Effects of L-methamphetamine treatment on cocaine- and food-maintained behavior in rhesus monkeys

Structural and functional perspectives on interactions between synthetic cathinones and monoamine transporters

Synthetic cathinone derivatives comprise a family of psychoactive compounds structurally related to amphetamine. Over the last decade, clandestine chemists have synthesized a consistent stream of innovative cathinone derivatives to outpace governmental regulatory restrictions. Many of these unregulated substances are produced and distributed as designer drugs. Two of the principal chemical scaffolds exploited to expand the synthetic cathinone family are methcathinone and α-pyrrolidinopentiophenone (or α-pyrrolidinovalerophenone, α-PVP). These compounds' main physiological targets are monoamine transporters, where they promote addiction by potentiating dopaminergic neurotransmission. This chapter describes techniques used to study the pharmacodynamic properties of cathinones at monoamine transporters in vitro. Biochemical techniques described include uptake inhibition and release assays in rat brain synaptosomes and in mammalian expression systems. Electrophysiological techniques include current measurements using the voltage clamp technique. We describe a Ca2+ mobilization assay wherein voltage-gated Ca2+ channels function as reporters to study the action of synthetic cathinones at monoamine transporters. We discuss results from systematic structure-activity relationship studies on simple and complex cathinones at monoamine transporters with an emphasis on identifying structural moieties that modulate potency and selectivity at these transporters. Moreover, different profiles of selectivity at monoamine transporters directly predict compounds associated with behavioral and subjective effects within animals and humans. In conclusion, clarification of the structural aspects of compounds which modulate potency and selectivity at monoamine transporters is critical to identify and predict potential addictive drugs. This knowledge may allow prompt allocation of resources toward drugs that represent the greatest threats after drugs are identified by forensic laboratories.

Reinforcing and Stimulant-Like Effects of Methamphetamine Isomers in Rhesus Macaques

Monoamine releasers such as d-methamphetamine (d-MA) can reduce cocaine use in laboratory studies and have been forwarded for the management of cocaine use disorder (CUD). However, the proven abuse liability of d-MA has limited enthusiasm for clinical use. The levorotatory isomer of MA, l-MA, appears to have lesser stimulant effects, possibly due to its preferential norepinephrine-releasing properties compared with dopamine. The present study evaluated the abuse potential of l-MA by comparing its reinforcing effects with known stimulant drugs of abuse in nonhuman primates. Adult rhesus macaques (N = 4) responded for intravenous injections of cocaine, d-MA, methcathinone (MCAT), or l-MA under a fixed-ratio (FR) schedule of reinforcement; reinforcing effectiveness was evaluated using behavioral economic demand procedures. In a separate cohort (N = 9), daily activity and food-reinforced responding were assessed during 100 days of treatment with daily dosages of l-MA (2.3 mg/kg per day, i.v.) or d-MA (0.74 mg/kg per day, i.v.) previously shown to decrease cocaine self-administration. Results show that all drugs maintained self-administration, with peak injections reaching ∼100 inj per session for cocaine, MCAT, and d-MA and ∼50 inj per session for l-MA . In demand studies, self-administration of each drug gradually decreased as FR size increased. The exponential model of demand indicated that the reinforcing effectiveness of l-MA was significantly less than the other drugs studied. Chronic l-MA treatment did not appreciably alter daily activity and only transiently suppressed food-reinforced responding. These data, coupled with previous findings that l-MA effectively reduces stimulant self-administration, suggest that l-MA, or other norepinephrine-preferring releasers, may serve as agonist medication for CUD with lesser abuse liability than common psychostimulants. SIGNIFICANCE STATEMENT: Development of pharmacotherapies for cocaine use disorder remains a formidable challenge. Agonist-based therapies show promise, but enthusiasm is tempered by the abuse liability of previously proposed medications. This study evaluated the abuse liability and chronic treatment effects of methamphetamine's levorotatory isomer (l-MA). l-MA demonstrated lower abuse liability compared with commonly abused stimulants and produced few untoward effects. In the context of recent studies demonstrating that l-MA attenuates stimulant self-administration, these findings support l-MA's potential as a pharmacotherapy for stimulant addiction.

