Sex hormones and development of the CO₂ventilatory response. What can we learn from amphibians?

Introduction: The ability to sense CO 2 and adjust ventilation to ensure stable CO 2 levels within tissues is a vital homeostatic function. In many vertebrates, the reflexive ventilatory response to CO 2 is modest during early life and increases during development. The mechanisms driving this developmental process are unknown, but the fact that it coincides with sexual maturation suggests that sex hormones contribute to the increase in CO 2 chemoreflex. The main structures are located within the central nervous system and because sex hormones act on numerous tissues, addressing the impact of these hormones on the respiratory control network is challenging. Reduced ( in vitro ) preparations are ideally suited to address this question, but because mammalian preparations are viable only in newborns, we opted for brainstem preparations from American bullfrogs ( Lithobates catesbeianus ). In addition to showing developmental change in CO 2 chemoreflex in vitro, this preparation remains fully functional for at least 24h at all developmental stages. Objective: To determine whether exposing isolated brainstems from American bullfrogs to sex hormones potentiates the fictive breathing response to CO 2 . Based on previous observations made in rats, we focused on estrogens. Methods: Experiments were performed on American bullfrogs at three developmental stages: tadpoles (TK stages VIII-XVII), juvenile frogs and adult frogs. Tadpoles and frogs were anesthetized (MS-222 1 g/l). Brains were harvested and the isolated brainstem preparation was placed in a recording chamber with circulating artificial cerebrospinal fluid at physiological values (98% O 2 ; 2% CO 2 ; 20°C; pH = 7,90). We placed suction electrodes on cranial nerves V and X to record respiratory-related motor activity. The brains underwent chronic (18h) exposure to 10 -7 M E 2 and 10 -9 M PPT, a selective agonist of estrogen receptor α. They were also exposed to hypercapnia (5% CO2; pH = 7,50). Results: The fictive CO 2 ventilatory response varies during the development, at its lowest in tadpoles, at its highest in juvenile frogs and lowering again for adult frogs. Time control experiments showed that respiratory-related neural activity remained stable throughout the protocol. Chronic exposure to drugs did not affect the hypercapnic response of the preparations. Conclusion: The fictive CO 2 ventilatory response of American bullfrogs varies with their developmental stage. However, E 2 and PPT do not seem to have an effect on their fictive CO 2 ventilatory response. This research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.