We have previously demonstrated that hypothermic newborn piglets have a depressed ventilatory response to hypoxia. This is accompanied by a decrease in the extracellular glutamate (GLU) concentration in the nucleus tractus solitarius (NTS)(Pediatr Res 37: 342A, 1995). To assess whether microinjection of L-GLU into the NTS of newborn piglets can reverse the hypoxic ventilatory depression observed during hypothermia, 7 anesthetized, paralyzed, mechanically ventilated piglets < 7 days old were studied at a brain temperature of 35± 0.5°C. L-GLU (50 nmol bolus over 10 sec, followed by 200 nmol/min for 10 min) or the vehicle solution were administered in random order into the area of the NTS upon initiation of hypoxia. Central respiratory output was assessed by measuring burst frequency and moving time average area of phrenic nerve activity. Minute phrenic output (MPO), arterial blood pressure (ABP), heart rate (HR), and arterial blood gases were obtained in room air (RA) and during isocapnic hypoxia (FiO2= 0.10). Mean± SD values for MPO (au/min) were: Table
During hypoxia, a significant decrease in MPO was observed with vehicle administration, while MPO increased significantly after GLU microinjection. Changes in ABP, HR, pH, and PaO2 were not different between groups. These data support the hypothesis that the depressed neonatal ventilatory response to hypoxia observed during hypothermia is due to a decrease in the glutamatergic neurotransmission in the NTS.
