1H MRS-detectable metabolic brain changes and reduced impulsive behavior in adult rats exposed to methylphenidate during adolescence

The impact of methylphenidate on choice impulsivity is inversely associated with corpus callosum fiber integrity across sexes

BACKGROUND

Choice impulsivity represents preference towards smaller immediate rewards over larger delayed rewards. Extensive literature demonstrates that choice impulsivity can be manipulated using dopaminergic agonists such as methylphenidate (MPH), and that females exhibit elevated choice impulsivity compared to males. Sex differences are also frequently reported with respect to brain white matter (WM) fiber integrity. It has yet to be determined whether sex differences also exist in the impact of MPH on choice impulsivity, and whether these putative differences are accounted for by the integrity of differential WM fibers.

METHODS

Forty-eight healthy young adults completed the delay discounting (DD) task twice during MRI-DTI scans after receiving either MPH or placebo in a double-blind, placebo-controlled, within-subject design. WM fiber integrity was assessed using automated fiber quantification (AFQ) and tract-based spatial statistics (TBSS).

RESULTS

Compared to placebo, MPH yielded significantly reduced choice impulsivity in males but not in females. DTI data revealed reduced integrity in multiple WM fibers in females compared to males. Interestingly, the impact of MPH on choice impulsivity was negatively associated with fiber integrity in the forceps major of the corpus callosum for males only and positively associated with fiber integrity in the forceps minor of the corpus callosum for females only.

CONCLUSIONS

Taken together, results uncover sex-specific effects of MPH on choice impulsivity, accounted for by inverse associations between choice impulsivity under MPH and the structural integrity of distinct segments of the corpus callosum. These findings highlight the need to consider sex differences in the neurobiological mechanisms of impulsivity.

Contrasting effects of DOI and lisuride on impulsive decision-making in delay discounting task

RATIONALE

The 5-HT2A receptor is the major target of classic hallucinogens. Both DOI (2,5-dimethoxy-4-iodoamphetamine) and lisuride act at 5-HT2A receptors, and lisuride shares comparable affinity with DOI and acts as a partial agonist at 5-HT2A receptors. However, not like DOI, lisuride lacks hallucinogenic properties. Impulsive decision-making refers to the preference for an immediate small reinforcer (SR) over a delayed large reinforcer (LR).

OBJECTIVES

The current study aims to compare the effects of DOI and lisuride on impulsive decision-making and further to investigate the possible receptor mechanisms responsible for the actions of the two drugs.

METHODS

Impulsive decision-making was evaluated in male Sprague-Dawley rats by the percentage of choice for the LR in delay discounting task (DDT). Delay to the LR changed in an ascending order (0, 4, 8, 16, and 32 s) across one session.

RESULTS

DOI (0.5 and 1.0 mg/kg) increased impulsive decision-making, and the effects of DOI (1.0 mg/kg) were blocked by the 5-HT2A receptor antagonist ketanserin (1.0 mg/kg) rather than the 5-HT2C receptor antagonist SB-242084 (1.0 mg/kg). Contrarily, lisuride (0.1, 0.3, and 0.5 mg/kg) decreased impulsive decision-making. The effects of lisuride (0.3 mg/kg) were not antagonized by ketanserin (1.0 mg/kg), selective 5-HT1A antagonist WAY-100635 (1.0 mg/kg), or selective dopamine D4 receptor antagonist L-745870 (1.0 mg/kg) but were attenuated by the selective dopamine D2/D3 receptor antagonist tiapride (40 mg/kg).

CONCLUSIONS

DOI and lisuride have contrasting effects on impulsive decision-making via distinct receptors. DOI-induced increase of impulsivity is mediated by the 5-HT2A receptor, while lisuride-induced inhibition of impulsivity is regulated by the dopamine D2/D3 receptor.

Prepubertal methylphenidate leads to sex-dependent differences in probabilistic discounting

Prescription psychostimulants, such as methylphenidate (MPH), have served as a first line treatment for ADHD and associated developmental disorders since 1961. Psychostimulants has been shown to improve attention, response inhibition, and reduce hyperactivity in patients with ADHD, as well as in non-clinical human populations and animals. While there is a considerable amount of preclinical research investigating the effects of stimulant medications on reward sensitivity and basic learning in male rats, less is understood about their effects in females. Further, there are competing theories on the long-term cognitive impact of MPH, specifically in children who do not have ADHD. To this end, Long-Evans female and male rats were exposed to methylphenidate (0, 2.5, 5 mg/kg, BID, IP) for 20 days during early development (PD10-29). After discontinuation of MPH into adulthood, rats (beginning PD 60) were trained and tested for risk-preference using a 2-choice probabilistic discounting task. For this task, rats were given an option between a 'large-risky' choice (3 sugar pellets delivered on a probabilistic VR schedule) and 'small-certain' choice (1 sugar pellet delivered on a FR schedule). Rats were subsequently tested on an open field conflict test. The results demonstrate that prepubertal exposure to MPH can have lasting effects on decision-making. Specifically, female rats treated with 2.5 mg/kg MPH displayed a decrease in preference for the risky option, whereas male rats treated with the same dose showed an overall increase in preference compared to sex-matched controls. Irrespective of sex, rats treated with 2.5 mg/kg MPH also demonstrated a decrease in anxiety/inhibitory behavior on the modified open field test compared to controls. These results were not due to differences in locomotor behavior. Overall, the study contributes to the growing body of evidence to suggest that MPH exposure early in development can have a sex-dependent impact on decision-making in adulthood.

