Isolated bullfrog rostral brainstems exhibit a DAMGO‐insensitive lung‐like episodic rhythm in the presence of bicuculline

Abstract

Respiratory behavior in terrestrial animals is primarily controlled via neural circuits in the medulla. A site in the amphibian rostral medulla at the level of the abducens nerve (CN VI) is believed to be homologous to the pre‐Bötzinger complex in mammals. The μ‐opioid agonist DAMGO has a suppressive effect, and the GABAA receptor antagonist bicuculline has an excitatory effect on lung activity in mammals and amphibians. However, the locations of action in amphibians are unknown. Therefore, we examined the effect of DAMGO and bicuculline on isolated rostral brainstem tissue.We isolated brainstems from bullfrog tadpoles, placed them in a recording chamber, and allowed them to recover for 1 hour perfused with artificial cerebral spinal fluid (aCSF) equilibrated with 98.5% O2 and 1.5% CO2 (pH 7.8). During this time, neural activity from the facial (CN VII) and hypoglossal (CN XII) cranial nerves was recorded. Following recovery, brainstems were transected just rostral to the trigeminal (CN V) nerve, and at the level of the glossopharyngeal nerve (CN IX), removing the caudal medulla. Then activity from the trigeminal (CN V) and facial (CN VII) nerves was recorded. A cocktail containing either DAMGO (120 nM) and bicuculline methochloride (5 μM) or bicuculline alone was subsequently bath applied for 30 minutes.The transection alone resulted in a loss of discernable respiratory rhythm. However, bicuculline application caused a lung‐like episodic rhythm to reappear. Contrary to the suppressive effect DAMGO has on lung activity in other preparations (including intact isolated amphibian brainstems), this rhythm was not affected by the addition of DAMGO in our transected preparation. This work informs the comparison of amphibian and mammalian respiratory oscillators and the consideration of their homology and differences.Support or Funding InformationNIH under Award Numbers UL1GM118991, TL4GM118992, or RL5GM118990.