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Neuromuscular inactivity of the respiratory control system during overwintering in bullfrogs, Lithobates catesbeianus

In clinical disorders, inactivity typically results in loss of normal neuromuscular function. In contrast, hibernating, estivating, and overwintering animals use compensatory mechanisms and/or avoidance strategies to preserve motor function in the face of inevitable dormancy. Semiaquatic frogs typically overwinter in ponds that may be covered by ice and, therefore, do not regulate lung breathing for weeks to months. Despite a lack of lung breathing, motor function of the respiratory system in American bullfrogs, Lithobates catesbeianus, remains fully competent during the winter and operates adequately to match ventilation to metabolism in early spring. Unlike overwintering hibernators with unloaded limb muscles and putatively inactive locomotor systems, frogs may continue producing respiratory motor activity during cold‐submergence to maintain a functional respiratory motor system. Inactivity avoidance could occur because anuran amphibians have respiratory mechanics that, counterintuitively, can produce ventilatory motor patterns under water. Therefore, we hypothesized that bullfrogs generate respiratory motor patterns during cold‐submergence as a mechanism to avoid disuse and preserve motor performance. To test this hypothesis, we measured activity of respiratory muscles (buccal floor compressor, posterior m. intermandibularis and glottal dilator, m. dilator laryngis) via chronic electromyography (EMG) in freely behaving bullfrogs at 20°C and while submerged at 2°C (n=8). As expected, activation of the glottal dilators coincided with large amplitude activity of the buccal floor compressors to produce lung ventilation. In contrast, small amplitude oscillatory activity of the buccal floor compressors occurred without activation of glottal dilators, indicative of buccal ventilation. We found that bullfrogs took 5.5±1.4 breaths per minute when surfacing at 20°C, and we confirmed that underwater “ventilation cycles” occur at a rate of 1.7±0.2 events per minute at 20°C. At 2°C, bullfrogs neither surfaced to ventilate their lungs (2°C vs. 20°C; p<0.0001; one‐way ANOVA), nor did they activate either group of respiratory muscles when submerged at 2°C for 10 days (2°C vs. 20°C; p<0.0001; one‐way ANOVA). Tonic activity characteristic of resting muscle tone did not change after submergence at 2°C in buccal floor compressors or the glottal dilators, for at least 10 days (buccal floor, p=0.4, one‐way ANOVA; glottal dilator, p=0.55, one‐way ANOVA). In conclusion, bullfrogs experience bona fide respiratory motor inactivity during cold‐submergence associated with overwintering. Underwater neuromuscular activation is, therefore, an unlikely mechanism to preserve respiratory motor function after overwintering, but normal function may be maintained through compensation at the level of the motor network.