Discriminative Stimulus Effects of Binary Drug Mixtures: Studies with Cocaine, MDPV, and Caffeine

Effects of the Synthetic Cathinone α-Pyrrolidinobutiothiophenone (α-PBT) on Discriminative Stimulus Effects and Intracranial Self-Stimulation Thresholds in Male Rats

Recently, the abuse of synthetic cathinones is increasing among young people. α-Pyrrolidinobutiothiophenone (α-PBT), a synthetic cathinone, is a designer drug that is freely traded online with no legal restrictions. Moreover, there is currently no scientific basis for legal regulation. Here, we examined the addictive properties of α-PBT using a drug discrimination (DD) task. We also investigated the role of α-PBT in brain stimulation reward (BSR) using an intracranial self-stimulation (ICSS) paradigm in rats. Initially, the rats were trained to discriminate between cocaine and saline. After the discrimination training criteria were met, we determined the dose-effect curves of cocaine and conducted generalization tests with α-PBT and α-pyrrolidinopentiothiophenone (α-PVT) using a cumulative dosing protocol. In a separate set of studies, we examined the dopaminergic mechanisms underlying the function of α-PBT as an interoceptive stimulus (17.8 mg/kg) by intraperitoneally injecting either the dopamine (DA) D1 antagonist SCH23390 (0.06 and 0.12 mg/kg) or the D2 antagonist eticlopride (0.05 and 0.1 mg/kg) 15 min before DD testing. Brain reward function was measured using an ICSS procedure to examine the effects of α-PBT on ICSS threshold under the frequency-rate procedure. Our results showed that α-PBT functioned as a discriminative cue similar to cocaine in rats. More importantly, SCH23390 abolished the effects of α-PBT as an interoceptive stimulus in a dose-dependent manner in rats trained to press a lever to receive cocaine. Similarly, eticlopride dose-dependently attenuated the effect of α-PBT used as a discriminative cue. Additionally, cumulative α-PBT administration dose-dependently lowered ICSS thresholds compared with those in saline-treated rats. Furthermore, α-PBT-induced potentiation of BSR was abolished by pretreatment with both SCH23390 and eticlopride. Taken together, our results suggest that α-PBT can function as a cocaine-like discriminative cue via the activation of D1 and D2 receptors. α-PBT also appears to influence BSR by reducing the brain reward threshold via changes in D1 and D2 receptors. The present study suggests that α-PBT could have addictive properties through DA D1 and D2 receptors and thus poses a threat to humans.

Synthetic Cathinones: Epidemiology, Toxicity, Potential for Abuse, and Current Public Health Perspective

Synthetic cathinones, derived from cathinone found in the plant Catha edulis, represent the second largest and most frequently seized group of new psychoactive substances. They are considered as β-keto analogs of amphetamine, sharing pharmacological effects with amphetamine and cocaine. This review describes the neurotoxic properties of synthetic cathinones, encompassing their capacity to induce neuroinflammation, dysregulate neurotransmitter systems, and alter monoamine transporters and receptors. Additionally, it discusses the rewarding and abuse potential of synthetic cathinones drawing from findings obtained through various preclinical animal models, contextualized with other classical psychostimulants. The review also offers an overview of current abuse trends of synthetic cathinones on the illicit drug market, specifying the aspects covered, and underscores the risks they pose to public health. Finally, the review discusses public health initiatives and efforts to reduce the hazards of synthetic cathinones, including harm reduction methods, education, and current clinical management strategies.

