Binge-like acquisition of 3,4-methylenedioxypyrovalerone (MDPV) self-administration and wheel activity in rats

The pharmacology and neurotoxicology of synthetic cathinones

The synthetic cathinones are man-made compounds derived from the naturally occurring drug cathinone, which is found in the khat plant. The drugs in this pharmacological class that will be the focus of this chapter include mephedrone, MDPV, methcathinone and methylone. These drugs are colloquially known as "bath salts". This misnomer suggests that these drugs are used for health improvement or that they have legitimate medical uses. The synthetic cathinones are dangerous drugs with powerful pharmacological effects that include high abuse potential, hyperthermia and hyperlocomotion. These drugs also share many of the pharmacological effects of the amphetamine class of drugs including methamphetamine, amphetamine and MDMA and therefore have high potential to cause damage to the central nervous system. The synthetic cathinones are frequently taken in combination with other psychoactive drugs such as alcohol, marijuana and the amphetamine-like stimulants, creating a situation where heightened pharmacological and neurotoxicological effects are likely to occur. Despite the structural features shared by the synthetic cathinones and amphetamine-like stimulants, including their actions at monoamine transporters and receptors, the effects of the synthetic cathinones do not always match those of the amphetamines. In particular, the synthetic cathinones are far less neurotoxic than their amphetamine counterparts, they produce a weaker hyperthermia, and they cause less glial activation. This chapter will briefly review the pharmacology and neurotoxicology of selected synthetic cathinones with the aim of delineating key areas of agreement and disagreement in the literature particularly as it relates to neurotoxicological outcomes.

Effects of repeated binge intake of the pyrovalerone cathinone derivative 3,4-methylenedioxypyrovalerone on prefrontal cytokine levels in rats - a preliminary study

Drugs of abuse activate neuroimmune signaling in addiction-related regions of the brain, including the prefrontal cortex (PFC) which mediates executive control, attention, and behavioral inhibition. Traditional psychostimulants including methamphetamine and cocaine are known to induce PFC inflammation, yet the effects of synthetic cathinone derivatives are largely unexplored. In this study, we examined the ability of repeated binge-like intake of the pyrovalerone cathinone derivative 3,4-methylenedioxypyrovalerone (MDPV) to alter cytokine profiles in the PFC. Male and female rats were allowed to intravenously self-administer MDPV (0.05 mg/kg/infusion) or saline as a control under conditions of prolonged binge-like access, consisting of three 96 h periods of drug access interspersed with 72 h of forced abstinence. Three weeks following cessation of drug availability, PFC cytokine levels were assessed using antibody arrays. Employing the unsupervised clustering and regression analysis tool CytoMod, a single module of co-signaling cytokines associated with MDPV intake regardless of sex was identified. With regards to specific cytokines, MDPV intake was positively associated with PFC levels of VCAM-1/CD106 and negatively associated with levels of Flt-3 ligand. These findings indicate that prolonged MDPV intake causes changes in PFC cytokine levels that persist into abstinence; however, the functional ramifications of these changes remain to be fully elucidated.

Sex Chromosome Complement and Estradiol Modify Cocaine Self-Administration Behaviors in Male Mice

INTRODUCTION

Women are more vulnerable to cocaine's reinforcing effects and have a more rapid course to addiction after initial cocaine use as compared to men. Studies in rodents similarly indicate an enhanced sensitivity to the reinforcing effects of cocaine in females versus males. Levels of estradiol (E2) are correlated with vulnerability to the rewarding actions of cocaine. Here, we asked if sex chromosome complement (SCC) influences vulnerability to cocaine use.

METHODS

We used the four-core genotype mouse that produces gonadal males and females with either XX or XY SCC. Mice were gonadectomized and implanted with either an estradiol (E2) or cholesterol-filled pellet. This allowed us to determine the effects of SCC in the absence (cholesterol-treated) and presence of tonic high physiological hormone levels (estradiol). Acquisition of cocaine self-administration was determined over a 12-day period using an escalated dose procedure (0.3 mg/kg/infusion, sessions 1-6; 0.6 mg/kg/infusion, sessions 6-12).

RESULTS

Without estradiol treatment, a greater percentage of castrated XY mice acquired cocaine self-administration and did so at a faster rate than XX castrates and ovariectomized XY females. These same XY males acquired sooner, infused more cocaine, and directed more nose pokes to the rewarded nose-poke hole than XX castrates and XY males receiving E2.

CONCLUSION

Our results suggest that in gonadal male mice, SCC and estradiol can modulate the reinforcing effects of cocaine which may influence the likelihood of cocaine use.

Effects of α-pyrrolidino-phenone cathinone stimulants on locomotor behavior in female rats

The α-pyrrolidino-phenone cathinone stimulants first came to widespread attention because of bizarre behavior consequent to the use of α-pyrrolidinopentiophenone (α-PVP, "flakka") reported in popular press. As with other designer drugs, diversification of cathinones has been driven by desirable subjective effects, but also by attempts to stay ahead of legal controls of specific molecules. The α-pyrrolidinohexiophenone (α-PHP) and α-pyrrolidinopropiophenone (α-PPP) compounds have been relatively under-investigated relative to α-PVP and provide a key opportunity to also investigate structure-activity relationships, i.e., how the extension of the alpha carbon chain may affect potency or efficacy. Female rats were used to contrast the effects of α-PHP and α-PPP with those of α-PVP in altering wheel activity and effects on spontaneous locomotion, temperature and intracranial self-stimulation reward. The α-PPP, α-PHP and α-PVP compounds (5, 10 mg/kg, i.p.) suppressed wheel activity. Inhalation of α-PHP or α-PVP also suppressed wheel activity, but for an abbreviated duration compared with the injection route. Spontaneous activity was increased, and brain reward thresholds decreased, in a dose-dependent manner by all three compounds; only small decrements in body temperature were observed. These data show that all three of the α-pyrrolidino-phenone cathinones exhibit significant stimulant-like activity in female rats. Differences were minor and abuse liability is therefore likely to be equivalent for all three α-pyrrolidino-phenones.

Influence of Contingent and Noncontingent Drug Histories on the Development of High Levels of MDPV Self-Administration

A subset of rats that self-administer 3,4-methylenedioxypyrovalerone (MDPV) develop unusually high levels of drug taking. A history of responding maintained by cocaine, but not food, prevents the development of this high-responder phenotype; however, it is unclear how histories of noncontingent cocaine exposure or self-administering drugs from other pharmacological classes would affect its development. In the current studies, 5 groups of male Sprague-Dawley rats were used to determine whether histories of responding maintained by drugs from different pharmacological classes (e.g., MDPV, cocaine, fentanyl, nicotine, or ketamine) would differentially impact the development of the high-responder phenotype when MDPV was available for self-administration. Two additional groups were used to determine whether noncontingent exposure to cocaine would prevent the development of the high-responder phenotype when MDPV was available for self-administration, and whether noncontingent exposure to MDPV would facilitate the development of the high-responder phenotype when cocaine was available for self-administration. Consistent with previous reports, a history of response-contingent cocaine, and to a lesser extent noncontingent cocaine, prevented the MDPV high-responder phenotype; however, when responding was initially maintained by fentanyl, nicotine, or ketamine, the MDPV high-responder phenotype developed in ∼45% of rats. By manipulating behavioral and pharmacological histories prior to evaluating MDPV self-administration, the current studies provide additional evidence that a history of response-contingent (or noncontingent) cocaine can prevent the transition from well regulated to aberrant drug-taking when responding is maintained by MDPV. Although the mechanism(s) that underlies this novel high-responder phenotype are unknown, elucidation may provide insight into individual differences relating to substance use disorder. SIGNIFICANCE STATEMENT: A subset of outbred Sprague-Dawley rats self-administer high levels of the synthetic cathinone 3,4-methylenedioxypyrovalerone (MDPV). Understanding the behavioral and/or pharmacological factors that can prevent the development of dysregulated MDPV self-administration may provide insight into individual differences in vulnerability to develop a substance use disorder.

