The effects of adolescent methylphenidate self-administration on responding for a conditioned reward, amphetamine-induced locomotor activity, and neuronal activation

Oral Methylphenidate Treatment of an Adolescent ADHD Rat Model Does Not Alter Cocaine-Conditioned Place Preference during Adulthood: A Negative Report

The stimulant, methylphenidate (MPH), is commonly used to treat attention deficit hyperactivity disorder (ADHD) and has been increasingly prescribed for school age children and adolescents. Concerns regarding its long-term effects on later substance use disorders (SUDs) have been raised. Previous animal studies have produced contradictory results regarding whether early exposure to MPH increases or protects against SUD in adulthood. The goal of our study was to determine if clinically relevant doses of MPH during adolescence alter cocaine responsiveness in adulthood in a rat model of ADHD, the spontaneous hypertensive rat (SHR). We pretreated SHRs with saline or MPH (2.5 mg/kg once or twice day) via oral gavage during their dark cycle from postnatal day 35 (p35) to p44. Adult rats (p80) were assessed in an eight-session cocaine-conditioned place preference test (CPP). Four doses of cocaine were administered via intraperitoneal injection (i.p.) during the conditioning sessions: 1, 5, 10 and 20 mg/kg. Once per day MPH treatment had a small sensitizing effect on baseline general locomotor activity in a novel environment at p80 as well as a limited suppressive effect on reward-specific locomotor activity as measured by the decreased preference to enter the cocaine-paired chamber. This treatment did not have any effect on the amount of time that rats chose to spend in the cocaine-paired chamber. Twice per day MPH treatment had no effect on locomotion or drug-preference. Our results suggest that MPH treatment of ADHD rats during adolescence does not alter preference for cocaine in adulthood.

Responding for conditioned reinforcement in C57BL/6 and CD-1 mice, and Sprague-Dawley rats: Effects of methylphenidate and amphetamine

RATIONALE

Characterization of responding for conditioned reinforcement in mice is important to implement genetic tools in examining the neurobiological mechanisms underlying reward-related learning and incentive motivation.

METHODS

Inbred C57BL/6 mice, outbred CD-1 mice, and outbred Sprague-Dawley rats underwent Pavlovian conditioning in which a conditioned stimulus (CS) was paired with saccharin. Subsequently, subjects were allowed to respond for that CS in tests of responding for conditioned reinforcement. Experiments measured the effects of methylphenidate (MPH) and amphetamine (AMPH) on lever pressing for conditioned reinforcement in mice and rats. We further examined the stability of responding for conditioned reinforcement in mice after repeated testing and the extinction of this behaviour following omission of the reinforcer. We also determined whether the CS exhibited reinforcing properties if it was not paired with saccharin.

RESULTS

C57BL/6 and CD-1 mice learned to respond for a conditioned reinforcer similarly to rats, and the behaviour was stable over time. MPH increased responding in CD-1 mice and rats, but not in C57BL/6 mice. AMPH only increased responding in rats. Responding was reduced following omission of the conditioned reinforcer, and responding was only established when the CS was paired with saccharin.

CONCLUSIONS

These experiments characterize a conditioned reinforcement test which produces stable responding in two different mouse backgrounds. These findings also show that dopaminergic psychomotor stimulants can differently affect rats and mice in tests of responding for conditioned reinforcement.

Dopamine reuptake transporter (DAT) "inverse agonism"--a novel hypothesis to explain the enigmatic pharmacology of cocaine

The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable subjective effects such as drug "highs" or euphoria in drug-experienced human volunteers. Moreover they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to conclude that the highly unusual stimulant profile of cocaine and related compounds, eg methylphenidate, is not mediated by monoamine reuptake inhibition alone. We describe the experimental findings which suggest cocaine serves as a negative allosteric modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of transport. This results in a firing-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an inverse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a new class of monoaminergic drugs; DAT "inverse agonists". This article is part of the Special Issue entitled 'CNS Stimulants'.

The effects of nicotine exposure during Pavlovian conditioning in rats on several measures of incentive motivation for a conditioned stimulus paired with water

RATIONALE

Nicotine enhances approach toward and operant responding for conditioned stimuli (CSs), but the effect of exposure during different phases of Pavlovian incentive learning on these measures remains to be determined.