Impacts of Self-Administered 3,4-Methylenedioxypyrovalerone (MDPV) Alone, and in Combination with Caffeine, on Recognition Memory and Striatal Monoamine Neurochemistry in Male Sprague Dawley Rats: Comparisons with Methamphetamine and Cocaine

Recent data suggest that 3,4-methylenedioxypyrovalerone (MDPV) has neurotoxic effects; however, the cognitive and neurochemical consequences of MDPV self-administration remain largely unexplored. Furthermore, despite the fact that drug preparations that contain MDPV often also contain caffeine, little is known regarding the toxic effects produced by the co-use of these two stimulants. The current study investigated the degree to which self-administered MDPV or a mixture of MDPV+caffeine can produce deficits in recognition memory and alter neurochemistry relative to prototypical stimulants. Male Sprague Dawley rats were provided 90 min or 12 h access to MDPV, MDPV+caffeine, methamphetamine, cocaine, or saline for 6 weeks. Novel object recognition (NOR) memory was evaluated prior to any drug self-administration history and 3 weeks after the final self-administration session. Rats that had 12 h access to methamphetamine and those that had 90 min or 12 h access to MDPV+caffeine exhibited significant deficits in NOR, whereas no significant deficits were observed in rats that self-administered cocaine or MDPV. Striatal monoamine levels were not systematically affected. These data demonstrate synergism between MDPV and caffeine with regard to producing recognition memory deficits, highlighting the importance of recapitulating the manner in which drugs are used (e.g., in mixtures containing multiple stimulants, binge-like patterns of intake).

Cardiovascular and Locomotor Effects of Binary Mixtures of Common "Bath Salts" Constituents: Studies with Methylone, MDPV, and Caffeine in Rats

Background and Purpose

The use of "Bath Salts" drug preparations has been associated with high rates of toxicity and death. Preparations often contain mixtures of drugs including multiple synthetic cathinones or synthetic cathinones and caffeine; however, little is known about whether interactions among "Bath Salts" constituents contribute to the adverse effects often reported in users.

Experimental Approach

This study used adult male Sprague-Dawley rats to characterize the cardiovascular effects, locomotor effects, and pharmacokinetics of methylone, MDPV, and caffeine, administered alone and as binary mixtures. Dose-addition analyses were used to determine the effect levels predicted for a strictly additive interaction for each dose pair.

Key Results

Methylone, MDPV, and caffeine increased heart rate and locomotion, with methylone producing the largest increase in heart rate, MDPV producing the largest increase in locomotor activity, and caffeine being the least effective in stimulating heart rate and locomotor activity. MDPV and caffeine increased mean arterial pressure, with caffeine being more effective than MDPV. The nature of the interactions between methylone and MDPV tended toward sub-additivity for all endpoints, whereas interactions between MDPV or methylone and caffeine tended to be additive or sub-additive for cardiovascular endpoints, and additive or supra-additive for increases in locomotion. No pharmacokinetic interactions were observed between individual constituents, but methylone displayed non-linear pharmacokinetics at the largest dose evaluated.

Conclusion and Implications

These findings demonstrate that the composition of "Bath Salts" preparations can impact both cardiovascular and locomotor effects and suggest that such interactions among constituent drugs could contribute to the "Bath Salts" toxidrome reported by human users.

Impacts of Self-Administered 3,4-Methylenedioxypyrovalerone (MDPV) Alone, and in Combination with Caffeine, on Recognition Memory and Striatal Monoamine Neurochemistry in Male Sprague-Dawley Rats: Comparisons with Methamphetamine and Cocaine

Recent data suggest that 3,4-methylenedioxypyrovalerone (MDPV) has neurotoxic effects; however, the cognitive and neurochemical consequences of MDPV self-administration remain largely unexplored. Furthermore, despite the fact that drug preparations that contain MDPV often also contain caffeine, little is known regarding the toxic effects produced by the co-use of these two stimulants. The current study investigated the degree to which self-administered MDPV, or a mixture of MDPV+caffeine can produce deficits in recognition memory and alter neurochemistry relative to prototypical stimulants. Male Sprague-Dawley rats were provided 90-min or 12-h access to MDPV, MDPV+caffeine, methamphetamine, cocaine, or saline for 6 weeks. Novel object recognition (NOR) memory was evaluated prior to any drug self-administration history and 3 weeks after the final self-administration session. Rats that had 12-h access to methamphetamine and those that had 90-min or 12-h access to MDPV+caffeine exhibited significant deficits in NOR, whereas no significant deficits were observed in rats that self-administered cocaine or MDPV. Striatal mono-amine levels were not systematically affected. These data demonstrate synergism between MDPV and caffeine with regard to producing recognition memory deficits and lethality, highlighting the importance of recapitulating the manner in which drugs are used (e.g., in mixtures containing multiple stimulants, binge-like patterns of intake).

