Chronic treatment with extended release methylphenidate does not alter dopamine systems or increase vulnerability for cocaine self-administration: a study in nonhuman primates

Neuroprotection in late life attention-deficit/hyperactivity disorder: A review of pharmacotherapy and phenotype across the lifespan

For decades, psychostimulants have been the gold standard pharmaceutical treatment for attention-deficit/hyperactivity disorder (ADHD). In the United States, an astounding 9% of all boys and 4% of girls will be prescribed stimulant drugs at some point during their childhood. Recent meta-analyses have revealed that individuals with ADHD have reduced brain volume loss later in life (>60 y.o.) compared to the normal aging brain, which suggests that either ADHD or its treatment may be neuroprotective. Crucially, these neuroprotective effects were significant in brain regions (e.g., hippocampus, amygdala) where severe volume loss is linked to cognitive impairment and Alzheimer's disease. Historically, the ADHD diagnosis and its pharmacotherapy came about nearly simultaneously, making it difficult to evaluate their effects in isolation. Certain evidence suggests that psychostimulants may normalize structural brain changes typically observed in the ADHD brain. If ADHD itself is neuroprotective, perhaps exercising the brain, then psychostimulants may not be recommended across the lifespan. Alternatively, if stimulant drugs are neuroprotective, then this class of medications may warrant further investigation for their therapeutic effects. Here, we take a bottom-up holistic approach to review the psychopharmacology of ADHD in the context of recent models of attention. We suggest that future studies are greatly needed to better appreciate the interactions amongst an ADHD diagnosis, stimulant treatment across the lifespan, and structure-function alterations in the aging brain.

MicroPET/CT assessment of neurochemical effects in the brain after long-term methylphenidate treatment in nonhuman primates

Methylphenidate (MPH) is a psychostimulant approved by the FDA to treatment Attention-Deficit Hyperactivity Disorder (ADHD). MPH is believed to exert its pharmacological effects via preferential blockade of the dopamine transporter (DAT) and the norepinephrine transporter (NET), resulting in increased monoamine levels in the synapse. We used a quantitative non-invasive PET imaging technique to study the effects of long-term methylphenidate use on the central nervous system (CNS). We conducted microPET/CT scans on young adult male rhesus monkeys to monitor changes in the dopaminergic system. We used [¹⁸F] AV-133, a ligand for the vesicular monoamine transporter 2 (VMAT2), and [¹⁸F]FESP a ligand for the D₂ and 5HT₂ receptors. In this study we evaluated the effects if chronic MPH treatment in the nonhuman primates (NHP). Two-year-old, male rhesus monkeys were orally administered MPH diluted in the electrolyte replenisher, Prang, twice a day, five days per week (M-F) over an 8-year period. The dose of MPH was gradually escalated from 0.15 mg/kg initially to 2.5 mg/kg/dose for the low dose group, and 1.5 mg/kg to 12.5 mg/kg/dose for the high dose group (Rodriguez et al., 2010). Scans were performed on Mondays, about 60 h after their last treatment, to avoid the acute effects of MPH. Tracers were injected intravenously ten minutes before microPET/CT scanning. Sessions lasted about 120 min. The Logan reference tissue model was used to determine the Binding Potential (BP) of each tracer in the striatum with the cerebellar cortex time activity curve as an input function. Both MP treatment groups had a lower [¹⁸F] AV-133 BP, although this failed to reach statistical significance. MPH treatment did not have a significant effect on The BP of [¹⁸F] FESP in the striatum. Long-term administration of MPH did not significant change any of the marker of monoamine function used here. These data suggest that, despite lingering concerns, long-term use of methylphenidate does not negatively impact monoamine function. This study also demonstrates that microPET imaging can distinguish differences in binding potentials of a variety of radiotracers in the CNS of NHPs. This approach may provide minimally-invasive biomarkers of neurochemical processes associated with chronic exposure to CNS medications. (Supported by NCTR).

Pharmacotherapeutic strategies for treating cocaine use disorder-what do we have to offer?

BACKGROUND

Cocaine use contines to be a significant public health problem world-wide. However, despite substantial research efforts, no pharmacotherapies are approved for the treatment of cocaine use disorder (CUD).

ARGUMENT

Studies have identified positive signals for a range of medications for treating CUD. These include long-acting amphetamine formulations, modafinil, topiramate, doxazosin and combined topiramate and mixed amphetamine salts extended-release (MAS-ER). However, valid conclusions about a medication's clinical efficacy require nuanced approaches that take into account behavioural phenotypes of the target population (frequency of use, co-abuse of cocaine and other substances, genetic subgroups, psychiatric comorbidity), variables related to the medication (dose, short-/long-acting formulations, titration speed, medication adherence) and other factors that may affect treatment outcomes. Meta-analyses frequently do not account for these co-varying factors, which contributes to a somewhat nihilistic view on pharmacotherapeutic options for CUD. In addition, the predominant focus on abstinence, which is difficult for most patients to achieve, may overshadow more nuanced therapeutic signals.

CONCLUSION

While there is an emphasis on finding new medications with novel mechanisms of action for treating CUD, currently available medications deserve further investigation based on the existing literature. Evaluating refined metrics of treatment success in well-defined subgroups of patients, and further exploring combination therapies and their synergy with behavioural/psychosocial interventions, are promising avenues to establishing effective therapies for CUD.

Is Attention-Deficit/Hyperactivity Disorder a Risk Syndrome for Parkinson's Disease?