Cannabidiol Modulates the Motivational and Anxiety-Like Effects of 3,4-Methylenedioxypyrovalerone (MDPV) in Mice

3,4-Methylenedioxypyrovalerone (MDPV) is a new psychoactive substance (NPS) and the most widespread and life-threatening synthetic cathinone of the "bath salts". Preclinical research has proven the cocaine-like psychostimulant effects of MDPV and its potential for abuse. Cannabidiol (CBD) is a non-psychotropic phytocannabinoid that has emerged as a new potential treatment for drug addiction. Here, we tested the effects of CBD (20 mg/kg) on MDPV (2 mg/kg)-induced conditioned place preference and MDPV (0.05 and 0.075 mg/kg/infusion) self-administration paradigms. In addition, we assessed the effects of the co-administration of CBD and MDPV (3 and 4 mg/kg) on anxiety-like behaviour using the elevated plus maze (EPM). CBD mitigated the MDPV-induced conditioned place preference. On the contrary, CBD administration throughout the MDPV (0.075 mg/kg/infusion) self-administration increased drug-seeking and taking behaviours, but only in the high-responders group of mice. Furthermore, CBD exerted anxiolytic-like effects, exclusively in MDPV-treated mice. Taken together, our results indicate that CBD modulation of MDPV-induced motivational responses in mice varies depending on the requirements of the learning task, resulting in a complex response. Therefore, further research attempting to decipher the behavioural and molecular interactions between CBD and MDPV is needed.

Acute MDPV Binge Paradigm on Mice Emotional Behavior and Glial Signature

3,4-Methylenedioxypyrovalerone (MDPV), a widely available synthetic cathinone, is a popular substitute for classical controlled drugs of abuse, such as methamphetamine (METH). Although MDPV poses public health risks, its neuropharmacological profile remains poorly explored. This study aimed to provide evidence on that direction. Accordingly, C57BL/6J mice were exposed to a binge MDPV or METH regimen (four intraperitoneal injections every 2 h, 10 mg/kg). Locomotor, exploratory, and emotional behavior, in addition to striatal neurotoxicity and glial signature, were assessed within 18–24 h, a known time-window encompassing classical amphetamine dopaminergic neurotoxicity. MDPV resulted in unchanged locomotor activity (open field test) and emotional behavior (elevated plus maze, splash test, tail suspension test). Additionally, striatal TH (METH neurotoxicity hallmark), Iba-1 (microglia), GFAP (astrocyte), RAGE, and TLR2/4/7 (immune modulators) protein densities remained unchanged after MDPV-exposure. Expectedly, and in sheer contrast with MDPV, METH resulted in decrease general locomotor activity paralleled by a significant striatal TH depletion, astrogliosis, and microglia arborization alterations (Sholl analysis). This comparative study newly highlights that binge MDPV-exposure comes without evident behavioral, neurochemical, and glial changes at a time-point where METH-induced striatal neurotoxicity is clearly evident. Nevertheless, neuropharmacological MDPV signature needs further profiling at different time-points, regimens, and brain regions.

MDPV self-administration in female rats: influence of reinforcement history

RATIONALE

A subset of male rats that self-administer 3,4-methylenedioxypyrovalerone (MDPV) have unusually high levels of drug intake; however, factor(s) that influence this behavior (e.g., reinforcement history and sex) are unknown.

OBJECTIVES

Characterize the reinforcing potency and effectiveness of MDPV in female rats to determine whether (1) a subset of females also develop high levels of MDPV self-administration (i.e., a high-responder phenotype) and (2) the degree to which the high-responder phenotype is influenced by various reinforcement histories (i.e., responding for cocaine or food).

METHODS

Female Sprague Dawley rats initially responded for MDPV (0.032 mg/kg/infusion), cocaine (0.32 mg/kg/infusion), or food (45-mg grain pellet) under fixed ratio (FR) 1 and FR5 schedules of reinforcement. After 20 sessions, the cocaine- and food-history rats responded for MDPV for 20 additional sessions. Dose-response curves for MDPV were generated under FR5 and progressive ratio (PR) schedules of reinforcement.

RESULTS

A subset of rats responding for MDPV developed high levels of MDPV intake. A history of responding for cocaine, but not food, inhibited the development of high levels of MDPV intake. Large individual differences were observed in the level of self-administration when MDPV was available under an FR5, but not PR, schedule of reinforcement.

CONCLUSIONS

MDPV functions as a powerful reinforcer in female rats, as has been previously reported in male rats. The substantial variability in MDPV self-administration between subjects may be related to individual differences in human drug-taking behavior.

Sex chromosome complement influences vulnerability to cocaine in mice

Women acquire cocaine habits faster and are more motivated to obtain drug than men. In general, female rodents acquire intravenous cocaine self-administration (SA) faster and show greater locomotor responses to cocaine than males. Sex differences are attributed to differences in circulating estradiol. We used the four core genotype (FCG) mouse to ask whether sex chromosome complement influences vulnerability to cocaine's reinforcing and/or locomotor-activating effects. The FCG cross produces ovary-bearing mice with XX or XY genotypes (XXF, XYF) and testes-bearing mice with XX or XY genotypes (XXM, XYM). A greater percentage of gonadal females acquired cocaine SA via infusions into jugular catheters as compared with XYM mice, but XXM mice were not significantly different than any other group. Discrimination of the active versus inactive nose poke holes and cocaine intake were in general greater in gonadal females than in gonadal males. Progressive ratio tests for motivation revealed an interaction between sex chromosomes and gonads: XYM mice were more motivated to self-administer cocaine taking more infusions than mice in any other group. Locomotor responses to cocaine exposure revealed effects of sex chromosomes. After acute exposure, activity was greater in XX than in XY mice and the reverse was true for behavioral sensitization. Mice with XY genotypes displayed more activity than XX mice when given cocaine after a 10-day drug-free period. Our data demonstrate that sex chromosome complement alone and/or interacting with gonadal status can modify cocaine's reinforcing and locomotor-activating effects. These data should inform current studies of sex differences in drug use.

Methylenedioxymethamphetamine-like discriminative stimulus effects of pyrrolidinyl cathinones in rats

BACKGROUND

Synthetic cathinone derivatives are used as alternatives both for stimulant drugs such as cocaine and methamphetamine and for club drugs such as 3,4-methylenedioxymethamphetamine (MDMA), but little is known about their MDMA-like subjective effects.

METHODS

In order to determine their similarity to MDMA, the discriminative stimulus effects of 10 pyrrolidinyl cathinones (α-pyrrolidinopropiophenone, 4'-methyl-α-pyrrolidinopropiophenone (4'-MePPP), α-pyrrolidinobutiophenone, 3',4'-methylenedioxy-α-pyrrolidinobutyrophenone (MD-PBP), α-pyrrolidinovalerophenone, 3,4-methylenedioxy-pyrovalerone (MDPV), α-pyrrolidinopentiothiophenone, napthylpyrovalerone (naphyrone), α-pyrrolidinohexiophenone, and 4'-methyl-α-pyrrolidinohexiophenone (4'-MePHP)) were assessed in Sprague-Dawley rats trained to discriminate 1.5 mg/kg racemic ±-MDMA from vehicle.