Relative reinforcing effects of cocaine and 3,4-methylenedioxypyrovalerone (MDPV) under a concurrent access self-administration procedure in rats

BACKGROUND

Recent evidence suggesting that polysubstance use is the norm rather than the exception highlights the need for a better understanding of interactions amongst the abuse-related effects of commonly co-abused drugs. Synthetic cathinones remain one of the most popular families of novel psychoactive substances and are typically used in preparations containing multiple stimulants. Evaluating the reinforcing effects of drugs under both single-operant procedures and procedures in which alternatives are available can provide a more complete characterization of their reinforcing effects and economic interactions.

METHODS

These studies utilized a drug-versus-drug choice procedure in 18 male Sprague-Dawley rats to evaluate economic interactions between the synthetic cathinone, MDPV, and cocaine in addition to how a history of concurrent access impacts reinstatement behavior.

RESULTS

When equi-effective doses of MDPV and cocaine were made concurrently available, approximately half of the subjects responded exclusively on the MDPV-reinforced lever whereas the other half responded exclusively on the cocaine-reinforced lever. Allocation of responding was reversed when the cost of the preferred drug increased, or the cost of the non-preferred drug decreased. Drug-paired cues and MDPV, cocaine, and methamphetamine pretreatments reinstated responding on both drug levers, regardless of preference.

CONCLUSION

These data demonstrate that MDPV and cocaine act as economic substitutes and suggest that measures of reinforcing effectiveness determined under a progressive ratio schedule of reinforcement can predict drug choice. These data also suggest that environmental stimuli associated with a particular drug might stimulate class-specific drug-seeking, however, further studies are needed to test the generality of this claim.

Application of dose-addition analyses to characterize the abuse-related effects of drug mixtures

Polysubstance use makes up a majority of drug use, yet relatively few studies investigate the abuse-related effects of drug mixtures. Dose-addition analyses provide a rigorous and quantitative method to determine the nature of the interaction (i.e., supraadditive, additive, or subadditive) between two or more drugs. As briefly reviewed here, studies in rhesus monkeys have applied dose-addition analyses to group level data to characterize the nature of the interaction between the reinforcing effects of stimulants and opioids (e.g., mixtures of cocaine + heroin). Building upon these foundational studies, more recent work has applied dose-addition analyses to better understand the nature of the interaction between caffeine and illicit stimulants such as MDPV and methamphetamine in rats. In addition to utilizing a variety of operant procedures, including drug discrimination, drug self-administration, and drug-primed reinstatement, these studies have incorporated potency and effectiveness ratios as a method for both statistical analysis and visualization of departures from additivity at both the group and individual subject level. As such, dose-addition analyses represent a powerful and underutilized approach to quantify the nature of drug-drug interactions that can be applied to a variety of abuse-related endpoints in order to better understand the behavioral pharmacology of polysubstance use.

Modeling Stimulant and Opioid Co-use in Rats Provided Concurrent Access to Methamphetamine and Fentanyl

