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Brief Communications
Cannabidiol, a Nonpsychotropic Component of Cannabis,
Inhibits Cue-Induced Heroin Seeking and Normalizes
Discrete Mesolimbic Neuronal Disturbances
Yanhua Ren,
1
John Whittard,
1
Alejandro Higuera-Matas,
2
Claudia V. Morris,
1
and Yasmin L. Hurd
1
1
Departments of Psychiatry and Pharmacology and Systems Therapeutics, Mount Sinai School of Medicine, New York, New York 10029-6574,
and
2
Psychobiology Department, School of Psychology, Universidad Nacional de Educacio´n a Distancia, 28040 Madrid, Spain
There remains debate regarding the impact of cannabis on neuropsychiatric disorders. Here, we examined the effects of cannabidiol
(CBD), a nonpsychoactive constituent of cannabis, on heroin self-administration and drug-seeking behavior using an experimental rat
model. CBD (5–20 mg/kg) did not alter stable intake of heroin self-administration, extinction behavior, or drug seeking induced by a
heroin prime injection. Instead, it specifically attenuated heroin-seeking behavior reinstated by exposure to a conditioned stimulus cue.
CBD had a protracted effect with significance evident after 24 h and even 2 weeks after administration. The behavioral effects were
paralleled by neurobiological alterations in the glutamatergic and endocannabinoid systems. Discrete disturbances of AMPA GluR1 and
cannabinoid type-1 receptor expression observed in the nucleus accumbens associated with stimulus cue-induced heroin seeking were
normalized by CBD treatment. The findings highlight the unique contributions of distinct cannabis constituents to addiction vulnera-
bility and suggest that CBD may be a potential treatment for heroin craving and relapse.
Introduction
There continues to be major controversy as to whether canna-
bis should be legalized given some of its medicinal benefits,
which are countered by the negative impact of cannabis on
physical and mental health. Early cannabis use is associated
with the development of psychotic disorders (Andreasson et
al., 1987; Arseneault et al., 2002; Hall and Degenhardt, 2008) and
we and others have used experimental animal models to show
that ⌬
9
-tetrahydrocannabinol (THC), the psychoactive compo-
nent of cannabis, can serve as a gateway to the subsequent poten-
tiated use of heroin (Solinas et al., 2004; Ellgren et al., 2007). Most
studies regarding cannabis have focused on THC with limited
attention given to other constituent compounds of the cannabis
plant. Cannabidiol (CBD) is also enriched in cannabis (Mechoulam,
1970), but in contrast to THC, it is nonpsychotomimetic and has
antipsychotic and anxiolytic properties (Crippa et al., 2004;
Zuardi et al., 2006).
In this study, we evaluated CBD effects in relation to addiction
vulnerability using a drug self-administration model such that
rats could directly control their drug intake and drug-seeking
behavior, which characterizes the chronic, relapsing disorder
of drug dependence. We focused on the potential influence of
CBD on heroin-related behaviors given the strong neurobio-
logical interactions between the cannabinoid and opioid sys-
tems (Rodriguez et al., 2001; Schoffelmeer et al., 2006). CBD was
studied during different behavioral phases—maintenance, ex-
tinction, and relapse. Various factors can induce drug relapse and
we specifically examined the impact of drug-associated environ-
mental cue and heroin prime that are well documented to promote
drug-seeking behavior and reinstate drug intake in experimental
animal models (See, 2002; Shaham et al., 2003) and to induce
drug craving in humans (Childress et al., 1993; Sinha et al., 2000).
Neurobiological correlates to the behavioral effects were also
evaluated in the striatum, a region critical for reward, goal-
directed behavior, and habit formation (Everitt and Robbins,
2005).
Materials and Methods
Animals. Male Long–Evans rats, weighing 230 –250 g at the beginning of
the experiment, were obtained from Taconic. They were housed in a
humidity- and temperature-controlled environment on a reversed 12 h
light/dark cycle (lights off at 9:00 A.M.) with ad libitum access to food and
water. Rats were allowed to acclimate in their new environment and were
handled daily for 1 week before the start of the experiment. All proce-
dures were conducted in accordance with the National Institutes of
Health Guide for the Care and Use of Laboratory Animals and approved
by the local Animal Care and Use Committee.
