Neuropsychopharmacology (2016) 41, 1513–1520
© 2016 American College of Neuropsychopharmacology. All rights reserved 0893-133X/16
www.neuropsychopharmacology.org
Varenicline-Induced Elevation of Dopamine in Smokers: A
Preliminary [11C]-(+)-PHNO PET Study
Patricia Di Ciano1, Mihail Guranda1, Dina Lagzdins1, Rachel F Tyndale2,3,4,5, Islam Gamaleddin1,6,
Peter Selby4,7,8,9,10, Isabelle Boileau5,9,11 and Bernard Le Foll*,1,3,4,5,7,9,10
1
Translational Addiction Research Laboratory, CAMH, Toronto, ON, Canada; 2Pharmacogenetics Laboratory, CAMH, Toronto, ON, Canada;
Department of Pharmacology and Toxicology, University of Toronto, Toronto, ON, Canada; 4Department of Psychiatry, University of Toronto,
Toronto, ON, Canada; 5Campbell Family Mental Health Research Institute, CAMH, Toronto, ON, Canada; 6Institute of Environmental Studies and
Research, Ain Shams University, Cairo, Egypt; 7Ambulatory Care and Structured Treatment Program, CAMH, Toronto, ON, Canada; 8Dalla Lana
School of Public Health, Toronto, ON, Canada; 9Institute of Medical Sciences, University of Toronto, Toronto, ON, Canada; 10Department of Family
and Community Medicine, University of Toronto, Toronto, ON, Canada; 11Addiction Imaging Group, Research Imaging Centre, CAMH, Toronto, ON,
Canada
3
Varenicline, a nicotinic partial agonist, is the most effective treatment for tobacco use disorder. However, its mechanism of action is still
unclear and may involve stimulating dopaminergic transmission. Here we used PET imaging with [11C]-(+)-PHNO to explore for the first
time the impact of varenicline on dopamine transmission in the D2-rich striatum and D3-rich extra-striatal regions and its relationship with
craving, withdrawal and smoking. Eleven treatment-seeking smokers underwent two PET scans with [11C]-(+)-PHNO, each following 12-h
overnight smoking abstinence both prior to receiving varenicline and following 10–11 days of varenicline treatment (ie, at steady-state drug
levels). Subjective measures of craving and urges to smoke were also assessed on the days of the PET scans. Varenicline treatment
significantly reduced [11C]-(+)-PHNO binding in the dorsal caudate (p = 0.008) and reduced some craving measures. These findings
provide the first evidence that varenicline is able to increase DA levels in the human brain, a factor that may contribute to its therapeutic
efficacy.
Neuropsychopharmacology (2016) 41, 1513–1520; doi:10.1038/npp.2015.305; published online 28 October 2015
INTRODUCTION
Smoking is a public health concern, yet pharmacological
treatment strategies remain only partly effective. Current
first-line medications include nicotine replacement therapy
(NRT) and prescription medications such as bupropion and
varenicline, with moderate effect sizes (Cahill et al, 2012).
NRT reduces acute nicotine withdrawal while bupropion and
varenicline appear to decrease the urge to smoke by reducing
withdrawal symptoms and blunting the rewarding effects of
smoking (Brandon et al, 2011; Gonzales et al, 2006; Jorenby
et al, 2006; Patterson et al, 2009). Exploring the neurobiological underpinning of these treatments is warranted as they
are poorly understood in humans.
Dopamine (DA) is a neurotransmitter believed to be
important in the final common path in drug dependence,
including nicotine (Pich et al, 1997). DA is believed to
mediate both the rewarding (Brody et al, 2004; Corrigall et al,
1992; Le Foll et al, 2014) and withdrawal-associated
*Correspondence: Dr B Le Foll, Translational Addiction Research
Laboratory, Centre for Addiction and Mental Health, 33 Russell Street,
Toronto, ON M5S 2S1, Canada, Tel: +416 535 8501, x34772, Fax:
+416 595 6922, E-mail: Bernard.lefoll@camh.ca
Received 21 July 2015; revised 2 September 2015; accepted 20
September 2015; accepted article preview online 7 October 2015
(Rada et al, 2001; Rahman et al, 2004) effects of nicotine.
