Perinatal Fluoxetine Exposure Impairs the CO2 Chemoreflex. Implications for Sudden Infant Death Syndrome

Microglial reactivity in brainstem chemosensory nuclei in response to hypercapnia

Microglia, the resident immune cells of the CNS, surveil, detect, and respond to various extracellular signals. Depending on the nature of these signals, an integrative microglial response can be triggered, resulting in a phenotypic transformation. Here, we evaluate whether hypercapnia modifies microglia phenotype in brainstem respiratory-related nuclei. Adult C57BL/6 inbred mice were exposed to 10% CO2 enriched air (hypercapnia), or pure air (control), for 10 or 30 min and immediately processed for immunohistochemistry to detect the ubiquitous microglia marker, ionized calcium binding adaptor molecule 1 (Iba1). Hypercapnia for thirty, but not 10 min reduced the Iba1 labeling percent coverage in the ventral respiratory column (VRC), raphe nucleus (RN), and nucleus tractus solitarius (NTS) and the number of primary branches in VRC. The morphological changes persisted, at least, for 60 min breathing air after the hypercapnic challenge. No significant changes were observed in Iba1+ cells in the spinal trigeminal nucleus (Sp5) and the hippocampus. In CF-1 outbred mice, 10% CO2 followed by 60 min of breathing air, resulted in the reduction of Iba1 labeling percent coverage and the number and length of primary branches in VRC, RN, and NTS. No morphological change was observed in Iba1+ cells in Sp5 and hippocampus. Double immunofluorescence revealed that prolonged hypercapnia increased the expression of CD86, an inflammatory marker for reactive state microglia, in Iba1+ cells in VRC, RN, and NTS, but not in Sp5 and hippocampus in CF-1 mice. By contrast, the expression of CD206, a marker of regulatory state microglia, persisted unmodified. In brainstem, but not in hippocampal microglia cultures, hypercapnia increased the level of IL1β, but not that of TGFβ measured by ELISA. Our results show that microglia from respiratory-related chemosensory nuclei, are reactive to prolonged hypercapnia acquiring an inflammatory-like phenotype.

Development of the Placenta and Brain Are Affected by Selective Serotonin Reuptake Inhibitor Exposure During Critical Periods

Selective serotonin reuptake inhibitors (SSRIs) are usually prescribed to treat major depression and anxiety disorders. Fetal brain development exhibits dependency on serotonin (5-hydroxytryptamine, 5-HT) from maternal, placental, and fetal brain sources. At very early fetal stages, fetal serotonin is provided by maternal and placental sources. However, in later fetal stages, brain sources are indispensable for the appropriate development of neural circuitry and the rise of emergent functions implied in behavior acquisition. Thus, susceptible serotonin-related critical periods are recognized, involving the early maternal and placental 5-HT synthesis and the later endogenous 5-HT synthesis in the fetal brain. Acute and chronic exposure to SSRIs during these critical periods may result in short- and long-term placental and brain dysfunctions affecting intrauterine and postnatal life. Maternal and fetal cells express serotonin receptors which make them susceptible to changes in serotonin levels influenced by SSRIs. SSRIs block the serotonin transporter (SERT), which is required for 5-HT reuptake from the synaptic cleft into the presynaptic neuron. Chronic SSRI administration leads to pre- and postsynaptic 5-HT receptor rearrangement. In this review, we focus on the effects of SSRIs administered during critical periods upon placentation and brain development to be considered in evaluating the risk-safety balance in the clinical use of SSRIs.

Serotonin and the ventilatory effects of etonogestrel, a gonane progestin, in a murine model of congenital central hypoventilation syndrome

INTRODUCTION

Congenital Central Hypoventilation Syndrome, a rare disease caused by PHOX2B mutation, is associated with absent or blunted CO₂/H⁺ chemosensitivity due to the dysfunction of PHOX2B neurons of the retrotrapezoid nucleus. No pharmacological treatment is available. Clinical observations have reported non-systematic CO₂/H⁺ chemosensitivity recovery under desogestrel.

