Interactions between hexobarbital and thiopental in male rats evaluated with an anaesthesia threshold.

The anaesthetic interaction between hexobarbital and thiopental in different combinations were investigated in male rats by using the isobolografic method. Data on dose, serum or brain concentrations at an EEG-criterion induced with a threshold method were utilized. The criterion was defined by a burst suppression in the EEG of 1 sec. or more (the "silent second"). With rats more than 115 days of age a synergistic interaction (potentiation) was obtained when a low dose of hexobarbital and a high dose of thiopental were used. The relationship between serum and brain concentrations of hexobarbital and thiopental in the different tested combinations gave no evidence for a pharmacokinetic interaction. Thus the synergistic interaction was located in the brain and probably related to the mechanisms of action. The same tests performed on rats at an age of approximately 90 days gave not only a potentiation when a low dose of hexobarbital and a high dose of thiopental were used, but also a potentiation when a high dose of hexobarbital and a low dose of thiopental were used. These results indicate an age-related change in sensitivity in the CNS between 90 and 115 days of age. Further investigations of this age-related change showed that this change from a potentiation to an additive interaction occurred between the age of 85 and 96 days.