That is, increased DA is associated with ratings of positive
subjective measures (Montgomery et al, 2007), and decreased
DA is believed to mediate the negative state during
withdrawal (Hildebrand et al, 1998; Le Foll et al, 2014). As
a partial agonist of the α4β2* acetylcholine nicotinic receptor
(Coe et al, 2005), it is possible that varenicline-induced
elevations in DA levels may contribute to its therapeutic
effects (Coe et al, 2005). However, the first study to evaluate
the impact of subchronic administration of varenicline in
animals that were not dependent on nicotine found no effect
of varenicline on basal DA levels but an ability of varenicline
to decrease nicotine-induced DA release in animals (Ericson
et al, 2009). More recently, it has been reported that
varenicline increased DA firing rates in nicotine-dependent
animals that were in acute withdrawal (Perez et al, 2015), an
effect consistent with its partial agonist profile. To date, the
impact of varenicline on DA transmission in the human
brain has not been reported.
Neuroreceptor imaging techniques with PET provide a
method to investigate changes in DA in the human brain
in vivo (Laruelle, 2000; Laruelle et al, 2002; Martinez and
Narendran, 2010). The traditional radioligand for dopamine
D2/3 receptors (DRD2/3) is [11C]-raclopride that has relatively
low sensitivity to detect changes in DA (Martinez and
Dopamine and varenicline
P Di Ciano et al
1514
Narendran, 2010). However, [11C]-(+)-PHNO (Wilson et al,
2005) is a DRD2/3 agonist that is also the most sensitive PET
tracer to detect relatively small fluctuations in DA (Gallezot
et al, 2012; Ginovart et al, 2006, 2007; Narendran et al,
2006; Willeit et al, 2006). We have recently shown that
[11C]-(+)-PHNO has an enhanced ability to detect elevations
in DA induced by smoking (Le Foll et al, 2014), with a
preferential effect in the limbic striatum. In addition,
[11C]-(+)-PHNO allows for the measurement of occupancy
of DA receptors in D2- and D3-rich areas.
[11C]-(+)-PHNO binding in the substantia nigra (SN) and
ventral pallidum (VP) is believed to represent 100 and 75%
binding to DRD3, respectively, while [11C]-(+)-PHNO
binding in the dorsal caudate and dorsal putamen is
accounted for by DRD2, and the VST is about 50% of each
(Ginovart et al, 2007; Rabiner et al, 2009; Tziortzi et al,
2011).
The purpose of the present study was to evaluate whether
treatment with varenicline affects DA transmission in the
D2-rich striatum and D3-rich extra-striatal regions and
whether this change was associated with changes in
subjective measures. Participants underwent two PET scans
with [11C]-(+)-PHNO after overnight abstinence from
tobacco. The first PET scan was done before varenicline
treatment and the second PET scan was done after
10–11 days of varenicline (at steady state). Subjective
questionnaires that measured craving and reward were
administered, and objective measures (ie, varenicline and
plasma cotinine) were collected.
past 12 months exceeding limits for participants in research
with PET; and (9) Allergy to varenicline.
Procedure
PET/MRI scans. After enrolment in the study, participants
had two PET scans and a 30-min MRI on a 3 Tesla GE MRI
scanner (Discovery MR750, GE, Milwaukee, USA) for region
of interest (ROI) delineation. On the day of the first PET
scan, participants were scanned after overnight (12 h)
abstinence from tobacco and given varenicline to take home,
with the instructions to start taking the medication the next
day (see dosing paradigm below). Seven days later,
participants returned to collect refills of blister packs of
varenicline and the second PET day was scheduled
10–11 days after the first PET day, after reaching maintenance doses of varenicline. Participants were asked to
refrain from smoking for 12 h prior to each PET visit, and
smoking abstinence was confirmed by breath CO levels
below 10 p.p.m. Alcohol abstinence was also confirmed by a
breath alcohol measure. At the start of each PET scan, a
sample of blood was drawn to measure plasma levels of
varenicline, cotinine, and nicotine. Participants completing
the second PET scan were followed every 2 weeks for a total
duration of 12 weeks for treatment of tobacco dependence.
During follow-up visits, subjects received behavioral support
adapted from the manual from the Mayo Clinic Guide
(Smoke Free and Livin'it), were given blister packs of
medication, and provided breath CO and completed
questionnaires (see below). A final visit was scheduled
3 months after varenicline was discontinued.
