3,4-Methylenedioxypyrovalerone High-Responder Phenotype as a Tool to Evaluate Candidate Medications for Stimulant Use Disorder.

Despite decades of research, there are no medications approved by the United States Food and Drug Administration to treat stimulant use disorders. Self-administration procedures are widely used to screen candidate medications for stimulant use disorder, although preclinical reductions in stimulant self-administration have not translated to meaningful reductions in stimulant use in humans. One possible reason for this discordance is that most preclinical studies evaluate candidate medications under conditions that promote predictable, and well-regulated patterns of drug-taking rather than the dysregulated and/or compulsive patterns of drug-taking characteristic of a stimulant use disorder. A subset of rats ("high-responders") that self-administer 3,4-methelyendioxypyrovalerone (MDPV), a monoamine uptake inhibitor, develop high levels of dysregulated drug-taking consistent with behaviors related to stimulant use disorders. Because MDPV acts on dopamine, serotonin (5-HT), and sigma receptor systems, the current studies compared the potency and effectiveness of a dopamine D3 receptor partial agonist (VK4-40) or antagonist (VK4-116), a sigma receptor antagonist (BD1063), a dopamine D2/D3/sigma receptor antagonist (haloperidol), and a 5-HT2C receptor agonist (CP-809,101) to reduce MDPV (0.0032-0.1 mg/kg/infusion) self-administration in high- and low-responding rats as well as rats self-administering cocaine (0.032-1 mg/kg/infusion). VK4-40, VK4-116, haloperidol, and CP-809,101 were equipotent and effective at reducing drug-taking in all three groups of rats, including the high-responders; however, VK4-116 and CP-809,101 were less potent at reducing drug-taking in female compared with male rats. Together, these studies suggest that drugs targeting dopamine D3 or 5-HT2C receptors can effectively reduce dysregulated patterns of stimulant use, highlighting their potential utility for treating stimulant use disorders. SIGNIFICANCE STATEMENT: There are no United States Food and Drug Administration-approved treatments for stimulant use disorder, perhaps in part because candidate medications are most often evaluated in preclinical models using male subjects with well-regulated drug-taking. In an attempt to better model aberrant drug taking, this study found compounds acting at dopamine D3 or 5-HT2C receptors can attenuate drug-taking in male and female rats that self-administered two different stimulants and exhibited either a high or low substance use disorder-like phenotype.

Interactions between impulsivity and MDPV self-administration in rats.

Synthetic cathinones, such as 3,4-methylenedioxypyrovalerone (MDPV), are recreational drugs of abuse often identified in 'bath salts' preparations. Humans report compulsive patterns of bath salts use, and previous work suggests that a subset of rats develop unusually high levels of MDPV self-administration. This study aims to test the hypothesis that high levels of impulsivity (e.g., inability to withhold responding for a sucrose reward) will predispose rats to high levels of MDPV self-administration relative to rats with lower levels of impulsivity. The 1-choice serial reaction time task (1-CSRTT) was used to assess impulsivity (i.e., premature responding) in 10 female and 10 male Sprague Dawley rats. Rats were then allowed to self-administer 0.032 mg/kg/inf MDPV or 0.32 mg/kg/inf cocaine, after which full dose-response curves for MDPV (0.001-0.1 mg/kg/inf) or cocaine (0.01-1 mg/kg/inf) were generated under a FR5 schedule of reinforcement. After a history of self-administering MDPV or cocaine, impulsivity was reassessed under the 1-CSRTT, prior to evaluating the acute effects of MDPV (0.032-0.32 mg/kg) or cocaine (0.1-1 mg/kg) on impulsivity. Level of impulsivity was not correlated with subsequent levels of either MDPV or cocaine self-administration, and level of drug self-administration was also not correlated with subsequent levels of impulsivity, although acute administration of MDPV and cocaine did increase premature responding. In failing to find direct relationships between either impulsivity and subsequent drug-taking behaviour, or drug-taking behaviour and subsequent assessments of impulsivity, these findings highlight the complexity inherent in the associations between impulsive behaviour and drug-taking behaviour in both animal models and humans.

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.

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.

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-HT2C receptor antagonist (SB242084), lorcaserin also has effects at 5-HT2A and 5-HT1A 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-HT2C (SB242084, 0.1 mg/kg), 5-HT2A (MDL100907, 0.1 mg/kg), and 5-HT1A (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-HT2C (but not 5-HT1A or 5-HT2A) receptors blocked the effects of lorcaserin on cocaine and MDPV self-administration. Taken together, these data provide additional support for further development of 5-HT2C receptor agonists, such as lorcaserin, for the treatment of stimulant abuse.

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.

Selective Pharmacological Augmentation of Hippocampal Activity Produces a Sustained Antidepressant-Like Response without Abuse-Related or Psychotomimetic Effects.

