Imaging of the super high affinity binding sites for [3H]Ro15–4513 in rat hippocampus: comparison between in vitro and in vivo binding

Abstract The super high affinity binding sites for [ 3 H]Ro15–4513, a partial inverse agonist of central benzodiazepine receptors, were analyzed in rat hippocampus both in vivo and in vitro. An ultra high sensitive method of autoradiography with an imaging plate system was employed for quantitative analysis of [ 3 H]Ro15–4513 binding. In in vitro binding, the super high affinity binding sites in the hippocampus were observed when the [ 3 H]Ro15–4513 concentration was below 0.5 nM. In vivo, the super high affinity binding sites were only found when the injected dose of Ro15–4513 was below 3.6 μ g/kg and almost disappeared when the dose was increased to 10 μ g/kg. These results both in vivo and in vitro indicate that there is a significant discrepancy between actual free ligand concentration in vivo and in vitro, and that concentrations in intact brain may be much lower than previously thought.