Effects of chronic oral methylphenidate on cocaine self-administration and
striatal dopamine D2 receptors in rodents
Panayotis K. Thanosa,b,⁎, Michael Michaelidesa,b, Helene Benvenistea,
Gene Jack Wanga, Nora D. Volkowb
aBehavioral Neuropharmacology and NeuroImaging Lab, Medical Department,
Building 490, Brookhaven National Laboratory, Upton, NY 11973-5000, United States
bLaboratory of Neuroimaging, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Department of Health and Human Services,
Park Building, Room 150, 12420 Parklawn Drive, MSC 8115, Bethesda, MD 20892-8115, United States
Received 21 December 2006; received in revised form 18 May 2007; accepted 25 May 2007
Available online 5 June 2007
Background: Methylphenidate (MP) and amphetamine, which are the mainstay for the treatment of ADHD, have raised concerns because of their
reinforcing effects and the fear that their chronic use during childhood or adolescence could induce changes in the brain that could facilitate drug
abuse in adulthood.
Methods: Here we measured the effects of chronic treatment (8 months) with oral MP (1 or 2 mg/kg), which was initiated in periadolescent rats
(postnatal day 30). Following this treatment, rats were tested on cocaine self-administration. In addition at 2 and 8 months of treatment we
measured dopamine D2 receptor (D2R) availability in the striatum using [11C]raclopride microPET (μPET) imaging.
Results: Animals treated for 8 months with 2 mg/kg of MP showed significantly reduced rates of cocaine self-administration at adulthood than
vehicle treated rats. D2R availability in the striatum was significantly lower in rats after 2 months of treatment with MP (1 and 2 mg/kg) but
significantly higher after 8 months of MP treatment than in the vehicle treated rats. In vehicle treated rats D2R availability decreased with age
whereas it increased in rats treated with MP. Because low D2R levels in the striatum are associated with a propensity for self-administration of
drugs both in laboratory animals and in humans, this effect could underlie the lower rates of cocaine self-administration observed in the rats given
8 months of treatment with MP.
Conclusions: Eight month treatment with oral MP beginning in adolescence decreased cocaine-self administration (1 mg/kg) during adulthood
which could reflect the increases in D2R availability observed at this life stage since D2R increases are associated with reduced propensity for
cocaine self administration. In contrast, two month treatment with MP started also at adolescence decreased D2R availability, which could raise
concern that at this life stage short treatments could possibly increase vulnerability to drug abuse during adulthood. These findings indicate that
MP effects on D2R expression in the striatum are sensitive not only to length of treatment but also to the developmental stage at which treatment is
given. Future studies evaluating the effects of different lengths of treatment on drug self-administration are required to assess optimal duration of
treatment regimes to minimize adverse effects on the propensity for drug self administration.
© 2007 Published by Elsevier Inc.
Keywords: Psychostimulant; Addiction; Impulsivity; Dopamine transporters; Attention deficit hyperactivity disorder; Drug abuse
commonly diagnosed and treated psychiatric disorder of child-
hood; its prevalence is estimated to be 5–10% of the general
population in the USA (Swanson et al., 1998). Methylphenidate
(MP), commonly known by brand name as Ritalin© or Ad-
derall© (which is a mix of dextro- and racemic amphetamine
Pharmacology, Biochemistry and Behavior 87 (2007) 426–433
⁎Corresponding author. Behavioral Neuropharmacology and NeuroImaging
Lab, Medical Department, Building 490, Brookhaven National Laboratory,
Upton, NY 11973-5000, United States. Tel.: +1 631 344 7364; fax: +1 631 344
E-mail address: email@example.com (P.K. Thanos).
URL: http://www.bnl.gov/thanoslab (P.K. Thanos).
0091-3057/$ - see front matter © 2007 Published by Elsevier Inc.
salts), and amphetamine (AMP) are the most frequently used
treatments for ADHD (Greenhill et al., 2002). Over the past
decade, the prescriptions for these stimulant medications have
increased from less than 2 million in 1991 to over 22 million in
2001, and now it is estimated that up to 6% of school-aged
children are treated with these drugs.
