The Sigma-1 Receptor Modulates Dopamine Transporter Conformation and Cocaine Binding and May Thereby Potentiate Cocaine Self-administration in Rats

The dopamine transporter (DAT) regulates dopamine (DA) neurotransmission by recapturing DA into the presynaptic terminals and is a principal target of the psychostimulant cocaine. The sigma-1 receptor (σR) is a molecular chaperone, and its ligands have been shown to modulate DA neuronal signaling, although their effects on DAT activity are unclear. Here, we report that the prototypical σR agonist (+)-pentazocine potentiated the dose response of cocaine self-administration in rats, consistent with the effects of the σR agonists PRE-084 and DTG (1,3-di--tolylguanidine) reported previously. These behavioral effects appeared to be correlated with functional changes of DAT. Preincubation with (+)-pentazocine or PRE-084 increased the B values of [H]WIN35428 binding to DAT in rat striatal synaptosomes and transfected cells. A specific interaction between σR and DAT was detected by co-immunoprecipitation and bioluminescence resonance energy transfer assays. Mutational analyses indicated that the transmembrane domain of σR likely mediated this interaction. Furthermore, cysteine accessibility assays showed that σR agonist preincubation potentiated cocaine-induced changes in DAT conformation, which were blocked by the specific σR antagonist CM304. Moreover, σR ligands had distinct effects on σR multimerization. CM304 increased the proportion of multimeric σRs, whereas (+)-pentazocine increased monomeric σRs. Together these results support the hypothesis that σR agonists promote dissociation of σR multimers into monomers, which then interact with DAT to stabilize an outward-facing DAT conformation and enhance cocaine binding. We propose that this novel molecular mechanism underlies the behavioral potentiation of cocaine self-administration by σR agonists in animal models.