Effects of chronic treatment with bupropion on self-administration of nicotine + cocaine mixtures in nonhuman primates

Chronic health problems associated with long-term nicotine use are the leading cause of preventable death in the United States. The use of tobacco products is 3-4 times greater among individuals with cocaine use disorder than that observed in the general population. This may reflect the propensity of nicotine to augment the reinforcing effects of cocaine. However, the mechanism of action of nicotine differs from that of cocaine, which presents a significant challenge for the development of pharmacotherapeutic interventions for the management of nicotine + cocaine polydrug abuse. Bupropion, an FDA-approved smoking cessation aid, has pharmacological actions at both monoamine transporters and nicotinic receptors, suggesting that it may be effective at decreasing nicotine + cocaine coabuse. Here, rhesus monkeys (n = 4) responded for food pellets and, separately, intravenous injections of nicotine, cocaine, or nicotine + cocaine mixtures under a second-order FR2(VR16:S) schedule of reinforcement during 7- to 10-day continuous treatment with saline or bupropion (1.0 and 1.8 mg/kg/hr). Results show that bupropion treatment dose-dependently decreased self-administration of nicotine combined with a low dose of cocaine (0.0032 mg/kg/inj); however, when the dose of cocaine in the mixture was higher (i.e., 0.01 mg/kg/inj), bupropion attenuated self-administration in only a subset of subjects. The effective dosage of bupropion increased responding for cocaine alone, nicotine alone, and for saline injections and significantly increased measures of daily activity. The apparent stimulant-like effects of bupropion at the dosage required to decrease cocaine + nicotine self-administration does not support its clinical use for the management of nicotine + cocaine polydrug abuse. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Further pharmacological comparison of D-methamphetamine and L-methamphetamine in rats: abuse-related behavioral and physiological indices

Previous preclinical research suggests that L-methamphetamine (L -MA) has potential therapeutic utility to treat psychostimulant abuse. This study examined potential abuse-related and adverse physiological effects of D -MA and L -MA alone and in combination in rats, as these effects had not been previously characterized. Potential abuse-related effects were examined in locomotor sensitization and conditioned place preference paradigms. Body temperature was monitored to assess the physiological effects of these drugs or drug combinations. In the locomotor study, D-MA induced locomotor sensitization to both D-MA and L -MA. L -MA induced locomotor sensitization only to D-MA. Responses to a combination of L-MA and D -MA were not differentially affected by L-MA or D-MA conditioning. In the conditioned place preference study, D-MA and L -MA each induced significant place preference. L -MA did not attenuate D-MA-induced place preference. In the body temperature study, D-MA induced hyperthermia and L -MA induced hypothermia. In combination, L -MA did not affect D-MA-induced hyperthermia. These data suggest that L -MA alone produces less abuse-related and adverse physiological effects than D-MA, but modulates and is modulated by concurrent and subsequent D-MA exposure, which may enhance the abuse liability of both drugs. These findings should be considered when L -MA is proposed for replacement therapy.

Effects of methamphetamine isomers on d-methamphetamine self-administration and food-maintained responding in male rats

RATIONALE

Methamphetamine (METH) abuse is generally attributed to the d-isomer. Self-administration of l-METH has been examined only in rhesus monkeys with a history of cocaine self-administration or drug-naïve rats using high toxic doses.

OBJECTIVES

In this study, the ability of l-METH and, for comparison, d-METH to engender self-administration in experimentally naïve rats, as well as to decrease d-METH self-administration and food-maintained responding, was examined.

METHODS

Male Sprague-Dawley rats were used in 3 separate experiments. In experiment 1, the acquisition of l- or d-METH self-administration followed by dose-response determinations was studied. In experiment 2, rats were trained to self-administer d-METH (0.05 mg/kg/infusion) and, then, various doses of l- or d-METH were given acutely prior to the session; the effect of repeated l-METH (30 mg/kg) also was examined. In experiment 3, rats were trained to respond for food reinforcement and, then, various doses of l- or d-METH were given acutely prior to the session; the effect of repeated l-METH (3 mg/kg) also was examined.

