The parabrachial nucleus (PBN) is thought to play an important role in cardiorespiratory control. However, the circumstances under which it affects ventilation are still not known. The purpose of the present study was to investigate how the PBN modulates the ventilatory responses to hypercapnia, hypoxia or a resistive load in awake rats with chemical lesions of the PBN. 2. In three groups of rats (with lateral PBN lesion, with Kölliker-Fuse nucleus lesion and control), ventilation was measured under various conditions. 3. There was no difference in the breathing of normal room air in any of the groups. However, the lesioned groups showed a reduced ventilatory response to hyperoxic hypercapnia (inspired CO2 fractions (FI,CO2) of 3, 5, 8 and 10%) and to graded hypoxia (inspired O2 fractions (FI,O2) of 16, 12, 10 and 8%) compared with the control group. The control group showed a biphasic response to sustained hypoxia (FI,O2 at 10% for 30 min), known as 'hypoxic depression', while the lesioned groups showed moderate ventilatory exaggeration throughout hypoxia. In response to a resistive load, the lateral PBN lesion group showed no change in ventilatory compensation. 4. The PBN appeared to have a considerable influence on ventilation stimulated in various ways during wakefulness.