Intravenous heroin self-administration. The self-administration proce-
dure was performed as described previously (Ellgren et al., 2007; Spano et
al., 2007). Briefly, catheters (CamCath) were implanted into the right
jugular vein under isoflurane anesthesia (2.4 –2.7%; Baxter). Following 1
week of recovery from surgery, self-administration training began during
the dark phase of the light/dark cycle in operant equipment which was
fitted with infrared beams to measure locomotor behavior (MED Asso-
ciates). Animals were allowed 3 h daily access to heroin (30
g/kg/infu-
Received Aug. 31, 2009; revised Oct. 1, 2009; accepted Oct. 3, 2009.
This work was supported by National Institutes of Health Grant DA19350 and funds from Mount Sinai School of
Medicine. We thank Drs. Michelle Jacobs and Didier Jutras Aswad as well as other members of the Hurd lab for their
input and support.
Correspondence should be addressed to Yasmin Hurd, Departments of Psychiatry, Pharmacology and Systems
Therapeutics, and Neuroscience, Mount Sinai School of Medicine, One Gustave Levy Place, Box 1603, New York, NY
10029-6574. E-mail: Yasmin.Hurd@mssm.edu.
DOI:10.1523/JNEUROSCI.4291-09.2009
Copyright © 2009 Society for Neuroscience 0270-6474/09/2914764-06$15.00/0
14764 •The Journal of Neuroscience, November 25, 2009 •29(47):14764 –14769
sion, diacetylmorphine-HCl; obtained from NIDA Drug Supply) under a
fixed ratio-1 reinforcement schedule in which one active lever press re-
sulted in a single drug infusion (85
l over 5 s) and activation of a white
conditioned stimulus light situated above the active lever. During train-
ing, animals were food restricted (20 g/d) and subsequently given ad
libitum access to food after stable heroin intake behavior was achieved.
Stable self-administration behavior was defined as at least 10 responses
on the active lever and at least a 2:1 ratio in active:inactive lever presses for
three consecutive sessions, with ⬍15% variation. After stable heroin in-
take behavior was achieved, the animals were divided into different
groups balanced for the number of active lever presses to subsequently
evaluate the effects of CBD (5, 20 mg/kg, i.p. dissolved in 3% Tween 80;
NIDA Drug Supply) or vehicle (3% Tween 80) on heroin self-
administration behavior.
In addition to the effects of CBD on heroin intake, the impact on
drug-seeking behavior was also evaluated. After the drug maintenance
phase, animals were kept drug-free in their home cage for 2 weeks. At the
end of the drug abstinence period, drug-seeking behavior was studied
during reinstatement sessions initiated by exposure to the conditioned
stimulus light cue or a heroin prime injection (0.25 mg/kg, i.p.). Heroin
was not delivered during the drug-seeking sessions, which lasted 1 h,
but the number of responses on both the active and inactive lever were
recorded.
CBD effects were also evaluated in regard to extinction of heroin self-
administration behavior. The extinction sessions were conducted at the
end of the maintenance phase of heroin self-administration, which lasted
⬃2 weeks. The testing conditions were the same as during training, ex-
cept that presses on the previously active lever was replaced by saline
infusion and there was no activation of the cue light.
Overall, 155 rats were trained to self-administer heroin in this study
and 18 were excluded because of loss of catheter patency, poor health, or
failure to acquire heroin self-administration.
Postmortem brain studies. To test neurobiological systems associated
with CBD’s effects on cue-induced heroin-seeking behavior, we studied
the striatum in postmortem brain samples 1 h after the drug-seeking
session in which rats were given vehicle or CBD. A group of saline ani-
mals were also included that were processed through the same behavioral
paradigm; the rats did not show active saline self-administration behav-
ior. At the end of drug-seeking sessions, rats were quickly killed by brief
CO
2
exposure and decapitation. Brains were rapidly removed, frozen in
isopentane (approximately ⫺50°C) for 10 s, and stored at ⫺80°C until
processed. Coronal sections (20
m thick) of the striatum were cut using
a refrigerated cryostat (⫺15°C; Frigocut 2800E, Leica Instruments) ac-
cording to the atlas of Paxinos and Watson (2005) and mounted onto
Superfrost Plus slides (Brain Research Laboratories). The sections were
stored at ⫺30°C until processed.