RESULTS

Compounds with no substitutions on the phenyl ring and the thiophene produced 44-67% MDMA-appropriate responding. In contrast, the substituted pyrrolidinyl cathinones produced a range of MDMA-appropriate responding dependent upon the length of the alpha side chain. 4'-MePPP, with a single carbon on the alpha position, produced 99.8% MDMA-appropriate responding, MD-PBP (two carbons) produced 83%, naphyrone (three carbons) produced 71%, MDPV (three carbons) produced, 66%, and 4'-MePHP (four carbons) produced 47%.

CONCLUSIONS

Many cathinone compounds have discriminative stimulus effects similar to those of MDMA. However, the pyrrolidine substitution appears to reduce serotonergic effects, with a commensurate decrease in MDMA-like effects. Substitutions on the phenyl ring appear to be able to restore MDMA-like responding, but only in compounds with short alpha side chains. These findings agree with earlier findings of increasing dopaminergic effects and stronger reinforcing effects with increasing side chain. Assessment of more compounds is necessary to establish the replicability/robustness of this phenomenon. These findings may be of use in predicting which compounds will have MDMA/club drug-like effects versus psychostimulant-like effects.

3,4-Methylenedioxypyrovalerone: Neuropharmacological Impact of a Designer Stimulant of Abuse on Monoamine Transporters

Methylenedioxypyrovalerone (MDPV) is an abused synthetic cathinone, commonly referred to as a "bath salt." Because the dopamine (DA) transporter (DAT) and vesicular monoamine transporter-2 (VMAT-2) are key regulators of both the abuse and neurotoxic potential of structurally and behaviorally related agents, the impact of MDPV on these transporters was investigated. Results revealed that a single in vivo MDPV administration rapidly (within 1 hour) and reversibly increased both rat striatal DAT and VMAT-2 activity, as assessed via [³H]DA uptake in synaptosomes and synaptic vesicles, respectively, prepared from treated rats. There was no evidence of an MDPV-induced increase in plasmalemmal membrane DAT surface expression. Plasma concentrations of MDPV increased dose-dependently as assessed 1 hour after 2.5 and 5.0 mg/kg (s.c.) administration and returned to levels less than 10 ng/ml by 18 hours after 2.5 mg/kg (s.c.). Neither pretreatment with a D1 receptor (SCH23390), a D2 receptor (eticlopride), nor a nicotinic receptor (mecamylamine) antagonist attenuated the MDPV-induced increase in DAT activity. In contrast, eticlopride pretreatment attenuated both the MDPV-induced increase in VMAT-2-mediated DA uptake and an associated increase in cytoplasmic-associated vesicle VMAT-2 immunoreactivity. SCH23390 did not attenuate the MDPV-induced increase in VMAT-2 activity. Repeated MDPV injections did not cause persistent DAergic deficits, as assessed 7 to 8 days later. The impact of MDPV on striatal and hippocampal serotonergic assessments was minimal. Taken together, these data contribute to a growing pharmacological rubric for evaluating the ever-growing list of designer cathinone-related stimulants. The profile of MDPV compared with related psychostimulants is discussed. SIGNIFICANCE STATEMENT: Pharmacological characterization of the synthetic cathinone, 3,4-methylenedioxypyrovalerone (MDPV; commonly referred to as a "bath salt"), is critical for understanding the abuse liability and neurotoxic potential of this and related agents. Accordingly, the impact of MDPV on monoaminergic neurons is described and compared with that of related psychostimulants.

Self-administration of the synthetic cathinones 3,4-methylenedioxypyrovalerone (MDPV) and α-pyrrolidinopentiophenone (α-PVP) in rhesus monkeys

RATIONALE

The availability and abuse of synthetic analogues of cathinone have increased dramatically around the world. Synthetic cathinones, such as 3,4-methylenedioxypyrovalerone [MDPV] and α-pyrrolidinopentiophenone [α-PVP], are cocaine-like inhibitors of monoamine transporters and common constituents of "bath salts" or "flakka" preparations. Studies in rats suggest that MDPV and α-PVP are 3 to 4-fold more effective reinforcers than cocaine; however, comparisons of the relative reinforcing effectiveness of MDPV and α-PVP have not been reported in other species.

OBJECTIVES

Accordingly, in the present study, 4 adult male rhesus monkeys responding under a progressive ratio schedule of reinforcement were used to characterize the reinforcing effects of MDPV and α-PVP and to compare directly these effects with those of cocaine and methamphetamine.

RESULTS

MDPV was the most potent reinforcer, followed by α-PVP, methamphetamine, and cocaine. α-PVP was the most effective reinforcer, followed by MDPV, cocaine, and methamphetamine. In addition to making more responses to obtain MDPV and α-PVP, monkeys also responded for longer periods of time when MDPV or α-PVP was available compared with when either cocaine or methamphetamine was available for infusion.

CONCLUSIONS

These studies confirm recent reports from rodents and provide strong evidence that the synthetic cathinones MDPV and α-PVP are capable of maintaining high levels of responding for prolonged periods of time, and that they function as more effective reinforcers than either cocaine or methamphetamine. The relative strength of these reinforcing effects may account for the high rates of "bath salts" use reported in humans.

Contribution of monoaminergic mechanisms to the discriminative stimulus effects of 3,4-methylenedioxypyrovalerone (MDPV) in Sprague-Dawley rats

RATIONALE

3,4-Methylenedioxypyrovalerone (MDPV) is a popular synthetic cathinone reported to have a high abuse potential. Recent preclinical research indicates the psychopharmacology of MDPV is comparable to cocaine. Despite a recent influx of research on the psychopharmacology of MDPV, few studies have employed preclinical drug discrimination methods to discern the neurochemical mechanisms involved in its interoceptive stimulus effects.

OBJECTIVE

The aim of this study was to evaluate a variety of monoaminergic agents for substitution, potentiation, or antagonism in rats trained to discriminate MDPV.

METHODS

Male Sprague-Dawley rats were trained to discriminate 0.5 (experiment 1) or 1 mg/kg MDPV (experiment 2) from saline under an FR 20 schedule of food reinforcement. In experiment 1, MDMA, MDA, and their respective optical isomers (0.75-3 mg/kg), cocaine (2.5-20 mg/kg), GBR 12909 (5-40 mg/kg), and desipramine (3.2-10 mg/kg) were assessed for substitution. GBR 12909 (40 mg/kg) and desipramine (3.2 mg/kg) were subsequently assessed for potentiation of the MDPV cue. In experiment 2, stimulus antagonism tests were conducted with dopamine antagonists (Sch 23390, haloperidol) and serotonin antagonists (pirenperone, MDL100907, WAY 100635).

RESULTS

The MDMA and MDA enantiomers produced divergent results, with virtually no substitution by (-)-MDMA or (-)-MDA, partial substitution with (+)-MDA, and full substitution with (+)-MDMA, as well as full substitution by the racemates, (±)-MDMA and (±)-MDA. Consistent with previous findings, cocaine fully substituted for MDPV. Although no dose of GBR 12909 or desipramine substituted for MDPV, these reuptake inhibitors enhanced the discriminative stimulus effects of lower MDPV doses. Both D1 (Sch 23390) and D2 (haloperidol) DA antagonists attenuated 1 mg/kg MDPV discrimination, whereas none of the 5-HT antagonists assessed altered MDPV discrimination.