OBJECTIVES

These studies examined the effects of administering nicotine early, late or throughout Pavlovian conditioning trials on discriminated approach behavior, nicotine-enhanced responding for conditioned reinforcement, extinction, and the reinstatement of responding for conditioned reinforcement. We also tested the effect of nicotine on approach to a lever-CS in a Pavlovian autoshaping procedure and for this CS to serve as a conditioned reinforcer.

METHODS

Thirsty rats were exposed to 13 conditioning sessions where a light/tone CS was paired with the delivery of water. Nicotine was administered either prior to the first or last seven sessions, or throughout the entire conditioning procedure. Responding for conditioned reinforcement, extinction, and the reinstatement of responding by the stimulus and nicotine were compared across exposure groups. Separately, the effects of nicotine on conditioned approach toward a lever-CS during autoshaping, and responding for that CS as a conditioned reinforcer, were examined.

RESULTS

Nicotine exposure was necessary for nicotine-enhanced responding for conditioned reinforcement and the ability for nicotine and the stimulus to additively reinstate responding on the reinforced lever. Nicotine increased contacts with a lever-CS during autoshaping, and removal of nicotine abolished this effect. Prior nicotine exposure was necessary for nicotine-enhanced responding reinforced by the lever.

CONCLUSIONS

Enhancements in the motivating properties of CSs by nicotine occur independently from duration and timing effects of nicotine exposure during conditioning.

Methylphenidate and cocaine self-administration produce distinct dopamine terminal alterations

Methylphenidate (MPH) is a commonly abused psychostimulant prescribed for the treatment of attention deficit hyperactivity disorder. MPH has a mechanism of action similar to cocaine (COC) and is commonly characterized as a dopamine transporter (DAT) blocker. While there has been extensive work aimed at understanding dopamine (DA) nerve terminal changes following COC self-administration, very little is known about the effects of MPH self-administration on the DA system. We used fast scan cyclic voltammetry in nucleus accumbens core slices from animals with a 5-day self-administration history of 40 injections/day of either MPH (0.56 mg/kg) or COC (1.5 mg/kg) to explore alterations in baseline DA release and uptake kinetics as well as alterations in the interaction of each compound with the DAT. Although MPH and COC have similar behavioral effects, the consequences of self-administration on DA system parameters were found to be divergent. We show that COC self-administration reduced DAT levels and maximal rates of DA uptake, as well as reducing electrically stimulated release, suggesting decreased DA terminal function. In contrast, MPH self-administration increased DAT levels, DA uptake rates and DA release, suggesting enhanced terminal function, which was supported by findings of increased metabolite/DA tissue content ratios. Tyrosine hydroxylase messenger RNA, protein and phosphorylation levels were also assessed in both groups. Additionally, COC self-administration reduced COC-induced DAT inhibition, while MPH self-administration increased MPH-induced DAT inhibition, suggesting opposite pharmacodynamic effects of these two drugs. These findings suggest that the factors governing DA system adaptations are more complicated than simple DA uptake blockade.

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

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

A preclinical evaluation of the discriminative and reinforcing properties of lisdexamfetamine in comparison to D-amfetamine, methylphenidate and modafinil