Age differences in brain structural and metabolic responses to binge ethanol exposure in fisher 344 rats

An overarching goal of our research has been to develop a valid animal model of alcoholism with similar imaging phenotypes as those observed in humans with the ultimate objective of assessing the effectiveness of pharmacological agents. In contrast to our findings in humans with alcohol use disorders (AUD), our animal model experiments have not demonstrated enduring brain pathology despite chronic, high ethanol (EtOH) exposure protocols. Relative to healthy controls, older individuals with AUD demonstrate accelerating brain tissue loss with advanced age. Thus, this longitudinally controlled study was conducted in 4-month old (equivalent to ~16-year-old humans) and 17-month old (equivalent to ~45-year-old humans) male and female Fisher 344 rats to test the hypothesis that following equivalent alcohol exposure protocols, older relative to younger animals would exhibit more brain changes as evaluated using in vivo structural magnetic resonance imaging (MRI) and MR spectroscopy (MRS). At baseline, total brain volume as well as the volumes of each of the three constituent tissue types (i.e., cerebral spinal fluid (CSF), gray matter, white matter) were greater in old relative to young rats. Baseline metabolite levels (except for glutathione) were higher in older than younger animals. Effects of binge EtOH exposure on brain volumes and neurometabolites replicated our previous findings in Wistar rats and included ventricular enlargement and reduced MRS-derived creatine levels. Brain changes in response to binge EtOH treatment were more pronounced in young relative to older animals, negating our hypothesis. Higher baseline glutathione levels in female than male rats suggest that female rats are perhaps protected against the more pronounced changes in CSF and gray matter volumes observed in male rats due to superior metabolic homeostasis mechanisms. Additional metabolite changes including low inositol levels in response to high blood alcohol levels support a mechanism of reversible osmolarity disturbances due to temporarily altered brain energy metabolism.

Juvenile exposure to methylphenidate and guanfacine in rats: effects on early delay discounting and later cocaine-taking behavior

RATIONALE

Both methylphenidate (MPH), a catecholamine reuptake blocker, and guanfacine, an alpha2A agonist, are used to treat attention-deficit hyperactivity disorder (ADHD). Childhood impulsivity, including delay discounting, is associated with increased substance use during adolescence. These effects can be mitigated by juvenile exposure to MPH, but less is known about the long-term effects of developmental exposure to guanfacine in males and females.

OBJECTIVE

This study aims to determine sex differences and dose-dependent effects of juvenile exposure to MPH or guanfacine on delay-discounting and later cocaine self-administration.

METHODS

The dose-dependent effects of vehicle, MPH (0.5, 1, and 2 mg/kg p.o.) or guanfacine (0.003, 0.03, and 0.3 mg/kg, i.p.) on discounting were determined in male and female Sprague-Dawley rats beginning at postnatal day (P)20. At P90, the amount, motivation, and sensitivity to cocaine following early drug exposure were determined with self-administration.

RESULTS

Guanfacine, but not MPH, significantly reduced weight by 22.9 ± 4.6% in females. MPH dose dependently decreased delay discounting in both juvenile males and females, while guanfacine was only effective in males. Discounting was associated with cocaine self-administration in vehicle males (R² = -0.4, P < 0.05) and self-administration was reduced by guanfacine treatment (0.3 mg/kg). Guanfacine significantly decreased cocaine sensitivity in both sexes.

CONCLUSIONS

These data suggest that MPH is effective in reducing delay discounting in both sexes. Due to both weight loss and ineffectiveness on discounting in females, guanfacine should be used only in males to reduce delay discounting and later cocaine use.

Inside the Developing Brain to Understand Teen Behavior From Rat Models: Metabolic, Structural, and Functional-Connectivity Alterations Among Limbic Structures Across Three Pre-adolescent Stages

Adolescence is an age of transition when most brain structures undergo drastic modifications, becoming progressively more interconnected and undergoing several changes from a metabolic and structural viewpoint. In the present study, three MR techniques are used in rats to investigate how metabolites, structures and patterns of connectivity do change. We focused in particular on areas belonging to the limbic system, across three post-weaning developmental stages: from "early" (PND 21-25) to "mid" (i.e., a juvenile transition, PND 28-32) and then to "late" (i.e., the adolescent transition, PND 35-39). The rs-fMRI data, with comparison between early and mid (juvenile transition) age-stage rats, highlights patterns of enhanced connectivity from both Striata to both Hippocampi and from there to (left-sided) Nucleus accumbens (NAcc) and Orbitofrontal Cortex (OFC). Also, during this week there is a maturation of pathways from right Striatum to ipsilateral NAcc, from right OFC to ipsilateral NAcc and vice versa, from left Prefrontal Cortex to ipsilateral OFC and eventually from left Striatum, NAcc and Prefrontal Cortex to contralateral OFC. After only 1 week, in late age-stage rats entering into adolescence, the first pathway mentioned above keeps on growing while other patterns appear: both NAcc are reached from contralateral Striatum, right Hippocampus from both Amygdalae, and left NAcc -further- from right Hippocampus. It's interesting to notice the fact that, independently from the age when these connections develop, Striata of both hemispheres send axons to both Hippocampi and both NAcc sides, both Hippocampi reach left NAcc and OFC and finally both NAcc sides reach right OFC. Intriguingly, the Striatum only indirectly reaches the OFC by passing through Hippocampus and NAcc. Data obtained with DTI highlight how adolescents' neurite density may be affected within sub-cortical gray matter, especially for NAcc and OFC at "late" age-stage (adolescence). Finally, levels of metabolites were investigated by 1H-MRS in the anterior part of the hippocampus: we put into evidence an increase in myo-inositol during juvenile transition and a taurine reduction plus a total choline increase during adolescent transition. In this paper, the aforementioned pattern guides the formulation of hypotheses concerning the correlation between the establishment of novel brain connections and the emergence of behavioral traits that are typical of adolescence.

