An alternative approach to the identification of respiratory central chemoreceptors in the brainstem

Central chemoreceptors (CCRs) play a crucial role in autonomic respiration. Although a variety of brainstem neurons are CO₂ sensitive, it remains to know which of them are the CCRs. In this article, we discuss a potential alternative approach that may allow an access to the CCRs. This approach is based on identification of specific molecules that are CO₂ or pH sensitive, exist in brainstem neurons, and regulate cellular excitability. Their molecular identity may provide another measure in addition to the electrophysiologic criteria to indicate the CCRs. The inward rectifier K⁺ channels (Kir) seem to be some of the CO₂ sensing molecules, as they regulate membrane potential and cell excitability and are pH sensitive. Among homomeric Kirs, we have found that even the most sensitive Kir1.1 and Kir2.3 have pK∼6.8, suggesting that they may not be capable of detecting hypocapnia. We have studied their biophysical properties, and identified a number of amino acid residues and molecular motifs critical for the CO₂ sensing. By comparing all Kirs using the motifs, we found the same amino acid sequence in Kir5.1, and demonstrated the pH sensitivity in heteromeric Kir4.1 and Kir5.1 channels to be pK∼7.4. In current clamp, we show evidence that the Kir4.1–Kir5.1 can detect P_CO2 changes in either hypercapnic or hypocapnic direction. Our in-situ hybridization studies have indicated that they are coexpressed in brainstem cardio–respiratory nuclei. Thus, it is likely that the heteromeric Kir4.1–Kir5.1 contributes to the CO₂/pH sensitivity in these neurons. We believe that this line of research intended to identify CO₂ sensing molecules is an important addition to current studies on the CCRs.