Mechanisms of inactivity-induced respiratory plasticity

描述(申请人提供)：指导这一提议的基本假设是，减少对呼吸运动神经元的突触输入会引起代偿性可塑性，将呼吸运动输出保持在与生活相适应的范围内。我们在本项目期间的具体目标是研究导致静止诱导的膈神经运动促进(IPMF)的细胞机制，iPMF是指在膈神经活动长期减少后，膈肌爆发幅度持续增加。在麻醉大鼠身上，将研究两种不同的减少膈神经活动的方法：一种是减少呼吸网络的整体活动(低碳酸血症)，另一种是专门减少到膈运动神经元的脊髓突触输入(C2轴突传导阻滞)。这些方法引起的iPMF表现出惊人的相似之处，但也可能有重要的差异。低碳酸血症和C2传导阻滞均可诱发iPMF(即波幅增加)，但只有低碳酸血症才能诱发膈神经爆发频率易化，提示iPMF可能是由多种机制引起的，这取决于神经活动是否局部或整体减少。在本项目中，我们将重点研究导致iPMF的脊髓机制。我们的工作模型是，减少对膈运动神经元的突触输入刺激膈运动核(目标1)释放TNF1，激活膈运动神经元或其附近的非典型PKC(APKC)亚型，从而产生iPMF(目标2和3)。我们进一步提出，iPMF与其他形式的神经可塑性类似，也受到调控的限制。通过对Spraogue Dawley大鼠一个独特的亚系的研究，我们将获得关于限制iPMF的机制的关键见解。具体地说，我们假设更大的结构性NMDA-谷氨酸受体活性限制了这个大鼠亚株(AIM 4)的iPMF，可能是由于遗传或表观遗传因素。由于未能诱导iPMF可能导致对人类健康具有重要意义的呼吸控制障碍，如长时间机械支持后的机械脱机失败，因此结构性NMDA受体活性的差异可能会区分成功脱机的患者和未脱机的患者。对导致iPMF的细胞级联的详细了解对于了解这种高度新颖的可塑性形式的生理作用至关重要，更重要的是，为治疗呼吸控制疾病的药物干预确定有希望的治疗靶点。 公共卫生相关性：由于呼吸是生命所必需的，在长时间的机械支持后未能恢复足够的呼吸是一个严重的临床问题。在这个项目中，我们将研究一种非常新的机制，通过呼吸努力减少的时间段诱导脊髓可塑性。通过对这一机制的详细了解，我们希望了解呼吸机脱机失败的神经基础，并为难以恢复无辅助呼吸的患者开发治疗方法。

DESCRIPTION (provided by applicant): The fundamental hypothesis guiding this proposal is that reduced synaptic inputs to respiratory motor neurons elicits compensatory plasticity, preserving respiratory motor output in a range compatible with life. Our specific goal in the present project period is to investigate cellular mechanisms giving rise to inactivity-induced phrenic motor facilitation (iPMF), a persistent increase in phrenic burst amplitude following prolonged decreases in phrenic neural activity. Two distinct methods of reducing phrenic activity will be studied in anesthetized rats: one that reduces overall activity in the respiratory network (hypocapnia) and another that specifically decreases spinal synaptic inputs to phrenic motor neurons (C2 axon conduction block). The iPMF evoked by these methods exhibits striking similarities, yet may have important differences. Hypocapnia and C2 conduction block both elicit iPMF (i.e., increased amplitude), but only hypocapnia elicits phrenic burst frequency facilitation suggesting the possibility of that iPMF arises from multiple mechanisms depending on whether neural activity was reduced locally versus globally. In this project, we will focus on spinal mechanisms leading to iPMF. Our working model is that reduced synaptic input to phrenic motor neurons stimulates TNF1 release in the phrenic motor nucleus (Aim 1), activating atypical PKC (aPKC) isoforms in or near phrenic motor neurons that give rise to iPMF (Aims 2 and 3). We further propose that iPMF is subject to regulatory constraints, similar to other forms of neuroplasticity. By investigations of a unique sub-strain of Sprague Dawley rats, we will gain critical insights concerning mechanisms that constrain iPMF. In specific, we hypothesize that greater constitutive NMDA-glutamateric receptor activity constrains iPMF in this rat sub-strain (Aim 4), possibly due to genetic or epigenetic factors. Since failure to elicit iPMF may contribute to ventilatory control disorders of importance to human health, such as ventilatory weaning failure following prolonged ventilatory support, differences in constitutive NMDA receptor activity may differentiate patients that successfully wean from ventilatory support versus those that do not. A detailed understanding of cellular cascades giving rise to iPMF is essential to understand the physiological role of this highly novel form of plasticity, and-importantly-to identify promising therapeutic targets for pharmacological interventions to treat respiratory control disorders. PUBLIC HEALTH RELEVANCE: Since breathing is necessary for life, failure to restore adequate breathing after prolonged periods of ventilatory support represents a serious clinical problem. In this project we will investigate a highly novel mechanism of spinal cord plasticity induced by periods of reduced breathing effort. Through a detailed understanding of this mechanism, we hope to understand the neural basis of ventilator weaning failure and to develop treatments for patients that have difficulty resuming unassisted breathing.