Postsynaptic mechanisms of CO2 responses in parafacial respiratory neurons of newborn rats

The central chemoreceptors for respiratory control in the medulla sense changes in CO2 concentration and regulate respiratory activity. Neurons that express a transcription factor, Phox2b, in the parafacial region of the rostral and ventrolateral medulla are excited by hypercapnic stimulation and are proposed to play an important role in central chemoreception. In this study, we show evidence that Phox2b-expressing parafacial neurons in neonatal rats were sensitive to hypercapnia via direct action on the postsynaptic membrane without contribution of putative presynaptic or other calcium-dependent components. Since these parafacial neurons are also a part of the respiratory rhythm generator in neonates, they are essential for postnatal survival, which is probably due to their contribution to central chemoreception as well as The parafacial respiratory group (pFRG) in the rostral ventrolateral medulla of the newborn rat is predominantly composed of pre-inspiratory (Pre-I) neurons and is involved in respiratory rhythm generation. The subgroup located close to the ventral surface (at least partially overlapping the retrotrapezoid nucleus, RTN) expresses the Phox2b transcription factor and responds to hypercapnic stimulation with strong depolarization, which suggests it has a role in central chemoreception. Although a CO2 response of pFRG/RTN neurons has been confirmed in the presence of tetrodotoxin (TTX), it is unknown whether the depolarization involved in this response is induced by a direct postsynaptic response of pFRG/RTN neurons or by any presynaptic components mediated by Ca2+-dependent mechanisms. In this study, we examined the effects of ATP or substance P receptor antagonists on hypercapnic responses of rostral pFRG/RTN neurons. We tested effects of Cd2+ and low Ca2+-high Mg2+ in the presence of TTX. The experiments were performed in in vitro brainstem-spinal cord preparations from newborn rats in which Pre-I neurons reflect the discharge pattern of the pFRG. We found that ATP receptor and substance P receptor antagonists do not block membrane potential responses to hypercapnic stimulation (2%-> 8%) of pFRG/RTN neurons in the rostral parafacial region. Moreover, rostral pFRG/RTN neurons were depolarized by hypercapnia under conditions where the contribution of presynaptic components was inhibited in the presence of TTX and Cd2+ or in a low Ca2+-high Mg2+ solution containing TTX and Cd2+. All cases (except some cases in a low Ca2+-high Mg2+ solution) of membrane depolarization by hypercapnic stimulation were accompanied with an increase in input resistance. These neurons were predominantly Phox2b immunoreactive. Our findings suggest that the response of pFRG/RTN neurons to hypercapnia is induced by direct action on the postsynaptic membrane via closing of K+ channels.