Synthetic cathinones are an emerging class of novel psychoactive substances whose rising rates of abuse have made them a significant public health issue. Recently, synthetic cathinones have emerged as popular drugs of abuse in the United States and Europe. Illicit drug dealers have synthesized stable analogues of cathinones and marketed them via the Internet as “legal high” alternatives to commonly abused psychostimulants. Some adverse effects of synthetic cathinone intoxication include depression, anxiety, myocardial infarction, cardiac dysrhythmias, pulmonary edema, renal failure, stroke and death. Two synthetic cathinones that have gained popularity over the past decade are 4-methylmethcathinone (mephedrone, MEPH) and 3,4-methylenedioxypyrovalerone (MDPV). Analogous to other amphetamines and cathinones, MEPH is a chiral molecule with two enantiomers, R-MEPH and S-MEPH. Pharmacological differences can exist between enantiomers. Given the enantiomeric specificity that exists between the enantiomers of MEPH, the overall goal of this dissertation was to investigate potential therapeutic effects of S-MEPH and to determine the reinforcing effects and potential abuse liability of each enantiomer of MEPH. To date, there are no approved medications for psychostimulant abuse despite high rates of relapse even with treatment. The lack of pharmacotherapies suggests that new approaches for psychostimulant abuse remain to be identified. Further exploration into the stereochemistry of MEPH will characterize a new approach to manage psychostimulant abuse during acute withdrawal that causes anxiety and relapse. The first experimental chapter of this thesis (Chapter 2) evaluated the effects of S-MEPH on cocaine and MDPV withdrawal-induced anxiety- and depression-like behavior in the elevated plus maze (EPM) and forced swim test (FST), respectively. EPM and FST were used to test the hypothesis that S-MEPH reduces anxiety-like and depression-like behaviors in rats withdrawn from a 10-day chronic binge cocaine paradigm (10 mg/kg, 3x/day in 1-hr intervals) or binge MDPV paradigm (1 mg/kg, 3x/day in 1-hr intervals). Control animals received saline injections. Both cocaine and MDPV induced heightened anxiety-like behavior on the EPM compared to saline controls. Rats withdrawn from chronic cocaine exposure received treatment with 10 mg/kg S-MEPH (COC SM 10). S-MEPH increased the percentage of time spent on the open arms compared to treatment with saline (COC SAL). Similar efficacy was observed for 10 mg/kg of S-MEPH where rats withdrawn from chronic MDPV exposure and treated with S-MEPH during the drug-free period (MDPV SM 10) increased the percentage of time spent on the open arms compared to treatment with saline (MDPV SAL). Treatment with a higher dose of S-MEPH (30 mg/kg) also produced an increase in time spent on the open arms for rats withdrawn from either cocaine (COC SM 30) or MDPV (MDPV SM 30). However, treatment with 30 mg/kg S-MEPH by itself (SAL S-MEPH) caused a slight, but significant, decrease in time spent on the open arms compared to saline controls (SAL SAL). Cocaine and MDPV withdrawn rats spent more time in the immobile state, indicative of depression-like behavior in the FST compared to saline controls. Subsequent treatment with 10 mg/kg S-MEPH (COC SM 10), following withdrawal from chronic cocaine exposure, reduced the time spent in the immobile state compared to treatment with saline (COC SAL). Similar efficacy was observed for 10 mg/kg of S-MEPH where rats withdrawn from chronic MDPV exposure and treated with S-MEPH during the drug-free period (MDPV SM 10) decreased the time spent in the immobile state compared to treatment with saline (MDPV SAL). Treatment with a higher dose of S-MEPH (30 mg/kg) also produced a decrease in immobile state for rats withdrawn from either cocaine (COC SM 30) or MDPV (MDPV SM 30). However, treatment with 30 mg/kg S-MEPH by itself (SAL S-MEPH) caused a reduction in time spent in the immobile compared to saline controls (SAL SAL). The present data suggest S-MEPH has therapeutic potential during early withdrawal from psychostimulant abuse and may be a possible structural and pharmacological template to develop maintenance therapy for psychostimulant abuse. Although dopamine (DA) and serotonin (5-HT) release is the primary mechanism of action of S-MEPH, in vitro studies assessed the receptor binding and activity of S-MEPH to elucidate a possible secondary mechanism that could contribute to the clinical effects of S-MEPH. Indeed, standard antidepressants (e.g. fluoxetine) have a dual mechanism of action, and not only increase 5-HT levels, but also block 5-HT2C receptors that help to prevent the initial onset of anxiety reported by patients. Binding and functional assays were performed at the National Institute of Mental Health Psychoactive Drug Screening Program (PDSP). For radioligand binding, S-MEPH was screened at 10 μM for binding to a battery of receptors including a range of 5-HT, DA, sigma, kappa opioid, adrenergic, muscarinic, and nicotinic receptor subtypes. It was found that S-MEPH binds to 5-HT2 receptors (2A, 2B, 2C) but showed negligible binding for dopaminergic, adrenergic and nicotinic receptors. Functional assays revealed S-MEPH has no agonist activity. These results suggest that S-MEPH may be a possible structural and pharmacological template to develop a maintenance therapy for addicts with acute anxiety and depression during early withdrawal by enhancing the release of 5-HT and/or through 5-HT2 receptor interactions. Chapter 3 investigated whether S-MEPH can reinstate drug-seeking behavior in rats with a history of cocaine exposure. Using the drug intravenous self-administration (IVSA) model, rats were trained to self-administer cocaine (0.375 mg/kg/infusion) under FR-1 schedule of reinforcement during daily 2-hour sessions for 12 days. Following acquisition of cocaine self-administration, rats were subject to daily 2-hour extinction sessions. After meeting extinction criteria (less than 15 active lever presses), rats underwent drug-primed reinstatement, where non-contingent injections of R-MEPH (10 mg/kg), S-MEPH (10, 20, 30 mg/kg), or cocaine (10 mg/kg) were administered prior to the reinstatement session. S-MEPH (10, 20, 30 mg/kg) prior to the reinstatement session was not efficacious in producing drug-seeking behavior. R-MEPH-prime injection reinstated cocaine-seeking behavior. Rats reinstated responding after cocaine-prime injection. After observing differences in priming injections with R-MEPH and S-MEPH during reinstatement to cocaine-seeking (Chapter 3), we further examined the reinforcing effects and potential for abuse of S-MEPH in Chapter 4. Fixed-ratio 1 (FR-1) (acquisition) and progressive-ratio (motivation) schedules of reinforcement were used to determine the rewarding properties and motivational incentive for R-MEPH, S-MEPH as well as racemic MEPH. Rats were trained to self-administer MEPH (0.5 mg/kg/infusion) under a fixed ratio (FR-1) schedule in daily 2-hour sessions for 14 days. After acquisition, rats underwent dose substitution to examine effects of R-MEPH (0.5 mg/kg/infusion) and S-MEPH (0.25, 0.5, 2. mg/kg/ infusion). Dose-substitution studies revealed the lowest (0.25 mg/kg/ infusion) and middle (0.5 mg/kg/ infusion) dose of S-MEPH significantly increased the number of reinforcers earned, while the highest (2.0 mg/kg/ infusion) dose decreased the number of reinforcers earned during the two hour sessions compared to acquisition of MEPH. Following the observations that rats readily acquire S-MEPH when a history of MEPH administration is already established, in a separate cohort of rats, acquisition of S-MEPH self-administration was studied using three doses of S-MEPH (0.25, 0.5, 2.0 mg/kg/infusion) and R-MEPH (0.5 mg/kg/infusion) on a fixed-ratio 1 (FR-1) and progressive-ratio schedule of reinforcement using drug naïve rats. In this cohort, 50-kHz ultrasonic vocalizations (USVs) were recorded during FR-1 sessions to assess potential stereospecific differences in drug-associated positive affect. Rats were trained to self-administer R-MEPH (0.5 mg/kg/ infusion) or S-MEPH (0.25, 0.5, 2.0 mg/kg/ infusion) for 10 days followed by progressive-ratio for 10 days to determine motivation for drug taking. Rats readily self-administer both enantiomers of MEPH under a FR-1 schedule. Equivalent doses of R-MEPH and S-MEPH did not significantly differ in number of infusions and active lever presses. Self-administration of R-MEPH elicited greater rates of 50-kHz USVs compared to S-MEPH. Progressive-ratio studies determined that R-MEPH- trained rats had significantly higher breakpoints than that of S-MEPH- trained rats, indicating an increase in motivation to work for reinforcer. These data suggest that escalation of S-MEPH intake occurred when a previous history of MEPH was established; rats exposed to MEPH are compensating with escalating their intake of S-MEPH to reach the previous euphoric state during acquisition of MEPH. These findings indicate that rats readily acquire both enantiomers of MEPH, which signify an abuse liability for these drugs. However, these data suggest that R-MEPH, rather than the S-MEPH, is responsible for the rewarding and reinforcing effects seen with the racemic form of MEPH, due to the lack of motivation to work for the S-enantiomer. In conclusion, during early withdrawal from chronic cocaine or MDPV, rats demonstrated heightened anxiety-like behavior on the EPM and increased depression-like behavior in FST. Binding and functional assays indicate affinity for the 5-HT2 receptors (2A, 2B, 2C) and no agonist activity of S-MEPH. Results show R-MEPH reinstates cocaine-seeking behavior and S-MEPH does not reinstate cocaine-seeking. Furthermore, R-MEPH and S-MEPH both have reinforcing effects, with R-MEPH having significantly greater reinforcing strength compared to S-MEPH. These studies identified unique differences in the behavioral profile of MEPH enantiomers. This data not only expands the body of literature on the stereospecific effects of MEPH, but also developing the pharmacological profile of MDPV in the context of dependence and addiction. Continued research is needed profiling MEPH and MDPV to develop efficacious pharmacotherapeutics for treating psychostimulant abuse to reduce relapse rates. / Pharmacology
| Identifer | oai:union.ndltd.org:TEMPLE/oai:scholarshare.temple.edu:20.500.12613/1607 |
| Date | January 2017 |
| Creators | Khalid, Helene |
| Contributors | Rawls, Scott M., Ashby, Barrie, Kirby, Lynn, Unterwald, Ellen M., Reitz, Allen Bernard, 1956- |
| Publisher | Temple University. Libraries |
| Source Sets | Temple University |
| Language | English |
| Detected Language | English |
| Type | Thesis/Dissertation, Text |
| Format | 140 pages |
| Rights | IN COPYRIGHT- This Rights Statement can be used for an Item that is in copyright. Using this statement implies that the organization making this Item available has determined that the Item is in copyright and either is the rights-holder, has obtained permission from the rights-holder(s) to make their Work(s) available, or makes the Item available under an exception or limitation to copyright (including Fair Use) that entitles it to make the Item available., http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | http://dx.doi.org/10.34944/dspace/1589, Theses and Dissertations |
Page generated in 0.0036 seconds