MP and AMP increase extracellular dopamine (DA) in the
brain, as do cocaine and methamphetamine, the most commonly
abused stimulant drugs. MP (like cocaine) increases DA by
blockingDAtransporters (DAT) (Volkow etal.,1995)and AMP
terminals (Jones et al., 1998). Both increase DA in the nucleus
accumbens (NAc), which is thought to underlie the reinforcing
MP and AMP are self-administered by animals (Bergman et al.,
1989; Johanson and Schuster, 1975) and sometimes used re-
creationally by humans (Foley et al., 2000; Kollins et al., 2001).
This has raised concerns about the abuse liability of MP and
AMP and the potential that their chronic use during adolescence
may induce long-term changes that facilitate drug abuse in
adulthood. One clinical study has shown diminished rates of
substance abuse in ADHD children and adolescents treated with
stimulant medications compared with controls (Wilens et al.,
2003). Short-term (5–7 days) ip methylphenidate treatment in
adolescent rats was shown to enhance reactivity to cocaine and
vulnerability to cocaine self-administration (Brandon et al.,
2001). So while it is recognized that ADHD per se constitutes a
risk for substance abuse, the role of stimulant treatment during
adolescence on drug abuse during adulthood is still not properly
understood. Chronic oral MP treatment in adolescent rats fol-
lowed by access to intravenous cocaine self-administration as
adults remains to be examined.
Animal studies investigating the effects of chronic exposure
to MP during the developmental periods corresponding to
human childhood and adolescence have shown changes in the
function of brain DA cells, as well as changes in behavior
(including enhanced sensitivity to stressful stimuli, decreased
response to natural reinforcers, and decreased threshold for
helplessness) and altered responses to the reinforcing properties
of cocaine in adulthood (Bolanos et al., 2003; Brandon et al.,
2001, 2003; Carlezon et al.,2003). The latter effect depended on
when the initial exposure occurred: when administered to young
rats at an age corresponding to childhood in humans, MP
exposure decreased the reinforcing properties of cocaine (Carle-
zon et al., 2003), but when administered to rats atan age roughly
et al., 2001).
MP has also been shown to produce changes in gene expres-
sion consistent with the effects of other psychostimulants. In
particular, striatal dose-dependent increases in the expression of
immediate–early genes (c-fos and zif 268; which encode trans-
cription factors) have been shown to be induced by both acute
and chronic MP (Brandon and Steiner, 2003; Yano and Steiner,
2005a,b), as well as synaptic plasticity factor Homer 1A. Daily
MP (10 mg/kg, ip) treatment for a week led to increases in c-fos
and zif268 gene expression as well as dynorphin expression in
the striatum of adolescent rats, indicating that short term expo-
sure to MP had some significant epigenetic effects which could
lead to a cascade of molecular and cellular changes associated
with DA receptor overstimulation.
Here we assess the effects of chronic oral MP treatment in the
adolescent rat. We gave MP orally since this is the route of
doses (1 mg/kg and 2 mg/kg) that have been shown to be com-
parable to those used clinically (Gerasimov et al., 2000). We
measured striatal D2R availability in-vivo using [11C]raclopride
μPET imaging; since low D2R levels are associated with a
vulnerability to the reinforcing effects of drugs of abuse in
laboratory animals and in humans (Volkow, 1997; Volkow et al.,
1999a,b; Volkow et al., 2002) whereas high levels appear to be
protective against drug self-administration. Also, chronic treat-
ment with cocaine, which is pharmacologically similar to MP
(Volkow et al., 1999a,b) has been previously shown to induce
long lasting decreases in D2R in non human primates, (Nader
etal.,2002)and cocaine abusers have long lasting reductions in
D2R (Volkow et al., 1997). We also measured the effects of
chronic MP treatment on subsequent cocaine self-administra-
tion behavior. Thus, this study was conducted to measure the
striatal D2R availability and 2) cocaine self-administration
behavior. We hypothesized that chronic MP exposure would
result in a lower D2R availability and an increase in cocaine
2. Materials and methods
Male 4-week old Sprague–Dawley rats (Taconic Farms)
were individually housed in a temperature-humidity controlled
environment under a 12-hour light/12-hour dark cycle. Animals
were randomly assigned into 3 groups (N=6/group): 2 mg/kg
MP, 1 mg/kg MP, and water (control) and were given access to
Purina Rat Chow and water ad libitum. All experiments were
conducted in conformity with the National Academy of Sci-
ences Guide for Care and Use of Laboratory Animals (National
Academy of Sciences NRC et al., 1996) and Brookhaven
National Laboratory Institutional Animal Care and Use Com-
mittee protocols. Drinking bottles were filled with the appropri-
ate solution, with a volume equal to the mean volume consumed
by the control animals. All solutions were prepared fresh daily
for corresponding doses of MP. Body weight was recorded daily
throughout the experiment. All MP-treated animals were given
volumes of the MP in water solution based on the volume
consumed by the control animals of similar age and weight that
drank a water solution. MP solutions were prepared fresh daily
according to these volumes to achieve the desired concentra-
tions. We did not see any indication of taste-aversion given that
all animals drank the entire solution daily.
