Effect of MDMA (ecstasy) on activity and cocaine conditioned place preference in adult and adolescent rats

Modulation of Gut Microbiome in Ecstasy/MDMA-Induced Behavioral and Biochemical Impairment in Rats and Potential of Post-Treatment with Anacyclus pyrethrum L. Aqueous Extract to Mitigate Adverse Effects

The use of illicit substances continues to pose a substantial threat to global health, affecting millions of individuals annually. Evidence suggests the existence of a 'brain-gut axis' as the involving connection between the central nervous system and gut microbiome (GM). Dysbiosis of the GM has been associated with the pathogenesis of various chronic diseases, including metabolic, malignant, and inflammatory conditions. However, little is currently known about the involvement of this axis in modulating the GM in response to psychoactive substances. In this study, we investigated the effect of MDMA (3,4-methylenedioxymethamphetamine, "Ecstasy")-dependence on the behavioral and biochemical responses, and the diversity and abundance of the gut microbiome in rats post-treated (or not) with aqueous extract of Anacyclus pyrethrum (AEAP), which has been reported to exhibit anticonvulsant activity. The dependency was validated using the conditioned place preference (CPP) paradigm, behavioral, and biochemical tests, while the gut microbiota was identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The CPP and behavioral tests confirmed the presence of MDMA withdrawal syndrome. Interestingly, treatment with AEAP led to a compositional shift in the GM compared to the MDMA-treated rats. Specifically, the AEAP group yielded a higher relative abundance of Lactobacillus and Bifidobacter, while animals receiving MDMA had higher levels of E. coli. These findings suggest that A. pyrethrum therapy may directly modulate the gut microbiome, highlighting a potential target for regulating and treating substance use disorders.

MDMA and memory, addiction, and depression: dose-effect analysis

RATIONALE

±3,4-Methylenedioxymethamphetamine (MDMA) is a recreational drug that shows substantial promise as a psychotherapeutic agent. Still, there is some concern regarding its behavioral toxicity, and its dose-effect relationship is poorly understood. We previously explored the role of dose in the cognitive effects of MDMA in a systematic review of existing literature and found no evidence in animals that MDMA impairs memory at low doses (< 3 mg/kg) but mixed results at high doses (≥ 3 mg/kg). Since this review comprised mostly of single-dose studies and an assortment of methodologies, an empirical dose-ranging study on this topic is warranted.

OBJECTIVES

The current study aims to evaluate the conclusion from our systematic review that 3 mg/kg may be the threshold for MDMA-induced amnesia, and to further understand the dose-effect relationship of MDMA on behavioral assays of memory, addiction, and depression.

METHODS

We systematically examined the effects of 0.01 to 10 mg/kg MDMA on Pavlovian fear conditioning; behavioral sensitization, conditioned place preference, and conditioned responding; and the Porsolt forced swim test in mice.

RESULTS

High doses of MDMA (≥ 3 mg/kg) produced amnesia of fear conditioning memory, some evidence of an addictive potential, and antidepressant effects, while low doses of MDMA (≤ 1 mg/kg) had no effect on these behaviors.

CONCLUSIONS

The present dose-ranging study provides further evidence that 3 mg/kg is the threshold for MDMA-induced amnesia. These findings, in addition to our systematic review, demonstrate that careful selection of MDMA dose is critical. High doses (≥ 3 mg/kg) should likely be avoided due to evidence that they can produce amnesia and addiction. Conversely, there is little evidence to suggest that low doses, which are usually administered in clinical studies (approximately 1-2 mg/kg), will lead to these same adverse effects. Ultra-low doses (< 1 mg/kg) are likely even safer and should be investigated for therapeutic effects in future studies.

Combining ecstasy and ethanol: higher risk for toxicity? A review

Ecstasy use is commonly combined with ethanol consumption. While combination drug use in general is correlated with a higher risk for toxicity, the risk of the specific combination of ecstasy (3,4-methylenedioxymethamphetamine (MDMA)) and ethanol is largely unknown. Therefore, we have reviewed the literature on changes in MDMA pharmacokinetics and pharmacodynamics due to concurrent ethanol exposure in human, animal and in vitro studies. MDMA pharmacokinetics appear unaffected: the MDMA blood concentration after concurrent exposure to MDMA and ethanol was comparable to lone MDMA exposure in multiple human placebo-controlled studies. In contrast, MDMA pharmacodynamics were affected: locomotor activity increased and body temperature decreased after concurrent exposure to MDMA and ethanol compared to lone MDMA exposure. Importantly, these additional ethanol effects were consistently observed in multiple animal studies. Additional ethanol effects have also been reported on other pharmacodynamic aspects, but are inconclusive due to a low number of studies or due to inconsistent findings. These investigated pharmacodynamic aspects include monoamine brain concentrations, neurological (psychomotor function, memory, anxiety, reinforcing properties), cardiovascular, liver and endocrine effects. Although only a single or a few studies were available investigating these aspects, most studies indicated an aggravation of MDMA-induced effects upon concurrent ethanol exposure. In summary, concurrent ethanol exposure appears to increase the risk for MDMA toxicity. Increased toxicity is due to an aggravation of MDMA pharmacodynamics, while MDMA pharmacokinetics is largely unaffected. Although a significant attenuation of the MDMA-induced increase of body temperature was observed in animal studies, its relevance for human exposure remains unclear.

Use and abuse of dissociative and psychedelic drugs in adolescence

Adolescence is a period of profound developmental changes, which run the gamut from behavioral and neural to physiological and hormonal. It is also a time at which there is an increased propensity to engage in risk-taking and impulsive behaviors like drug use. This review examines the human and preclinical literature on adolescent drug use and its consequences, with a focus on dissociatives (PCP, ketamine, DXM), classic psychedelics (LSD, psilocybin), and MDMA. It is the case for all the substances reviewed here that very little is known about their effects in adolescent populations. An emerging aspect of the literature is that dissociatives and MDMA produce mixed reinforcing and aversive effects and that the balance between reinforcement and aversion may differ between adolescents and adults, with consequences for drug use and addiction. However, many studies have failed to directly compare adults and adolescents, which precludes definitive conclusions about these consequences. Other important areas that are largely unexplored are sex differences during adolescence and the long-term consequences of adolescent use of these substances. We provide suggestions for future work to address the gaps we identified in the literature. Given the widespread use of these drugs among adolescent users, and the potential for therapeutic use, this work will be crucial to understanding abuse potential and consequences of use in this developmental stage.