MATERIALS AND METHODS
Participants
All procedures were approved by the Centre for Addiction
and Mental Health (CAMH) Research Ethics Board and
complied with the Helsinki Declaration of 1975 (as revised in
1983). The study was approved by the Institutional Research
Ethics Board. Thirteen participants (adult males or females
21–45 years) were recruited from the community and
provided written informed consent prior to participating in
any study procedures. All met the following inclusion
criteria: (1) Nicotine dependent as assessed by smoking at
least 10 cigarettes a day, a baseline score of ⩾ 4 on the
Fagerstrom Test of Nicotine Dependence and expired carbon
monoxide (CO) levels of at least 10 p.p.m.; (2) motivated to
quit within the next 30 days; and (3) treatment-seekers who
were willing to use varenicline as a quit aid. Exclusion
criteria were: (1) Previous use of medication for smoking
cessation within the past month; (2) Abnormal physical
examination, 12-lead or routine routine blood tests or a
condition that may impede memory and attention; (3) Past/
present axis I psychiatric diagnoses as per MINIInternational Neuropsychiatric Interview version 5.0 and
the Hamilton Depression Rating Scale; (4) Magnetic
resonance (MR) scanning contraindication; (5) Claustrophobia; (6) Current pregnancy/breastfeeding; (7) Current use or
use during the previous month of medication that may affect
the central nervous system or positive during drug screening
for drugs of abuse or abuse of alcohol or drugs of abuse
within the past 3 months; (8) Exposure to radiation in the
Neuropsychopharmacology
Subjective measures. During each PET scan visit, participants completed the Minnesota Nicotine Withdrawal Scale
(MNWS; assesses urge to smoke, depressed mood, irritability, anxiety, difficulty concentrating, restlessness,
increased appetite, and sleep) and the Tobacco Craving
Questionnaire (TCQ; relief from withdrawal (factor 1),
anticipation of positive outcomes (factor 2), control over
tobacco use (factor 3) and intention to smoke for positive
outcomes (factor 4) both before and after each PET scan).
The patient health questionnaire (PHQ-9; to assess depressive symptoms) was also administered on each PET scan day.
All these questionnaires were also given on day 7 and on the
follow-up visits with the exception of the TCQ. Participants
kept a daily log of the number of cigarettes smoked. At each
visit, participants were asked to indicate how many cigarettes
they had smoked on each day for the 7 days prior to the visit,
and an expired breath CO reading was taken. Participants
were also asked how much alcohol and caffeine they had
during the past 7 days.
Drug administration. Varenicline was administered as
prescribed in clinical practice. For the first 3 days, participants took 0.5 mg orally once a day in the morning and then
twice a day on days 4–7. After that, 1 mg varenicline was
taken orally twice a day. The target quit date was set at the
second PET scan visit (days 10–11 of taking varenicline).
Concomitant medications and adverse events were assessed
at each visit.
Dopamine and varenicline
P Di Ciano et al
1515
varenicline. Varenicline measured by this method provided
identical levels as observed using other methods.
Table 1 Demographic Variables
Sex
Male
7
Female
4
Race
Black
2
Caucasian
6
Mixed
1
Asian
1
Unknown
1
Mean ± SEM
Age
Cigarettes
FTND
Pack-years
37 ± 1.85
104 ± 11.36
5.68+0.55
16.86+2.15
CO level
17.18+2.34
Years smoking
22.64+1.99
Motivation to quit
9.36+0.31
Abbreviations: Cigarettes, number of cigarettes smoked per week; CO, exhaled
carbon monoxide, parts per million (p.p.m.); FTND, Fagerstrom Test of Nicotine
Dependence; motivation to quit, on a scale of 0–10 with 10 being the most
motivated; pack-years, years × packs/day. Values are presented as n or
mean ± SEM.
PET Image Acquisition
The radiosynthesis of [11C]-(+)-PHNO has been described
in detail elsewhere (Wilson et al, 2005). PET scans were
performed using a Siemens-Biograph HiRez XVI
(Siemens Molecular Imaging, Knoxville, TN, USA) PET/CT
camera system, which measures radioactivity in 81
brain sections with a reconstructed pixel size of
1.07 × 1.07 × 2.00 mm3 each with an in-plane resolution of
5 mm full-width at half maximum. A transmission scan was
acquired and the emission scan, acquired in 32-bit list mode,
began after bolus injection of [11C]-(þ)-PHNO (duration of
the bolus injection approximately 2 min). Emission data
were reconstructed by 2D filtered back projection to yield
dynamic images with 15 1-min frames and 15 5-min frames.
