School of Physical Education, Yunnan University, Kunming, China.
Key Laboratory of Adolescent Health Assessment and Exercise Intervention of Ministry of Education, School of Physical Education and Health Care, East China Normal University, Shanghai, China.
Front Neural Circuits. 2021 Apr 6;15:657445. doi: 10.3389/fncir.2021.657445. eCollection 2021.
Serotonergic (5-HT) neurons in the medulla play multiple functional roles associated with many symptoms and motor activities. The descending serotonergic pathway from medulla is essential for initiating locomotion. However, the ionic properties of 5-HT neurons in the medulla remain unclear. Using whole-cell patch-clamp technique, we studied the biophysical and modulatory properties of persistent inward currents (PICs) in 5-HT neurons of medulla in ePet-EYFP transgenic mice (P3-P6). PICs were recorded by a family of voltage bi-ramps (10-s duration, 40-mV peak step), and the ascending and descending PICs were mirrored to analyze the PIC hysteresis. PICs were found in 77% of 5-HT neurons (198/258) with no significant difference between parapyramidal region ( = 107) and midline raphe nuclei (MRN) ( = 91) in either PIC onset (-47.4 ± 10 mV and -48.7 ± 7 mV; = 0.44) or PIC amplitude (226.9 ± 138 pA and 259.2 ± 141 pA; = 0.29). Ninety-six percentage (191/198) of the 5-HT neurons displayed counterclockwise hysteresis and four percentage (7/198) exhibited the clockwise hysteresis. The composite PICs could be differentiated as calcium component (Ca_PIC) by bath application of nimodipine (25 μM), sodium component (Na_PIC) by tetrodotoxin (TTX, 2 μM), and TTX- and dihydropyridine-resistance component (TDR_PIC) by TTX and nimodipine. Ca_PIC, Na_PIC and TDR_PIC all contributed to upregulation of excitability of 5-HT neurons. 5-HT (15 μM) enhanced the PICs, including a 26% increase in amplitude of the compound currents of Ca_PIC and TDR_PIC ( < 0.001, = 9), 3.6 ± 5 mV hyperpolarization of Na_PIC and TDR_PIC onset ( < 0.05, = 12), 30% increase in amplitude of TDR_PIC ( < 0.01), and 2.0 ± 3 mV hyperpolarization of TDR_PIC onset ( < 0.05, = 18). 5-HT also facilitated repetitive firing of 5-HT neurons through modulation of composite PIC, Na_PIC and TDR_PIC, and Ca_PIC and TDR_PIC, respectively. In particular, the high voltage-activated TDR_PIC facilitated the repetitive firing in higher membrane potential, and this facilitation could be amplified by 5-HT. Morphological data analysis indicated that the dendrites of 5-HT neurons possessed dense spherical varicosities intensively crossing 5-HT neurons in medulla. We characterized the PICs in 5-HT neurons and unveiled the mechanism underlying upregulation of excitability of 5-HT neurons through serotonergic modulation of PICs. This study provided insight into channel mechanisms responsible for the serotonergic modulation of serotonergic neurons in brainstem.
延髓中的 5-羟色胺能（5-HT）神经元在与许多症状和运动活动相关的功能中发挥着多种作用。来自延髓的下行 5-HT 通路对于启动运动至关重要。然而，延髓中 5-HT 神经元的离子特性仍不清楚。使用全细胞膜片钳技术，我们在 ePet-EYFP 转基因小鼠（P3-P6）的延髓中研究了 5-HT 神经元中持续内向电流（PICs）的生物物理和调制特性。通过一系列电压双斜坡（持续时间 10s，峰步 40mV）记录 PICs，并镜像上升和下降 PICs 以分析 PIC 滞后。在 258 个 5-HT 神经元中有 77%（198/258）发现了 PICs，在旁锥体区（=107）和中缝核（MRN）（=91）之间，PIC 的起始（-47.4±10mV 和-48.7±7mV；=0.44）或 PIC 幅度（226.9±138pA 和 259.2±141pA；=0.29）没有显著差异。96%（191/198）的 5-HT 神经元显示逆时针滞后，4%（7/198）显示顺时针滞后。复合 PIC 可以通过浴用尼莫地平（25μM）区分钙成分（Ca_PIC）、河豚毒素（TTX，2μM）区分钠成分（Na_PIC）以及 TTX 和二氢吡啶抗性成分（TDR_PIC）。Ca_PIC、Na_PIC 和 TDR_PIC 均有助于增加 5-HT 神经元的兴奋性。5-HT（15μM）增强了 PICs，包括 Ca_PIC 和 TDR_PIC 复合电流幅度增加 26%（<0.001，=9）、Na_PIC 和 TDR_PIC 起始的 3.6±5mV 超极化（<0.05，=12）、TDR_PIC 幅度增加 30%（<0.01）和 TDR_PIC 起始的 2.0±3mV 超极化（<0.05，=18）。5-HT 还通过调制复合 PIC、Na_PIC 和 TDR_PIC 以及 Ca_PIC 和 TDR_PIC，分别促进 5-HT 神经元的重复放电。特别是，高电压激活的 TDR_PIC 促进了在较高膜电位下的重复放电，并且这种易化可以通过 5-HT 放大。形态数据分析表明，5-HT 神经元的树突具有密集的球形膨体，强烈穿过延髓中的 5-HT 神经元。我们对 5-HT 神经元中的 PICs 进行了表征，并揭示了通过 5-HT 调制 PICs 增加 5-HT 神经元兴奋性的机制。这项研究为大脑脑干中 5-HT 神经元的 5-HT 调制所涉及的通道机制提供了深入了解。