Reward-centricity and attenuated aversions: An adolescent phenotype emerging from studies in laboratory animals

Adolescence is an evolutionarily conserved developmental period, with neural circuits and behaviors contributing to the detection, procurement, and receipt of rewards bearing similarity across species. Studies with laboratory animals suggest that adolescence is typified by a "reward-centric" phenotype-an increased sensitivity to rewards relative to adults. In contrast, adolescent rodents are reportedly less sensitive to the aversive properties of many drugs and naturally aversive stimuli. Alterations within the mesocorticolimbic dopamine and endocannabinoid systems likely contribute to an adolescent reward-sensitive, yet aversion-resistant, phenotype. Although early hypotheses postulated that developmental changes in dopaminergic circuitry would result in a "reward deficiency" syndrome, evidence now suggests the opposite: that adolescents are uniquely poised to seek out hedonic stimuli, experience greater "pleasure" from rewards, and consume rewarding stimuli in excess. Future studies that more clearly define the role of specific brain regions and neurotransmitter systems in the expression of behaviors toward reward- and aversive-related cues and stimuli are necessary to more fully understand an adolescent-proclivity for and vulnerability to rewards and drugs of potential abuse.

Influence of the Novelty-Seeking Endophenotype on the Rewarding Effects of Psychostimulant Drugs in Animal Models

Novelty seeking (NS), defined as a tendency to pursue novel and intense emotional sensations and experiences, is one of the most relevant individual factors predicting drug use among humans. High novelty seeking (HNS) individuals present an increased risk of drug use compared to low novelty seekers. The NS endophenotype may explain some of the differences observed among individuals exposed to drugs of abuse in adolescence. However, there is little research about the particular response of adolescents to drugs of abuse in function of this endophenotype, and the data that do exist are inconclusive. The present work reviews the literature regarding the influence of NS on psychostimulant reward, with particular focus on adolescent subjects. First, the different animal models of NS and the importance of this endophenotype in adolescence are discussed. Later, studies that have used the most common animal models of reward (self-administration, conditioned place preference paradigms) to evaluate how the NS trait influences the rewarding effects of psychostimulants are reviewed. Finally, possible explanations for the enhanced risk of developing substance dependence among HNS individuals are discussed. In conclusion, the studies referred to in this review show that the HNS trait is associated with: (1) increased initial sensitivity to the rewarding effects of psychostimulants, (2) a higher level of drug craving when the subject is exposed to the environmental cues associated with the drug, and (3) enhanced long-term vulnerability to relapse to drug consumption after prolonged abstinence.

Dorsal hippocampal NMDA receptors mediate the interactive effects of arachidonylcyclopropylamide and MDMA/ecstasy on memory retrieval in rats

A combination of cannabis and ecstasy may change the cognitive functions more than either drug alone. The present study was designed to investigate the possible involvement of dorsal hippocampal NMDA receptors in the interactive effects of arachidonylcyclopropylamide (ACPA) and ecstasy/MDMA on memory retrieval. Adult male Wistar rats were cannulated into the CA1 regions of the dorsal hippocampus (intra-CA1) and memory retrieval was examined using the step-through type of passive avoidance task. Intra-CA1 microinjection of a selective CB1 receptor agonist, ACPA (0.5-4ng/rat) immediately before the testing phase (pre-test), but not after the training phase (post-training), impaired memory retrieval. In addition, pre-test intra-CA1 microinjection of MDMA (0.5-1μg/rat) dose-dependently decreased step-through latency, indicating an amnesic effect of the drug by itself. Interestingly, pre-test microinjection of a higher dose of MDMA into the CA1 regions significantly improved ACPA-induced memory impairment. Moreover, pre-test intra-CA1 microinjection of a selective NMDA receptor antagonist, D-AP5 (1 and 2μg/rat) inhibited the reversal effect of MDMA on the impairment of memory retrieval induced by ACPA. Pre-test intra-CA1 microinjection of the same doses of D-AP5 had no effect on memory retrieval alone. These findings suggest that ACPA or MDMA consumption can induce memory retrieval impairment, while their co-administration improves this amnesic effect through interacting with hippocampal glutamatergic-NMDA receptor mechanism. Thus, it seems that the tendency to abuse cannabis with ecstasy may be for avoiding cognitive dysfunction.

The novelty-seeking phenotype modulates the long-lasting effects of adolescent MDMA exposure

Exposure to drugs such as ethanol or cocaine during adolescence induces alterations in the central nervous system that are modulated by the novelty-seeking trait. Our aim was to evaluate the influence of this trait on the long-term effects of MDMA administration during adolescence on spontaneous behavior and conditioned rewarding effects in adulthood. Adolescent mice were classified as high or low novelty seekers (HNS or LNS) according to the hole-board test and received either MDMA (0, 10 or 20mg/kg PND 33-42) or saline. Three weeks later, having entered adulthood (PND>68), one set of mice performed the elevated plus maze and social interaction tests, while another set performed the conditioning place preference (CPP) test induced by cocaine-(1mg/kg) or MDMA-(1mg/kg). Only HNS mice treated with MDMA during adolescence acquired CPP in adulthood with a non-effective dose of cocaine or MDMA. Although it did not produce changes in motor activity, exposure to MDMA during adolescence was associated with more aggressive behaviors (threat and attack) and increased social contacts in HNS mice, while an anxiolytic effect was noted in LNS mice pre-treated with the highest dose of MDMA (20mg/kg). Administration of MDMA (10 or 20mg/kg) induced a decrease in DA levels in the striatum in LNS mice only and lower striatal serotonin levels in mice treated with the highest MDMA dose. Our findings show that adolescent MDMA exposure results in higher sensitivity to the conditioned reinforcing properties of MDMA and cocaine in adult HNS mice, which suggests that the relationship between exposure to MDMA in adolescence and a higher probability of substance is a feature of high novelty seekers only.

The preclinical pharmacology of mephedrone; not just MDMA by another name

The substituted β-keto amphetamine mephedrone (4-methylmethcathinone) was banned in the UK in April 2010 but continues to be used recreationally in the UK and elsewhere. Users have compared its psychoactive effects to those of 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy'). This review critically examines the preclinical data on mephedrone that have appeared over the last 2-3 years and, where relevant, compares the pharmacological effects of mephedrone in experimental animals with those obtained following MDMA administration. Both mephedrone and MDMA enhance locomotor activity and change rectal temperature in rodents. However, both of these responses are of short duration following mephedrone compared with MDMA probably because mephedrone has a short plasma half-life and rapid metabolism. Mephedrone appears to have no pharmacologically active metabolites, unlike MDMA. There is also little evidence that mephedrone induces a neurotoxic decrease in monoamine concentration in rat or mouse brain, again in contrast to MDMA. Mephedrone and MDMA both induce release of dopamine and 5-HT in the brain as shown by in vivo and in vitro studies. The effect on 5-HT release in vivo is more marked with mephedrone even though both drugs have similar affinity for the dopamine and 5-HT transporters in vitro. The profile of action of mephedrone on monoamine receptors and transporters suggests it could have a high abuse liability and several studies have found that mephedrone supports self-administration at a higher rate than MDMA. Overall, current data suggest that mephedrone not only differs from MDMA in its pharmacological profile, behavioural and neurotoxic effects, but also differs from other cathinones.

