Hypoxia-Tolerance of neural function after hibernation in bullfrogs: Responses to Graded Oxygen Deprivation and tissue metabolic demand

Neural circuit function is vulnerable to low oxygen levels; however, the adult American bullfrog (Aquarana catesbeianus) shows remarkable plasticity in response to hibernation, allowing neural networks to function ~20-times longer without oxygen by relying solely on anaerobic glycolysis. While past studies have focused on sudden and acute anoxia in natural environments, oxygen often changes more gradually, induced by slower changes in ambient conditions or local changes in tissue metabolism (e.g., altered neural activity). Using graded oxygen reductions and a sensor for partial pressures of oxygen (pO 2 ), we monitored real-time changes in tissue pO 2 and correlated them rhythmic activity of in vitro brainstem preparations in control (n=12) and hibernating frogs (n=4). As oxygen levels were lowered in controls, tissue pO 2 dropped, and burst activity decreased in proportion. In addition, we identified that most brains have a critical tissue Po2 near 15-25 mmHg, below which activity became “seizure-like,” highlighting a threshold for circuit failure in control frogs. To address how local changes in metabolic demands influence tissue Po2, we increased network activity by introducing isoproterenol, a β-adrenoreceptor agonist, which elevated burst frequency. This served to raise metabolic demand and accelerate oxygen depletion within the tissue (repeated measures ANOVA; p<0.01). Moreover, the frequency of non-respiratory (seizure-like) bursts also increased during isoproteronol treatment, which further contributed to the drop in tissue pO2. In contrast, tetrodotoxin (TTX), a sodium channel blocker that silences neural output, raised tissue pO 2 levels (repeated measures ANOVA; p<0.01) levels as a consequence of reduced energy consumption. Preliminary findings in 4 hibernators indicate that graded hypoxia elicits similar Po2 changes within the tissue as controls, but no changes in burst rate and no critical points that elicit seizure-like activity, even during anoxia (0 mmHg in tissue). These findings underscore the energy sensitivity of bullfrog neural circuits and suggest that this animal can dramatically reshape its oxygen demands while maintaining neural performance across a wide range of tissue Po2 values. Understanding the mechanisms underlying this adaptation not only provides insights into amphibian physiology but also has potential implications for medical research, such as developing strategies that induce similar types of plasticity that improves function during energy stress before ischemic strokes occur. Future work will access the activity-dependence of tissue Po2 in hibernators, as well as the mechanisms underlying this metabolic plasticity response. National Institutes of Health (R01NS114514). 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.