Animal models indicate that serotonin (5-HT) release onto motoneurons facilitates motor output, particularly during strong motor activities. However, evidence for 5-HT effects during human movement are limited. This study examined how antagonism of the 5-HT2 receptor, which is a 5-HT receptor that promotes motoneuron excitability, affects human movement. Ten healthy participants (24.2 ± 1.9 yr) ingested 8 mg of cyproheptadine (competitive 5-HT2 antagonist) in a double-blinded, placebo-controlled, repeated-measures design. Transcranial magnetic stimulation (TMS) of the motor cortex was used to elicit motor evoked potentials (MEPs) from biceps brachii. First, stimulus-response curves (90-160% active motor threshold) were obtained during very weak elbow flexions (10% of maximal). Second, to determine if 5-HT effects are scaled to the intensity of muscle contraction, TMS at a fixed intensity was applied during elbow flexions of 20, 40, 60, 80 and 100% of maximal. Cyproheptadine reduced the size of MEPs across the stimulus-response curves (P = 0.045). Notably, MEP amplitude was 22.3% smaller for the cyproheptadine condition for the strongest TMS intensity. In addition, cyproheptadine reduced maximal torque (P = 0.045), lengthened the biceps silent period during maximal elbow flexions (P = 0.037), and reduced superimposed twitch amplitude during moderate-intensity elbow flexions (P = 0.035). This study presents novel evidence that 5-HT2 receptors influence corticospinal-motoneuronal output, which was particularly evident when a large number of descending inputs to motoneurons were active. While it is likely that antagonism of 5-HT2 receptors reduces motoneuron gain to ionotropic inputs, supraspinal mechanisms may have also contributed to the study findings.

中文翻译：

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5-HT2 受体拮抗作用降低人皮质脊髓运动神经元输出

动物模型表明，5-羟色胺 (5-HT) 释放到运动神经元上可促进运动输出，尤其是在强烈的运动活动期间。然而，人类运动过程中 5-HT 效应的证据有限。该研究检查了 5-HT2 受体（一种促进运动神经元兴奋性的 5-HT 受体）的拮抗作用如何影响人体运动。在双盲、安慰剂对照、重复测量设计中，10 名健康参与者（24.2 ± 1.9 岁）摄入了 8 mg 赛庚啶（竞争性 5-HT2 拮抗剂）。运动皮层的经颅磁刺激 (TMS) 用于从肱二头肌中引发运动诱发电位 (MEP)。首先，在非常微弱的肘部屈曲（最大值的 10%）期间获得刺激-反应曲线（90-160% 主动运动阈值）。第二，为了确定 5-HT 效应是否与肌肉收缩强度成比例，在肘部屈曲 20%、40%、60%、80% 和 100% 的最大值期间应用了固定强度的 TMS。赛庚啶降低了刺激-反应曲线上 MEP 的大小 (P = 0.045)。值得注意的是，对于最强 TMS 强度的赛庚啶条件，MEP 幅度小 22.3%。此外，赛庚啶降低了最大扭矩（P = 0.045），延长了最大肘关节屈曲期间的二头肌静默期（P = 0.037），并减少了中等强度肘关节屈曲期间的叠加抽搐幅度（P = 0.035）。这项研究提出了新的证据，表明 5-HT2 受体影响皮质脊髓运动神经元的输出，当运动神经元的大量下行输入处于活动状态时，这一点尤其明显。