The serotonin type 3 receptor (5-HT3) is a ligand-gated ion channel that converts the binding of the neurotransmitter serotonin (5-HT) into a transient cation current that mediates fast excitatory responses in peripheral and central nervous systems. Information regarding the activation and modulation of the human 5-HT3 type A receptor has been based only on macroscopic current measurements because of its low ion conductance. By constructing a high-conductance human 5-HT3A receptor, we here revealed mechanistic information regarding the orthosteric activation by 5-HT and by the partial agonist tryptamine, and the allosteric activation by the terpenoids, carvacrol, and thymol. Terpenoids potentiated macroscopic currents elicited by the orthosteric agonist and directly elicited currents with slow-rising phases and submaximal amplitudes. At the single-channel level, activation by orthosteric and allosteric agonists appeared as openings in quick succession (bursts) that showed no ligand concentration dependence. Bursts were grouped into long-duration clusters in the presence of 5-HT and even longer in the presence of terpenoids, whereas they remained isolated in the presence of tryptamine. Kinetic analysis revealed that allosteric and orthosteric activation mechanisms can be described by the same scheme that includes transitions of the agonist-bound receptor to closed intermediate states before opening (priming). Reduced priming explained the partial agonism of tryptamine; however, equilibrium constants for gating and priming were similar for 5-HT and terpenoid activation. Thus, our kinetic analysis revealed that terpenoids are efficacious agonists for 5-HT3A receptors. These findings not only extend our knowledge about the human 5-HT3A molecular function but also provide novel insights into the mechanisms of action of allosteric ligands, which are of increasing interest as therapeutic drugs in all the superfamily.

中文翻译：

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人类 5-HT3A 受体的正构和变构激活

3 型血清素受体 (5-HT3) 是一种配体门控离子通道，可将神经递质血清素 (5-HT) 的结合转化为瞬时阳离子电流，介导外周和中枢神经系统的快速兴奋反应。由于其低离子电导，关于人 5-HT3 A 型受体的激活和调节的信息仅基于宏观电流测量。通过构建高电导人类 5-HT3A 受体，我们在这里揭示了有关 5-HT 和部分激动剂色胺的正构激活以及萜类化合物、香芹酚和百里酚的变构激活的机制信息。萜类化合物增强了由正构激动剂引发的宏观电流，并直接引发了具有缓慢上升阶段和次最大振幅的电流。在单通道水平上，正构和变构激动剂的激活表现为快速连续（爆发）的开口，没有显示配体浓度依赖性。在 5-HT 存在的情况下，爆发被分成长时间的簇，在萜类化合物的存在下甚至更长，而在色胺的存在下，它们仍然是孤立的。动力学分析表明，变构和正构激活机制可以通过相同的方案来描述，包括激动剂结合受体在打开（启动）之前转变为闭合中间状态。启动减少解释了色胺的部分激动作用；然而，门控和引发的平衡常数对于 5-HT 和萜类化合物的激活是相似的。因此，我们的动力学分析表明萜类化合物是 5-HT3A 受体的有效激动剂。