Analogs of benztropine (BZT) bind with high affinity to the dopamine transporter (DAT) but are distinct from cocaine in their behavioral activity. Because of these differences, and their potential as medical treatments for cocaine abuse, we have studied these compounds. Previously studied analogs of BZT have slow onsets of action that may contribute to their atypical effects. In one study BZTs with a relatively fast onset of effects were studied. Only one of the compounds increased locomotor activity, and the increases were modest compared to those of 10-20 mg/kg cocaine. In rats trained to discriminate 10 mg/kg cocaine from saline none of the compounds produced greater than 40% cocaine-like responding up to two hours after injection. None of the compounds produced place-conditioning when examined up to 90 min after injection, indicating minimal abuse liability. The compounds had high affinities at the DAT, with uniformly lower affinities at norepinephrine and serotonin transporters, suggesting minimal contribution of those sites to the behavioral effects of the compounds. The compounds also had affinity for sigma receptors that may have contributed to their behavioral effects. Together the results indicate that a slow onset of action is not a necessary condition for reduced cocaine-like effects of atypical DAT ligands. We have most recently focused on targets other than the well-known antimuscarinic and antihistaminic effects of the parent compound, those having been previously shown to be unlikely contributors to the atypical effects of the BZT analogs. A study of 3HJHW007, an N-butyl BZT analog, found multiple binding sites in striatum from DAT KO mice (Kopajtic et al. 2010). In contrast to the binding of standard DAT ligands, the binding of 3HJHW007 was independent of Na concentration indicating that it was not a traditional DAT site. The identity of those sites was not fully determined in that study, though several sites were found to be unlikely contributors to the behavioral effects of JHW007. In addition, time-course studies of the cocaine-antagonist effects of JHW007 indicated blockade of cocaines locomotor-stimulant effects within 10-min of administration, a time when in vivo DAT occupancy of JHW007 was relatively low, suggesting the importance of off-target actions. Recent studies focused on other off-target sites common to several BZT analogs for which broad spectrum screens had been conducted. Among other targets that might be involved in the reduced cocaine-like behavioral effects are DA D2 receptors, cannabinoid CB1 receptors, and Rs. As part of a more complete characterization of the cocaine-antagonist effects of JHW007, we examined antagonism of the locomotor-stimulant effects of JHW007. As part of that study, it was found that JHW007 antagonized the locomotor stimulant effects of cocaine in both DA D2 wild-type (WT) and knockout (KO) mice, indicating a lack of involvement of this target in the atypical DA uptake inhibition produced by JHW007. Previous studies suggested that the cocaine-antagonist effects of JHW007 could be due at least in part to its activity as a positive modulator of CB1 receptors. In our studies the cocaine antagonist effects of JHW007 were studied in both CB1 WT and KO mice. In both lines of mice, JHW007 blocked the locomotor-stimulant effects of cocaine, suggesting that CB1 receptor mediated allosteric effects do not play a role in the antagonist effects of JHW007. An previous study of drug interactions with cocaine found antagonism of its locomotor stimulant effects by several R antagonists. In a more recent study the R antagonist, BD1008, blocked the locomotor activation produced by cocaine, suggesting again that R antagonist effects may contribute to the decreased cocaine-like efficacy and the antagonist effects of JHW007 and other BZT analogs. Unpublished radioligand binding studies show affinity of all BZT analogs tested for Rs a result that is being followed with studies of the in vivo antagonist effects of various BZT analogs. In summary, several off-target effects of various BZT analogs have been examined. Among the common effects off-target effects, R antagonist actions appear at this time to be the most likely candidates for contributing to atypical effects of BZT analogs. One simpler hypothesis of atypical DAT inhibitors is that they may produce substantial hyperlocomotion or stereotypies alone or in combination with cocaine that interfere with other characteristic DAT-mediated effects and result in an apparent antagonism of cocaine self-administration. That hypothesis was examined in a study that compared excess behavioral activity (hyperlocomotion or stereotypy) induced by standard DAT inhibitors and BZT analogs alone or in combination with cocaine. Pretreatments with the standard DA uptake inhibitor methylphenidate dose-dependently shifted the cocaine self-administration dose-effect curve leftward with shifts obtained at doses that, administered alone, decreased food-maintained behavior, increased locomotor activity, and produced stereotyped behavior. In contrast, the N-substituted BZT analogs (JHW007, AHNl-055, AHN2-005) decreased cocaine self-administration at doses that were virtually without effects on food-maintained responding. The BZT analogs alone had minimal effects on locomotor activity and stereotypies and did not appreciably change the effects of cocaine on these measures when administered in combination with cocaine. Thus, the decreases in cocaine self-administration produced by the N-substituted BZT analogs appear to be due to an antagonism of the reinforcing effects of cocaine rather than due to interference from competing behavioral overstimulation.
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