In situ hybridization histochemistry. cDNA fragments of cannabinoid
CB
1
receptor (CB
1
R) (NM_012784) and mGluR5 (NM_017012.1) were
obtained from rat brain total RNA by reverse transcription-PCR using
the following primer pairs: CB1, sense: 5⬘-GGGTTACAGCCTCCTTC-
ACA-3⬘, antisense: 5⬘-TGTCTCAGGTCCTTGCTCCT-3⬘; mGluR5,
sense: 5⬘-CTGTAATACGACTCACTATAGCCCAAGCATTCGAGAAGT-
CTA-3⬘, antisense: 5⬘-GGGATTTAGGTGACACTATAGCCAGGATG-
ATGTACACC TT-3⬘. The RNA probe was transcribed in the presence of
[
35
S]-uridine 5⬘-[
␣
-thio]triphosphate (spe-
cific activity 1000 –1500 Ci/mmol; New En-
gland Nuclear). The in situ hybridization
procedure was similar to previously published
protocols (Hurd, 2003; Ellgren et al., 2007).
Briefly, the labeled probe was applied to the
brain sections at a concentration of 2 ⫻10
3
cpm/mm
2
of the coverslip area. Two adjacent
sections from each subject were studied. Hy-
bridization was performed overnight at 55°C in
a humidified chamber. After in situ hybridiza-
tion, the slides were apposed to Imaging Plates
(Fujifilm) along with
14
C-standards (Ameri-
can Radiolabeled Chemicals). The films were
developed with FLA-7000 phosphoimaging
analyzer (Fujifilm) and the images were ana-
lyzed by MultiGauge software (Fujifilm).
Immunohistochemistry. Brain sections were rinsed twice in PBS for 10
min each and then blocked using 5% (v/v) heat-inactivated horse serum
(Invitrogen) diluted in PBS at room temperature for 2 h. Sections were
incubated overnight at 4°C with antibodies directed against CB
1
R(1
g/ml polyclonal, Millipore) or GluR1 (1
g/ml polyclonal, Millipore)
diluted in 2.5% (v/v) heat-inactivated horse serum in PBS. Following
four washes in PBS containing 0.05% (v/v) Tween 20 (MP Biomedicals),
sections were incubated with anti-rabbit or anti-mouse IRDye800CW
(1:1000; Jackson ImmunoResearch Laboratories) for2hatroom tem-
perature. Following four washes, slides were allowed to dry for at least 1 h
in the dark before imaging using the Odyssey infrared imaging system
(LI-COR) at a 21
m resolution and an offset of 1.0 mm. The level of
nonspecific binding of the secondary antibodies, as assessed with brain
sections incubated without primary antibody, was negligible (data not
shown). An excess of blocking peptide (Millipore) was incubated with
the GluR1 antibody before staining to determine the specificity of the
antibody. The staining pattern with the blocking peptide was similar to
that seen with brain sections incubated with secondary antibody alone
(data not shown).
Data analysis. For in situ hybridization studies, the mRNA expression
level was estimated within the nucleus accumbens (NAc) (bregma ⫹2.28
to ⫹1.60 mm) and caudate–putamen (bregma ⫹2.28 to ⫹1.60 mm) in
accordance with the Paxinos and Watson rat brain atlas (Paxinos and
Watson, 2005). Results were expressed as disintegrations per minute per
milligram of tissue with reference to the coexposed standard, and values
obtained from duplicate brain sections for each subject were averaged.
For immunohistochemistry studies, the images were quantified using
average integrated intensity values derived from regions of interest using
the Odyssey application software (version 2.0; LI-COR).
Statistical analysis. For the acquisition, maintenance, and extinction of
heroin self-administration studies, data were analyzed using a two-way
ANOVA for repeated measures followed, when appropriate, by planned
comparison tests with Bonferroni correction. For the drug-seeking stud-
ies, one-way ANOVA was used. The significance level was set at p⬍0.05.
