Assessing the role of dopamine in the differential neurotoxicity patterns of methamphetamine, mephedrone, methcathinone and 4-methylmethamphetamine

Abstract

Methamphetamine and mephedrone are designer drugs with high abuse liability and they share extensive similarities in their chemical structures and neuropharmacological effects. However, these drugs differ in one significant regard: methamphetamine elicits dopamine neurotoxicity and mephedrone does not. From a structural perspective, mephedrone has a β-keto group and a 4-methyl ring addition, both of which are lacking in methamphetamine. Our previous studies found that methcathinone, which contains only the β-keto substituent, is neurotoxic, while 4-methylmethamphetamine, which contains only the 4-methyl ring substituent, elicits minimal neurotoxicity. In the present study, it was hypothesized that the varying neurotoxic potential associated with these compounds is mediated by the drug-releasable pool of dopamine, which may be accessed by methamphetamine more readily than mephedrone, methcathinone, and 4-methylmethamphetamine. To test this hypothesis, l-DOPA and pargyline, compounds known to increase both the releasable pool of dopamine and methamphetamine neurotoxicity, were combined with mephedrone, 4-methylmethamphetamine and methcathinone. Methamphetamine was also tested because of its ability to increase releasable dopamine. All three regimens significantly enhanced striatal neurotoxicity and glial reactivity for 4-methylmethamphetamine. Methcathinone neurotoxicity and glial reactivity were enhanced only by l-DOPA. Mephedrone remained non-neurotoxic when combined with either l-DOPA or pargyline. Body temperature effects of each designer drug were not altered by the combined treatments. These results support the conclusion that the neurotoxicity of 4-methylmethamphetamine, methcathinone and methamphetamine may be differentially regulated by the drug-releasable pool of dopamine due to β-keto and 4-methyl substituents, but that mephedrone remains non-neurotoxic despite large increases in this pool of dopamine. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Keywords: 4-Methylmethamphetamine; Dopamine; Mephedrone; Methamphetamine; Methcathinone; Neurotoxicity.

Fig. 1

Comparative structures of MEPH, METH, and intermediate structures. Diagram depicts the structures of the related compounds methamphetamine (no structural substituents), 4-methlymethamphetamine (4-methyl), methcathinone (β-keto), and mephedrone (4-methyl and β-keto). Reprinted from Anneken et al. (2017) with permission from the American Society for Pharmacology and Experimental Therapeutics.

Fig. 2

Effects of 4 MM ± L-DOPA, PARG or METH on markers of striatal dopaminergic toxicity. Levels of (A) DA, (B) DAT, and (C) TH were determined in the striatum following treatment with 4 MM, +/−L-DOPA (L–D), pargyline (PARG), or METH. *p < 0.05 **p < 0.01 ****p < 0.0001 compared to controls. ^##p < 0.01 ^###p < 0.001 ^####p < 0.0001 compared to 4MM. ^@@@ p < 0.001 ^@@@@ p < 0.0001 compared to METH. Data are presented as mean + SEM. N = 5–16 mice per group.

Fig. 3

Effects of MeCa ± L-DOPA, PARG or METH on markers of striatal dopaminergic toxicity. Levels of (A) DA, (B) DAT, and (C) TH were determined in the striatum following treatment with MeCa, +/−L-DOPA (L–D), pargyline (PARG), or METH. *p < 0.05 **p < 0.01 ***p < 0.001 ****p < 0.0001 compared to controls. ^#p < 0.05 ^##p < 0.01 compared to MeCa. Data are presented as mean + SEM. N = 5–16 mice per group.

Fig. 4

Effects of MEPH ± L-DOPA and PARG on markers of striatal dopaminergic toxicity. Levels of (A) DA, (B) DAT, and (C) TH were determined in the striatum following treatment with MEPH, +/−L-DOPA (L–D) or pargyline (PARG) are shown above. METH and MEPH + METH data from a prior study (Angoa-Perez et al., 2013) are included for comparison (reprinted with permission from John Wiley and Sons, Inc.) These data are not statistically analyzed in the present figure, but METH elicited significant toxic effects on all three measures in the prior study, while MEPH significantly enhanced each marker of toxicity when co-administered with METH. *p < 0.05 compared to controls. Data are presented as mean + SEM. N = 5–16 mice per group.

Fig. 5

Effects of MEPH, 4 MM, and MeCa ± L-DOPA, PARG, or METH on striatal Iba-1 immunoreactivity. Levels of Iba-1 immunofluorescence are shown in the upper panels (arbitrary units) for 4 MM (A), MeCa (B) and MEPH (C) + SEMs. Representative photomicrographs (20X) of each treatment condition stained against Iba-1 (red) and DAPI (counterstain; blue) are shown in the lower panels (D). *p < 0.05 **p < 0.01 ***p < 0.001 ****p < 0.0001 compared to controls. ^#p < 0.05 ^##p < 0.01 ^###p < 0.001 compared to MEPH/4MM/MeCa. Data are presented as mean + SEM. N = 3–5 mice per group.

Fig. 6

Effects of MEPH, 4 MM, and MeCa ± L-DOPA, PARG, or METH on striatal GFAP immunoreactivity. Levels of GFAP immunofluorescence (mean + SEM) are shown in the upper panels (arbitrary units) for 4 MM (A), MeCa (B) and MEPH (C) + SEMs. Representative photomicrographs (20X) of each treatment condition stained against GFAP (green) and DAPI (counterstain; blue) are shown in the lower panels (D). *p < 0.05 **p < 0.01 ***p < 0.001 ****p < 0.0001 compared to controls. ^#p < 0.05 ^####p < 0.0001 compared to MEPH/4MM/ MeCa. ^@ p < 0.05 ^@@ p < 0.01 denotes comparison to METH. Data are presented as mean + SEM. N = 3–5 mice per group.

Fig. 7

Effects of MEPH, 4 MM, and MeCa ± L-DOPA, PARG, or METH on core body temperature. Core body temperatures of mice treated with 4 MM (A), MeCa (B), MEPH (C), ± L-DOPA, pargyline (PARG), or METH were recorded every 20 min. MEPH, 4 MM, MeCa, and METH were administered at time points 40, 160, 280, and 400 min. L-DOPA and PARG were administered 20 min prior to time point 0, and L-DOPA was also administered at time points 100, 220, and 340 min. Data are presented as mean temperature in C. SEMs were <5% of the mean for each group and are omitted for clarity. L-DOPA, PARG, and METH controls were run with each experiment and included for analysis, but each is included on only one graph for clarity. Significance versus controls (p < 0.05) is indicated with open symbols. Data for MEPH + METH from Angoa-Perez et al. (2013) were reprinted for comparison, with permission from John Wiley and Sons, Inc. These were not analyzed in the present figure, but induced a significant hyperthermia compared to controls in the original publication. N = 4–9 mice per group.