Characterization of the binding sites for ¹²³I-ADAM and the relationship to the serotonin transporter in rat and mouse brains using quantitative autoradiography

Imaging of serotonin transporter (SERT) in the central nervous system may provide an important tool to evaluate some psychiatric disorders. Recently, a novel ¹²³I-labeled radiotracer, 2-((2-((dimethylamino)methyl)phenyl) thio)-5-iodophenylamine (¹²³I-ADAM), has been developed that exhibited a high selectivity for SERT. The aim of this study was to characterize the biodistribution and specificity of ¹²³I-ADAM to SERT using quantitative autoradiography in both control and neurotoxin-treated animals. Methods: ¹²³I-ADAM (74 MBq) was injected intravenously into the mice to access its biodistribution in the brain via quantitative autoradiography. Further, rats with serotonin depleted by intraperitoneal injection of p-chloroamphetamine (PCA) were used to evaluate the specificity of ¹²³I-ADAM to SERT. The levels of biogenic amines were then measured and correlated with quantitative ¹²³I-ADAM labeling in control and PCA-treated rat brains. Results: The autoradiographic results showed that ¹²³I-ADAM accumulated in SERT-rich brain areas after systemic injection, including the globus pallidus, thalamus, hypothalamus, substantia nigra, interpeduncular nucleus, amygdala, and raphe nucleus. The dorsal raphe nucleus had the highest initial uptake with a peak specific binding ratio (i.e., [target - cerebellum]/cerebellum) at 120 min after injection. ¹²³I-ADAM uptake was dramatically decreased in the hippocampus, thalamus, amygdala, geniculate nuclei, hypothalamus, raphe nucleus, and substantia nigra in PCA-lesioned rats. The decrement in radioactivity was more prominent at higher dosages of PCA and was in parallel with the changes in amounts of serotonin and 5-hydroxyindoleacetic acid in the prefrontal cortex. Conclusion: This study demonstrates that regional distribution of ¹²³I-ADAM radioactivity is similar to the SERT localization in both rat and mouse brains. We also validated that destruction on central serotonergic neurons after PCA treatment inhibits the uptake of ¹²³I-ADAM in serotonin-rich brain regions. High specific binding to SERT in vivo makes ¹²³I-ADAM an appropriate radiotracer for solitary studies of serotonin functions in living humans.