Preadolescent methylphenidate versus cocaine treatment differ in the expression of cocaine-induced locomotor sensitization during adolescence and adulthood

Neurobehavioral changes arising from early life dopamine signaling perturbations

Dopamine (DA) signaling is critical to the modulation of multiple brain functions including locomotion, reinforcement, attention and cognition. The literature provides strong evidence that altered DA availability and actions can impact normal neurodevelopment, with both early and enduring consequences on anatomy, physiology and behavior. An appreciation for the developmental contributions of DA signaling to brain development is needed to guide efforts to preclude and remedy neurobehavioral disorders, such as attention-deficit/hyperactivity disorder, addiction, bipolar disorder, schizophrenia and autism spectrum disorder, each of which exhibits links to DA via genetic, cellular and/or pharmacological findings. In this review, we highlight research pursued in preclinical models that use genetic and pharmacological approaches to manipulate DA signaling at sensitive developmental stages, leading to changes at molecular, circuit and/or behavioral levels. We discuss how these alterations can be aligned with traits displayed by neuropsychiatric diseases. Lastly, we review human studies that evaluate contributions of developmental perturbations of DA systems to increased risk for neuropsychiatric disorders.

Emergence of sex differences in the development of substance use and abuse during adolescence

Substance use and abuse begin during adolescence. Male and female adolescent humans initiate use at comparable rates, but males increase use faster. In adulthood, more men than women use and abuse addictive drugs. However, some women progress more rapidly from initiation of use to entry into treatment. In animal models, adolescent males and females consume addictive drugs similarly. However, reproductively mature females acquire self-administration faster, and in some models, escalate use more. Sex/gender differences exist in neurobiologic factors mediating both reinforcement (dopamine, opioids) and aversiveness (CRF, dynorphin), as well as intrinsic factors (personality, psychiatric co-morbidities) and extrinsic factors (history of abuse, environment especially peers and family) which influence the progression from initial use to abuse. Many of these important differences emerge during adolescence, and are moderated by sexual differentiation of the brain. Estradiol effects which enhance both dopaminergic and CRF-mediated processes contribute to the female vulnerability to substance use and abuse. Testosterone enhances impulsivity and sensation seeking in both males and females. Several protective factors in females also influence initiation and progression of substance use including hormonal changes of pregnancy as well as greater capacity for self-regulation and lower peak levels of impulsivity/sensation seeking. Same sex peers represent a risk factor more for males than females during adolescence, while romantic partners increase risk for women during this developmental epoch. In summary, biologic factors, psychiatric co-morbidities as well as personality and environment present sex/gender-specific risks as adolescents begin to initiate substance use.

Preadolescent drd1-EGFP mice exhibit cocaine-induced behavioral sensitization

In adult mice, repeated cocaine administration induces behavioral sensitization measured as increased horizontal locomotor activity. Cocaine-induced locomotor sensitization has been well characterized in adult mice. In adult animals, the D1 dopamine receptor is important for mediating effects of cocaine. The effect of cocaine on D1 receptor expression and function in preadolescent animals is less understood. The recently described drd1-enhanced green fluorescent protein (drd1-EGFP) reporter mouse is a useful model for performing such mechanistic studies; however, preadolescent drd1-EGFP mice have not been characterized previously. Here we studied cocaine-induced locomotor sensitization in preadolescent drd1-EGFP reporter mice. We administered 15mg/kg cocaine three times daily at 1h intervals for seven consecutive days beginning on postnatal day 23 to drd1-EGFP reporter mice and the commonly used C57BL/6 mice. Under this regimen, preadolescent mice of both strains exhibited cocaine-induced locomotor sensitization; however, by day 7 the cocaine-induced locomotor activity in the drd1-EGFP mice was maintained for a longer duration compared to the C57BL/6 mice. The preadolescent drd1-EGFP mice also exhibited elevated basal locomotor activity in a novel environment and had higher D1 and D2 dopamine receptor mRNA levels in the caudate nucleus compared to the C57BL/6 mice. The cocaine-induced locomotor sensitization was not retained when the drd1-EGFP mice were maintained cocaine-free for two weeks suggesting that in preadolescent drd1-EGFP mice the cocaine-induced changes do not persist.