The emission scan lasted for 90 min. The raw data were
reconstructed by filtered-back projection. A custom-fitted
thermoplastic mask (Tru-Scan Imaging, USA) was made for
each subject to reduce movement during the acquisition.
A total of ~ 370 ± 40 MBq (approximately 10 ± 1 mCi) of
[11C]-(+)-PHNO was injected as a bolus into an
antecubital vein.
Plasma Levels of Varenicline, Nicotine and Cotinine
Consistent with its elimination half-life of approximately
24 h, steady-state conditions for varenicline are reached
within 4 days of repeat dosing (Faessel et al, 2010). Plasma
levels of nicotine, cotinine and varenicline, were measured by
LC/MS/MS using previously established methods (St Helen
et al, 2012; Tanner et al, 2015) for nicotine, cotinine and
3-hydroxycotinine modified to additionally detect
PET Image Analysis
ROI delineation and time activity curve analyses were
performed using ROMI (details in (Rusjan et al, 2006).
ROI included the dorsal caudate (DC), dorsal putamen (DP),
ventral striatum (VST) as well as globus pallidus (GP;
whole), VP, and SN. Delineation is described elsewhere
(Boileau et al, 2012). [11C]-(+)-PHNO specific binding
(BPND) was estimated in each ROI using the simplified
reference tissue method (SRTM; Lammertsma and Hume,
1996), with cerebellar cortex (excluding vermis) as reference
region. Parameter estimation was performed using PMOD
(Version 2.8.5; PMOD Technologies, Zurich, Switzerland).
Percentage of occupancy was calculated as [11C]-(+)-PHNO
binding after varenicline/([11C]-(+)-PHNO binding at
baseline − 1).
Voxel-wise parameter estimation of [11C]-(+)-PHNO
binding was generated using the basis function implementation of SRTM (Lammertsma and Hume, 1996), with the
tissue time activity curve of cerebellar cortex as the reference
region. Normalized BPND maps (SPM8; Wellcome Trust
Centre for Neuroimaging, London, UK) were statistically
investigated to assess significant contrasts between
conditions at every voxel using paired sample t-test analysis.
The threshold for significant clusters was set to a family-wise
error corrected p = 0.05.
Comparisons between [11C]-(+)-PHNO BPND in ROIs
were conducted by using repeated-measures Day (two levels;
baseline, post-varenicline) × Region (six levels; ROIs: SN, GP,
VP, VST, DC, DP) ANOVAs (SPSS 20.0, SPSS, USA).
Sphericity was assessed with Mauchly test, and corrections
were made when indicated. Bonferonni-corrected paired
t-tests were conducted between the baseline scan and scan
under varenicline for each ROI.
Subjective questionnaire data obtained prior to and
following each PET scan were averaged for each PET scan
day. Questionnaires were analyzed with Day (two levels;
baseline PET scan days vs PET scan after 11 days of
treatment with varenicline) × Measure (six levels; TCQ1,
TCQ2, TCQ3, TCQ4, MNWS, and PHQ) ANOVAs. All
objective measures (number of cigarettes smoked and plasma
cotinine) were analyzed with ANOVAs on the effect of Day
(two levels). Correlations between questionnaire measures
and BPND were conducted for any ROIs that had any
significant changes in BPND.
RESULTS
Thirteen participants were recruited for this study. One
participant withdrew after the first scan. For another
participant, there was a movement artifact in the PET scan
that prevented proper determination of [11C]-(+)-PHNO
BPND, and data were not included. A total of 11 subjects were
included in the final analysis.
All participants tested negative for drugs of abuse at the
time of scanning. Varenicline was detected in all participants
on the day of second PET scan. Demographic variables
are provided in Table 1. A mixed ANOVA of Measure
Neuropsychopharmacology
Dopamine and varenicline
P Di Ciano et al
1516
4
Baseline
varenicline
3.5
BPND
3
2.5
2
*
1.5
1
0.5
SN
VP
GP
VST
DP
DC
Region of Interest
11
Figure 1 Mean ± SEM [ C]-(+)-PHNO BPND before varenicline (open
bars) and after 10–11 days of varenicline (filled bars) presented for individual
participants for ROIs (DC, dorsal caudate; DP, dorsal putamen: GP, globus
pallidus; SN, substantia nigra; VP, ventral pallidum; VST, ventral striatum).
