Medullary 5-HT neurons: Switch from tonic respiratory drive to chemoreception during postnatal development

Abstract

Serotonin (5-HT) neurons contribute to respiratory chemoreception in adult mice, but it is unclear whether they play a similar role in neonatal mice. We studied breathing during development in Lmx1bf/f/p mice, which lack 5-HT neurons. From postnatal days 1–7 (P1–P7), ventilation of Lmx1bf/f/p mice breathing room air was 50% of WT mice (p < 0.001). By P12, baseline ventilation increased to a level equal to WT mice. In contrast, the hypercapnic ventilatory response (HCVR) of neonatal Lmx1bf/f/p and WT mice were equal to each other, but were both much less than adult WT mice. By P21 the HCVR of WT mice increased to near adult levels, but the HCVR of Lmx1bf/f/p mice had not changed, and was 42% less than WT mice. Primary cell cultures were prepared from the ventromedial medulla of neonatal mice, and patch-clamp recordings were made from neurons identified as serotonergic by expression of a reporter gene. In parallel with developmental changes of the HCVR in vivo, 5-HT neurons had little chemosensitivity to acidosis until 12 days in vitro (DIV), after which their response increased to reach a plateau around 25 DIV. Neonatal Lmx1bf/f/p mice displayed high mortality and decreased growth rate, and this worsened in hypoxia. Mortality was decreased in hyperoxia. These results indicate that maturation of 5-HT neurons contributes to development of respiratory CO2/pH chemoreception during the first few weeks of life in mice in vivo. A defect in the 5-HT system in early postnatal life decreases survival due in part to hypoxia.

Development of a Novel FIJI‐Based Method to Investigate Neuronal Circuitry in Neonatal Mice

“…Our results suggest that the number of Nkx2.2 ‐derived afferents to the PreBötC during the first three weeks of postnatal life decreases. These data are in line with recent findings that serotonergic medullary neurons switch their function from rhythm generation to chemoreception between P12 and P21 (Cerpa et al, ). Olig3 regulates the development of dorsal interneurons and precerebellar neurons in the hindbrain (Liu et al, ; Zechner et al, ).…”

Chemosensitivity of Phox2b‐expressing retrotrapezoid neurons is mediated in part by input from 5‐HT neurons

“…), and the hypercapnic ventilatory response increases over this age range in rodents in vivo , as does chemosensitivity of 5‐HT neurons in vitro (Wang & Richerson, ; Cerpa et al . ).…”

“…Cell culture has advantages for studying neuronal electrophysiology compared to brain slices, such as lack of recent traumatic and ischaemic injury (Richerson & Messer, ), more stable and longer duration patch clamp recordings, ability to study neurons that are older and more developmentally mature (Cerpa et al . ), and better long‐term control of experimental conditions such as prolonged or repeated exposure to drugs. However, neuronal properties can sometimes be altered in cell culture, making it important to verify that the neuronal properties to be studied are comparable to those in brain slices.…”

“…; Cerpa et al . ), in part to enhance stability of patch clamp recordings. To determine whether chemosensitivity was dependent on 5‐HT receptor activation at a temperature closer to normal body temperature, the above experiments were repeated with bath temperature = 31°C (Fig.…”

“…Cultured cells were allowed to mature for at least 20 days prior to using them for experiments (Cerpa et al . ).…”