Recent epidemiological evidence indicates that diagnosis of attention-deficit/hyperactivity disorder (ADHD) is associated with increased risk for diseases of the basal ganglia and cerebellum, including Parkinson's disease (PD). The evidence reviewed here indicates that deficits in striatal dopamine are a shared component of the causal chains that produce these disorders. Neuropsychological studies of adult ADHD, prodromal PD, and early-stage PD reveal similar deficits in executive functions, memory, attention, and inhibition that are mediated by similar neural substrates. These and other findings are consistent with the possibility that ADHD may be part of the PD prodrome. The mechanisms that may mediate the association between PD and ADHD include neurotoxic effects of stimulants, other environmental exposures, and Lewy pathology. Understanding the nature of the association between PD and ADHD may provide insight into the etiology and pathogenesis of both disorders. The possible contribution of stimulants to this association may have important clinical and public health implications.

Pharmacokinetic and pharmacodynamic analysis of d-amphetamine in an attention task in rodents

Amphetamine is a common therapeutic agent for alleviating the core symptoms associated with attention-deficit hyperactivity disorder (ADHD) in children and adults. The current study used a translational model of attention, the five-choice serial reaction time (5-CSRT) procedure with rats, to examine the time-course effects of d-amphetamine. Effects of different dosages of d-amphetamine were related to drug-plasma concentrations, fashioned after comprehensive pharmacokinetic/pharmacodynamic assessments that have been employed in clinical investigations. We sought to determine whether acute drug-plasma concentrations that enhance performance in the 5-CSRT procedure are similar to those found to be therapeutic in patients diagnosed with ADHD. Results from the pharmacokinetic/pharmacodynamic assessment indicate that d-amphetamine plasma concentrations associated with improved performance on the 5-CSRT procedure overlap with those that have been reported to be therapeutic in clinical trials. The current findings suggest that the 5-CSRT procedure may be a useful preclinical model for predicting the utility of novel ADHD therapeutics and their effective concentrations.

Long-term effects of stimulant exposure on cerebral blood flow response to methylphenidate and behavior in attention-deficit hyperactivity disorder

Stimulant prescription rates for attention deficit hyperactivity disorder (ADHD) are increasing, even though potential long-term effects on the developing brain have not been well-studied. A previous randomized clinical trial showed short-term age-dependent effects of stimulants on the DA system. We here assessed the long-term modifying effects of age-of-first-stimulant treatment on the human brain and behavior. 81 male adult ADHD patients were stratified into three groups: 1) early stimulant treatment (EST; <16 years of age) 2) late stimulant treatment (LST: ≥23 years of age) and 3) stimulant treatment naive (STN; no history of stimulant treatment). We used pharmacological magnetic resonance imaging (phMRI) to assess the cerebral blood flow (CBF) response to an oral methylphenidate challenge (MPH, 0.5 mg/kg), as an indirect measure of dopamine function in fronto-striatal areas. In addition, mood and anxiety scores, and recreational drug use were assessed. Baseline ACC CBF was lower in the EST than the STN group (p = 0.03), although CBF response to MPH was similar between the three groups (p = 0.23). ADHD symptom severity was higher in the STN group compared to the other groups (p < 0.01). In addition, the EST group reported more depressive symptoms (p = 0.04), but not anxiety (p = 0.26), and less recreational drug use (p = 0.04). In line with extensive pre-clinical data, our data suggest that early, but not late, stimulant treatment long-lastingly affects the human brain and behavior, possibly indicating fundamental changes in the dopamine system.

Changes in Endogenous Dopamine Induced by Methylphenidate Predict Functional Connectivity in Nonhuman Primates

Dopamine (DA) levels in the striatum are increased by many therapeutic drugs, such as methylphenidate (MPH), which also alters behavioral and cognitive functions thought to be controlled by the PFC dose-dependently. We linked DA changes and functional connectivity (FC) using simultaneous [¹⁸F]fallypride PET and resting-state fMRI in awake male rhesus monkeys after oral administration of various doses of MPH. We found a negative correlation between [¹⁸F]fallypride nondisplaceable binding potential (BP_ND) and MPH dose in the head of the caudate (hCd), demonstrating increased extracellular DA resulting from MPH administration. The decreased BP_ND was negatively correlated with FC between the hCd and the PFC. Subsequent voxelwise analyses revealed negative correlations with FC between the hCd and the dorsolateral PFC, hippocampus, and precuneus. These results, showing that MPH-induced changes in DA levels in the hCd predict resting-state FC, shed light on a mechanism by which changes in striatal DA could influence function in the PFC.SIGNIFICANCE STATEMENT Dopamine transmission is thought to play an essential role in shaping large scale-neural networks that underlie cognitive functions. It is the target of therapeutic drugs, such as methylphenidate (Ritalin), which blocks the dopamine transporter, thereby increasing extracellular dopamine levels. Methylphenidate is used extensively to treat attention deficit hyperactivity disorder, even though its effects on cognitive functions and their underlying neural mechanisms are not well understood. To date, little is known about the link between changes in dopamine levels and changes in functional brain organization. Using simultaneous PET/MR imaging, we show that methylphenidate-induced changes in endogenous dopamine levels in the head of the caudate predict changes in resting-state functional connectivity between this structure and the prefrontal cortex, precuneus, and hippocampus.

