In vivo binding of [¹²⁵I] RTI‐55 to dopamine transporters: Pharmacology and regional distribution with autoradiography

Abstract

Previous studies have demonstrated that para-substituted WIN 35,065-2 analogs of cocaine show high binding affinity for dopamine uptake sites both in vitro and in vivo, and inhibit DA uptake in vitro. These analogs also produce potent cocaine-like behavioral effects in various procedures. The purpose of the present studies was to evaluate the iodinated WIN 35,065-2 analog [125I]RTI-55 as an in vivo ligand for the DA transporter. Following intravenous injection in mice, [125I]RTI-55 showed highest accumulation in areas with high densities of dopamine uptake sites. Light microscopic autoradiography was used to examine binding with higher resolution. Displacement studies demonstrated that [125I]RTI-55 binding in dopamine containing regions, striatum and olfactory tubercles, was saturable and inhibited by other cocaine analogs. GBR 12909 and WIN 35,428 significantly inhibited [125I]RTI-55 binding in striatum, while paroxetine significantly inhibited hypothalamic binding but had little effect in striatum. The latter finding suggests that [125I]RTI-55 also binds to the serotonin transporter. Haloperidol had no effect on [125I]RTI-55 binding in any brain region measured. In addition, treatment of animals with the dopamine neurotoxin MPTP caused significant reductions in striatal [125I]RTI-55 binding. The results of these studies indicate that [125I]RTI-55 binds primarily to the dopamine transporter in the mouse striatum in vivo.

Advanced brain dopamine transporter imaging in mice using small-animal SPECT/CT

“…Thus, BP analysis can be considered valid 4 h post- 123 I-β-CIT injection in the mouse brain. These results agree with the earlier autoradiography works [7-9]. However, our results show moderate increase in BP, which is evident at the 7-h imaging point.…”

“…BP increased over the time period followed, similar to previous rat studies with SPECT [11,12] and autoradiography work with both rats [22] and mice [7-9]. In healthy human subjects, the evolvement of BP is similar, differing only by time scale [23,24], due to the slower metabolism of humans compared with mice.…”

“…123 I-2β-carbomethoxy-3β-(4-iodophenyl)tropane ( 123 I-β-CIT), a SPECT ligand for DATs, has not yet been validated in mice, which are frequently induced to model several diseases. Previous mouse studies are ex vivo autoradiography works [7-9], in which follow-up of the same animals was not possible and test-retest measurement was disabled. Whereas, Mochizuki et al used a simple probe system offering only a single projection and therefore cannot differentiate between the brain structures [10].…”

Non-amine dopamine transporter probe [3H]tropoxene distributes to dopamine-rich regions of monkey brain

“…The distribution of [ 3 H]tropoxene in brain corresponded closely to the distribution observed with selective DAT probes that contain an amine nitrogen in their structure, including [ 3 H]CFT Kaufman et al, 1991;Madras et al, 1989a,b), [ 125 I]altropane (Madras et al, 1998a,b), [ 125 I]RTI-121 (Little et al, 1995;Staley et al, 1995), and [ 125 I]RTI-55 (Cline et al, 1992;Staley et al, 1994Staley et al, , 1995. The distribution of [ 3 H]tropoxene also corresponded with the brain distribution of the DAT previously detected with antibodies (Ciliax et al, 1995;Freed et al, 1995;Nirenberg et al, 1996Nirenberg et al, , 1997a to the DAT.…”

Dopamine transporter SPECT: How to remove subjectivity?

“…14,22 To obtain the V3" index, ROIs need to be outlined in the striatum representing the compartment of specific and nonspecific ligand uptake and a reference region with absent (or low) DAT density (e.g., occipital cortex, cerebellum) representing the nonspecific radioligand binding compartment. 23,24 The use of ROI techniques together with the visual assessment is recommended by the European Nuclear Medicine Association to be included in a report of a patients' DAT-SPECT investigation to minimize sources of error which might arise from sole visual rating. 25 Potential pitfalls might originate from the use of noncontinuous color tables or inappropriate thresholding leading to the over/underestimation of findings or introduction of artifacts.…”