Dopamine Toxicity Following Chronic Co‐exposure to ‘Bath Salt’ Constituents MDPV, Mephedrone, and Methylone

Abstract

In recent years, the global abuse of synthetic designer drugs has rapidly increased to become a public health threat. Popularized under the term ‘bath salts,’ synthetic cathinones continue to be one of the most frequently abused classes of designer drugs. According to DEA statistics, bath salts commonly contain one or a mixture of the following synthetic cathinones, 3, 4‐methylenedioxypyrovalerone (MDPV), 4‐methylmethcathinone (mephedrone), and 3, 4‐methylenedioxymethcathinone (methylone). Although these compounds were permanently classified as schedule I controlled substances in the Synthetic Drug Abuse Prevention Act of 2012 (DEA, 2011), they are currently not controlled by international laws and abuse continues to thrive with internet serving as a central marketplace (Karila, et. al. 2015). Pharmacological research regarding synthetic cathinone abuse has primarily focused on the individual mechanisms and associated toxicities of MDPV, mephedrone, and methylone, despite DEA statistics and numerous case reports indicating that many of the frequently abused bath salt ‘brands’ contain various mixtures of these compounds (Araujo, et. al. 2015; DEA, 2011). Given the reported dissimilarities between MDPV, mephedrone, and methylone in regards to selectivity and mechanism of action at the dopamine transporter, it was hypothesized that chronic co‐exposure to the synthetic cathinones would produce significantly altered effects on dopaminergic tone as compared to individual exposure. Our previous work has demonstrated that repeated, combined exposure to MDPV, mephedrone, and methylone, induces significant reductions in dopamine (DA) within the nucleus accumbens (NAc), striatum (STR), substantia nigra pars compacta (SNpc), and ventral tegmental area (VTA) brain regions, a result that was not observed following chronic individual administration. The present study aims to correlate the observed DA depletions following co‐synthetic cathinone exposure with changes in the expression of key proteins involved in DA synthesis, storage, release, and uptake. For this study, adolescent male Swiss‐Webster mice were administered 10 mg/kg intraperitoneal injections of MDPV, mephedrone, or the cathinone cocktail (comprised of MDPV, mephedrone, and methylone) every other day for 14 days (7 injections total). Although the transporter selectivity profile for methylone is similar to mephedrone, releasing assays have shown that it is only half as potent at the dopamine transporter (DAT) and thus was not individually examined in this study (Baumann, et. al. 2012). Brains were removed and microdissections of the NAc, STR, SNpc, and VTA were collected 48 hours after the final exposure. Tissue expression levels of tyrosine hydroxylase (TH), monoamine oxidase B (MAO‐B), catechol‐O‐methyltransferase (COMT), dopamine transporter (DAT‐1) and vesicular monoamine transporter 2 (VMAT2), were determined using Protein simple western blot analysis. Our analyses revealed significantly reduced expression levels of these key proteins involved in DA synthesis, metabolism, and transport in all collected brain regions following repeated dosing of the cathinone cocktail. These data, along with our previous findings, suggest that chronic combined exposure to MDPV, mephedrone, and methylone may be toxic to dopaminergic neurons of the mesolimibic and nigrostriatal brain pathways.