ATP and mechanisms of central CO2 chemosensitivity

Abstract

ATP release from the surface of the ventro-lateral medulla (VLM) is integral to the hypercapnic response in vivo and can be seen in vitro. By employing horizontal slices of the ventral medulla containing the ventral chemosensitive nuclei, I have developed a model that consistently evokes hypercapnia-induced ATP release in vitro. Using this preparation I have studied CO2-triggered ATP release by means of microelectrode biosensors. I conclude that it is the change in PCO2 itself, and not associated pH changes that accompany it, that is directly responsible for eliciting ATP release from the surface of the VLM. In addition ATP release from this region may have a role in the response to hypocapnia as well as hypercapnia. Using pharmacological agents I have demonstrated that gating of connexin hemichannels mediates ATP release. The dorso-ventral distribution of Cx26 ascertained via quantitative PCR and immunofluorescence makes this hemichannel the most likely candidate. Dye loading the cells responsible for ATP release with carboxyfluorescein, which co-localised with Cx26, revealed these cells reside in the pia mater and subpial astrocytes. Application of gap-junction antagonists, with selectivity towards connexin 26, greatly reduced ATP release in response to elevated CO2 in vitro and in vivo and reduced the tone of ATP at the VLM surface. Moreover, by loading Cx26 expressing HeLa cells with ATP, I was able to recapitulate the entire in vivo response. Therefore I propose that ATP is released from sub-pial astrocytes and leptomeningeal cells through connexin 26 hemichannels in response to alterations in PCO2. Here Cx26 performs a dual role, as both the chemosensory transducer and the conduit for ATP release