Background: Selective augmentation of hippocampal activity in ways similar to that caused by ketamine may have therapeutic advantages over ketamine, which has psychotomimetic and reinforcing effects likely due to effects outside the hippocampus (i.e., off-target effects). Methods: Here we evaluated the antidepressant-like response to a negative allosteric modulator of α5 subunit- containing gamma aminobutyric acid subtype A receptors, L-655,708, as these receptors are expressed to a much greater extent in the hippocampus than in other brain areas. Results: Systemic administration of L-655,708 produced a sustained antidepressant-like effect in the forced swim test that was comparable with that of ketamine and was blocked by hippocampal inactivation with lidocaine. However, in contrast to ketamine, L-655,708 did not affect prepulse inhibition of startle, nor did it maintain responding in rats trained to self-administer i.v. ketamine. Conclusion: Taken together, these findings suggest that activation of the hippocampus by L-655,708 produces an antidepressant-like effect in the absence of any psychotomimetic or abuse-related effects.

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.

Effects of Lorcaserin on Cocaine and Methamphetamine Self-Administration and Reinstatement of Responding Previously Maintained by Cocaine in Rhesus Monkeys.

Stimulant abuse is a serious public health issue for which there is no effective pharmacotherapy. The serotonin2C [5-hydroxytryptamine2C (5-HT2C)] receptor agonist lorcaserin decreases some abuse-related effects of cocaine in monkeys and might be useful for treating stimulant abuse. The current study investigated the effectiveness of lorcaserin to reduce self-administration of either cocaine or methamphetamine and cocaine-induced reinstatement of extinguished responding. Four rhesus monkeys responded under a progressive-ratio (PR) schedule in which the response requirement increased after each cocaine infusion (32-320 µg/kg/infusion). A separate group of four monkeys responded under a fixed-ratio (FR) schedule for cocaine (32 µg/kg/infusion) and reinstatement of extinguished responding was examined following administration of noncontingent infusions of cocaine (0.1-1 mg/kg) that were combined with response-contingent presentations of the drug-associated stimuli. Finally, three monkeys responded under a FR schedule for methamphetamine (0.32-100 µg/kg/infusion). Lorcaserin (3.2 mg/kg) significantly decreased the final ratio completed (i.e., decreased break point) in monkeys responding under the PR schedule and reduced the reinstatement of responding for drug-associated stimuli following a noncontingent infusion of cocaine; these effects did not appear to change when lorcaserin was administered daily. The same dose of lorcaserin decreased responding for methamphetamine in two of the three monkeys, and the effect was maintained during daily lorcaserin administration; larger doses given acutely (10-17.8 mg/kg) significantly decreased responding for methamphetamine, although that effect was not sustained during daily lorcaserin administration. Together, these results indicate that lorcaserin might be effective in reducing cocaine and methamphetamine abuse and cocaine relapse at least in some individuals.

Lorcaserin Reduces the Discriminative Stimulus and Reinforcing Effects of Cocaine in Rhesus Monkeys.

Cocaine abuse and obesity are serious public health problems, and studies suggest that both dopamine and serotonin systems are involved in regulating the consumption of drugs and food. Lorcaserin has serotonin (5-HT)2C receptor agonist actions, is approved by the U.S. Food and Drug Administration for treating obesity, and might be effective for treating cocaine abuse. These studies characterized the pharmacokinetic and behavioral profiles of lorcaserin (intragastric administration) and determined the effectiveness of lorcaserin to alter discriminative stimulus and reinforcing effects of cocaine (intravenous administration) in rhesus monkeys. Administered acutely, lorcaserin dose-dependently increased the occurrence of yawning while decreasing spontaneous activity and operant responding for food. These effects appeared within 30-60 minutes of administration and began to dissipate by 240 minutes, a time course closely matching plasma concentrations of lorcaserin. In monkeys discriminating cocaine from saline, lorcaserin alone did not occasion cocaine-appropriate responding but shifted the cocaine dose-response curve to the right and down in two of three monkeys. When administered acutely, lorcaserin dose-dependently decreased the rate at which monkeys responded for infusions of cocaine. When administered chronically, 3.2 mg/kg lorcaserin reduced the rate of cocaine-maintained responding by 50% for the duration of a 14-day treatment period. Together, these results show that lorcaserin attenuates the discriminative stimulus effects of cocaine after acute administration and the reinforcing effects of cocaine after acute and repeated administration, consistent with the view that it might have utility in treating cocaine abuse.

