Characterization of the dopamine transporter in nonhuman primate brain: homogenate binding, whole body imaging, and ex vivo autoradiography using [125I] and [123I]IPCIT.

IPCIT [2 beta-carboisopropoxy-3 beta-(4-iodophenyl)tropane; also designated RTI-121] is the isopropyl ester of beta-CIT [2 beta-carbomethoxy-3 beta-(4-iodophenyl) tropane]. Although beta-CIT binds to dopamine (DA), serotonin (5-HT) and norepinephrine (NE) transporters, IPCIT has been reported to be selective for the DA transporter. IPCIT was labeled with 125I and its receptor binding to membranes prepared from baboon striatum was compared with that of [125I] beta-CIT. These studies confirmed the relative selectivity of IPCIT for the DA transporter in comparison to 5-HT and NE transporters. The nonspecific binding of [125I]IPCIT was almost four times greater than that of [125I] beta-CIT. The biodistribution of IPCIT was examined in two baboons with whole body imaging for 24-30 h after administration of 3 mCi of 123I-labeled tracer. The brain uptake peaked within the first hour at 9.2% of the injected dose and the majority of activity in the body cleared through the hepatobiliary system. The distribution of activity within the brain was examined with ex vivo autoradiography in one monkey injected with [123I]IPCIT. Activity was concentrated in the caudate and putamen and had values of 5 and 7 microCi/cm3 per microCi/g, respectively. The distribution in brain regions receiving moderately dense serotonergic innervation (e.g. superior colliculus and thalamus) had levels of activity equivalent to that in cerebellum. This study confirmed the in vitro and in vivo selectivity of IPCIT for the DA transporter but also showed that [125I]IPCIT had higher in vitro nonspecific binding than [125I] beta-CIT.

Methyl 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylate in vitro binding to dopamine and serotonin transporters under "physiological" conditions.

Methyl 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylate ([123I]beta-CIT) is a single photon emission computed tomographic radiotracer for in vivo labeling of dopamine (DA) and serotonin (5-HT) transporters. Single photon emission computed tomographic experiments in nonhuman primates showed that [123I]beta-CIT in vivo binding to DA transporters had a much slower washout than binding to 5-HT transporters. This observation was not predicted from previously published in vitro studies. These studies, performed at 22 degrees C in nonphysiological buffer, reported similar affinity of [125I]beta-CIT for DA and 5-HT transporters. We now report [125I]beta-CIT binding parameters to fresh rat membranes at 22 degrees C and 37 degrees C, in a buffer mimicking the composition of cerebrospinal fluid. At both temperatures, binding to DA transporters was best fit by a two-site model, whereas binding to 5-HT transporters was compatible with one population of sites. At 22 degrees C, [125I]beta-CIT showed similar affinity to high-affinity DA (0.39 nM) and 5-HT transporter sites (0.47 nM). Increasing the incubation temperature from 22 degrees C to 37 degrees C reduced binding to DA transporters by 60%, whereas binding to 5-HT transporters was only marginally affected. In vitro kinetic experiments failed to detect significant differences in on or off rates that could explain the observed in vivo kinetics. These experiments thus failed to explain [125I]beta-CIT in vivo uptake kinetics, suggesting the existence of specific factors affecting the in vivo situation.

Active and inactive enantiomers of 2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane: comparison using homogenate binding and single photon emission computed tomographic imaging.

2 beta-Carbomethoxy-3 beta-(4-iodophenyl)tropane (beta-CIT; also designated RTI-55) is an analog of cocaine that has been developed as a single photon emission computed tomography radiotracer that labels dopamine and serotonin transporters. We have prepared the 125I- and 123I-labeled ([1R] "active" and [1S] "inactive") enantiomers of beta-CIT. Total homogenate binding of the 125I-labeled inactive isomer to baboon caudate and cortex was approximately equal to nonspecific binding of the active isomer in cortex and much lower than total binding of the active isomer in caudate. However, inactive isomer homogenate binding in caudate was somewhat higher than in cortex, and during single photon emission computed tomography scanning in vivo striatal (1S)-[123I]beta-CIT uptake was also slightly greater than in cortex. Following intravenous administration of the 123I-labeled enantiomers, the plasma clearances of the active and inactive enantiomers were not significantly different. Single photon emission computed tomography imaging demonstrated that a bolus dose of nonradioactive (1R)-beta-CIT rapidly displaced the uptake of (1R)-[123I]beta-CIT. In contrast, the brain uptake of (1S)-[123I]beta-CIT was not displaced by nonradioactive (1R)-beta-CIT using either a bolus ("kinetic") or bolus plus constant infusion ("equilibrium") paradigm for administration of the radiotracer. In scans with bolus administration of radiotracer, peak striatal uptake of the active isomer was approximately twice that of the inactive isomer. In comparison to the 123I-labeled active tracer, the inactive tracer showed earlier times to peak activity and faster washouts of activity in all brain regions. These studies demonstrate beta-CIT stereoselectivity using both homogenate binding and in vivo imaging and suggest that the inactive enantiomer may be a useful measure of the kinetics of both blood-brain barrier transport and nonspecific binding.

The effect of lithium on luteinizing hormone and testosterone in man.

Serum luteinizing hormone and testosterone were determined weekly during the course of a comparison of the effects of lithium versus placebo on impulsive aggressive behavior in 16-24 year-old male prisoners. The duration of drug treatment for each individual was up to 3 months. A significant reduction of serious agressive behavioral incidents occurred in the third month on lithium and was accompanied by a significant rise in serum luteinizing hormone, with no change in serum testosterone.