[A model of the central control of respiration]

Central respiratory drive is very much dependent upon the CO2-tension, the H+-content and the ionic composition of the blood and the extracellular fluid of the brain. Ventilation is linearly related in the steady state to the H+-content in the cerebrospinal fluid (CSF). Semiaquatic turtles are an excellent model to study central chemical control of ventilation, and in particular their tolerance to asphyxia. Their ability to maintain prolonged dives is seemingly incongruous with highly-developed mechanisms of central chemical control of ventilation. Experiments were performed on four groups of turtles subjected to two hours of either apneic dives, hypercapnia, anoxia or anoxia plus hypercapnia. One additional group was breathing room air and served as control. At the end of the two-hour period the animals were immediately decapitated and the heads instantly frozen in liquid nitrogen. Brain tissue was removed from the skull and free aminoacids were measured chromatographically. Gamma-aminobutyric acid (GABA) increased significantly in those animals subjected to anoxia (p less than 0.01). These results suggest that the central ventilatory drive during diving and related experimental conditions may be related to alterations in brain concentrations of aminoacid neurotransmitters. GABA is a potent inhibitor of respiratory responses which may function under physiologic and pathophysiologic circumstances to modify ventilatory drive. The role of taurine is not yet clear and has to be further investigated.