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

Illicit drug preparations often include more than one pharmacologically active compound. For example, cocaine and synthetic cathinones [e.g., 3,4-methylenedioxypyrovalerone (MDPV)] are often mixed with caffeine before sale. Caffeine is likely added to these preparations because it is inexpensive and legal; however, caffeine might also mimic or enhance some of the effects of cocaine or MDPV. In these studies, male Sprague-Dawley rats were trained to discriminate 10 mg/kg cocaine from saline, and the discriminative stimulus effects of cocaine, caffeine, and MDPV were evaluated alone and as binary mixtures (cocaine and caffeine, MDPV and caffeine, and cocaine and MDPV) at fixed-dose ratios of 3:1, 1:1, and 1:3 relative to the dose of each drug that produced 50% cocaine-appropriate responding. Dose-addition analyses were used to determine the nature of the drug-drug interactions for each mixture (e.g., additive, supra-additive, or subadditive). Although additive interactions were observed for most mixtures, supra-additive interactions were observed at the 50% effect level for the 1:1 mixture of cocaine and caffeine and at the 80% effect level for all three mixtures of cocaine and caffeine, as well as for the 3:1 and 1:3 mixtures of cocaine and MDPV. These results demonstrate that with respect to cocaine-like discriminative stimulus effects, caffeine can function as a substitute in drug preparations containing either cocaine or MDPV, with enhancements of cocaine-like effects possible under certain conditions. Further research is needed to determine whether similar interactions exist for other abuse-related or toxic effects of drug preparations, including cocaine, synthetic cathinones, and caffeine.

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).

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).

Effects of access condition on substance use disorder-like phenotypes in male and female rats self-administering MDPV or cocaine.

Substance use disorder (SUD) is a heterogeneous disorder, where severity, symptoms, and patterns of substance use vary across individuals. Yet, when rats are allowed to self-administer drugs such as cocaine under short-access conditions, their behavior tends to be well-regulated and homogeneous in nature; though individual differences can emerge when rats are provided long- or intermittent-access to cocaine. In contrast to cocaine, significant individual differences emerge when rats are allowed to self-administer 3,4-methylenedioxypyrovalerone (MDPV), even under short-access conditions, wherein ~30% of rats rapidly transition to high levels of drug-taking. This study assessed the SUD-like phenotypes of male and female Sprague Dawley rats self-administering MDPV (0.032 mg/kg/infusion) or cocaine (0.32 mg/kg/infusion) by comparing level of drug intake, responding during periods of signaled drug unavailability, and sensitivity to footshock punishment to test the hypotheses that: (1) under short-access conditions, rats that self-administer MDPV will exhibit a more robust SUD-like phenotype than rats that self-administered cocaine; (2) female rats will have a more severe phenotype than male rats; and (3) compared to short-access, long- and intermittent-access to MDPV or cocaine self-administration will result in a more robust SUD-like phenotype. After short-access, rats that self-administered MDPV exhibited a more severe phenotype than rats that self-administered cocaine. Though long- and intermittent-access to cocaine and MDPV self-administration altered drug-taking patterns, manipulating access conditions did not systematically alter their SUD-like phenotype. Evidence from behavioral and quantitative autoradiography studies suggest that these differences are unlikely due to changes in expression levels of dopamine transporter, dopamine D2 or D3 receptors, or 5-HT1B, 5-HT2A, or 5-HT2C receptors, though these possibilities cannot be ruled out. These results show that the phenotype exhibited by rats self-administering MDPV differs from that observed for rats self-administering cocaine, and suggests that individuals that use MDPV and/or related cathinones may be at greater risk for developing a SUD, and that short-access MDPV self-administration may provide a useful method to understand the factors that mediate the transition to problematic or disordered substance use in humans.

Repeated administration of a mutant cocaine esterase: effects on plasma cocaine levels, cocaine-induced cardiovascular activity, and immune responses in rhesus monkeys.

Previous studies have demonstrated the capacity of a long-acting mutant form of a naturally occurring bacterial double mutant cocaine esterase (DM CocE) to antagonize the reinforcing, discriminative, convulsant, and lethal effects of cocaine in rodents and reverse the increases in mean arterial pressure (MAP) and heart rate (HR) produced by cocaine in rhesus monkeys. This study was aimed at characterizing the immunologic responses to repeated dosing with DM CocE and determining whether the development of anti-CocE antibodies altered the capacity of DM CocE to reduce plasma cocaine levels and ameliorate the cardiovascular effects of cocaine in rhesus monkeys. Under control conditions, intravenous administration of cocaine (3 mg/kg) resulted in a rapid increase in the plasma concentration of cocaine (n = 2) and long-lasting increases in MAP and HR (n = 3). Administration of DM CocE (0.32 mg/kg i.v.) 10 min after cocaine resulted in a rapid hydrolysis of cocaine with plasma levels below detection limits within 5 to 8 min. Elevations in MAP and HR were significantly reduced within 25 and 50 min of DM CocE administration, respectively. Although slight (10-fold) increases in anti-CocE antibodies were observed after the fourth administration of DM CocE, these antibodies did not alter the capacity of DM CocE to reduce plasma cocaine levels or ameliorate cocaine's cardiovascular effects. Anti-CocE titers were transient and generally dissipated within 8 weeks. Together, these results suggest that highly efficient cocaine esterases, such as DM CocE, may provide a novel and effective therapeutic for the treatment of acute cocaine intoxication in humans.