The role of the dorsal striatum in choice impulsivity

It has long been recognized that the dorsal striatum is an essential brain region for control of action selection based on action-outcome contingency learning, particularly when the available actions are bound to rewarding outcomes. In principle, intertemporal choice in the delay-discounting task-a validated measure of choice impulsivity-involves reward-associated actions that require the recruitment of the dorsal striatum. Here, we conjecture about ways the dorsal striatum is involved in choice impulsivity. Based on a selective body of studies, we begin with a brief history of research on choice impulsivity and the dorsal striatum, and then provide a comprehensive summary of contemporary studies utilizing human neuroimaging and animal models to search for links between choice impulsivity and the dorsal striatum. In particular, we discuss in-depth the converging evidence for the associations of choice impulsivity with the reward valuation coded by the caudate, a ventral-to-dorsal gradient in the dorsal striatum, the origins of striatal afferents, and developmental maturation of frontostriatal connectivity during adolescence.

Adolescent Exposure to Methylphenidate Increases Impulsive Choice Later in Life

Background: The psychostimulant methylphenidate (MPH) is known to temporarily reduce impulsive choice and promote self-control. What is not sufficiently understood is how repeated treatment with MPH affects impulsive choice in the long run, and whether any such effect is contingent on exposure at certain developmental stages. Methods: Using an animal model for impulsive choice, we examined first whether giving MPH through early adolescence alters delay discounting, an operational measure of impulsive choice, later in adulthood. We then tested whether equivalent long-term effects are observed if exposure to the drug occurred during adulthood. Starting on postnatal day 25 or postnatal day 60, male rats received one of a range of doses of MPH for 10 consecutive days. Twenty-six days later, all rats were trained to choose between a lever that produced a small immediate reward and a lever that produced a large reward after a range of delays. Results: Rats showed a long-term decrease in the selection of the delayed larger reward when treated with moderate doses of MPH during early adolescence, but not when treated with the lower or higher doses. In contrast, no differences were observed in the selection of the delayed larger reward in animals that were treated with various doses of MPH during adulthood. Conclusions: Our findings suggest effects of MPH on impulsive choice that are contingent on dosage and on the developmental period of exposure. When administered during adolescence, moderate doses of MPH increase impulsive choice long after the end of treatment, whereas these same doses administered during adulthood were without effect.

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.

Acute and long-term effects of adolescent methylphenidate on decision-making and dopamine receptor mRNA expression in the orbitofrontal cortex

Though commonly used as a treatment for ADHD, the psychostimulant methylphenidate (MPH) is also misused and abused in adolescence in both clinical and general populations. Although MPH acts via pathways activated by other drugs of abuse, the short- and long-term effects of MPH on reward processing in learning and decision-making are not clearly understood. We examined the effect of adolescent MPH treatment on a battery of reward-directed behaviors both in adolescence during its administration and in adulthood after its discontinuation. We further measured whether MPH had lasting effects on dopamine receptor mRNA expression in orbitofrontal cortex (OFC) that may correspond with behavior. Long-Evans rats were injected with MPH (0, 1, 2.5, or 5mg/kg IP) twice daily from middle to late adolescence (PD38-57). During adolescence, the high dose of MPH reduced preference for large rewards in a Reward Magnitude Discrimination task, but did not affect preference for smaller-sooner rewards in a Delay Discounting task. In adulthood, after discontinuation of MPH, animals previously treated with the moderate dose of MPH showed improved acquisition, but not reversal, in a Reversal Learning task. MPH exposure did not increase preference for large-risky rewards in a Risk task in adulthood. We then quantified mRNA expression of D1, D2, and D3 receptors in the OFC using qPCR. MPH increased mRNA expression of dopamine D3 receptor subtype, but not D1 or D2. Overall, these results indicate that MPH has both immediate and lasting effects on reward-dependent learning and decisions, as well as dopaminergic function in rodents.

Adolescent methylphenidate treatment differentially alters adult impulsivity and hyperactivity in the Spontaneously Hypertensive Rat model of ADHD

Impulsivity and hyperactivity are two facets of attention deficit/hyperactivity disorder (ADHD). Impulsivity is expressed as reduced response inhibition capacity, an executive control mechanism that prevents premature execution of an intermittently reinforced behavior. During methylphenidate treatment, impulsivity and hyperactivity are decreased in adolescents with ADHD, but there is little information concerning levels of impulsivity and hyperactivity in adulthood after adolescent methylphenidate treatment is discontinued. The current study evaluated impulsivity, hyperactivity as well as cocaine sensitization during adulthood after adolescent methylphenidate treatment was discontinued in the Spontaneously Hypertensive Rat (SHR) model of ADHD. Treatments consisted of oral methylphenidate (1.5mg/kg) or water vehicle provided Monday-Friday from postnatal days 28-55. During adulthood, impulsivity was measured in SHR and control strains (Wistar Kyoto and Wistar rats) using differential reinforcement of low rate (DRL) schedules. Locomotor activity and cocaine sensitization were measured using the open-field assay. Adult SHR exhibited decreased efficiency of reinforcement under the DRL30 schedule and greater levels of locomotor activity and cocaine sensitization compared to control strains. Compared to vehicle, methylphenidate treatment during adolescence reduced hyperactivity in adult SHR, maintained the lower efficiency of reinforcement, and increased burst responding under DRL30. Cocaine sensitization was not altered following adolescent methylphenidate in adult SHR. In conclusion, adolescent treatment with methylphenidate followed by discontinuation in adulthood had a positive benefit by reducing hyperactivity in adult SHR rats; however, increased burst responding under DRL compared to SHR given vehicle, i.e., elevated impulsivity, constituted an adverse consequence associated with increased risk for cocaine abuse liability.