Behavioral daily rhythmic activity pattern of adolescent female rat is modulated by acute and chronic cocaine

Cocaine is one of well-known drugs of abuse, and many children experience early exposure to cocaine. Because of an immature neuronal system in adolescents, they may react differently to repeated cocaine administration compared to adults. Most of the published papers report the effect of cocaine on adult male rats and this paper focused on the effects of cocaine on the 24 h locomotor activity rhythm patterns activity of adolescent Sprague Dawley (SD) female rats. Changes in the locomotor activity rhythm patterns could indicate that cocaine elicits long-term changes in the clock genes of the body that regulate different physiological processes. The objective of this study was to investigate whether cocaine in adolescent female rats modulated their daily activity pattern. Animals were divided into control (saline), 3.0, 7.5, 15.0 mg/kg cocaine groups. On experimental day 1 (ED 1), all groups were given saline injection. From ED 2 to ED 7, either saline or cocaine (3.0, 7.5, or 15.0 mg/kg) was given daily. ED 8 to ED 10 were the washout days, where no injection was given. On ED 11, the animals were injected with saline or with the same dose of cocaine as they were treated on ED 2 to ED 7. Each animal's locomotor activities was recorded nonstop following saline or cocaine injection for 11 consecutive days using the open field assay. In conclusion, it was observed that all three groups receiving repeated cocaine administration (3.0, 7.5, and 15.0 mg/kg) displayed significantly altered locomotor activity rhythm patterns.

Differential effects of methylphenidate and cocaine on GABA transmission in sensory thalamic nuclei

Methylphenidate (MPH) is widely used to treat children and adolescents diagnosed with attention deficit/hyperactivity disorder. Although MPH shares mechanistic similarities to cocaine, its effects on GABAergic transmission in sensory thalamic nuclei are unknown. Our objective was to compare cocaine and MPH effects on GABAergic projections between thalamic reticular and ventrobasal (VB) nuclei. Mice (P18-30) were subjected to binge-like cocaine and MPH acute and sub-chronic administrations. Cocaine and MPH enhanced hyperlocomotion, although sub-chronic cocaine-mediated effects were stronger than MPH effects. Cocaine and MPH sub-chronic administration altered paired-pulse and spontaneous GABAergic input differently. The effects of cocaine on evoked paired-pulse GABA-mediated currents changed from depression to facilitation with the duration of the protocols used, while MPH induced a constant increase throughout the administration protocols. Thalamic reticular nucleus GAD67 and VB Ca(V) 3.1 protein levels were measured using western blot to better understand their link to increased GABA release. Both proteins were increased by sub-chronic administration of cocaine. MPH showed effects on GABAergic transmission that seems less disruptive than cocaine. Unique effects of cocaine on postsynaptic VB calcium currents might explain deleterious cocaine effects on sensory thalamic nuclei. These results suggest that cocaine and MPH produced distinct presynaptic alterations on GABAergic transmission.

A review of psychostimulant-induced neuroadaptation in developing animals

The effects of clinically relevant doses of commonly prescribed stimulants methylphenidate (MPH), d-amphetamine (d-AMPH), and dl-AMPH or mixed amphetamine salts (MAS) such as Adderall, on short- and long-term gene neuroadaptations in developing animals have not been widely investigated. In the present review, the effects of oral stimulant administration were compared with those of the subcutaneous or intra-peritoneal route. A selective set of studies between 1979 and 2010, which incorporated in their design developmental period, clinically relevant doses of stimulants, and repeated daily doses were reviewed. These studies indicate that neuroadaptation to chronic stimulants includes blunting of stimulated immediate early gene expression, sensitivity of younger (prepubertal) brain to smaller dosages of stimulants, and the persistence of some effects, especially behavioral neuroadaptations, into adulthood. In addition, oral amphetamines (MAS) have more profound effects than does oral MPH. Further animal developmental studies are required to understand potential long-term neuroadaptations to low, daily oral doses of stimulants. Implications for clinical practice were also discussed.

Responding during signaled availability and nonavailability of iv cocaine and food in rats: age and sex differences

RATIONALE

Research suggests that age and sex are vulnerability factors for drug abuse. However, few studies have systematically examined their interaction.

OBJECTIVE

The purpose of the present study was to examine male and female, adult and adolescent rats under a procedure that measures responding during periods of signaled availability and nonavailability of iv cocaine and food reinforcers.

