Simplified schematic representation of the ventral aspect of the rat brainstem. The retrotrapezoid nucleus and midline raphe nuclei contain brainstem carbon dioxide-sensitive neurons (central chemoreceptors) that activate premotor neurons of the ventral respiratory group (VRG) that includes the pre-Bötzinger complex, an area with respiratory rhythm–generating neurons. Afferent sensory input from the peripheral chemoreceptors of the carotid bodies activates the nucleus tractus solitaries (red arrow), which also projects to the VRG. The VRG send signals to respiratory motoneurons in the spinal cord and phrenic nucleus that control intercostal muscles and the diaphragm. Another structure containing respiratory neurons is the pontine respiratory group (parabrachialis medialis and Kolliker–Fuse nucleus) that is implicated in volume and rate control. Other areas involved in ventilatory control (such as the locus coeruleus and areas in the cerebellum) are not depicted. The locations of action of ampakines, 5-hydroxytryptamine (5HT) agonists, and phosphodiesterase-4 inhibitors are indicated by blue, red, and green colors, respectively. Data redrawn from Dahan et al.¹ with permission.