Adolescents and alcohol: acute sensitivities, enhanced intake, and later consequences

Adolescence is an evolutionarily conserved developmental period characterized by notable maturational changes in the brain along with various age-related behavioral characteristics, including the propensity to initiate alcohol and other drug use and consume more alcohol per occasion than adults. After a brief review of adolescent neurobehavioral function from an evolutionary perspective, the paper will turn to assessment of adolescent alcohol sensitivity and consequences, with a focus on work from our laboratory. After summarizing evidence showing that adolescents differ considerably from adults in their sensitivity to various effects of alcohol, potential contributors to these age-typical sensitivities will be discussed, and the degree to which these findings are generalizable to other drugs and to human adolescents will be considered. Recent studies are then reviewed to illustrate that repeated alcohol exposure during adolescence induces behavioral, cognitive, and neural alterations that are highly specific, replicable, persistent and dependent on the timing of the exposure. Research in this area is in its early stages, however, and more work will be necessary to characterize the extent of these neurobehavioral alterations and further determine the degree to which observed effects are specific to alcohol exposure during adolescence.

Behavioural and neurochemical comparison of chronic intermittent cathinone, mephedrone and MDMA administration to the rat

The synthetic cathinone derivative, mephedrone, is a controlled substance across Europe. Its effects have been compared by users to 3,4-methylenedioxymethamphetamine (MDMA), but little data exist on its pharmacological properties. This study compared the behavioural and neurochemical effects of mephedrone with cathinone and MDMA in rats. Young-adult male Lister hooded rats received i.p. cathinone (1 or 4 mg/kg), mephedrone (1, 4 or 10mg/kg) or MDMA (10mg/kg) on two consecutive days weekly for 3 weeks or as a single acute injection (for neurochemical analysis). Locomotor activity (LMA), novel object discrimination (NOD), conditioned emotional response (CER) and prepulse inhibition of the acoustic startle response (PPI) were measured following intermittent drug administration. Dopamine, 5-hydroxytryptamine (5-HT) and their major metabolites were measured in striatum, frontal cortex and hippocampus by high performance liquid chromatography 7 days after intermittent dosing and 2h after acute injection. Cathinone (1, 4 mg/kg), mephedrone (10mg/kg) and MDMA (10mg/kg) induced hyperactivity following the first and sixth injections and sensitization to cathinone and mephedrone occurred with chronic dosing. All drugs impaired NOD and mephedrone (10mg/kg) reduced freezing in response to contextual re-exposure during the CER retention trial. Acute MDMA reduced hippocampal 5-HT and 5-HIAA but the only significant effect on dopamine, 5-HT and their metabolites following chronic dosing was altered hippocampal 3,4-dihydroxyphenylacetic acid (DOPAC), following mephedrone (4, 10mg/kg) and MDMA. At the doses examined, mephedrone, cathinone, and MDMA induced similar effects on behaviour and failed to induce neurotoxic damage when administered intermittently over 3 weeks.

Metabolic profiling of urine and blood plasma in rat models of drug addiction on the basis of morphine, methamphetamine, and cocaine-induced conditioned place preference

The metabolic profiles of urine and blood plasma in drug-addicted rat models based on morphine (MOR), methamphetamine (MA), and cocaine (COC)-induced conditioned place preference (CPP) were investigated. Rewarding effects induced by each drug were assessed by use of the CPP model. A mass spectrometry (MS)-based metabolomics approach was applied to urine and plasma of MOR, MA, and COC-addicted rats. In total, 57 metabolites in plasma and 70 metabolites in urine were identified by gas chromatography-MS. The metabolomics approach revealed that amounts of some metabolites, including tricarboxylic acid cycle intermediates, significantly changed in the urine of MOR-addicted rats. This result indicated that disruption of energy metabolism is deeply relevant to MOR addiction. In addition, 3-hydroxybutyric acid, L-tryptophan, cystine, and n-propylamine levels were significantly changed in the plasma of MOR-addicted rats. Lactose, spermidine, and stearic acid levels were significantly changed in the urine of MA-addicted rats. Threonine, cystine, and spermidine levels were significantly increased in the plasma of COC-addicted rats. In conclusion, differences in the metabolic profiles were suggestive of different biological states of MOR, MA, and COC addiction; these may be attributed to the different actions of the drugs on the brain reward circuitry and the resulting adaptation. In addition, the results showed possibility of predict the extent of MOR addiction by metabolic profiling. This is the first study to apply metabolomics to CPP models of drug addiction, and we demonstrated that metabolomics can be a multilateral approach to investigating the mechanism of drug addiction.

Oxycodone-induced conditioned place preference and sensitization of locomotor activity in adolescent and adult mice

Nonmedical use of the prescription opioid oxycodone has become a major public health problem in the United States, with special concern for adolescents. Although adults and adolescents have different sensitivities for drugs, little is known about the rewarding effects of oxycodone in adolescents compared to adults, even in rodent models. Here, we investigate sensitivity to oxycodone by the conditioned place preference assay of conditioned reward, and effect on the locomotor activity in adolescent (4 weeks old) and adult (10 weeks old) C57BL/6J mice. Mice of both ages were trained with multiple doses of oxycodone (0, 0.3, 1, and 3 mg/kg) and showed conditioned preference in a dose-dependent manner. The adult mice developed conditioned preference to the lowest dose tested (0.3 mg/kg), but adolescent mice did not. Dose-dependent oxycodone-induced increases in locomotor activity were observed across the conditioning session. Interestingly, adolescent mice developed greater sensitization to the locomotor-activating effects of oxycodone than adult mice. Thus differences in sensitivity to oxycodone, such as the lower initial sensitivity for conditioned preference but greater locomotor sensitization in adolescent mice, may indicate contributing factors in oxycodone abuse and later addiction in human adolescents.

Age-dependent time courses of recovery for motor functions following acute toluene intoxication in rats

Toluene is a psychoactive chemical found in many household products including adhesives and thinners. Inhalation of these vapors can cause euphoria and impairments in motor control and neurological functioning. Misuse and abuse of toluene is most common in children, which may in part be due to an age-dependent neurobehavioral sensitivity to toluene. Here we assessed the effects of acute binge-like toluene inhalations (15 or 30 min; ∼5,000 ppm) on tasks that examine locomotion, exploration, balance, gait, and neurological functioning for adolescent (1 month), young adult (2-3 months), adult (5-6 months), and older adult (10-12 months) rats. Both motor and neurological functions were impaired following acute toluene inhalation at all ages. However, only the duration to recover from deficits in motor functions differed among age groups, with adolescent and young adult rats requiring notably longer recovery times than older rats. Our results are suggestive of an age-dependent vulnerability to the intoxicating effects of toluene.

The effects of abused drugs on adolescent development of corticolimbic circuitry and behavior

Adolescence is a period of significant neurobiological change that occurs as individuals transition from childhood to adulthood. Because the nervous system is in a relatively labile state during this stage of development, it may be especially sensitive to experience-induced plasticity. One such experience that is relatively common to adolescents is the exposure to drugs of abuse, particularly alcohol and psychostimulants. In this review, we highlight recent findings on the long-lasting effects of exposure to these drugs during adolescence in humans as well as in animal models. Whenever possible, our focus is on studies that use comparison groups of adolescent- and adult-exposed subjects as this is a more direct test of the hypothesis that adolescence represents a period of enhanced vulnerability to the effects of drug-induced plasticity. Lastly, we suggest areas of future investigation that are needed and methodological concerns that should be addressed.

