Analyzing the brainstem circuits for respiratory chemosensitivity in freely moving mice

Regulation of systemic PCO₂ is a life-preserving homeostatic mechanism. In the medulla oblongata, the retrotrapezoid nucleus (RTN) and rostral medullary Raphe are proposed as CO₂ chemosensory nuclei mediating adaptive respiratory changes. Hypercapnia also induces active expiration, an adaptive change thought to be controlled by the lateral parafacial region (pF_L). Here, we use GCaMP6 expression and head-mounted mini-microscopes to image Ca²⁺ activity in these nuclei in awake adult mice during hypercapnia. Activity in the pF_L supports its role as a homogenous neuronal population that drives active expiration. Our data show that chemosensory responses in the RTN and Raphe differ in their temporal characteristics and sensitivity to CO₂, raising the possibility these nuclei act in a coordinated way to generate adaptive ventilatory responses to hypercapnia. Our analysis revises the understanding of chemosensory control in awake adult mouse and paves the way to understanding how breathing is coordinated with complex non-ventilatory behaviours.