MP hydrochloride (Sigma, St. Louis, MO) was dissolved in
427P.K. Thanos et al. / Pharmacology, Biochemistry and Behavior 87 (2007) 426–433
Cocaine hydrochloride (1 mg/kg; Sigma, St. Louis, MO) was
dissolved in saline (0.9% NaCl).
2.3.1. Experiment 1. D2R availability
184.108.40.206. D2R μPET imaging.
performed using a μPET R4 Scanner (CTI Concorde, Knox-
ville, TN). Each animal was anesthetized and injected intra-
venously with [11C]raclopride (a D2R-specific ligand) and
scanned two times: a) after 2 months of treatment (3 months of
age) and b) after 8 months of treatment (9 months of age).
Raclopride was purchased from Sigma and [11C]raclopride was
synthesized in the Brookhaven National Laboratory Cyclotron
as previously described (Fowler and Volkow, 1998; Fowler
et al., 1999).
and xylazine (10 mg/kg) and placed in a custom stereotaxic head
μPET assessment of D2R was
then injected via the tail vein with [11C]raclopride (334+/
−76 μCi; specific activity, 1.4–7.0 mCi/nmol). Injected volumes
were approximately 400 μl. Total acquisition time was 60 min
(24 frames: 6 frames, 10 s; 3 frames, 20 s; 8 frames, 60 s; 4
frames, 300 s; 3 frames, 600 s), and data were acquired in fully
3-dimensional mode with maximum axial acceptance angle
(+/−28°). Images were constructed using Fourier rebinning
(Matej et al., 1998) followed by 2-dimensional filtered back-
projection with a ramp filter cutoff at Nyquist frequency.
220.127.116.11. MRI imaging.
tive image analysis using μMRI-μPET image coregistration, a
was acquired on a 9.4T Bruker Biospin Avance scanner (Bruker
Biospin Corporation, Massachusetts, USA) using the RARE
pulse sequence with an acceleration factor of 8 and TE/TR=40/
(FOV) of 3.84×3.84 cm2, which gave an in plane resolution of
For accurate qualitative and quantita-
Fig. 1. Representative [11C]raclopride μPET images across groups following chronic oral treatment with MP or vehicle.
428 P.K. Thanos et al. / Pharmacology, Biochemistry and Behavior 87 (2007) 426–433
150 μm. The slice thickness was 0.9 mm with a 0.1 mm gap
between slices; and the total acquisition time was 11 min.
ware Suite (PMOD Technologies, Switzerland), μPET images
were co-registered with the previously generated μMRI tem-
plate. Initially the two images were co-registered using the
method of Mutual Information (MI) (Woods et al., 1992),
(Woods et al., 1993); through a MI algorithm implemented
within the PMOD environment, followed by a manual adjust-
ment of the μPET image in all three planes (sagital, coronal,
transversal) so that the Harderian Glands (HG) and caudate
putamen from both imaging modalities matched. Using the
Region of Interest (ROI) method; ROIs were selected for the left
and right striatum (ST) and the cerebellum (CB) using the HG
as a reference point. Specifically the STand CB for each animal
were identified as 8 and 12 slices respectively, caudal to the HG
(slice thickness, 1.2 mm). It has been shown previously that the
HG (located just rostral of the brain), because of their uptake of
radioactivity, are useful markers in rodent PET studies (Thanos
et al., 2002; 2004; Hume et al., 1996; Fukuyama et al., 1998;
Kuge et al., 1997).
