Psychostimulants given in adolescence modulate their effects in adulthood using the open field and the wheel-running assays

The effect of environment on cross-sensitization between methylphenidate and amphetamine in female rats

Methylphenidate (MPD) and amphetamine (AMP) are both psychostimulants that are often used to treat behavioral disorders. More recently, it has also been increasingly used illicitly for recreation as well as to improve intellectual performance. Many factors such as age, gender, genetic background, and environment govern the development of behavioral sensitization to MPD and cross-sensitization with other drugs, which are experimental behavioral markers indicating potential of substance dependence and abuse. This study examines the effects of the environment and age when MPD was exposed in adulthood alone as well as in adolescence into adulthood on cross-sensitization with AMP in female SD rats by randomizing animals to either receive the drug in a home cage or a test cage during adolescence, adulthood, or both. In a 34 day experiment, 16 groups of animals starting in adolescence were treated with saline on experimental day one (ED1), followed by a 6 day (ED2-ED7) treatment with either saline, 0.6 mg/kg AMP, 0.6, 2.5, or 10.0 mg/kg MPD. Experimental groups were then subject to a 3-day washout period (ED8-ED10) and then a retreatment with the respective drug on ED11 in adolescence (P-38 to P-49). Experiments continued in the same animal groups now in adulthood (P-60) with a saline treatment (ED1), followed by the same sequence of treatments in adolescence (ED2-ED11;P-61 to P-69). A rechallenge with the same AMP or MPD dose was performed on ED11 (P-70) followed by a single exposure to 0.6 mg/kg AMP on ED12 (P-71) to assess for cross sensitization between MPD and AMP. Animals treated with MPD in both adolescence and adulthood and in the last experimental day of AMP (ED12) showed higher intensity of cross-sensitivity between MPD and AMP as compared to animals treated with MPD only in adulthood. AMP and MPD treatment in adolescence and into adulthood in the home or test cage resulted in significantly higher responses to the drug as compared to those treated only in adulthood. Overall, we conclude that environmental alteration and adolescent exposure to MPD appeared to increase the risk of cross-sensitization to AMP in female SD rats i.e, using MPD in adolescence may increase the probability of becoming dependent on drugs of abuse. This further indicates that age, sex, and environment all influence the response to MPD and AMP, and further work is needed to elucidate the risks associated with MPD and AMP use.

Evaluation of the effects of chemotherapy-induced fatigue and pharmacological interventions in multiple mouse behavioral assays

Fatigue is a common symptom in many diseases and disorders and can reduce quality of life, yet lacks an adequate pharmacological intervention. To identify and develop such interventions, and to better understand fatigue, additional preclinical research is necessary. However, despite numerous mouse behavioral assays reportedly detecting fatigue-like behavior, the assumption that fatigue-like behavior is detected in many assays has not been validated through a cross-assay study. Thus, we modeled fatigue in mice by administering 5-fluorouracil, a chemotherapy drug known to cause fatigue in humans and fatigue-like behavior in mice, then evaluated its effects via voluntary wheel running activity (VWRA), locomotor activity in the open field test (OFT), immobility in the forced swim test (FST), and distance run in the treadmill fatigue test (TFT) and treadmill exercise capacity test. Additionally, taltirelin or methylphenidate was administered to alleviate fatigue-like behavior. As a result of 5-fluorouracil treatment, VWRA and the TFT were markedly reduced, indicating fatigue. The OFT, FST, and treadmill exercise capacity test, however, failed to detect fatigue-like behavior. Interestingly, both taltirelin and methylphenidate alleviated fatigue-like behavior in TFT. These data suggest that, of the current assays, only the TFT and VWRA should be expected to detect fatigue-like behavior. Moreover, this study provides additional evidence that taltirelin may provide a novel treatment for chemotherapy-induced fatigue and warrants further evaluation as an anti-fatigue therapeutic.