Deficient Purposeful Use of Forepaws in Female Mice Modelling Rett Syndrome

Rett syndrome (RTT) is a rare neurodevelopmental disorder, characterized by severe behavioural and physiological symptoms. Mutations in the methyl CpG binding protein 2 gene (MECP2) cause more than 95% of classic cases. Motor abnormalities represent a significant part of the spectrum of RTT symptoms. In the present study we investigated motor coordination and fine motor skill domains in MeCP2-308 female mice, a validated RTT model. This was complemented by the in vivo magnetic resonance spectroscopy (MRS) analysis of metabolic profile in behaviourally relevant brain areas. MeCP2-308 heterozygous female mice (Het, 10-12 months of age) were impaired in tasks validated for the assessment of purposeful and coordinated forepaw use (Morag test and Capellini handling task). A fine-grain analysis of spontaneous behaviour in the home-cage also revealed an abnormal handling pattern when interacting with the nesting material, reduced motivation to explore the environment, and increased time devoted to feeding in Het mice. The brain MRS evaluation highlighted decreased levels of bioenergetic metabolites in the striatal area in Het mice compared to controls. Present results confirm behavioural and brain alterations previously reported in MeCP2-308 males and identify novel endpoints on which the efficacy of innovative therapeutic strategies for RTT may be tested.

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.

MR imaging-detectable metabolic alterations in attention deficit/hyperactivity disorder: from preclinical to clinical studies

MR spectroscopy represents one of the most suitable in vivo tool to assess neurochemical dysfunction in several brain disorders, including attention deficit/hyperactivity disorder. This is the most common neuropsychiatric disorder in childhood and adolescence, which persists into adulthood (in approximately 30%-50% of cases). In past years, many studies have applied different MR spectroscopy techniques to investigate the pathogenesis and effect of conventional treatments. In this article, we review the most recent clinical and preclinical MR spectroscopy results on subjects with attention deficit/hyperactivity disorder and animal models, from childhood to adulthood. We found that the most investigated brain regions were the (pre)frontal lobes and striatum, both involved in the frontostriatal circuits and networks that are known to be impaired in this pathology. Neurometabolite alterations were detected in several regions: the NAA, choline, and glutamatergic compounds. The creatine pool was also altered when an absolute quantitative protocol was adopted. In particular, glutamate was increased in children with attention deficit/hyperactivity disorder, and this can apparently be reversed by methylphenidate treatment. The main difficulties in reviewing MR spectroscopy studies were in the nonhomogeneity of the analyzed subjects, the variety of the investigated brain regions, and also the use of different MR spectroscopy techniques. As for possible improvements in future studies, we recommend the use of standardized protocols and the analysis of other brain regions of particular interest for attention deficit hyperactivity disorder, like the hippocampus, limbic structures, thalamus, and cerebellum.

Paracetamol impairs the profile of amino acids in the rat brain

In our experiment we investigated the effect of subcutaneous administration of paracetamol on the levels of amino acids in the brain structures. Male Wistar rats received for eight weeks paracetamol at two doses: 10 mg/kg b.w. (group P10, n=9) and 50 mg/kg b.w. per day s.c. (group P50, n=9). The regional brain concentrations of amino acids were determined in the prefrontal cortex, hippocampus, hypothalamus and striatum of control (Con, n=9) and paracetamol-treated groups using HPLC. Evaluation of the biochemical results indicated considerable decrease of the content of amino acids in the striatum (glutamine, glutamic acid, taurine, alanine, aspartic acid) and hypothalamus (glycine) between groups treated with paracetamol compared to the control. In the prefrontal cortex paracetamol increased the level of γ-aminobutyric acid (GABA). The present study demonstrated significant effect of the long term paracetamol treatment on the level of amino acids in the striatum, prefrontal cortex and hypothalamus of rats.

Mapping pathological phenotypes in reelin mutant mice

Autism Spectrum Disorders (ASD) are neurodevelopmental disorders with multifactorial origin characterized by social communication deficits and the presence of repetitive behaviors/interests. Several studies showed an association between the reelin gene mutation and increased risk of ASD and a reduced reelin expression in some brain regions of ASD subjects, suggesting a role for reelin deficiency in ASD etiology. Reelin is a large extracellular matrix glycoprotein playing important roles during development of the central nervous system. To deeply investigate the role of reelin dysfunction as vulnerability factor in ASD, we assessed the behavioral, neurochemical, and brain morphological features of reeler male mice. We recently reported a genotype-dependent deviation in the ultrasonic vocal repertoire and a general delay in motor development of reeler pups. We now report that adult male heterozygous (Het) reeler mice did not show social behavior and communication deficits during male-female social interactions. Wildtype and Het mice showed a typical light/dark locomotor activity profile, with a peak during the central interval of the dark phase. However, when faced with a mild stressful stimulus (a saline injection) only Het mice showed an over response to stress. In addition to the behavioral studies, we conducted high performance liquid chromatography and magnetic resonance imaging and spectroscopy to investigate whether reelin mutation influences brain monoamine and metabolites levels in regions involved in ASD. Low levels of dopamine in cortex and high levels of glutamate and taurine in hippocampus were detected in Het mice, in line with clinical data collected on ASD children. Altogether, our data detected subtle but relevant neurochemical abnormalities in reeler mice supporting this mutant line, particularly male subjects, as a valid experimental model to estimate the contribution played by reelin deficiency in the global ASD neurobehavioral phenotype.

