Modulation of [3H]flunitrazepam binding to rat cerebellar benzodiazepine receptors by phosphatidylserine

The influence of phosphatidylserine on ligand binding to the benzodiazepine/GABA receptor complex was assessed in rat cerebellar synaptic membranes and in a detergent-solubilized membrane preparation. Intact synaptic membranes or membranes solubilized with the zwitterionic detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]propanesulfonate) were incubated with a range of concentrations of phosphatidylserine for 2 h at 4 degrees C, prior to use in radioligand binding assays. Phosphatidylserine, an endogenous membrane phospholipid, facilitated the site-specific binding of [3H]flunitrazepam to synaptic membranes and CHAPS-solubilized preparations. In addition, phosphatidylserine inhibited the facilitation of [3H]flunitrazepam binding induced by either cartazolate or gamma-aminobutyric acid (GABA). Although the maximum effect (38% facilitation of [3H]flunitrazepam binding; greater than 90% inhibition of the cartazolate action) was produced using 130 microM phosphatidylserine, a significant enhancement of [3H]flunitrazepam binding could be observed upon preincubation of synaptic membranes with concentrations of phosphatidylserine as low as 5 microM. These results suggest that endogenous phosphatidylserine may play a role in the regulation of benzodiazepine/GABA receptor function, possibly through modulation of the mechanisms which functionally link the various components of this complex receptor system.