Oral methylphenidate establishes a conditioned place preference in rats

Methylphenidate attenuates signs of evoked neuropathic pain in animal model

Methylphenidate is a psychostimulant that increases dopamine and noradrenaline levels. Recent studies have shown that methylphenidate potentiates the effect of morphine and together suppress acute and chronic pain. In clinical practice, methylphenidate has been used as a treatment for ADHD and changes of pain threshold have been noted in these patients. The aim of this study was to determine the effect of methylphenidate in an animal model of peripheral neuropathic pain. Neuropathic pain was modeled by the chronic constriction of the sciatic nerve (CCI) in Wistar rats. We evaluated the effect of methylphenidate (1 mg/kg, s.c.) on evoked pain (reflex tests - plantar test, vonFrey test and operant test - thermal place preference) and on spontaneous pain (conditioned place preference). CCI induced thermal, mechanical and cold hyperalgesia/allodynia. Methyphenidate suppressed mechanical and cold hyperalgesia/allodynia, while had no effect on thermal one. Therefore, methylphenidate seems to be a new potential pharmacotherapy for the treatment of neuropathic pain.

Methylphenidate as a treatment option for substance use disorder: a transdiagnostic perspective

A transition in viewing mental disorders from conditions defined as a set of unique characteristics to one of the quantitative variations on a collection of dimensions allows overlap between disorders. The overlap can be utilized to extend to treatment approaches. Here, we consider the overlap between attention-deficit/hyperactivity disorder and substance use disorder to probe the suitability to use methylphenidate as a treatment for substance use disorder. Both disorders are characterized by maladaptive goal-directed behavior, impaired cognitive control, hyperactive phasic dopaminergic neurotransmission in the striatum, prefrontal hypoactivation, and reduced frontal cortex gray matter volume/density. In addition, methylphenidate has been shown to improve cognitive control and normalize associated brain activation in substance use disorder patients and clinical trials have found methylphenidate to improve clinical outcomes. Despite the theoretical basis and promising, but preliminary, outcomes, many questions remain unanswered. Most prominent is whether all patients who are addicted to different substances may equally profit from methylphenidate treatment.

Pre-clinical evidence that methylphenidate increases motivation and/or reward preference to search for high value rewards

Methylphenidate is a stimulant used to treat attention deficit and hyperactivity disorder (ADHD). In the last decade, illicit use of methylphenidate has increased among healthy young adults, who consume the drug under the assumption that it will improve cognitive performance. However, the studies that aimed to assess the methylphenidate effects on memory are not consistent. Here, we tested whether the effect of methylphenidate on a spatial memory task can be explained as a motivational and/or a reward effect. We tested the effects of acute and chronic i.p. administration of 0.3, 1 or 3 mg/kg of methylphenidate on motivation, learning and memory by using the 8-arm radial maze task. Adult male Wistar rats learned that 3 of the 8 arms of the maze were consistently baited with 1, 3, or 6 sucrose pellets, and the number of entries and reentries into reinforced and non-reinforced arms of the maze were scored. Neither acute nor chronic (20 days) methylphenidate treatment affected the number of entries in the non-baited arms. However, chronic, but not acute, 1-3 mg/kg methylphenidate increased the number of reentries in the higher reward arms, which suggests a motivational/rewarding effect rather than a working memory deficit. In agreement with this hypothesis, the methylphenidate treatment also decreased the approach latency to the higher reward arms, increased the approach latency to the low reward arm, and increased the time spent in the high, but not low, reward arm. These findings suggest that methylphenidate may act more as a motivational enhancer rather than a cognitive enhancer in healthy people.

The potent psychomotor, rewarding and reinforcing properties of 3-fluoromethamphetamine in rodents

3-fluoromethamphetamine (3-FMA), a derivative of methamphetamine (METH), produces behavioral impairment and deficits in dopaminergic transmission in the striatum of mice. The abuse potential of 3-FMA has not been fully characterized. The aim of this study was to evaluate the effects of 3-FMA on locomotor activity as well as its rewarding and reinforcing properties in the conditioned place preference (CPP) and self-administration procedures. Intravenous (i.v.) administration of 3-FMA (0.5 and 1.0 mg/kg) significantly increased locomotor activity in a dose-dependent manner in rats. In the CPP procedure, intraperitoneal administration of 3-FMA (10 and 30 mg/kg) produced a significant alteration in place preference in mice. In the self-administration paradigms, 3-FMA showed drug-taking behavior at the dose of 0.1 mg/kg/infusion (i.v.) during 2 hr sessions under fixed ratio schedules and high breakpoints at the dose of 0.3 and 1.0 mg/kg/infusion (i.v.) during 6 hr sessions under progressive ratio schedule of reinforcement in rats. A priming injection of 3-FMA (0.4 mg/kg, i.v.), METH (0.2 mg/kg, i.v.), or cocaine (2.0 mg/kg, i.v.) reinstated 3-FMA-seeking behavior after an extinction period in 3-FMA-trained rats during 2 hr session. Taken together, these findings demonstrate robust psychomotor, rewarding and reinforcing properties of 3-FMA, which may underlie its potential for compulsive use in humans.