Emotional and risk seeking behavior after prepuberal subchronic or adult acute stimulation of 5-HT7-Rs in Naples High Excitability rats

We report here the results of studies aimed to investigate the involvement of serotonin receptor 7 subtype (5-HT7-R) in the modulation of emotional response in Naples High-Excitability (NHE) rat, a validated model for hyperactivity and impaired attention. A range of dosages (0.0, 0.125, 0.250, or 0.500 mg/kg) of LP-211, a selective agonist of 5-HT7-Rs, has been evaluated in animals at different age (adolescence and adulthood). Male NHE and random bred (NRB) control rats were tested in an Elevated Zero-Maze (EZM) after LP-211 treatment in two different regimens: at the issue of adolescent, subchronic exposure (14 intraperitoneal [i.p.] injections, once/day, pnd 31-44, tested on pnd 45--Exp. 1) or as adult, acute effect (15 min after i.p. injection--Exp. 2). Adolescent, subchronic LP-211 at 0.500 mg/kg dosage increased the frequency of head-dips only in NHE rats. Drug effect on time spent and entries in open EZM quadrants were revealed with adult, acute administration of 0.125 mg/kg LP-211 (both strains), indicating a tendency toward anxiolytic effects. In conclusion, data demonstrate that subchronic stimulation of 5-HT7-Rs during prepuberal period increases novelty-seeking/risk-taking propensity in NHE adults. These sequels are revealing increased disinhibition and/or motivation to explore in the NHE rats, which are characterized by a hyperactive dopaminergic system. These data may open new perspectives in studying mechanism of risk-seeking behavior.

Age-dependent effects of repeated amphetamine exposure on working memory in rats

Cognitive dysfunction is a hallmark of chronic psychostimulant misuse. Adolescents may have heightened risk of developing drug-induced deficits because their brains are already undergoing widespread changes in anatomy and function as a normal part of development. To address this hypothesis, we performed two sets of experiments where adolescent and young adult rats were pre-exposed to saline or amphetamine (1 or 3mg/kg) and subsequently tested in a prefrontal cortex (PFC)-sensitive working memory task. A total of ten injections of AMPH or saline (in control rats) were given every other day over the course of 19 days. After rats reached adulthood (>90 days old), cognitive performance was assessed using operant-based delayed matching-to-position (DMTP) and delayed nonmatching-to-position (DNMTP) tasks. DNMTP was also assessed following challenges with amphetamine (0.3-1.25mg/kg), and ketamine (5.0-10mg/kg). In experiment one, we also measured the locomotor response following the first and tenth pre-exposure to amphetamine and after an amphetamine challenge given at the conclusion of operant testing. Compared to adult-exposed groups, adolescents were less sensitive to the psychomotor effects of amphetamine. However, they were more vulnerable to exposure-induced cognitive impairments. For example, adolescent-exposed rats displayed delay-dependent deficits in accuracy, increased sensitivity to proactive interference, and required more training to reach criterion. Drug challenges produced deficits in DNMTP performance, but these were not dependent on pre-exposure group. These studies demonstrate age of exposure-dependent effects of amphetamine on cognition in a PFC-sensitive task, suggesting a heightened sensitivity of adolescents to amphetamine-induced neuroplasticity.

Stimulant drugs trigger transient volumetric changes in the human ventral striatum

The ventral striatum (VStr) integrates mesolimbic dopaminergic and corticolimbic glutamatergic afferents and forms an essential component of the neural circuitry regulating impulsive behaviour. This structure represents a primary target of psychostimulant medication, the first-choice treatment for attention-deficit/hyperactivity disorder (ADHD), and is biochemically modified by these drugs in animals. However, the effects of stimulants on the human VStr remain to be determined. We acquired anatomical brain MRI scans from 23 never-medicated adult patients with ADHD, 31 adult patients with a history of stimulant treatment and 32 control subjects, and VStr volumes were determined using individual rater-blinded region of interest delineation on high-resolution neuroanatomical scans. Furthermore, we also extracted VStr volumes before and after methylphenidate treatment in a subsample of the medication-naïve adult patients as well as in 20 never-medicated children with ADHD. We observed smaller VStr volumes in adult patients with a history of stimulant treatment in comparison to never-medicated patients. Moreover, our longitudinal analyses uncovered a reduction of grey matter volume in the bilateral VStr in adult patients after exposure to methylphenidate, which was followed by volumetric recovery to control level. In children, the same pattern of VStr volume changes was observed after treatment with methylphenidate. These findings suggest that the altered VStr volumes previously observed in patients with ADHD may represent a transitory effect of stimulant exposure rather than an intrinsic feature of the disorder. More generally, these data show that stimulant drugs can render plastic volume changes in human VStr neuroanatomy.

Long-term effects of chronic oral Ritalin administration on cognitive and neural development in adolescent wistar kyoto rats

The diagnosis of Attention Deficit Hyperactivity Disorder (ADHD) often results in chronic treatment with psychostimulants such as methylphenidate (MPH, Ritalin^®). With increases in misdiagnosis of ADHD, children may be inappropriately exposed to chronic psychostimulant treatment during development. The aim of this study was to assess the effect of chronic Ritalin treatment on cognitive and neural development in misdiagnosed “normal” (Wistar Kyoto, WKY) rats and in Spontaneously Hypertensive Rats (SHR), a model of ADHD. Adolescent male animals were treated for four weeks with oral Ritalin^® (2 × 2 mg/kg/day) or distilled water (dH₂O). The effect of chronic treatment on delayed reinforcement tasks (DRT) and tyrosine hydroxylase immunoreactivity (TH-ir) in the prefrontal cortex was assessed. Two weeks following chronic treatment, WKY rats previously exposed to MPH chose the delayed reinforcer significantly less than the dH₂O treated controls in both the DRT and extinction task. MPH treatment did not significantly alter cognitive performance in the SHR. TH-ir in the infralimbic cortex was significantly altered by age and behavioural experience in WKY and SHR, however this effect was not evident in WKY rats treated with MPH. These results suggest that chronic treatment with MPH throughout adolescence in “normal” WKY rats increased impulsive choice and altered catecholamine development when compared to vehicle controls.

