Exploring a Role for Organic Cation Transporter 3 in Ethanol and Cocaine Co‐Abuse

Abstract

According to the National Institute on Drug Abuse, concurrent use of cocaine and alcohol is a major cause for emergency hospitalization, underscoring a vital need to understand the mechanistic basis of this highly addictive, and dangerous drug combination. It is well-known that both cocaine (blocker of dopamine, norepinephrine and serotonin transporters, DAT, NET and SERT, respectively) and ethanol (EtOH) increase extracellular dopamine (DA), norepinephrine and serotonin. However, we and others, find no evidence for EtOH interacting with DAT, NET or SERT, suggesting that EtOH may be acting elsewhere to inhibit uptake of these monoamines. Organic cation transporter 3 (OCT3) is emerging as an important player in regulation of monoamine signaling. However, whether OCT3 contributes to actions of abused drugs that act primarily by increasing extracellular levels of monoamines, remains unclear. To establish proof-of-principle for this idea, we conducted studies to determine the ability of EtOH to inhibit uptake of the prototypical cation [3H]1-methyl-4-phenylpyridinium ([3H]MPP+) into cultured HEK293 cells stably overexpressing human OCT3, and found the IC50 to be 4 mM. Extracellular brain concentrations of EtOH reported after doses that produce effects ranging from mild sedation to full anesthesia in rodents range from 0.5-35 mM. Thus, these data provide support for OCT3-dependent actions of EtOH, and show that concentrations of EtOH reaching brain following behaviorally relevant doses may be sufficient to engage OCT3. We also found that cocaine does not have activity at OCT3 (Mayer et al., 2018, PMID: 29773909) raising the possibility that EtOH may interact with OCT3 to inhibit uptake of monoamines, potentially increasing the addictive properties of cocaine and propagating the concurrent use of these drugs. To this end we used high-speed chronoamperometry to interrogate the effects of local application of EtOH, cocaine, and their combination on clearance of exogenously applied DA from extracellular fluid in striatum of male and female constitutive OCT3+/+ and OCT3-/- mice in vivo. We found that in both male and female OCT3+/+ mice, EtOH inhibited DA clearance and enhanced the ability of cocaine to inhibit DA clearance, effects that were lost in OCT3-/- mice. Preliminary data in male OCT3+/+ mice reveal that the combination of EtOH (1000 mg/kg) and cocaine (3.2 mg/kg), doses which by themselves did not induce conditioned place preference (CPP), produced robust CPP. Consistent with females being more sensitive to the rewarding effect of cocaine, OCT3+/+ females developed CPP to this dose of cocaine, and EtOH appeared to augment this effect. CPP studies in OCT3-/- mice are ongoing. Taken together, results so far suggest that potentiation of the neurochemical and behavioral effects of cocaine by EtOH are OCT3 dependent. OCT3 may be a putative target for therapeutic intervention in the treatment of EtOH and cocaine co-abuse.