Sinead E. Shortall

Behavioural and neurochemical comparison of chronic intermittent cathinone, mephedrone and MDMA administration to the rat

The synthetic cathinone derivative, mephedrone, is a controlled substance across Europe. Its effects have been compared by users to 3,4-methylenedioxymethamphetamine (MDMA), but little data exist on its pharmacological properties. This study compared the behavioural and neurochemical effects of mephedrone with cathinone and MDMA in rats. Young-adult male Lister hooded rats received i.p. cathinone (1 or 4 mg/kg), mephedrone (1, 4 or 10mg/kg) or MDMA (10mg/kg) on two consecutive days weekly for 3 weeks or as a single acute injection (for neurochemical analysis). Locomotor activity (LMA), novel object discrimination (NOD), conditioned emotional response (CER) and prepulse inhibition of the acoustic startle response (PPI) were measured following intermittent drug administration. Dopamine, 5-hydroxytryptamine (5-HT) and their major metabolites were measured in striatum, frontal cortex and hippocampus by high performance liquid chromatography 7 days after intermittent dosing and 2h after acute injection. Cathinone (1, 4 mg/kg), mephedrone (10mg/kg) and MDMA (10mg/kg) induced hyperactivity following the first and sixth injections and sensitization to cathinone and mephedrone occurred with chronic dosing. All drugs impaired NOD and mephedrone (10mg/kg) reduced freezing in response to contextual re-exposure during the CER retention trial. Acute MDMA reduced hippocampal 5-HT and 5-HIAA but the only significant effect on dopamine, 5-HT and their metabolites following chronic dosing was altered hippocampal 3,4-dihydroxyphenylacetic acid (DOPAC), following mephedrone (4, 10mg/kg) and MDMA. At the doses examined, mephedrone, cathinone, and MDMA induced similar effects on behaviour and failed to induce neurotoxic damage when administered intermittently over 3 weeks.

Lost in translation: preclinical studies on 3,4-methylenedioxymethamphetamine provide information on mechanisms of action, but do not allow accurate prediction of adverse events in humans.

3,4‐Methylenedioxymethamphetamine (MDMA) induces both acute adverse effects and long‐term neurotoxic loss of brain 5‐HT neurones in laboratory animals. However, when choosing doses, most preclinical studies have paid little attention to the pharmacokinetics of the drug in humans or animals. The recreational use of MDMA and current clinical investigations of the drug for therapeutic purposes demand better translational pharmacology to allow accurate risk assessment of its ability to induce adverse events. Recent pharmacokinetic studies on MDMA in animals and humans are reviewed and indicate that the risks following MDMA ingestion should be re‐evaluated. Acute behavioural and body temperature changes result from rapid MDMA‐induced monoamine release, whereas long‐term neurotoxicity is primarily caused by metabolites of the drug. Therefore acute physiological changes in humans are fairly accurately mimicked in animals by appropriate dosing, although allometric dosing calculations have little value. Long‐term changes require MDMA to be metabolized in a similar manner in experimental animals and humans. However, the rate of metabolism of MDMA and its major metabolites is slower in humans than rats or monkeys, potentially allowing endogenous neuroprotective mechanisms to function in a species specific manner. Furthermore acute hyperthermia in humans probably limits the chance of recreational users ingesting sufficient MDMA to produce neurotoxicity, unlike in the rat. MDMA also inhibits the major enzyme responsible for its metabolism in humans thereby also assisting in preventing neurotoxicity. These observations question whether MDMA alone produces long‐term 5‐HT neurotoxicity in human brain, although when taken in combination with other recreational drugs it may induce neurotoxicity.

