Modifiability of Conduction Across Preganglionic Axonal Branch Points

项目总结 脊髓交感节前神经元(SPN)存在于胸腰段脊髓。他们 是中枢神经系统同情输出的最终仲裁者。SPN轴突分支发出高度分化的多节段 椎旁交感链神经节节后神经元的投射。分歧提供了一种 将中枢交感神经指令放大到数量更大的三叉神经节的机制。假设是这样的 这种尖峰传导在多节段分支点是可靠的。这是基于体外录制的 来自室温(T°)的交感神经链，其中尖峰幅度和宽度的大幅增加确保 分支点导通的高安全系数。随着T°的增加会导致传导故障，很可能 SPN分支点传导也是T°敏感的。 在躯体感觉系统中，分支点为控制轴突传导和 突触外含有α5的GABA受体(GABA受体)也参与其中。SPN还表达α5-GABAAR，以及 它们在突触前轴突分支点的表达可能类似地控制发散，从而控制反应 对PNS的扩增。我们记录了成年小鼠体外胸段SPN轴突跨神经节的发散。 刺激附着的腹根后的椎旁链。最初的实验观察到传导 阻断依赖于被穿越的链状神经节数目、T°、频率和GABAAR拮抗剂。结果 支持轴突发散作为交感神经获得控制中的一个可修改的输出阶段。 [SA1]我们假设跨分支点的节前轴突传导阻滞位于 神经调节控制的结构性活性，含α5的GABA。我们对AESs进行了实验 应用GABAAR激动剂、拮抗剂和内源性药物后轴突募集的变化 神经类固醇变构调节剂，并在放电频率的生理范围内进行测试。 [SA2]许多脊髓损伤(SCI)患者都有体温调节功能障碍。即使是很小的海拔 在躯体T°可损害棘波传导的情况下，SPN轴突功能在脊髓损伤人群中可能尤为明显 很脆弱。我们假设跨分支点的传导对T°核心的更大范围的变化很敏感 并将在与低温或高温一致的T°范围内测试T°对传导阻滞的影响。 [SA3]我们假设脊髓损伤导致了促进跨分支传导的变化，包括：(A) GABAAR活性增加和(B)穗宽加宽。以上结果将与之后看到的结果进行比较 T2胸段脊髓损伤早期(1-3天)和慢性(4-6周)时间点。 探索促进或阻止跨分支点传导的机制对于理解 SPN信号放大是否是硬连接的或生理上可修改的(例如，行为状态相关) 以及脊髓损伤后的可塑性是否改变了这一重要的交感输出阶段的功能。

PROJECT SUMMARY Spinal cord sympathetic preganglionic neurons (SPNs) are found in the thoracic and lumbar spinal cord. They are the final arbiters of CNS sympathetic output. SPN axons branch to issue highly divergent multisegmental projections on paravertebral sympathetic chain ganglia postganglionic neurons (PNs). Divergence provides a mechanism for amplification of CNS sympathetic commands to the numerically much greater PNs. It is assumed that spike conduction is reliable across multisegmental branch points. This was based on ex vivo recordings from the sympathetic chain at room temperature (T°) where large increases in spike amplitude and width ensure a high safety factor for branch point conduction. As increasing T° promotes conduction failures, it is likely that SPN branch point conduction is also T°-sensitive. In somatosensory systems, branch points provide an important site for control of axonal conduction and extrasynaptic α5-containing GABAA receptors (GABAARs) are implicated. SPNs also express α5-GABAARs, and their expression at presynaptic axonal branch points may similarly control divergence and hence response amplification to PNs. We recorded from SPN axons diverging across ganglia in the adult mouse ex vivo thoracic paravertebral chain following stimulation of attached ventral roots. Initial experiments observed that conduction block was dependent on; number of traversed chain ganglia, T°, frequency, and GABAAR antagonists. Results support axonal divergence as a modifiable output stage in sympathetic gain control. [SA1] We hypothesize that preganglionic axonal conduction block across branch points is under neuromodulatory control by constitutively active, α5-containing GABAAR. We undertake experiments to aess axonal recruitment changes following application of GABAAR agonists, antagonists and endogenous neurosteroid allosteric modulators and tested across the physiological range of firing frequencies. [SA2] Many spinal cord injured (SCI) individuals have thermoregulatory dysfunction. As even small elevations in body T° can compromise spike conduction, SPN axonal function in the SCI population may be particularly vulnerable. We hypothesize that conduction across branch points is sensitive to the broader changes in T°core and will test the effect of T° on conduction block over a range of T° consistent with hypo- or hyperthermia. [SA3] We hypothesize that SCI leads to changes that promote conduction across branch points, including (a) increased GABAAR activity and (b) spike width broadening. Results above will be compared to those seen after T2 thoracic SCI at early (1-3 days) and chronic (4-6 weeks) time points. Exploring mechanisms that promote or prevent conduction across branch points is critical to understanding whether SPN signal amplification is hard-wired or physiologically modifiable (e.g. behavioral state dependent) and whether plasticity after SCI alters function at this important sympathetic output stage.