Qualitative assessment of μPET images was performed, and
representative μPET images of each rat were acquired. The
image analysis was performed using the Fusion, PxMOD and
Kinetic programs included in the PMOD software suite (PMOD
Technologies, Zurich, Switzerland). Quantitative analysis of the
μPET images consisted of the MRTM0 (Multilinear Reference
Tissue Model) Ichise Binding Potential (BP) which has been
shown to be a reliable measure of D2R receptor occupancy
(Ichise et al., 2003).
Using the Pixel-Wise Modeling Soft-
2.3.2. Experiment 2. Cocaine self-administration
Surgery was performed prior to operant training on animals
at 9 months of age. Rats were anesthetized with 100/10 mg/kg
Ketamine/Xylazine (Fort Dodge, TX/Lloyd Laboratories, IA).
A 4-centimeter lateral incision was made along the chest and the
right jugular vein was dissected out and isolated from the
surrounding tissue. A 20-gauge heparin tipped, silicon catheter
(Instech Solomon, PA) was implanted into the jugular vein and
anchored to the surrounding fascia with a 7–0 silk suture
(Biosurgery, MA). The venous line was flushed with normal
saline to test patency and to compensate for volumetric blood
loss. The distal catheter tubing was subcutaneously routed to the
dorsal region of the animal, where it was attached to a back-
mounted 22-G cannula (Plastics One, VA). The line was flushed
daily with saline and filled with heparin (80 IU/ml) /cefazolin
(200 mg/ml) locking solution (Sigma, USA/G.C. Hanford, NY)
to prevent clotting and thrombus formation. Animals were
given a one-week recovery period before they are started on the
operant conditioning task.
After recovery, animals were put on a fixed-ratio food-
training schedule using operant boxes (Coulbourn Instruments,
PA) until animals reached a stable FR4 lever pressing for food
baseline (achieved a criteria of b20% variation in the mean
number of food pellets for 3 consecutive days) before being
started on cocaine self-administration.
Cocaine self-administration (1 mg/kg/0.1 ml infusion) ses-
sions were carried out under a FR1 schedule, with a 30 second
time out period between consecutive reinforcements (increased
from 5 s time out for food). Session duration was always 2 h,
and rats were run daily for 15 days. Self-administration data was
acquired using Graphic State software (Coulbourn Instruments,
PA). Cumulative lever-press and drug infusion data were re-
corded for each session.
3.1. Experiment 1
Mean weight across each treatment group was assessed after
8 months of treatment. A one-way ANOVA revealed no signi-
ficant differences in mean weight between groups (F=0.383;
18 g (2 mg/kg); 400+16 g (1 mg/kg); and 413+21 g (Vehicle).
3.1.2. D2R μPET Imaging
Fig. 1 shows the [11C]raclopride images for representative
groups of rats at 2 and at 8 months of treatment with either MP
or vehicle. A two-way ANOVA and subsequent Holm–Sidak
multiple comparison procedures were used to examine D2R
availability at 2 and 8 months of treatment. In order to maximize
statistical power, statistical observations used in the ANOVA
consisted of the individual ROI's of each brain. A statistically
significant main effect was observed with respect to treatment
(F=5.535; df=2.239; p=0.004), time (F=9.268; df=1.239:
p=0.003), and in the interaction between these two factors
(F=38.995; df=2.239; pb0.001). There were no significant
differences in D2R binding between the 1 and the 2 mg/kg dose
treated rats after 2 months of treatment. However, pairwise
comparisons during this time revealed significantly lower D2R
binding between the 1 mg/kg dose (t=4.237; pb0.001) and the
Fig. 2. Mean (+SEM) [11C] raclopride D2R binding levels using the MRTM0
Ichise Binding Potential (BP) across treatment and age groups.⁎denotes
significant differences as outlined in the figure with a p value equal to or less
than 0.001 using Holm–Sidak pairwise comparisons.
