Abstract
The influence of GABA on the affinity of flunitrazepam (FLU) for benzodiazepine receptor subtypes (type I and II) was studied by measurement of the competitive inhibition of [ 3H]FLU and [ 3H]propyl beta-carboline-3-carboxylate ([ 3H]PCC) binding. When assays were carried out at 0°C using a low concentration (0.040 nM) of [ 3H]PCC so that the type I receptors were selectively labelled, no significant effect of GABA (10 −4 M) on the FLU [ 3H] PCC competition curve was detected. In contrast, when assays were carried out at 0°C using [ 3H]FLU or a high concentration of [ 3H]PCC to achieve [ 3H]ligand receptor occupancy of both type I and type II receptors, GABA (10 −4 M) caused a significant increase in the affinity of FLU as measured by FLU [ 3H] FLU and FLU [ 3H] PCC competition experiments. Collectively, these data suggest that the influence of GABA on benzodiazepine receptor binding is mediated, primarily, by the type II receptor. It was also noted that the PCC [ 3H] FLU competition curve had a Hill coefficient of approximately 1 at 37°C as compared to the results of experiments at 0°C during which a Hill coefficient of approximately 0.7 was calculated.
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