For molecular and protein studies, statistical comparison was performed
by one-way ANOVA.
Results
CBD specifically affects heroin-seeking behavior induced by
conditioned cue
Experiment 1
Rats quickly learned to self-administer heroin such that a signif-
icantly higher number of responses were observed on the active
lever at the fourth training session (Fig. 1a)(p⬍0.001) with
stable heroin self-administered behavior acquired after the sixth
session (lever presses ⫻training session interaction; F
(15,315)
⫽
15.57, p⬍0.001). After acquisition of stable self-administration
behavior, animals were divided into subgroups with equal heroin
intake behavior to evaluate the effects of CBD or vehicle treat-
ment. Initial CBD pilot studies indicated a greater protracted
effect of the drug (data not shown), and thus the effects of CBD
Figure 1. CBD effects on heroin self-administration. a, Rats readily maintained stable self-administered heroin from approxi-
mately the sixth training session. b,c, CBD (5–20 mg/kg, i.p.) did not affect the number of lever presses (b) or locomotor activity
(c) during maintenance of heroin self-administration. Data represent mean ⫾SEM; n⫽7–9/group.
Ren et al. •Cannabidiol Inhibits Heroin-Seeking Behavior J. Neurosci., November 25, 2009 •29(47):14764 –14769 • 14765
administration were examined after 30 min and 24 h time peri-
ods. The maintenance of heroin intake behavior was not affected
following CBD administration (5 and 20 mg/kg, i.p.) at either
time point (Fig. 1b). CBD also failed to alter locomotor activity
(Fig. 1c), which was simultaneously monitored during the self-
administration session.
To assess the potential impact of CBD on heroin-seeking be-
havior, rats were re-exposed to the heroin self-administration
chamber and stimulus light cue 14 d following drug abstinence.
During the drug-seeking session, no drug reinforcement was ob-
tained upon lever pressing. Vehicle-treated animals showed ro-
bust lever pressing during the drug-seeking session, however, this
response was inhibited by a single CBD injection administered
24 h (Fig. 2a) (5 mg/kg, p⬍0.05), but not
30 min (data not shown), before the ses-
sion. The impact of CBD on drug-seeking
behavior was very specific, affecting only
active lever presses following the light cue
exposure (Fig. 2a)(p⬍0.01). Various
time periods were evaluated in another set
of animals after CBD injections (5 mg/kg
daily over 3 d) in the reinstatement ses-
sions. Intriguingly, there remained a sig-
nificant decrease in active, but not
inactive, lever presses for heroin seeking
monitored even 2 weeks after the last CBD
treatment (Fig. 2b)(p⬍0.001).
To determine the effects of CBD on
other conditions known to induce relapse,
additional sets of animals were tested dur-
ing a drug-seeking session that was initi-
ated by a heroin prime injection (0.25 mg/
kg, i.p.). CBD, both acute and 24 h after injection, had no effect
on either active or inactive lever presses following the heroin
prime (data not shown).
Experiment 2
The impact of CBD on extinction of heroin self-administration
was also examined in which the context of the self-administration
chamber, but not the cue light, was extinguished. CBD injections
given 24 h before the initial extinction session and daily through-
out the extinction training did not alter the rate of decline in the
active lever-pressing or alter the inactive lever press responses
compared with vehicle control animals (Fig. 3a). After a 2 week
abstinence period, re-exposure to the cue light reinstated lever
pressing in vehicle animals, but this was attenuated in animals
administered CBD 24 h before the relapse session (Fig. 3b)(p⬍
0.01). Animals carried through the extinction paradigm were also
exposed to a drug-seeking session that was initiated by a heroin
prime injection (0.25 mg/kg, i.p.). CBD, both acute (data not
shown) and 24 h after injection, had no effect on either active or
inactive lever presses (Fig. 3c) induced by the heroin prime.
Altogether, these results suggest that CBD has a protracted
neurobiological effect to counter long-lasting neuroadaptations
that specifically govern conditioned cue-induced drug-seeking
behavior and relapse.