429P.K. Thanos et al. / Pharmacology, Biochemistry and Behavior 87 (2007) 426–433
2 mg/kg dose (t=3.516; p=0.001) treated rats as compared to
the age-matched vehicle group (Fig. 2). D2R availability was
shown to have increased for both the 2 mg/kg (t=6.847; pb
0.001) and the 1 mg/kg dose (t=4.567; pb0.001) treated
groups in a dose-dependent manner from 2 to 8 months of
treatment while it decreased in the vehicle treated rats (t=5.200;
pb0.001) (Fig. 2). Furthermore, after 8 months of treatment the
group treated with 2 mg/kg showed greater D2R binding avail-
ability compared to the group treated with 1 mg/kg (t=3.393;
p=0.001) and the vehicle treated group (t=9.225; pb0.001).
Finally, after 8 months of treatment, the group treated with
1 mg/kg showed greater D2R binding compared to the vehicle
treated group (t=5.832; pb0.001).
3.2. Experiment 2
There were no differences between groups in self-administra-
tion behavior during operant training. During cocaine self-
administration sessions, MP treated rats displayed significantly
lower mean active lever responses compared to vehicle treated
groups (F=162.754; df=2.33; pb0.001). Subsequent pairwise
comparisons (Holm–Sidak) between the 3 groups revealed that
there was a significant difference in the mean number of active
lever responses between the 2 mg/kg and 1 mg/kg MP group
(t=16.608;pb0.01) and betweenthe 2 mg/kgandvehicle treated
rats (t=14.413; pb0.05) (Fig. 3). There were no significant
treated animals. The rats treated with 2 mg/kg pressed the lever
significantly less than the animals given 1 mg/kg MP or the
vehicle treated animals.
Cocaine intake was examined in terms of the number of
cocaine infusions. A one-way ANOVA between groups for
mean total number of infusions revealed a significant difference
(F=30.966; df=2.33; pb0.001; Fig. 3). Subsequent pairwise
comparisons (Holm–Sidak) between the 3 groups revealed that
2 mg/kg MP treated rats had significantly lower number of
infusions than both the 1 mg/kg MP (t=6.925; pb0.05) and the
vehicle groups (t=6.714; pb0.05). There were no differences
in the number of cocaine infusions between the rats treated with
1 mg/kg and the vehicle treated rats.
This is the first study to evaluate the effects of 8 months of
duration of MP to 2 weeks and concentrated on assessing the
effects at one point in adulthood. Also different from other
studies this is the first to evaluate the effects of chronic MP
treatment on D2R availability. We show that daily oral treatment
with MP initiated during the period corresponding to adoles-
cence in the rat resulted in: (1) a decrease of striatal D2R avail-
ability at 2 months of treatment and an increase at 8 months of
MP (2 mg/kg and 1 mg/kg) treatment. (2) a decrease in the self-
administration of cocaine following 8 months of MP (2 mg/kg)
4.1. Effects of chronic MP after 2 months of treatment
Our imaging data after 2 months of MP treatment showed a
significantly lower D2R availability in both MP treated groups
of rats relative to age-matched vehicle-treated rats. The reduc-
since [11C]raclopride competes with DA in binding to D2R it
could also reflect an increase in synaptic DA. Future studies are
required to assess if there are changes in DA release after
2 months of MP treatment given at this developmental stage.
Also, it is important to examine possible neuroadaptations in
Fig. 3. A. Mean number (+SEM) of active lever responses and subsequent
infusions across treatment and age groups.⁎denotes significant differences
between the high (2 mg/kg) MP dose and the other two treatments with a p value
less than 0.05 using Holm–Sidak pairwise comparisons. B. Mean Reinforced
Lever Presses across session. C. Mean Cocaine Infusions across session.
430P.K. Thanos et al. / Pharmacology, Biochemistry and Behavior 87 (2007) 426–433
other neurotransmitter systems, such as the noradrenergic sys-
tem (Shaywitz et al., 1982; Wargin et al., 1983; Swanson et al.,
1999; Vitiello, 2001).
