ROLE OF GLUTAMATE AS THE CENTRAL NEUROTRANSMITTER IN THE HYPOXIC VENTILATORY RESPONSE

Recent data suggest that the increase in ventilation during hypoxia may be related to the release of the excitatory amino acid neurotransmitter glutamate centrally. To further investigate this, we studied the effects of MK-801, a selective noncompetitive N-methyl-D-aspartate receptor antagonist, on the hypoxic ventilatory response in lightly anesthetized spontaneously breathing intact dogs. The cardiopulmonary effects of sequential ventriculocisternal perfusion (VCP) at the rate of 1 ml/min with mock cerebrospinal fluid (CSF, control) and MK-801 (2 mM) were compared during normoxia and 8 min of hypoxic challenge with 12% O2. Minute ventilation ((V(E)), tidal volume (VT), and respiratory frequency (f) were recorded continuously, and hemodynamic parameters [heart rate (HR), blood pressure (MAP), cardiac output (CO), pulmonary arterial pressure, and pulmonary capillary wedge pressure] were measured periodically. Each dog served as its own baseline control before and after each period of sequential VCP under the two different O2 conditions. During 15 min of normoxia, there were no significant changes in the cardiopulmonary parameters with mock CSF VCP, whereas with MK-801 VCP for 15 min, VE decreased by approximately 27%, both by reductions in VT and f (17 and 9.5%, respectively). HR, MAP, and CO were unchanged. During 8 min of hypoxia with mock CSF VCP, VE increased by 171% associated with increased VT and f (25 and 125%, respectively). HR, MAP, and CO were likewise augmented. In contrast, the hypoxic response during MK-801 VCP was characterized by an increased VE of 84%, mainly by a rise in f by 83%, whereas the VT response was abolished. The cardiovascular excitation was also inhibited. In addition, we showed that these observations were not related to the effects of anesthesia in our VCP model using time control experiments in separate animals. We also demonstrated that these findings were not due to a generalized depression of central respiratory drive, because ventilation increased in response to central administration of acetylcholine in three other dogs. These findings support a major role for glutamate as the central excitatory neurotransmitter in the hyperventilatory response to hypoxia.