Barbiturate tolerance and dependence: effects on synaptosomal sodium transport and membrane fluidity

DBA mice were fed lab chow containing phenobarbital for seven or eight days. Upon withdrawal of the phenobarbital diet, dependence was evidenced by appearance of hypothermia, handling-induced convulsions and lethal seizures. Functional tolerance was determined by injecting phenobarbital into mice treated with the phenobarbital diet or a pair-fed control diet and measuring the brain concentration of phenobarbital at the time of loss of righting reflex and the time of regaining righting reflex. Both measures demonstrated that chronic consumption of phenobarbital resulted in functional tolerance. When the diet was withdrawn for two days, tolerance was no longer present, indicating a rapid reversal of the adaptive changes. The veratridine-stimulated uptake of 24Na by isolated brain synaptosomes was used as a measure of membrane function. Sodium uptake was inhibited in vitro by pentobarbital and ethanol, and the inhibitory effects of these drugs were attenuated by chronic in vivo phenobarbital treatment. The fluidity of brain synaptic plasma membranes was estimated by the fluorescence polarization of the fluorescent probe molecules 1-(4-trimethylammonium phenyl)-6-phenyl-1,3,5-hexatriene and 1,6-diphenyl-1,3,5-hexatriene. Synaptic membranes from mice treated chronically with phenobarbital did not differ from those of control mice with regard to either the baseline fluorescence polarization of the probes or the decrease in fluorescence polarization produced by in vitro exposure to phenobarbital or ethanol. Taken together, these results indicate that although chronic phenobarbital ingestion resulted in tolerance and dependence (studied in vivo), and adaptation of sodium channels (studied in vitro), there was no evidence that these changes were due to alterations in the membrane physical properties.