Effects of GABA receptor blockage on the respiratory response to hypoxia in sedated newborn piglets.

Brain gamma-aminobutyric acid (GABA) levels increase during hypoxia, which may modulate the ventilatory response to hypoxia. To test the possibility that the depressed neonatal ventilatory response to hypoxia may be related to increased central nervous system GABA activity, 26 sedated spontaneously breathing newborn piglets (age 5 +/- 1 day, wt 1.7 +/- 0.4 kg) were studied. Minute ventilation (VE), oxygen consumption, heart rate, arterial blood pressure, and arterial blood gases were measured in room air and after 1, 5, and 10 min of hypoxia (inspired O2 fraction 0.10) before drug intervention. Immediately after these measurements, an infusion of saline or the GABA alpha-receptor blocker (bicuculline, 0.3 mg/kg iv) or beta-receptor blocker (CGP-35348, 100-300 mg/kg iv) was administered while animals were hypoxic. All measurements were repeated at 1, 5, and 10 min after initiation of the drug infusion. Basal VE was similar among groups. During hypoxia, VE increased significantly in the animals that received either a GABA alpha- or beta-receptor blocker but not in those receiving saline. Changes in arterial Po2, oxygen consumption, heart rate, and arterial blood pressure were similar among groups before and after saline or GABA antagonist infusion. These results suggest that the decrease in ventilation during the biphasic ventilatory response to hypoxia in the neonatal piglet is in part mediated through the depressant effect of GABA on the central nervous system.