Dopamine, serotonin and impulsivity

Impulsive people have a strong urge to act without thinking. It is sometimes regarded as a positive trait but rash impulsiveness is also widely present in clinical disorders such as attention deficit hyperactivity disorder (ADHD), drug dependence, mania, and antisocial behaviour. Contemporary research has begun to make major inroads into unravelling the brain mechanisms underlying impulsive behaviour with a prominent focus on the limbic cortico-striatal systems. With this progress has come the understanding that impulsivity is a multi-faceted behavioural trait involving neurally and psychologically diverse elements. We discuss the significance of this heterogeneity for clinical disorders expressing impulsive behaviour and the pivotal contribution made by the brain dopamine and serotonin systems in the aetiology and treatment of behavioural syndromes expressing impulsive symptoms.

Brain processes in discounting: consequences of adolescent methylphenidate exposure

Traits of inattention, impulsivity, and motor hyperactivity characterize children diagnosed with attention-deficit/hyperactivity disorder (ADHD), whose inhibitory control is reduced. In animal models, crucial developmental phases or experimental transgenic conditions account for peculiarities, such as sensation-seeking and risk-taking behaviors, and reproduce the beneficial effects of psychostimulants. An "impulsive" behavioral profile appears to emerge more extremely in rats when forebrain dopamine (DA) systems undergo remodeling, as in adolescence, or with experimental manipulation tapping onto the dopamine transporter (DAT). Ritalin(®) (methylphenidate, MPH), a DAT-blocking drug, is prescribed for ADHD therapy but is also widely abused by human adolescents. Administration of MPH during rats' adolescence causes a long-term modulation of their self-control, in terms of reduced intolerance to delay and diminished proneness for risk when reward is uncertain. Exactly the opposite profile emerges when exogenous alteration of DAT levels is achieved via lentiviral transfection. Both adolescent MPH exposure and DAT-targeting transfection lead to enduring hyperfunction of dorsal striatum and hypofunction of ventral striatum. Together with upregulation of prefronto-cortical phospho-creatine, striatal upregulation of selected genes (like serotonin 7 receptor gene) suggests that enhanced inhibitory control is generated by adolescent MPH exposure. Operant tasks, which assess the balance between motivational drives and inhibitory self-control, are thus useful for investigating reward-discounting processes and their modulation by DAT-targeting tools. In summary, due to the complexity of human studies, preclinical investigations of rodent models are necessary to understand better both the neurobiology of ADHD-like symptoms' etiology and the long-term therapeutic safety of adolescent MPH exposure.

Methylphenidate and fluphenazine, but not amphetamine, differentially affect impulsive choice in spontaneously hypertensive, Wistar-Kyoto and Sprague-Dawley rats

Impulsivity is one of the core symptoms of attention-deficit/hyperactivity disorder (ADHD). The spontaneously hypertensive rat (SHR), a putative animal model of ADHD, has been used to investigate the neurobiology of impulsivity, although this model has been questioned over concerns that use of Wistar-Kyoto rats (WKY) as a comparison strain may exaggerate effects. The present study compared SHR, WKY and standard, outbred Sprague-Dawley (SD) rats on a delay discounting task where the primary measure was mean adjusted delay (MAD), or the indifference point (in sec) between choice of an immediate delivery of 1 grain-based pellet versus 3 pellets delivered after varying delays. The acute dose effects of the ADHD medications amphetamine (0.1-1.0 mg/kg) and methylphenidate (1.0-10 mg/kg) were then determined; in addition, the effect of the dopamine receptor antagonist fluphenazine (0.1-1.0 mg/kg) was also assessed for comparison with the indirect agonists. While there were no strain differences in the rate of task acquisition or stabilization of baseline MAD scores, SHR had significantly lower MAD scores than WKY but not SD due to the greater individual variability of MAD scores in SD. Although amphetamine did not alter MAD scores in any strain, methylphenidate selectively increased MAD scores in WKY and fluphenazine selectively increased MAD scores in SHR. WKY were also more sensitive than SHR and SD to the response-impairing effects of each drug. The finding that SHR showed a decrease in impulsivity following fluphenazine, but not following either amphetamine or methylphenidate, suggests that delay discounting in SHR may not represent a valid predictive model for screening effective ADHD medications in humans.

