Potential bullfrog homologue to the chemosensitive mammalian retrotrapezoid nucleus (1092.4)

Abstract

Vertebrate breathing is controlled by a highly conserved brainstem neural network. Air breathing is generally driven by elevated CO2 in body tissues/fluids and is present in all vertebrate classes. Bullfrogs transition from water to air breathing during metamorphosis, and exhibit central CO2 sensitivity throughout development. In mammals, many sites contribute to this process, and their relative contributions vary according to CO2/pH level, arousal state, and development. Potential central CO2 sensors have been extensively investigated and described in mammals but not amphibians. The retrotrapezoid nucleus (RTN) lies on the medullary surface near the facial nucleus and is crucial for chemosensitivity in developing mammals. The locus coeruleus and classical “rostral” and “caudal” medullary chemosensitive regions have been identified in frogs. Furthermore, the “rostral” and “caudal” sites correspond to the locations of the mammalian RTN and Pre‐Botzinger Complex, respectively, suggesting potential homology. Immunohistochemical markers for the mammalian RTN include presence of Phox2b transcription factor and vesicular glutamate transporter 2 (VGLUT2), and absence of tyrosine hydroxylase (TH), the enzymatic marker of dopamine synthesis. We tested the hypothesis that a chemosensitive region homologous to the mammalian RTN is present in bullfrog. We identified this area by staining for RTN markers: presence of Phox2b and VGLUT2, and absence of TH. We describe this region in different developmental stages and identify metamorphic changes. We also assessed chemosensitivity of this region with cFos, a marker of neural activity. Establishing this as a homologous chemosensory site between amphibians and mammals provides insight into evolutionary origins of chemosensory development and function.Grant Funding Source: Supported by NSF IOS‐1022442