Development of avian intrapulmonary chemoreceptors

Although avian intrapulmonary chemoreceptors (IPC) have been studied extensively in adults, the maturation of IPC CO₂ sensitivity during development is completely unknown. To begin investigating IPC development we asked two fundamental questions: (1) Are IPC capable of sensing CO₂ during early development, and, if so, how early? And, (2) does IPC CO₂ sensitivity during early development exhibit postnatal maturation compared to IPC discharge characteristics in adult ducks? We addressed these questions by recording from single IPC Anas platyrhynchos ducklings beginning approximately 6 h prior to internal pipping through 4 days of postnatal development. We then compared mean IPC discharge characteristics during early development with mean IPC activity from adult ducks greater than 12 weeks old. In total, we recorded 28 individual IPC from 5 ducklings and 12 adult ducks. Results show that IPC were capable of responding to rapid step changes in CO₂ before hatching occurred, during the paranatal developmental period. We also found that mean IPC activity during early development had increased peak discharge frequencies, greater spike frequency adaptation, and less tonic CO₂ sensitivity when compared to adults (P ≤ 0.05). These results suggest that during early development phasic IPC CO₂ sensitivity is fully developed, yet tonic IPC CO₂ sensitivity exhibits postnatal maturation possibly associated with hatching. These results also suggest that the mechanisms that underlie phasic and tonic IPC action potential discharge, and therefore the degree of partial spike frequency adaptation, may be independent processes with different developmental trajectories.