Concurrent use of stimulants (e.g., methamphetamine) and opioids (e.g., fentanyl) has become increasingly common in recent years and continues to pose an enormous health burden, worldwide. Despite the prevalence, relatively little is known about interactions between the reinforcing effects of stimulants and opioids in this pattern of polysubstance use. The goals of the current study were to evaluate the relative reinforcing and relapse-related effects of methamphetamine and fentanyl using a concurrent access, drug-vs.-drug choice procedure. Male Sprague-Dawley rats were first allowed to acquire self-administration for either 0.1 mg/kg/infusion methamphetamine or 0.0032 mg/kg/infusion fentanyl, independently, after which concurrent access to both drugs was provided. When training doses of methamphetamine and fentanyl were concurrently available, a subset of rats self-administered both drugs, either within a session or alternating across sessions, whereas the remaining rats responded exclusively for one drug. When the cost of the preferred drug was increased (i.e., unit dose reduced), or the cost of the non-preferred drug was decreased (i.e., unit dose increased), choice was largely allocated toward the cheaper alternative. Following extinction of responding, methamphetamine- and fentanyl-paired cues reinstated responding on both levers. Responding reinstated by a priming injection of methamphetamine or fentanyl allocated more responding to the lever previously reinforced by the priming drug. The current studies suggest that choice of methamphetamine and fentanyl is largely allocated to the cheaper alternative, although more co-use was observed than would be expected for economic substitutes. Moreover, they lay the groundwork for more fully evaluating interactions between commonly co-abused drugs (e.g., stimulants and opioids) in order to better understand the determinants of polysubstance use and develop effective treatment strategies for individuals suffering from a polysubstance use disorder.

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.

Discriminative stimulus effects of 3,4-methylenedioxypyrovalerone (MDPV) and structurally related synthetic cathinones

The 3,4-methylenedioxypyrovalerone (MDPV), and other structurally related synthetic cathinones, are popular alternatives to prototypical illicit psychostimulants, such as cocaine and methamphetamine. These drugs are often referred to as 'bath salts' and function either as cocaine-like inhibitors of monoamine uptake, or amphetamine-like substrates for dopamine, norepinephrine and serotonin transporters. These studies used male Sprague-Dawley rats trained to discriminate MDPV from saline to evaluate the substitution profiles of structurally related synthetic cathinones, cocaine, and other direct-acting dopamine and noradrenergic receptor agonists in order to characterize the relative contributions of dopamine, norepinephrine and serotonin to the discriminative stimulus effects of MDPV. As expected, each of the cathinones and cocaine dose-dependently increased MDPV-appropriate responding, with a rank-order potency that was positively correlated with their potency to inhibit dopamine and norepinephrine, but not serotonin, a relationship that is consistent with the rank order to maintain self-administration. The dopamine D2/3 receptor-preferring agonist quinpirole produced a modest increase in MDPV-appropriate responding, whereas the dopamine D1/5 receptor agonist, SKF 82958, nonselective dopamine receptor agonist, apomorphine, as well as the α-1, and α-2 adrenergic receptor agonists, phenylephrine and clonidine, respectively, failed to increase MDPV-appropriate responding at doses smaller than those that suppressed responding altogether. Although these studies do not support a role for serotonergic or adrenergic systems in mediating/modulating the discriminative stimulus effects of MDPV, convergent evidence is provided to suggest that the discriminative stimulus effects of MDPV are primarily mediated by its capacity to inhibit dopamine uptake, and the subsequent activation of dopamine D2 or D3 receptors.

Psychoactive Drugs-From Chemical Structure to Oxidative Stress Related to Dopaminergic Neurotransmission. A Review

Nowadays, more and more young people want to experience illegal, psychoactive substances, without knowing the risks of exposure. Besides affecting social life, psychoactive substances also have an important effect on consumer health. We summarized and analyzed the published literature data with reference to the mechanism of free radical generation and the link between chemical structure and oxidative stress related to dopaminergic neurotransmission. This review presents data on the physicochemical properties, on the ability to cross the blood brain barrier, the chemical structure activity relationship (SAR), and possible mechanisms by which neuronal injuries occur due to oxidative stress as a result of drug abuse such as "bath salts", amphetamines, or cocaine. The mechanisms of action of ingested compounds or their metabolites involve intermediate steps in which free radicals are generated. The brain is strongly affected by the consumption of such substances, facilitating the induction of neurodegenerative diseases. It can be concluded that neurotoxicity is associated with drug abuse. Dependence and oxidative stress are linked to inhibition of neurogenesis and the onset of neuronal death. Understanding the pathological mechanisms following oxidative attack can be a starting point in the development of new therapeutic targets.