The fate of bacterial cocaine esterase (CocE): an in vivo study of CocE-mediated cocaine hydrolysis, CocE pharmacokinetics, and CocE elimination.

Cocaine abuse and toxicity remain widespread problems in the United States. Currently cocaine toxicity is treated only symptomatically, because there is no Food and Drug Administration-approved pharmacotherapy for this indication. To address the unmet need, a stabilized mutant of bacterial cocaine esterase [T172R/G173Q-CocE (DM-CocE)], which hydrolyzes cocaine into inactive metabolites and has low immunogenic potential, has been developed and previously tested in animal models of cocaine toxicity. Here, we document the rapid cocaine hydrolysis by low doses of DM-CocE in vitro and in vivo, as well as the pharmacokinetics and distribution of the DM-CocE protein in rats. DM-CocE at 50.5 µg/kg effectively eliminated 4 mg/kg cocaine within 2 min in both male and female rats as measured by mass spectrometry. We expanded on these findings by using a pharmacologically relevant dose of DM-CocE (0.32 mg/kg) in rats and monkeys to hydrolyze convulsant doses of cocaine. DM-CocE reduced cocaine to below detection limits rapidly after injection; however, elimination of DM-CocE resulted in peripheral cocaine redistribution by 30 to 60 min. Elimination of DM-CocE was quantified by using [³5S] labeling of the enzyme and was found to have a half-life of 2.1 h in rats. Minor urinary output of DM-CocE was also observed. Immunohistochemistry, Western blotting, and radiography all were used to elucidate the mechanism of DM-CocE elimination, rapid proteolysis, and recycling of amino acids into all tissues. This rapid elimination of DM-CocE is a desirable property of a therapeutic for cocaine toxicity and should reduce the likelihood of immunogenic or adverse reactions as DM-CocE moves toward clinical use.

Lack of abuse potential in a highly selective dopamine D3 agonist, PF-592,379, in drug self-administration and drug discrimination in rats.

Dopamine D3-preferring agonists are commonly used to treat Parkinson's disease and restless leg syndrome; however, laboratory animal studies suggest that they may possess a moderate abuse potential. These studies aimed to compare the highly selective, full D3 agonist PF-592,379 to the less selective D3 agonist 7-OH-DPAT, and the indirect dopamine agonist cocaine in drug self-administration and discrimination assays. Although rats readily acquired high rates of fixed ratio (FR)1 responding for cocaine, experimentally naive rats failed to acquire responding when 7-OH-DPAT or PF-592,379 was made available during an 18-session acquisition period. Cocaine also maintained dose-dependent levels of responding when available under a FR5 or a progressive ratio (PR) schedule of reinforcement. Although 7-OH-DPAT maintained modest levels of responding when substituted under a FR5, it failed to maintain significant levels of PR responding. PF-592,379 maintained saline-like rates of responding when substituted under FR5 or PR schedules of reinforcement. Similar behavioral profiles were observed in cocaine discrimination assays, with 7-OH-DPAT partially substituting for cocaine, and PF-592,379 producing saline-like effects over a wide range of doses. Together, the results of these studies predict that highly selective D3 agonists, such as PF-592,379, will have low abuse potential in humans.

Self-administration of agonists selective for dopamine D2, D3, and D4 receptors by rhesus monkeys.

Dopamine receptor mechanisms are believed to play a role in the reinforcing effects of cocaine and other drugs of abuse. The lack of receptor-selective agonists has made it difficult to determine the role of the individual dopamine receptors in mediating these reinforcing effects. In this study, rhesus monkeys with a history of intravenous cocaine self-administration were tested for the reinforcing effects of several D(3)-preferring agonists, a D(2)-preferring agonist, and a D(4) agonist. The D(2)-preferring agonist did not maintain responding in any monkeys, and the D(4) agonist was self-administered at low rates, just above those maintained by saline, in one monkey. The D(3)-preferring agonists were self-administered by approximately half of the animals, although at lower rates than cocaine. These results indicate that the apparent limited reinforcing effectiveness of D(2)-like agonists requires activity at D(3) receptors. Previous data from this laboratory and others also suggest that these drugs may not serve as reinforcers directly; the behavior may be maintained by response-contingent delivery of stimuli previously paired with cocaine. The ability of drug-related stimuli to maintain responding apparently differs among monkeys and other organisms, and may be related to individual differences in drug-taking behavior in humans.