Connexin hemichannel‐mediated CO2‐dependent release of ATP in the medulla oblongata contributes to central respiratory chemosensitivity

Arterial , a major determinant of breathing, is detected by chemosensors located in the brainstem. These are important for maintaining physiological levels of in the blood and brain, yet the mechanisms by which the brain senses CO2 remain controversial. As ATP release at the ventral surface of the brainstem has been causally linked to the adaptive changes in ventilation in response to hypercapnia, we have studied the mechanisms of CO2‐dependent ATP release in slices containing the ventral surface of the medulla oblongata. We found that CO2‐dependent ATP release occurs in the absence of extracellular acidification and correlates directly with the level of . ATP release is independent of extracellular Ca2+ and may occur via the opening of a gap junction hemichannel. As agents that act on connexin channels block this release, but compounds selective for pannexin‐1 have no effect, we conclude that a connexin hemichannel is involved in CO2‐dependent ATP release. We have used molecular, genetic and immunocytochemical techniques to demonstrate that in the medulla oblongata connexin 26 (Cx26) is preferentially expressed near the ventral surface. The leptomeninges, subpial astrocytes and astrocytes ensheathing penetrating blood vessels at the ventral surface of the medulla can be loaded with dye in a CO2‐dependent manner, suggesting that gating of a hemichannel is involved in ATP release. This distribution of CO2‐dependent dye loading closely mirrors that of Cx26 expression and colocalizes to glial fibrillary acidic protein (GFAP)‐positive cells. In vivo, blockers with selectivity for Cx26 reduce hypercapnia‐evoked ATP release and the consequent adaptive enhancement of breathing. We therefore propose that Cx26‐mediated release of ATP in response to changes in is an important mechanism contributing to central respiratory chemosensitivity. The brain can detect CO2 in the blood and adjust our breathing so that the level of CO2, and hence the acidity of the blood, remains within physiological limits. Although the areas of the brain involved in detecting CO2 are known, we do not understand how CO2 is detected. We show that a connexin channel, Cx26, which often forms a passageway between cells, but in this case acts as a hemichannel that opens to the extracellular space, is expressed in glial cells of the CO2‐sensing areas of the brain. Cx26 opens in proportion to the level of CO2 to release ATP, which then excites the neurons that control breathing. Our results identify