METHODS

Adolescent and adult rats lever pressed for iv infusions of cocaine or food pellets under a procedure with three components of signaled availability of the reinforcer alternating with two components of signaled nonavailability. Adolescent rats were removed and then later retested under the same conditions as adults, and a group of adult rats was also removed and retested after a similar number of days. A subset of rats earning cocaine infusions under the initial test was later retested with food pellets under the same behavioral task to assess the influence of prior cocaine exposure on subsequent responding for a nondrug reinforcer.

RESULTS

Adolescents (vs. adults) made more responses during periods of signaled iv cocaine availability and nonavailabiltiy, and adult females responded more than adult males during these periods. Responding during periods of signaled nonavailability of iv cocaine and food did not differ between the initial and subsequent retest conditions in adult rats. Further, adult males and females exposed to cocaine during adolescence responded more during periods of food availability compared to cocaine-naïve adults.

CONCLUSION

These results indicate that sex and age are vulnerability factors in cocaine abuse, and cocaine exposure during critical developmental stages can have long-lasting effects.

Blockade of adenosine A(1) receptors prevents methylphenidate-induced impairment of object recognition task in adult mice

Methylphenidate (MPH) is the preferred treatment used for attention-deficit/hyperactivity disorder (ADHD). Recently, misuse for MPH due to its apparent cognitive enhancer properties has been reported. Adenosine is a neuromodulator known to exert influence on the dopaminergic neurotransmission, which is the main pharmacological target of MPH. We have reported that an overdosage of MPH up-regulates adenosine A(1) receptors in the frontal cortex, but this receptor was not involved in its anxiolytic effects. In this study, the role of adenosine A(1) receptor was investigated on MPH-induced effects on aversive and recognition memory in adult mice. Adult mice received acute and chronic (15 days) administration of methylphenidate (5mg/kg, i.p.), or an acute overdosage (50mg/kg, i.p) in order to model misuse. Memory was assessed in the inhibitory avoidance and object recognition task. Acute administration 5mg/kg improved whereas 50mg/kg disrupted recognition memory and decreased performance in the inhibitory avoidance task. Chronic administration did not cause any effect on memory, but decreased adenosine A(1) receptors immunocontent in the frontal cortex. The selective adenosine A(1) receptor antagonist, (DPCPX 1mg/kg, i.p.), prevented methylphenidate-triggered recognition memory impairment. Our findings showed that recognition memory rather than aversive memory was differently affected by acute administration at both doses. Memory recognition was fully impaired by the overdosage, suggesting that misuse can be harmful for cognitive functions. The adenosinergic system via A(1) receptors may play a role in the methylphenidate actions probably by interfering with dopamine-enhancing properties of this drug.

Adenosine A1 receptors are modified by acute treatment with methylphenidate in adult mice

In recent years misuse of methylphenidate (MPH) has been reported. The main pharmacological target of methylphenidate is the dopaminergic system. Adenosine is a neuromodulator that influences the dopaminergic neurotransmission, but studies on MPH and adenosine are still lacking. In this study, adult mice were acutely treated with MPH (5mg/kg, i.p.) and to model misuse, they received an acute overdosage (50mg/kg, i.p). The involvement of adenosine A(1) receptors in anxiety-related behavior and locomotor and exploratory activity was examined. The administration of methylphenidate (5 and 50mg/kg) 30 min before the exposure to open field arena did not modify locomotor activity. The anxiolytic-like behavior was observed with both doses of MPH as revealed by the increase on the number of entries and the time spent in the open arms in the elevated plus-maze. Pre treatment with selective adenosine A(1) receptor antagonist (DPCPX 1mg/kg, i.p.) did not prevent anxiolytic effect caused by MPH 50mg/kg. Immunoblotting of frontal cortex and hippocampal extracts revealed that MPH 50mg/kg increased 88% adenosine A(1) receptor density in the frontal cortex. Extracts from hippocampus did not reveal any differences in the adenosine A(1) receptor density. Our findings ruled out the participation of adenosine A(1) receptors on the MPH-triggered anxiolytic effects. However, the density of adenosine A(1) receptors increased in a brain area strictly involved in the MPH-mediated effects. Thus, the adenosinergic system may play a role in the methylphenidate actions in the central nervous system.

