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

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.

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.

Characterization of 3,4-methylenedioxypyrovalerone discrimination in female Sprague-Dawley rats

3,4-Methylenedioxypyrovalerone (MDPV), one of several synthetic cathinones, is a popular constituent of illicit 'bath salts'. In preclinical studies utilizing drug discrimination methods with male rodents, MDPV has been characterized as similar to both cocaine and 3,4-methylenedioxymethamphetamine-hydrochloride (MDMA). Whereas few drug discrimination studies have utilized female rats, the current study evaluated the discriminative stimulus effects of MDPV in 12 adult female Sprague-Dawley rats trained to discriminate 0.5 mg/kg MDPV from saline under a fixed ratio 20 schedule of food reinforcement. Stimulus substitution was assessed with MDPV and its enantiomers, other synthetic cathinones [alpha pyrrolidinopentiophenone-hydrochloride(α-PVP), 4-methylmethcathinone (4-MMC)], other dopamine agonists (cocaine, [+)-methamphetamine] and serotonin agonists [MDMA, lysergic acid diethylamide (LSD)] Stimulus antagonism was assessed with the dopamine D1 receptor antagonist, Sch 23390 and the D2 receptor antagonist, haloperidol. Cocaine and (+)-methamphetamine engendered full stimulus generalization to MDPV with minimal effects on response rate. LSD produced partial substitution, whereas MDMA and 4-MMC produced complete substitution, and all these serotonergic compounds produced dose-dependent response suppression. (S)-MDPV and α-PVP engendered full substitution with similar potency to the racemate, while (R)-MDPV failed to substitute up to 5 mg/kg. Both Sch 23390 and haloperidol attenuated the discrimination of low MDPV doses and essentially shifted the dose-response curve to the right but failed to block discrimination of the training dose. These findings are generally consistent with previous reports based exclusively on male rodents. Moreover, they confirm the contribution of dopaminergic mechanisms but do not rule out the possible contribution of other neurotransmitter actions to the interoceptive stimulus effects of MDPV.

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.

Conditioned place preference following concurrent treatment with 3, 4-methylenedioxypyrovalerone (MDPV) and methamphetamine in male and female Sprague-Dawley rats

Synthetic cathinones gained initial popularity on the illicit drug market as a result of attempts to evade legal restrictions on other commonly abused psychostimulants. A body of published research has determined that the psychopharmacology of the synthetic cathinone 3, 4-methylenedioxypyrovalerone (MDPV) is comparable to cocaine and methamphetamine (METH). Few preclinical studies have systematically investigated concurrent use of synthetic cathinones with other psychostimulant drugs. The present study utilized conditioned place preference (CPP), a rodent model of conditioned drug reward, to evaluate the effects of concurrent treatment with MDPV and METH. Male (N = 72) and female (N = 105) Sprague-Dawley rats underwent a two-compartment biased CPP procedure, with one trial per day for eight consecutive days. Subjects were randomly assigned to the following treatment groups: saline, METH (1 mg/kg), MDPV (1, 3.2, 5.6 mg/kg) or a mixture consisting of METH (1 mg/kg) and MDPV (1, 3.2, 5.6 mg/kg). All treatments increased locomotor activity during drug conditioning trials, and most treatments produced higher activity increases in females compared to males. Although the level of CPP established by MDPV and MDPV + METH mixtures varied between males and females, sex differences were not statistically significant. Although none of the MDPV+METH mixtures produced stronger CPP than either substance alone, some mixtures of MDPV and METH produced higher increases in locomotor activity compared to either drug alone. Further studies with higher doses may be warranted to determine if concurrent use of MDPV and METH pose an enhanced risk for abuse.