RESULTS

Reliable acquisition of l- and d-METH self-administration was obtained at unit doses of 0.5 and 0.05 mg/kg/infusion respectively. The dose-response function for l-METH self-administration was flattened and shifted rightward compared with d-METH self-administration, with peak responding for l- and d-METH occurring at unit doses of 0.17 and 0.025 respectively. l-METH also was approximately 10-fold less potent than d-METH in decreasing d-METH self-administration and 2-fold lower in decreasing food-maintained responding. Tolerance did not occur to repeated l-METH pretreatments on either measure.

CONCLUSIONS

As a potential pharmacotherapeutic, l-METH has less abuse liability than d-METH and its efficacy in decreasing d-METH self-administration and food-maintained responding is sustained with repeated treatment.

Methamphetamine-associated difficulties in cognitive control allocation may normalize after prolonged abstinence

Chronic heavy methamphetamine use likely causes dopaminergic neurotoxicity, which is commonly thought to result in cognitive control deficits. Both of these alterations may persist even after the use is discontinued, but tend to (partly) improve with increasing duration of abstinence. While several studies have demonstrated that the reinstatement of comparatively normal dopaminergic signaling may take months, if not years, the amelioration of cognitive deficits has predominantly been investigated in much shorter intervals of several weeks to less than half a year. Against this background, we set out to investigate the effects on prolonged abstinence in n = 27 abstinent former methamphetamine users in a cross-sectional design using behavioral and neurophysiological measures of cognitive control. Our behavioral results suggest that former users struggled to identify and adapt to different degrees of cognitive control requirements, which made their behavioral performance less expedient than that of healthy controls. On the neurophysiological level, this was reflected by reduced modulations of the N2-N450 amplitude in response to high vs. low cognitive control requirements. Yet, those effects could only be observed in methamphetamine users who had been abstinent for a relatively short time (mean 9.9; max. 18 months), but not in former users who had been abstinent two years or longer. While this finding alone does not allow for causal inferences, it suggests that the amelioration of control deficits may take longer than what is commonly investigated (1-6 months). Hence, some of the statements about permanent/irreversible dopamine-dependent executive dysfunctions in former methamphetamine users should be interpreted with caution.

Methamphetamine Users Show No Behavioral Deficits in Response Selection After Protracted Abstinence

Introduction: Chronic recreational methamphetamine use causes dopaminergic neurotoxicity, which has been linked to impairments in executive functioning. Within this functional domain, response selection and the resolution of associated conflicts have repeatedly been demonstrated to be strongly modulated by dopamine. Yet, it has never been investigated whether chronic methamphetamine use leads to general impairments in response selection (i.e., irrespective of consumption-associated behavior) after substance use is discontinued. Materials and Methods: We tested n = 24 abstinent methamphetamine users (on average 2.7 years of abstinence) and n = 24 individually matched controls in a cross-sectional design with a flanker task. Results: Compared to healthy controls, former methamphetamine consumers had significantly slower reaction times, but did not show differences in the size of the flanker or Gratton effect, or post-error slowing. Complementary Bayesian analyses further substantiated this lack of effects despite prior consumption for an average of 7.2 years. Discussion: The ability to select a correct response from a subset of conflicting alternatives, as well as the selective attention required for this seem to be largely preserved in case of prolonged abstinence. Likewise, the ability to take previous contextual information into account during response selection and to process errors seem to be largely preserved as well. Complementing previously published finding of worse inhibition/interference control in abstinent consumers, our results suggest that not all executive domains are (equally) impaired by methamphetamine, possibly because different cognitive processes require different levels of dopamine activity.

Cocaine-like discriminative stimulus effects of "norepinephrine-preferring" monoamine releasers: time course and interaction studies in rhesus monkeys

RATIONALE

The therapeutic potential of monoamine releasers with prominent dopaminergic effects is hindered by their high abuse liability.

OBJECTIVES

The present study examined the effects of several novel "norepinephrine (NE)-preferring" monoamine releasers relative to non-selective monoamine releasers, d-amphetamine and d-methamphetamine, in rhesus monkeys trained to discriminate cocaine. NE-preferring releasers were approximately 13-fold more potent for NE compared to dopamine release and ranged in potency for serotonin release (PAL-329 < l-methamphetamine < PAL-169).

