Acute methylphenidate treatments reduce sucrose intake in restricted-fed bingeing rats

Stimulation of Dopamine D4 Receptors in the Nucleus Accumbens Shell Increases Palatable Food Intake in Satiated Male Rats: Modulation by NMDA and AMPA Receptors

BACKGROUND/OBJECTIVES

Palatability significantly influences food consumption, often leading to overeating and obesity by activating the brain's reward systems. The nucleus accumbens (NAc) plays a central role in this process, modulating reward mechanisms primarily via dopamine through D2-like receptors (D2R, D3R, D4R). While the involvement of D2 receptors in feeding is well-documented, the role of D4 receptors (D4Rs) is less clear.

METHODS

Male Wistar rats received intra-NAc shell microinjections of the D4R agonist PD-168077 and the antagonist L-745870. This study also examined the modulation between D4R and glutamatergic transmission by administration of NMDA, NMDA receptor antagonist AP-5, AMPA, and AMPA receptor antagonist CNQX.

RESULTS

PD-168077 increased sweet solution intake by 46%, an effect that was reversed by L-745870. Pre-treatment with NMDA prevented the stimulatory effect of PD-168077, whereas the NMDA receptor antagonist AP-5 had no such effect. Additionally, AMPA administration reduced sweet solution intake by 63%, counteracting the effect of PD-168077, while the AMPA receptor antagonist CNQX, on its own, increased intake by 40%.

CONCLUSIONS

These findings suggest that D4Rs promote hedonic feeding by modulating glutamatergic transmission in the NAc shell, highlighting the complexity of D4R involvement in food intake regulation. This study underscores the potential of targeting D4Rs for therapeutic interventions in eating disorders and obesity, though further research is essential to clarify the precise mechanisms through which D4R modulates AMPA and NMDA receptor activity in feeding behavior.

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

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

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Can We Selectively Reduce Appetite for Energy-Dense Foods? An Overview of Pharmacological Strategies for Modification of Food Preference Behavior

Excessive intake of food, especially palatable and energy-dense carbohydrates and fats, is largely responsible for the growing incidence of obesity worldwide. Although there are a number of candidate antiobesity drugs, only a few of them have been proven able to inhibit appetite for palatable foods without the concurrent reduction in regular food consumption. In this review, we discuss the interrelationships between homeostatic and hedonic food intake control mechanisms in promoting overeating with palatable foods and assess the potential usefulness of systemically administered pharmaceuticals that impinge on the endogenous cannabinoid, opioid, aminergic, cholinergic, and peptidergic systems in the modification of food preference behavior. Also, certain dietary supplements with the potency to reduce specifically palatable food intake are presented. Based on human and animal studies, we indicate the most promising therapies and agents that influence the effectiveness of appetite-modifying drugs. It should be stressed, however, that most of the data included in our review come from preclinical studies; therefore, further investigations aimed at confirming the effectiveness and safety of the aforementioned medications in the treatment of obese humans are necessary.

Novel pharmacologic treatment in acute binge eating disorder - role of lisdexamfetamine

Binge eating disorder (BED) is the most common eating disorder and an important public health problem. It is characterized by recurrent episodes of excessive food consumption accompanied by a sense of loss of control over the binge eating behavior without the inappropriate compensatory weight loss behaviors of bulimia nervosa. BED affects both sexes and all age groups and is associated with medical and psychiatric comorbidities. Until recently, self-help and psychotherapy were the primary treatment options for patients with BED. In early 2015, lisdexamfetamine dimesylate, a prodrug stimulant marketed for attention deficit hyperactive disorder, was the first pharmacologic agent to be approved by the US Food and Drug Administration for the treatment of moderate or severe BED in adults. This article summarizes BED clinical presentation, and discusses the pharmacokinetic profile, efficacy, and safety of lisdexamfetamine dimesylate in the treatment of BED in adults.

Insights revealed by rodent models of sugar binge eating

Binge eating is seen across the spectrum of eating disorder diagnoses as well as among individuals who do not meet diagnostic criteria. Analyses of the specific types of foods that are frequently binged upon reveal that sugar-rich items feature prominently in binge-type meals, making the effects of binge consumption of sugar an important focus of study. One avenue to do this involves the use of animal models. Foundational and recent studies of animal models of sugar bingeing, both outlined here, lend insight into the various neurotransmitters and neuropeptides that may participate in or be altered by this behavior. Further, several preclinical studies incorporating sugar bingeing paradigms have explored the utility of pharmacological agents that target such neural systems for reducing sugar bingeing in an effort to enhance clinical treatment. Indeed, the translational implications of findings generated using animal models of sugar bingeing are considered here, along with potential avenues for further study.

