小球层中特定的抑制性中间神经元子集，它们同时释放多巴胺和GABA，以影响OSN输入和M/TC输出之间的信息传递增益。这些多巴胺(DA)细胞可接受OSN的直接输入，并在肾小球网络中承担两种主要的抑制功能：反馈抑制OSN的释放和前馈抑制肾小球神经元。它们也是异质性的，我们以前的工作确定了两个主要的亚型：1)主要的(&gt；98%)小胞体类型，完全缺乏轴突，局部分支，可以通过胚胎和成年神经发生产生；2)更罕见的大胞体类型，具有轴突和广泛的分支，仅在胚胎发育早期产生。这两种类型都经历了经验依赖的可塑性变化：我们和其他人描述了它们的结构、功能和基因表达的经验依赖的变化。这包括我们的初步发现，在MMZ诱导的OSN再生过程中，OB DA神经元中酪氨酸羟化酶(TH)-多巴胺合成的限速酶-的水平降低，潜在地减少了这些细胞对嗅觉信息流的抑制影响。因此，我们推测，这些神经元的可塑性，特别是在主要的无轴突亚型中，其小杆非常局部地限制了它们的影响范围，影响了OSN重新连接过程中肾小球内回路的功能。具体地说，我们假设，降低OB DA神经元对局部肾小球网络整体抑制作用的可塑性机制会增加这些网络的输入输出增益，以促进最初弱再生输入的下游使用。靶电路中的可塑性是帮助还是阻碍功能恢复？重新生长的轴突是否会遇到一个适应并能够适当处理它们提供的新信息的网络？或者，塑料中央机制会产生异常的电路活动，从而将新连接的输入转变为功能失调的输出？我们将在这里提出这些问题，利用嗅觉系统独特的自然再生能力，以及著名的OB DA神经元的可塑性。我们将在体外和体内对这些细胞进行研究和操作，以验证我们的主要假设：目标中间神经元的可塑性有助于自然发生的神经元再生过程中的功能恢复。

In the mammalian olfactory system, olfactory sensory neurons (OSNs) in the nose send axons to their central target, the olfactory bulb (OB). There, initial stages of sensory processing in modular units called glomeruli control the gain of information transmission to the OB's output neurons, mitral/tufted cells (M/TCs). After the wholescale death of OSNs due to injury, infection, or toxic damage, the entire OSN population can be restored, and their axons can grow into the brain in order to re-establish olfactory function. Using a reliable, reproducible, and selective means of inducing such de- and regeneration with a single dose of the olfactotoxin methimazole (MMZ), we have identified key milestones in this anatomical and functional recoveryHowever, in this well-described, entirely naturally occurring mammalian model of successful neuronal regeneration, we currently know almost nothing about the role played by plasticity in target circuits. The OB is an extremely plastic part of the brain throughout life, and our recent work has identified OB cell-type-specific plasticity occurring after even brief periods of sensory deprivation. This involves a specific subset of inhibitory interneurons in the OB's glomerular layer, which release both dopamine and GABA to influence the gain of information transmission between OSN inputs and M/TC outputs. These dopaminergic (DA) cells can receive direct input from OSNs, and undertake two main inhibitory functions in glomerular networks: feedback inhibition of OSN release, and feedforward inhibition of glomerular neurons. They are also heterogeneous, with our previous work identifying two major subtypes: 1) a predominant (>98%) small-soma type, which entirely lacks an axon, ramifies locally, and can be generated via both embryonic and adult neurogenesis; and 2) a rarer, large-soma type, which is axon-bearing and broadly ramifying, and is exclusively generated in early embryonic developmentBoth types undergo experience-dependent plastic changes: we and others have described experience-dependent alterations in their structure, function, and gene expression. This includes our preliminary findings that levels of tyrosine hydroxylase (TH) - the rate-limiting enzyme for dopamine synthesis - are decreased in OB DA neurons during MMZ-induced OSN regeneration, potentially reducing these cells' inhibitory influence on the nose-to-brain flow of olfactory information. We therefore postulate that plasticity in these neurons, especially within the predominant anaxonic subtype whose small arbours limit their sphere of influence very locally, influences intra-glomerular circuit function during OSN re-connection. Specifically, we hypothesise that plastic mechanisms that reduce the overall inhibitory action of OB DA neurons on local glomerular networks produce an increase in those networks' input output gain, in order to facilitate the downstream use of initially weak regenerating input.Does plasticity in target circuits help or hinder functional recovery? Do re-grown axons encounter a network which is adapted to and can appropriately process the new information they provide? Or do plastic central mechanisms produce aberrant circuit activity which turns newly re-connected inputs into dysfunctional output? We will ask precisely these questions here, taking advantage of the unique natural regenerative capacity of the olfactory system, as well as the renowned plastic capabilities of OB DA neurons. We will study and manipulate these cells both ex vivo and in vivo, to test our primary hypothesis: 'That plasticity in target interneurons contributes to functional recovery during naturally occurring neuronal regeneration'.