Evolution of the Study of Methylphenidate and Its Actions on the Adult Versus Juvenile Brain

OBJECTIVE

Methylphenidate (MPH) is the most often prescribed medication for treatment of ADHD. However, many of its specific cellular and molecular mechanisms of action, as well as developmental consequences of treatment, are largely unknown. This review provides an overview of current understanding of MPH efficacy, safety, and dosage in adult and pediatric ADHD patients, as well as adult animal studies and pioneering studies in juvenile animals treated with MPH.

METHOD

A thorough review of the current literature on MPH efficacy and safety in children, adults, and animal models was included. Results of studies were compared and contrasted.

RESULTS

While MPH is currently considered safe, there is a lack of knowledge of potential developmental consequences of early treatment, as well as differences in drug actions in the developing versus mature brain system.

CONCLUSION

This review emphasizes the need for further research into the age-dependent activities and potency of MPH, and a need for tighter control and clinical relevance in future studies.

Adolescent exposure to cocaine, amphetamine, and methylphenidate cross-sensitizes adults to methamphetamine with drug- and sex-specific effects

The increasing availability, over-prescription, and misuse and abuse of ADHD psychostimulant medications in adolescent populations necessitates studies investigating the long-term effects of these drugs persisting into adulthood. Male and female C57Bl/6J mice were exposed to amphetamine (AMPH) (1.0 and 10 mg/kg), methylphenidate (MPD) (1.0 and 10 mg/kg), or cocaine (COC) (5.0 mg/kg) from postnatal day 22 to 31, which represents an early adolescent period. After an extended period of drug abstinence, adult mice were challenged with a subacute methamphetamine (METH) dose (0.5 mg/kg), to test the long-term effects of adolescent drug exposures on behavioral cross-sensitization using an open field chamber. There were no sex- or dose-specific effects on motor activity in adolescent, saline-treated controls. However, AMPH, MPD, and COC adolescent exposures induced cross-sensitization to a subacute METH dose in adulthood, which is a hallmark of addiction and a marker of long-lasting plastic changes in the brain. Of additional clinical importance, AMPH-exposed male mice demonstrated increased cross-sensitization to METH in contrast to the female-specific response observed in MPD-treated animals. There were no sex-specific effects after adolescent COC exposures. This study demonstrates differential drug, dose, and sex-specific alterations induced by early adolescent psychostimulant exposure, which leads to behavioral alterations that persist into adulthood.

Behavioral and neuronal recording of the nucleus accumbens in adolescent rats following acute and repetitive exposure to methylphenidate

The nucleus accumbens (NAc) has been shown to play a key role in the brain's response to methylphenidate (MPD). The present study focuses on neuronal recording from this structure. The study postulates that repetitive exposure to the same dose of MPD will elicit in some rats behavioral sensitization and in others tolerance. Furthermore, the study postulates that NAc neuronal activity recorded from animals expressing behavioral tolerance after repetitive MPD exposure will be significantly different from NAc neuronal activity recorded from animals expressing behavioral sensitization after repetitive MPD exposure at doses of 0.6, 2.5, 5.0, and 10.0 mg/kg. To test this, behavioral and neuronal activity was recorded concomitantly from the NAc of freely behaving adolescent rats (postnatal day 40) before and after acute and repetitive administration of four different MPD doses. Comparing the acute MPD effect to the repetitive MPD effect revealed that the acute response to MPD exhibited dose-response characteristics: an increase in behavioral activity correlated with increasing MPD doses. On the other hand, following repetitive MPD exposure, some animals exhibited attenuated behavior (tolerance), while others exhibited further increases in the recorded behavior (sensitization). Moreover, the neuronal activity following repetitive MPD exposure recorded in animals exhibiting behavioral sensitization was significantly different from neuronal activity recorded in animals exhibiting behavioral tolerance. This implies that when studying the effects of repetitive MPD administration on adolescent rats, it is advisable to simultaneously record both neuronal and behavioral activity and to evaluate all data based on the animals' behavioral response to the repetitive MPD exposure.