Influence of abstinence and intervals between extinction trials on the expression of cocaine-conditioned place preference in adolescent rats

RATIONALE

Disruption of acquired drug-cue associations can effectively decrease relapse. The benefits of extinction training as opposed to abstinence have been reported. Timing of extinction trials is an important variable. Finding an effective extinction regimen can optimize addiction therapies.

OBJECTIVE

To determine the effects of different drug-free periods on cocaine-conditioned place preference (CPP) in rats that either did or did not receive non-reinforced exposure to drug-associated stimuli.

MATERIALS AND METHODS

Male adolescent rats were trained for cocaine-CPP (5, 10, or 15 mg/kg, i.p.) in a biased manner for 8 days and then tested following different intervals.

RESULTS

Rats treated with 15 mg/kg cocaine displayed high and equal CPP on the first test, performed 1, 4, 7, or 14 days following conditioning. Expression of CPP during the test performed 1 day after conditioning was equal in the groups conditioned with 5, 10, or 15 mg/kg cocaine. When the interval before the first test was extended to 14 days, the group treated with 5 mg/kg did not show CPP. Rats treated with the three doses and tested repeatedly at 1, 7, and 14 days did not display CPP on the third test. CPP after treatment with 10 or 15 mg/kg cocaine was already extinguished in the second test but only for an interval of 1-14 days.

CONCLUSIONS

Maintenance of CPP was evident at least 2 weeks after forced abstinence. Extinguished CPP can be obtained after a single extinction trial, performed close to original training and followed by prolonged abstinence. However, with low doses of cocaine, abstinence alone may be sufficient to disrupt drug-cue associations.

Differential alteration of the effects of MDMA (ecstasy) on locomotor activity and cocaine conditioned place preference in male adolescent rats by social and environmental enrichment

RATIONALE

Ecstasy (MDMA) is used predominately by adolescents and young adults. Young MDMA users are more likely than non-users to use other drugs, including cocaine. The response to stimulant drugs can be affected by environmental factors; however, little information exists about the role that housing plays in mediating effects of MDMA in adolescence.

OBJECTIVES

The present experiment examined whether social and environmental factors alter effects of MDMA on activity and cocaine reward.

METHODS

Male adolescent rats were housed on PND 23. Isolated rats were housed alone (1 rat/cage) in an impoverished environment with no toys (II) or enriched with toys (IE). Social rats were housed three/cage with (SE3) or without (SI3) toys. Starting on PND 29, 5 mg/kg MDMA or saline was injected and activity was measured for 60 min once daily for five consecutive days. On PND 36-40, cocaine CPP was conducted.

RESULTS

Saline vehicle-induced activity of II rats was higher than other groups, and all groups became sensitized to the locomotor-stimulant effects of MDMA. In II rats, maximal CPP was increased after MDMA pre-exposure compared to vehicle. Environmental enrichment blocked this; however, dose-effect curves for cocaine CPP shifted to the left in both IE and SE3 rats. In rats with just social enrichment, there were no effects of MDMA on cocaine CPP.

CONCLUSION

Drug prevention and treatment strategies should take into account different environments in which adolescents live. These findings show that MDMA increases cocaine reward in male adolescents, and social enrichment diminishes, while environmental enrichment enhances this.

Adolescent male rats are less sensitive than adults to the anxiogenic and serotonin-releasing effects of fenfluramine

Risk taking behavior increases during adolescence, which is also a critical period for the onset of drug abuse. The central serotonergic system matures during the adolescent period, and its immaturity during early adolescence may contribute to adolescent risk taking, as deficits in central serotonergic function have been associated with impulsivity, aggression, and risk taking. We investigated serotonergic modulation of behavior and presynaptic serotonergic function in adult (67-74 days old) and adolescent (28-34 days old) male rats. Fenfluramine (2 mg/kg, i.p.) produced greater anxiogenic effects in adult rats in both the light/dark and elevated plus maze tests for anxiety-like behavior, and stimulated greater increases in extracellular serotonin in the adult medial prefrontal cortex (mPFC) (1, 2.5, and 10 mg/kg, i.p.). Local infusion of 100 mM potassium chloride into the mPFC also stimulated greater serotonin efflux in adult rats. Adult rats had higher tissue serotonin content than adolescents in the prefrontal cortex, amygdala, and hippocampus, but the rate of serotonin synthesis was similar between age groups. Serotonin transporter (SERT) immunoreactivity and SERT radioligand binding were comparable between age groups in all three brain regions. These data suggest that lower tissue serotonin stores in adolescents limit fenfluramine-stimulated serotonin release and so contribute to the lesser anxiogenic effects of fenfluramine.

Repeated exposure to MDMA and amphetamine: sensitization, cross-sensitization, and response to dopamine D₁- and D₂-like agonists

RATIONALE

Acute exposure to (±) 3, 4-methylenedioxymethamphetamine (MDMA) produces hyperlocomotion that is preferentially expressed in the periphery of closed chambers. Following repeated administration, however, a sensitized hyperlocomotor response is preferentially expressed in the center of an activity box, so that the response resembles the more generalized activity that is produced by D-amphetamine (AMPH).

OBJECTIVES

The present study was designed to determine whether common neuroadaptations underlie the acute and sensitized responses to MDMA and AMPH.

METHODS

Rats were pretreated with five daily injections of MDMA (10.0 mg/kg), AMPH (2.0 mg/kg), or saline. Following a 2-day drug-free period, dose-response curves for hyperactivity produced by MDMA (2.5-10.0 mg/kg), AMPH (0.5-2.0 mg/kg), SKF-81297 (1.0-2.0 mg/kg), or quinpirole (0.25-1.0 mg/kg) were obtained.

RESULTS

Effects of MDMA and AMPH were increased by pretreatment with both drugs. The sensitized response following MDMA exposure was preferentially expressed in the center compartment, but, following AMPH pretreatment, the sensitized response was observed in both compartments. Cross-sensitization was unidirectional; AMPH pretreatment failed to sensitize to the effects of MDMA, but MDMA pretreatment sensitized to the effects of AMPH. MDMA and AMPH pretreatment produced marginal increases in the effects of SKF-81297. The response to quinpirole was, however, greater following MDMA, but not AMPH, pretreatment.

CONCLUSIONS

These data suggest that repeated MDMA exposure produces sensitization via a unique neurochemical effect.

Role of the dopaminergic system in the acquisition, expression and reinstatement of MDMA-induced conditioned place preference in adolescent mice

Background

The rewarding effects of 3,4-methylenedioxy-metamphetamine (MDMA) have been demonstrated in conditioned place preference (CPP) procedures, but the involvement of the dopaminergic system in MDMA-induced CPP and reinstatement is poorly understood.

