Placebo effects on human ?-opioid activity
Tor D. Wager*†, David J. Scott‡, and Jon-Kar Zubieta‡§
*Department of Psychology, Columbia University, 1190 Amsterdam Avenue, New York, NY 10027; and Departments of§Radiology and‡Psychiatry and
Molecular and Behavioral Neuroscience Institute, University of Michigan, 205 Zina Pitcher Place, Ann Arbor, MI 48109-0720
Communicated by Edward E. Smith, Columbia University, New York, NY, March 15, 2007 (received for review October 26, 2006)
a manner reversible by opioid antagonists, but little is known
about the central brain mechanisms of opioid release during
placebo treatment. This study examined placebo effects in pain by
using positron-emission tomography with [11C]carfentanil, which
measures regional ?-opioid receptor availability in vivo. Noxious
thermal stimulation was applied at the same temperature for
placebo and control conditions. Placebo treatment affected endog-
enous opioid activity in a number of predicted ?-opioid receptor-
rich regions that play central roles in pain and affect, including
periaqueductal gray and nearby dorsal raphe and nucleus cunei-
formis, amygdala, orbitofrontal cortex, insula, rostral anterior
cingulate, and lateral prefrontal cortex. These regions appeared to
be subdivided into two sets, one showing placebo-induced opioid
activation specific to noxious heat and the other showing placebo-
induced opioid reduction during warm stimulation in anticipation
of pain. These findings suggest that a mechanism of placebo
analgesia is the potentiation of endogenous opioid responses to
noxious stimuli. Opioid activity in many of these regions was
correlated with placebo effects in reported pain. Connectivity
analyses on individual differences in endogenous opioid system
activity revealed that placebo treatment increased functional con-
nectivity between the periaqueductal gray and rostral anterior
cingulate, as hypothesized a priori, and also increased connectivity
among a number of limbic and prefrontal regions, suggesting
increased functional integration of opioid responses. Overall, the
results suggest that endogenous opioid release in core affective
brain regions is an integral part of the mechanism whereby
expectancies regulate affective and nociceptive circuits.
neuroimaging ? periaqueductal gray ? expectancy ? affective neuroscience
ascribed to it. They are particularly strong in experimental and
clinical studies of pain. However, much remains to be learned
about the neural and cognitive mechanisms by which placebo
treatments have their effects. Placebo analgesic treatments elicit
expectations of pain relief, which are thought to change the
affective and motivational context in which nociceptive signals
are interpreted (1–6). Although ample evidence exists that
placebo expectancies reduce reported pain, the neurobiology of
how expectancies interact with nociceptive brain processes is
Early pharmacological studies showed that placebo analgesia is
reduced by the opioid antagonist naloxone (7), suggesting that
endogenous opioids play a crucial role. Follow-up work pointed to
both opioid and nonopioid mechanisms of placebo (8). An emerg-
ing idea is that expectancy-based placebo effects are opioid-
mediated, but conditioned placebo effects may depend on other
mechanisms (9–13). Although these studies support a strong role
for opioids, little is known about the brain mechanisms that link
expectancy with opioid release and pain relief in humans. Opioid-
mediated placebo effects are thought to involve the periaqueductal
gray (PAG), which contains many of the brain’s opioid-containing
neurons and has been linked in a large animal literature with pain
lacebo effects are treatment effects caused not by the
physical properties of a treatment but by the meaning
of frontal stimulation in rats, and PAG is a target for human
therapeutic treatments for chronic pain (22–25).
MRI (fMRI) activity in lateral prefrontal cortex (PFC) and or-
bitofrontal cortex (OFC). The magnitude of these activations was
correlated with increases in the midbrain around the PAG, sup-
porting the hypothesis that PAG and opioids play a role in
expectancy-based placebo (26). However, the only previous direct
study of regional placebo-induced opioid activity found effects in
regions related to motivation and reward value, including the
nucleus accumbens (NAC) and anterior insula (aINS), but not the
PAG (27). Thus, involvement of PAG opioid activity in placebo
analgesia has not yet been directly demonstrated, and the relation-
ships between endogenous opioid activity in PAG and other brain
regions, such as PFC and rostral anterior cingulate (rACC), are
In this study, we aimed to address these gaps in knowledge by
testing placebo effects on ?-opioid activity in PAG and other
opioid-rich regions of interest (ROIs). We delivered identical
thermal stimuli to patches of skin treated with identical, phar-
macologically inert placebo and control creams. The conditions
differed only in the instructions: Participants were told that the
placebo treatment was a ‘‘highly effective pain reliever,’’ and that
the control treatment would ‘‘have no effect on pain.’’ We
examined placebo-induced changes in ?-opioid receptor binding
potential (BP) as measured with11C-radiolabeled carfentanil (a
?-opioid receptor-selective agonist) and positron-emission to-
mography (PET). With this method, endogenous opioids inter-
fere with the binding of [11C]carfentanil to the receptors,
resulting in reductions in BP (28). Thus, the reduction in BP for
placebo vs. control treatment is a measure of placebo-induced
opioid system activation (27).