[11C]-(+)-PHNO BPND was significantly decreased in the DC after
treatment with varenicline (*po0.05, paired t-tests).
(mass injected, amount injected, specific activity) × Day
(PET baseline, PET after varenicline) revealed no effects,
suggesting that there were no differences in scan parameters
across scans or between groups (mass injected (μg)
2.09 ± 0.09; amount injected (mCi) 9.45 ± 0.25; specific
activity (mCi/μmol) 1695.59 ± 99.81).
Changes in Binding Potential
A two-way repeated-measures Region (six levels) × Day
(two levels) revealed no interaction and no effect of Day.
t-Tests corrected for multiple comparisons (Bonferroni)
indicated that BPND differed between baseline and while
taking varenicline only in the DC (Figure 1; p = 0.008;
Bonferroni uncorrected to 0.0083; Cohen’s D: VST: 0.168;
VP: 0.41; SN: 0; GP: 0.53; DC: 0.49; DP: 0.20).
Voxel-Wise Analyses
In the voxel-wise statistical analyses (SPM8), we identified
small clusters of lower [11C]-(+)-PHNO BPND in the
varenicline condition as compared with baseline in areas
corresponding to the ventral lateral nucleus of the thalamus
and the head/body of the caudate nucleus (on left voxel 48
with peak threshold of 4.15 and on the right a cluster of 57
voxels with a peak threshold of 4.01; Figure 2). No other
clusters of significantly lower [11C]-(+)-PHNO BPND was
significant after correction for multiple comparisons.
Objective and Subjective Measures
On the first PET session, prior to taking varencline, the
number of cigarettes smoked in the 7 days prior was 98 ± 12.
On the day of the second PET scan, after taking varenicline
for 11 days, the number of cigarettes smoked in the 7 days
prior was 93 ± 8. A one-way ANOVA revealed no significant
effect of Day for the objective measures of number of
Neuropsychopharmacology
Figure 2 Voxel-wise comparison between baseline and varenicline
illustrating lower [11C]-(+)-PHNO BPND in the DC (tmax = 5.92; p = 0.05).
The image is not corrected for multiple comparisons.
cigarettes smoked or cotinine levels (day 1: 210 = 42;
day 11: 149 ± 29). A two-way repeated-measures Day
(two levels) × Measure (six levels; TCQ1, TCQ2, TCQ3,
TCQ4, MNWS, PHQ) ANOVA revealed a significant
interaction (F(5, 45) = 5.040, p = 0.001). Main effects of Day
were found for TCQ2 (F(1, 9) = 10.229, po0.011) and TCQ4
(F(1, 10) = 3.494, p = 0.091). For the MNWS and PHQ,
main effects of Day approached significance (MNWS:
F(1, 10) = 3.564, p = 0.088; PHQ: F(1, 10) = 3.494, p = 0.091).
Bonferroni-corrected t-tests (corrected p = 0.0083) revealed
differences between the 2 days only for TCQ factor 4
(p = 0.006), suggesting that the intention and planning to
smoke for positive outcomes was reduced after varenicline.
The effect of Day for TCQ factor 2 was not statistically
significant after correction for multiple comparisons
(p = 0.011, see Figure 3). One participant was missing
subjective data from day 1 and was not included in this
analysis.
Correlations
Correlations (Figure 4) between changes in BPND in the DC
and subjective measures (PHQ, TCQ1, TCQ2, TCQ3, TCQ4,
MNWS) revealed that the correlation between changes in
BPND in the DC and changes in TCQ factor 2 (anticipation
of positive outcomes) approached, but did not reach,
significance (r2 = − 0.505, p = 0.136). One participant was
missing subjective data from day 1 and was not included in
the analyses. A significant correlation between change in
plasma cotinine and change in BPND in the DC was found
(r2 = − 0.742, p = 0.009)
DISCUSSION
The purpose of the present study was to investigate the
effects of varenicline treatment on [11C]-(+)-PHNO BPND in
Dopamine and varenicline
P Di Ciano et al
20
18
16
14
12
10
8
6
4
2
0
DC and TCQ2
0.05
*
TCQf1
TCQf2
TCQf3
TCQf4
Change in BPND
Score
1517
Before varenicline
After varenicline
MNWS
Subjective Measure
Figure 3 Mean ± SEM scores on the Tobacco Craving Questionnaire
(TCQ) factors 1–4 and the Minnesota Nicotine Withdrawal Scale (MNWS)
presented before (open squares) and after (closed squares) treatment with
varenicline. *Significant effects of treatment with varenicline were found in
the scores on the TCQ factor 4 (intention and planning to smoke for
positive outcomes; t-test, p = 0.006). TCQ factor 2 (anticipation of positive
outcomes) was significant but did not survive corrections for multiple
comparisons (paired t-test: p = 0.011; Bonferroni p = 0.0083).