Single and Repeated Administration of Methylphenidate Modulates Synaptic Plasticity in Opposite Directions via Insertion of AMPA Receptors in Rat Hippocampal Neurons

Methylphenidate (MPH) is widely used in the treatment of Attention Deficit Hyperactivity Disorder. Several lines of evidence support that MPH can modulate learning and memory processes in different ways including improvement and impairment of test performances. A relevant factor in the efficacy of treatment is whether administration is performed once or several times. In this study we demonstrate opposite effects of MPH on performance of preadolescent rats in the Morris Water Maze test. Animals treated with a single dose (1 mg/kg) performed significantly better compared to controls, while in animals treated with repetitive administration at the same concentration performance was reduced. We found that hippocampal LTP in slices from rats treated with a single dose was increased, while LTP from rats treated with repetitive injections of MPH was lower than in controls. Using Western blot of CA1 areas from potentiated slices of rats treated with a single dose we found a significant increase of phosphorylation at Ser845 of GluA1 subunits, associated to an increased insertion of GluA1-containing AMPARs in the plasma membrane. These receptors were functional, because AMPA-dependent EPSCs recorded on CA1 were enhanced, associated to a significant increase in short-term plasticity. In contrast, CA1 samples from rats injected with MPH during six consecutive days, showed a significant decrease in the phosphorylation at Ser845 of GluA1 subunits associated to a lower insertion of GluA1-containing AMPARs. Accordingly, a reduction of the AMPA-mediated EPSCs and short-term plasticity was also observed. Taken together, our results demonstrate that single and repeated doses with MPH can induce opposite effects at behavioral, cellular, and molecular levels. The mechanisms demonstrated here in preadolescent rats are relevant to understand the effects of this psychostimulant in the treatment of ADHD.

Abuse liability assessment in preclinical drug development: predictivity of a translational approach for abuse liability testing using methylphenidate in four standardized preclinical study models

OBJECTIVES

Preclinical abuse liability assessment of novel clinical CNS-active candidates involves several tests, addressing different aspects characteristic for abuse potential, which are considered predictive for substance abuse of these candidates, thus ensuring an appropriate translational approach. To demonstrate how such a strategy could work, a known drug of abuse, methylphenidate was evaluated in a full rodent test battery, comprising four test models, and in accordance with the requirements of the FDA, ICH and EMA guidelines.

METHODS

Methylphenidate was tested orally at 2.5, 5 or 10mg/kg for its physical dependence potential in a repeated dose non-precipitated withdrawal test, for its drug profiling in a drug discrimination learning procedure (single escalating doses), and for its reinforcing properties in a conditioned place preference test (alternate dosing days) and an intravenous self-administration procedure (0.05 to 1mg/kg/IV infusion during 5 daily 1-h test sessions). The stimulant d-amphetamine served as positive control and was administered subcutaneously at 0.8mg/kg in the first three test models. In the intravenous self-administration procedure rats were habituated to intravenously self-administer d-amphetamine at 0.06mg/kg/IV infusion prior to methylphenidate substitution.

RESULTS

Cessation of subchronic dosing up to 10mg/kg methylphenidate led to sustained or even exacerbated effects on locomotion and behavior, body temperature, body weight, food consumption, and alteration of the diurnal rhythm during withdrawal. Clear generalization to d-amphetamine was obtained in the drug discrimination test at 5 and 10mg/kg. Distinct reinforcing properties were present in the conditioned place preference test at 10mg/kg and in the intravenous self-administration study from 0.05mg/kg/IV infusion onwards. The maximum plasma exposure after oral administration of methylphenidate over the dose ranges tested in the present rat studies covered at least 1.9-fold to 18.9-fold the recommended human therapeutic exposure of 10ng/ml, a plasma level that is considered representative of the human efficacious methylphenidate dose. The ratio Cmax Hu/rat calculated from the intravenous self-administration data ranged from 14.9 to 576.5. Consequently the regulatory requirements, stating that preclinical drug abuse liability studies should include high doses that produce plasma levels that are multiples of the therapeutic dose were fulfilled (FDA, EMA, ICH).

DISCUSSION

The presented preclinical models, implemented within a drug development environment, were considered highly predictive to assess the abuse potential of methylphenidate, and in accordance with the regulatory requirements of drug licensing authorities in terms of appropriate methods, dose selection and subsequent plasma exposure.

Methylphenidate place conditioning in adolescent rats: an analysis of sex differences and the dopamine transporter

In two experiments, we analyzed the effects of methylphenidate (MPH) on conditioned place preference (CPP) in adolescent male and female rats, and the effects of MPH on the dopamine transporter (DAT). In Experiment 1, male and female rats were conditioned for 5 consecutive days from postnatal day (P)44 to P48 with saline, 1, or 5mg/kg MPH. On the post conditioning preference test, the group administered the 1mg/kg dose of MPH resulted in no significant preference compared to controls, whereas the 5mg/kg dose of MPH produced a robust significant preference for the paired context, but there were no sex differences. Analysis of the DAT revealed that animals conditioned with the 5mg/kg dose of MPH demonstrated a significant decrease of the dopamine transporter (DAT) in the nucleus accumbens and striatum compared to controls. In Experiment 2, animals were conditioned using an every second day paradigm from P33-41 to model a previous MPH treatment regimen that had revealed sex differences in behavioral sensitization. MPH produced an increased preference for the paired context on a post-conditioning preference test in Experiment 2, but as in Experiment 1, no sex differences were observed. These data show that a relatively high dose of MPH has rewarding associative effects in both adolescent male and female rats reliably across two different conditioning paradigms and ages in adolescence, but no sex difference. In addition, MPH results in a significant decrease of the DAT in drug reward brain areas which has implications toward plasticity of the brain's reward system.

