Phox2b-expressing retrotrapezoid neurons are intrinsically responsive to H+ and CO2

Abstract

Central respiratory chemoreceptors sense changes in CO2/H+ and initiate the adjustments to ventilation required to preserve brain and tissue pH. The cellular nature of the sensors (neurons and/or glia) and their CNS location are not conclusively established but the glutamatergic, Phox2b-expressing neurons located in the retrotrapezoid nucleus (RTN) are strong candidates. However, a direct demonstration that RTN neurons are intrinsically sensitive to CO2/H+, required for designation as a chemosensor, has been lacking. To address this, we tested the pH sensitivity of RTN neurons that were acutely dissociated from two lines of Phox2b-GFP BAC transgenic mice. All GFP-labeled cells assayed by reverse transcriptase-PCR (n=40) were Phox2b+, VGlut2+, TH-, and ChAT-, the neurochemical phenotype previously defined for chemosensitiveRTNneurons in vivo.We found that most dissociatedRTNneurons from both lines of mice were CO2/H+-sensitive (~79%), with discharge increasing during acidification and decreasing during alkalization. The pHsensitive cells could be grouped into two populations characterized by similar pH sensitivity but different basal firing rates, as previously observed in recordings from GFP-labeled RTN neurons in slice preparations. In conclusion, these data indicate that RTN neurons are inherently pH-sensitive, as expected for a respiratory chemoreceptor. © 2013 the authors.