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.

Interactions between reinforcement history and drug-primed reinstatement: Studies with MDPV and mixtures of MDPV and caffeine

Many drugs of abuse are mixed with other psychoactive substances (e.g., caffeine) prior to their sale or use. Synthetic cathinones (e.g., 3,4-methylenedioxypyrovalerone [MDPV]) are commonly mixed with caffeine or other cathinones (e.g., 3,4-methylenedioxy-N-methylcathinone [methylone]), and these "bath salts" mixtures (e.g., MDPV + caffeine) can exhibit supra-additive interactions with regard to their reinforcing and discriminative stimulus properties. However, little is known about relapse-related effects of drug mixtures. In these studies, male Sprague-Dawley rats self-administered 0.032 mg/kg/inf MDPV or a mixture of MDPV + caffeine (0.029 + 0.66 mg/kg/inf, respectively) and then underwent multiple rounds of extinction and reinstatement testing to evaluate the influence of reinforcement history and drug-associated stimuli on the effectiveness of saline (drug-paired stimuli alone), MDPV (0.032-1.0 mg/kg), caffeine (1.0-32 mg/kg), and mixtures of MDPV:caffeine (in 3:1, 1:1, and 1:3 ratios, relative to each drug's ED₅₀ ) to reinstate responding. Dose-addition analyses were used to determine the nature of the drug-drug interaction for each mixture. MDPV and caffeine dose-dependently reinstated responding and were equally effective, regardless of reinforcement history. Most fixed ratio mixtures of MDPV + caffeine exhibited supra-additive interactions, reinstating responding to levels greater than was observed with caffeine and/or MDPV alone. Drug-associated stimuli also played a key role in reinstating responding, especially for caffeine. Together, these results demonstrate that the composition of drug mixtures can impact relapse-related effects of drug mixtures, and "bath salts" mixtures (MDPV + caffeine) may be more effective at promoting relapse-related behaviors than the constituents alone. Further research is needed to determine how other polysubstance reinforcement histories can impact relapse-related behaviors.

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.

Behavioral economic analysis of the reinforcing effects of "bath salts" mixtures: studies with MDPV, methylone, and caffeine in male Sprague-Dawley rats

RATIONALE

"Bath salts" preparations often contain combinations of synthetic cathinones (e.g., 3,4-methylenedioxymethcathinone [methylone], 3,4-methylenedioxypyrovalerone [MDPV]), and caffeine, and evidence suggests that mixtures of synthetic cathinones and caffeine (e.g., MDPV + caffeine or methylone + caffeine) can be more potent and/or effective reinforcers than predicted for an additive interaction.

OBJECTIVE

To use demand curve analyses to compare the reinforcing effectiveness of MDPV and methylone to mixtures of MDPV + caffeine and methylone + caffeine.

METHODS

Male Sprague-Dawley rats acquired methylone self-administration (0.32 mg/kg/inf) under a fixed ratio (FR) 1 schedule of reinforcement and generated full dose-response curves for methylone (0.01-1 mg/kg/inf) under an FR5 schedule of reinforcement. Demand curves were then obtained for methylone, MDPV, caffeine, and methylone + caffeine and MDPV + caffeine mixtures by increasing the FR across sessions according to the following series: 3, 10, 18, 32, 56, 100, 178, etc. RESULTS: Self-administration of methylone was rapidly acquired by 87.5% of rats and was maintained across a range of doses, producing an inverted U-shaped dose-response curve. Rank order demand for the individual constituents was MDPV > methylone > caffeine. Demand for the 3:1 (but not 10:1) methylone + caffeine mixture was greater than that for methylone alone, and demand for MDPV alone was similar to both MDPV + caffeine mixtures evaluated.

CONCLUSIONS

These studies provide additional evidence that although methylone is an effective reinforcer, combining methylone with caffeine results in an enhanced reinforcing effectiveness compared to methylone alone. Thus, abused "bath salts" preparations containing synthetic cathinones and caffeine may have higher abuse liability than preparations containing only synthetic cathinones.

