Control of lung ventilation following overwintering conditions in bullfrogs,Lithobates catesbeianus

Abstract

Ranid frogs in northern latitudes survive winter at cold temperatures in aquatic habitats often completely covered by ice. Cold-submerged frogs survive aerobically for several months relying exclusively on cutaneous gas exchange while maintaining temperature-specific acidbase balance. Depending on the overwintering hibernaculum, frogs in northern latitudes could spend several months without access to air, the need to breathe or the chemosensory drive to use neuromuscular processes that regulate and enable pulmonary ventilation. Therefore, we performed experiments to determine whether aspects of the respiratory control system of bullfrogs, Lithobates catesbeianus, are maintained or suppressed following minimal use of air breathing in overwintering environments. Based on the necessity for control of lung ventilation in early spring, we hypothesized that critical components of the respiratory control system of bullfrogs would be functional following simulated overwintering. We found that bullfrogs recently removed from simulated overwintering environments exhibited similar resting ventilation when assessed at 24°C compared with warm-acclimated control bullfrogs. Additionally, ventilation met resting metabolic and, presumably, acid-base regulation requirements, indicating preservation of basal respiratory function despite prolonged disuse in the cold. Recently emerged bullfrogs underwent similar increases in ventilation during acute oxygen lack (aerial hypoxia) compared with warm-acclimated frogs; however, CO 2 -related hyperventilation was significantly blunted following overwintering. Overcoming challenges to gas exchange during overwintering have garnered attention in ectothermic vertebrates, but this study uncovers robust and labile aspects of the respiratory control system at a time point correlating with early spring following minimal to no use of lung breathing in coldaquatic overwintering habitats.

Environmentally induced return to juvenile‐like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

“…This strongly implies that phenotypic plasticity decreases CO 2 /pH sensitivity in response to aquatic overwintering by acting on chemosensory mechanisms (be they intrinsic or extrinsic to the neuron) and not general dysfunction of neurons in the respiratory control network. Collectively, these data indicate that aquatic overwintering conditions lead to a (at least temporary) reduction of ventilatory (Santin & Hartzler, a ) and central (Figs ) CO 2 chemosensitivity, at least in part, by decreasing CO 2 ‐induced firing responses of presumed respiratory control neurons (Figs and ).…”

“…Several lines of evidence suggest that mechanisms underlying CO 2 chemosensitivity of lung breathing are reduced following overwintering. First, ventilatory responses to CO 2 challenge are reduced following aquatic overwintering in vivo through inability to increase breathing frequency (Santin & Hartzler, a ). Second, although we showed that aquatic overwintered bullfrogs had a higher baseline lung frequency, reduced brainstem chemosensitivity was unlikely to have occurred as a result of a ‘ceiling effect’.…”

“…We next took advantage of the brainstem-spinal cord preparation to understand whether the reduction in CO 2 sensitivity that we previously observed in the intact animal (Santin & Hartzler, 2016a) is reflected in the hypercapnic sensitivity of the brainstem. The in vitro brainstem preparation from adult bullfrogs undergoes increased fictive lung breathing during hypercapnia (Harris et al 2002;, while the brain of premetamorphic tadpoles undergoes little to no absolute change (Torgerson et al 1997;Taylor et al 2003b).…”

“…After bullfrogs emerge from submerged overwintering conditions, breathing operates normally at rest, indicating functional respiratory rhythmogenic processes, but has diminished CO 2 sensitivity (Santin & Hartzler, 2016a). In response to increased brain and arterial CO 2 , air breathing vertebrates undergo hyperventilation primarily mediated by brainstem chemoreceptors (Torgerson et al 1997;Taylor et al 2003a,b;Sundin et al 2007;Milsom, 2010).…”

Motor inactivity in hibernating frogs: Linking plasticity that stabilizes neuronal function to behavior in the natural environment

“…Interestingly, once frogs emerge from 2 to 3 months in this environment simulated in the lab, they breathe normally to match their resting metabolic demands at a warm body temperature (Figure 1b). In addition, the motor system retains a sufficient scope to increase beyond baseline during hypoxic challenge, a common respiratory stimulant (Figure 1c; Santin & Hartzler, ). These results show that frogs maintain the capacity for a range of breathing outputs despite the complete absence of rhythmic motor function for several months.…”