ROIs in treatment-seeking smokers. PET scans were
performed before and after 10–11 days of varenicline
treatment (at steady state). Subjective mood and craving
questionnaires as well objective measures of plasma levels of
varenicline and cotinine were also taken. After treatment
with varenicline, [11C]-(+)-PHNO BPND was significantly
decreased in the DC, as assessed with voxel-wise and ROI
analyses. Subjective measures of TCQ factor 4 (intention to
smoke for positive outcomes) were also decreased, while
TCQ factor 2 (anticipation of positive outcomes from
smoking) failed to reach significance after correction for
multiple comparisons.
This study revealed that [11C]-(+)-PHNO BPND was lower
in DC after treatment with varenicline, as compared with
baseline in treatment-seeking smokers. Changes in BPND
could be obtained not only by changes in DA but possibly
also by changes in DA receptor expression. However, the
latter is unlikely here as preclinical studies suggest that if
varenicline has any effects on DA receptor expression it
would be an increase in receptor number (Crunelle et al,
2012), which would then be producing an increase in
[11C]-(+)-PHNO binding. Therefore, it is more likely that the
lowest [11C]-(+)-PHNO BPND reflects an elevation of DA in
DC. This effect was consistently found with both our ROI
analysis and with the voxel-wise analysis approach. The
voxel-wise approach is aimed at detecting differences in
neuroreceptor ligand binding at the voxel level, with no
a priori anatomical hypothesis, and enables circumvention of
some limitations of ROI placement, as well as investigation
of regions not included in our ROI template. It should be
noted that, in this study, participants were abstinent from
tobacco use, in order to avoid the confound of tobaccoinduced elevations in DA (Le Foll et al, 2014).
Reported intention to smoke for the positive effects of
tobacco (TCQ factor 4) were decreased. This decrease in the
positive effects of smoking is consistent with both preclinical
(George et al, 2011; Le Foll et al, 2012) and clinical (Gonzales
et al, 2006; Jorenby et al, 2006) studies that have demonstrated that varenicline affects the rewarding properties of
smoking. Given that positive reactions to smoking have been
0
-10
-5
-0.05
0
5
-0.1
-0.15
r 2= 0.651, p=0.057
-0.2
-0.25
Change in Score
Figure 4 Correlations changes in TCQ factor 2 and changes in BPND in
the DC approached significance. Improvements in the TCQ score were
correlated with decreased BPND after varenicline (ie, increased DA).
shown to predict relapse (Strong et al, 2011), decreases in
measures of positive reinforcement may provide an explanation as to the efficacy of varenicline (Gonzales et al, 2006;
Jorenby et al, 2006). In addition, varenicline also decreased
the negative reinforcement of withdrawal (Gonzales et al,
2006; Jorenby et al, 2006). However, changes in brain DA
and subjective appraisals of smoking occurred in the absence
of any changes in plasma cotinine or the number of
cigarettes smoked per day from baseline. Thus changes in
subjective values or BPND do not reflect alterations in
smoking habits per se. This is compelling as it suggests that
varenicline may act to alter the brain and subjective response
prior to quitting, thus enabling the mechanism by which
smokers may subsequently quit. It may also explain the
delayed quitting observed up to 4 weeks after taking
varenicline (Agboola et al, 2010, 2015; Kasza et al, 2013). It
has been shown that treatment with the nicotine patch prior
to the quit date can improve smoking cessation (Rose et al,
2009); in smokers who did not decrease smoking prior to
their quit date while undergoing nicotine replacement
therapy, they could be ‘rescued’ by bupropion augmentation
of the patch or with varenicline treatment alone (Rose and
Behm, 2013). Future studies can address how brain response
and subjective measures can predict the success of smoking
cessation interventions.