Contribution of Clinical Neuroimaging to the Understanding of the Pharmacology of Methylphenidate

Methylphenidate (MPH) is currently the most widely used molecule in the pharmacologic treatment of attention-deficit hyperactivity disorder (ADHD). Although experience of its application now extends over several decades, its psychotropic nature, prolonged use in children, and chemical relation to amphetamines still raise doubts in the minds of prescribers and the families of the patients. Brain imaging has shed considerable light on the neuropharmacology of MPH. The two main in vivo neuroimaging techniques are positron-emission tomography (PET) and magnetic resonance imaging (MRI), and these can be applied in both animal models and humans. The present review seeks to show how human molecular and functional imaging has contributed to determining not only the molecular targets of MPH, and the action kinetics of the various pharmaceutical forms available, but also the connectivity and brain networks activated by treatment. We also discuss the perspectives opened up by new hybrid PET-MRI techniques that enable multimodal tracking of the impact of methylphenidate on neurotransmission.

Regulation of emotional response in juvenile monkeys treated with fluoxetine: MAOA interactions

Juvenile male rhesus macaques received therapeutic doses of fluoxetine daily from one to three years of age and were compared to vehicle-treated controls (N=16/group). Genotyping for monoamine oxidase A (MAOA) polymorphisms was used to form subgroups (N=8) with high and low expression of the gene. Behavioral responses were scored during 30-second exposures to pictures differing in affective content. As expected from its therapeutic effect, fluoxetine decreased the behavioral response to emotionally evocative pictures. A 44% reduction in number of expressive behaviors was seen, but only in subjects with low expression MAOA polymorphisms. In general, this effect occurred for pictures of varying affective content and was not due to altered occurrence of one specific behavior or type of behavior. The drug*genotype interaction was seen after one and two years of treatment and did not reverse one year after discontinuation of dosing. Two potential translational implications are suggested: (1) MAOA genetic polymorphisms may be the source of some of the variability in response to fluoxetine treatment in children; (2) extended fluoxetine treatment during juvenile brain development may result in persistent effects on emotional regulation.

The effect of methylphenidate intake on brain structure in adults with ADHD in a placebo-controlled randomized trial

BACKGROUND

Based on animal research several authors have warned that the application of methylphenidate, the first-line drug for the treatment of attention-deficit/hyperactivity disorder (ADHD), might have neurotoxic effects potentially harming the brain. We investigated whether methylphenidate application, over a 1-year period, results in cerebral volume decrease.

METHODS

We acquired structural MRIs in a double-blind study comparing methylphenidate to placebo. Global and regional brain volumes were analyzed at baseline, after 3 months and after 12 months using diffeomorphic anatomic registration through exponentiated lie algebra.

RESULTS

We included 131 adult patients with ADHD into the baseline sample, 98 into the 3-month sample (54 in the methylphenidate cohort and 44 in the placebo cohort) and 76 into the 1-year sample (37 in the methylphenidate cohort and 29 in the placebo cohort). Methylphenidate intake compared with placebo did not lead to any detectable cerebral volume loss; there was a trend toward bilateral cerebellar grey matter increase.

LIMITATIONS

Detecting possible neurotoxic effects of methylphenidate might require a longer observation period.

CONCLUSION

There is no evidence of grey matter volume loss after 1 year of methylphenidate treatment in adult patients with ADHD.

Neural Correlates of Exposure to Cocaine Cues in Rhesus Monkeys: Modulation by the Dopamine Transporter

BACKGROUND

A major goal of treatments for cocaine addiction is to reduce relapse-associated cravings, which are typically induced by environmental stimuli associated with cocaine use and related to changes in dopamine neurotransmission.

METHODS

The present study used an animal model of cocaine seeking to determine functional consequences of cue exposure using fluorodeoxyglucose positron emission tomography and to relate findings to juvenile levels of dopamine transporter and D₂-like receptor availabilities determined before any drug exposure. Adult male rhesus monkeys (N = 11) self-administered cocaine (0.2 mg/kg per injection) under a second-order schedule of reinforcement, in which responding was maintained by conditioned reinforcers. Positron emission tomography scans assessing glucose utilization, a marker of functional activation, were conducted during cocaine-cue responding and food-reinforced responding in a context where cocaine was never available.

RESULTS

Compared with the noncocaine condition, we found significant functional activation in the medial prefrontal cortex, anterior cingulate, precuneus region of the parietal cortex, and striatum-findings similar to those reported in humans who abuse cocaine. Furthermore, these functional activations in the prefrontal, cingulate, and parietal cortex measured during cocaine-cue responding were significantly correlated with juvenile measures of dopamine transporter availability, whereas no significant relationship with prior D₂-like receptor availability was observed in any brain region.

CONCLUSIONS

The similarity between the present findings and findings in humans who use cocaine supports the use of this model for examination of factors that affect the development and intensity of cue-induced drug seeking and provides evidence for potential biomarkers for the evaluation of potential treatments (behavioral and pharmacologic) for cocaine abuse.

