5-HYDROXYTRYPTAMINE MODULATES CENTRAL RESPIRATORY ACTIVITY IN THE NEWBORN RAT - AN INVITRO STUDY

Respiratory activity was recorded in cervical ventral roots during in vitro experiments performed on superfused brainstem-cervical cord preparations from newborn rats aged 0-3 days. The effects of 5-HT agents added to the bathing medium on respiratory frequency were analysed. Adding 5-hydroxytryptamine (5-HT, 30-mu-M) or 5-HT precursors (either L-tryptophane, L-Trp, 50-mu-M, or 5-hydroxytryptophane, 5-HTP, 50-mu-M) elicited increases in respiratory frequency. Precursor effects were blocked by a 5-HT antagonist (methysergide, 70-mu-M) and significantly reduced by pretreatment with a 5-HT synthesis inhibitor (p-chlorophenylalanine, PCPA, i.p. 300 mg/kg at birth). Adding drugs known to block (methysergide, 40-mu-M) or to potentiate (fluoxetine, 100-mu-M) any endogenous 5-HT effects decreased or increased respiratory frequency, respectively. These results suggest that the 5-HT biosynthesis mechanisms remain functional in vitro and that a continuous release of endogenous 5-HT exerts excitatory modulation on the respiratory rhythm generator.