METHODS

Here, we used a preclinical model of Congenital Central Hypoventilation Syndrome, the retrotrapezoid nucleus conditional Phox2b mutant mouse, to investigate whether etonogestrel, the active metabolite of desogestrel, led to a restoration of chemosensitivity by acting on serotonin neurons known to be sensitive to etonogestrel, or retrotrapezoid nucleus PHOX2B residual cells that persist despite the mutation. The influence of etonogestrel on respiratory variables under hypercapnia was investigated using whole-body plethysmographic recording. The effect of etonogestrel, alone or combined with serotonin drugs, on the respiratory rhythm of medullary-spinal cord preparations from Phox2b mutants and wildtype mice was analyzed under metabolic acidosis. c-FOS, serotonin and PHOX2B were immunodetected. Serotonin metabolic pathways were characterized in the medulla oblongata by ultra-high-performance liquid chromatography.

RESULTS

We observed etonogestrel restored chemosensitivity in Phox2b mutants in a non-systematic way. Histological differences between Phox2b mutants with restored chemosensitivity and Phox2b mutant without restored chemosensitivity indicated greater activation of serotonin neurons of the raphe obscurus nucleus but no effect on retrotrapezoid nucleus PHOX2B residual cells. Finally, the increase in serotonergic signaling by the fluoxetine application modulated the respiratory effect of etonogestrel differently between Phox2b mutant mice and their WT littermates or WT OF1 mice, a result which parallels with differences in the functional state of serotonergic metabolic pathways between these different mice.

DISCUSSION

Our work thus highlights that serotonin systems were critically important for the occurrence of an etonogestrel-restoration, an element to consider in potential therapeutic intervention in Congenital Central Hypoventilation Syndrome patients.

Analyzing the brainstem circuits for respiratory chemosensitivity in freely moving mice

Regulation of systemic PCO2 is a life-preserving homeostatic mechanism. In the medulla oblongata, the retrotrapezoid nucleus (RTN) and rostral medullary Raphe are proposed as CO2 chemosensory nuclei mediating adaptive respiratory changes. Hypercapnia also induces active expiration, an adaptive change thought to be controlled by the lateral parafacial region (pFL). Here, we use GCaMP6 expression and head-mounted mini-microscopes to image Ca2+ activity in these nuclei in awake adult mice during hypercapnia. Activity in the pFL supports its role as a homogenous neuronal population that drives active expiration. Our data show that chemosensory responses in the RTN and Raphe differ in their temporal characteristics and sensitivity to CO2, raising the possibility these nuclei act in a coordinated way to generate adaptive ventilatory responses to hypercapnia. Our analysis revises the understanding of chemosensory control in awake adult mouse and paves the way to understanding how breathing is coordinated with complex non-ventilatory behaviours.

Prenatal fluoxetine has long lasting, differential effects on respiratory control in male and female rats

Serotonin (5-HT) is an important modulator of brain networks that control breathing. The selective serotonin reuptake inhibitor fluoxetine (FLX) is the first-line antidepressant drug prescribed during pregnancy. We investigated the effects of prenatal FLX on baseline breathing, ventilatory and metabolic responses to hypercapnia and hypoxia as well as number of brainstem 5-HT and tyrosine hydroxylase (TH) neurons of rats during postnatal development (P0-82). Prenatal FLX exposure of males showed a lower baseline that appeared in juveniles and remained in adulthood, with no sleep-wake state dependency. Prenatal FLX exposure of females did not affect baseline breathing. Juvenile male FLX rats showed increased CO₂ and hypoxic ventilatory responses, normalizing by adulthood. Alterations in juvenile-FLX treated males were associated with greater number of 5-HT neurons in the ROB and RMAG. Adult FLX-exposed males showed greater number of 5-HT neurons in the RPA and TH neurons in the A5, while reduced number of TH neurons in A7. Prenatal FLX exposure of female rats was associated with greater hyperventilation induced by hypercapnia at P0-2 and juveniles whereas P12-14 and adult FLX (NREM sleep) rats showed an attenuation of the hypercapnic hyperventilation.FLX-exposed females had fewer 5-HT neurons in the RPA and reduced TH A6 density at P0-2; and greater number of TH neurons in the A7 at P12-14. These data indicate that prenatal FLX exposure affects the number of neurons of some monoaminergic regions in the brain and results in long lasting, sex specific changes in baseline breathing pattern and ventilatory responses to respiratory challenges.

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

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 Lmx1b^f/f/p mice, which lack 5-HT neurons. From postnatal days 1-7 (P1-P7), ventilation of Lmx1b^f/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 Lmx1b^f/f/p and WT mice was 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 Lmx1b^f/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 12days in vitro (DIV), after which their response increased to reach a plateau around 25 DIV. Neonatal Lmx1b^f/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.