Peer social interaction is facilitated in juvenile rhesus monkeys treated with fluoxetine

Fluoxetine improves social interactions in children with autism, social anxiety and social phobia. It is not known whether this effect is mediated directly or indirectly by correcting the underlying pathology. Genetics may also influence the drug effect. Polymorphisms of the MAOA (monoamine oxidase A) gene interact with fluoxetine to influence metabolic profiles in juvenile monkeys. Juvenile nonhuman primates provide an appropriate model for studying fluoxetine effects and drug*gene interactions in children. Male rhesus monkeys 1-3 years of age living in permanent social pairs were treated daily with a therapeutic dose of fluoxetine or vehicle (n = 16/group). Both members of each social pair were assigned to the same treatment group. They were observed for social interactions with their familiar cagemate over a 2-year dosing period. Subjects were genotyped for MAOA variable number of tandem repeats (VNTR) polymorphisms categorized for high or low transcription rates (hi-MAOA, low-MAOA). Fluoxetine-treated animals spent 30% more time in social interaction than vehicle controls. Fluoxetine significantly increased the duration of quiet interactions, the most common type of interaction, and also of immature sexual behavior typical of rhesus in this age group. Specific behaviors affected depended on MAOA genotype of the animal and its social partner. When given fluoxetine, hi-MOAO monkeys had more social invitation and initiation behaviors and low-MAOA subjects with low-MAOA partners had more grooming and an increased frequency of some facial and vocal expressive behaviors. Fluoxetine may facilitate social interaction in children independent of remediation of psychopathology. Common genetic variants may modify this effect.

Exposure to methylphenidate during infancy and adolescence in non-human animals and sensitization to abuse of psychostimulants later in life: a systematic review

INTRODUCTION

Attention deficit hyperactivity disorder (ADHD) is a neuropsychiatric pathology that has an important prevalence among young people and is difficult to diagnose. It is usually treated with methylphenidate, a psychostimulant with a mechanism of action similar to that of cocaine. Previous studies show that repeated use of psychostimulants during childhood or adolescence may sensitize subjects, making them more prone to later abuse of psychostimulant drugs such as cocaine and methamphetamine.

OBJECTIVE

To review experimental studies in non-human models (rodents and monkeys) treated with methylphenidate during infancy or adolescence and tested for reinforcing effects on psychostimulant drugs in adulthood.

METHOD

Systematic collection of data was performed on four databases (Web of Knowledge, PsycARTICLE, PubMed and SciELO). The initial search identified 202 articles published from 2009 to 2014, which were screened for eligibility. Seven articles met the inclusion criteria and were reviewed in this study.

RESULTS

The findings indicate that early exposure to methylphenidate has an effect on an ADHD animal model, specifically, on spontaneously hypertensive strain rats, especially those tested using the self-administration paradigm.

CONCLUSION

Future studies should prioritize the spontaneously hypertensive rat strain - an animal model of ADHD. Experimental designs comparing different behavioral paradigms and modes of administration using this strain could lead to improved understanding of the effects of exposure to methylphenidate during childhood and adolescence.

Dissociable rate-dependent effects of oral methylphenidate on impulsivity and D2/3 receptor availability in the striatum

We have previously shown that impulsivity in rats is linked to decreased dopamine D2/3 receptor availability in the ventral striatum. In the present study, we investigated, using longitudinal positron emission tomography (PET), the effects of orally administered methylphenidate (MPH), a first-line treatment for attention deficit hyperactivity disorder, on D2/3 receptor availability in the dorsal and ventral striatum and related these changes to impulsivity. Rats were screened for impulsive behavior on a five-choice serial reaction time task. After a baseline PET scan with the D2/3 ligand [(18)F]fallypride, rats received 6 mg/kg MPH, orally, twice each day for 28 d. Rats were then reassessed for impulsivity and underwent a second [(18)F]fallypride PET scan. Before MPH treatment, we found that D2/3 receptor availability was significantly decreased in the left but not the right ventral striatum of high-impulse (HI) rats compared with low-impulse (LI) rats. MPH treatment increased impulsivity in LI rats, and modulated impulsivity and D2/3 receptor availability in the dorsal and ventral striatum of HI rats through inverse relationships with baseline levels of impulsivity and D2/3 receptor availability, respectively. However, we found no relationship between the effects of MPH on impulsivity and D2/3 receptor availability in any of the striatal subregions investigated. These findings indicate that trait-like impulsivity is associated with decreased D2/3 receptor availability in the left ventral striatum, and that stimulant drugs modulate impulsivity and striatal D2/3 receptor availability through independent mechanisms.

A preliminary study on methylphenidate-regulated gene expression in lymphoblastoid cells of ADHD patients

OBJECTIVES

Methylphenidate (MPH) is a commonly used stimulant medication for treating attention-deficit/hyperactivity disorder (ADHD). Besides inhibiting monoamine reuptake there is evidence that MPH also influences gene expression directly.

METHODS

We investigated the impact of MPH treatment on gene expression levels of lymphoblastoid cells derived from adult ADHD patients and healthy controls by hypothesis-free, genome-wide microarray analysis. Significant findings were subsequently confirmed by quantitative Real-Time PCR (qRT PCR) analysis.

RESULTS

The microarray analysis from pooled samples after correction for multiple testing revealed 138 genes to be marginally significantly regulated due to MPH treatment, and one gene due to diagnosis. By qRT PCR we could confirm that GUCY1B3 expression was differential due to diagnosis. We verified chronic MPH treatment effects on the expression of ATXN1, HEY1, MAP3K8 and GLUT3 in controls as well as acute treatment effects on the expression of NAV2 and ATXN1 specifically in ADHD patients.

CONCLUSIONS

Our preliminary results demonstrate MPH treatment differences in ADHD patients and healthy controls in a peripheral primary cell model. Our results need to be replicated in larger samples and also using patient-derived neuronal cell models to validate the contribution of those genes to the pathophysiology of ADHD and mode of action of MPH.