Corticosterone treatment before puberty sensitizes the effect of oral methylphenidate on locomotor activity in preadolescence and produces differential effects in adulthood

The first objective of this study was to examine the effects of early exposure to methylphenidate and then those of re-exposure in adulthood. The second was to analyze the effect of corticosterone treatment during pre-puberty on oral methylphenidate consumption and, consequently, the effect of this psychostimulant on locomotor activity in preadolescent and adult rats. Experiment 1: from 31 to 39 days of postnatal age (PA), Wistar rats were exposed to either oral methylphenidate or water. Experiment 2: from 24 to 39 days PA, the rats received either corticosterone (2.0 mg/kg/day/subject) or a saline solution. From 31 to 39 days PA, rats were exposed to either methylphenidate or water. During adulthood, all rats in experiments 1 and 2 were exposed to either methylphenidate or water, and subsequently exposed to a free-choice condition of the same two substances.

RESULTS

Experiment 1. Methylphenidate increased locomotor activity (LA) regardless of age. In adulthood, higher methylphenidate consumption was observed in the group that had not been exposed to this substance, compared to the early methylphenidate-exposed group. Experiment 2. Corticosterone did not affect methylphenidate consumption during preadolescence or adulthood; however, the LA induced by methylphenidate was higher in the preadolescents that had been treated with corticosterone+methylphenidate than in the animals treated only with methylphenidate. In adulthood, methylphenidate produced higher LA in the animals previously treated with corticosterone+methylphenidate than in those that had received previous treatment exclusively with corticosterone. These results suggest that preadolescent corticosterone exposure produced a sensitizing effect of methylphenidate on LA in preadolescence. The differential effect on LA in adulthood depended on whether the corticosterone was administered with or without methylphenidate in preadolescence, which would suggest an enduring effect of the early synergic action between these two substances.

Stress-induced reinstatement of amphetamine-conditioned place preference and changes in tyrosine hydroxylase in the nucleus accumbens in adolescent rats

Drug abuse among humans often begins during adolescence. Exposure to psychostimulants during this age period may have long-term consequences which can render the organism more susceptible to drug abuse and relapse later in life. It has been demonstrated that exposure to stress can promote relapse to drug use even after long periods of withdrawal. The reinstatement of conditioned place preference (CPP) is a useful animal model for studying relapse. In humans and animals, changes in tyrosine hydroxylase (TH) have been related to drug addiction. Our study examined whether amphetamine-induced CPP during adolescence could be reinstated by exposure to stress 1 (adolescence) and 30 (adulthood) days after the extinction test. We also investigated TH levels following the reinstatement of CPP. Our results showed that amphetamine-induced CPP during adolescence can be reinstated by stress exposure 1day (P42, end of adolescence) but not 30days after extinction (P71, adulthood). Moreover the reinstatement of AMPH-induced CPP by stress exposure occurred in the presence of decreased TH in the nucleus accumbens. In conclusion, our data add new evidence that neuroadaptations on TH may mediate relapse to drug-seeking behavior induced by stress within adolescence.

Juvenile methylphenidate exposure and factors that influence incentive processing

Methylphenidate (MPH) is one of the few psychotropic agents approved for use in pediatric populations, underscoring the importance of elucidating any long-term consequences following exposure to this agent. Here, we examined the influence of several variables (i.e. age of assessment, age of exposure, sex, route of administration) on the effect of chronic low-dose MPH (2 mg/kg, twice daily) exposure on place conditioning to cocaine. Juvenile exposure to MPH, but not later exposure, resulted in aversions to cocaine-paired environments when assessed in young adult male rats, but not those entering adolescence. Juvenile MPH enhanced place preferences for cocaine-paired environments in female adolescent rats. The route of administration (i.p. injection or oral ingestion) did not produce enduring differential effects on behavior, and D-MPH was confirmed as the active enantiomer. These observations add to the growing literature on the enduring effects of MPH exposure, and highlight the need for more research in females.