With this design, we can address another important unresolved
issue. In Zubieta et al. (27), placebo and control conditions were
designed to produce equal subjective pain by using an adaptive
procedure. Because noxious input differed for placebo and control
conditions, it is possible that (placebo ? control) opioid effects
could be caused by differences in input or even minimized because
Author contributions: T.D.W. and J.-K.Z. designed research; T.D.W. and D.J.S. performed
research; T.D.W. and J.-K.Z. contributed new reagents/analytic tools; T.D.W. and D.J.S.
analyzed data; and T.D.W., D.J.S., and J.-K.Z. wrote the paper.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
Abbreviations: aINS, anterior insula; BP, binding potential; CH, control with painful heat;
CW, control with nonpainful warmth; DLPFC, dorsolateral PFC; DRN, dorsal raphe nucleus;
NAC, nucleus accumbens; NCF, nucleus cuneiformis; OFC, orbitofrontal cortex; PAG, peri-
aqueductal gray; PET, positron-emission tomography; PFC, prefrontal cortex; PH, placebo
with painful heat; pgACC, pregenual anterior cingulate; PW, placebo with nonpainful
warmth; rACC, rostral anterior cingulate; ROI, region of interest; SVC, small volume
†To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2007 by The National Academy of Sciences of the USA
June 26, 2007 ?
vol. 104 ?
of the psychological equivalence of the conditions. Here, we
examine whether there are placebo effects on opioid activity when
noxious input is equivalent.
Placebo expectancy may affect opioid activity in several ways,
three of which are shown in Fig. 1A. Placebo may enhance
endogenous opioid release caused by noxious stimulation (mech-
anism no. 1), in which case placebo-induced ?-opioid activity
should be higher during hot than warm stimulation; thus, a tem-
perature (hot vs. warm) ? placebo (placebo vs. control) interaction
is predicted. This effect is consistent with studies showing larger
placebo effects with more intense pain (29). Alternatively, appli-
cation of the placebo cream may itself trigger endogenous opioid
activity (mechanism no. 2), which should result in evidence for
placebo-induced opioid increases with both hot and warm stimu-
lation. Finally, a function of opioid analgesia may be to prepare an
organism to deal with an imminent threat, in which case pain
anticipation may result in opioid release (mechanism no. 3; Fig. 1A
Right). In this case, placebo may reduce threat (30, 31) and thus
reduce ?-opioid release during the anticipatory warm stimulation
period. Different brain regions may independently show evidence
2) in opioid-rich regions such as PAG, OFC, and aINS.
In addition to regional opioid activity, we sought to test for
placebo effects on the functional integration of opioid responses in
frontal, limbic, and brainstem regions, which relate to different
components of the endogenous analgesic response (28). We ex-
pected frontal regions, which may maintain expectancies for pain
relief, to be more correlated with limbic regions and PAG under
placebo conditions. Connectivity between frontal cortex and brain-
stem appears to play an important role in the modulation of pain
by attention (32, 33) and placebo (26), as predicted from animal
analgesia (36). In the present study, multivariate analyses, which
combined nonmetric multidimensional scaling (NMDS) and per-
mutation tests with hierarchical clustering, allowed us to identify
several functional opioid subsystems and to test for the modulation
of connectivity within and between subsystems by placebo.
Pain-Specific Placebo Effects. The four conditions in the study were
(CH), placebo with nonpainful warmth (PW), and control with
nonpainful warmth (CW). Placebo treatment led to significant
shown in supporting information (SI) Fig. 6C. In the brain, we first
tested for opioid activation differences in the temperature ?
placebo interaction contrast, (CH ? PH) ? (CW ? PW), in which
positive values indicate larger placebo-induced opioid activation in
heat relative to warm stimulation (mechanism no. 1 in Fig. 1A).