Nucleus accumbens neuronal activity correlates to the animal's behavioral response to acute and chronic methylphenidate

Acute and chronic methylphenidate (MPD) exposure was recorded simultaneously for the rat's locomotor activity and the nucleus accumbens (NAc) neuronal activity. The evaluation of the neuronal events was based on the animal's behavior response to chronic MPD administration: 1) Animals exhibiting behavioral sensitization, 2) Animals exhibiting behavioral tolerance. The experiment lasted for 10days with four groups of animals; saline, 0.6, 2.5, and 10.0mg/kg MPD. For the main behavioral findings, about half of the animals exhibited behavioral sensitization or behavioral tolerance to 0.6, 2.5, and/or 10mg/kg MPD respectively. Three hundred and forty one NAc neuronal units were evaluated. Approximately 80% of NAc units responded to 0.6, 2.5, and 10.0mg/kg MPD. When the neuronal activity was analyzed based on the animals' behavioral response to chronic MPD exposure, significant differences were seen between the neuronal population responses recorded from animals that expressed behavioral sensitization when compared to the NAc neuronal responses recorded from animals exhibiting behavioral tolerance. Three types of neurophysiological sensitization and neurophysiological tolerance can be recognized following chronic MPD administration to the neuronal populations. Collectively, these findings show that the same dose of chronic MPD can elicit either behavioral tolerance or behavioral sensitization. Differential statistical analyses were used to verify our hypothesis that the neuronal activity recorded from animals exhibiting behavioral sensitization will respond differently to MPD compared to those animals exhibiting behavioral tolerance, thus, suggesting that it is essential to record the animal's behavior concomitantly with neuronal recordings.

Acute administration of methylphenidate alters the prefrontal cortex neuronal activity in a dose-response characteristic

The prefrontal cortex (PFC) is part of the collective structures known as the motive circuit. The PFC acts to enhance higher cognitive functions as well as mediate the effects of psychostimulants. Previous literature shows the importance of PFC neuronal adaptation in response to acute and chronic psychostimulant exposure. The PFC receives input from other motive circuit structures, including the ventral tegmental area, which mediates and facilitates the rewarding effects of psychostimulant exposure. PFC neuronal and locomotor activity from freely behaving rats previously implanted with permanent semimicroelectrodes were recorded concomitantly using a telemetric (wireless) recording system. Methylphenidate (MPD) is used as a leading treatment for behavioral disorders and more recently as a cognitive enhancer. Therefore, the property of MPD dose response on PFC neuronal activity was investigated. The results indicate that MPD modulates PFC neuronal activity and behavioral activity in a dose-dependent manner. PFC neuronal responses to 0.6 mg/kg elicited mainly a decrease in PFC neuronal activity, while higher MPD doses (2.5 and 10.0 mg/kg) elicited mainly increased neuronal activity in response to MPD. The correlation between MPD effects on PFC neuronal activity and animal behavior is discussed.

Acute and chronic psychostimulant treatment modulates the diurnal rhythm activity pattern of WKY female adolescent rats

The psychostimulants considered the gold standard in the treatment of attention deficit hyperactivity disorder, one of the most common childhood disorders, are also finding their way into the hands of healthy young adults as brain augmentation to improve cognitive performance. The possible long-term effects of psychostimulant exposure in adolescence are considered controversial, and thus, the objective of this study was to investigate whether the chronic exposure to the psychostimulant amphetamine affects the behavioral diurnal rhythm activity patterns of female adolescent Wistar-Kyoto (WKY) rat. The hypothesis of this study is that change in diurnal rhythm activity pattern is an indicator for the long-term effect of the treatment. Twenty-four rats were divided into two groups, control (N = 12) and experimental (N = 12), and kept in a 12:12-h light/dark cycle in an open-field cage. After 5-7 days of acclimation, 11 days of consecutive non-stop behavioral recordings began. On experimental day 1 (ED1), all groups were given an injection of saline. On ED2 to ED7, the experimental group was injected with 0.6 mg/kg amphetamine followed by 3 days of washout from ED8 to ED10, and amphetamine re-challenge on ED11 similar to ED2. The locomotor movements were counted by the computerized animal activity monitoring system, and the cosinor statistical test analysis was used to fit a 24-h curve of the control recording to the activity pattern after treatment. The horizontal activity, total distance, number of stereotypy, vertical activity, and stereotypical movements were analyzed to find out whether the diurnal rhythm activity patterns were altered. Data obtained using these locomotor indices of diurnal rhythm activity pattern suggest that amphetamine treatment significantly modulates the locomotor diurnal rhythm activity pattern of female WKY adolescent rats.

