Kinetics of [3H]flunitrazepam binding to membrane-bound benzodiazepine receptors

The kinetics of [3H]flunitrazepam binding to synaptosomal (P2) membranes from rat cerebral cortex was studied. The pseudo-first order association at 0 degrees was curvilinear and the association rate was increased with increasing concentrations of ligand. The association curve was resolved best as two linear exponentials, representing a fast and a slow component. The percentage of each association component varied with the ligand concentration. Association was slightly decreased in the presence of gamma-aminobutyric acid. Dissociation at 0 degrees, whether equilibrium binding had occurred at 0 degrees or 37 degrees, was also biphasic. The dissociation of the complexes was more rapid at higher concentrations of ligand. Dissociation was slower in the presence of gamma-aminobutyric acid, and this effect was more pronounced at higher concentrations of ligand. Prolonged preincubation at 37 degrees either in the presence or in the absence of [3H]flunitrazepam also decreased the dissociation rate. Dissociation was faster before equilibrium than after equilibrium had been reached. These results are discussed in terms of one homogeneous type of benzodiazepine receptor with two interconvertible conformations.