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，鼻子中的嗅觉感觉神经元（OSNs）将轴突发送到它们的中心目标嗅球（OB）。在那里，被称为肾小球的模块单元的感觉处理的初始阶段控制着信息传递到OB的输出神经元，二尖瓣/簇状细胞（M/TCs）的增益。在OSN因损伤、感染或毒性损伤而全部死亡后，整个OSN群体可以恢复，其轴突可以生长到大脑中，以重建嗅觉功能。使用一种可靠的、可重复的、选择性的方法，用单剂量的嗅觉毒素甲巯咪唑（MMZ）诱导这种去细胞和再生，我们已经确定了这种解剖和功能恢复的关键里程碑。然而，在这种描述良好的、完全自然发生的成功神经元再生的哺乳动物模型中，我们目前对可塑性在目标回路中所起的作用几乎一无所知。OB在一生中都是大脑中极具可塑性的部分，我们最近的工作已经确定，即使在短暂的感觉剥夺之后，OB细胞类型的特异性可塑性也会发生。这涉及OB肾小球层中特定的抑制性中间神经元子集，它们释放多巴胺和GABA以影响OSN输入和M/TC输出之间信息传递的增益。这些多巴胺能（DA）细胞可以接受OSN的直接输入，并在肾小球网络中承担两种主要抑制功能：反馈抑制OSN释放和前馈抑制肾小球神经元。它们也是异质的，我们之前的工作确定了两种主要亚型：1)主要的（>98%）小体型，完全缺乏轴突，局部分支，可以通过胚胎和成人神经发生产生；2)一种更罕见的大体细胞类型，具有轴突和广泛分支，只在早期胚胎发育中产生。这两种类型都经历经验依赖的可塑性变化：我们和其他人已经描述了它们的结构、功能和基因表达的经验依赖改变。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）（多巴胺合成的限制性酶）的水平降低，可能降低了这些细胞对嗅觉信息从鼻子到大脑流动的抑制作用。因此，我们假设这些神经元的可塑性，特别是在主要的无轴突亚型中，其小乔木限制了它们的影响范围，在OSN重新连接期间影响肾小球内回路功能。具体来说，我们假设降低OB DA神经元对局部肾小球网络的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，从而促进下游使用最初较弱的再生输入。目标回路的可塑性是帮助还是阻碍功能恢复？重新生长的轴突会遇到一个适应并能适当处理它们提供的新信息的网络吗？或者是可塑性中枢机制产生了异常的电路活动，将新重新连接的输入变成了功能失调的输出？我们将利用嗅觉系统独特的自然再生能力，以及OB - DA神经元著名的可塑性能力，在这里提出这些问题。我们将在体外和体内研究和操作这些细胞，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'.