Behavioral and dorsal raphe neuronal activity following acute and chronic methylphenidate in freely behaving rats

Concomitant behavioral and dorsal raphe (DR) neuronal activity were recorded following acute and chronic dose response of methylphenidate (MPD) in freely moving rats previously implanted with permanent semi-microelectrodes using telemetric (wireless) technology. On experimental day (ED) 1, the neuronal and locomotor activity were recorded after saline (baseline) and MPD (0.6, 2.5 or 10.0mg/kg) injection (i.p.). Animals were injected daily with a single dose of MPD for five consecutive days (ED 2-6) to elicit behavioral sensitization or tolerance. After three washout days, the neuronal and locomotor activity recording was resumed on ED 10 followed by saline and MPD rechallenge injection. The main findings were: (1) the same dose of chronic MPD administration elicited behavioral sensitization in some animals and behavioral tolerance in others. (2) 46%, 56% and 73% of DR units responded to acute 0.6, 2.5 and 10.0mg/kg MPD respectively. (3) 89%, 70% and 86% of DR units changed their baseline activity on ED 10 compared to that on ED 1 in the 0.6, 2.5 and 10.0mg/kg MPD groups respectively. (4) A significant difference in ED 10 baseline activity was observed in the DR neuronal population recording from animals expressing behavioral sensitization compared to that of animals expressing behavioral tolerance. (5) 89%, 78% and 88% of DR units responded to chronic 0.6, 2.5 and 10.0mg/kg MPD respectively. (6) The DR neuronal population recording following acute MPD on ED 1 and rechallenge MPD on ED 10 from animals expressing behavioral sensitization was significantly different from the neuronal population recorded from animals exhibited behavioral tolerance. The correlation between the DR neuronal activity and animal's behavior following chronic MPD exposure suggested that the DR neuronal activity may play an important role in the expression of behavioral sensitization and tolerance induced by chronic MPD administration.

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

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

Methylphenidate modulates the locus ceruleus neuronal activity in freely behaving rat

The electrophysiological properties of the locus coeruleus (LC) neurons in response to acute and chronic administration of methylphenidate (MPD) were investigated. The extracellular LC neuronal activities were recorded from non-anesthetized, freely behaving rats previously implanted bilaterally with permanent semi microelectrodes. The main findings were: (1) On experimental day 1 (ED1), 87% (94/108) of LC units significantly changed their firing rate after initial (acute) MPD (2.5mg/kg, i.p.) administration. The majority of the responsive units (80%, 75/94) increased their firing rate; (2) Daily MPD (2.5mg/kg) injection was given on ED2 through ED6 followed by 3 washout days (ED7 to 9). On ED10, all LC units exhibited a significant change of their baseline activity compared to their baseline activity on ED1; (3) MPD rechallenge on ED10 elicits 94% (101/108) of LC units significantly changed their firing rate; the majority of them (78%, 79/101) increased their firing rate; (4) The effect of rechallenge MPD administration on ED10 were compared to the effect of initial MPD on ED1, 98% of the LC units exhibited a significant change in their firing rate. 41% (43/106) of them exhibited a significant increase in their firing rate while 59% (63/106) units significantly decreased their firing rate which can be interpreted as electrophysiological sensitization or tolerance respectively. In conclusion, the majority of LC neurons significantly increased their firing rate after acute and chronic MPD administration. This data demonstrated that enhanced LC neuronal activities play important role in the effect of MPD.