CONCLUSIONS

These findings indicate MDPV's interoceptive stimulus effects are mediated predominantly by dopaminergic actions, although serotonergic and/or noradrenergic modulation of these effects cannot be ruled out. Further investigations into the neurochemical actions involved in the discriminative stimulus effects of MDPV may serve to inform medication discovery and development for the treatment of MDPV abuse.

Binge-like acquisition of α-pyrrolidinopentiophenone (α-PVP) self-administration in female rats

RATIONALE

The synthetic cathinone α-pyrrolidinopentiophenone (α-PVP) has been associated with bizarre public behavior in users. Association of such behavior with extended binges of drug use motivates additional investigation, particularly since a prior study found that half of male rats experience a binge of exceptionally high intake, followed by sustained lower levels of self-administration during the acquisition of intravenous self-administration (IVSA) of a related drug, 3,4-methylenedioxypyrovalerone.

OBJECTIVES

The binge-like acquisition pattern is novel for rat IVSA; thus, the present study sought to determine if this effect generalizes to IVSA of α-PVP in female rats.

METHODS

Female Wistar rats were trained in IVSA of α-PVP (0.05 mg/kg/inf) in experimental chambers containing an activity wheel. Groups were trained with the wheels fixed (No-Wheel group), fixed for the initial 5 days of acquisition or free to move throughout acquisition (Wheel group). The groups were next subjected to a wheel access switch and then all animals to dose-substitution (0.0125-0.3 mg/kg/inf) with the wheels alternately fixed and free to move.

RESULTS

Approximately half of the rats initiated their IVSA pattern with a binge day of exceptionally high levels of drug intake, independent of wheel access condition. Wheel activity was much lower in the No-Wheel group in the wheel switch post-acquisition. Dose-effect curves were similar for wheel access training groups, for binge/no binge phenotypic subgroups and were not altered with wheel access during the dose-substitution.

CONCLUSION

This confirms the high reinforcer effectiveness of α-PVP in female rats and the accompanying devaluation of wheel activity as a naturalistic reward.

Characterization of the discriminative stimulus effects of 3,4-methylenedioxypyrovalerone in male Sprague-Dawley rats

Recreational use of 3,4-methylenedioxypyrovalerone (MDPV) in the early 2000s prompted numerous scientific investigations of its behavioral and neurochemical effects. The purpose of this study was to further characterize the interoceptive stimulus effects of MDPV using a validated in-vivo drug-detection assay. Male Sprague-Dawley rats were trained to discriminate 0.3 mg/kg MDPV from saline under a fixed ratio 20 (FR 20) schedule of food reinforcement. After stimulus control was established with MDPV (∼35 training sessions), substitution tests were commenced with drugs from several chemical classes, including drugs with predominantly dopaminergic actions [MDPV, D-amphetamine, (+)-methamphetamine, (-)-cocaine], drugs with predominantly serotonergic actions [(+)-lysergic acid diethylamide, (+)-fenfluramine], and drugs with both serotonergic and dopaminergic actions (3,4-methylenedioxymethamphetamine, 4-methylmethcathinone). Full substitution for the 0.3 mg/kg MDPV cue was observed with D-amphetamine, (+)-methamphetamine, and (-)-cocaine. Surprisingly, the 5-HT releaser (+)-fenfluramine fully substituted in half the subjects, but completely suppressed responding in the remaining subjects. 3,4-Methylenedioxymethamphetamine, 4-methylmethcathinone, and (+)-lysergic acid diethylamide failed to fully substitute for MDPV. These results indicate that the MDPV cue is similar to cues produced by drugs with predominantly dopamine-increasing effects and perhaps serotonin-releasing effects among individual subjects. Given these findings, further research is warranted to directly assess the contributions of dopamine and serotonin receptor isoforms to the discriminative stimulus functions of MDPV.

Conjugate Vaccine Immunotherapy for Substance Use Disorder

Substance use disorder, especially in relation to opioids such as heroin and fentanyl, is a significant public health issue and has intensified in recent years. As a result, substantial interest exists in developing therapeutics to counteract the effects of abused drugs. A promising universal strategy for antagonizing the pharmacology of virtually any drug involves the development of a conjugate vaccine, wherein a hapten structurally similar to the target drug is conjugated to an immunogenic carrier protein. When formulated with adjuvants and immunized, the immunoconjugate should elicit serum IgG antibodies with the ability to sequester the target drug to prevent its entry to the brain, thereby acting as an immunoantagonist. Despite the failures of first-generation conjugate vaccines against cocaine and nicotine in clinical trials, second-generation vaccines have shown dramatically improved performance in preclinical models, thus renewing the potential clinical utility of conjugate vaccines in curbing substance use disorder. This review explores the critical design elements of drug conjugate vaccines such as hapten structure, adjuvant formulation, bioconjugate chemistry, and carrier protein selection. Methods for evaluating these vaccines are discussed, and recent progress in vaccine development for each drug is summarized.

Neurocognitive dysfunction following repeated binge-like self-administration of the synthetic cathinone 3,4-methylenedioxypyrovalerone (MDPV)

Synthetic cathinones, frequently referred to as "bath salts", have significant abuse potential, and recent evidence suggests that these novel psychoactive substances can also produce cognitive deficits as well as cytotoxic effects. However, most of these latter findings have been obtained either using high concentrations in vitro or following non-contingent high dose administration in vivo. The present study utilized a model of long-term voluntary binge-like self-administration to determine potential detrimental effects of synthetic cathinones on cognitive function and their known underlying neural circuits, collectively referred to as neurocognitive dysfunction. Male Sprague-Dawley rats were allowed to self-administer the cocaine-like synthetic cathinone 3,4-methylenedioxypyrovalerone (MDPV, 0.03 mg/kg/infusion i.v.) in 96-hr sessions, or saline as a control. A total of five 96-hr sessions were conducted, each separated by 3 days of abstinence in the home cage. Three weeks following the last 96-hr session, animals underwent assessment of cognitive function using spatial object recognition (SOR) and novel object recognition (NOR) tasks, after which brains were harvested and assessed for neurodegeneration using FluoroJade C (FJC). Compared to animals self-administering saline, animals self-administering MDPV demonstrated (1) robust drug intake that escalated over time, (2) deficits in NOR but not SOR, and (3) neurodegeneration in the perirhinal and entorhinal cortices. These results indicate that repeated binge-like intake of MDPV can induce neurocognitive dysfunction. In addition, utilization of rodent models of extended binge-like intake may provide insight into potential mechanisms and/or approaches to prevent or reverse the detrimental effects of abused substances on cognitive and neurobiological functioning. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

High ambient temperature facilitates the acquisition of 3,4-methylenedioxymethamphetamine (MDMA) self-administration

RATIONALE

MDMA alters body temperature in rats with a direction that depends on the ambient temperature (T_A). The thermoregulatory effects of MDMA and T_A may affect intravenous self-administration (IVSA) of MDMA but limited prior reports conflict.

OBJECTIVE

To determine how body temperature responses under high and low T_A influence MDMA IVSA.

METHODS

Male Sprague-Dawley rats were trained to IVSA MDMA (1.0mg/kg/infusion; 2-h sessions; FR5 schedule of reinforcement) under T_A 20°C or 30°C. Radiotelemetry transmitters recorded body temperature and activity during IVSA.

