Department of Biological Sciences, San José State University, San Jose, CA, USA.
J Physiol. 2021 Jun;599(11):2953-2967. doi: 10.1113/JP281182. Epub 2021 Apr 18.
Muscle spindle afferents are slowly adapting low threshold mechanoreceptors that report muscle length and movement information critical for motor control and proprioception. The rapidly adapting cation channel PIEZO2 has been identified as necessary for muscle spindle afferent stretch sensitivity, although the properties of this channel suggest that additional molecular elements are necessary for mediating the complex slowly adapting response of muscle spindle afferents. We report that glutamate increases muscle spindle afferent static sensitivity in an ex vivo mouse muscle nerve preparation, although blocking glutamate packaging into vesicles by the sole vesicular glutamate transporter, VGLUT1, either pharmacologically or by transgenic knockout of one allele of VGLUT1 decreases muscle spindle afferent static but not dynamic sensitivity. Our results confirm that vesicle-released glutamate is an important contributor to maintained muscle spindle afferent excitability and may suggest a therapeutic target for normalizing muscle spindle afferent function.
Muscle spindle afferents are slowly adapting low threshold mechanoreceptors that have both dynamic and static sensitivity to muscle stretch. The exact mechanism by which these neurons translate muscle movement into action potentials is not well understood, although the PIEZO2 mechanically sensitive cation channel is essential for stretch sensitivity. PIEZO2 is rapidly adapting, suggesting the requirement for additional molecular elements to maintain firing during stretch. Spindle afferent sensory endings contain glutamate-filled synaptic-like vesicles that are released in a stretch- and calcium-dependent manner. Previous work has shown that glutamate can increase and a phospholipase-D coupled metabotropic glutamate antagonist can abolish firing during static stretch. Here, we test the hypothesis that vesicle-released glutamate is necessary for maintaining muscle spindle afferent excitability during static but not dynamic stretch. To test this hypothesis, we used a mouse muscle-nerve ex vivo preparation to measure identified muscle spindle afferent responses to stretch and vibration. In C57BL/6 adult mice, bath applied glutamate significantly increased the firing rate during the plateau phase of stretch but not during the dynamic phase of stretch. Blocking the packaging of glutamate into vesicles by the sole vesicular glutamate transporter, VGLUT1, either with xanthurenic acid or by using a transgenic mouse with only one copy of the VGLUT1 gene (VGLUT1 ), decreased muscle spindle afferent firing during sustained stretch but not during vibration. Our results suggest a model of mechanotransduction where calcium entering the PIEZO2 channel can cause the release of glutamate from synaptic-like vesicles, which then helps to maintain afferent depolarization and firing.
肌梭传入纤维是一种缓慢适应的低阈值机械感受器，可报告对运动控制和本体感觉至关重要的肌肉长度和运动信息。快速适应的阳离子通道 PIEZO2 已被确定为肌梭传入纤维拉伸敏感性所必需的，尽管该通道的特性表明，介导肌梭传入纤维复杂的缓慢适应反应还需要其他分子元件。我们报告说，在离体小鼠肌肉神经制备中，谷氨酸可增加肌梭传入纤维的静态敏感性，尽管通过单独的囊泡谷氨酸转运体 VGLUT1 药理学或转基因敲除 VGLUT1 的一个等位基因来阻断谷氨酸包装到囊泡中，会降低肌梭传入纤维的静态敏感性，但不会降低动态敏感性。我们的结果证实，囊泡释放的谷氨酸是维持肌梭传入纤维兴奋性的重要贡献者，并且可能为正常化肌梭传入纤维功能提供治疗靶标。
肌梭传入纤维是一种缓慢适应的低阈值机械感受器，对肌肉拉伸具有动态和静态敏感性。这些神经元将肌肉运动转化为动作电位的确切机制尚不清楚，尽管 PIEZO2 机械敏感阳离子通道对于拉伸敏感性是必需的。PIEZO2 是快速适应的，这表明需要其他分子元件来维持在拉伸过程中的发射。梭内感觉末梢包含充满谷氨酸的突触样小泡，这些小泡以拉伸和钙依赖性的方式释放。先前的工作表明，谷氨酸可以增加并且与磷酯酶 D 偶联的代谢型谷氨酸受体拮抗剂可以在静态拉伸期间消除发射。在这里，我们测试了这样一个假设，即囊泡释放的谷氨酸对于维持肌梭传入纤维在静态但不是动态拉伸期间的兴奋性是必需的。为了验证该假设，我们使用 C57BL/6 成年小鼠的离体肌肉神经制备来测量对拉伸和振动的已识别肌梭传入纤维的反应。在成年 C57BL/6 小鼠中，浴施加的谷氨酸可显著增加拉伸平台期的放电频率，但不增加拉伸动态期的放电频率。通过唯一囊泡谷氨酸转运体 VGLUT1（黄嘌呤酸）或使用只有一个 VGLUT1 基因（VGLUT1 ）拷贝的转基因小鼠来阻止谷氨酸包装到囊泡中，降低了持续拉伸过程中的肌梭传入纤维放电频率，但降低了振动过程中的放电频率。我们的结果提出了一种机械转导模型，其中进入 PIEZO2 通道的钙可以引起突触样小泡释放谷氨酸，这有助于维持传入纤维去极化和发射。
