Time-dependent changes in spontaneous respiratory activity in turtle brainstems in vitro.

Our goal was to determine whether time-dependent changes in respiratory motor output in vitro could be minimized by altering bath solution composition. Adult turtle brainstems were bathed in standard solution, nutrient-rich Dulbecco's Eagle media (100 or 25% concentration), or standard solution with phenylbiguanide (PBG, 5-HT3 agonist which increases respiratory drive). Except for a 63% frequency increase in PBG solution, hypoglossal bursts were unaltered within 100 min of observation. Respiratory activity was abolished within 7 h in 100% Dulbecco's compared with a mean of 24-31 h in other test solutions. At 12 h, burst frequency decreased faster in standard solution and 25% Dulbecco's (-0.28+/-0.07 and -0.13+/-0.05 bursts/h, respectively) compared with PBG solution (-0.09+/-0.04 bursts/h); amplitude declined at approximately 2%/h in all solutions. The tendency for episodic discharge decreased gradually in standard solution, but was eliminated in 25% Dulbecco's and PBG solution. Certain bath solutions may minimize time-dependent frequency reductions but may also cause breathing pattern changes.

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 microM) reversibly decreased integrated burst amplitude by approximately 45% (P < 0.05); burst frequency decreased in a dose-dependent manner with 20 microM abolishing bursts in 9 of 13 preparations (P < 0.05). These 5-HT-dependent effects were mimicked by application of a 5-HT(1A) agonist, but not a 5-HT(1B) agonist, and were abolished by the broad-spectrum 5-HT antagonist, methiothepin. During 5-HT (20 microM) washout, frequency rebounded to levels above the original baseline for 40 min (P < 0.05) and remained above baseline for 2 h. A 5-HT(3) antagonist (tropesitron) blocked the post-5-HT rebound and persistent frequency increase. A 5-HT(3) agonist (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 altered. The 5-HT(3) receptor-dependent persistent frequency increase represents a unique model of plasticity in vertebrate rhythm generation.