A simple probe measures the pharmacokinetics of[125I]RTI-55 in mouse brain in vivo

A simple external radiation detector system was used to assess brain dopamine and serotonin transporters in mice in vivo using [125I]RTI-55. The results were compared to ex vivo dissection data. Methyl 3beta-(4-iodophenyl) tropane-2beta-carboxic acid methyl ester (RTI-55 or beta-CIT), a high-affinity cocaine antagonist, binds to presynaptic dopamine and serotonin transporters in the brain. Radiotracer accumulation increased for the first 150 min after intravenous injection and then remained constant. When unlabeled RTI-55 was injected, either before or 60 min after radiotracer administration, a significant decrease in tracer accumulation was observed. [125I]RTI-55 binding was also displaced by blocking doses of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine dihydrochloride (GBR 12909) and paroxetine. The results were similar to the ex vivo dissection data. The results demonstrate the feasibility of using the probe detector system to study the presynaptic transporter system in vivo in the mouse brain. The technique is applicable to other cerebral neurotransmitter systems.