Methodology/Principal Findings

In this study, the effects of the DA D1 antagonist SCH 23390 (0.125 and 0.250 mg/kg), the DA D2 antagonist Haloperidol (0.1 and 0.2 mg/kg), the D2 antagonist Raclopride (0.3 and 0.6 mg/kg) and the dopamine release inhibitor CGS 10746B (3 and 10 mg/kg) on the acquisition, expression and reinstatement of a CPP induced by 10 mg/kg of MDMA were evaluated in adolescent mice. As expected, MDMA significantly increased the time spent in the drug-paired compartment during the post-conditioning (Post-C) test, and a priming dose of 5 mg/kg reinstated the extinguished preference. The higher doses of Haloperidol, Raclopride and CGS 10746B and both doses of SCH 23390 blocked acquisition of the MDMA-induced CPP. However, only Haloperidol blocked expression of the CPP. Reinstatement of the extinguished preference was not affected by any of the drugs studied. Analysis of brain monoamines revealed that the blockade of CPP acquisition was accompanied by an increase in DA concentration in the striatum, with a concomitant decrease in DOPAC and HVA levels. Administration of haloperidol during the Post-C test produced increases in striatal serotonin, DOPAC and HVA concentrations. In mice treated with the higher doses of haloperidol and CGS an increase in SERT concentration in the striatum was detected during acquisition of the CPP, but no changes in DAT were observed.

Conclusions/Significance

These results demonstrate that, in adolescent mice, the dopaminergic system is involved in the acquisition and expression of MDMA-induced CPP, but not in its reinstatement.

Adolescent pre-exposure to ethanol and 3,4-methylenedioxymethylamphetamine (MDMA) increases conditioned rewarding effects of MDMA and drug-induced reinstatement

Many adolescents often take ethanol (EtOH) in combination with 3,4-methylenedioxymethylamphetamine (MDMA). In the present work, we used a mouse model to study the effect of repeated pre-exposure during adolescence to EtOH (2 g/kg), MDMA (10 or 20 mg/kg) or EtOH + MDMA on the rewarding and reinstating effects of MDMA in the conditioned place preference (CPP) paradigm. Pre-exposure to EtOH, MDMA or both increased the rewarding effects of a low dose of MDMA (1.25 mg/kg). These pre-treatments did not affect the acquisition of the CPP induced by 5 mg/kg of MDMA. However, the CPP was more persistent in mice pre-exposed to both doses of MDMA or to EtOH + MDMA20. After extinction of the CPP induced by 5 mg/kg of MDMA, reinstatement was observed in all groups with a priming dose of 2.5 mg/kg of MDMA, in the groups pre-exposed to EtOH or MDMA alone with a priming dose of 1.25 mg/kg, and in the groups pre-treated with MDMA alone with a priming dose of 0.625 mg/kg. Pre-treatment during adolescence with MDMA or EtOH induced long-term changes in the level of biogenic amines [dihydroxyphenyl acetic acid, homovanillic acid, dopamine turnover, serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindole acetic acid (5-HIAA) in the striatum, and 5-HT and 5-HIAA in the cortex] after the first reinstatement test, although these effects depended on the dose used during conditioning. These results suggest that exposure to EtOH and MDMA during adolescence reinforces the addictive properties of MDMA.

Locomotor activity changes in female adolescent and adult rats during repeated treatment with a cannabinoid or club drug

Adolescents and young adults of both sexes are the primary consumers of "club" drugs; yet, most of the mechanistic preclinical research in this area has been performed in adult male rodents. The purpose of this study was to evaluate the acute and repeated effects of drugs that are commonly abused by adolescents in female adolescent and adult rats in a rodent model of behavioral sensitization. During two five-day periods separated by a two-day break, rats were injected daily with saline or with one of the following drugs: cocaine (7 or 15 mg/kg), ketamine (3 or 10 mg/kg), 3,4-methylenedioxymethamphetamine (MDMA) (3, 10, or 30 mg/kg), or Δ(9)-tetrahydrocannabinol (THC) (0.03, 0.1, 0.3 or 1 mg/kg) and their locomotor activity was measured. Cocaine increased activity across days in both age groups. Whereas ketamine produced progressive increases in activity with repeated administration in rats of both ages, MDMA increased, and then decreased, activity in the chronic dosing regimen in female adolescents only. Tolerance to the initial stimulatory effects of low doses of THC was observed at both ages. The results with THC are similar to those obtained for male rats tested under identical conditions in a previous study; however, in contrast with the present results in females, male adolescent rats in the previous study failed to develop behavioral sensitization to ketamine. Together, these results suggest that age and sex strongly influence the progressive adaptive changes that occur with repeated administration of some, but not all, of these commonly abused substances.

Mephedrone (4-methylmethcathinone, 'meow'): acute behavioural effects and distribution of Fos expression in adolescent rats

Mephedrone (4-methylmethcathinone) is a novel recreational drug that has rapidly increased in popularity in recent years. Users report mephedrone as having the stimulant-like qualities of methamphetamine and cocaine, combined with the prosocial, entactogenic effects of 3,4-methylenedioxymethamphetamine (MDMA). Anecdotal and case study reports indicate that mephedrone may have the potential to engender compulsive patterns of use as well as toxicity in overdose. However, there have been almost no neuropharmacological investigations of the drug up to this point. Here we examined the effects of two different mephedrone doses [15 and 30 mg/kg, intraperitoneal (IP)] relative to the well-known stimulant methamphetamine (2 mg/kg IP) in adolescent rats. Rats were injected, assessed for locomotor activity for 60 minutes and then tested in a 10-minute social preference test (measuring time spent in close proximity to a real rat versus a dummy rat). Their brains were then processed using Fos immunohistochemistry to determine patterns of brain activation. Results showed that mephedrone caused profound locomotor hyperactivity at both dose levels while tending to reduce social preference. Patterns of Fos expression with mephedrone resembled a combination of those observed with methamphetamine and MDMA, with particularly strong Fos expression in the cortex, dorsal and ventral striatum, ventral tegmental area (typical of both MDMA and methamphetamine) and supraoptic nucleus (typical of MDMA). These results demonstrate for the first time the powerful stimulant effects of mephedrone in animal models and its capacity to activate mesolimbic regions. These results also provide some empirical basis to user reports that mephedrone subjectively resembles a MDMA/methamphetamine hybrid.

Adolescent neurobehavioral characteristics, alcohol sensitivities, and intake: Setting the stage for alcohol use disorders?

The transition to adolescence is characterized by rapid biological transformations that include not only the hormonal and physiological changes of puberty but also dramatic changes in the brain as well. Similar neural and physiological changes are associated with the transition from immaturity to maturity across a variety of mammalian species, along with a variety of common adolescent-typical behavioral characteristics. Among the neural systems undergoing alterations during adolescence are those that modulate sensitivity to a variety of alcohol effects, potentially increasing the propensity for relatively high levels of adolescent alcohol use, which in turn may set the stage for later alcohol use disorders. This article reviews research on adolescent alcohol sensitivities and suggests possible implications of these findings for the frequent initiation and relatively high levels of alcohol intake seen at this age.