Lisdexamfetamine dimesylate, which consists of L-lysine covalently bound to D-amfetamine, is the first prodrug for treating ADHD. Its metabolic conversion to yield D-amfetamine by rate-limited, enzymatic hydrolysis is unusual because it is performed by peptidases associated with red blood cells. Other stimulants shown to be effective in managing ADHD include D-amfetamine, methylphenidate and modafinil. All have the potential for misuse or recreational abuse. The discriminative and reinforcing effects of these compounds were determined in rats using a 2-choice, D-amfetamine (0.5 mg/kg, i.p.)-cued drug-discrimination test, and by substitution for intravenous cocaine in self-administration. Lisdexamfetamine (0.5-1.5 mg/kg [D-amfetamine base], p.o.) generalised to saline when tested 15 min post-dosing, but dose-dependently generalised to d-amfetamine at 60 min. At 120 min, its D-amfetamine-like effects were substantially diminished. At 15 min, methylphenidate (3.0-10 mg/kg, p.o.) and D-amfetamine (0.1-1.5 mg/kg, p.o.) dose-dependently generalised to the intraperitoneal D-amfetamine cue. Switching to the intraperitoneal route reduced the interval required for lisdexamfetamine to be recognised as D-amfetamine-like, but did not alter its potency. Switching to intraperitoneal injection increased the potency of methylphenidate and D-amfetamine by 3.4× and 2.2×, respectively. Modafinil (50-200 mg/kg, i.p.) generalised partially, but not fully, to d-amfetamine. Methylphenidate (0.1, 0.3, 1.0 mg/kg/injection, i.v.) maintained robust self-administration at the 2 highest doses. Neither lisdexamfetamine (0.05, 0.15 or 0.5 mg/kg/injection [D-amfetamine base], i.v.) nor modafinil (0.166, 0.498 or 1.66 mg/kg/injection, i.v.) served as reinforcers. The results reveal important differences between the profiles of these stimulants. Lisdexamfetamine did not serve as a positive reinforcer in cocaine-trained rats, and although it generalised fully to D-amfetamine, its discriminative effects were markedly influenced by its unusual pharmacokinetics.

Strain differences in self-administration of methylphenidate and sucrose pellets in a rat model of attention-deficit hyperactivity disorder

Despite its abuse potential, methylphenidate (MPH) is widely prescribed for treatment of attention-deficit hyperactivity disorder (ADHD). The purpose of the present study was to examine MPH self-administration in a rat model of ADHD. Experiment 1 examined the acquisition of MPH self-administration and assessed the MPH dose-effect curve in spontaneously hypertensive rats (SHR), an inbred rat model of ADHD, Wistar Kyotos (WKY), the progenitor strain for SHR, and Sprague-Dawley (SD), an outbred control strain. Experiment 2 replicated Experiment 1, but replaced MPH infusions with sucrose pellets. Initial acquisition of MPH self-administration was greater in SHR and SD than WKY. However, with extended training using an incrementing fixed ratio schedule SHR and WKY did not differ in responding for MPH using the training dose (0.3 mg/kg/infusion) or other unit doses, except that SHR showed greater responding than WKY at 0.1 mg/kg/infusion. SHR also showed greater acquisition and maintenance of sucrose-reinforced responding compared with both WKY and SD. Greater initial acquisition of MPH self-administration in SHR than WKY may not be due to a strain-specific difference in sensitivity to the reinforcing effect of MPH.

Nicotine-induced enhancement of responding for conditioned reinforcement in rats: role of prior nicotine exposure and α4β2 nicotinic receptors

RATIONALE

Stimuli associated with nicotine can become motivationally significant and may play a role in tobacco dependence. Previous work indicates that nicotine enhances responding for a conditioned reinforcer (CR).

OBJECTIVES

These studies examined the effects of prior exposure to nicotine on responding for a CR, persistence of this response, and the role of α4β2 or α7 nicotinic receptor subtypes.

METHODS

Water deprived rats were given 13 Pavlovian conditioning sessions where a light/tone conditioned stimulus (CS) was paired with the delivery of water. Then, rats were presented with two levers: one delivered the CS (now a CR), the other was inactive. Experiments examined the effect of nicotine administered prior to Pavlovian conditioning sessions on approach behavior during CS presentations, operant responding for CR in the presence and absence of nicotine, and the persistence of responding for CR. The effects of nicotinic acetylcholine receptor (nAChR) antagonism with mecamylamine and α4β2 or α7 nAChR antagonism with dihydro-beta-erythroidine (DHβE) or methyllycaconitine (MLA) on nicotine-enhanced responding for CR were examined.

RESULTS

Nicotine enhanced approach behavior during CS presentations and potentiated operant responding for CR, an effect sensitized as a result of nicotine exposure during conditioning. Responding for CR and its potentiation by nicotine was stable over multiple tests. Enhanced responding for the CR induced by nicotine was blocked by mecamylamine and DHβE, but not MLA.

CONCLUSIONS

Nicotine enhances Pavlovian discriminated approach and shows sensitized nicotine-induced enhancements in responding for CR, an effect depending on α4β2 nAChRs.

