Serotonergic modulation of the respiratory rhythm generator at birth: an in vitro study in the rat.

In order to investigate the mechanisms through which serotonin (5-HT) modulates the activity of the respiratory rhythm generator, respiratory activity was recorded from cervical ventral roots of the superfused isolated brainstem-spinal cord preparation of the newborn rat. Replacing the normal bathing medium by a medium containing 5-HT (30 microM) increased the respiratory frequency by 70% of the control value. Intact pontomedullary structures are necessary for this effect to take place, however, since the 5-HT-induced increases in respiratory frequency were no longer observed after elimination (section and electrolytic lesion) of the caudal ventro-lateral pons containing the A5 areas. Local applications of 5-HT (dual bath, microdialysis and microinjection experiments) revealed, however, that 5-HT acts at the medullary level and that its effects are not due to a diffuse action on all the neurons of the medullary respiratory centers but to a specific action focusing on structures located in the rostral ventro-lateral medulla.