RESULTS

MDMA intake increased under both T_A during acquisition, but to a greater extent in the 30°C group. The magnitude of hypothermia was initially equivalent between groups but diminished over training in the 30°C group. Within-session activity was initially lower in the 30°C group, but by the end of acquisition and maintenance, activity was similar for both groups. When T_A conditions were swapped, the hot-trained group increased MDMA IVSA under 20°C T_A and a modest decrease in drug intake was observed in the cold-trained group under 30°C T_A. Subsequent non-contingent MDMA (1.0-5.0mg/kg, i.v.) found that rats with higher MDMA IVSA rates showed blunted hypothermia compared with rats with lower IVSA levels; however, within-session activity did not differ by group. High T_A increased intracranial self-stimulation thresholds in a different group of rats and MDMA reduced thresholds below baseline at low, but not high, T_A.

CONCLUSIONS

High T_A appears to enhance acquisition of MDMA IVSA through an aversive effect and not via thermoregulatory motivation.

Effects of orally self-administered bath salt constituent 3,4-methylenedioxypyrovalerone (MDPV) in mice

Synthetic cathinones in bath salts products are psychostimulant drugs of abuse, and 3,4-methylenedioxypyrovalerone (MDPV) is a common constituent of these products. Oral MDPV has been show to stimulate locomotor activity but reinforcing, locomotor and appetitive stimulus effects of oral MDPV are unknown. Choice procedures evaluated preference for 0.03, 0.10, 0.30, and 1.00mg/mL MDPV solutions versus 0.10mg/mL quinine solution or water. To verify that oral MDPV produced pharmacological effects, locomotor activity was monitored during and after consumption of water, quinine, or MDPV solutions. Conditioned place preference (CPP) tested the apparent appetitive effects of a preferred concentration of oral MDPV with locomotor stimulant effects (0.30mg/mL), using water as a control, and compared with results from intraperitoneally-administered MDPV. Consumption of MDPV solutions (0.03-1.00mg/mL) was low when the alternative fluid was water, but a history of MDPV consumption increased MDPV choice. When paired with a quinine control solution, MDPV solutions (0.03-0.30mg/mL) were almost exclusively preferred, and treatment with the catecholamine synthesis inhibitor αMPT decreased MDPV choice. Consumption of MDPV concentrations (0.1-1.0mg/mL) stimulated locomotor activity. Chronic (10day) access to 0.30mg/mL MDPV resulted in escalated consumption, but locomotor effects did not systematically change across the access period. Finally, consumption of 0.30mg/mL MDPV elicited CPP with a magnitude similar to the preference observed following intraperitoneal administration of MDPV. Consistent with human abuse patterns, oral MDPV has reinforcing effects in the mouse which are most likely related to its psychostimulant-like pharmacological profile.

Relative reinforcing effects of second-generation synthetic cathinones: Acquisition of self-administration and fixed ratio dose-response curves in rats

"Bath salts" preparations contain synthetic cathinones which interact with monoamine transporters and function as either monoamine uptake inhibitors or releasers. 3,4-Methylenedioxypyrovalerone (MDPV), 3,4-methylenedioxymethcathinone (methylone), and 4-methylmethcathinone (mephedrone) were three of the most common cathinones (i.e., "first-generation" cathinones); however, after the US Drug Enforcement Administration placed them under Schedule I regulations, they were replaced with structurally related cathinones that were not subject to regulations (i.e., "second-generation" cathinones). Although the reinforcing effects of some second-generation cathinones have been described (e.g., α-pyrrolidinopentiophenone [α-PVP]), little is known about how structural modifications, particularly those involving the methylenedioxy moiety and α-alkyl side chain, impact the abuse liability of other second-generation cathinones (e.g., α-pyrrolidinopropiophenone [α-PPP], 3,4-methylenedioxy-α-pyrrolidinobutiophenone [MDPBP], and 3,4-methylenedioxy-α-pyrrolidinopropiophenone [MDPPP]). The present study used male Sprague-Dawley rats (n = 12 per drug) to directly compare: (1) the acquisition of responding for α-PVP (0.032 mg/kg/inf), α-PPP (0.32 mg/kg/inf), MDPBP (0.1 mg/kg/inf), and MDPPP (0.32 mg/kg/inf) under a fixed ratio (FR) 1 schedule of reinforcement; and (2) full dose-response curves for each drug to maintain responding under an FR5 schedule of reinforcement. The average number of days (∼4 days) and percentage (100%) of rats that acquired self-administration was similar for each drug. The observed rank order potency to maintain responding under an FR5 schedule of reinforcement (α-PVP ≈ MDPBP>α-PPP > MDPPP) is consistent with their potencies to inhibit dopamine uptake. These are the first studies to report on the reinforcing effects of the unregulated second-generation cathinones MDPBP, MDPPP, and α-PPP and indicate all three compounds are readily self-administered, suggesting each possesses high potential for abuse. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Individual Differences in the Relative Reinforcing Effects of 3,4-Methylenedioxypyrovalerone under Fixed and Progressive Ratio Schedules of Reinforcement in Rats

The recreational use of designer drugs, including synthetic cathinones (bath salts), is associated with high levels of abuse and toxicity, and represents a growing threat to public health. 3,4-Methylenedioxypyrovalerone (MDPV) is a cocaine-like monoamine uptake inhibitor, and one of the most widely available and abused synthetic cathinones. The present study used male Sprague-Dawley rats to directly compare: (1) the acquisition of responding for MDPV and cocaine under a fixed ratio (FR) 1 schedule of reinforcement; (2) full dose-response curves for MDPV and cocaine under a FR5 schedule; and (3) progressive ratio (PR) schedules of reinforcement. Self-administration of MDPV and cocaine was acquired at comparable rates, and by a similar percentage of rats. Compared with cocaine, MDPV was ∼10-fold more potent and ∼3-fold more effective at maintaining responding (PR; final ratio completed). Unlike cocaine, for which little variability was observed among rats, the FR5 dose-response curve for MDPV was shifted ∼3-fold upward for a subset of rats (high-responders) relative to other rats with identical histories (low-responders). Compared with low-responding rats, high responders also self-administered more cocaine under the FR5 schedule, and earned significantly more MDPV, cocaine, and methamphetamine under a PR schedule of reinforcement. In addition to functioning as a significantly more effective reinforcer than either cocaine or methamphetamine, MDPV also appears to be unique in its capacity to establish an enduring phenotype in rats, characterized by unusually high levels of drug intake. Although the factors underlying this high-responder phenotype are unclear, they might be related to individual differences in human drug-taking behavior.

Reinforcing Effects of Cathinone NPS in the Intravenous Drug Self-Administration Paradigm

Since the mid- to late 2000s, there has been a dramatic rise in the use and abuse of synthetic derivatives of cathinone, a stimulant alkaloid found in the African shrub Catha edulis. Cathinone novel psychoactive substances (NPS), also referred to as synthetic cathinones or "bath salt"-type drugs, have gained popularity among drug users due to their potency, low cost, ease of procurement, and diverse array of evolving chemical structures. While the ability of cathinone NPS to produce psychotomimetic effects, multiple organ system toxicity, and death in humans is well documented, there has been limited scientific investigation into the reinforcing effects and abuse liability of these drugs. In this chapter, we will summarize the existing literature on the reinforcing effects of cathinone NPS in rodents using the intravenous self-administration (IVSA) paradigm. We will also compare the ability of cathinone NPS to serve as reinforcers to that of classical psychostimulants such as cocaine, methamphetamine, and methylenedioxymethamphetamine (MDMA). The chapter will conclude with a summary and indications for future avenues of research on cathinone NPS.