Adolescent amphetamine exposure elicits dose-specific effects on monoaminergic neurotransmission and behaviour in adulthood

Despite the growing non-medical consumption of amphetamine (Amph) during adolescence, its long-term neurobiological and behavioural effects have remained largely unexplored. The present research sought to characterize the behavioural profile and electrophysiological properties of midbrain monoaminergic neurons in adult rodents after Amph exposure during adolescence. Adolescent rats were administered vehicle, 0.5, 1.5, or 5.0 mg/kg.d Amph from postnatal day (PND) 30-50. At adulthood (PND 70), rats were tested in an open-field test (OFT) and elevated plus maze (EPM), paralleled by in-vivo extracellular recordings of serotonin (5-HT), dopamine (DA) and norepinephrine (NE) neurons from the dorsal raphe nucleus, ventral tegmental area, and locus coeruleus, respectively. 5-HT firing in adulthood was increased in rats that had received Amph (1.5 mg/kg.d) during adolescence. At this regimen, DA firing activity was increased, but not NE firing. Conversely, the highest Amph dose regimen (5.0 mg/kg.d) enhanced NE firing, but not DA or 5-HT firing rates. In the OFT, Amph (1.5 mg/kg.d) significantly increased the total distance travelled, while the other doses were ineffective. In the EPM, all three Amph doses increased time spent in the open arms and central platform, as well as the number of stretch-attend postures made. Repeated adolescent exposure to Amph differentially augments monoaminergic neuronal firing in a dose-specific fashion in adulthood, with corresponding alterations in locomotion, risk assessment (stretch-attend postures and central platform occupancy) and risk-taking behaviours (open-arm exploration). Thus, adolescent Amph exposure induces long-lasting neurophysiological alterations that may have implications for drug-seeking behaviour in the future.

Cue effects on methylphenidate self-administration in rats

Associations between drugs and the stimuli paired with drugs have been proposed as primary factors in drug addiction and relapse. Previous research has found cues paired with drug infusions are important for many classes of drugs. The purpose of the present experiment was to determine if a cue light was necessary to engender reliable self-administration of methylphenidate (MPH), which is a widely prescribed drug for attention deficit hyperactivity disorder. Rats were given access to MPH (0.3 mg/kg/infusion) or saline for self-administration. Half of the rats in each group had infusions paired with a cue light, whereas the other half did not. Two additional groups of rats received MPH infusions noncontingently; one group's lever pressing turned on the cue light, and the other group's lever pressing had no consequence. Both MPH and the cue functioned as weak reinforcers on their own. The group that lever pressed for MPH paired with a cue light pressed significantly more for MPH than any other group, indicating that the cue and MPH had a synergistic effect on self-administration when combined. Taken together, these results indicate that MPH has reinforcing properties on its own, but that environmental cues also play an important role in enhancing MPH self-administration.

Juvenile exposure to methylphenidate reduces cocaine reward and alters netrin-1 receptor expression in adulthood

The guidance cue netrin-1 acting on mesocorticolimbic dopamine (DA) neurons through its receptor DCC (deleted in colorectal cancer) has been implicated in the neuronal plasticity induced by psychostimulant drugs. We examined in C57/BL6 mice the effects of repeated juvenile methylphenidate (MPH) exposure on cocaine-reward sensitivity in adulthood and determined whether early MPH treatment alters adult expression of DCC in mesocorticolimbic DA regions. By using place conditioning, we show that adult mice exposed to MPH during the juvenile period are less sensitive to cocaine-reward compared to saline-controls. Furthermore, by means of immunoblotting, we demonstrate that early MPH treatment attenuates adult DCC expression in the ventral tegmental area (VTA) selectively. These results support previous evidence that developmental MPH treatment diminishes cocaine-reward in adulthood and are the first to suggest that DCC in the VTA may participate in this enduring effect.

Chronic atomoxetine treatment during adolescence decreases impulsive choice, but not impulsive action, in adult rats and alters markers of synaptic plasticity in the orbitofrontal cortex

RATIONALE

Impulsivity is a key symptom of attention-deficit hyperactivity disorder (ADHD). The use of the norepinephrine reuptake inhibitor, atomoxetine, to treat ADHD suggests that the activity of the norepinephrine transporter (NET) may be important in regulating impulsive behavior. Many ADHD patients receive chronic drug treatment during adolescence, a time when frontal brain regions important for impulse control are undergoing extensive development.

