Comparison of the Kinetics of [3H]Diazepam and [3H]Flunitrazepam Binding to Cortical Synaptosomal Membranes

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Abstract
[3H]Diazepam and [3H]flunitrazepam ([3H]FNP) binding to washed and frozen synaptosomal membranes from rat cerebral cortex were compared. In Tris-citrate buffer, γ-aminobutyric acid (GABA) and NaCl both increased [3H]diazepam binding more than [3H]FNP binding. GABA and pentobarbital both enhanced this effect of NaCl. Because of the extremely rapid dissociation of [3H]diazepam in the absence of NaCl and GABA, the Bmax (maximal binding capacity) was smaller by the filtration assay than by the centrifugation assay. [3H]FNP, which dissociates more slowly, had the same Bmax in both assays. [3H]Diazepam association had two components, and was faster than [3H]FNP association. [3H]Diazepam dissociation, which also had two components, was faster than that of [3H]FNP, and also had a greater fraction of rapidly dissociating species. [3H]FNP dissociation was similar when initiated by diazepam, flunitrazepam, clonazepam, or Ro15-1788, which is a benzodiazepine antagonist. [3H]Diazepam dissociation with Ro15-1788, flunitrazepam, or clonazepam was slower than with diazepam. GABA and NaCl, but not pentobarbital, increased the percentage of slowly dissociating species. This effect of NaCl was potentiated by GABA and pentobarbital. The results support the cyclic model of benzodiazepine receptors existing in two interconvertible conformations, and suggest that, distinct from their binding affinity, some ligands (like flunitrazepam) are better than others (like diazepam) in inducing the conversion of the receptor to the higher-affinity state.