Heterozygous mutations of the transcription factor PHOX2B are responsible for Congenital Central Hypoventilation Syndrome, a neurological disorder characterized by inadequate respiratory response to hypercapnia and life-threatening hypoventilation during sleep. Although no cure is currently available, it was suggested that a potent progestin drug provides partial recovery of chemoreflex response. Previous in vitro data show a direct molecular link between progestins and PHOX2B expression. However, the mechanism through which these drugs ameliorate breathing in vivo remains unknown. Here, we investigated the effects of chronic administration of the potent progestin drug Etonogestrel (ETO) on respiratory function and transcriptional activity in adult female rats. We assessed respiratory function with whole-body plethysmography and measured genomic changes in brain regions important for respiratory control. Our results show that ETO reduced metabolic activity, leading to an enhanced chemoreflex response and concurrent increased breathing cycle variability at rest. Furthermore, ETO-treated brains showed reduced mRNA and protein expression of PHOX2B and its target genes selectively in the dorsal vagal complex, while other areas were unaffected. Histological analysis suggests that changes occurred in the solitary tract nucleus (NTS). Thus, we propose that the NTS, rich in both progesterone receptors and PHOX2B, is a good candidate for ETO-induced respiratory modulation.

Breathing movements driven by the brainstem rhythmogenic neural networks rely on chemosensory inputs to produce a motor output that is homeostatically adjusted to the prevailing metabolic demand. The central CO ₂ chemoreflex is a critical component of respiratory neural circuitry, as defects in these sensors cause hypoventilation syndromes, which are typically difficult to manage pharmacologically. Progesterone has long been known to stimulate breathing in both sexes, and remarkably, a potent progestin drug, etonogestrel (ETO), has been shown to enhance CO ₂ ‐chemosensitivity in animal models and in a subset of female patients affected by congenital central hypoventilation syndrome. Our recent work shows that systemic chronic ETO treatment recovered the CO ₂ chemoreflex in female rats in which <80% of neurons of the key CO ₂ chemosensory structure of the retrotrapezoid nucleus (RTN) were eliminated, whereas male rats with similar lesions and ETO serum levels did not show recovery of the CO ₂ chemoreflex. Interestingly, female respiratory recovery was associated with increased expression of the pH sensors Task2 and Gpr4 mRNA in the surviving RTN neurons, suggesting that progestin drugs may recover the CO ₂ chemoreflex responses in a sex‐specific fashion.

PHOX2B is a transcription factor essential for the development of different classes of neurons in the central and peripheral nervous system. Heterozygous mutations in the PHOX2B coding region are responsible for the occurrence of Congenital Central Hypoventilation Syndrome (CCHS), a rare neurological disorder characterised by inadequate chemosensitivity and life-threatening sleep-related hypoventilation. Animal studies suggest that chemoreflex defects are caused in part by the improper development or function of PHOX2B expressing neurons in the retrotrapezoid nucleus (RTN), a central hub for CO₂ chemosensitivity. Although the function of PHOX2B in rodents during development is well established, its role in the adult respiratory network remains unknown. In this study, we investigated whether reduction in PHOX2B expression in chemosensitive neuromedin-B (NMB) expressing neurons in the RTN altered respiratory function. Four weeks following local RTN injection of a lentiviral vector expressing the short hairpin RNA (shRNA) targeting Phox2b mRNA, a reduction of PHOX2B expression was observed in Nmb neurons compared to both naive rats and rats injected with the non-target shRNA. PHOX2B knockdown did not affect breathing in room air or under hypoxia, but ventilation was significantly impaired during hypercapnia. PHOX2B knockdown did not alter Nmb expression but it was associated with reduced expression of both Task2 and Gpr4, two CO₂/pH sensors in the RTN. We conclude that PHOX2B in the adult brain has an important role in CO₂ chemoreception and reduced PHOX2B expression in CCHS beyond the developmental period may contribute to the impaired central chemoreflex function.