Institute of Neurosciences, The Fourth Military Medical University, 17 Chang Le Xi Road, Xi'an 710032, PR China.
Neuroscience. 2010 Jun 16;168(1):61-73. doi: 10.1016/j.neuroscience.2010.03.023. Epub 2010 Mar 18.
The pre-Bötzinger complex (pre-BötC), a functionally defined subregion in the ventrolateral medulla oblongata, is a presumed kernel of normal respiratory rhythmogenesis. However, less is known about the pre-BötC's contribution to respiratory neuroplasticity. The most frequently studied model for respiratory neuroplasticity is episodic hypoxia-induced phrenic long-term facilitation, which is 5-HT(2A) receptors (5-HT(2A)R)-dependent. We hypothesized that preconditioning with chronic intermittent hypoxic (CIH) would activate the 5-HT/5-HT(2A)R system and the downstream protein kinase C (PKC) pathway in the pre-BötC. Animals were exposed to alternating 5 min of hypobaric hypoxia and 5 min of normoxia for 10 h/day for 7 days. Hypobaric hypoxia was achieved by continuous air evacuation to reach a pressure of 210-220 mm Hg, corresponding to an altitude of 9000-10000 m. In contrast to the CIH model, a group of animals were pretreated with chronic sustaining hypoxia (CSH), a protocol of continuous hypobaric hypoxia at 360 mm Hg, corresponding to an altitude of about 6000 m, for 10 h/day for 7 days. Immunoreactivity of 5-HT and 5-HT(2A)R was examined in the pre-BötC, identified by the presence of neurokinin-1 receptor (NK1R). We found that 15.5% of 5-HT-immunoreactive (ir) terminals were in contact with NK1R-ir neurons. Asymmetric synapses could be identified between them. 38.7% of NK1R-ir dendrites were also immunoreactive for 5-HT(2A)R, which was distributed along the inner surface of the plasma membrane in control animals. CIH challenge increased the expressions of 5-HT and 5-HT(2A)R in the pre-BötC, an increase in the expressed 5-HT(2A)R that was not detected in this region in CSH animals. Specifically, 5-HT(2A)R was distributed not only along the inner surface, but also along the outer surface, or directly on the plasma membrane, a pattern not detectable in control animals. 5-HT(2A)R was also detectable in the invaginations of the plasma membrane, where receptor endocytosis or exocytosis might occur, indicating CIH-induced higher trafficking of 5-HT(2A)R. Concurrently, there was an up-regulation of phospho-PKC theta (P-PKCtheta) in the pre-BötC, suggesting a 5-HT/5-HT(2A)R-activated PKC mechanism that may contribute to hypoxia-induced respiratory neuroplasticity in the pre-BötC. The close association of P-PKCtheta with the postsynaptic density implicates a postsynaptic mechanism mediating respiratory neuroplasticity in the pre-BötC.
前脑桥呼吸中枢（pre-BötC）是延髓腹外侧部的一个功能定义的亚区，被认为是正常呼吸节律发生的核心。然而，关于 pre-BötC 对呼吸神经可塑性的贡献知之甚少。呼吸神经可塑性最常研究的模型是间歇性缺氧诱导的膈神经长期易化，这是 5-羟色胺 2A 受体（5-HT2AR）依赖性的。我们假设慢性间歇性低氧（CIH）预处理会激活 pre-BötC 中的 5-HT/5-HT2AR 系统和下游蛋白激酶 C（PKC）途径。动物每天接受 10 小时的交替 5 分钟低压缺氧和 5 分钟常氧处理，共 7 天。通过连续排空空气来实现低压缺氧，达到 210-220 毫米汞柱的压力，相当于 9000-10000 米的海拔高度。与 CIH 模型不同，一组动物接受慢性持续缺氧（CSH）预处理，这是一种连续低压缺氧 360 毫米汞柱的方案，相当于约 6000 米的海拔高度，每天 10 小时共 7 天。通过神经激肽-1 受体（NK1R）的存在，在 pre-BötC 中检测到 5-HT 和 5-HT2AR 的免疫反应性。我们发现，5-HT 免疫反应性（ir）终末中有 15.5%与 NK1R-ir 神经元接触。可以识别它们之间的不对称突触。38.7%的 NK1R-ir 树突也对 5-HT2AR 呈免疫反应性，在对照动物中，5-HT2AR 沿质膜内表面分布。CIH 挑战增加了 pre-BötC 中 5-HT 和 5-HT2AR 的表达，而在 CSH 动物中，该区域未检测到这种增加的表达 5-HT2AR。具体而言，5-HT2AR 不仅分布在内表面，还分布在外表面，或直接分布在质膜上，这在对照动物中是无法检测到的。质膜的内陷中也可以检测到 5-HT2AR，这可能发生受体内吞或胞吐作用，表明 CIH 诱导 5-HT2AR 更高的转运。同时，pre-BötC 中磷酸化蛋白激酶 Cθ（P-PKCθ）上调，提示 5-HT/5-HT2AR 激活的 PKC 机制可能有助于 pre-BötC 中的低氧诱导呼吸神经可塑性。P-PKCθ 与突触后密度的密切关联暗示了介导 pre-BötC 呼吸神经可塑性的突触后机制。