Synthetic Cathinone Use Among Polysubstance Users: Indirect Indicator of Indiscriminate Drug Taking or Preferred Drug of Abuse?

A survey pertaining to synthetic cathinone (SC) use was completed by 499 polysubstance users enrolled in a residential recovery program in the Southeastern United States. Of the final sample, 28% reported ever SC use. SC-users, compared with nonusers, were more often younger (32.7 vs. 36.0, p = .001), White (93.4% vs. 80.8%, p = .001), and on probation/parole since 2010 (80.9% vs. 70.9%, p = .032). SC-users evidenced extensive drug histories and were less likely to be enrolled in an urban-based program, compared to a rural, Appalachian-based program (73.8% vs. 86.6%, p = .001). Use of synthetic cannabinoids (adjusted odds ratio [AOR] = 1.9, p = .044), kratom (AOR = 1.7, p = .045), and inhalants (AOR = 2.3, p = .001) were significantly associated with SC use. Approximately 23% of SC-users preferred SCs to amphetamines or cocaine; however, only 3.6% ranked SC as their most preferred drug. Past-year SC use declined to 6.6%. Among polysubstance users in this sample, SC use may be a potential indicator of versatile and indiscriminate drug-taking.

The abuse-related effects of pyrrolidine-containing cathinones are related to their potency and selectivity to inhibit the dopamine transporter

Synthetic cathinones are common constituents of abused "bath salts" preparations and represent a large family of structurally related compounds that function as cocaine-like inhibitors or amphetamine-like substrates of dopamine (DAT), norepinephrine (NET), and serotonin (SERT) transporters. Preclinical evidence suggests that some cathinones (e.g., MDPV and α-PVP) are more effective reinforcers than prototypical stimulant drugs of abuse, such as cocaine or methamphetamine. Although the reinforcing potency of these cathinones is related to their potency to inhibit DAT, less is known about the pharmacological determinants of their unusually high reinforcing effectiveness. To this end, we tested the hypothesis that reinforcing effectiveness of cathinone stimulants is positively correlated with their selectivity for DAT relative to SERT. Uptake inhibition assays in rat brain synaptosomes were used to directly compare the potency of MDPV, MDPBP, MDPPP, α-PVP, α-PPP, and cocaine at DAT, NET, and SERT, whereas intravenous self-administration in rats was used to quantify relative reinforcing effectiveness of the drugs using progressive ratio (PR) and behavioral economic procedures. All cathinones were more potent at DAT than NET or SERT, with a rank order for selectivity at DAT over SERT of α-PVP > α-PPP > MDPV > MDPBP > MDPPP > cocaine. These synthetic cathinones were more effective reinforcers than cocaine, and the measures of reinforcing effectiveness determined by PR and demand curve analyses were highly correlated with selectivity for DAT over SERT. Together, these studies provide strong and convergent evidence that the abuse potential of stimulant drugs is mediated by uptake inhibition at DAT, with activity at SERT serving as a negative modulator of reinforcing effectiveness.

Effects of lorcaserin and buspirone, administered alone and as a mixture, on cocaine self-administration in male and female rhesus monkeys