The preclinical pharmacology of mephedrone; not just MDMA by another name

The substituted β‐keto amphetamine mephedrone (4‐methylmethcathinone) was banned in the UK in April 2010 but continues to be used recreationally in the UK and elsewhere. Users have compared its psychoactive effects to those of 3,4‐methylenedioxymethamphetamine (MDMA, ‘ecstasy’). This review critically examines the preclinical data on mephedrone that have appeared over the last 2–3 years and, where relevant, compares the pharmacological effects of mephedrone in experimental animals with those obtained following MDMA administration. Both mephedrone and MDMA enhance locomotor activity and change rectal temperature in rodents. However, both of these responses are of short duration following mephedrone compared with MDMA probably because mephedrone has a short plasma half‐life and rapid metabolism. Mephedrone appears to have no pharmacologically active metabolites, unlike MDMA. There is also little evidence that mephedrone induces a neurotoxic decrease in monoamine concentration in rat or mouse brain, again in contrast to MDMA. Mephedrone and MDMA both induce release of dopamine and 5‐HT in the brain as shown by in vivo and in vitro studies. The effect on 5‐HT release in vivo is more marked with mephedrone even though both drugs have similar affinity for the dopamine and 5‐HT transporters in vitro. The profile of action of mephedrone on monoamine receptors and transporters suggests it could have a high abuse liability and several studies have found that mephedrone supports self‐administration at a higher rate than MDMA. Overall, current data suggest that mephedrone not only differs from MDMA in its pharmacological profile, behavioural and neurotoxic effects, but also differs from other cathinones.

Differential effects of cathinone compounds and MDMA on body temperature in the rat, and pharmacological characterization of mephedrone-induced hypothermia.

Recreational users report that mephedrone has similar psychoactive effects to 3,4‐methylenedioxymethamphetamine (MDMA). MDMA induces well‐characterized changes in body temperature due to complex monoaminergic effects on central thermoregulation, peripheral blood flow and thermogenesis, but there are little preclinical data on the acute effects of mephedrone or other synthetic cathinones.

Contribution of serotonin and dopamine to changes in core body temperature and locomotor activity in rats following repeated administration of mephedrone.

The psychoactive effects of mephedrone are commonly compared with those of 3,4‐methylenedioxymethamphetamine, but because of a shorter duration of action, users often employ repeated administration to maintain its psychoactive effects. This study examined the effects of repeated mephedrone administration on locomotor activity, body temperature and striatal dopamine and 5‐hydroxytryptamine (5‐HT) levels and the role of dopaminergic and serotonergic neurons in these responses. Adult male Lister hooded rats received three injections of vehicle (1 ml/kg, i.p.) or mephedrone HCl (10 mg/kg) at 2 h intervals for radiotelemetry (temperature and activity) or microdialysis (dopamine and 5‐HT) measurements. Intracerebroventricular pre‐treatment (21 to 28 days earlier) with 5,7‐dihydroxytryptamine (150 µg) or 6‐hydroxydopamine (300 µg) was used to examine the impact of 5‐HT or dopamine depletion on mephedrone‐induced changes in temperature and activity. A final study examined the influence of i.p. pre‐treatment (−30 min) with the 5‐HT1A receptor antagonist WAY‐100635 (0.5 mg/kg), 5‐HT1B receptor antagonist GR 127935 (3 mg/kg) or the 5‐HT7 receptor antagonist SB‐258719 (10 mg/kg) on mephedrone‐induced changes in locomotor activity and rectal temperature. Mephedrone caused rapid‐onset hyperactivity, hypothermia (attenuated on repeat dosing) and increased striatal dopamine and 5‐HT release following each injection. Mephedrone‐induced hyperactivity was attenuated by 5‐HT depletion and 5‐HT1B receptor antagonism, whereas the hypothermia was completely abolished by 5‐HT depletion and lessened by 5‐HT1A receptor antagonism. These findings suggest that stimulation of central 5‐HT release and/or inhibition of 5‐HT reuptake play a pivotal role in both the hyperlocomotor and hypothermic effects of mephedrone, which are mediated in part via 5‐HT1B and 5‐HT1A receptors.

Cathinone increases body temperature, enhances locomotor activity, and induces striatal c-fos expression in the Siberian hamster.