Dopaminergic system dysfunction in recreational dexamphetamine users

Dexamphetamine (dAMPH) is a stimulant drug that is widely used recreationally as well as for the treatment of attention-deficit hyperactivity disorder (ADHD). Although animal studies have shown neurotoxic effects of dAMPH on the dopaminergic system, little is known about such effects on the human brain. Here, we studied the dopaminergic system at multiple physiological levels in recreational dAMPH users and age, gender, and IQ-matched dAMPH-naïve healthy controls. We assessed baseline D2/3 receptor availability, in addition to changes in dopamine (DA) release using single-photon emission computed tomography and DA functionality using pharmacological magnetic resonance imaging, following a dAMPH challenge. Also, the subjective responses to the challenge were determined. dAMPH users displayed significantly lower striatal DA D2/3 receptor binding compared with healthy controls. In dAMPH users, we further observed a blunted DA release and DA functionality to an acute dAMPH challenge, as well as a blunted subjective response. Finally, the lower D2/3 availability, the more pleasant the dAMPH administration was experienced by control subjects, but not by dAMPH users. Thus, in agreement with preclinical studies, we show that the recreational use of dAMPH in human subjects is associated with dopaminergic system dysfunction. These findings warrant further (longitudinal) investigations and call for caution when using this drug recreationally and for ADHD.

Methylphenidate treatment beyond adolescence maintains increased cocaine self-administration in the spontaneously hypertensive rat model of attention deficit/hyperactivity disorder

Past research with the spontaneously hypertensive rat (SHR) model of attention deficit/hyperactivity disorder showed that adolescent methylphenidate treatment enhanced cocaine abuse risk in SHR during adulthood. The acquisition of cocaine self-administration was faster, and cocaine dose-response functions were shifted upward under fixed-ratio and progressive ratio schedules compared to adult SHR that received adolescent vehicle treatment or to control strains that received adolescent methylphenidate treatment. The current study determined if extending treatment beyond adolescence would ameliorate long-term consequences of adolescent methylphenidate treatment on cocaine abuse risk in adult SHR. Treatments (vehicle or 1.5mg/kg/day oral methylphenidate) began on postnatal day 28. Groups of male SHR were treated with vehicle during adolescence and adulthood, with methylphenidate during adolescence and vehicle during adulthood, or with methylphenidate during adolescence and adulthood. The group receiving adolescent-only methylphenidate was switched to vehicle on P56. Cocaine self-administration began on postnatal day 77, and groups receiving methylphenidate during adolescence and adulthood were treated either 1-h before or 1-h after daily sessions. At baseline under a fixed-ratio 1 schedule, cocaine self-administration (2h sessions; 0.3mg/kg unit dose) did not differ among the four treatment groups. Under a progressive ratio schedule (4.5h maximum session length; 0.01-1.0mg/kg unit doses), breakpoints for self-administered cocaine in SHR receiving the adult methylphenidate treatment 1-h pre-session were not different from the vehicle control group. However, compared to the vehicle control group, breakpoints for self-administered cocaine at the 0.3 and 1.0mg/kg unit doses were greater in adult SHR that received adolescent-only methylphenidate or received methylphenidate that was continued into adulthood and administered 1-h post-session. These findings suggest that extending methylphenidate treatment beyond adolescence does not ameliorate explicitly the long-term consequences of adolescent methylphenidate treatment. Pre-session methylphenidate may mask temporarily the detection of an increase in cocaine self-administration following chronic methylphenidate treatment.

Prior methylphenidate self-administration alters the subsequent reinforcing effects of methamphetamine in rats

Methylphenidate (MPD) is clinically effective in treating the symptoms of attention-deficit hyperactivity disorder; however, its relatively widespread availability has raised public health concerns on nonmedical use of MPD among certain adult populations. Most preclinical studies investigate whether presumed therapeutically relevant doses of MPD alter sensitivity to the reinforcing effects of other drugs, but it remains unclear whether doses of MPD likely exceeding therapeutic relevance impact the subsequent reinforcing effects of drugs. To begin to address this question, the effect of prior MPD self-administration (0.56 mg/kg/infusion) on the subsequent reinforcing effects of methamphetamine (METH, 0.032 or 0.1 mg/kg/infusion) was investigated in male Sprague-Dawley rats. For comparison, it was also determined whether prior experimenter-administered MPD, injected daily at a presumed therapeutically relevant dose (2 mg/kg), altered the subsequent reinforcing effects of METH. Results indicated that, under the current conditions, only a history of MPD self-administration increased sensitivity to the subsequent reinforcing effects of METH. Furthermore, MPD did not impact food-maintained responding, suggesting that the effect of MPD might be specific to drug reinforcers. These data suggest that short-term, nonmedical use of MPD might alter the positive reinforcing effects of METH in a manner relevant to vulnerability to drug use in humans.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

PET studies in nonhuman primate models of cocaine abuse: translational research related to vulnerability and neuroadaptations

The current review highlights the utility of positron emission tomography (PET) imaging to study the neurobiological substrates underlying vulnerability to cocaine addiction and subsequent adaptations following chronic cocaine self-administration in nonhuman primate models of cocaine abuse. Environmental (e.g., social rank) and sex-specific influences on dopaminergic function and sensitivity to the reinforcing effects of cocaine are discussed. Cocaine-related cognitive deficits have been hypothesized to contribute to high rates of relapse and are described in nonhuman primate models. Lastly, the long-term consequences of cocaine on neurobiology are discussed. PET imaging and longitudinal, within-subject behavioral studies in nonhuman primates have provided a strong framework for designing pharmacological and behavioral treatment strategies to aid drug-dependent treatment seekers. Non-invasive PET imaging will allow for individualized treatment strategies. Recent advances in radiochemistry of novel PET ligands and other imaging modalities can further advance our understanding of stimulant use on the brain. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Chronic methylphenidate exposure during adolescence reduces striatal synaptic responses to ethanol