Acute and chronic methylphenidate alters prefrontal cortex neuronal activity recorded from freely behaving rats

Today's students around the world are striking deals to buy and sell the drug methylphenidate (MPD) for cognitive enhancement. Our knowledge on the effects of MPD on the brain is very limited. The present study was designed to investigate the acute and chronic effect of MPD on the prefrontal cortex (PFC) neurons. On experimental day 1 (ED1) recordings were obtained following saline injections and after 2.5 mg/kg MPD. On ED2 through ED6, daily single 2.5 mg/kg MPD was given followed by 3 washout days (ED7 to 9). On ED10, neuronal recordings were resumed from the same animal after saline and MPD injection similar to that obtained at ED1. Ninety PFC units were recorded, all responded to the initial MPD injection, 66 units (73%) increased their activity at ED10. Recordings were resumed for the 66 units that increased their firing rate at ED1, and following MPD injection 54 units (82%) exhibited significant increases in their baseline firing rates compared to ED1 baseline. When these 54 units were rechallenged (chronic effect) with MPD, 39/54 (72%) exhibited reduction in their firing rate which can be interpreted as tolerance. From the 24 (27%) units that responded to MPD at ED1 by decreasing their activity, 14 units (58%) exhibited a decrease in their baseline firing rates at ED10 compared to ED1 baseline. However, following MPD rechallenge of these 14 units, 11 units (79%) exhibited an increase in their firing rate which is interpreted as sensitization. In conclusion, all PFC units modified their neural baseline activity.

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

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

Nucleus accumbens neuronal activity in freely behaving rats is modulated following acute and chronic methylphenidate administration

Methylphenidate (MPD) is a psychostimulant that enhances dopaminergic neurotransmission in the central nervous system by using mechanisms similar to cocaine and amphetamine. The mode of action of brain circuitry responsible for an animal's neuronal response to MPD is not fully understood. The nucleus accumbens (NAc) has been implicated in regulating the rewarding effects of psychostimulants. The present study used permanently implanted microelectrodes to investigate the acute and chronic effects of MPD on the firing rates of NAc neuronal units in freely behaving rats. On experimental day 1 (ED1), following a saline injection (control), a 30 min baseline neuronal recording was obtained immediately followed by a 2.5 mg/kg i.p. MPD injection and subsequent 60 min neuronal recording. Daily 2.5 mg/kg MPD injections were given on ED2 through ED6 followed by 3 washout days (ED7 to ED9). On ED10, neuronal recordings were resumed from the same animal after a saline and MPD (rechallenge) injection exactly as obtained on ED1. Sixty-seven NAc neuronal units exhibited similar wave shape, form and amplitude on ED1 and ED10 and their firing rates were used for analysis. MPD administration on ED1 elicited firing rate increases and decreases in 54% of NAc units when compared to their baselines. Six consecutive MPD administrations altered the neuronal baseline firing rates of 85% of NAc units. MPD rechallenge on ED10 elicited significant changes in 63% of NAc units. These alterations in firing rates are hypothesized to be through mechanisms that include D1 and D2-like DA receptor induced cellular adaptation and homeostatic adaptations/deregulation caused by acute and chronic MPD administration.

Acute and chronic methylphenidate modulates the neuronal activity of the caudate nucleus recorded from freely behaving rats

