Relative efficacies of 1,4-diazepines on GABA-stimulated chloride influx in rat brain vesicles

Summary The effects of 1, 4-diazepines with two annelated heterocyles [brotizolam (WE 941), ciclotizolam (WE 973) and WE 1008] on gamma-aminobutyric acid (GABA) -stimulated chloride influx into rat brain membrane vesicles were examined. Brotizolam enhanced GABA (30 μM) -stimulated 36 Cl − influx (146.1% of control), while ciclotizolam and WE 1008 showed only a small enhancement (119.3% and 119.1%, respectively) of GABA-stimulated 36 Cl − uptake. Brotizolam resulted in a left shift of the GABA dose response curve at lower concentrations of GABA (10 μM), while at higher concentrations of GABA (1 mM), brotizolam caused a reduction of the maximal response. The enhancement of GABA-stimulated 36 Cl − uptake by brotizolam (0.1 μM) was antagonized by Ro 15–1788. At higher concentration of GABA (300 μM), brotizolam inhibited GABA-stimulated 36 Cl − uptake in a dose dependent manner and Ro15–1788 failed to antagonize this effect. These results suggest that 1) brotizolam produces an enhancement of GABA (30 μM) -stimulated chloride influx through the benzodiazepine receptor. 2) brotizolam inhibition of GABA (300 μM) -stimulated chloride influx involves an additional mechanism, and 3) the sedativehypnotic action of brotizolam may be related to its high efficacy at the benzodiazepine/GABA-gated chloride channel.