Dopamine (DA) plays an important role in integrative functions contributing to adaptive behaviors. In support of this essential function, DA modulates synaptic plasticity in different brain areas, including the striatum. Many drugs used for cognitive enhancement are psychostimulants, such as methylphenidate (MPH), which enhance DA levels. MPH treatment is of interest during adolescence, a period of enhanced neurodevelopment during which the DA system is in a state of flux. Recent epidemiological studies report the co-abuse of MPH and ethanol in adolescents and young adults. Although repeated MPH treatment produces enduring changes that affect subsequent behavioral responses to other psychostimulants, few studies have investigated the interactions between MPH and ethanol. Here we addressed whether chronic therapeutic exposure to MPH during adolescence predisposed mice to an altered response to ethanol and whether this was accompanied by altered DA release and striatal plasticity. C57BL/6J mice were administered MPH (3-6 mg/kg/day) via the drinking water between post-natal days 30 and 60. Voltammetry experiments showed that sufficient brain MPH concentrations were achieved during adolescence in mice to increase the DA clearance in adulthood. The treatment also increased long-term depression and reduced the effects of ethanol on striatal synaptic responses. Although the injection of 0.4 or 2 g/kg ethanol dose-dependently decreased locomotion in control mice, only the higher dose decreased locomotion in MPH-treated mice. These results suggested that the administration of MPH during development promoted long-term effects on synaptic plasticity in forebrain regions targeted by DA. These changes in plasticity might, in turn, underlie alterations in behaviors controlled by these brain regions into adulthood.

Psychostimulants and brain dysfunction: a review of the relevant neurotoxic effects

Psychostimulants abuse is a major public concern because is associated with serious health complications, including devastating consequences on the central nervous system (CNS). The neurotoxic effects of these drugs have been extensively studied. Nevertheless, numerous questions and uncertainties remain in our understanding of these toxic events. Thus, the purpose of the present manuscript is to review cellular and molecular mechanisms that might be responsible for brain dysfunction induced by psychostimulants. Topics reviewed include some classical aspects of neurotoxicity, such as monoaminergic system and mitochondrial dysfunction, oxidative stress, excitotoxicity and hyperthermia. Moreover, recent literature has suggested new phenomena regarding the toxic effects of psychostimulants. Thus, we also reviewed the impact of these drugs on neuroinflammatory response, blood-brain barrier (BBB) function and neurogenesis. Assessing the relative importance of these mechanisms on psychostimulants-induced brain dysfunction presents an exciting challenge for future research efforts. This article is part of the Special Issue entitled 'CNS Stimulants'.

Long-term oral methylphenidate treatment in adolescent and adult rats: differential effects on brain morphology and function

Methylphenidate is a widely prescribed psychostimulant for treatment of attention deficit hyperactivity disorder (ADHD) in children and adolescents, which raises questions regarding its potential interference with the developing brain. In the present study, we investigated effects of 3 weeks oral methylphenidate (5 mg/kg) vs vehicle treatment on brain structure and function in adolescent (post-natal day [P]25) and adult (P65) rats. Following a 1-week washout period, we used multimodal magnetic resonance imaging (MRI) to assess effects of age and treatment on independent component analysis-based functional connectivity (resting-state functional MRI), D-amphetamine-induced neural activation responses (pharmacological MRI), gray and white matter tissue volumes and cortical thickness (postmortem structural MRI), and white matter structural integrity (postmortem diffusion tensor imaging (DTI)). Many age-related differences were found, including cortical thinning, white matter development, larger dopamine-mediated activation responses and increased striatal functional connectivity. Methylphenidate reduced anterior cingulate cortical network strength in both adolescents and adults. In contrast to clinical observations from ADHD patient studies, methylphenidate did not increase white matter tissue volume or cortical thickness in rat. Nevertheless, DTI-based fractional anisotropy was higher in the anterior part of the corpus callosum following adolescent treatment. Furthermore, methylphenidate differentially affected adolescents and adults as evidenced by reduced striatal volume and myelination upon adolescent treatment, although we did not observe adverse treatment effects on striatal functional activity. Our findings of small but significant age-dependent effects of psychostimulant treatment in the striatum of healthy rats highlights the importance of further research in children and adolescents that are exposed to methylphenidate.

A positron emission tomography study of nigro-striatal dopaminergic mechanisms underlying attention: implications for ADHD and its treatment