4.2. Effects of Chronic MP after 8 months of treatment
In contrast to the decrease in D2R availability observed after
two months of MP treatment, animals tested after 8 months of
treatment showed increased D2R availability when compared
with animals treated with vehicle. The differences in the results
obtained at 2 and 8 months are likely to reflect the difference in
the duration of treatment. Furthermore, it is possible that these
results also reflect differences on the effects of chronic MP dose
and as a function of the developmental stage of the animal
(youngadolescence versusyoung adulthood),butthis remains to
in adult animals. Finally, these differences may also be related to
changes in synaptic DA concentrations, which can compete with
[11C]raclopride for D2R binding sites. Previous studies have
making thereceptor unavailableforbinding with [11C]raclopride
(Laruelle, 2000; Sun et al., 2003). Eight months of treatment
with MP was also shown to significantly reduce cocaine-self
administration for the 2 mg/kg dose though there was no effect
for the 1 mg/kg MP dose. Unfortunately, we can not compare
that obtained after 2 months of treatment since animals were not
tested (for cocaine self-administration) at this earlier time point
to avoid confounds from potential long term effects secondary to
cocaine exposure during this early developmental stage.
4.3. Significance of MP induced changes in D2R and cocaine
In clinical studies we and others have shown decreases in
D2R in substance abusers (cocaine, methamphetamine, alco-
hol, and heroin; reviewed (Volkow et al., 2004). Also we had
shown that in non-drug abusing subjects D2R availability
modulates the rewarding responses to intravenous MP. Speci-
fically, subjects with low D2R availability tended to report MP
as pleasant, whereas those with high D2R availability tended to
report it as unpleasant. These findings have led us to hypothe-
size that low D2R availability in striatum increases the vul-
nerability whereas high D2R protect against substance abuse
(Volkow et al., 2004). Indeed in rodents trained to self-ad-
minister alcohol, D2R overexpression in nucleus accumbens
resulted in marked reductions in alcohol intake both in rats that
were genetically predisposed to self administering alcohol
(Thanos et al., 2004), as well as in those that were not (Thanos
et al., 2001). An inverse relationship between D2R availability
and vulnerability to the reinforcing effects of cocaine (Nader
and Czoty, 2005) and amphetamine (Ginovart et al., 1999) has
also been reported in nonhuman primates. Morgan and col-
leagues have shown that social environmental interventions
that lead to increases in D2R availability led to decreases in
cocaine self-administration (Morgan et al., 2002) whereas in-
terventions that resulted in low D2R availability led to a
facilitation in cocaine administration (Morgan et al., 2002).
Moreover a recent study documents an inverse relationship
between D2R availability and propensity to self-administer
cocaine (Nader et al., 2006). Thus, these studies provide
indirect evidence that the high D2R availability in animals
treated chronically with MP could underlie their attenuated
administration of cocaine. Our data showing increases in D2R
availability at 8 months of treatment would therefore suggest
that chronic MP treatment (started in adolescence) attenuated
cocaine self-administration during adulthood. Indeed the lower
rates of cocaine self-administration in the animals treated for
8 months with MP (2 mg/kg ip) support this. In contrast, the
lower D2R availability at 2 months of MP treatment would
suggest that shorter lengths of treatment or the age at which the
treatment effects are evaluated (early versus late adulthood)
could result in different effects. In this case a lowering of D2R
availability could make the animals more vulnerable to drug
self-administration during early adulthood (after 2 months of
treatment). Unfortunately, we did not evaluate cocaine self-
administration at this time point which would have allowed us
to corroborate if low D2R availability was associated with
increased cocaine consumption. Prior studies in rodents had
reported that 2 weeks of MP treatment led to a reduction
in cocaine and methamphetamine preference (Carlezon et al.,
2003; Kuczenski and Segal, 2002; Andersen et al., 2002) but
another study reported that it increased cocaine self-adminis-
tration (Brandon et al., 2001). These apparent discrepancies are
likely to reflect a) differences in the stage of development when
MP was given (pre-adolescence versus adolescent rats); b) the
different tests used to assess cocaine's rewarding effects (con-
ditioned place preference versus self-administration) and c) the
route of administration of MP (ip versus oral). On the other
hand, clinical studies suggest that there is no increased risk for
stimulant abuse associated with ADHD stimulant pharmaco-
therapy (Biederman and Spencer, 1999; Loney et al., 2002).
The findings from this study pertain to the question of whether
clinical treatment with stimulants in adolescence increases the
risk for drug abuse in young adulthood. While our findings
would support a reduction in the risk for cocaine self-adminis-
tration we have to be cautious since drug intake in the real life
situation is much more complex than in a laboratory setting.