Varenicline and tobacco smoke affect DA differently.
Varenicline appears to produce elevations in DA in a D2-rich
areas (DC) but not in D3-rich areas (SN, VP). In contrast,
tobacco produced elevations in both limbic striatum and in
D3-rich area (VP), but not in the DC, as assessed with
[11C]-(+)-PHNO (Le Foll et al, 2014). These differential
effects may be due to the different pharmacological agonist
properties (full agonist for nicotine vs partial agonist for
varenicline) or the involvement of different subtypes of
nicotinic acetylcholine receptors (Coe et al, 2005). Another
possibility is that varenicline predominantly stimulated
nicotinic acetylcholine receptors located in DA neurons that
preferentially project to DC. Further studies should explore
for such effects. Regardless of the receptor target (D2 or D3),
[11C]-(+)-PHNO has been shown to have a greater sensitivity
in detecting smaller changes in synaptic DA levels as
compared with [11C]-raclopride (Ginovart et al, 2007;
Narendran et al, 2006; Willeit et al, 2006). This is supported
by the direct comparison of the dose–effect of amphetamine
Neuropsychopharmacology
Dopamine and varenicline
P Di Ciano et al
1518
(0.1, 0.5, and 2 mg/kg; i.v.) on binding of [11C]-(+)-PHNO
and [11C]-raclopride in cats (Ginovart et al, 2006) and
humans (Shotbolt et al, 2012).
To our knowledge, no study has investigated the effects of
bupropion or NRT using [11C]-(+)-PHNO. Previous studies
measuring the impact of bupropion on DA transmission
using [11C]-raclopride found no effect on ventral
caudate/nucleus accumbens binding induced by bupropion
(Brody et al, 2010). Limited work has been carried out
assessing the impact of NRT on dopamine transmission with
PET. Nicotine gum (Takahashi et al, 2008), but not nicotine
spray (Montgomery et al, 2007), was effective in decreasing
[11C]-raclopride in striatal area. As compared with the
traditional radiotracer [11C]-raclopride, [11C]-(+)-PHNO
allows for greater sensitivity in the measurement of
change in DA levels (Gallezot et al, 2012;
Ginovart et al, 2006, 2007; Narendran et al, 2006;
Willeit et al, 2006). It is possible that our ability to detect
significant elevations in DA induced by varenicline was
due to the use of [11C]-(+)-PHNO vs [11C]-raclopride.
As response to treatment for varenicline and for NRT
has
been
related
to
nicotine
metabolic
rate
(Lerman et al, 2015), further studies exploring the impact
of nicotine metabolic rate on [11C]-(+)-PHNO would be
informative.
Partial agonists are believed to be able to decrease druginduced elevations in DA and also to elevate DA levels
during withdrawal (Childress and O'Brien, 2000). Therefore,
increases in DA levels in the DC in the present study may
reflect a reversal of attenuated DA levels normally seen
during withdrawal. It should be noted that the present study
did not measure whether varenicline could decrease elevations in DA levels induced by smoking in humans. Further
studies could explore this as a further test of the partial
agonist properties of varenicline.
This study has several limitations. First, the study has a
limited sample size. However, despite this limited sample
size, we were able to detect significant changes in
[11C]-(+)-PHNO BPND in the DC that were still significant
after corrections for multiple testing. But it is possible that
with a larger sample size we would have detected an effect of
varenicline in more brain areas and also had more power to
detect changes in subjective measures. Indeed, previous
studies have found alterations in measures of withdrawal
following varenicline (Gonzales et al, 2006; Jorenby et al,
2006). Further, we were not able to analyze the data in terms
of those who respond to treatment compared with those who
do not respond to treatment (only three responded to
treatment). Future investigations may reveal the predictive
relationship between early changes in BPND and response to
treatment in smokers, as already identified in subjects with
cocaine use disorders undergoing behavioral treatment
(Martinez et al, 2011).
