Mechanism of the respiratory action of pentobarbital at the medullary and pontine levels.

A functional differentiation of the respiratory action of pentobarbital was made by applying the drug in cats to the ventral surface of the medulla and to the dorso-rostral surface of the pons. The involvement of a GABAergic mechanism was assessed by studying the interaction of bicuculline with pentobarbital at both levels of the brain-stem. In the medulla, pentobarbital (4 to 16 mg) caused an immediate and dose-dependent depression of tidal volume down to apnea with a minimal or no change in frequency. In the pons, the depression affected selectively the frequency (-45%) without modification in amplitude. Medullary structures were much more sensitive to the action of pentobarbital. Doses that induced apnea in the medulla did not attain 50% depression of the minute volume in the pons. Bicuculline (300 micrograms) completely antagonized the effect of pentobarbital in both regions, suggesting that a GABAergic mechanism may be involved in the respiratory action of pentobarbital.