Abuse and dependence liability analysis of methylphenidate in the spontaneously hypertensive rat model of attention-deficit/hyperactivity disorder (ADHD): what have we learned?

Methylphenidate is the most prescribed stimulant medication for attention-deficit/hyperactivity disorder (ADHD). Despite the well documented clinical benefits of the drug, several questions remain unanswered concerning the effects of extended methylphenidate use (e.g. can methylphenidate be abused by ADHD patients? does repeated methylphenidate treatment produce addiction?). Preclinical studies can help address the long-term safety of clinical treatments, moreover animal studies provide valuable information on the details of drug actions. The spontaneously hypertensive rat (SHR), bred from normotensive Wistar Kyoto rat strain, is considered as the best validated and the most widely used animal model of ADHD. We reviewed the findings of research reports that investigated the abuse and dependence liability of methylphenidate in SHR. In particular, we surveyed the studies which investigated the effects of methylphenidate pretreatment on subsequent methylphenidate-induced conditioned place preference or self-administration for they may give insights into the abuse or dependence liability of long-term methylphenidate treatment in ADHD.

Interactive effects of methylphenidate and alcohol on discrimination, conditioned place preference and motor coordination in C57BL/6J mice

INTRODUCTION

Prior research indicates methylphenidate (MPH) and alcohol (ethanol, EtOH) interact to significantly affect responses humans and mice. The present studies tested the hypothesis that MPH and EtOH interact to potentiate ethanol-related behaviors in mice.

METHODS

We used several behavioral tasks including: drug discrimination in MPH-trained and EtOH-trained mice, conditioned place preference (CPP), rota-rod and the parallel rod apparatus. We also used gas chromatographic methods to measure brain tissue levels of EtOH and the D- and L-isomers of MPH and the metabolite, ethylphenidate (EPH).

RESULTS

In discrimination, EtOH (1 g/kg) produced a significant leftward shift in the MPH generalization curve (1-2 mg/kg) for MPH-trained mice, but no effects of MPH (0.625-1.25 mg/kg) on EtOH discrimination in EtOH-trained mice (0-2.5 g/kg) were observed. In CPP, the MPH (1.25 mg/kg) and EtOH (1.75 g/kg) combination significantly increased time on the drug paired side compared to vehicle (30.7 %), but this was similar to MPH (28.8 %) and EtOH (33.6 %). Footslip errors measured in a parallel rod apparatus indicated that the drug combination was very ataxic, with footslips increasing 29.5 % compared to EtOH. Finally, brain EtOH concentrations were not altered by 1.75 g/kg EtOH combined with 1.25 mg/kg MPH. However, EtOH significantly increased D-MPH and L-EPH without changing L-MPH brain concentrations.

CONCLUSIONS

The enhanced behavioral effects when EtOH is combined with MPH are likely due to the selective increase in brain D-MPH concentrations. These studies are consistent with observations in humans of increased interoceptive awareness of the drug combination and provide new clinical perspectives regarding enhanced ataxic effects of this drug combination.

Methylphenidate and μ opioid receptor interactions: a pharmacological target for prevention of stimulant abuse

Methylphenidate (MPH) is one of the most commonly used and highly effective treatments for attention deficit hyperactivity disorder (ADHD) in children and adults. As the therapeutic use of MPH has increased, so has its abuse and illicit street-use. Yet, the mechanisms associated with development of MPH-associated abuse and dependence are not well understood making it difficult to develop methods to help its mitigation. As a result, many ADHD patients especially children and youth, that could benefit from MPH treatment do not receive it and risk lifelong disabilities associated with untreated ADHD. Therefore, understanding the mechanisms associated with development of MPH addiction and designing methods to prevent it assume high public health significance. Using a mouse model we show that supra-therapeutic doses of MPH produce rewarding effects (surrogate measure for addiction in humans) in a conditioned place preference paradigm and upregulate μ opioid receptor (MOPR) activity in the striatum and nucleus accumbens, brain regions associated with reward circuitry. Co-administration of naltrexone, a non-selective opioid receptor antagonist, prevents MPH-induced MOPR activation and the rewarding effects. The MPH-induced MOPR activation and rewarding effect require activation of the dopamine D1 but not the D2-receptor. These findings identify the MOPR as a potential target for attenuating rewarding effects of MPH and suggest that a formulation that combines naltrexone with MPH could be a useful pharmaceutical approach to alleviate abuse potential of MPH and other stimulants.