No abstract

Dopaminergic mechanisms are thought to play a central role in the reinforcing effects of cocaine. The present study examined the reinforcing effects of 3beta-(4-chlorophenyl)tropane-2beta-carboxylic acid phenyl ester (RTI-113), a long-acting, selective, high-affinity dopamine uptake inhibitor. Additionally, the effects of RTI-113 pretreatment on cocaine self-administration were determined. Monkeys were trained to respond under a second-order schedule for intravenous cocaine administration (0.10 or 0.17 mg/kg/infusion). When responding was stable, cocaine (0.0030-1.0 mg/kg/infusion) and RTI-113 (0.010-0.30 mg/kg/infusion) were substituted for the cocaine training dose. Cocaine and RTI-113 were equipotent for their reinforcing effects. However, cocaine maintained higher response rates in two of the three monkeys tested. When administered as a pretreatment, RTI-113 (0.10-0.30 mg/kg) dose-dependently reduced responding maintained by two doses of cocaine. Drug effects on behavior were related to dopamine transporter (DAT) occupancy in monkey striatum during neuroimaging with positron emission tomography. DAT occupancy was determined by displacement of 8-(2-[(18)F]fluroethyl)2beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane (FECNT). DAT occupancy was between 65-76% and 94-99% for doses of cocaine and RTI-113 that maintained maximum response rates, respectively. DAT occupancy did not differ markedly across RTI-113 pretreatment doses and ranged between 72-84%. The results suggest that the pharmacokinetic profile of RTI-113 (i.e., long-acting) may influence its ability to maintain self-administration, and therefore its abuse liability. Additionally, high DAT occupancy is required for RTI-113 to reduce cocaine-maintained responding.

The results validate the use of microdialysis in awake primates using repeated sampling of the same anatomical site and demonstrate orderly changes in extracellular dopamine following administration of dopamine uptake inhibitors.

2Beta-carbo(2-fluoroethoxy)-3beta-(4'-((Z)-2-iodoethenyl)phenyl)nortropane (betaFEpZIENT, 1) was synthesized as a serotonin transporter (SERT) imaging agent for both positron emission tomography (PET) and single photon emission computerized tomography (SPECT). The binding affinity of 1 to human monoamine transporters showed a high affinity for the SERT (Ki = 0.08 nM) with respect to the dopamine transporter (DAT) (Ki = 13 nM) and the norepinephrine transporter (NET) (Ki = 28 nM). In vivo biodistribution and blocking studies performed in male rats demonstrated that [123I]1 was selective and specific for SERT. In vivo microPET brain imaging studies in an anesthetized monkey with [18F]1 showed high uptake in the diencephalon and brainstem with peak uptake achieved at 120 min. A chase study with (R,S)-citalopram.HBr displaced [18F]1 radioactivity from all SERT-rich brain regions. A chase study with the DAT ligand 2beta-carbophenoxy-3beta-(4-chlorophenyl)tropane (9, RTI-113) failed to displace [18F]1, indicating that [18F]1 is specific to the SERT. The in vivo evaluation of [18F]1 indicates that this radiotracer is a good candidate for mapping and quantifying CNS SERT.