Specifically in real life drug intake reflects a selection among
various competing reinforcers and thus changes in the sensi-
tivity of reward centers in the brain not only to cocaine but also
to other rewards could still result in an increase risk for drug
abuse. This is relevant since prior studies have shown that
chronic MP not only decreases the sensitivity to the rewarding
properties of cocaine but also to that of natural reinforcers
(Bolanos et al., 2003). It has been proposed that the therapeutic
effects of psychostimulants in ADHD reflect in part their ability
to enhance the saliency of reinforcers while they exert their
pharmacological effects (Solanto 1998; Wilkison et al., 1995).
4.4. Study limitations
The present study examined D2R availability in rats longi-
tudinally after 2 and 8 months of MP treatment however we
431 P.K. Thanos et al. / Pharmacology, Biochemistry and Behavior 87 (2007) 426–433
only examined cocaine self-administration in these same rats
after 8 months of MP treatment. Thus one limitation to our
study was that we did not evaluate cocaine self-administration
after 2 months of MP treatment. Future experiments will
examine the relationship between the low D2R availability
observed after 2 months of treatment with cocaine self-ad-
ministration. More studies evaluating the effects of different
to assess optimal duration of treatment regimes to minimize
adverse effects on the propensity for drug self administration.
In this study we only tested one dose of cocaine and thus we
can not rule out the possibility that the decrease in cocaine-self
administration in the MP treated rats (2 mg//kg) was due to an
enhanced sensitivity to the rewarding effects of cocaine (thus
animals required less of the drug) rather than a decrease in the
reinforcing effects of the drug. This is because cocaine self-
administration is characterized by an inverted U-shaped dose
decreased rewarding effects of cocaine would depend on where
one is on the curve. However, regardless of the mechanism(s)
responsible for the decrease in cocaine self-administration (i.e.
decrease rewarding properties of cocaine or an increased sensi-
tivity to its reinforcing effects) the net result was a significant
decline in cocaine consumption. Though unlikely, since there is
no evidence that chronic treatment with MP in ADHD induces
withdrawal upon discontinuation, we cannot rule out the pos-
sibility that withdrawal may have affected the responses to
When assessing D2R availability using μPET, we used ke-
tamine as an anesthetic. There is some discussion as to whether
or not ketamine binds to the D2R in-vivo. It has previously been
shown that surgical concentrations of isoflurane, halothane,
ethanol and ketamine inhibit high affinity states of the D2R as
well as other G-protein linked receptors in-vitro (Kapur and
Seeman, 2002; Seeman and Kapur, 2003). We do not feel that
this raises a major concern for our data since all animals were
administered ketamine during the scanning period and therefore
any effects on the D2R would apply to all groups. However, this
does raise the question of whether or not there were any inter-
actions between ketamine, [11C]raclopride, MP exposure and
age in our experiment. Future studies using the above anes-
thetics should take this into account.
It is important to point out that the findings from this study
cannot be directly extrapolated to the treatment regimes used for
ADHD since in general these are of shorter relative duration.
However, what this paper addresses is the effects of long-term
treatment with MP in DA function of the adult brain. It is also
important to point out that the current and other preclinical
studies are done in healthy animals and not in rodent models of
This study provides evidence that although chronic MP
treatment during the adolescent–adulthood transition may lead
to changes in striatum D2R levels, that have been associated
with a greater vulnerability to drug abuse (D2R decreases), over
the long-term, MP treatment may produce an increase in D2R
availability which in this study was associated with an attenu-
ation of cocaine self-administration. This study also documen-
ted a reversal of the age associated reduction in D2R with
chronic MP that deserves further investigation to evaluate its
merits as a potential intervention to help prevent the reductions
in D2R that occurs with age, which have been shown to con-
tribute to the decline in locomotor and cognitive function in the
elderly (Volkow et al., 1998).
Program (AA 11034 & AA07574, AA07611) and by the U.S.
Department of Energy under contract DE-AC02-98CH10886.
We thank Joanna Fowler, Colleen Shea and Millard C. Jane for
radiotracer synthesis and PET operations.
Andersen SL, Arvanitogiannis A, Pliakas AM, LeBlanc C, Carlezon Jr WA.
Altered responsiveness to cocaine in rats exposed to methylphenidate during
development. Nat Neurosci 2002;5:13–4.
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