Methylphenidate (MPD) is currently one of the most prescribed drug therapies for attention deficit/hyperactivity disorder (ADHD) and moreover is abused for cognitive enhancement and used for recreation by the young and adults. Methylphenidate is used for prolonged periods of time and its mechanism of action on the brain is still unknown. The main action of MPD is known to act on the motive circuit of the brain, and one of these structures is the caudate nucleus (CN). The objective of this study was to investigate the neurophysiological properties of the CN neurons in response to acute and chronic administration of MPD in freely behaving animals, previously implanted with permanent semi microelectrodes. Twenty-six rats were permanently implanted with semi microelectrodes into the CN using general anesthesia. On experimental day one (ED1) the rat was placed into the testing chamber, and neuronal activity was recorded using a wireless (telemetric) headstage device following both a saline and a 2.5 mg/kg MPD injection. From ED2 to ED6 daily injections of 2.5 mg/kg MPD were administered without recordings to induce a chronic effect of the drug, preceded by three days of washout (ED7-ED9) where no injections were given. On ED10 rats were placed back into the testing chamber, the wireless headstage device was attached to skull cap and recordings were resumed for 1 h each following both a saline and re-challenge administration of 2.5 mg/kg MPD. Sixty-seven CN neuronal recorded units from twenty-six animals with identical shape and amplitude at ED1 and ED10 were evaluated. All the 67 CN units responded to MPD administration, 70% (47/67) CN units exhibited an increase in activity following initial 2.5 mg/kg MPD administration and 30% (20/67) exhibited a decrease in neuronal activity. On ED10 all the CN units showed a significant change in their firing rate baseline compared to ED1 baseline, 52% (35/67) exhibiting an increase in their ED10 baseline activity compared to ED1 baseline activity and 48% (32/67) of the CN units at ED10 exhibited decreasing activity. All the CN units responded significantly to MPD rechallenge at ED10, 57% (38/67) of the units exhibited increased neuronal activity while 43% (29/67) exhibited decreasing neuronal activity. The results indicate that the majority of the CN units exhibited neurophysiological sensitization.

Sex differences in the behavioral response to methylphenidate in three adolescent rat strains (WKY, SHR, SD)

Methylphenidate (MPD) is the most widely used drug in the treatment of attention-deficit hyperactivity disorder (ADHD). ADHD has a high incidence in children and can persist in adolescence and adulthood. The relation between sex and the effects of acute and chronic MPD treatment was examined using adolescent male and female rats from three genetically different strains: spontaneously hyperactive rat (SHR), Wistar-Kyoto (WKY) and Sprague-Dawley (SD). Rats from each strain and sex were randomly divided into a control group that received saline injections and three MPD groups that received either 0.6 or 2.5 or 10mg/kg MPD injections. All rats received saline on experimental day 1 (ED1). On ED2 to ED7 and ED11, the rats were injected either with saline or MPD and received no treatment on ED8-ED10. The open field assay was used to assess the dose-response of acute and chronic MPD administration. Significant sex differences were found. Female SHR and SD rats were significantly more active after MPD injections than their male counterparts, while the female WKY rats were less active than the male WKY rats. Dose dependent behavioral sensitization or tolerance to MPD treatment was not observed for SHR or SD rats, but tolerance to MPD was found in WKY rats for the 10mg/kg MPD dose. The use of dose-response protocol and evaluating different locomotor indices provides the means to identify differences between the sexes and the genetic strain in adolescent rats. In addition these differences suggest that the differences to MPD treatment between the sexes are not due to the reproductive hormones.

Behavioral sensitization and cross-sensitization between methylphenidate amphetamine, and 3,4-methylenedioxymethamphetamine (MDMA) in female SD rats

The psychostimulants amphetamine and methylphenidate (MPD/Ritalin) are the drugs most often used to treat attention deficit hyperactivity disorder (ADHD). In addition, students of all ages take these drugs to improve academic performance but also abuse them for pleasurable enhancement. In addition, other psychostimulants such as 3,4-methylenedioxymethamphetamine (MDMA/ecstasy) are used/abused for similar objectives. One of the experimental markers for the potential of a drug to produce dependence is its ability to induce behavioral sensitization and cross sensitization with other drugs of abuse. The objective of this study is to use identical experimental protocols and behavioral assays to compare in female rats the effects of amphetamine, MPD and MDMA on locomotor activity and to determine if they induce behavioral sensitization and/or cross sensitization with each other. The main findings of this study are as follows: (1) acute amphetamine, MPD and MDMA all elicited increases in locomotor activity; (2) chronic administration of an intermediate dose of amphetamine or MPD elicited behavioral sensitization; (3) chronic administration of MDMA elicited behavioral sensitization in some animals and behavioral tolerance in others; (4) cross sensitization between MPD and amphetamine was observed; and (5) MDMA did not show either cross sensitization or cross tolerance with amphetamine. In conclusion, these results suggest that MDMA acts by different mechanisms compared to MPD and amphetamine.