Cocaine use disorder is a serious public health issue for which there is no effective pharmacotherapy. One strategy to speed development of medications for cocaine use disorder is to repurpose drugs already approved for use in humans based on their ability to interact with targets known to be important for addiction. Two such drugs, lorcaserin (Belviq; a drug with serotonin [5-HT]2C receptor agonist properties) and buspirone (Buspar; a drug with 5-HT1A receptor partial agonist and dopamine D3/D4 receptor antagonist properties) can produce modest decreases in cocaine self-administration in rhesus monkeys. The current study evaluated the effectiveness of mixtures of lorcaserin and buspirone (at fixed dose ratios of 3:1, 1:1, and 1:3 relative to each drug's ID50) to reduce responding for 0.032 mg/kg/inf cocaine under a progressive ratio schedule of reinforcement in 2 male and 2 female rhesus monkeys. Dose addition analyses were used to determine if the effects of the drug mixtures differed from those predicted for an additive interaction between lorcaserin and buspirone. Dose-dependent reductions of cocaine self-administration were observed when lorcaserin and buspirone were administered alone, as well as when they were administered as 3:1, 1:1, and 1:3 fixed ratio mixtures of lorcaserin + buspirone. The effects of the 1:1 mixture of lorcaserin + buspirone on cocaine self-administration were supraadditive, whereas the effects of 3:1 and 1:3 mixtures were additive. Together, these results indicate that a combination therapy containing a mixture of lorcaserin and buspirone might be more effective than either drug alone at treating cocaine use disorder. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Reinforcing effects of abused 'bath salts' constituents 3,4-methylenedioxypyrovalerone and α-pyrrolidinopentiophenone and their enantiomers

Synthetic cathinones found in abused 'bath salts' preparations are chiral molecules. Racemic 3,4-methylenedioxypyrovalerone (MDPV) and α-pyrrolidinopentiophenone (α-PVP) are two common constituents of these preparations that have been reported to be highly effective reinforcers; however, the relative contribution of each enantiomer toward these effects has not been determined. Thus, male Sprague-Dawley rats were trained to respond for racemic MDPV or α-PVP (n=9/drug), with full dose-response curves for the racemate and the S and R enantiomers of MDPV and α-PVP generated under a progressive ratio schedule of reinforcement. Racemic mixtures of both MDPV and α-PVP as well as each enantiomer maintained responding in a dose-dependent manner, with racemic MDPV and α-PVP being equipotent. The rank order of potency within each drug was S enantiomer>racemate ≫ R enantiomer. Although both enantiomers of α-PVP were as effective as racemic α-PVP, R-MDPV was a slightly less effective reinforcer than both S and racemic MDPV. The results of these studies provide clear evidence that both enantiomers of MDPV and α-PVP function as highly effective reinforcers and likely contribute toward the abuse-related effects of 'bath salts' preparations containing racemic MDPV and/or α-PVP.

Reinforcing Effects of Binary Mixtures of Common Bath Salt Constituents: Studies with 3,4-Methylenedioxypyrovalerone (MDPV), 3,4-Methylenedioxymethcathinone (methylone), and Caffeine in Rats

Bath salts use is associated with high rates of abuse, toxicity, and death. Bath salt preparations often contain mixtures of drugs including multiple synthetic cathinones (eg, 3,4-methylenedioxypyrovalerone (MDPV) or 3,4-methylenedioxymethcathinone (methylone)) or synthetic cathinones and caffeine; however, little is known about whether interactions among bath salt constituents contribute to the abuse-related effects of bath salts preparations. This study used male Sprague-Dawley rats responding under a progressive ratio schedule to quantify the reinforcing effectiveness of MDPV, methylone, and caffeine, administered alone and as binary mixtures (n=12 per mixture). Each mixture was evaluated at four ratios (10 : 1, 3 : 1, 1 : 1, and 1 : 3) relative to the mean ED₅₀ for each drug alone. Dose-addition analyses were used to determine the predicted, additive effect for each dose pair within each drug mixture. MDPV, methylone, and caffeine maintained responding in a dose-dependent manner, with MDPV being the most potent and effective, and caffeine being the least potent and effective of the three bath salts constituents. High levels of responding were also maintained by each of the bath salts mixtures. Although the nature of the interactions tended toward additivity for most bath salts mixtures, supra-additive (3 : 1 MDPV : caffeine, and 3 : 1 and 1 : 1 methylone : caffeine) and sub-additive (3 : 1, 1 : 1, and 1 : 3 MDPV : methylone) interactions were also observed. Together, these findings demonstrate that the composition of bath salts preparations can have an impact on both their reinforcing potency and effectiveness, and suggest that such interactions among constituent drugs could contribute to the patterns of use and effects reported by human bath salts users.