Related to the relatively small sample size is the inability in
the present study to look at individual differences. One of
these is gender effects (Cosgrove et al, 2014). It has been
shown, using [11C]raclopride, that DA levels in the ventral
striatum are increased in males as compared with females
during cigarette smoking. In the present study, the sample
consisted of seven males and four females, and thus the data
may disproportionately represent one gender, with not
enough power to compare the two. It is noteworthy that
Neuropsychopharmacology
effects found in the present study in the VST were minimal
(Cohen’s D of 0.168), and thus it is possible that gender
effects are more pronounced during smoking than withdrawal per se, as studied in the present study. Future studies
will need to determine whether gender differences exist in
the neurochemical response to treatment approaches for
smoking cessation.
Further, there are some limitations owing to the PET
scanning parameters. The injected mass of the radiotracer
was slightly above the limit suggested by others
(Gallezot et al, 2012), potentially leading to underestimation
of DA occupancy in both conditions (Shotbolt et al, 2012).
The fact that the mass injected was similar in the two
conditions suggests that it did not interfere with the
results. Although our analysis based on the brain area
suggests that varenicline has an impact predominantly at the
level of the DRD2 receptor in the DC, we do not have a clear
explanation as to why there would be no effect at the level of
the DRD3. In our previous studies, we have found that
DRD3 levels are increased in the brains of people with
psychostimulant use disorder (Boileau et al, 2012;
Payer et al, 2013).
It should be considered, given the lack of a control group,
that the present results may be due, perhaps in part, to a
placebo effect. The placebo effect is common in the
treatment of pain, depression, and Parkinson’s disease and
is set up by an expectation on the part of the participant that
a treatment will be successful (for reviews, see Lidstone and
Stoessl, 2007; Murray and Stoessl, 2013). Particularly
powerful in establishing a placebo effect are environmental
considerations such as a hospital and controlled clinical trial
setting, both of which were true here. With the use of
[11C]raclopride, it has been shown that the placebo effect can
be associated with increases in DA in the dorsal and
ventral striatum, the same brain regions that are involved
in reward expectation (de la Fuente-Fernandez et al, 2001,
2002, 2006; de la Fuente-Fernandez and Stoessl, 2002;
Strafella et al, 2001, 2003, 2006). Given the increase in DA
observed in the DC in the present study, the
possibility that this may be partly explained by a placebo
effect cannot be ruled out. However, this suggestion is
somewhat tempered by the fact the placebo effect also
elevates DA in the VST, something that was not found in the
present study.
CONCLUSIONS
The purpose of the present study was to determine
whether steady-state levels of varenicline can increase DA
levels in the brain of abstinent smokers. It was found that DA
was increased in the DC after treatment with varenicline
and that this increase was (nearly) correlated with decreases
in ratings of the positive effects of smoking. These
findings indicate for the first time that varenicline increases
DA transmission in human smokers scanned under
abstinence. This effect may contribute to its therapeutic
efficacy, and future studies would be needed to determine
this relationship, especially as varenicline negatively affects
positive ratings of smoking (Agboola et al, 2010, 2015;
Cahill et al, 2012).
Dopamine and varenicline
P Di Ciano et al
1519
FUNDING AND DISCLOSURE
In the past 3 years, RFT has consulted for Apotex on issues
unrelated to smoking. In the previous 3 years, PS has
received funding from the Centre for Addiction and Mental
Health, Canadian Cancer Society, Canadian Institutes of
Health Research, Health Canada, the Association of Faculties
of Medicine of Canada, Ontario Ministry of Health and
Long-Term Care, Ontario Brain Institute, Ontario Lung
Association, Cancer Care Ontario, Ontario Institute for
Cancer Research, National Institutes of Health, Workplace
Safety and Insurance Board, Pfizer, McLaughlin Centre for
Molecular Medicine, and Shoppers Drug Mart. BLF has
received support unrelated to the current project from Pfizer,
Bioprojet, and Mettrum and various public funding agenices.
This work was funded in part by an Ontario Lung
Association-Pfizer grant awarded to BLF and a CIHR grant
TMH-109787 to RFT. We acknowledge the support of the
Endowed Chair in Addictions for the Department of
Psychiatry (to RFT), the Campbell Family Mental Health
Research Institute of CAMH, the CAMH Foundation, the
Canada Foundation for Innovation (nos. 20289 and 16014),
and the Ontario Ministry of Research and Innovation. We
also acknowledge the support of the Clinician Scientist Salary
support Program from the Department of Family and
Community Medicine (to PS). The other authors declare
no conflict of interest.
ACKNOWLEDGMENTS
We thank Greg Staios and Alexandra Andric for their
assistance with this investigation.
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