Department of Animal Morphology and Physiology, College of Agricultural and Veterinary Sciences, São Paulo State University, Unesp. Jaboticabal, SP 14884-900, Brazil.
Department of Pediatrics, Institut Universitaire de Cardiologie et de Pneumologie de Québec, Université Laval, Québec, QC, Canada, G1V 4G5.
J Exp Biol. 2021 Apr 15;224(8). doi: 10.1242/jeb.240804. Epub 2021 Apr 16.
In pre-metamorphic tadpoles, the neural network generating lung ventilation is present but actively inhibited; the mechanisms leading to the onset of air breathing are not well understood. Orexin (ORX) is a hypothalamic neuropeptide that regulates several homeostatic functions, including breathing. While ORX has limited effects on breathing at rest, it potentiates reflexive responses to respiratory stimuli mainly via ORX receptor 1 (OX1R). Here, we tested the hypothesis that OX1Rs facilitate the expression of the motor command associated with air breathing in pre-metamorphic bullfrog tadpoles (Lithobates catesbeianus). To do so, we used an isolated diencephalic brainstem preparation to determine the contributions of OX1Rs to respiratory motor output during baseline breathing, hypercapnia and hypoxia. A selective OX1R antagonist (SB-334867; 5-25 µmol l-1) or agonist (ORX-A; 200 nmol l-1 to 1 µmol l-1) was added to the superfusion media. Experiments were performed under basal conditions (media equilibrated with 98.2% O2 and 1.8% CO2), hypercapnia (5% CO2) or hypoxia (5-7% O2). Under resting conditions gill, but not lung, motor output was enhanced by the OX1R antagonist and ORX-A. Hypercapnia alone did not stimulate respiratory motor output, but its combination with SB-334867 increased lung burst frequency and amplitude, lung burst episodes, and the number of bursts per episode. Hypoxia alone increased lung burst frequency and its combination with SB-334867 enhanced this effect. Inactivation of OX1Rs during hypoxia also increased gill burst amplitude, but not frequency. In contrast with our initial hypothesis, we conclude that ORX neurons provide inhibitory modulation of the CO2 and O2 chemoreflexes in pre-metamorphic tadpoles.
在变态前期的蝌蚪中，产生肺通气的神经网络已经存在，但被积极抑制；导致空气呼吸开始的机制尚不清楚。食欲素（ORX）是一种下丘脑神经肽，调节包括呼吸在内的几种稳态功能。虽然 ORX 在休息时对呼吸的影响有限，但它主要通过 ORX 受体 1（OX1R）增强对呼吸刺激的反射反应。在这里，我们测试了这样一个假设，即 OX1Rs 促进与变态前期牛蛙蝌蚪（Lithobates catesbeianus）空气呼吸相关的运动指令的表达。为此，我们使用分离的脑桥延髓脑片来确定 OX1Rs 在基线呼吸、高碳酸血症和低氧血症期间对呼吸运动输出的贡献。选择性 OX1R 拮抗剂（SB-334867；5-25 μmol l-1）或激动剂（ORX-A；200 nmol l-1 至 1 μmol l-1）被添加到灌流介质中。实验在基础条件下（用 98.2% O2 和 1.8% CO2 平衡的介质）、高碳酸血症（5% CO2）或低氧血症（5-7% O2）下进行。在休息状态下，鳃，但不是肺，运动输出被 OX1R 拮抗剂和 ORX-A 增强。高碳酸血症单独不能刺激呼吸运动输出，但它与 SB-334867 的组合增加了肺爆发频率和幅度、肺爆发次数和每个爆发的次数。低氧血症单独增加了肺爆发频率，而其与 SB-334867 的组合增强了这种作用。在低氧血症期间 OX1Rs 的失活也增加了鳃爆发幅度，但不增加频率。与我们最初的假设相反，我们得出结论，ORX 神经元对变态前期蝌蚪的 CO2 和 O2 化学感受器反射提供抑制性调节。
