Modifiability of Conduction Across Preganglionic Axonal Branch Points

项目摘要 脊髓交感节前神经元（Spinal cord sympathetic preganglionic neurons，SPNs）存在于胸、腰段脊髓中。他们 是中枢神经系统交感神经输出的最终仲裁者。SPN轴突分支发出高度发散的多节段 椎旁交感链神经节节后神经元（PN）的投射。分歧提供了一个 中枢神经系统交感神经的命令放大的机制，以数字上更大的PN。假设 棘波传导在多节段分支点上是可靠。这是基于体外记录 从交感神经链在室温（T°），其中大幅增加尖峰幅度和宽度，确保 分支点传导的高安全系数。由于增加T°促进传导故障，因此很可能 SPN分支点传导也是T°敏感的。 在躯体感觉系统中，分支点提供了控制轴突传导的重要部位， 突触外含α5的GABAA受体（GABAAR）也参与其中。SPN也表达α5-GABAAR， 它们在突触前轴突分支点的表达可以类似地控制发散， 放大到PN。我们记录了成年小鼠离体胸神经节的SPN轴突分叉 刺激附着的腹根后的椎旁链。最初的实验观察到， 阻滞依赖于穿过的链式神经节的数量、T°、频率和GABAAR拮抗剂。结果 支持轴突发散作为交感神经增益控制中可修改输出级。 [SA1]我们假设跨分支点的节前轴突传导阻滞是在 神经调节控制的组成性活性，α5含GABAAR。我们进行实验， 应用GABAAR激动剂、拮抗剂和内源性GABAAR受体后轴突募集的变化 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）的个体具有体温调节功能障碍。即使是很小的海拔 在身体T°可以损害尖峰传导，脊髓损伤人群中的SPN轴突功能可能特别 脆弱我们假设，通过分支点的传导对T°核心的更广泛变化敏感 并将在与体温过低或过高一致的T°范围内测试T°对传导阻滞的影响。 [SA3]我们假设SCI导致了促进分支点传导的变化，包括（a） 增加GABAAR活性和（B）峰宽增宽。以上结果将与 早期（1-3天）和慢性（4-6周）时间点的T2胸脊髓损伤。 探索促进或阻止跨分支点传导的机制对于理解 SPN信号放大是硬连线的还是生理上可修改的（例如，行为状态相关的） 以及SCI后的可塑性是否改变了这一重要交感神经输出阶段的功能。

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.