The dopamine reuptake inhibitor MRZ-9547 increases progressive ratio responding in rats

Drugs that are able to shift effort-related decision making in intact rats towards high-effort response options are largely unknown. Here, we examined the effects of two candidate drugs, MRZ-9547 and its l-enantiomer MRZ-9546 on progressive ratio (PR) responding using two different tasks, a standard PR task that involves increasing ratio requirements and a PR/chow feeding choice task in which animals can lever press for preferred food pellets under a PR schedule or approach freely available less preferred lab chow. Furthermore, we assessed the mechanisms of action of both drugs using in vitro-assay methods and in vivo-microdialysis. Results reveal that MRZ-9547 is a selective dopamine transporter (DAT) inhibitor that moderately stimulated striatal dopamine release. MRZ-9546 was a much less potent DAT inhibitor. Furthermore, MRZ-9547 dose dependently increased the tendency to work for food reinforcement both in the standard PR task and the PR/chow feeding choice task, MRZ-9546 was considerably less active. Relative to MRZ-9547, other DAT-interfering drugs had only moderate (methylphenidate) or marginal (modafinil, d-amphetamine) stimulant effects on PR responding in either task. Collectively, our data demonstrate that the DAT inhibitor MRZ-9547 can markedly stimulate PR responding and shift effort-related decision making in intact rats towards high-effort response options. An analysis of effort-related decision making in rodents could provide an animal model for motivational dysfunctions related to effort expenditure such as fatigue, e.g. in Parkinson's disease or major depression. Our findings suggest that DAT inhibitors such as MRZ-9547 could be potentially useful for treating energy-related symptoms in neurological or neuropsychiatric disorders.

Acoustic noise improves motor learning in spontaneously hypertensive rats, a rat model of attention deficit hyperactivity disorder

The spontaneously hypertensive (SH) rat model of ADHD displays impaired motor learning. We used this characteristic to study if the recently described acoustic noise benefit in learning in children with ADHD is also observed in the SH rat model. SH rats and a Wistar control strain were trained in skilled reach and rotarod running under either ambient noise or in 75 dBA white noise. In other animals the effect of methylphenidate (MPH) on motor learning was assessed with the same paradigms. To determine if acoustic noise influenced spontaneous motor activity, the effect of acoustic noise was also determined in the open field activity paradigm. We confirm impaired motor learning in the SH rat compared to Wistar SCA controls. Acoustic noise restored motor learning in SH rats learning the Montoya reach test and the rotarod test, but had no influence on learning in Wistar rats. Noise had no effect on open field activity in SH rats, but increased corner time in Wistar. MPH completely restored rotarod learning and performance but did not improve skilled reach in the SH rat. It is suggested that the acoustic noise benefit previously reported in children with ADHD is shared by the SH rat model of ADHD, and the effect is in the same range as that of stimulant treatment. Acoustic noise may be useful as a non-pharmacological alternative to stimulant medication in the treatment of ADHD.

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.

Hypophagia and induction of serotonin transporter gene expression in raphe nuclei of male and female rats after short-term fluoxetine treatment

Serotonin (5-HT) is one of the regulators of feeding in humans. Drugs acting on the serotoninergic system are used to treat bulimia nervosa and to enhance the effect of hypocaloric diets in overweight subjects. They act rapidly to normalise feeding when used to treat eating-related problems. To explore the role of the 5-HT transporter (serotonin transporter (SERT)) in the short-term action of serotonin selective reuptake inhibitor fluoxetine, rats were i.p. given the drug for five consecutive days. Acute administration of fluoxetine in male and female rats produced a strong reduction in food intake, an effect that held up when daily treatment was maintained for five consecutive days. This reduction translated into a diminution of body weight that was statistically significant in the case of the males. As a reflection of the body weight change in rats killed after the fifth daily drug injection, retroperitoneal fat pad also decreased; a diminution that was statistically significant in the case of male rats. In these conditions, plasma leptin levels of both male and female rats were lower than in untreated animals. While acute fluoxetine administration did not modify SERT gene expression, subchronic drug treatment increased the content of SERT mRNA in the midbrain raphe complex of both rat genders. These findings may contribute to explain the role of SERT in fluoxetine action on binging and as an adjunct to hypocaloric diets.

