CO2-sensitive Cx26 hemichannels in dorsal raphe-ventral tegmental area pathway mediate hypercapnic arousal

Abstract

Hypercapnic arousal is a life-preserving reflex that promotes awakening in response to elevated PCO2, yet the cellular mechanisms by which CO2 is detected remain incompletely understood. Serotonergic neurons of the dorsal raphe (DR) have been implicated in hypercapnic responses, but the molecular sensor that directly detects CO2 within this circuit is unknown. Here, we show that the CO2-gated connexin 26 (Cx26) hemichannel is expressed on serotonergic terminals projecting from the DR to the ventral tegmental area (VTA). Brief elevations in PCO2 modulated the excitability of VTA dopamine neurons through serotonergic signaling without altering glutamatergic transmission, and these effects were abolished by pharmacological inhibition of hemichannels. By expressing Cx26 and genetically encoded GRAB5HT sensors in cultured cells, we further demonstrate that Cx26 hemichannels opened by elevated PCO2 are directly permeable to serotonin. Selective removal of CO2 sensitivity from Cx26 in DR serotonergic neurons delayed arousal from sleep during transient hypercapnia and reduced activation of VTA dopamine neurons in vivo. Together, these findings identify Cx26 hemichannels as a direct CO2 sensor that enables serotonin release through a channel co-synapse and contributes to hypercapnic arousal.