Juvenile and adult rats differ in cocaine reward and expression of zif268 in the forebrain

Adolescents are more likely to experiment with and become addicted to drugs of abuse. A number of studies indicate that the developmental forebrain may be responsible for making adolescents vulnerable to the addictive properties of such drugs. The aim of this study was to first compare behavioral responses to novelty and cocaine between juvenile and adult rats and then compare levels of the immediate-early gene zif268 activation in several forebrain areas via in situ hybridization. We found that juveniles demonstrated higher locomotion scores and required a higher dose of cocaine than adults to establish a conditioned place preference. Additionally, at this higher dose, juvenile rats exhibited higher levels of zif268 mRNA in the prefrontal cortex compared with adults. A developmental effect for increased zif268 mRNA was also observed in the striatum and nucleus accumbens, but there was no interaction with the cocaine dose. These findings hold interesting implications for the study of the molecular mechanisms underlying juvenile drug addiction.

Rewards, aversions and affect in adolescence: emerging convergences across laboratory animal and human data

The adolescent transition is associated with increases in reward- and sensation-seeking, peer-directed social interactions, and risk-taking, with exploratory use of alcohol and other drugs often beginning at this time. These age-related behaviors may have biological roots embedded in the evolutionary past, with similar adolescent-typical characteristics evident across a variety of mammalian species. Drawing across human behavioral and fMRI data and studies conducting in laboratory animals, this review examines processing of rewards, aversions, and affect in adolescence. Evidence for both hyper- and hypo-reactivity during adolescence in the processing of rewards is reviewed, along with possible contributors to these differences. Indications of sometimes heightened reward reactivity during adolescence are contrasted with frequent attenuations in adolescent sensitivity to aversive stimuli. At the same time, adolescents appear particularly prone to becoming emotionally aroused, especially in social contexts. Emerging evidence hints that exaggerated adolescent reactivity in reward and affective systems may be promoted in part by unusual strong cross-reactivity between these systems during adolescence. Such age-related propensities may promote adolescent risk taking, especially in social and exciting contexts, and contribute to adolescent-typical propensities to attach greater benefit and less cost to risky behaviors such as alcohol and drug use than individuals at other ages.

Pretreatment with Δ9-tetrahydrocannabinol (THC) increases cocaine-stimulated activity in adolescent but not adult male rats

Marijuana (Cannabis sativa) remains one of the most widely used illegal drugs, with adolescents being particularly vulnerable to its use and abuse. In spite of this, most studies are conducted in adult animals even though the effects might be quite different in adolescents. Additionally, the use of marijuana often precedes the use of other psychoactive drugs including cocaine, especially when marijuana exposure begins during early adolescence. The purpose of this study was to examine the effects of repeated Δ9-tetrahydrocannabinol (THC), the major active ingredient in marijuana, in adolescents compared to adults and to determine its subsequent effects on cocaine-stimulated activity. To this end, adolescent (postnatal day PND 34) and adult (PND 66) rats were administered 3 mg/kg/day THC for 8 days and locomotor activity was measured on days 1, 2, 7 and 8 after dosing. On day 12 (4 days after the last dose of THC), rats were injected with escalating doses of cocaine and behavior was recorded. Results show that THC depressed locomotor activity in adult rats but not in adolescents. However, following a cocaine challenge, adolescents exposed to THC showed increased locomotor responses to cocaine compared to chronic vehicle-injected controls. This was not seen in adults. These results show that the effects of cocaine are enhanced after THC in adolescents, but not adults, and that this might account for the greater transition to cocaine after early, as opposed to later, marijuana use.

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.

Residual performance impairments in adult rats trained on an object discrimination task subsequent to cocaine administration during adolescence

The present study was conducted to determine whether cognitive impairments in adult rats treated with cocaine during adolescence demonstrated in previous investigations extend to tests of object discrimination learning. Accordingly, 30-day-old male Long-Evans rats were injected subcutaneously with either 10 or 20 mg/kg cocaine or received control injections of saline for 7-8 consecutive days. An extended abstinence period was then introduced (mean = 70.7 ± 9.8 days) before subjects, who were now young adults (mean = 106.3 ± 10.2 days old), were assessed for acquisition of a two-choice object discrimination task. Using a correctional learning procedure conducted in a water maze, subjects were trained (eight trials per day for 10 days) to approach one of two multi-dimentional 'junk' objects. Although all animals acquired the discrimination to a reasonable extent, cocaine-treated subjects exhibited lower percentages of correct choices over the course of training (10 mg/kg = 59.6 ± 7.2% and 20 mg/kg = 59.4 ± 4.9%) relative to the saline control group (67.5 ± 4.9%). Further analyses revealed that saline-treated subjects acquired proficient discrimination performance earlier during the course of training, achieving an approximate 72% performance rate after only 3 days of training. This was in contrast to the two cocaine-treated groups needing 7 days of training to achieve comparable levels of performance. In addition, saline-treated subjects required significantly fewer trials (20.8 ± 8.9) than either cocaine-treated group (10 mg/kg = 52.2 ± 11.9 and 20 mg/kg = 63.3 ± 8.7) to reach an 87.5% correct response criterion (i.e. 7-correct-out-of-8-consecutive-trials) and performed at a higher above-chance level (13.5%) than either cocaine-treated group (3.6% and 5.3% for the 10 and 20 mg/kg cocaine groups, respectively). These findings demonstrate the existence of cognitive impairments in adulthood subsequent to cocaine exposure during adolescence despite a prolonged drug-free interval. Speculation regarding the neurobiological basis for this effect, especially with regard to alterations to prefrontal circuitry, is provided.

MDMA ("ecstasy") abuse as an example of dopamine neuroplasticity

A number of reviews have focused on the short- and long-term effects of MDMA and, in particular, on the persistent deficits in serotonin neurotransmission that accompany some exposure regimens. The mechanisms underlying the serotonin deficits and their relevance to various behavioral and cognitive consequences of MDMA use are still being debated. It has become clear, however, that some individuals develop compulsive and uncontrolled drug-taking that is consistent with abuse. For other drugs of abuse, this transition has been attributed to neuroadaptations in central dopamine mechanisms that occur as a function of repeated drug exposure. A question remains as to whether similar neuroadaptations occur as a function of exposure to MDMA and the impact of serotonin neurotoxicity in the transition from use to abuse. This review focuses specifically on this issue by first providing an overview of human studies and then reviewing the animal literature with specific emphasis on paradigms that measure subjective effects of drugs and self-administration as indices of abuse liability. It is suggested that serotonin deficits resulting from repeated exposure to MDMA self-administration lead to a sensitized dopaminergic response to the drug and that this sensitized response renders MDMA comparable to other drugs of abuse.

Sensitivity to ethanol and other hedonic stimuli in an animal model of adolescence: implications for prevention science?