Assessing binge eating. An analysis of data previously collected in bingeing rats

As interest in the study of binge eating has increased, several measures of bingeing have been developed for use in animal models. Two of the measures that have been used to distinguish binge-type from normal intake in animal studies are: (1) comparing intake at a given point in time between groups, and (2) assessing escalation of intake across time within groups. Here we use both of these measures to reanalyze data from 10 previous bingeing experiments conducted in our lab. Additionally, the data from two of these studies were then restructured in order to evaluate the use of these measures in binge eating prone (BEP) and resistant (BER) rats, as described by others. Analyses comparing intake at a given point in time indicated bingeing in all 10 studies, while comparisons of escalation indicated bingeing in 9 out of 10 studies. The goal of this study was to compare and contrast the two measures, identify the strengths and weaknesses of each, and determine their appropriateness for a given set of potential outcomes. The results indicate that both intake and escalation are useful measures. However, their limitations need to be taken into consideration when attempting to operationalize binge-type eating in animal models.

Pharmacological management of binge eating disorder: current and emerging treatment options

Growing evidence suggests that pharmacotherapy may be beneficial for some patients with binge eating disorder (BED), an eating disorder characterized by repetitive episodes of uncontrollable consumption of abnormally large amounts of food without inappropriate weight loss behaviors. In this paper, we provide a brief overview of BED and review the rationales and data supporting the effectiveness of specific medications or medication classes in treating patients with BED. We conclude by summarizing these data, discussing the role of pharmacotherapy in the BED treatment armamentarium, and suggesting future areas for research.

Pubertal delay in male nonhuman primates (Macaca mulatta) treated with methylphenidate

Juvenile male rhesus monkeys treated with methylphenidate hydrochloride (MPH) to evaluate genetic and behavioral toxicity were observed after 14 mo of treatment to have delayed pubertal progression with impaired testicular descent and reduced testicular volume. Further evaluation of animals dosed orally twice a day with (i) 0.5 mL/kg of vehicle (n = 10), (ii) 0.15 mg/kg of MPH increased to 2.5 mg/kg (low dose, n = 10), or (iii) 1.5 mg/kg of MPH increased to 12.5 mg/kg (high dose, n = 10) for a total of 40 mo revealed that testicular volume was significantly reduced (P < 0.05) at months 15 to 19 and month 27. Testicular descent was significantly delayed (P < 0.05) in the high-dose group. Significantly lower serum testosterone levels were detected in both the low- (P = 0.0017) and high-dose (P = 0.0011) animals through month 33 of treatment. Although serum inhibin B levels were increased overall in low-dose animals (P = 0.0328), differences between groups disappeared by the end of the study. Our findings indicate that MPH administration, beginning before puberty, and which produced clinically relevant blood levels of the drug, impaired pubertal testicular development until ∼5 y of age. It was not possible to resolve whether MPH delayed the initiation of the onset of puberty or reduced the early tempo of the developmental process. Regardless, deficits in testicular volume and hormone secretion disappeared over the 40-mo observation period, suggesting that the impact of MPH on puberty is not permanent.

The interactive effects of methylphenidate and ethanol on ethanol consumption and locomotor activity in mice

The concomitant use of alcohol (EtOH) and the psychotherapeutic agent dl-methylphenidate (MPH) has risen as a consequence of an increase in ADHD diagnoses within the drinking age population. It was recently found that the combination of MPH and EtOH increases the self-report of pleasurable feelings relative to MPH alone. This finding raises concerns regarding the combined abuse liability for these two widely used drugs. The present behavioral study reports on the development of an adult male C57BL/6J (B6) mouse model to further characterize this MPH-EtOH interaction. We examined the effects of MPH on EtOH consumption in a limited access paradigm and EtOH stimulation of locomotor activity. B6 mice consumed about 2g/kg EtOH daily and MPH dose-dependently reduced drinking. The most effective dose of MPH was 1.25mg/kg, which produced a 41% decrease in drinking and had no effect on locomotor activity. However, when the 1.25mg/kg dose of MPH was combined with a stimulatory dose of ethanol (1.75g/kg) by intraperitoneal injection, there was a significantly enhanced stimulation of locomotor activity. The drug combination increased activity compared to the vehicle or MPH injections by 45% and increased the activity relative to EtOH alone by an additional 25%. The results of the EtOH and MPH interactions observed with the mouse model appear to be behaviorally relevant and suggest several converging mechanisms that may underlie MPH-EtOH interactions.