Protracted 'anti-addictive' effects of adolescent phenylpropanolamine exposure in C57BL/6J mice

Exposure to the once highly prevalent over-the-counter (OTC) sympathomimetic phenylpropanolamine (PPA; +/--norephedrine) during pre-adolescence alters the developmental trajectory of catecholamine and amino acid neurotransmitter systems in the nucleus accumbens (NAC) that culminate in a 'pro-addictive' phenotype in adulthood. Thus, the present study sought to extend these earlier data by examining the long-term consequences of repeated PPA treatment during adolescence upon the behavioral and neurochemical responses to cocaine. For this, C57BL/6J mice were pre-treated with PPA (0-40 mg/kg) during postnatal days 35-44, and the capacity of cocaine (4 x 15 mg/kg) to elicit a conditioned place-preference, as well as behavioral and neurochemical sensitization within the NAC, were then assessed in adulthood. While adolescent PPA exposure did not influence spontaneous locomotor activity or the motor responses to either acute or repeated cocaine (4 x 15 mg/kg), PPA pre-exposure dose-dependently reduced the expression of a conditioned place-preference. As observed previously for juvenile PPA treatment, adolescent PPA administration blunted the dopamine and norepinephrine response to acute cocaine, prevented the development of catecholamine sensitization but did not influence cocaine-induced elevations in serotonin. However, unlike juvenile PPA treatment, adolescent PPA also prevented the development of glutamate sensitization within the NAC. These data provide evidence that adolescent exposure to a formerly prevalent OTC sympathomimetic produces protracted effects upon cocaine-induced changes in NAC glutamate transmission that may reduce vulnerability to cocaine addiction in later life and further the hypothesis that early exposure to sympathomimetic drugs may be an environmental factor contributing to the etiology of addiction.

Environmental modulation of ethanol-induced locomotor activity: Correlation with neuronal activity in distinct brain regions of adolescent and adult Swiss mice

Drug abuse is a concerning health problem in adults and has been recognized as a major problem in adolescents. Induction of immediate-early genes (IEG), such as c-Fos or Egr-1, is used to identify brain areas that become activated in response to various stimuli, including addictive drugs. It is known that the environment can alter the response to drugs of abuse. Accordingly, environmental cues may trigger drug-seeking behavior when the drug is repeatedly administered in a given environment. The goal of this study was first to examine for age differences in context-dependent sensitization and then evaluate IEG expression in different brain regions. For this, groups of mice received i.p. ethanol (2.0 g/kg) or saline in the test apparatus, while other groups received the solutions in the home cage, for 15 days. One week after this treatment phase, mice were challenged with ethanol injection. Acutely, ethanol increased both locomotor activity and IEG expression in different brain regions, indistinctly, in adolescent and adult mice. However, adults exhibited a typical context-dependent behavioral sensitization following repeated ethanol treatment, while adolescent mice presented gradually smaller locomotion across treatment, when ethanol was administered in a paired regimen with environment. Conversely, ethanol-treated adolescents expressed context-independent behavioral sensitization. Overall, repeated ethanol administration desensitized IEG expression in both adolescent and adult mice, but this effect was greatest in the nucleus accumbens and prefrontal cortex of adolescents treated in the context-dependent paradigm. These results suggest developmental differences in the sensitivity to the conditioned and unconditioned locomotor effects of ethanol.

Juvenile methylphenidate modulates reward-related behaviors and cerebral blood flow by decreasing cortical D3 receptors

Attention deficit hyperactivity disorder is associated with reduced cortical blood flow that is reversible with exposure to the psychostimulant methylphenidate (MPH). D3 dopamine receptors modulate stimulant-induced changes in blood flow and are associated with reward processing during young adulthood, but their role in the enduring effects of MPH during development is unknown. Rats were given vehicle (VEH) or MPH (2 mg/kg between postnatal days 20-35) and assessed in young adulthood for regional cerebral blood volume (rCBV) after MPH challenge and mRNA expression levels of dopamine receptors. To probe D3 receptor involvement, juvenile subjects were exposed to VEH, MPH, the D3-preferring agonist +/-7-OHDPAT (0.3 mg/kg), the D3 antagonist nafadotride (Naf; 0.05, 0.5 or 5.0 mg/kg) or a Naf (0.05 mg/kg)/MPH combination, and assessed biochemically and behaviorally. Juvenile MPH exposure increased MPH-induced rCBV in the cingulate and medial prefrontal cortex and thalamus in adulthood. Behaviorally, juvenile MPH- or +/-7-OHDPAT-exposed subjects demonstrated an aversion to cocaine-associated environments, which was prevented by juvenile co-treatment with MPH and Naf, or with adult cortical microinjections of +/-7-OHDPAT. Cortical D3 mRNA levels significantly decreased by 23.8 +/- 6.7% in MPH-treated subjects and normalized with combined Naf/MPH treatment, with no change in the other dopamine receptors. Enhanced cortical responsiveness to psychostimulants may occur through a reduction in D3 receptors, which in turn reduces drug-seeking behavior. These data provide evidence for a postnatal sensitive period when juvenile MPH exposure is able to alter cortical development.

