Influence of CO2 and HCO3− on Basal Activity and Acid Sensitivity of Chemosensitive Neurons in the Retrotrapezoid Nucleus

Abstract

Central chemoreception is the mechanism by which the brain regulates breathing in response to changes in tissue CO2/H+. A region of the brainstem called the retrotrapezoid nucleus (RTN) is an important site of chemoreception. The mechanisms of RTN chemoreception involve direct H+‐mediated activation of chemosensitive neurons, and indirect modulation of chemosensitive neurons by CO2/H+‐dependent ATP‐purinergic signaling. Recent evidence identifies astrocytes as the source of CO2/H+‐evoked ATP release, possibly by a mechanism involving CO2‐mediated opening of connexin 26 hemichannels. However, the extent to which ATP contributes to RTN chemoreception is questionable; some studies report that ATP release from astrocytes is required for chemosensitive RTN neurons to sense changes in CO2/H+, whereas other studies showed that blocking ATP receptors in the RTN decreased (~ 30%) but did not eliminate CO2/H+‐sensitivity of RTN chemoreceptors. Based on evidence that CO2 is required for ATP release by RTN astrocytes, we chose to investigate the contribution of purinergic to RTN chemoreception by comparing the firing responses of chemosensitive RTN neurons to graded acidifications in the presence and absence of CO2 and HCO3−. Cell‐attached recordings were used to identify RTN chemoreceptors in brainstem slices (300 μm thick) isolated from rat pups (P7–11 days postnatal) incubated in a bicarbonate (26 mM) buffered solution by their characteristic response to CO2; i.e., low basal activity in 5% CO2 (pH=7.3; 0.7 ± 0.1 Hz) and high activity in 10% CO2 (pH=7.0; 1.7 ± 0.3 Hz) or 15% CO2 (pH=6.9; 2.2 ± 0.4 Hz). After returning to control conditions (5% CO2; pH=7.3) switching to a bicarbonate‐free HEPES buffered solution (pH=7.3) decreased baseline activity by 0.5 ± 0.4 Hz (T3=5.175, p=0.014; paired t‐test). In HEPES buffer, acidifications to pH=7.0 and pH=6.9 increased activity by 0.65 ± 0.2 Hz and 0.8 ± 0.2 Hz, respectively. In some cases, during prolonged exposure to HEPES baseline activity increased to near control levels, suggesting that removal of CO2/HCO3− from the perfusate only transiently suppressed chemoreceptor activity. These results suggest that RTN chemoreceptors respond more vigorously to hypercapnic acidosis compared to the same acidification in HEPES buffer. This finding is consistent with the possibility that CO2‐evoked ATP release from astrocytes contributes in part to RTN chemoreception. These results also suggest that HCO3− controls baseline activity of RTN chemoreceptors, possibly by influencing intracellular pH or regulating HCO3−‐sensitive K+ channels (i.e., KCNQ).Support or Funding InformationNIH Grant HL104101