Age-related patterns of sensitivity to appetitive and aversive stimuli seemingly have deep evolutionary roots, with marked developmental transformations seen during adolescence in a number of relatively ancient brain systems critical for motivating and directing reward-related behaviors. Using a simple animal model of adolescence in the rat, adolescents have been shown to be more sensitive than their adult counterparts to positive rewarding effects of alcohol, other drugs, and certain natural stimuli, while being less sensitive to the aversive properties of such stimuli. Adolescent-typical alcohol sensitivities may be exacerbated further by a history of prior stress or alcohol exposure as well as by genetic vulnerabilities, permitting relatively high levels of adolescent alcohol use and perhaps an increased probability for the emergence of abuse disorders. A number of potential (albeit tentative) implications of these basic research findings for prevention science are considered.

Repeated intermittent methylenedioxymethamphetamine exposure protects against the behavioral and neurotoxic, but not hyperthermic, effects of an MDMA binge in adult rats

We have recently shown that chronic intermittent exposure of adolescent rats to 3,4-methylenedioxymethamphetamine (MDMA or Ecstasy) completely blocks the reduction in serotonin transporter (SERT) binding and the hypoactivity seen following a subsequent MDMA binge treatment. The present study determined whether a similar neuroprotective effect also occurs in rats given the same intermittent MDMA exposure in adulthood. Adult male Sprague-Dawley rats were given either MDMA (10 mg/kg x 2) or saline, every fifth day, from postnatal day (PD) 60 to PD 85. The MDMA-induced latency until seminal plug production was reduced over the course of intermittent treatments. After a 1-week wash-out period, animals received either a low- or high-dose MDMA binge (2.5 or 5.0 mg/kg x 4). Core body temperature was measured during and after the binge to determine the effects of MDMA pretreatment on MDMA-induced hyperthermia. Spontaneous motor activity was determined the next day, and cortical and hippocampal samples were collected at 1 week postbinge to measure serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations as well as [3H]citalopram binding to SERT. Hyperthermia occurred more rapidly and seminal discharge was more common in the MDMA-pretreated group compared to the MDMA-naïve group in animals given the low-dose binge. MDMA preexposure protected animals from the reductions in cortical 5-HT levels and SERT binding produced by the high-dose binge and blocked the postbinge hypoactivity. These findings indicate that chronic, intermittent MDMA exposure in adulthood induces neuroprotective effects similar to those seen with adolescent treatment. However, there was also evidence for drug-induced sensitization in adults that was not observed in adolescents. Thus, altered drug sensitivity in chronic Ecstasy users may depend not only on the frequency and pattern of use but also on the age of the user.

An in vivo microdialysis assessment of concurrent MDMA and cocaine administration in Sprague-Dawley rats

BACKGROUND AND RATIONALE

Despite the popularity of polysubstance abuse among recreational methylendioxymethamphetamine (MDMA) users, relatively few controlled experimental studies have documented the neurobehavioral effects of MDMA in combination with other abused substances.

OBJECTIVE

In this study, the combined acute effects of MDMA and cocaine were examined by conducting in vivo microdialysis in the rat nucleus accumbens while simultaneously monitoring locomotor activity.

METHODS

Male Sprague-Dawley rats were administered cocaine (10 or 20 mg/kg, i.p.), MDMA (1.5 or 3.0 mg/kg, i.p.), or one of four combinations of cocaine and MDMA during microdialysis experiments. Locomotor activity was monitored, and dialysis samples were collected every 30 min for 3 h prior to injections, for one 30-min period following saline injections, and for an additional 3-h period following drug injections. Samples were analyzed for dopamine content by high-performance liquid chromatography with electrochemical detection.

RESULTS

Significant differences in locomotor activity and dopamine efflux were found among treatment groups, with some MDMA/cocaine combinations producing significantly greater increases compared to single doses of cocaine or MDMA within the first 30 min after injection.

CONCLUSION

Considering the popularity of polysubstance use among recreational MDMA users, the clinical implications of the current findings warrant further investigation.

Is ecstasy a drug of dependence?

This paper examines the evidence for an MDMA or "ecstasy" dependence syndrome. Animal evidence suggests that MDMA may be a less potent reinforcer than other drugs, but that it does have dependence potential. This suggests that (a) ecstasy dependence might be less likely than dependence upon other drugs; and (b) factors related to the behavioural and psychological aspects of reward and dependence may make a relatively greater contribution for ecstasy than for other drugs, where physically centred (and better understood) features of dependence may be more salient. Human evidence supports this proposition. Some people report problems with their use, but the literature suggests that physical features play a more limited role than psychological ones. Tolerance is apparent, and withdrawal is self-reported, but it is unclear whether these reports distinguish sub-acute effects of ecstasy intoxication from symptoms reflective of neuroadaptive processes underlying a "true" withdrawal syndrome. Studies examining the structure of dependence upon ecstasy suggest it may be different from drugs such as alcohol, methamphetamine and opioids. Consistent with studies of hallucinogens, a two-factor structure has been identified with factors suggestive of "compulsive use" and "escalating use". Regardless of the nature of any dependence syndrome, however, there is evidence to suggest that a minority of ecstasy users become concerned about their use and seek treatment. Further controlled studies are required to investigate this phenomenon.

Motivational systems in adolescence: possible implications for age differences in substance abuse and other risk-taking behaviors

Adolescence is an evolutionarily conserved developmental phase characterized by hormonal, physiological, neural and behavioral alterations evident widely across mammalian species. For instance, adolescent rats, like their human counterparts, exhibit elevations in peer-directed social interactions, risk-taking/novelty seeking and drug and alcohol use relative to adults, along with notable changes in motivational and reward-related brain regions. After reviewing these topics, the present paper discusses conditioned preference and aversion data showing adolescents to be more sensitive than adults to positive rewarding properties of various drugs and natural stimuli, while less sensitive to the aversive properties of these stimuli. Additional experiments designed to parse specific components of reward-related processing using natural rewards have yielded more mixed findings, with reports of accentuated positive hedonic sensitivity during adolescence contrasting with studies showing less positive hedonic affect and reduced incentive salience at this age. Implications of these findings for adolescent substance abuse will be discussed.

Acquisition and reinstatement of MDMA-induced conditioned place preference in mice pre-treated with MDMA or cocaine during adolescence

Those who take ecstasy are more likely to consume other drugs than non-users with cocaine abuse being reported by 75.5% of high school student MDMA (+/- 3,4-methylenedioxymetamphetamine hydrochloride) users. The aim of this work was to evaluate the effects of exposure during adolescence to MDMA, cocaine or to both drugs on the MDMA-induced conditioned place preference (CPP) in adult mice. Animals received two daily administrations of saline, 10 mg/kg of MDMA, 25 mg/kg of cocaine or 10 mg/kg of MDMA plus 25 mg/kg of cocaine over 3 days (from PD28 to 30). Three weeks after pre-treatment, the MDMA-induced CPP procedure was initiated (PD52). Acquisition of CPP was induced with a sub-threshold dose of MDMA (1.25 mg/kg) only in animals treated during adolescence with MDMA alone. Preference was established in all the groups after conditioning with 10 mg/kg of MDMA, while the time required to achieve extinction was longer in those pre-treated with cocaine or MDMA alone (46 and 28 sessions, respectively). Moreover, preference was reinstated with progressively lower priming doses of MDMA in mice pre-treated with MDMA or cocaine alone. These results demonstrate that early exposure to MDMA or cocaine induces long-lasting changes that last until adulthood and modify the response of animals to MDMA.