Cathinone is a β-keto alkaloid that is the major active constituent of khat, the leaf of the Catha edulis plant that is chewed recreationally in East Africa and the Middle East. Related compounds, such as methcathinone and mephedrone have been increasing in popularity as recreational drugs, resulting in the recent proposal to classify khat as a Class C drug in the UK. There is still limited knowledge of the pharmacological effects of cathinone. This study examined the acute effects of cathinone on core body temperature, locomotor and other behaviors, and neuronal activity in Siberian hamsters. Adult male hamsters, previously implanted with radio telemetry devices, were treated with cathinone (2 or 5mg/kg i.p.), the behavioral profile scored and core body temperature and locomotor activity recorded by radio telemetry. At the end of the study, hamsters received vehicle or cathinone (5mg/kg) and neuronal activation in the brain was determined using immunohistochemical evaluation of c-fos expression. Cathinone dose-dependently induced significant (p<0.0001) increases in both temperature and locomotor activity lasting 60-90min. Cathinone (2mg/kg) increased rearing (p<0.02), and 5mg/kg increased both rearing (p<0.001) and lateral head twitches (p<0.02). Both cathinone doses decreased the time spent at rest (p<0.001). The number of c-fos immunopositive cells were significantly increased in the striatum (p<0.0001) and suprachiasmatic nucleus (p<0.05) following cathinone, indicating increased neuronal activity. There was no effect of cathinone on food intake or body weight. It is concluded that systemic administration of cathinone induces significant behavioral changes and CNS activation in the hamster.

Ecstasy cannot be assumed to be 3,4-methylenedioxyamphetamine (MDMA)

This is a rebuttal by the authors (Green et al., pp. 1523–1536 of this issue) to a commentary by Parrott, pp. 1518–1520 of this issue. To view the article by Green et al. visit http://dx.doi.org/10.1111/j.1476‐5381.2011.01819.x. To view the commentary by Parrott visit http://dx.doi.org/10.1111/j.1476‐5381.2012.01941.x

Caffeine alters the behavioural and body temperature responses to mephedrone without causing long-term neurotoxicity in rats

Administration of caffeine with 3,4-methylenedioxymethamphetamine (MDMA) alters the pharmacological properties of MDMA in rats. The current study examined whether caffeine alters the behavioural and neurochemical effects of mephedrone, which has similar psychoactive effects to MDMA. Rats received either saline, mephedrone (10 mg/kg), caffeine (10 mg/kg) or combined caffeine and mephedrone intraperitoneally twice weekly on consecutive days for three weeks. Locomotor activity (days 1 and 16), novel object discrimination (NOD, day 2), elevated plus maze (EPM) exploration (day 8), rectal temperature changes (day 9) and pre-pulse inhibition (PPI) of acoustic startle response (day 15) were assessed. Seven days after the final injection, brain regions were collected for the measurement of 5-hydroxytryptamine (5-HT), dopamine and their metabolites. Combined caffeine and mephedrone further enhanced the locomotor response observed following either drug administered alone, and converted mephedrone-induced hypothermia to hyperthermia. Co-administration also abolished mephedrone-induced anxiogenic response on the EPM, but had no effect on NOD or PPI. Importantly, no long-term neurotoxicity was detected following repeated mephedrone alone or when co-administered with caffeine. In conclusion, the study suggests a potentially dangerous effect of concomitant caffeine and mephedrone, and highlights the importance of taking polydrug use into consideration when investigating the acute adverse effect profile of popular recreational drugs.

P.6.d.001 The effects of MDMA pre-treatment on behavioural responses to mephedrone in the rat

cathinone derivatives to be used recreationally with users comparing its psychostimulant effects to those of 3,4-methylenedioxymethamphetamine (MDMA, reduced head dips in both groups irrespective of pretreatment Head dips ‘ecstasy’; Winstock et al., 2011).  In contrast, MDMA reduced open arm entries and head dips in vehicle pretreated groups only Acutely MDMA releases 5-HT and dopamine from nerve endings and can cause long term neurotoxic damage in the forebrain (Green et al., 2003). It also induces behavioural sensitisation in rats (Aberg et al., 2007). Initial studies investigating the effects of mephedrone in rats have revealed that it also releases neuronal 5-HT and dopamine (Kehr et al, 2011; Baumann et al., 2012). In addition, the majority of MDMA pre-treatment has differential effects on mephedrone-induced *p<0.05, **p<0.01 compared to respective vehicle, Bonferroni post-hoc following Two way ANOVA