Effects of barbiturates and ethanol on the physical properties of brain membranes

Synaptic plasma membranes (SPM) and myelin were prepared from mouse brain and their physical properties evaluated by fluorescence probes. 1,6,-Diphenyl-1,3,5-hexatriene (DPH) was used as a probe of the membrane core and endogenous tryptophan as a probe of membrane proteins. In vitro addition of pentobarbital or ethanol decreased the fluorescence polarization of DPH in SPM. These decreases were not due to changes in fluorescence lifetime and indicate that both drugs increased the rotation mobility of the probe in the membrane core. This action was shared by other barbiturates and their potencies were correlated with their lipid solubility. In contrast to the effect on SPM, pentobarbital increased the fluorescence polarization of DPH in SPM phospholipids but had little effect on the fluorescence polarization of DPH in a total lipid extract from SPM. Pentobarbital did not affect fluorescence polarization of DPH in myelin. Analysis of the temperature dependence of pentobarbital effects indicated that the initial rigidity of the membranes was one factor determining whether the drug decreased, increased or did not change the membrane fluidity. Pentobarbital decreased the fluorescence intensity of SPM tryptophan, but did not affect the fluorescence of free tryptophan. Ethanol did not alter tryptophan fluorescence. Thus, pentobarbital, but not ethanol, produced a detectable perturbation of synaptic proteins. These results indicate that both pentobarbital and ethanol fluidized the hydrophobic core of the membrane, but pentobarbital differed from ethanol in its action on very fluid lipid domains and on membrane proteins.