Endogenous serotonin modulates the fetal respiratory rhythm: an in vitro study in the rat

The aim of the present work was to know whether the excitatory modulation of the central respiratory rhythm generator by serotonin (5-HT) previously found to occur in the newborn rat, is already functional during the fetal life. Experiments were performed at embryonic day 18 (D18) and 20-21 (D20-21; full-term day 21) on the fetal rat brainstem-spinal cord preparation in which the ability to generate central respiratory activity in vitro persists. Replacing the normal medium which bathed the preparation by a medium containing 5-HT increased the respiratory frequency (RF) within 2-3 min in a dose-dependent manner in both D18 and D20-21 fetuses but the effect was particularly drastic at D18. Applying a medium containing the 5-HT antagonist, methysergide, to block the effect of endogenous 5-HT, if any, reduced the RF within 2-3 min and the reduction was especially drastic at D18 where respiratory arrests occurred for several minutes in most of the experiments. Applying a medium containing either the 5-HT reuptake inhibitor fluoxetine to potentiate the effect of endogenous 5-HT or the 5-HT precursor, L-tryptophan, to activate 5-HT biosynthesis mechanisms, increased the RF. To define the type of 5-HT receptors involved in the modulation of the RF, experiments were conducted with specific 5-HT agonists and antagonists. Both 5-HT1 (8-OH-DPAT, buspirone) and 5-HT2 agonists (DOI, alpha-methyl-5-HT) increased the RF but only the 5-HT1A agonist 8-OH-DPAT was efficient at submicromolar concentrations. Applying the 5-HT1A antagonist NAN-190 alone decreased the RF and even elicited respiratory arrests while the 5-HT2 antagonist ketanserin was inefficient. NAN-190 pre-treatment blocked the increase in the RF due to 8-OH-DPAT and 5-HT. Taken as a whole these results clearly indicate that endogenous 5-HT exerts an excitatory modulation on the respiratory rhythm generator via activation of medullary 5-HT1A receptors well before birth, as soon as D18 where the modulation is particularly potent.