Emergence of air breathing in vertebrates

背景：从水呼吸到空气呼吸的转变可以说是动物生理上最壮观的变化。然而，这种转变需要中枢神经系统内的重大转变，而这方面的文献仍然很少。我的研究计划的长期目标是阐明导致脊椎动物空气呼吸出现的神经变化。因为牛蛙处于水和空气呼吸的过渡阶段，所以它们为探索这一迷人的现象提供了一个独特的动物模型。在我最后一个NSERC授权期内，我们测试了一个假设，即调节青蛙变态的内分泌和环境信号“揭示”了驱动肺通气的运动指令。我和我的学员证明，缺氧和甲状腺激素增加了蝌蚪孤立脑干中有效空气呼吸的发生。我们在未来5年的主要目标是通过评估性类固醇对驱动空气呼吸的运动指令的影响来完成和扩大这一富有成效的努力。原理：两栖动物是脊椎动物中第一个利用脊髓内的神经元产生主动呼气和辅助肺通气的动物。然而，并不是在所有物种中都观察到这一点，因为目前的数据表明牛蛙已经失去了这种能力。然而，青蛙可以招募脊髓神经元来发出求偶声的事实让我们重新考虑了这一观点。由于性类固醇对发声很重要，并且是大脑发育的有力决定因素，我们将测试一种假设，即性类固醇的升高对于牛蛙的脊髓神经元的募集是必要的，从而驱动肺通气。在这里，实验将在成年青蛙和发育中的蝌蚪身上进行；雄性激素和雌性激素的水平将在实验中对两性动物进行控制。我们将进行神经解剖学追踪研究，以评估脊髓运动神经元与产生呼吸活动的大脑部分之间的连通性。这些实验将辅以电生理学研究，以评估选择性激活/抑制目标脑区对呼吸活动的影响。研究和培训的影响：到下一个授权期结束时，可以合理地预期我和我的学员将对青蛙空气呼吸出现的神经内分泌决定因素有一个全面的了解。我们的研究也将为呼吸节律发生和主动呼气产生的基本原理提供新的线索。这项工作的潜在影响是重大的：最近对基于性别的研究的兴趣暴露了我们对性激素在呼吸神经生物学中的贡献缺乏了解。总的来说，这个研究项目将继续为学员提供学习动物生理学和神经科学方面的优秀技能的绝佳机会。

BACKGROUND:    The transition from water to air breathing is arguably the most spectacular change in animal physiology. However, this transition requires significant transformations within the central nervous system that are still poorly documented. The long-term objective of my research program is to elucidate the neurological changes that contribute to the emergence of air breathing in vertebrates. Because they lie at the transition between water and air breathing, bullfrogs offer a unique animal model to explore this fascinating phenomenon. During my last NSERC granting term we tested the hypothesis that the endocrine and environmental signals regulating metamorphosis in frogs "reveal" the motor command driving lung ventilation. My trainees and I showed that hypoxia and thyroid hormones augment the occurrence of fictive air breaths in isolated brainstems from tadpoles. Our main objective over the next 5 years is to complete and expand this fruitful endeavour by evaluating the impacts of sex steroids on the motor command driving air breathing. RATIONALE:    Amphibians are the first vertebrates in which neurons located within the spinal cord are recruited to produce active expiration and assist lung ventilation. However, this is not observed in all species as the current data suggest that bullfrogs have lost this ability. However, the fact that frogs can recruit spinal neurons to produce mating calls brings us to reconsider this view. Since sex steroids are important for vocalisation and are powerful determinants of brain development we will test the hypothesis that elevation of sex steroids is necessary for the recruitment of spinal neurons to drive lung ventilation in bullfrogs. Here, experiments will be performed in adult frogs and developing tadpoles; the levels of androgens and estrogens will be manipulated experimentally in animals of both sexes. We will perform neuroanatomical tracing studies to evaluate the connectivity between spinal motoneurons and the parts of the brain that generate respiratory activity. These experiments will be complemented by electrophysiological studies to evaluate the impacts of selective activation/inhibition of target brain areas on respiratory activity. IMPACT OF RESEARCH AND TRAINING:    By the end of the next granting term, it is reasonable to anticipate that my trainees and I will have provided a comprehensive understanding of the neuroendocrine determinants of the emergence of air breathing in frogs. Our research will also shed new light on respiratory rhythmogenesis and the basic principles responsible for the production of active expiration. The potential impact of this work is significant: the recent interest for sex-based research has exposed our lack of knowledge on the contribution of sex hormones in respiratory neurobiology. As a whole, this research program will continue to provide trainees excellent opportunities to learn highly desirable skills in animal physiology and neuroscience.