Reversible depression of ventilation and cardiovascular function by ventriculocisternal perfusion with gamma-aminobutyric acid in dogs.
Gamma-aminobutyric acid (GABA) is a putative central neurotransmitter that depresses respiratory neurons and has a metabolism in the brain that is tied to CO2 fixation and H+ metabolism. Therefore, the effect of 3 concentrations of GABA (10, 30, and 50 mM) in different groups of pentobarbital-anesthetized dogs was investigated by ventriculocisternal perfusion for 15 to 45 min. During multiple perfusion sequences, tidal volume (VT) and respiratory frequency were recorded continuously, whereas heart rate (HR), mean systemic arterial pressure (Psa), cardiac output, mean pulmonary arterial pressure, and pulmonary capillary wedge pressure were monitored periodically. Minute ventilation decreased by a reduction in VT. The mean VT (+/- SEM) decreased after 15 min of GABA perfusion from 365.9 +/- 19.5 to 151.0 +/- 15.0 ml with 50 mM GABA in mock CSF, from 272.8 +/- 25.1 to 110.6 +/- 7.4 with 30 mM GABA, and from 223.6 +/- 22.3 to 155.3 +/- 21.8 with 10 mM GABA. A decrease in mean inspiratory flow was associated with the reduction in VT. The decrease in ventilation was associated with respiratory acidosis. At each GABA concentration, mean Psa decreased, whereas HR fell only with 50 mM. Other cardiovascular parameters did not change. Perfusion with mock CSF alone restored cardiorespiratory depression caused by GABA. Mean Psa fell with GABA whether ventilation was kept constant mechanically or not. These results support the hypothesis of a GABA-sensitive mechanism via a population of receptors that affect respiratory and cardiovascular function and are accessible by ventriculocisternal perfusion.