METHODS

Adult rhesus macaques were trained to discriminate 0.4 mg/kg, IM cocaine on a 30-response fixed ratio schedule of food reinforcement. Substitution studies determined the extent to which test drugs produced cocaine-like discriminative stimulus effects and their time course. Drug interaction studies determined whether pretreatment with test drugs altered the discriminable effects of cocaine.

RESULTS

Results show that cocaine, d-amphetamine, and d-methamphetamine dose-dependently substituted for cocaine with similar potencies. Among the "NE-preferring" releasers, PAL-329 and l-methamphetamine also dose-dependently substituted for cocaine but differed in potency. PAL-169 failed to substitute for cocaine up to a dose that disrupted responding. When administered prior to cocaine, only d-amphetamine and PAL-329 significantly shifted the cocaine dose-effect function leftward indicating enhancement of cocaine's discriminative stimulus effects.

CONCLUSIONS

These data suggest that greater potency for NE relative to dopamine release (up to 13-fold) does not interfere with the ability of a monoamine releaser to produce cocaine-like discriminative effects but that increased serotonin release may have an inhibitory effect. Further characterization of these and other "NE-preferring" monoamine releasers should provide insight into their potential for the management of cocaine addiction.

Comparison of some behavioral effects of d- and l-methamphetamine in adult male rats

RATIONALE

Both l- and d-methamphetamine (l- and d-MA) are more potent to release norpepinephrine (NE) than dopamine, and the selectivity is greater for l-MA than d-MA. Little is known of the in vivo pharmacology of l-MA.

OBJECTIVE

This study compared the effects of l-MA and d-MA in assays of nociception, behavioral disruption, and impulsivity.

METHODS

Antinociceptive effects of d- and l-MA were examined in two pain assays: the warm water tail withdrawal test for acute nociception and the von Frey test in complete Freund's adjuvant (CFA)-treated rats for chronic inflammatory pain. Food-maintained operant responding and locomotion tests were used to assess generalized behavioral disruption. The 5-choice serial reaction time test (5-CSRTT) was used to assess drug-induced effects on impulse control. A delay discounting procedure was used to determine drug-induced changes in sensitivity to reinforcer delay (impulsive choice).

RESULTS

l-MA (3.2-10 mg/kg) produced dose-dependent antinociception in both pain assays, decreased the rate of food-maintained operant responding, and decreased locomotor activity at a higher dose (17.8 mg/kg). In contrast, d-MA (0.32-3.2 mg/kg) did not produce antinociception in either assay, produced biphasic effects on response rate, and increased locomotor activity. In the 5-CSRTT, d-MA but not l-MA produced significant increase in premature responses. In the delay discounting procedure, both drugs did not affect the delay function at doses that did not increase omissions.

CONCLUSIONS

These data suggest that d- and l-MA have different behavioral profiles. Consideration should be given to these differences in future studies when l-MA is proposed for potential therapies.

Evaluation of the "Pipeline" for Development of Medications for Cocaine Use Disorder: A Review of Translational Preclinical, Human Laboratory, and Clinical Trial Research

Cocaine use disorder is a persistent public health problem for which no widely effective medications exist. Self-administration procedures, which have shown good predictive validity in estimating the abuse potential of drugs, have been used in rodent, nonhuman primate, and human laboratory studies to screen putative medications. This review assessed the effectiveness of the medications development process regarding pharmacotherapies for cocaine use disorder. The primary objective was to determine whether data from animal and human laboratory self-administration studies predicted the results of clinical trials. In addition, the concordance between laboratory studies in animals and humans was assessed. More than 100 blinded, randomized, fully placebo-controlled studies of putative medications for cocaine use disorder were identified. Of the 64 drugs tested in these trials, only 10 had been examined in both human and well-controlled animal laboratory studies. Within all three stages, few studies had been conducted for each drug and when multiple studies had been conducted conclusions were sometimes contradictory. Overall, however, there was good concordance between animal and human laboratory results when the former assessed chronic drug treatment. Although only seven of the ten reviewed drugs showed fully concordant results across all three types of studies reviewed, the analysis revealed several subject-related, procedural, and environmental factors that differ between the laboratory and clinical trial settings that help explain the disagreement for other drugs. The review closes with several recommendations to enhance translation and communication across stages of the medications development process that will ultimately speed the progress toward effective pharmacotherapeutic strategies for cocaine use disorder.