Through the combined use of ¹⁸F-fallypride positron emission tomography and magnetic resonance imaging this study examined the neural mechanisms underlying the attentional deficits associated with attention deficit/hyperactivity disorder and their potential reversal with a single therapeutic dose of methylphenidate. Sixteen adult patients with attention deficit/hyperactivity disorder and 16 matched healthy control subjects were positron emission tomography and magnetic resonance imaging scanned and tested on a computerized sustained attention task after oral methylphenidate (0.5 mg/kg) and placebo administration in a within-subject, double-blind, cross-over design. Although patients with attention deficit/hyperactivity disorder as a group showed significant attentional deficits and reduced grey matter volume in fronto-striato-cerebellar and limbic networks, they had equivalent D₂/D₃ receptor availability and equivalent increases in endogenous dopamine after methylphenidate treatment to that observed in healthy control subjects. However, poor attentional performers drawn from both the attention deficit/hyperactivity disorder and the control groups had significantly reduced left caudate dopamine activity. Methylphenidate significantly increased dopamine levels in all nigro-striatal regions, thereby normalizing dopamine levels in the left caudate in low performers. Behaviourally, methylphenidate improved sustained attention in a baseline performance-dependent manner, irrespective of diagnosis. This finding was accompanied by an equally performance-dependent effect of the drug on dopamine release in the midbrain, whereby low performers showed reduced dopamine release in this region. Collectively, these findings support a dimensional model of attentional deficits and underlying nigro-striatal dopaminergic mechanisms of attention deficit/hyperactivity disorder that extends into the healthy population. Moreover, they confer midbrain dopamine autoreceptors a hitherto neglected role in the therapeutic effects of oral methylphenidate in attention deficit/hyperactivity disorder. The absence of significant case–control differences in D₂/D₃ receptor availability (despite the observed relationships between dopamine activity and attention) suggests that dopamine dysregulation per se is unlikely to be the primary cause underlying attention deficit/hyperactivity disorder pathology in adults. This conclusion is reinforced by evidence of neuroanatomical changes in the same set of patients with attention deficit/hyperactivity disorder.

Adolescent atomoxetine treatment in a rodent model of ADHD: effects on cocaine self-administration and dopamine transporters in frontostriatal regions

Cocaine abuse and attention deficit/hyperactivity disorder (ADHD) are often comorbid. Preclinical research indicates that medial prefrontal (mPFC) and orbitofrontal (OFC) cortices are important neural substrates for both disorders. Using the spontaneously hypertensive rat (SHR) model of ADHD, we reported that adolescent treatment with the stimulant methylphenidate, a dopamine (DAT) and norepinephrine (NET) transporter inhibitor, enhanced cocaine self-administration during adulthood, and was associated with increased DAT function in mPFC. This study investigates the effects of atomoxetine ((R)-N-methyl-γ-(2-methylphenoxy)-benzenepropanamine hydrochloride) treatment, a selective NET inhibitor, during adolescence on cocaine self-administration and on DAT function and cell-surface expression in mPFC and OFC during adulthood. SHR acquired cocaine self-administration faster than Wistar-Kyoto and Wistar. Across cocaine doses, SHR earned more cocaine infusions and had higher progressive-ratio breakpoints than Wistar-Kyoto and Wistar, demonstrating that the SHR phenotype models comorbid ADHD and cocaine abuse. Prior atomoxetine treatment did not augment cocaine self-administration in SHR, but acquisition was enhanced in Wistar-Kyoto. No strain differences were found for DAT kinetic parameters or cellular localization in the vehicle controls. Atomoxetine did not alter DAT kinetic parameters or localization in SHR mPFC. Rather, atomoxetine decreased V(max) and DAT cell surface expression in SHR OFC, indicating that inhibition of NET by atomoxetine treatment during adolescence indirectly reduced DAT function and trafficking to the cell surface in OFC, specifically in the ADHD model. Thus, atomoxetine, unlike methylphenidate, does not enhance vulnerability to cocaine abuse in SHR and may represent an important alternative for teens with ADHD when drug addiction is a concern.

Effects of chronic methylphenidate in adolescence on later methylphenidate self-administration in rhesus monkeys

Many children diagnosed with attention deficit hyperactivity disorder are treated with methylphenidate (MPH), despite limited information on later vulnerability to drug abuse. A previous study in adolescent monkeys treated with MPH for 1 year did not indicate differences in acquisition to cocaine reinforcement compared with controls. The present study extended this characterization to include MPH self-administration. Adolescent male rhesus monkeys treated previously with a sustained-release formulation of MPH (beginning at ∼30 months old) and control monkeys (n=8/group) were used. All had previous experience of self-administering cocaine under a fixed-ratio 30 schedule of reinforcement. Responding was maintained by food (1.0-g banana-flavored pellets) and MPH (saline, 0.001-0.1 mg/kg/injection) was substituted for food for at least five consecutive sessions. MPH functioned as a reinforcer in all monkeys; there were no differences between groups in MPH self-administration. These findings extend earlier research with cocaine reinforcement showing that MPH treatment in adolescent monkeys does not increase future reinforcing effects of stimulant drugs.

Functional genomics of attention-deficit/hyperactivity disorder (ADHD) risk alleles on dopamine transporter binding in ADHD and healthy control subjects

BACKGROUND

The main aim of this study was to examine the relationship between dopamine transporter (DAT) binding in the striatum in individuals with and without attention-deficit/hyperactivity disorder (ADHD), attending to the 3'-untranslated region of the gene (3'-UTR) and intron8 variable number of tandem repeats (VNTR) polymorphisms of the DAT (SLC6A3) gene.

METHODS

Subjects consisted of 68 psychotropic (including stimulant)-naïve and smoking-naïve volunteers between 18 and 55 years of age (ADHD n = 34; control subjects n = 34). Striatal DAT binding was measured with positron emission tomography with 11C altropane. Genotyping of the two DAT (SLC6A3) 3'-UTR and intron8 VNTRs used standard protocols.

RESULTS

The gene frequencies of each of the gene polymorphisms assessed did not differ between the ADHD and control groups. The ADHD status (t = 2.99; p<.004) and 3'-UTR of SLC6A3 9 repeat carrier status (t = 2.74; p<.008) were independently and additively associated with increased DAT binding in the caudate. The ADHD status was associated with increased striatal (caudate) DAT binding regardless of 3'-UTR genotype, and 3'-UTR genotype was associated with increased striatal (caudate) DAT binding regardless of ADHD status. In contrast, there were no significant associations between polymorphisms of DAT intron8 or the 3'-UTR-intron8 haplotype with DAT binding.

