Serotonin elicits long-lasting enhancement of rhythmic respiratory activity in turtle brain stems in vitro

Brain stem preparations from adult turtles were used to determine how bath- applied serotonin (5-HT) alters respiration-related hypoglossal activity in a mature vertebrate. 5-HT (5-20 muM) reversibly decreased integrated burst amplitude by similar to 45% (P < 0.05); burst frequency decreased in a dos e-dependent manner with 20 <mu>M abolishing bursts in 9 of 13 preparations (P < 0.05). These 5-HT-dependent effects were mimicked by application of a 5-HT1A agonist, but not a 5-HT1B agonist, and were abolished by the broad-s pectrum 5-HT antagonist, methiothepin. During 5-HT (20 <mu>M) washout, freq uency rebounded to levels above the original baseline for 40 min (P < 0.05) and remained above baseline for 2 h. A 5-HT3 antagonist (tropesitron) bloc ked the post-5-HT rebound and persistent frequency increase. A 5-HT3 agonis t (phenylbiguanide) increased frequency during and after bath application ( P < 0.05). When phenylbiguanide was applied to the brain stem of brain stem /spinal cord preparations, there was a persistent frequency increase (P < 0 .05), but neither spinal-expiratory nor -inspiratory burst amplitude were a ltered. The 5-HT3 receptor-dependent persistent frequency increase represen ts a unique model of plasticity in vertebrate rhythm generation.