Changes in in vivo binding of 3H-Ro 15-1788 in mouse brain by reserpine

The effects of reserpine on the in vivo binding of 3H-Ro 15-1788, (Ro 15-1788:ethyl 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5a][1,4]benzodiazepine-3-carboxylate) a selective benzodiazepine antagonist, in the mouse brain were investigated. The biodistributions of tracer amounts of 3H-Ro 15-1788 in mice were significantly altered by pretreatment with reserpine (2.5 or 5.0 mg/kg, 24 h before the tracer administration). The time courses of radioactivity in the brain and the blood following i.v. injection of 3H-Ro 15-1788 with carrier Ro 15-1788 were not changed by pretreatment with reserpine, which suggested that the specific binding process might be altered by reserpine. The degree of alteration in the in vivo binding of 3H-Ro 15-1788 seemed to be dependent upon the dose of reserpine and the duration after the treatment of reserpine. The maximum changes in the biodistribution of 3H-Ro 15-1788 were observed at 1 day after injection of reserpine. The body temperature and the brain monoamine contents (dopamine, norepinephrine and 5-hydroxytryptamine) in mice were measured as indicators of pharmacological effects of reserpine, and good relationships to the degree of changes in the biodistribution of 3H-Ro 15-1788 and either the body temperature or brain monoamine contents, were observed. Furthermore, the changes in the biodistribution of 3H-Ro 15-1788 in the reserpinized mice were significantly suppressed by anti-depressant imipramine treatment. These results suggest that it would be possible to detect the in vivo drug interaction with brain benzodiazepine receptors in the living human brain using 11C-Ro 15-1788 and positron emission tomography (PET).