Are adolescents more vulnerable to drug addiction than adults? Evidence from animal models

BACKGROUND AND RATIONALE

Epidemiological evidence suggests that people who begin experimenting with drugs of abuse during early adolescence are more likely to develop substance use disorders (SUDs), but this correlation does not guarantee causation. Animal models, in which age of onset can be tightly controlled, offer a platform for testing causality. Many animal models address drug effects that might promote or discourage drug intake and drug-induced neuroplasticity.

METHODS

We have reviewed the preclinical literature to investigate whether adolescent rodents are differentially sensitive to rewarding, reinforcing, aversive, locomotor, and withdrawal-induced effects of drugs of abuse.

RESULTS AND CONCLUSIONS

The rodent model literature consistently suggests that the balance of rewarding and aversive effects of drugs of abuse is tipped toward reward in adolescence. However, increased reward does not consistently lead to increased voluntary intake: age effects on voluntary intake are drug and method specific. On the other hand, adolescents are consistently less sensitive to withdrawal effects, which could protect against compulsive drug seeking. Studies examining neuronal function have revealed several age-related effects but have yet to link these effects to vulnerability to SUDs. Taken together, the findings suggest factors which may promote recreational drug use in adolescents, but evidence relating to pathological drug-seeking behavior is lacking. A call is made for future studies to address this gap using behavioral models of pathological drug seeking and for neurobiologic studies to more directly link age effects to SUD vulnerability.

Differential effects of methamphetamine and cocaine on conditioned place preference and locomotor activity in adult and adolescent male rats

Human and animal laboratory studies show that adolescents and adults respond differently to drugs and that drug administration during adolescence leads to different behavioral effects than during adulthood. Although there are a number of studies on the effects of cocaine, little is known about the effects of methamphetamine in adolescent vs adult rats. In the present study, sensitivity to the conditioned reward of multiple doses of methamphetamine or cocaine was evaluated in male adolescent (PND 34) and adult (PND 66) rats using a conditioned place preference (CPP) paradigm. In addition, the locomotor-activating effects of methamphetamine were determined across a 5-day period of administration. After 3 days of training with cocaine, both adolescent and adult male rats developed CPP to cocaine, however, the dose-effect curve for cocaine CPP was shifted to the left in adolescent compared to adult rats. In contrast to the development of CPP to cocaine in both groups after 3 days of conditioning, methamphetamine CPP occurred only in adolescent, and not in adult rats. After 5 days of training, however, both adolescent and adult rats exhibited identical responses to multiple doses of methamphetamine and a significant CPP was observed in both groups. Daily administration of methamphetamine increased locomotor activity in both adolescent and adult rats, with a greater effect seen in the adults. In neither group, was there evidence of a significant sensitization to the locomotor-activating effects of methamphetamine. These data show that adolescents are more sensitive to psychostimulant reward and thus to the conditioned rewarding properties of cocaine or methamphetamine than adults. A better understanding of this difference may lead to age-specific preventions and treatments for psychostimulant abuse.

Age-dependent differences in sensitivity and sensitization to cannabinoids and 'club drugs' in male adolescent and adult rats

Lifelong substance abuse is often initiated during adolescence; yet, most pre-clinical research in this area has been conducted in adult animals. Substantial evidence exists that the brain development that continues throughout adolescence may result in pharmacological responses that differ in a crucial manner from those of adults. The goal of this study was to evaluate age differences in motor activity following acute and repeated administration of drugs that are commonly abused by adolescents, including cocaine, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), and the club drugs, ketamine and 3,4-methylenedioxymethamphetamine (MDMA). Adolescent and adult male rats were injected once daily with saline or with a dose of one of the test drugs for two 5-day dosing periods, separated by a 2-day drug holiday during which they remained in their home cages. Following each injection, rats were placed in a locomotor chamber for a 20-minute session. The potencies of cocaine, ketamine and MDMA for producing motor stimulation were less in male adolescents than in male adults. Furthermore, sensitization to the club drug, ketamine, developed after repeated dosing in adults, but not adolescents. In contrast, adolescents were initially more sensitive to the stimulatory effects of low doses of Delta(9)-THC than were adults, although rapid tolerance occurred. These results suggest that adolescents are less sensitive to the acute and repeated stimulant effects of some, but not all, of the drugs that are preferentially abused by this age group. This differential sensitivity may contribute to the different patterns of use that have been noted in adolescent versus adult drug abusers.

Evaluation of age and sex differences in locomotion and catalepsy during repeated administration of haloperidol and clozapine in adolescent and adult rats

Adolescence is associated with characteristic behavioral patterns as well as with substantial neuronal pruning and re-organization of the brain. Recent research has determined that the effects of various centrally active drugs differ in adolescents and adults. This study examined the motor effects of two prototypic antipsychotics in adult [>postnatal day 70 (PN70)] and adolescent (PN30-PN39) rats. Rats were injected daily with saline, 0.3 mg/kg haloperidol, or 10 mg/kg clozapine for 10 days and activity and catalepsy were measured. Adolescents of both sexes were less sensitive to the cataleptic effects of haloperidol than were adults. Male adolescents were also less sensitive to the cataleptic effects of clozapine, although this difference was transitory. In contrast, female adults showed decreased sensitivity to clozapine's effects, differing from all other groups. These results suggest that adolescents of both sexes may be less sensitive to the extrapyramidal motor effects of haloperidol. Translational implications of the clozapine results are less clear; however, results suggest that developmental differences in neurochemical systems affected by clozapine that are also related to motor behavior may play a role. These results also emphasize the importance of age and sex as determinants of the pharmacological effects of these antipsychotics.

Chronic social stress in adolescence influenced both amphetamine conditioned place preference and locomotor sensitization

We previously reported that chronic social stress (SS) in adolescence, but not in adulthood, increased the locomotor-activating effects of nicotine in females, and not males, when tested in adulthood. However, SS rats had decreased locomotor response to nicotine when tested in adolescence. Here, we investigated age-related changes in the effects of SS on both conditioned place preference (CPP) and locomotor sensitization to amphetamine. In the CPP experiment, SS females tested in adolescence had increased preference for the 1.0 mg/kg dose of amphetamine, whereas SS rats of both sexes showed a decrease in CPP for the 0.5 mg/kg dose when tested as adults. Irrespective of time of testing, SS males and females had enhanced locomotor sensitization compared to controls. Thus, adolescent SS produced both immediate and enduring effects on behavioral responses to amphetamine, likely by altering the development of the mesocorticolimbic dopamine system, which holds implications for vulnerability to addiction.