The sigma-1 receptor modulates dopamine transporter conformation and cocaine binding and may thereby potentiate cocaine self-administration in rats

Abstract

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.

Structural basis for σ1 receptor ligand recognition

“…However, if α4 were to move to a greater degree, it could disrupt the oligomerization interface. This is consistent with prior data, which suggest that σ1 receptor agonists bias the receptor towards lower molecular weight states, while antagonists bias it towards higher molecular weight states 4,[22][23][24] . Additionally, molecular modeling by Yano et al predicted that (+)-pentazocine and other multimer-impeding ligands would occupy this space differentially from haloperidol and other multimer-promoting ligands 24 , which is consistent with our structural results.…”

“…While there are myriad proposed binding partners for the σ1 receptor 1,3,4,19,30 , the critical effectors of σ1 receptor signaling still need to be unambiguously established. Future work will need to focus on these functional questions in order to fully understand the function of the σ1 receptor and its potential as a therapeutic target.…”

“…Currently, the biochemical basis for agonism or antagonism at the σ 1 receptor is largely unknown, which complicates the unambiguous assignment of efficacy class for σ 1 ligands. The most well-documented biochemical difference between the two ligand classes is that antagonists increase the receptor's oligomeric state, while agonists decrease the oligomeric state 4,[22][23][24] . The structural data we present here show that these ligands occupy a different region of the binding pocket ( Fig.…”

“…Specifically, knockdown or antagonism of σ 1 receptor can potentiate G-protein coupled receptor (GPCR) signaling 2,3 , while agonists of the σ 1 receptor result in an IP3 receptor-dependent intracellular calcium flux 19 and inhibition of sodium 18 and potassium 20,21 channel currents. The σ 1 receptor exists in multiple oligomeric states, and reports suggest agonists causes a shift to monomeric or low molecular weight species, while antagonists bias the receptor towards high molecular weight species 4,[22][23][24] . However, the dominant physiologically relevant oligomeric forms and the precise way in which oligomerization is tied to agonist binding are unknown.…”

σ Receptor Effects of N-Substituted Benztropine Analogs: Implications for Antagonism of Cocaine Self-Administration

“…Patch-clamp and amperometry in cells and in vivo chronoamperometry revealed a s 1 Rmediated decrease in methamphetamine-stimulated DA efflux. Another study (Hong et al, 2017) showed that the s 1 R agonists (1)-pentazocine and PRE-084 increased the B max values of [ 3 H] WIN35,428 in radioligand binding assays both in cultured cells and rat striatal synaptosomes. Further, coimmunoprecipitation and bioluminescence resonance energy-transfer assays indicated an interaction between s 1 R and DAT in transfected cells.…”