Stereochemistry of mephedrone neuropharmacology: enantiomer-specific behavioural and neurochemical effects in rats

Abstract

Background and purpose: Synthetic cathinones, commonly referred to as 'bath salts', are a group of amphetamine-like drugs gaining popularity worldwide. 4-Methylmethcathinone (mephedrone, MEPH) is the most commonly abused synthetic cathinone in the UK, and exerts its effects by acting as a substrate-type releaser at monoamine transporters. Similar to other cathinone-related compounds, MEPH has a chiral centre and exists stably as two enantiomers: R-mephedrone (R-MEPH) and S-mephedrone (S-MEPH).

Experimental approach: Here, we provide the first investigation into the neurochemical and behavioural effects of R-MEPH and S-MEPH. We analysed both enantiomers in rat brain synaptosome neurotransmitter release assays and also investigated their effects on locomotor activity (e.g. ambulatory activity and repetitive movements), behavioural sensitization and reward.

Key results: Both enantiomers displayed similar potency as substrates (i.e. releasers) at dopamine transporters, but R-MEPH was much less potent than S-MEPH as a substrate at 5-HT transporters. Locomotor activity was evaluated in acute and repeated administration paradigms, with R-MEPH producing greater repetitive movements than S-MEPH across multiple doses. After repeated drug exposure, only R-MEPH produced sensitization of repetitive movements. R-MEPH produced a conditioned place preference whereas S-MEPH did not. Lastly, R-MEPH and S-MEPH produced biphasic profiles in an assay of intracranial self-stimulation (ICSS), but R-MEPH produced greater ICSS facilitation than S-MEPH.

Conclusions and implications: Our data are the first to demonstrate stereospecific effects of MEPH enantiomers and suggest that the predominant dopaminergic actions of R-MEPH (i.e. the lack of serotonergic actions) render this stereoisomer more stimulant-like when compared with S-MEPH. This hypothesis warrants further study.

Figure 1

R-MEPH acts more selectively on dopamine transporters than S-MEPH. Drug concentration-response effects of R-MEPH, S-MEPH or racemic MEPH on facilitating monoamine release of [³H]-MPP+ (A) and [³H]-5-HT (B) in vitro. Concentration–response curves (n = 3 per dose) were constructed by incubating rat brain synaptosomes preloaded with tritiated substrate in increasing concentrations of each MEPH enantiomer with synaptosomes preloaded with tritiated substrate.

Figure 2

Acute R-MEPH produces greater repetitive movements than acute S-MEPH. Following drug injection, rats were monitored for 90 min for repetitive movements and ambulatory activity. Data are presented as a time course in 5 min batches (A–D) or as total counts over 90 min + SEM (E–F). For total repetitive movements and ambulatory activity analyses, ***P < 0.001 compared with saline control group.

Figure 3

R-MEPH, but not S-MEPH, produces sensitization of repetitive movements. Rats (n = 8 per group) were given either saline or a repeated, variable-dose of R-MEPH or S-MEPH for 7 days, followed by a 10 day abstinence interval. After the abstinence interval, rats were challenged with either 15 mg·kg⁻¹ R-MEPH or S-MEPH. Repetitive movements (A) and ambulatory activity (B) were monitored in 5 min bins and expressed as counts + SEM. *P < 0.05 and **P < 0.01 comparing rats given repeated R-MEPH to acute R-MEPH as determined by two-way anova with Bonferonni post hoc tests.

Figure 4

R-MEPH, but not S-MEPH, produces dose-dependent place preference. Rats (n = 7–8 per group) underwent a bias-design conditioned place preference assay, where drug was administered for 4 days in the non-preferred compartment, as determined by a 30 min pre-test in a drug-naïve state. Data are presented as a preference score (seconds on drug-paired side post-conditioning minus preconditioning)(s) + SEM. Each panel represents a cohort of animals with every panel having its own saline control group. Dose–response curves for R-MEPH (B) and S-MEPH (C), as well as a comparison with MEPH enantiomers and racemic MEPH at 20 mg·kg⁻¹ (A), were performed. *P < 0.05 compared with saline control or indicated doses.

Figure 5

R-MEPH produces greater ICSS facilitation than S-MEPH. Left panels (A, C) show MEPH effects on full frequency–rate ICSS curves. Abscissae: frequency of electrical brain stimulation in log Hz. Ordinates: % maximum control reinforcement rate (%MCR). Drug doses indicated in keys are in units of mg·kg⁻¹. Solid symbols represent frequencies at which reinforcement rates were statistically different from vehicle rates as determined by two-way anova followed by Holm–Sidak post hoc test (P < 0.05). Right panels (B, D) show MEPH effects on a summary measure of ICSS performance. Abscissae: drug dose in mg·kg⁻¹. Ordinates: % baseline number of stimulations per component (% baseline ICSS). Arrows indicate statistically significant increases (up arrows) and/or decreases (down arrows) in ICSS relative to vehicle at any frequency as determined from full frequency–rate curves. All data show mean ± SEM for six rats.