Ontogeny of rhythmic breathing‐related motor activity in the Zebra Finch brainstem

Abstract

Activity in the CNS begins as a widely distributed and synchronous depolarization wave known as rhythmic spontaneous neural activity (rSNA). As growth and maturation proceed, rSNA differentiates into central pattern generators (CPGs) that produce individual rhythmic behaviors, such as breathing. While it is important to understand the mechanisms that underlie this progression, monitoring the uninterrupted development of breathing‐related motor circuits remains difficult due to the limited access of these circuits in the mammalian model and a lack of information about breathing‐related CPG behavior in other models, such as birds. Here, we use a novel brainstem spinal cord preparation from the altricial Zebra Finch embryo to study the early development and maturation of breathing‐related motor patterns conveyed in cranial nerve IX. We show that rSNA can be measured continuously from embryonic day 4 (E4) through external pipping (E14). rSNA starts at embryonic day 4 (E4) with low frequency (0.29 bursts/min) short duration (<2.5 sec) single bursts, and progresses to a more complex pattern consisting of episodes with short duration bursts coupled to slow frequency (0.2 burst/minute) long duration bursts (>80 sec) at E9. The activity returns to short duration (<2.5 sec), high frequency (6‐12 bursts/min) at hatching (E14). At E4 the activity is nicotinergic‐sensitive and in the older animals (>E7) the short duration bursts are completely blocked by NMDA and non‐NMDA antagonism. The long duration bursts only diminished by bath application of m‐type glutamate antagonists. These findings establish an extremely tractable new experimental model to study the development of central breathing behaviors in both health and disease.