The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens

Classic hallucinogens share pharmacology as serotonin 5-HT_2A, 5-HT_2B, and 5-HT_2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT_2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT_2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT_2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancing inhibitory activity via intrinsic mechanisms. Together with experiments that show that addictive drugs, such as cocaine, potentiate Kv1.x channels, thereby suppressing NAc shell GABAergic activity, this hypothesis provides a mechanism by which classic hallucinogen-mediated stimulation of 5-HT_2C receptors could thwart addiction. It also provides a potential reason for the non-addictive nature of classic hallucinogens.

Active vaccination attenuates the psychostimulant effects of α-PVP and MDPV in rats

Recreational use of substituted cathinones continues to be an emerging public health problem in the United States; cathinone derivatives α-pyrrolidinopentiophenone (α-PVP) and 3,4-methylenedioxypyrovalerone (MDPV), which have been linked to human fatalities and show high potential for abuse liability in animal models, are of particular concern. The objective of this study was to develop an immunotherapeutic strategy for attenuating the effects of α-PVP and MDPV in rats, using drug-conjugate vaccines created to generate antibodies with neutralizing capacity. Immunoconjugates (α-PVP-KLH and MDPV-KLH) or the control carrier protein, keyhole limpet hemocyanin (KLH), were administered to groups (N = 12) of male Sprague-Dawley rats on Weeks 0, 2 and 4. Groups were administered α-PVP or MDPV (0.0, 0.25, 0.5, 1.0, 5.0 mg/kg, i.p.) in acute drug challenges and tested for changes in wheel activity. Increased wheel activity produced by α-PVP or MDPV in the controls was attenuated in the α-PVP-KLH and MDPV-KLH vaccinated groups, respectively. Rectal temperature decreases produced by MDPV in the controls were reduced in duration in the MDPV-KLH vaccine group. A separate group (N = 19) was trained to intravenously self-administer α-PVP (0.05, 0.1 mg/kg/inf) and vaccinated with KLH or α-PVP-KLH, post-acquisition. Self-administration in α-PVP-KLH rats was initially higher than in the KLH rats but then significantly decreased following a final vaccine booster, unlike the stable intake of KLH rats. The data demonstrate that active vaccination provides functional protection against the effects of α-PVP and MDPV, in vivo, and recommend additional development of vaccines as potential therapeutics for mitigating the effects of designer cathinone derivatives.

Inhaled delivery of Δ(9)-tetrahydrocannabinol (THC) to rats by e-cigarette vapor technology

Most human Δ(9)-tetrahydrocannabinol (THC) use is via inhalation, and yet few animal studies of inhalation exposure are available. Popularization of non-combusted methods for the inhalation of psychoactive drugs (Volcano(®), e-cigarettes) further stimulates a need for rodent models of this route of administration. This study was designed to develop and validate a rodent chamber suitable for controlled exposure to vaporized THC in a propylene glycol vehicle, using an e-cigarette delivery system adapted to standard size, sealed rat housing chambers. The in vivo efficacy of inhaled THC was validated using radiotelemetry to assess body temperature and locomotor responses, a tail-flick assay for nociception and plasma analysis to verify exposure levels. Hypothermic responses to inhaled THC in male rats depended on the duration of exposure and the concentration of THC in the vehicle. The temperature nadir was reached after ∼40 min of exposure, was of comparable magnitude (∼3 °Celsius) to that produced by 20 mg/kg THC, i.p. and resolved within 3 h (compared with a 6 h time course following i.p. THC). Female rats were more sensitive to hypothermic effects of 30 min of lower-dose THC inhalation. Male rat tail-flick latency was increased by THC vapor inhalation; this effect was blocked by SR141716 pretreatment. The plasma THC concentration after 30 min of inhalation was similar to that produced by 10 mg/kg THC i.p. This approach is flexible, robust and effective for use in laboratory rats and will be of increasing utility as users continue to adopt "vaping" for the administration of cannabis.

Reinforcing and neurochemical effects of the "bath salts" constituents 3,4-methylenedioxypyrovalerone (MDPV) and 3,4-methylenedioxy-N-methylcathinone (methylone) in male rats

RATIONALE

3,4-Methylenedioxypyrovalerone (MDPV) and 3,4-methylenedioxy-N-methylcathinone (methylone) are synthetic drugs found in so-called "bath salts" products. Both drugs exert their effects by interacting with monoamine transporter proteins. MDPV is a potent uptake blocker at transporters for dopamine and norepinephrine while methylone is a non-selective releaser at transporters for dopamine, norepinephrine, and serotonin (5-HT).

OBJECTIVES

We hypothesized that prominent 5-HT-releasing actions of methylone would render this drug less reinforcing than MDPV.

METHODS

To test this hypothesis, we compared behavioral effects of MDPV and methylone using intravenous (i.v.) self-administration on a fixed-ratio 1 schedule in male rats. Additionally, neurochemical effects of the drugs were examined using in vivo microdialysis in nucleus accumbens, in a separate cohort of rats.

RESULTS

MDPV self-administration (0.03 mg/kg/inj) was acquired rapidly and reached 40 infusions per session, similar to the effects of cocaine (0.5 mg/kg/inj), by the end of training. In contrast, methylone self-administration (0.3 and 0.5 mg/kg/inj) was acquired slowly, and response rates only reached 20 infusions per session by the end of training. In dose substitution studies, MDPV and cocaine displayed typical inverted U-shaped dose-effect functions, but methylone did not. In vivo microdialysis revealed that i.v. MDPV (0.1 and 0.3 mg/kg) increased extracellular dopamine while i.v. methylone (1 and 3 mg/kg) increased extracellular dopamine and 5-HT.

CONCLUSIONS

Our findings support the hypothesis that elevations in extracellular 5-HT in the brain can dampen positive reinforcing effects of cathinone-type drugs. Nevertheless, MDPV and methylone are both self-administered by rats, suggesting these drugs possess significant abuse liability in humans.

Neurobiology of 3,4-methylenedioxypyrovalerone (MDPV) and α-pyrrolidinovalerophenone (α-PVP)

Synthetic cathinones are analogs of cathinone or β-ketoamphetamine - the major psychostimulant component of the shrub Catha edulis or khat. Cathinone analogs - though not termed as such - have been known for >100 years, but confusing chemical nomenclature often made the topic difficult to appreciate. In addition, many of the early analogs were prepared as synthetic precursors for the development of various other agents, and relatively few were pharmacologically evaluated. Cathinone is a close structural relative of amphetamine. Today, certain cathinone derivatives, synthetic cathinones, are known to produce central stimulant actions and represent a "new" class of drugs of abuse. Depending upon the nature of their terminal amine, α substituent, and aryl substituents, they seem to produce their effects via release or reuptake of various neurotansmitters including dopamine norepinephreine and/or serotonin. Two of the newest and most prominent members of the class are MDPV and its parent α-PVP ("flakka"). Both have been encountered on their own and in what might be constituents of what has been termed by a variety of names including psychoactive "bath salts". Here, we describe the nomenclature of synthetic cathinones, the mechanism(s) of action of MDPV and α-PVP, and their structure-activity relationships. In order to assist in forensic studies, and to identify novel substances requiring future pharmacological evaluation, the metabolism of these agents is also described. Finally, the preclinical behavioral actions of these two agents in a variety of behavioral assays, including rodent locomotor assays, self-administration studies, intracranial self-stimulation, conditioned place preference, and drug discrimination, is summarized. The results of these studies with MDPV and α-PVP are consistent with their acting as potent cocaine-like central stimulants with abuse liability.

