Dissecting the Influence of Two Structural Substituents on the Differential Neurotoxic Effects of Acute Methamphetamine and Mephedrone Treatment on Dopamine Nerve Endings with the Use of 4-Methylmethamphetamine and Methcathinone

Abstract

Mephedrone (MEPH) is a β-ketoamphetamine stimulant drug of abuse that is often a constituent of illicit bath salts formulations. Although MEPH bears remarkable similarities to methamphetamine (METH) in terms of chemical structure, as well as its neurochemical and behavioral effects, it has been shown to have a reduced neurotoxic profile compared with METH. The addition of a β-keto moiety and a 4-methyl ring substituent to METH yields MEPH, and a loss of direct neurotoxic potential. In the present study, two analogs of METH, methcathinone (MeCa) and 4-methylmethamphetamine (4MM), were assessed for their effects on mouse dopamine (DA) nerve endings to determine the relative contribution of each individual moiety to the loss of direct neurotoxicity in MEPH. Both MeCa and 4MM caused significant alterations in core body temperature as well as locomotor activity and stereotypy, but 4MM was found to elicit minimal dopaminergic toxicity only at the highest dose. By contrast, MeCa caused significant reductions in all markers of DA nerve-ending damage over a range of doses. These results lead to the conclusion that ring substitution at the 4-position profoundly reduces the neurotoxicity of METH, whereas the β-keto group has much less influence on this property. Although the mechanism(s) by which the 4-methyl substituent reduces METH-induced neurotoxicity remains unclear, it is speculated that this effect is mediated by a loss of DA-releasing action in MEPH and 4MM at the synaptic vesicle monoamine transporter, an effect that is thought to be critical for METH-induced neurotoxicity.

Graphical abstract

Fig. 1.

Comparative structures of METH, 4MM, MeCa, and MEPH. This figure demonstrates the structural differences between neurotoxic METH, non-neurotoxic MEPH, and their intermediates.

Fig. 2.

Effects of 4MM and MeCa on markers of dopaminergic toxicity. Mice were treated with 4MM (4 × 2.5, 5, 10, 20, or 40 mg/kg), MeCa (4 × 10, 20, 40, or 80 mg/kg), or saline vehicle every 2 hours for a total of four injections at the given dose. Levels of DA (A), DAT (B), TH (C), and GFAP (D) were determined 48 hours after drug exposure. Data are means ± S.E.M. (for 4MM: control, n = 18; 2.5 mg/kg, n = 8; 5 mg/kg, n = 8; 10 mg/kg, n = 9; 20 mg/kg, n = 12; 40 mg/kg, n = 12; for MeCa: control, n = 15; 10 mg/kg, n = 6; 20 mg/kg, n = 6; 40 mg/kg, n = 11; 80 mg/kg, n = 10). Each experiment was performed three times, with differing doses, and results were then pooled for analysis. METH and MEPH data were reprinted for comparison from Anneken et al. (2015) with permission from John Wiley and Sons, Inc. These data are not statistically analyzed in the present figure, but METH elicited highly significant toxic effects on each measure in the original experiments, whereas MEPH had no significant effects. One-way ANOVA, *P < 0.05 **P < 0.01 ***P < 0.001 ****P < 0.0001 compared with controls, with Bonferroni’s post-hoc test.

Fig. 3.

Effects of 4MM and MeCa on core body temperature. Mice were treated with (A) 4MM (4 × 2.5, 5, 10, 20, or 40 mg/kg), (C) MeCa (4 × 20, 40, or 80 mg/kg), or saline vehicle every 2 hours for a total of four injections. Body temperature was monitored beginning 40 minutes prior to the first injection via telemetry, and was measured at 20-minute intervals for the duration of the experiment. Data are mean temperature values with S.E.M. (<5% of the mean for each group) omitted for clarity (4MM: control, n = 4; 2.5–20 mg/kg, n = 4; 40 mg/kg, n = 5; for MeCa: control, n = 9; 20 mg/kg, n = 6; 40 mg/kg, n = 10; 80 mg/kg, n = 8). This experiment was performed once for 4MM, and repeated twice with differing doses for MeCa. The mean difference from control animals over the course of the entire experiment for each dose is shown in (B) and (D) ± S.E.M. Two-way repeated-measures ANOVA, ****P < 0.0001 compared with controls, with Bonferroni’s post-hoc test.

Fig. 4.

Effects of 4MM and MeCa on locomotor behavior. Mice were treated with 4MM (4 × 2.5, 5, 10, 20, or 40 mg/kg), MeCa (4 × 20, 40, or 80 mg/kg), or saline vehicle every 2 hours for a total of four injections. Horizontal activity (A), movement time (B), and stereotypy time (C) were monitored for 60 minutes following each injection. Data are mean values ± S.E.M. (4MM: control, n = 10; 2.5 mg/kg, n = 4; 5–40 mg/kg, n = 5; for MeCa: all groups: n = 4). This experiment was performed twice for 4MM and once for MeCa with differing doses, though owing to equipment size constraints, each experiment was performed over consecutive days. One-way ANOVA, *P < 0.05 **P < 0.01 ****P < 0.0001 compared with controls, with Bonferroni’s post-hoc test.