Social withdrawal and gambling-like profile after lentiviral manipulation of DAT expression in the rat accumbens

Dysfunction of brain dopamine transporter (DAT) has been associated with sensation seeking and impulse-control disorders. We recently generated a new animal model by stereotaxical inoculation of lentiviral vectors, which allowed localized intra-accumbal delivery of modulators for DAT gene: GFP (green fluorescent protein) control, silencers (Sil), a regulatable enhancer (DAT+), or both (DAT+Sil). Wistar male rats were followed both for socio-emotional profiles and for propensity to seek risky, uncertain rewards. Elevated anxiety and affiliation towards an unfamiliar partner emerged in Sil rats. Interestingly, in DAT+Sil rats (and Sil rats to a lesser extent) levels of playful social interaction were markedly reduced compared to controls. These DAT+Sil rats displayed a marked 'gambling-like' profile (i.e. preference for a large/uncertain over a small/sure reward), which disappeared upon doxycycline-induced switch-off onto DAT enhancer, but consistently reappeared with doxycycline removal. MRI-guided 1H-MRS (at 4.7 T) examinations in vivo (under anaesthesia) revealed changes in the bioenergetic metabolites (phosphocreatine and total creatine) for DAT+Sil rats, indicating a functional up-regulation of dorsal striatum (Str) and conversely a down-regulation of ventral striatum (i.e. nucleus accumbens, NAc). A combined profile of (1) enhanced proneness to gambling and (2) strong social withdrawal is thus associated with altered DAT-induced balance within forebrain dopamine systems. In fact, risk of developing a gambling-prone, social-avoidant psychopathology might be associated with (1) dominant semi-automatic strategies and/or habits, developed within Str circuits, and (2) reduced NAc function, with poorer feedback adjustment on decisions by aversive experiences.

Intermittent methylphenidate during adolescent development produces locomotor hyperactivity and an enhanced response to cocaine compared to continuous treatment in rats

The consequences of chronic methylphenidate (MPH) administration in adolescents for the treatment of attention-deficit/hyperactivity disorder (ADHD) remain to be fully understood. Studies in rats indicate that the pharmacokinetics of psychostimulant administration can powerfully influence the behavioral and neural consequences of chronic treatment. The purpose of the present study was to assess the effects of intermittent (0.8 or 1.6mg/kg, s.c., twice daily) versus continuous (1.6 or 3.2mg/kg/day via osmotic minipump) MP administration across four weeks of adolescent development in rats. Results indicate that intermittent treatment produced hyperactivity in a novel open field and increased sensitivity to both the reinforcing and locomotor-activating effects of cocaine. In contrast, continuous MPH resulted in a hypoactive response to the novel open field and a reduced sensitivity to both operant and non-contingent cocaine. To the extent that the continuous release condition models the sustained-release formulations utilized in human ADHD treatment, we interpret these data to indicate that sustained-release formulations are less likely to advance a risk of subsequent substance abuse.

Cognitive impulsivity in animal models: role of response time and reinforcing rate in delay intolerance with two-choice operant tasks

Impulsivity, a key symptom of ADHD (attention-deficit hyperactivity disorder), is also common in obsessive-compulsive and addictive disorders. There is rising interest in animal models of inhibitory-control impairment. Adolescent rats were tested daily in the intolerance-to-delay (ID) task (session 25 min, timeout 20 s), involving choice between either immediate small amount of food (SS), or larger amount of food after a delay (LL). The mixed 5-HT(1A/7) agonist (8-OH-DPAT, 0 or 0.060 mg/kg i.p.) was administered acutely just before the last three sessions at highest delays. In addition to the classical choice parameter (percent LL preference), the spontaneous waiting (termed response time, RT) occurring between end of a timeout (TO) and next nose-poke was calculated. The pace between consecutive reinforcer deliveries is given by the mean inter-trial interval (mITI, i.e. TO + RT). Hence, the impact of any given delay may be proportional to this pace and be expressed as delay-equivalent odds, i.e. the extent by which delays are multiples of the mITI. Data revealed that RT/mITI increased sharply from around 15 s/35 s to around 30 s/50 s when imposed delay changed from 30 s to 45 s (i.e. odds from 0.91 to 1.06). This suggests that rats adopted a strategy allowing them to keep in pace with perceived reinforcing rate. The increasing delay constraint directly influenced the length of rats' spontaneous waiting (RT) before next decision. For higher delays, with odds >1, rats shifted to a clear-cut SS preference, which is devoid of any exogenous temporal constraint. A challenge with 8-OH-DPAT (0 or 0.060 mg/kg i.p.) decreased impulsive choice but also increased RT. Thus, tapping onto 5-HT(1A/7) receptors slightly enhanced RT/mITI values, possibly reflecting ability of rats to cope with slower reinforcing rates and/or with delay-cancelled reward paces. In summary, delay-induced states of aversion may arise from the innate tendency to rely on a regular rate of reinforcement. Conversely, a drug-enhanced capacity to cope with delay may involve an internal ability to adjust expectancy about such a reinforcing rate.

Attention-deficit hyperactivity disorder (ADHD) and tuberous sclerosis complex

The neurobiological basis of attention-deficit hyperactivity disorder (ADHD) in tuberous sclerosis complex is still largely unknown. Cortical tubers may disrupt several brain networks that control different types of attention. Frontal lobe dysfunction due to seizures or epileptiform electroencephalographic discharges may perturb the development of brain systems that underpin attentional and hyperactive functions during a critical early stage of brain maturation. Comorbidity of attention-deficit hyperactivity disorder (ADHD) with mental retardation and autism spectrum disorders is frequent in children with tuberous sclerosis. Attention-deficit hyperactivity disorder (ADHD) may also reflect a direct effect of the abnormal genetic program. Treatment of children with tuberous sclerosis complex with combined symptoms of attention-deficit hyperactivity disorder (ADHD) and epilepsy may represent a challenge for clinicians, because antiepileptic therapy and drugs used to treat attention-deficit hyperactivity disorder (ADHD) may aggravate the clinical picture of each other.