OBJECTIVES

The current study aimed to determine the effects of chronic atomoxetine treatment during adolescence in rats on two distinct forms of impulsivity in adulthood and whether any behavioral changes were accompanied by alterations in mRNA or protein levels within the frontal cortices.

METHODS

Rats received daily injections of saline or atomoxetine (1 mg/kg) during adolescence (postnatal days 40-54). Two weeks later, animals were trained to perform either the delay-discounting test or the five-choice serial reaction time task (5CSRT).

RESULTS

Adolescent atomoxetine treatment caused a stable decrease in selection of small immediate rewards over larger delayed rewards (impulsive choice) in adulthood, but did not affect premature responding (impulsive action) in the 5CSRT. Chronic atomoxetine treatment also altered the ability of acute atomoxetine to modulate aspects of impulsivity, but did not change the response to d-amphetamine. Ex vivo analysis of brain tissue indicated that chronic atomoxetine decreased phosphorylation of CREB and ERK in the orbitofrontal cortex and decreased mRNA for BDNF and cdk5.

CONCLUSIONS

These data suggest that repeated administration of atomoxetine in adolescence can lead to stable decreases in impulsive choice during adulthood, potentially via modulating development of the orbitofrontal cortex.

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.

Enhanced incentive motivation for sucrose-paired cues in adolescent rats: possible roles for dopamine and opioid systems

Vulnerability to the effects of drugs of abuse during adolescence may be related to altered incentive motivation, a process believed to be important in addiction. Incentive motivation can be seen when a neutral stimulus acquires motivational properties through repeated association with a primary reinforcer. We compared adolescent (postnatal day (PND) 24-50) and adult (>PND 70) rats on a measure of incentive motivation: responding for a conditioned reinforcer (CR). Rats learned to associate the delivery of 0.1 ml of 10% sucrose with a conditioned stimulus (CS; light and tone); 30 pairings per day were given over 14 days. Then, we measured responding on a lever delivering the CS (now a CR) after injections of amphetamine (0, 0.25 or 0.5 mg/kg). We also examined responding for CR when the CS and sucrose were paired or unpaired during conditioning, and responding for the primary reinforcer (10% sucrose) in control experiments. Finally, we examined the effects of D(1) and D(2) dopamine receptor antagonists (SCH 39166 and eticlopride, respectively) and an opioid receptor antagonist (naltrexone) on responding for a CR in adolescent rats. Adolescents but not adults acquired responding for a CR, but adolescents responded less than adults for the primary reinforcer. Responding for a CR depended upon the pairing of the CS and sucrose during conditioning. Both dopamine and opioid receptor antagonists reduced responding for the CR. Therefore, incentive motivation may be enhanced in adolescents compared with adults, and incentive motivation may be mediated in part by both dopamine and opioid systems.

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.

Aberrant learning and memory in addiction

Over the past several years, drug addiction has increasingly been accepted to be a disease of the brain as opposed to simply being due to a lack of willpower or personality flaw. Exposure to addictive substances has been shown to create enduring changes in brain structure and function that are thought to underlie the transition to addiction. Specific genetic and environmental vulnerability factors also influence the impact of drugs of abuse on the brain and can enhance the likelihood of becoming an addict. Long-lasting alterations in brain function have been found in neural circuits that are known to be responsible for normal appetitive learning and memory processes and it has been hypothesized that drugs of abuse enhance positive learning and memory about the drug while inhibiting learning about the negative consequences of drug use. Therefore, the addict's behavior becomes increasingly directed towards obtaining and using drugs of abuse, while at the same time developing a poorer ability to stop using, even when the drug is less rewarding or interferes with functioning in other facets of life. In this review we will discuss the clinical evidence that addicted individuals have altered learning and memory and describe the possible neural substrates of this dysfunction. In addition, we will explore the pre-clinical evidence that drugs of abuse cause a progressive disorder of learning and memory, review the molecular and neurobiological changes that may underlie this disorder, determine the genetic and environmental factors that may increase vulnerability to addiction, and suggest potential strategies for treating addiction through manipulations of learning and memory.