Respiratory pattern in midline-lesioned brainstems and hemibrainstems from adult turtles

Discrete midline lesions uncouple left and right respiratory motor output in mammals, but not in frogs and lampreys. To address this question in reptiles, isolated adult turtle brainstems were cut along the midline while recording respiratory motor output (bursts of action potentials) on left and right hypoglossal (XII) nerves. XII motor bursts were synchronized as long as a small portion of the midline was still intact. When turtle brainstems were completely cut along the midline and separated into hemibrainstems, XII motor bursts were produced that could be abolished by mu-opioid receptor (MOR) activation or exposure to high pH (7.80) solution. Also, 13/57 hemibrainstems expressed episodic discharge (>1.75 bursts/episode). To test whether crossed connections were necessary to express a long-lasting increase in burst frequency (i.e., frequency plasticity), phenylbiguanide (PBG, 5-HT3 receptor agonist, 20 μM) was bath-applied to hemibrainstems. Although PBG significantly increased burst frequency by 0.43 ± 0.10 bursts/min after 60 min, no frequency plasticity was observed because burst frequency returned to near baseline levels after a 2-h washout. Thus, crossed connections in turtle brainstems synchronize respiratory motor output and are not required for normal respiratory pattern formation, but are required for PBG-dependent frequency plasticity.