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The abnormal metabolism of serotonin during the perinatal period alters respiratory network maturation at birth as revealed by comparing the monoamine oxidase A-deficient transgenic (Tg8) with the control (C3H) mice (Bou-Flores et al., 2000). To know whether these alterations occur only transiently or induce persistent respiratory dysfunction during adulthood, we studied the respiratory activity and regulations in adult C3H and Tg8 mice. First, plethysmographic and pneumotachographic analyses of breathing patterns revealed weaker tidal volumes and shorter inspiratory durations in Tg8 than in C3H mice. Second, electrophysiological studies showed that the firing activity of inspiratory medullary neurons and phrenic motoneurons is higher in Tg8 mice and that of the intercostal motoneurons in C3H mice. Third, histological studies indicated abnormally large cell bodies of Tg8 intercostal but not phrenic motoneurons. Finally, respiratory responses to hypoxia and lung inflation are weaker in Tg8 than in C3H mice. DL-p-chlorophenyl-alanine treatments applied to Tg8 mice depress the high serotonin level present during adulthood; the treated mice recover normal respiratory responses to both hypoxia and lung inflation, but their breathing parameters are not significantly affected. Therefore in Tg8 mice the high serotonin level occurring during the perinatal period alters respiratory network maturation and produces a permanent respiratory dysfunction, whereas the high serotonin level present in adults alters the respiratory regulatory processes. In conclusion, the metabolism of serotonin plays a crucial role in the maturation of the respiratory network and in both the respiratory activity and the respiratory regulations.

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The ventral surface of the caudal ventrolateral medulla (cVLM) has been shown to generate intense respiratory responses after surface acid‐base stimulation. With respect to their chemosensitive characteristics, cVLM neurons have been less studied than other rostral‐most regions of the brainstem. The purpose of these experiments was to determine the bioelectric responses of cVLM neurons to acidic stimuli and to determine their chemosensitive properties. Using extracellular and microiontophoretic techniques, we recorded electrical activities from 117 neurons in an area close to the ventral surface of the cVLM in anaesthetised rats. All neurons were tested for their sensitivity to H+. The fluorescent probe BCECF was used to measure extracellular pH changes produced by the microiontophoretic injection of H+ in brainstem slices. This procedure provided an estimation of the local changes in pH produced by microiontophoretic H+ application in the anaesthetised rat. Neurons coupled to the respiratory cycle, R (n= 51), were not responsive to direct stimulation with H+. Sixty‐six neurons that did respond to H+ stimulation were uncoupled from respiration, and identified as NR neurons. These neurons presented distinct ranges of H+ sensitivity. The neuronal sensitivity to H+ was mainly assessed by the slope of the stimulus‐response curve, where the steeper the slope, the higher the H+ sensitivity. On this basis, NR neurons were classed as being either weakly or highly sensitive to H+. NR neurons with a high H+ sensitivity (n= 12) showed an average value of 34.17 ± 7.44 spikes s−1 (100 nC)−1 (mean ±s.d.) for maximal slope and an EC50 of 126.76 ± 33 nC. Suprathreshold H+ stimulation of highly sensitive NR neurons elicited bursting pattern responses coupled to the respiratory cycle. The bursting responses, which were synchronised with the inspiratory phase and the early expiratory phase of the respiratory cycle, lasted for several seconds before returning to the steady state firing pattern characteristic of the pre‐stimulus condition. These NR neurons, which possess the capacity to detect distinct H+ concentrations in the extracellular microenvironment, are excellent candidates to serve in a chemoreceptor capacity in the caudal medulla.

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