Effects of adolescent exposure to cocaine on locomotor activity and extracellular dopamine and glutamate levels in nucleus accumbens of DBA/2J mice

Adolescents differ from adults in their acute sensitivity to several drugs of abuse, but little is known about the long-term neurobehavioral effects of adolescent drug exposure. To explore this further, we evaluated the locomotor responses to repeated cocaine administration in adolescent and adult male DBA/2J mice and alterations in extracellular levels of dopamine (DA) and glutamate (GLU) in the nucleus accumbens (NAc) in response to a subsequent cocaine challenge. Adolescent and adult mice were treated daily with saline or cocaine (10 mg/kg, i.p) for 9 consecutive days. Ten days following the last injection, animals were implanted with microdialysis probes and 24 h later microdialysis samples were collected before and after an acute cocaine challenge. Adolescents but not adults demonstrated development of behavioral sensitization to cocaine. Microdialysis procedures revealed that cocaine-treated mice displayed greater peak increases in extracellular DA in response to a subsequent cocaine challenge as compared to saline-treated mice, in contrast with lower peak increases in extracellular GLU. While adults exhibited greater peaks in extracellular DA in response to cocaine than adolescents did, adolescent mice presented a more rapid onset of peak extracellular DA levels than adults. Our results indicate differences in the behavioral and neurochemical responses to cocaine in adolescent versus adult mice, which may be relevant to the increased risk of developing addiction in humans who are exposed to drugs of abuse during adolescence.

Protracted 'pro-addictive' phenotype produced in mice by pre-adolescent phenylpropanolamine

For decades, the sympathomimetic phenylpropanolamine (PPA; +/- -norepinephrine) was an active ingredient found in popular children's over-the-counter (OTC) cold, cough, and allergy medications. To examine the possibility that pre-adolescent PPA exposure may induce neuroadaptations that influence behavioral and neurochemical responding to cocaine, C57BL/6J mice were pretreated with PPA (0-40 mg/kg) during postnatal days 21-31. The behavioral and neurochemical responses to acute and repeated cocaine (4 x 15 mg/kg) were then assessed in adulthood when the mice were 10 weeks of age. Whereas pre-adolescent PPA exposure did not influence the acute locomotor response to 15 mg/kg cocaine, PPA pre-exposure dose-dependently enhanced the expression of cocaine-induced place conditioning, reduced the expression of locomotor sensitization, but did not influence cocaine-induced stereotypy. Pre-adolescent PPA exposure completely prevented the capacity of cocaine to elevate extracellular levels of catecholamines in the nucleus accumbens, but facilitated the development of cocaine-induced glutamate sensitization. Neither acute nor repeated cocaine altered extracellular GABA levels in the accumbens of control mice; however, 15 mg/kg cocaine lowered GABA levels by approximately 40% in PPA pretreated mice and this effect showed tolerance with repeated cocaine administration. These data provide the first evidence that early exposure to an OTC compound produces protracted effects upon cocaine-induced changes in nucleus accumbens neurotransmission that may contribute to a 'pro-addictive' phenotype in adulthood.

Sensitization and cross-sensitization after chronic treatment with methylphenidate in adolescent Wistar rats

An increasing debate exists about the potential of early exposure to methylphenidate to increase the risk for drug abuse. In addition, little is known about the neurobiological effects of early exposure to methylphenidate. This study was designed to investigate whether chronic treatment with methylphenidate induces behavioral sensitization to subsequent methylphenidate and D-amphetamine challenge in adolescent Wistar rats. Young Wistar rats (P25) were treated with either methylphenidate (1, 2, or 10 mg/kg, intraperitoneally) or saline for 28 days. After 14 days of washout, animals were challenged with methylphenidate 2.5 mg/kg intraperitoneally or D-amphetamine 2 mg/kg intraperitoneally (P67). Locomotor behavior was assessed using the open field test. Rats chronically treated with methylphenidate in the adolescent period showed augmented locomotor sensitization to D-amphetamine but not to methylphenidate in the adult phase. These findings suggest that early exposure do methylphenidate might increase the risk for subsequent D-amphetamine abuse. Further studies focusing on the neurobiological effects of early exposure to methylphenidate are warranted.