CONCLUSIONS

The 3'-UTR but not intron8 VNTR genotypes were associated with increased DAT binding in both ADHD patients and healthy control subjects. Both ADHD status and the 3'-UTR polymorphism status had an additive effect on DAT binding. Our findings suggest that an ADHD risk polymorphism (3'-UTR) of SLC6A3 has functional consequences on central nervous system DAT binding in humans.

The effects of rearing environment and chronic methylphenidate administration on behavior and dopamine receptors in adolescent rats

Rearing young rodents in socially isolated or environmentally enriched conditions has been shown to affect numerous components of the dopamine system as well as behavior. Methylphenidate (MPH), a commonly used dopaminergic agent, may affect animals differently based on rearing environment. Here we examined the interaction between environment and chronic MPH treatment at clinically relevant doses, administered via osmotic minipump. Young Sprague Dawley rats (PND 21) were assigned to environmentally enriched, pair-housed, or socially isolated rearing conditions, and treated with either 0, 2, 4, or 8 mg/kg/day MPH for 3 weeks. At the end of the treatment period, animals were tested for locomotor activity and anxiety-like behavior. The densities of D1-like and D2-like receptors were measured in the striatum using in vitro receptor autoradiography. Locomotor activity and anxiety-like behavior were increased in isolated animals compared to pair-housed and enriched animals. The density of D1-like receptors was greater in isolated animals, but there were no differences between groups in D2-like receptor density. Finally, there were no effects of MPH administration on any reported measure. This study provides evidence for an effect of early rearing environment on the dopamine system and behavior, and also suggests that MPH administration may not have long-term consequences.

Is the treatment with psychostimulants in children and adolescents with attention deficit hyperactivity disorder harmful for the dopaminergic system?

A major concern regarding psychostimulant medication (amphetamine and methylphenidate) in the treatment of children and adolescents with attention deficit/hyperactivity disorder (ADHD) are the potential adverse effects to the developing brain, particularly in regard to dopaminergic brain function. The present review focuses on the pharmacology of these psychostimulants, their mode of action in the human brain and their potential neurotoxic effects to the developing brain in animals, particularly concerning DA brain function. The potential clinical significance of these findings for the treatment of ADHD in children and adolescents is discussed. Studies on sensitization to psychostimulants' rewarding effects, which is a process expected to increase the risk of substance abuse in humans, are not included. The available findings in non-human primates support the notion that the administration of amphetamine and methylphenidate with procedures simulating clinical treatment conditions does not lead to long-term adverse effects in regard to development, neurobiology or behaviour as related to the central dopaminergic system.

Effects of chronic methylphenidate on cocaine self-administration under a progressive-ratio schedule of reinforcement in rhesus monkeys

It has been hypothesized that drugs that serve as substrates for dopamine (DA) and norepinephrine (NE) transporters may be more suitable medications for cocaine dependence than drugs that inhibit DA and NE uptake by binding to transporters. Previous studies have shown that the DA/NE releaser d-amphetamine can decrease cocaine self-administration in preclinical and clinical studies. The present study examined the effects of methylphenidate (MPD), a DA uptake inhibitor, for its ability to decrease cocaine self-administration under conditions designed to reflect clinically relevant regimens of cocaine exposure and pharmacotherapy. Each morning, rhesus monkeys pressed a lever to receive food pellets under a fixed-ratio 50 schedule of reinforcement; cocaine was self-administered under a progressive-ratio schedule of reinforcement in the evening. After cocaine (0.003-0.56 mg/kg per injection, i.v.) dose-response curves were determined, self-administration sessions were suspended and MPD (0.003-0.0056 mg/kg per hour, i.v.; or 1.0-9.0 mg/kg p.o., b.i.d.) was administered for several weeks. A cocaine self-administration session was conducted every 7 days. When a MPD dose was reached that either persistently decreased cocaine self-administration or produced disruptive effects, the cocaine dose-effect curve was re-determined. In most cases, MPD treatment either produced behaviorally disruptive effects or increased cocaine self-administration; it took several weeks for these effects to dissipate. These data are consistent with the largely negative results of clinical trials with MPD. In contrast to the positive effects with the monoamine releaser d-amphetamine under identical conditions, these results do not support use of monoamine uptake inhibitors like MPD as a medication for cocaine dependence.

Methylphenidate amplifies the potency and reinforcing effects of amphetamines by increasing dopamine transporter expression

Methylphenidate (MPH) is commonly diverted for recreational use, but the neurobiological consequences of exposure to MPH at high, abused doses are not well defined. Here we show that MPH self-administration in rats increases dopamine transporter (DAT) levels and enhances the potency of MPH and amphetamine on dopamine responses and drug-seeking behaviours, without altering cocaine effects. Genetic overexpression of the DAT in mice mimics these effects, confirming that MPH self-administration-induced increases in DAT levels are sufficient to induce the changes. Further, this work outlines a basic mechanism by which increases in DAT levels, regardless of how they occur, are capable of increasing the rewarding and reinforcing effects of select psychostimulant drugs, and suggests that individuals with elevated DAT levels, such as ADHD sufferers, may be more susceptible to the addictive effects of amphetamine-like drugs.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.