Synthetic cathinone MDPV downregulates glutamate transporter subtype I (GLT-1) and produces rewarding and locomotor-activating effects that are reduced by a GLT-1 activator

Synthetic cathinones produce dysregulation of monoamine systems, but their effects on the glutamate system and the influence of glutamate on behavioral effects related to cathinone abuse are unknown. A principal regulator of glutamate homeostasis is glutamate transporter subtype 1 (GLT-1), an astrocytic protein that clears glutamate from the extracellular space and influences behavioral effects of established psychostimulants. We hypothesized that repeated administration of the synthetic cathinone, MDPV (3,4-methylenedioxypyrovalerone), would affect GLT-1 expression in the corticolimbic circuit, and that a GLT-1 activator (ceftriaxone, CTX) would reduce rewarding and locomotor-stimulant effects of MDPV in rats. GLT-1 protein expression in the nucleus accumbens (NAcc), but not prefrontal cortex (PFC), was decreased following withdrawal (2, 5 and 10 days) from repeated MDPV treatment, but not immediately after the last MDPV injection. CTX (200 mg/kg) pretreatment did not affect acute locomotor activation produced by MDPV (0.5, 1, 3 mg/kg). However, CTX (200 mg/kg) administered during a 7-day MDPV treatment paradigm attenuated the development of MDPV-induced sensitization of repetitive movements in rats challenged with MDPV following 11 days of drug abstinence. Pretreatment with CTX (200 mg/kg) during a 4-day MDPV (2 mg/kg) conditioned place preference (CPP) paradigm reduced the development of place preference produced by MDPV. The present data demonstrate dysregulation of corticolimbic glutamate transport systems during withdrawal from chronic MDPV exposure, and show that a GLT-1 transporter activator disrupts behavioral effects of MDPV that are related to synthetic cathinone abuse.

Sensitization to the motor stimulant effects of 3,4-methylenedioxypyrovalerone (MDPV) and cross-sensitization to methamphetamine in rats

BACKGROUND

In recent years, there has been a dramatic increase in abuse of the synthetic cathinone 3,4-methylenedioxypyrovalerone (MDPV), often in combination with other illicit stimulants.

PURPOSE

We sought to determine if repeated exposure to MDPV would produce sensitization to the motor stimulant effects of the drug, and whether cross-sensitization would develop with the stimulant effects of methamphetamine (METH).

STUDY DESIGN

Male Sprague-Dawley rats were administered MDPV (1 or 5 mg/kg) or saline once daily for 5 days at 24 hour intervals, or were administered MDPV (1 mg/kg) or saline once daily for 5 days at 48 hour intervals. For cross-sensitization experiments, rats were administered METH (1 mg/kg) or MDPV (1 or 5 mg/kg) once daily for 5 days at 48 hour intervals, and following a 5 day incubation period, were given an acute challenge injection of either MDPV (0.5 mg/kg) or METH (0.5 mg/kg), respectively.

RESULTS

Rats repeatedly administered MDPV (1 mg/kg) every 48 hours, but not every 24 hours, demonstrated increased motor activity when given either a subsequent challenge of MDPV (0.5 mg/kg i.p.) or METH (0.5 mg/kg), indicating the development of behavioral sensitization and cross-sensitization, respectively. Moreover, rats repeatedly administered METH (1 mg/kg) every 48 hours did not exhibit cross-sensitization to the motor stimulating effects of a subsequent challenge with MDPV (0.5 mg/kg).

CONCLUSION

These results suggest that specific patterns of MDPV administration may lead to lasting changes in behavioral responses to subsequent METH exposure.

Wheel running exercise attenuates vulnerability to self-administer nicotine in rats

BACKGROUND

Preventing or postponing tobacco use initiation could greatly reduce the number of tobacco-related deaths. While evidence suggests that exercise is a promising treatment for tobacco addiction, it is not clear whether exercise could prevent initial vulnerability to tobacco use. Thus, using an animal model, we examined whether exercise attenuates vulnerability to the use and reinforcing effects of nicotine, the primary addictive chemical in tobacco.

METHODS

Initial vulnerability was assessed using an acquisition procedure wherein exercising (unlocked running wheel, n=10) and sedentary (locked or no wheel, n=12) male adolescent rats had access to nicotine infusions (0.01-mg/kg) during daily 21.5-h sessions beginning on postnatal day 30. Exercise/sedentary sessions (2-h/day) were conducted prior to each of the acquisition sessions. The effects of exercise on nicotine's reinforcing effects were further assessed in separate groups of exercising (unlocked wheel, n=7) and sedentary (no wheel, n=5) rats responding for nicotine under a progressive-ratio schedule with exercise/sedentary sessions (2-h/day) conducted before the daily progressive-ratio sessions.

RESULTS

While high rates of acquisition of nicotine self-administration were observed among both groups of sedentary controls, acquisition was robustly attenuated in the exercise group with only 20% of exercising rats meeting the acquisition criterion within the 16-day testing period as compared to 67% of the sedentary controls. Exercise also decreased progressive-ratio responding for nicotine as compared to baseline and to sedentary controls.

CONCLUSIONS

Exercise may effectively prevent the initiation of nicotine use in adolescents by reducing the reinforcing effects of nicotine.

Methylenedioxypyrovalerone (MDPV) mimics cocaine in its physiological and behavioral effects but induces distinct changes in NAc glucose

Methylenedioxypyrovalerone (MDPV) is generally considered to be a more potent cocaine-like psychostimulant, as it shares a similar pharmacological profile with cocaine and induces similar physiological and locomotor responses. Recently, we showed that intravenous cocaine induces rapid rise in nucleus accumbens (NAc) glucose and established its relation to neural activation triggered by the peripheral drug actions. This study was conducted to find out whether MDPV, at a behaviorally equivalent dose, shares a similar pattern of NAc glucose dynamics. Using enzyme-based glucose sensors coupled with amperometery in freely moving rats, we found that MDPV tonically decreases NAc glucose levels, a response that is opposite to what we previously observed with cocaine. By analyzing Skin-Muscle temperature differentials, a valid measure of skin vascular tone, we found that MDPV induces vasoconstriction; a similar effect at the level of cerebral vessels could be responsible for the MDPV-induced decrease in NAc glucose. While cocaine also induced comparable, if not slightly stronger peripheral vasoconstriction, this effect was overpowered by local neural activity-induced vasodilation, resulting in rapid surge in NAc glucose. These results imply that cocaine-users may be more susceptible to addiction than MDPV-users due to the presence of an interoceptive signal (i.e., sensory cue), which may result in earlier and more direct reward detection. Additionally, while health complications arising from acute cocaine use are typically cardiovascular related, MDPV may be more dangerous to the brain due to uncompensated cerebral vasoconstriction.

Intravenous self-administration of entactogen-class stimulants in male rats

The intravenous self-administration (IVSA) of 3,4-methylenedioxymethamphetamine (MDMA) is inconsistent in rats, with up to half of subjects failing to acquire reliable drug intake. It is unknown if this changes under long-access conditions (6 h sessions) under which the IVSA of cocaine and methamphetamine escalates. The entactogen class cathinone stimulants which exhibit MDMA-like monoamine effects in the nucleus accumbens, mephedrone (4-methylmethcathinone) and methylone (3,4-methylenedioxymethcathinone), may support more reliable IVSA but results have been mixed. This study was designed to directly compare the IVSA of these three compounds. Groups of male Wistar rats were trained to self-administer mephedrone, methylone or MDMA (0.5 mg/kg/inf) under a Fixed-Ratio (FR) 1 schedule of reinforcement for 14 sessions. Following the acquisition interval, animals were evaluated in FR (0.0, 0.125, 0.25, 0.5, 1.0, 2.5 mg/kg/inf) and Progressive Ratio (PR; 0.125, 1.0 mg/kg/inf) dose-substitution procedures. Long access conditions escalated MDMA intake over the 6 h session but not in the first 2 h. In short access, drug intake was significantly higher in mephedrone-trained rats compared with either the methylone-trained or MDMA-trained groups during acquisition. Mephedrone resulted in the highest intakes during FR and PR dose-substitution in MDMA- and mephedrone-trained groups. Overall it was found that mephedrone is a more effective reinforcer than methylone or MDMA and represents a higher risk for compulsive use.

