THE EFFECT OF BENZODIAZEPINES ON THE BINDING OF [H-3] SCH-23390 INVIVO

The effect of flunitrazepam upon the binding of [H-3]SCH 23390 in vivo was investigated. Acute treatment with flunitrazepam decreased the binding of [H-3]SCH 23390 in the striatum in a dose-dependent manner. The time course of radioactivity in the striatum, cerebral cortex and cerebellum in controls and flunitrazepam (1 mg/kg)-treated mice was measured after intravenous injection of [H-3]SCH 23390. The binding kinetics were calculated, using the cerebellum as a reference region for the estimation of the amount of free ligand in the brain. Flunitrazepam significantly decreased the input rate constant to the receptor compartment and the dissociation rate constant in vivo. An in vivo displacement study, using carrier SCH 23390, also showed significant reduction in the dissociation rate constant of [H-3]SCH 23390 in vivo. The drug Ro 15-1788 reversed the effect of flunitrazepam, suggesting that this reduction in binding of [H-3]SCH 23390 was mediated by benzodiazepine receptors. To evaluate the relationship between the reduction in binding of [H-3]SCH 23390 in vivo and in vivo occupancy of benzodiazepine receptors, in vivo occupancy of benzodiazepine receptors was measured using [H-3]Ro 15-1788. A non-linear relationship was found between the reduction in dopamine D1 receptor binding in vivo and the occupancy of benzodiazepine receptors in vivo, indicating that benzodiazepines exerted the maximum change in dopamine receptor binding at a low fractional occupancy of receptors.