Early life experience alters behavioral responses to sweet food and accumbal dopamine metabolism

Neonatal handling in rats persistently alters behavioral parameters and responses to stress. Such animals eat more sweet food in adult life, without alterations in lab chow ingestion. Here, we show that neonatally handled rats display greater incentive salience to a sweet reward in a runway test; however they are less prone to conditioned place preference and show less positive hedonic reactions to sweet food. When injected with methylphenidate (a dopamine mimetic agent), non-handled rats increase their sweet food ingestion in the fasted state, while neonatally handled rats do not respond. We did not observe any differences regarding baseline general ambulatory activity between the groups. A lower dopamine metabolism in the nucleus accumbens was observed in handled animals, without differences in norepinephrine content. We suggest that early handling leads to a particular response to positive reinforcers such as palatable food, in a very peculiar fashion of higher ingestion but lower hedonic impact, as well as higher incentive salience, but diminished dopaminergic metabolism in the nucleus accumbens.

Consummatory, anxiety-related and metabolic adaptations in female rats with alternating access to preferred food

Avoidance of and relapse to palatable foods is a qualitative aspect of dieting, a putative risk factor for eating disorders or obesity. The present studies tested the hypotheses that rats with alternating access to highly preferred foods would show: (1) hypophagia, a function of the relative hedonic value of the underaccepted diet, (2) increased anxiety-like behavior and psychomotor arousal when preferred diet was unavailable, (3) obesity-like changes, and (4) stable individual differences in diet-switch-induced hypophagia. Preferences among three high-carbohydrate diets were determined in female Wistar rats (n=16). Adolescent rats (n=162) received the following weekly diet schedules: (1) continuous regular chow (7 days/week), (2) chow (5 days/week) followed by a more preferred diet (2 days/week), or (3) chow (5 days/week) followed by a less preferred chow (2 days/week). Some animals were yoke-restricted (75% calories) when provided chow to increase its rewarding properties. Diurnal locomotor activity was measured in a familiar environment, and anxiety-like behavior was assessed in the elevated plus-maze and defensive withdrawal tests. Rats withdrawn from the preferred diet showed hypophagia, anxiogenic-like behavior, increased locomotion, and weight loss. Chow hypophagia was progressive, individual-specific in magnitude, (partly) non-homeostatic in nature, and blunted by previous chow restriction. Despite eating less, rats cycled with the preferred diet became heavier, fatter, and diurnally less active, with greater feed efficiency and proinflammatory adipokine levels than chow controls. The present diet cycling procedure may model consummatory, anxiety-related, and metabolic effects of qualitative dieting in humans.

Genetic variance contributes to dopamine receptor antagonist-induced inhibition of sucrose intake in inbred and outbred mouse strains

Preference and intake of sucrose varies across inbred and outbred strains of mice. Pharmacological analyses revealed that the greatest sensitivity to naltrexone-induced inhibition of sucrose (10%) intake was observed in C57BL10/J and C57BL/6J strains, whereas 129P3/J, SWR/J and SJL/J strains displayed far less sensitivity to naltrexone-induced inhibition of sucrose intake. Given that dopamine D1 (SCH23390) and D2 (raclopride) receptor antagonism potently reduce sucrose intake in outbred rat and mouse strains, the present study examined the possibility of genetic variance in the dose-dependent (50-1600 nmol/kg) and time-dependent (5-120 min) effects of these antagonists upon sucrose (10%) intake in the eight inbred (BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J, SWR/J and 129P3/J) and one outbred (CD-1) mouse strains previously tested with naltrexone. SCH23390 significantly reduced sucrose intake across all five doses in 129P3/J and SJL/J mice, across four doses in C57BL/6J and BALB/cJ mice, across three doses in DBA/2J, SWR/J, C3H/HeJ and C57BL/10J mice, but only at the two highest doses in CD-1 mice. SCH23390 was 2-3-fold more potent in inhibiting sucrose intake in 129P3/J and SJL/J mice relative to CD-1 mice. In contrast, only the highest equimolar 1600 nmol/kg dose of raclopride significantly reduced sucrose intake in the BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J and 129P3/J, but not the SWR/J and CD-1 strains. The present and previous data demonstrate specific and differential patterns of genetic variability in inhibition of sucrose intake by dopamine and opioid antagonists, suggesting that distinct neurochemical mechanisms control sucrose intake across different mouse strains.

