Filogonio Renato, Saunders Sandy E, Gray Michael, Viteri Jose A, Santin Joseph M
Division of Biological Sciences, University of Missouri, Columbia, MO 65201, USA.
Department of Physical Medicine and Rehabilitation, University of Missouri-Columbia, Columbia, MO 65211, USA.
J Exp Biol. 2025 Mar 15;228(6). doi: 10.1242/jeb.249687. Epub 2025 Mar 24.
Many animals undergo prolonged dormancy periods to survive cold or dry environments. While humans and most laboratory-based mammals experience a loss of neuromuscular function during inactivity, hibernators possess physiological mechanisms to mitigate this loss. The American bullfrog provides an extreme model of this phenomenon, as brainstem circuits that generate breathing are completely inactive during underwater hibernation, during which motoneurons employ various types of synaptic plasticity to ensure adequate respiratory motor output in the spring. In addition to synapses, voltage-gated ion channels may undergo plasticity to boost neuronal output. Therefore, we hypothesized that motoneuron excitability would also be enhanced after hibernation via alterations in voltage-gated ion channels. We used whole-cell patch-clamp electrophysiology to measure membrane excitability and activities of several voltage-gated channels (K+, Ca2+, Na+) from motoneurons that innervate muscles of the buccal pump (hypoglossal) and glottal dilator (vagal). Surprisingly, compared with controls, overwintered hypoglossal motoneurons displayed multiple indices of reduced excitability (hyperpolarized resting membrane potential, lower firing rates, greater lag to first spike). Mechanistically, this occurred via enhanced voltage-gated K+ and reduced Ca2+ channel activity. In contrast, vagal motoneuron excitability was unaltered, but exhibited altered ion channel profiles which seemed to stabilize neuronal output, involving either reduced Ca2+ or K+ currents. Therefore, different motoneurons of the same neuromuscular behavior respond differently to overwintering by altering the function of voltage-gated channels. We suggest divergent responses may reflect different energetic demands of these neurons and/or their specific contribution to breathing and other orofacial behaviors.
许多动物会经历长时间的休眠期以在寒冷或干燥的环境中生存。虽然人类和大多数基于实验室的哺乳动物在不活动期间会出现神经肌肉功能丧失，但冬眠动物拥有减轻这种丧失的生理机制。美国牛蛙提供了这一现象的极端模型，因为在水下冬眠期间，产生呼吸的脑干回路完全不活动，在此期间运动神经元利用各种类型的突触可塑性来确保春季有足够的呼吸运动输出。除了突触，电压门控离子通道也可能发生可塑性变化以增强神经元输出。因此，我们假设冬眠后运动神经元的兴奋性也会通过电压门控离子通道的改变而增强。我们使用全细胞膜片钳电生理学来测量支配颊泵（舌下）和声门扩张肌（迷走）肌肉的运动神经元的膜兴奋性和几种电压门控通道（钾离子、钙离子、钠离子）的活性。令人惊讶的是，与对照组相比，越冬后的舌下运动神经元表现出多种兴奋性降低的指标（超极化静息膜电位、较低的放电频率、首次放电的更大延迟）。从机制上讲，这是通过增强电压门控钾离子通道和降低钙离子通道活性发生的。相比之下，迷走运动神经元的兴奋性未改变，但表现出离子通道特征的改变，这似乎稳定了神经元输出，涉及钙离子或钾离子电流的减少。因此，同一神经肌肉行为的不同运动神经元通过改变电压门控通道的功能对越冬有不同的反应。我们认为不同的反应可能反映了这些神经元不同的能量需求和/或它们对呼吸和其他口面部行为的特定贡献。
