Emergence of air breathing in vertebrates

从水呼吸到空气呼吸的转变可以说是动物生理学上最壮观的变化。鉴于水和空气之间物理化学性质的巨大差异，呼吸介质的成功转换需要产生和调节呼吸的神经回路的重大转变。以美国牛蛙（Lithobates catesbeianus）为主要模型系统，本研究计划的长期目标是了解与脊椎动物出现空气呼吸相关的呼吸控制系统内的神经生理学变化。 在生命早期，蝌蚪通过产生连续的、高频率的、低振幅的颊泵激活来刺激它们的鳃。随着肺的发育，口腔泵逐渐出现更有力但频率较低的激活，将空气推入肺中。在离体脑干（体外）从前变质蝌蚪进行的实验表明，驱动肺通气的电路存在之前，空气呼吸有助于气体交换，但被抑制。鉴于有证据表明产生肺部通气的运动命令的表达具有高度可塑性，我们建议将联合收割机电生理研究（分离的脑干制备、用膜片钳记录全细胞）与呼吸活动的整体动物测量相结合（体内）以解决以下具体目标： 1)三叉神经运动神经元的电特性和形态在发育过程中发生了显著的变化。 2)酸碱状态的发育变化有助于空气呼吸的出现。 3)甲状腺激素（TH）促进前变质蝌蚪呼吸空气的表达。 4)脑干缺氧足以促进前变质蝌蚪呼吸空气的表达。 为下一个资助周期提出的研究是我的研究计划中的一个重要里程碑，其成果将促进呼吸神经生物学的基础知识。通过将简单的实验与“最先进”的电生理方法相结合，该计划将帮助学员获得研究的基本专业知识以及非常理想的技能，无论他们的学术培训阶段如何。

The transition from water to air breathing is arguably the most spectacular change in animal physiology. Given the substantial differences in physico-chemical properties between water and air, a successful transition in respiratory medium requires significant transformations in the neural circuits that generate and regulate breathing. Using the American bullfrog (Lithobates catesbeianus) as our main model system, the long term objective of my research program is to understand the neurophysiological changes within the respiratory control system associated with the emergence of air breathing in vertebrates. During early life, tadpoles ventilate their gills by producing a continuous, high frequency, low amplitude activation of the buccal pump. As the lungs develop, a more forceful but less frequent activation of the buccal pump emerges progressively to push air into the lungs. Experiments performed on isolated brainstems (in vitro) from pre-metamorphic tadpoles show that the circuits driving lung ventilation are present well before air breathing contributes to gas exchange but is inhibited. In light of the evidence indicating that expression of the motor command producing lung ventilation is highly plastic, we propose to combine electrophysiological studies (isolated brainstem preparation, whole cell recording with patch clamp) with whole animal measurements of respiratory activity (in vivo) to address the following specific aims: 1) The electrical properties and morphology of trigeminal motoneurons undergo significant changes during development. 2) Developmental changes in acid-base status contribute to the emergence of air breathing. 3) Thyroid hormone (TH) facilitates the expression of air breathing in pre-metamorphic tadpoles. 4) Brainstem hypoxia is sufficient to facilitate expression of air breathing in pre-metamorphic tadpoles. The research proposed for the next grant cycle is an important milestone in my research program and the outcome will advance basic knowledge in respiratory neurobiology. By combining simple experiments with “state of the art” electrophysiological approaches, this program will help trainees acquire basic expertise in research as well as highly desirable skills regardless of the stage of their academic training.