Effects of active anti-methamphetamine vaccination on intravenous self-administration in rats

BACKGROUND

d-Methamphetamine (METH) addiction is a serious public health concern for which successful treatment remains elusive. Immunopharmacotherapy has been shown to attenuate locomotor and thermoregulatory effects of METH. The current study investigated whether active vaccination against METH could alter intravenous METH self-administration in rats.

METHODS

Male Sprague-Dawley rats (Experiment 1: N=24; Experiment 2: N=18) were vaccinated with either a control keyhole-limpet hemocyanin conjugate vaccine (KLH) or a candidate anti-METH vaccine (MH6-KLH) or. Effects of vaccination on the acquisition of METH self-administration under two dose conditions (0.05, 0.1mg/kg/inf) and post-acquisition dose-substitution (0, 0.01, 0.05, 0.20mg/kg/inf, Experiment 1; 0.01, 0.05, 0.10, 0.15mg/kg/inf, Experiment 2) during steady-state responding were investigated. Plasma METH concentrations were determined 30min after an acute challenge dose of 3.2mg/kg METH.

RESULTS

Active vaccination inhibited the acquisition of METH self-administration under the 0.1mg/kg/inf dose condition, with 66% of the MH6-KLH-vaccinated rats compared to 100% of the controls reaching criteria, and produced transient and dose-dependent effects on self-administration during the maintenance phase. Under the 0.05mg/kg/inf dose condition, MH6-KLH-vaccinated rats initially self-administered more METH than controls, but then self-administration decreased across the acquisition phase relative to controls; a subsequent dose-response assessment confirmed that MH6-KLH-vaccinated rats failed to acquire METH self-administration. Finally, plasma METH concentrations were higher in MH6-KLH-vaccinated rats compared to controls after an acute METH challenge, and these were positively correlated with antibody titers.

CONCLUSIONS

These data demonstrate that active immunopharmacotherapy for METH attenuates the acquisition of METH self-administration.

In vivo potency and efficacy of the novel cathinone α-pyrrolidinopentiophenone and 3,4-methylenedioxypyrovalerone: self-administration and locomotor stimulation in male rats

RATIONALE

Numerous substituted cathinone drugs have appeared in recreational use. This variety is often a response to legal actions; the scheduling of 3,4-methylenedioxypyrovalerone (MDPV; "bath salts") in the USA was followed by the appearance of the closely related drug α-pyrrolidinopentiophenone (alpha-PVP; "flakka").

OBJECTIVES

This study aimed to directly compare the efficacy and potency of alpha-PVP with that of MDPV.

METHODS

Groups of male Wistar rats were trained in the intravenous self-administration (IVSA) alpha-PVP or MDPV under a fixed-ratio 1 schedule of reinforcement. An additional group was examined for locomotor and body temperature responses to noncontingent administration of MDVP or alpha-PVP (1.0, 5.6, and 10.0 mg/kg, i.p.).

RESULTS

Acquisition of alpha-PVP (0.1 mg/kg/infusion) IVSA resulted in low, yet consistent drug intake and excellent discrimination for the drug-paired lever. Dose substitution (0.05-0.25 mg/kg/infusion) under a fixed-ratio 1 schedule confirmed potency was similar to MDPV in prior studies. In direct comparison to MDPV (0.05 mg/kg/infusion), rats trained on alpha-PVP (0.05 mg/kg/infusion) responded for more infusions but demonstrated similar drug-lever discrimination by the end of acquisition. However, the dose-response (0.018-0.56 mg/kg/infusion) functions of these drugs under a progressive-ratio schedule of reinforcement reflected identical efficacy and potency. Peak locomotor responses to MDPV or alpha-PVP were observed after the 1.0 mg/kg, i.p. dose and lasted ∼2 h. Modest body temperature decreases were of similar magnitude (∼0.75 °C) for each compound.

CONCLUSIONS

The potency and efficacy of MDPV and alpha-PVP were very similar across multiple assays, predicting that the abuse liability of alpha-PVP will be significant and similar to that of MDPV.

One day access to a running wheel reduces self-administration of D-methamphetamine, MDMA and methylone

BACKGROUND

Exercise influences drug craving and consumption in humans and drug self-administration in laboratory animals, but the effects can be variable. Improved understanding of how exercise affects drug intake or craving would enhance applications of exercise programs to human drug users attempting cessation.

METHODS

Rats were trained in the intravenous self-administration (IVSA) of D-methamphetamine (METH; 0.05 mg/kg/inf), 3,4-methylenedioxymethamphetamine (MDMA; 0.5 mg/kg/inf) or methylone (0.5 mg/kg/inf). Once IVSA was established, the effect of ∼ 22 h of wheel access in the home cage on subsequent drug taking was assessed in a two cohort crossover design.

RESULTS

Provision of home cage wheel access during the day prior to IVSA sessions significantly decreased the self-administration of METH, MDMA and methylone. At the individual level, there was no correlation between the amount a rat used the wheel and the size of the individual's decrease in drug intake.

CONCLUSIONS

Wheel access can reduce self-administration of a variety of psychomotor stimulants. It does so immediately, i.e., without a need for weeks of exercise prior to drug access. This study therefore indicates that future mechanistic investigations should focus on acute effects of exercise. In sum, the results predict that exercise programs can be used to decrease stimulant drug use in individuals even with no exercise history and an established drug taking pattern.

Intravenous self-administration of mephedrone, methylone and MDMA in female rats

Male rats will intravenously self-administer (IVSA) the substituted cathinone stimulants ("bath salts") mephedrone (4-methylmethcathione) and methylone (3,4-methylenedioxymethcathinone) robustly, whereas the IVSA of 3,4-methylenedioxymethamphetamine (MDMA) is inconsistent in many rat models. There are no data available on the self-administration of these drugs in female rats, thus a study was undertaken to contrast them directly. Groups of female Wistar rats were trained to self-administer mephedrone, methylone or MDMA (0.5 mg/kg/inf) under a Fixed-Ratio (FR) 1 schedule of reinforcement for 14 sessions. Following the acquisition interval, animals were evaluated in FR (0.0, 0.125, 0.25, 0.5, 1.0, 2.5 mg/kg/inf) and PR (0.125, 1.0 mg/kg/inf) dose-substitution procedures. The results show that female rats acquired the self-administration of all three compounds with intakes in mephedrone-trained rats that were significantly higher than that of methylone-trained or MDMA-trained rats. In dose-substitution under either FR or PR contingencies, however, the potencies of all three drugs were similar within the original training groups. The mephedrone-trained animals exhibited higher intakes of all drugs during dose-substitution, indicating lasting consequences of the training drug. Abuse liability of these three compounds is therefore predicted to be similar in established stimulant users but may differ in liability if they are primary drugs of initiation.