RESEARCH ARTICLE Small-Animal Single-Photon Emission Computed Tomographic Imaging of the Brain Serotoninergic Systems in Wild-Type and Mdr1a Knockout Rats

The pharmacokinetic properties of radiotracers are crucial for successful in vivo single-photon emission computed tomographic (SPECT) imaging. Our goal was to determine if MDR1A-deficient animals could allow better SPECT imaging outcomes than wild-type (WT) animals for a selection of serotoninergic radioligands. Thus, we compared the performances of 123I-p-MPPI, 123I-R91150, 123I-SB207710, and 123I-ADAM radioligands, for imaging of their respective targets (5-hydroxytryptamine [5-HT]1A, 5-HT2A, 5-HT4, and serotonin transporter [SERT]), in WT and Mdr1a knockout (KO) rats. With 123I-SB207710, virtually no SPECT signal was recorded in the brain of WT or KO animals. For 123I-p-MPPI, low nondisplaceable binding potentials (BPND, mean 6 SD) were observed in WT (0.49 6 0.25) and KO (0.89 6 0.52) animals. For 123I-ADAM, modest imaging contrast was observed in WT (1.27 6 0.02) and KO (1.31 6 0.09) animals. For 123I-R91150, the BPND were significantly higher inMdr1a KO (3.98 6 0.65) animals compared to WT animals (1.22 6 0.26). The pharmacokinetics of 123I-SB207710 and 123I-p-MPPI do not make them ideal tracers for preclinical SPECT neuroimaging. 123I-ADAM showed adequate brain uptake regardless of Mdr1a expression and appeared suitable for preclinical SPECT neuroimaging in both animal strains. The use of Mdr1a KO animals significantly improved the brain penetration of 123I-R91150, making this animal strain an interesting option when considering SPECT neuroimaging of 5-HT2A receptors in rat. I N THE PAST DECADE, intense research efforts havebeen dedicated to the development of imaging systems that meet the requirements of small-animal imaging i