5-HT₃ receptor-dependent modulation of respiratory burst frequency, regularity, and episodicity in isolated adult turtle brainstems

To determine the role of central serotonin 5-HT₃ receptors in respiratory motor control, respiratory motor bursts were recorded from hypoglossal (XII) nerve rootlets on isolated adult turtle brainstems during bath-application of 5-HT₃ receptor agonists and antagonists. mCPBG and PBG (5-HT₃ receptor agonists) acutely increased XII burst frequency and regularity, and decreased bursts/episode. Tropisetron and MDL72222 (5-HT₃ antagonists) increased bursts/episode, suggesting endogenous 5-HT₃ receptor activation modulates burst timing in vitro. Tropisetron blocked all mCPBG effects, and the PBG-induced reduction in bursts/episode. Tropisetron application following mCPBG application did not reverse the long-lasting (2 h) mCPBG-induced decrease in bursts/episode. We conclude that endogenous 5-HT₃ receptor activation regulates respiratory frequency, regularity, and episodicity in turtles and may induce a form of respiratory plasticity with the long-lasting changes in respiratory regularity.