CBD effects on mesolimbic CB1
and GluR1 receptors
CBD has been reported to be an inverse agonist at CB
1
Rs
(Thomas et al., 2007) and an agonist at the transient receptor
potential V1 (TRPV1) and TRPV2 (Qin et al., 2008). We exam-
ined the striatum, a region critical for reward, goal-directed be-
havior, and habit formation (Everitt and Robbins, 2005), in
animals following the drug-seeking session. TRP proteins and
mRNA levels were extremely low in the forebrain, though they
were detected in the dorsal root ganglion (data not shown). CB
1
R
mRNA expression was significantly increased in the ventral stri-
atum (NAc), the core subdivision in heroin rats that received
vehicle treatment before drug seeking (Fig. 4a)(p⬍0.05). The
heroin–CBD group showed decreased CB
1
R mRNA expression
in the NAc core and shell subdivisions compared with heroin–
vehicle animals even 2 weeks after the last CBD injection (Fig. 4a)
(p⬍0.05 and p⬍0.01). Moreover, this CBD effect was also
evident in the most medial division of the dorsal striatum ( p⬍
0.05), which is innervated by limbic cortices, but no CBD effects
were evident in the dorsolateral division that receives primarily
sensorimotor cortical input (supplemental Table 1, available at
www.jneurosci.org as supplemental material). Similar to alter-
ations of the CB
1
R transcript, CB
1
R protein levels tended to be
increased in the NAc of heroin–vehicle animals (Fig. 4b,c) (sig-
nificant in the lateral NAc shell; p⬍0.05), which were decreased
both 24 h and 2 weeks after CBD treatment [medial ( p⬍0.05)
and lateral ( p⬍0.01) NAc shell]. These findings suggest that
CBD’s effects on CB
1
R expression have a mesolimbic specificity
in the striatum. Interestingly, CBD administered in heroin-naive
animals acutely reduced CB
1
R expression in the NAc but not in
the dorsal striatum; no significant alterations were observed with
repeated CBD exposure (supplemental Table 2, available at www.
jneurosci.org as supplemental material).
Drug-seeking behavior and relapse have been strongly linked
to dysregulation of glutamate (Kalivas and Volkow, 2005;
LaLumiere and Kalivas, 2008). We studied mRNA and protein
levels of several markers relevant to glutamatergic function;
Figure 2. CBD inhibits cue-induced heroin-seeking behavior. a,b, CBD reduced the number
of active lever presses induced by exposure to a stimulus light cue 24 h before testing (a) and 2
weeks following last repeated CBD injection (b; 5 mg/kg, daily during the final3dofheroin
self-administration maintenance; 3⫻). Data represent mean ⫾SEM; n⫽7–9/group. *p⬍
0.05, **p⬍0.01, ***p⬍0.001 versus vehicle.
Figure 3. CBD effects on extinction behavior. a,b, CBD (10 and 20 mg/kg, i.p.) did not affect lever pressing during extinction
training (a; arrow represents the first day of extinction), but it inhibited lever pressing induced by exposure to a conditioned
stimuluslight cue after extinction of theenvironmental context (b; self-administrationchamber). c, CBD didnot alter lever pressing
induced by heroin prime (0.25 mg/kg, i.p.) after extinction. Data represent mean ⫾SEM; n⫽9 –11/group. **p⬍0.01, ***p⬍
0.001 versus vehicle.
