The interaction between ethanol and pregnanolone at induction of anaesthesia investigated with a threshold method in male rats.

1. An anaesthesia threshold was used to investigate the pharmacodynamic and pharmacokinetic interactions between ethanol and pregnanolone in male rats. 2. The criterion to determine threshold doses of pregnanolone was the first burst suppression of 1 s in the EEG. 3. Ethanol (0.5, 1.0, 1.5 and 2.0 g kg(-1)) was injected i.p. 15 min before pregnanolone infusion. Trunk blood, serum, cortex, cerebellum, hippocampus, striatum, brain stem, fat and muscle tissues obtained at criterion were used to determine ethanol (blood) and pregnanolone. Ethanol reduced threshold doses in a dose dependent linear manner. A similar reduction of pregnanolone tissue concentrations was only found in brain stem and striatum. Deviations consisted of larger decreases in serum, cerebellum and hippocampus after 0.5 g kg(-1) ethanol and in cerebellum, cortex and hippocampus after 2.0 g kg(-1) of ethanol. Positive correlations between dose and concentration of pregnanolone was recorded in brain stem, hippocampus, cerebellum and cortex. A kinetic component influenced the concentration in cortex. There was a correlation between dose and serum concentration of pregnanolone only after ethanol. In the muscle 0.5 g kg(-1) ethanol had no influence on pregnanolone concentration. 4. The linear, additive pharmacodynamic interaction could involve the GABA ionophore. A pharmacokinetic interaction was found in cortex. The retained high uptake of pregnanolone in muscle (after 0.5 g kg(-1)) corresponded to losses in other tissues (including serum). The reduced uptake of pregnanolone in cerebellum, cortex and hippocampus (after 2.0 g kg(-1)) was not due to a corresponding change in serum concentration. It was probably due to a reduced blood flow.