Gender differences in delay-discounting under mild food restriction

Impulsivity, a core symptom of attention-deficit hyperactivity disorder (ADHD), is tested in animal models by delay-discounting tasks. So far, mainly male subjects have been used in this paradigm at severe levels of food restriction. Here we studied the impulsive behaviour of CD-1 adult male and female mice at mild levels of food restriction. Mice maintained at 90 +/- 5% of ad libitum bodyweight, were tested in operant chambers provided with nose-poking holes. Nose poking in one hole resulted in the immediate delivery of one food pellet (small-soon, SS), whereas nose poking in the other hole delivered five food pellets after a delay (large-late, LL), which was increased progressively each day (0-150 s). Two subgroups emerged: individuals that shifted at short delays ("steep") and individuals that did not shift, even at the highest delays ("flat"). Analysis showed that "steep" females shifted at shorter delays than "steep" males, while no difference existed between males and females within the "flat" sub-population. In home-cage circadian activity as well as in a novelty-seeking test, females were more active than males. It can be concluded from these results that female mice are more impulsive than male mice under mild food restriction. This is in contrast with findings in earlier studies with more severe food restriction. Therefore, an alternative explanation is that females are more explorative, and that different features might be tested in delay-discounting paradigms, depending on restriction levels.

Affective status in relation to impulsive, motor and motivational symptoms: personality, development and physical exercise

The contributions of impulsive and risk-taking behaviour in depressive and bipolar disorders, motivational and motor behaviours in anhedonic and substance addictive states, and the factors, particularly distress and trauma, underlying the development of neuropathology in affective status are described from clinical, epidemiological and laboratory perspectives. In order to distinguish one case factor for biopsychological substrates of health, an array of self-reported characteristics, e.g., positive or negative affect, stress or energy, optimism, etc., that may be predictive or counterpredictive for the propensity for physical exercise and activity were analysed using a linear regression in twelve different studies. Several individual characteristics were found to be markedly and significantly predictive of the exercise propensity, i.e., positive affect, energy, health-seeking behaviour and character, while optimism was of lesser, though significant, importance. Several individual characteristics were found to be significantly counterpredictive: expression of BDI- and HAD-depression, major sleep problems and lack/negligence of health-seeking behaviour. The consequences of physical activity and exercise for both affective well-being, cognitive mobility and neurogenesis is noted, particularly with regard to developmental assets for younger individuals. Affective disorder states may be studied through analyses of personal characteristics that unfold predispositions for symptoms-profiles and biomarkers derived from properties of dysfunction, such as impulsiveness, temperament dimensions, anhedonia and 'over-sensitivity', whether interpersonal or to reward.

Peculiar response to methylphenidate in adolescent compared to adult rats: a phMRI study

RATIONALE

Adolescent rodents differ markedly from adults in several neuro-behavioural parameters. Moreover, 'paradoxical' responses to psychostimulants have been reported at this age.

OBJECTIVES

Thus, we investigated the responses of adolescent (post-natal day, PND, 34 to 43) and adult (PND >60) Sprague-Dawley male rats to the psychostimulant drug methylphenidate (MPH). We used pharmacological magnetic resonance imaging (phMRI) performed at 4.7 T under isoflurane anaesthesia. Following anatomical MRI, axial gradient echo images were collected continuously. After baseline recording (32 min), animals received MPH (0 or 4 mg/kg i.p.) and were recorded for further 32 min.

RESULTS

Region-specific changes in the blood-oxygenation level dependent (BOLD) signal were evident as a function of age. As expected, among adults MPH induced an increase of BOLD signal in nucleus accumbens (NAcc) and prefrontal cortex (PFC), with no effects in the hippocampus (Hip). Notably, among adolescents, MPH induced a marked and generalised decrease of BOLD signal, which occurred earlier in NAcc and PFC whilst being delayed in the Hip. Any bias in BOLD responses was excluded by the measurement of physiological parameters.

CONCLUSIONS

The present findings highlight the utility of phMRI in animal models. The peculiar negative BOLD effect found in adolescent rats may be suggestive of a reduced cerebro-vascular feedback and/or an increased MPH-induced neuronal activation. Data are relevant for a better understanding of brain/behavioural regulation during adolescent development. Moreover, a greater understanding of the differences between adult and adolescent drug responses will aid in the development of a more appropriate age-specific treatment strategy.

Increased impulsive behavior and risk proneness following lentivirus-mediated dopamine transporter over-expression in rats' nucleus accumbens

Multiple theories have been proposed for sensation seeking and vulnerability to impulse-control disorders [Zuckerman M, Kuhlman DM (2000) Personality and risk-taking: Common biosocial factors. J Pers 68:999-1029], and many of these rely on a dopamine system deficit. Available animal models reproduce only some behavioral symptoms and seem devoid of construct validity. We used lentivirus tools for over-expressing or silencing the dopamine transporter (DAT) and we evaluated the resulting behavioral profiles in terms of motivation and self-control. Wistar adult rats received stereotaxic inoculation of a lentivirus that allowed localized intra-accumbens delivery of a DAT gene enhancer/silencer, or the green fluorescent protein, GFP. These animals were studied for intolerance to delay, risk proneness and novelty seeking. As expected, controls shifted their demanding from a large reward toward a small one when the delivery of the former was increasingly delayed (or uncertain). Interestingly, in the absence of general locomotor effects, DAT over-expressing rats showed increased impulsivity (i.e. a more marked shift of demanding from the large/delayed toward the small/soon reward), and increased risk proneness (i.e. a less marked shift from the large/uncertain toward the small/sure reward), compared with controls. Rats with enhanced or silenced DAT expression did not show any significant preference for a novel environment. In summary, consistent with literature on comorbidity between attention-deficit/hyperactivity disorder and pathological gambling, we demonstrate that DAT over-expression in rats' nucleus accumbens leads to impulsive and risk prone phenotype. Thus, a reduced dopaminergic tone following altered accumbal DAT function may subserve a sensation-seeker phenotype and the vulnerability to impulse-control disorders.