Inhibition of Cocaine and 3,4-Methylenedioxypyrovalerone (MDPV) Self-Administration by Lorcaserin Is Mediated by 5-HT2C Receptors in Rats

Lorcaserin is a serotonin (5-HT)_2C receptor-preferring agonist approved by the US Food and Drug Administration to treat obesity. Lorcaserin decreases cocaine self-administration in rats and monkeys. Although this effect is partially inhibited by a 5-HT_2C receptor antagonist (SB242084), lorcaserin also has effects at 5-HT_2A and 5-HT_1A receptors, and the relative contribution of these receptors to its anti-cocaine effects has not been investigated. The goals of this study were to determine 1) the potency and effectiveness of lorcaserin to decrease self-administration of cocaine and 3,4-methylenedioxypyrovalerone (MDPV), a common “bath salts” constituent; and 2) the receptor(s) mediating the effects of lorcaserin on cocaine and MDPV self-administration. Male Sprague-Dawley rats (n = 6) were trained to self-administer MDPV under a progressive ratio schedule of reinforcement and maintained under this schedule with daily access to 0.32 mg/kg per infusion of cocaine or 0.032 mg/kg per infusion of MDPV. Dose-response curves for the effects of lorcaserin on cocaine and MDPV self-administration were generated by administering lorcaserin (0.1–5.6 mg/kg) 25 minutes before the start of the session. To assess the effects of 5-HT_2C (SB242084, 0.1 mg/kg), 5-HT_2A (MDL100907, 0.1 mg/kg), and 5-HT_1A (WAY100635, 0.178 mg/kg) receptor antagonists, they were administered 15 minutes before lorcaserin. Lorcaserin decreased cocaine and MDPV self-administration with equal potency. Antagonism of 5-HT_2C (but not 5-HT_1A or 5-HT_2A) receptors blocked the effects of lorcaserin on cocaine and MDPV self-administration. Taken together, these data provide additional support for further development of 5-HT_2C receptor agonists, such as lorcaserin, for the treatment of stimulant abuse.

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.

Role of monoaminergic systems and ambient temperature in bath salts constituent 3,4-methylenedioxypyrovalerone (MDPV)-elicited hyperthermia and locomotor stimulation in mice

3,4-Methylenedioxypyrovalerone (MDPV) is a common constituent of illicit bath salts products, and in vitro studies implicate monoamine transporters as mediators of its pharmacological effects. Locomotor and thermoregulatory effects of MDPV depend on ambient temperature, so the current studies aimed to gauge the involvement of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in MDPV-induced locomotor stimulation and hyperthermia in the mouse at different ambient temperatures. Mice were pretreated with the selective 5-HT-reuptake inhibitor fluoxetine (3 mg/kg), the NE-reuptake inhibitor desipramine (3 mg/kg), the DA-reuptake inhibitor bupropion (10 mg/kg), or saline, followed by 10 mg/kg MDPV while thermoregulation and locomotor activity were monitored via radiotelemetry. In other studies, mice were pretreated for three days with saline, 100 mg/kg of the tryptophan hydroxylase inhibitor para-chlorophenylalanine (p-CPA), or 100 mg/kg of the tyrosine hydroxylase inhibitor α-methyl-para-tyrosine (α-MPT) before receiving 10 mg/kg MDPV on the fourth day. All manipulations were conducted at both 20 °C and 28 °C ambient temperatures. MDPV increased locomotor activity under both ambient conditions and modestly increased core body temperature at 20 °C; however, neither pretreatment with monoamine reuptake inhibitors nor monoamine synthesis inhibitors significantly altered these effects. At 28 °C, MDPV induced a more pronounced hyperthermic effect which was attenuated by bupropion, desipramine, or fluoxetine pretreatment, but not by the monoamine synthesis inhibitors. These results suggest that MDPV may have a more complex pharmacological profile than suggested by in vitro studies, perhaps extending beyond interactions with monoamine transporters. A more thorough binding profile of MDPV at various brain recognition sites should be developed. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

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.