Behavioural and neurochemical comparison of chronic intermittent cathinone, mephedrone and MDMA administration to the rat
Introduction
Cathinone, the β-keto analogue of amphetamine, is the main pharmacologically active constituent of the leaves of the Khat shrub (Catha eludis) which originates from the Arabian peninsula and East Africa (ACMD, 2010). The leaves of the Khat plant are commonly chewed or sometimes brewed as tea for their psychostimulant effects by people endogenous to its natural habitat, although use has recently been introduced by immigrants to the United States, Canada, Australia and Europe in the form of harvested leaves (Feyissa and Kelly, 2008).
Synthetic cathinone derivatives have recently received substantial media attention, particularly 4-methylmethcathinone (mephedrone) which, since its introduction to Europe in 2007, has been implicated in a number of adverse events and deaths. This has led to its reclassification as a controlled substance in many European countries (EMCDDA, 2011). There is nevertheless evidence that mephedrone remains available for illicit recreational consumption (Van Hout and Bingham, 2012, Wood et al., 2012). Before the ban mephedrone was the most popular of the cathinone derivatives to be used recreationally due to its supposed psychostimulant properties. One online survey of 2289 experienced polydrug users (male: mean age=25.2 years; female: mean age=24.6 years) showed 42% had tried mephedrone at least once, with approximately 30% using it every two weeks or more frequently (Winstock et al., 2011b). Recreational users of mephedrone have compared its stimulant effects to those of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine (Winstock et al., 2011b).
Although considered a designer drug, mephedrone is not a newly discovered compound. First synthesised in 1929, mephedrone received little interest until 2007, when it started being sold on the internet as a plant food and in shops as a ‘legal high’. This coincided with strict restrictions on MDMA, which caused a decrease in its availability and also in the purity of ‘ecstasy’ tablets. For example, less than 50% of ecstasy tablets confiscated in the Netherlands in 2009 contained MDMA as their primary component, compared to 90% in previous years (Brunt et al., 2011). In many of these tablets MDMA was substituted by other compounds and in 2009 mephedrone was found to be the most prevalent new designer drug to be misleadingly sold as ecstasy. This decline in purity and availability of ecstasy tablets, as well as mephedrone's initial legal status, is thought to be the main reasons for its increased popularity.
Preliminary studies on mephedrone have indicated that, like MDMA, it acts on the brain monoaminergic systems and causes hyperactivity, conditioned place preference and reduced social preference in rats (Baumann et al., 2012, Kehr et al., 2011, Lisek et al.,, Martinez-Clemente et al., 2011, Motbey et al., 2012). Mephedrone also improves visuo-spatial memory and learning in non-human primates (Wright et al., accepted for publication) and impairs working memory in humans (Freeman et al., 2012). The current study examined the effects of mephedrone in behavioural paradigms regulated by monoamine function, and compared its effects to those of cathinone and MDMA in the adult rat. Doses of cathinone were selected from previous studies in an attempt to produce plasma levels similar to those reported in man (Feyissa and Kelly, 2008). While an MDMA dose of 10 mg/kg is higher than that required to produce plasma levels comparable to that observed in recreational users (Green et al., 2012) it was chosen to reflect the longer plasma half-life of the drug in humans compared with rats (Green et al., 2012). Although it has been reported that human users of mephedrone may on average ingest 0.5–4 g in a single session, this is generally a cumulative dose following binge dosing (Schifano et al., 2011, Winstock et al., 2011a). Since previous studies have shown that a significant release of dopamine occurs in the nucleus accumbens following a mephedrone dose of 3 mg/kg (Kehr et al., 2011) we selected doses of 1, 4 and 10 mg/kg of mephedrone to observe pharmacological effects. A dose schedule involving injection on two consecutive days each week for 3 weeks was used to mimic the weekend use of a recreational drug user. The effects of these compounds were investigated on locomotor activity and visual recognition memory, and since the amphetamines have been reported to affect associative memory and sensorimotor gating (Cappell et al., 1972, Vollenweider et al., 1999) these behaviours were also measured. In addition, rats were killed 7 days after the final drug administration and the concentration of dopamine, 5-HT and their metabolites were measured in the frontal cortex, striatum and hippocampus to ascertain if the drugs had any longer-term monoaminergic neurotoxicity such as previously reported following high doses of MDMA (Carvalho et al., 2012, Green et al., 2003). For comparison, the short-term (2 h post-injection) neurochemical effects of a single injection of cathinone, mephedrone or MDMA were also compared in separate groups of rats to measure brain regional changes in monoamines and metabolites.
Section snippets
Animals
All experiments used experimentally naïve young-adult male Lister hooded rats (170–305 g; Charles River UK or University of Nottingham Biomedical Services Unit) housed in groups of 4–5 per cage under constant environmental conditions (12 h light and dark cycle with lights on at 07.00 h, ambient temperature 21±2 °C and relative humidity 55±10%). Food and water were freely available. All experiments were conducted during the light phase. The dose and behavioural schedule was chosen to comply with the
Locomotor activity
Habituation to the activity boxes prior to injection on days 1 and 16 was confirmed by a decline in activity over the 60 min period on both days, with no significant difference observed between treatment groups (data not shown). On day 1, there was a main effect of drug treatment on locomotion (cathinone (1 and 4 mg/kg): F(2,21)=41.65, p≤0.0001, Fig. 1A; mephedrone and MDMA (10 mg/kg): F(2,19)=9.91, p≤0.001, Fig. 1E) which just failed to reach significance for 1 and 4 mg/kg mephedrone (F(2,21)
Discussion
In the current study, cathinone, mephedrone and MDMA all produced significant ambulatory hyperactivity when administered to rats and in the case of cathinone and mephedrone (where different doses were examined) these effects appeared to be dose dependent (Fig. 1). The hyperactivity induced by cathinone was greater in both magnitude and duration than that caused by mephedrone at the same dose. Dopaminergic neuronal activity plays a key role in locomotion and dopamine release has been reported in
Role of the funding source
This work was funded by the University of Nottingham, School of Biomedical Sciences; the University of Nottingham had no further involvement in the study design, in the collection, analysis or interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.
Contributors
S.E. Shortall, P.M. Wigmore, F.J. Ebling, A.R. Green, K.C.F. Fone, M.V. King designed and conceived the study. S.E. Shortall and M.V. King collected data with assistance from A.E. Macerola, R.T.R. Swaby, R. Jayson, C. Korsah, K.E. Pillidge. S.E. Shortall conducted statistical analysis and wrote the first draft of the manuscript. P.M. Wigmore, F.J.P. Ebling, A.R. Green, K.C.F. Fone, M.V. King supervised the work and critically reviewed the manuscript. All authors contributed to and have approved
Conflict of interest
Authors do not have any conflict of interest to report.
Acknowledgements
The authors would like to thank Mr. Ian Topham, Mrs. Karen Swift and Mr. Ben Pointer-Gleadhill for their technical assistance. This work was funded by the University of Nottingham.
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