14766 •J. Neurosci., November 25, 2009 •29(47):14764 –14769 Ren et al. •Cannabidiol Inhibits Heroin-Seeking Behavior
markers related to opioid transmission were also evaluated given
the relevance to heroin. Most showed only heroin-associated ef-
fects with no alterations induced by CBD. For example, mRNA
levels of mGluR5, abundantly expressed in medium spiny striatal
efferent neurons and intricately linked to endocannabinoid-
mediated synaptic plasticity (Katona and Freund, 2008), were
strongly downregulated to the same extent in the heroin–vehicle
and heroin–CBD groups (Fig. 4d)(p⬍0.001). In contrast,
AMPA GluR1 receptors, which are highly implicated in drug-
seeking behavior (Conrad et al., 2008), were significantly altered
only in the heroin–CBD animals; GluR2/3 were not robustly ex-
pressed in the striatum and CDB administration on its own did
not alter expression levels of any of the glutamatergic markers
studied (data not shown). As shown in Figure 4, eand f, heroin–
vehicle animals, with strong cue-induced relapse behavior, had
marked reduction of AMPA GluR1 protein expression in the NAc
core ( p⬍0.001), medial shell ( p⬍0.01), and lateral shell ( p⬍
0.05), with no significant effect in the dorsal striatum (supple-
mental Table 1, available at www.jneurosci.org as supplemental
material). However, 24 h after CBD, GluR1 protein expression
was significantly normalized in the NAc core ( p⬍0.001) and
medial shell ( p⬍0.05). A similar pattern was observed even 2
weeks following the last repeated CBD treatment, but the effect
was most evident in the NAc core (Fig. 4e)(p⬍0.01).
Discussion
The current study has revealed unique properties of the phyto-
cannabinoid CBD and underscores the contrasting characteris-
tics of the main constituents of cannabis in relation to addiction
vulnerability. Compared with the documented effects of THC to
enhance heroin self-administration (Solinas et al., 2004; Ellgren
et al., 2007), the present data demonstrated that CBD specifically
inhibited reinstatement of cue-induced heroin seeking. The spec-
ificity of CBD to cue-induced reinstatement was also emphasized
by the observation that the compound still inhibited drug relapse
behavior in animals extinguished to the environmental context
(self-administration chamber) previously associated with heroin.
The results are striking given the very selective and protracted
effects of CBD. Although CBD significantly altered drug-seeking
behavior promoted by conditioned cue, it failed to influence drug
seeking initiated by a heroin prime. Whether CBD induces some
perceptual alterations that compromise cue- but not priming-
induced reinstatement of drug seeking remains to be determined.
The apparent diminished impact of CBD in the presence of
heroin was also evident during the drug maintenance phase, in
which CBD did not modify stable heroin intake behavior.
Thus CBD does not appear to interfere with the reinforcing
effects of heroin at least on a FR-1 schedule. Interestingly, the
ability of CBD to reduce heroin-seeking behavior at least 2 weeks
after exposure was nevertheless still observed when CBD had
been administered during the active phase of heroin self-
administration. These findings emphasize that CBD retains its
effects to modulate neural mechanisms relevant to cue-induced
drug relapse vulnerability even in the presence of heroin.
The observation that CBD’s effects on cue-induced drug-
seeking behavior was apparent 24 h and 2 weeks, but not after 30
min, following administration may suggest delayed pharmaco-
logical actions of the drug. However, it is important to note that
behavioral effects have been observed immediately after admin-
istration of CBD at the dose range currently studied on, for ex-
ample, its anxiolytic properties (Guimara˜es et al., 1994; Moreira
et al., 2006). Moreover, acute administration of CBD has been
shown to enhance the extinction of cocaine- and amphetamine-
induced conditioned place preference without affecting learning
or retrieval (Parker et al., 2004). There was also no evidence in our
study that CBD affected extinction learning. CBD did not alter
the extinction of heroin seeking. The potential influence of CBD
Figure 4. CBD effects on CB
1
R and glutamate receptor expression in relation to cue-induced heroin-seeking behavior. a– c,CB
1
R RNA (a) and protein (band c) levels in the NAc following either
vehicleor CBD administration 24 h(1⫻) beforethe cue-induced drug-seekingsession. d–f, mGluR5RNA levels(d) and GluR1protein levels (eandf). Datarepresent mean ⫾SEM;n⫽4 – 8/group.
NAc-c, NAc core; M/L-NAc-s, medial/lateral NAc shell; CP, caudate–putamen (dorsal striatum); SA, self-administration. *p⬍0.05, **p⬍0.01, ***p⬍0.001 versus vehicle;
#
p⬍0.01 versus
heroin–vehicle.
Ren et al. •Cannabidiol Inhibits Heroin-Seeking Behavior J. Neurosci., November 25, 2009 •29(47):14764 –14769 • 14767
on psychostimulant-seeking behavior needs to be examined with
an operant self-administration procedure to determine the spec-
ificity of CBD to different classes of drugs and different relapse
models.
