Breathing is maintained and controlled by a network of neurons in the brainstem that generate respiratory rhythm and provide regulatory input. Central chemoreception, the mechanism for CO2 detection that provides an essential stimulatory input, is thought to involve neurons located near the medullary surface, whose nature is controversial. Good candidates are serotonergic medullary neurons and glutamatergic neurons in the parafacial region. Here, we show that mice bearing a mutation in Phox2b that causes congenital central hypoventilation syndrome in humans breathe irregularly, do not respond to an increase in CO2, and die soon after birth from central apnea. They specifically lack Phox2b-expressing glutamatergic neurons located in the parafacial region, whereas other sites known or supposed to be involved in the control of breathing are anatomically normal. These data provide genetic evidence for the essential role of a specific population of medullary interneurons in driving proper breathing at birth and will be instrumental in understanding the etiopathology of congenital central hypoventilation syndrome.brainstem ͉ congenital central hypoventilation syndrome ͉ neurodegenerative disease ͉ respiration

Heterozygous mutations of the transcription factor PHOX2B have been found in most patients with central congenital hypoventilation syndrome, a rare disease characterized by sleep-related hypoventilation and impaired chemosensitivity to sustained hypercapnia and sustained hypoxia. PHOX2B is a master regulator of autonomic reflex pathways, including peripheral chemosensitive pathways. In the present study, we used hyperoxic tests to assess the strength of the peripheral chemoreceptor tonic drive in Phox2bϩ/Ϫ newborn mice. We exposed 69 wild-type and 67 mutant mice to two hyperoxic tests (12-min air followed by 3-min 100% O2) 2 days after birth. Breathing variables were measured noninvasively using whole body flow plethysmography. The initial minute ventilation decrease was larger in mutant pups than in wild-type pups: Ϫ37% (SD 13) and Ϫ25% (SD 18), respectively, P Ͻ 0.0001. Furthermore, minute ventilation remained depressed throughout O2 exposure in mutants, possibly because of their previously reported impaired CO2 chemosensitivity, whereas it returned rapidly to the normoxic level in wild-type pups. Hyperoxia considerably increased total apnea duration in mutant compared with wild-type pups (P ϭ 0.0001). A complementary experiment established that body temperature was not influenced by hyperoxia in either genotype group and, therefore, did not account for genotype-related differences in the hyperoxic ventilatory response. Thus partial loss of Phox2b function by heterozygosity did not diminish the tonic drive from peripheral chemoreceptors.control of breathing; chemosensitivity; apnea CENTRAL CONGENITAL HYPOVENTILATION syndrome (CCHS or Ondine's curse) is a rare disease, generally present from birth and characterized by hypoventilation during sleep in the absence of primary neuromuscular or lung disease or of brain stem lesions (3). Throughout life, patients with CCHS have absent or markedly reduced ventilatory responses to sustained hypercapnia (29) and, to a lesser extent, to sustained hypoxia (29). These respiratory impairments are generally ascribed to impaired central integration of chemosensory inputs at the brain stem level, rather than to failure of chemoreceptor activity, which is at least partially present (15,22,37). The genetic basis for CCHS was discovered recently: most patients with CCHS have a heterozygous mutation of the PHOX2B gene (2,23,41,43). PHOX2B is a master regulator of the noradrenergic phenotype and of all neuronal relays of autonomic medullary reflex pathways (30), including peripheral chemosensitive pathways (9).Mice with a single functional Phox2b allele (i.e., Phox2bϩ/Ϫ mice) provide a unique opportunity to investigate the genotype-phenotype relationship in CCHS. Our laboratory's previous studies showed that Phox2bϩ/Ϫ mice had longer sleep apnea times than their wild-type littermates on postnatal day (P) 5 (P5) (11) and a weaker response to CO 2 on P2, followed by normalization before P10 (9). These ventilatory impairments were reminiscent of CCHS. However, the hyperpneic response t...