Abstract
Previous structure-activity relationship studies indicate that a series of cocaine analogs, 3β-aryltropanes with 2β-diarylmethoxy substituents, selectively bind to the dopamine transporter (DAT) with nanomolar affinities that are 10-fold greater than the affinities of their corresponding 2α-enantiomers. The present study compared these compounds to cocaine with respect to locomotor effects in mice, and assessed their ability to substitute for cocaine (10 mg/kg, i.p.) in rats trained to discriminate cocaine from saline. Despite nanomolar DAT affinity, only the 2β-Ph2COCH2-3β-4-Cl-Ph analog fully substituted for cocaine-like discriminative effects. Whereas all of the 2β compounds increased locomotion, only the 2β-(4-ClPh)PhCOCH2-3β-4-Cl-Ph analog had cocaine-like efficacy. None of the 2α-substituted compounds produced either of these cocaine-like effects. To explore the molecular mechanisms of these drugs, their effects on DAT conformation were probed using a cysteine-accessibility assay. Previous reports indicate that cocaine binds with substantially higher affinity to the DAT in its outward (extracellular)- compared with inward-facing conformation, whereas atypical DAT inhibitors, such as benztropine, have greater similarity in affinity to these conformations, and this is postulated to explain their divergent behavioral effects. All of the 2β- and 2α-substituted compounds tested altered cysteine accessibility of DAT in a manner similar to cocaine. Furthermore, molecular dynamics of in silico inhibitor-DAT complexes suggested that the 2-substituted compounds reach equilibrium in the binding pocket in a cocaine-like fashion. These behavioral, biochemical, and computational results show that aryltropane analogs can bind to the DAT and stabilize outward-facing DAT conformations like cocaine, yet produce effects that differ from those of cocaine.
Footnotes
- Received November 13, 2015.
- Accepted January 7, 2016.
The work reported herein was supported in various parts by funds as follows: Work by W.C.H., T.A.K., and J.L.K. was supported by the Intramural Research Program of the National Institutes of Health National Institute on Drug Abuse [ZIA DA000103-26]. W.C.H. was also supported by startup funds from Butler University. The work of J.D.M. and C.K.S. was supported by a grant from the National Institute on Drug Abuse [DA027806]. The efforts by J.D.M. and B.J. were funded in part by National Institutes of Health National Science Foundation grants [R01DA027806 and XSEDE MCB060069 supercomputers]. The work of M.L.T., L.X., and S.A.L. was supported by a grant from the National Institute on Drug Abuse [DA11528].
- U.S. Government work not protected by U.S. copyright
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