The protracted behavioral effects of CBD, in addition to its
specific influence on heroin-seeking behavior, strongly implied a
long-term impact on synaptic plasticity, the pathology of which is
hypothesized to underlie compulsive disorders such as drug ad-
diction. The endocannabinoid and glutamatergic systems have
been tightly linked to synaptic plasticity (Kauer and Malenka,
2007). In addition to its high potency at the CB
1
R (Thomas et al.,
2007), CBD has also been reported to alter the hydrolysis of the
endocannabinoid anandamide (Bisogno et al., 2001). The reduc-
tion of CB
1
R expression in the NAc when CBD was administered
alone was short-term though its impact was enduring in heroin-
seeking animals. Attenuation of the elevated expression CB
1
R
mRNA and protein levels in the NAc by CBD in heroin rats,
which paralleled the behavioral alterations, is consistent with the
observation that inhibition of the CB
1
R inhibits cue-induced
drug-seeking behavior (De Vries et al., 2003).
Various lines of evidence have clearly documented the critical
role of AMPA GluR1 in drug-seeking behavior though most
studies have focused on psychostimulant drugs (Anderson et al.,
2008; Conrad et al., 2008). Of the few investigations that have
evaluated opiates, the expression of AMPA receptors in prefron-
tal cortical synaptic membranes was reported to be reduced in
heroin-abstinent animals after re-exposure to heroin cues (Van
den Oever et al., 2008) and glutamate arising from the prefrontal
cortex was increased in the NAc core (LaLumiere and Kalivas,
2008). The specific cellular localization of the GluR1 was not
examined in the current study, which limits interpretations as to
the dynamic cellular distribution of AMPA receptors relevant to
drug-seeking behavior. Moreover, it is important to note that a
similar alteration of glutamate levels was reported in the NAc
core with both cue and heroin prime (LaLumiere and Kalivas,
2008). Thus, other mechanisms than NAc core glutamatergic dis-
turbances most likely underlie CBD’s apparent ability to alter
cue- but not priming-induced reinstatement of drug seeking.
Nevertheless, the observation that GluR1 disturbances in the NAc
associated with heroin seeking were absent in those administered
CBD is intriguing. Most studies have focused on the NAc core in
relation to glutamatergic involvement in drug-seeking behavior,
but CBD’s effects on GluR1 were also apparent in the NAc shell,
though to a weaker extent than the core. Further studies are re-
quired to determine the specific contribution of the NAc subre-
gions as well as other brain regions implicated in cue-induced
reinstatement to the actions of CBD. Although additional studies
are needed to fully elucidate the molecular mechanisms of CBD
in regard to its direct and indirect effects on heroin-seeking be-
havior, the mesolimbic specificity and protracted effects of CBD
on CB
1
R and GluR1 is interesting given the role of the limbic
system in goal-directed behavior.
Of the over 1 million opiate-dependent subjects today, only
less than a quarter of such individuals receive treatment, which
have traditionally targeted
-opioid receptors. Although such
treatment strategies including methadone have improved sub-
stance abuse outcome, they do not effectively block opiate crav-
ing in all patients (Walter et al., 2008) and thus are still associated
with high rates of relapse and ultimately the continued cycle of
opioid abuse. The fact that drug craving is generally triggered by
exposure to conditioned cues makes the current results particu-
larly fascinating. Moreover, the observation that CBD did not
cause gross motor impairment as evident by a lack of effect on
inactive lever presses or on locomotor behavior is consistent with
the weak side effects that have been reported with this compound
in humans (Consroe et al., 1991; Tomida et al., 2006). In addi-
tion, CBD lacks hedonic properties on its own (Parker et al.,
2004). Overall, the observations of this study suggest the potential
for CBD as a treatment strategy given its specificity to attenuate
cue-induced drug-seeking behavior, preferential impact on me-
solimbic neuronal populations, and enduring neural actions.
Clearly, greater attention needs be given to the potential role of
CBD in the treatment of addiction and other mental health
disorders.
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