Methylphenidate and nicotine focus responding to an informative discrete CS over successive sessions of appetitive conditioning

Methylphenidate (MP) and nicotine would be expected to improve associative learning, though previous evidence suggests that they should reduce the selectivity with which associations are formed. Here we tested their effects on learning the association between a conditioned stimulus (CS) and food (unconditioned stimulus, UCS) in male Wistar rats. The UCS was delivered immediately (0 s) following CS offset or after a 10 s trace. In addition to the measures of discrete CS conditioning, contextual and trace responding was measured in the inter-trial- and the inter-stimulus-interval, respectively. In all cases, conditioning was measured as nose poking for food. Both MP and nicotine improved the acquisition of discrete cue conditioning. Acquisition was accelerated (compared to saline) under 5 but not 1 mg/kg MP and 0.6, but not 0.4 mg/kg nicotine. In each case, this effect was observed in 0 s but not 10 s conditioned groups. For comparison, some earlier published data obtained following the same procedure with D-amphetamine were re-analysed in the same way. Amphetamine similarly improved conditioning in the 0 s group, in this case at 0.5, but not 1.5 mg/kg. Thus three different dopamine agonists increased the ability to focus responding to CS presentations over successive sessions of appetitive acquisition.

Intermittent access to preferred food reduces the reinforcing efficacy of chow in rats

Intermittent, extended access to preferred diets increases their intake. However, the effects of such access on the acceptance and reinforcing efficacy of otherwise satisfying alternatives is less known. To investigate the role of nonnutritional contributions to the hypophagia that follows removal of preferred food, male Wistar rats were fed a chow diet (Chow A/I), preferred to their regular chow (Chow), which was equally consumed under 1-choice conditions to an even more preferred chocolate-flavored, sucrose-rich diet (Preferred). Rats then learned to obtain Chow A/I pellets under a progressive ratio schedule of reinforcement and were assigned to two matched groups. Each week, one group (n = 15) was diet-cycled, receiving Chow A/I for 5 days followed by the Preferred diet for 2 days. Controls received Chow A/I daily (n = 14). Progressive ratio sessions were performed daily during the 5 days that all subjects received Chow A/I in the home cage. Across 5 wk, diet-cycled rats progressively ate less of the otherwise palatable Chow A/I diet. Hypophagia was not due to greater prior intake or weight gain, motor impairment, or facilitated satiation and was associated with changes in progressive ratio performance that suggested a reduced reinforcing efficacy of the Chow A/I diet in diet-cycled animals. By week 4, diet-cycled animals began to overeat the preferred diet, especially during the first 6 h of renewed access, resembling a deprivation effect. The results suggest that intermittent access to highly preferred food, as practiced by many restrained eaters, may progressively decrease the acceptability of less palatable foods, and may promote relapse to more rewarding alternatives.

Cocaine during adolescence enhances dopamine in response to a natural reinforcer

The use of cocaine during adolescent development could alter the normal growth of brain regions affected by cocaine, specifically the reward system, and impact the adult mesolimbic system. However, there is scant literature aimed at determining whether animals are more vulnerable to the adverse effects of drugs during adolescence. The present study investigated whether cocaine pretreatment in either adolescence or adulthood altered the dopaminergic response to a naturally reinforcing substance in adulthood. To evaluate the responsivity of the mesolimbic system after repeated cocaine, sucrose was offered during the dialysis procedure and dialysates were collected. Regardless of age all saline pretreated rats had significant increases in sucrose-induced extracellular dopamine (DA) levels in the nucleus accumbens septi (NAcc) as compared to baseline levels. Rats pretreated with cocaine as adults also had significant increases in DA levels after sucrose. Interestingly, sucrose intake significantly enhanced DA levels in cocaine pretreated adolescent rats as compared to all other conditions. The results from the present study show that in rats pretreated with cocaine during adolescence there is an enhanced response of the dopaminergic system in animals exposed to a naturally reinforcing substance. Therefore, cocaine exposure during adolescence results in long-term functional changes in the mesolimbic pathway. Future studies need to ascertain the underlying mechanisms and their potential role in cocaine addiction.