Of the 11 key ROIs, 8 showed significant positive values, indi-
volume corrected (SVC)], and none showed decreases. Across all
27 ROIs, 14 regions showed significant placebo-induced increases
and none showed decreases. At the set level, activation of three key
of significant ROIs to reach the P ? 0.05 corrected level. Thus,
highly significant (P ? 0.001 for both tests), demonstrating that
would be expected under chance.
ROIs are shown in green in Fig. 2 and SI Fig. 6B, and voxels that
reached corrected significance within ROIs (P ? 0.05, SVC) are
anterior cingulate (pgACC), multiple loci within OFC, aINS,
thalamus, dorsolateral PFC (DLPFC), and amygdala. The activa-
tion extent is shown in Fig. 2 and subsequent figures at P ? 0.005,
0.01, and 0.05 (two-tailed) in hot colors. Activation volumes and
center-of-mass coordinates in Montreal Neurological Institute
ranged from 5.0% of baseline BP (left DLPFC) to 16.4% (left
amygdala), large enough to be of practical significance and com-
parable to previous work (27).
Pain-Specific Placebo Effects Predict Reported Placebo Analgesia.We
expected correlations between temperature ? placebo interactions
in significant ROIs and reported placebo analgesia, demonstrating
BP differences between relative placebo ‘‘responders’’ and ‘‘non-
responders.’’ We first tested for correlations in the exact voxels that
showed the significant group effects above. Opioid activity effect
magnitudes were significantly negatively correlated with reported
placebo analgesia in four of these regions: PAG, right lateral OFC
(lOFC), pgACC, and rACC (partial r’s ? ?0.47 to ?0.77; SI Table
1), indicating, contrary to our initial hypotheses, greater placebo
opioid activation in low responders. We return to a discussion of
these effects below. Two other regions, left anterior OFC and
lOFC, showed negative trends (r’s ? ?0.57 to ?0.60). (For more
information on robust partial correlations, see SI Methods.) Only
1. Opioid potentiation
increases only in heat
2. Nonspecific release
increases in warm and
3. Preparation reduction
decreases in warm
40 min10 min
PET session 1
PET session 2
40 min10 min
effect. (B) Study procedures.
Study hypotheses and procedures. (A) Mechanisms by which placebo
(CW ? PW)]. ROI extent is shown in green, and significant voxels in or
contiguous with ROIs are shown in red/yellow (positive effects) or lavender/
blue (negative effects). Amy, amygdala; aOFC, anterior OFC; lOFC, lateral
OFC/inferior frontal border; mOFC, medial OFC; thal, thalamus.
Wager et al.
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one region showed a positive correlation, left DLPFC (r ? 0.79).
Correlation scatter plots are shown in SI Fig. 7.
Tests within key ROIs revealed three additional negatively
correlated regions: PAG, right lOFC, and right NAC (P ? 0.05,
set showed positive correlations, both within left DLPFC (set-level
both significant in the group and predicted the magnitude of
reported analgesia, supporting mechanism no. 1 (Fig. 1).
Placebo Effects in Heat Vs. Anticipation.Ifplacebopotentiatesopioid
release during pain (mechanism no. 1), then the effects reported
above should be caused by placebo-related increases in opioid
temperature ? placebo interactions could be caused by placebo-
induced opioid decreases during pain anticipation (PW ? CW;
in heat (CH ? PH) and warmth (CW ? PW) separately.
Results revealed separate sets of regions that showed effects in
PH) are shown in Fig. 3A and SI Table 2. During heat, opioid
medial OFC (mOFC), and lOFC bilaterally] and right amygdala,
and at lower thresholds, thalamus (P ? 0.005), rACC (P ? 0.005),
and NAC (P ? 0.01, all two-tailed). In these regions, there was
virtually no effect of placebo during warm stimulation (Fig. 3B). Of
these regions, significant negative correlations between average
(CH ? PH) and reported placebo analgesia were found in right
amygdala (r ? ?0.61), mOFC (r ? ?0.78), and lOFC (r ? ?0.74).
Voxel-wise searches within ROIs yielded the same set of regions
(P ? 0.05, SVC). Scatter plots and coordinates are shown in SI Fig.
8. Subsequent analyses suggested that negative correlations could
be caused by greater nonspecific release in control conditions in
placebo responders (SI Fig. 9 and SI Text), which was observed in
areas including medial PFC, aINS, mOFC, and lOFC.
A different set of regions showed evidence of placebo-induced
anticipatory decreases (PW ? CW; mechanism no. 3) (Fig. 3C).
These regions include left amygdala, left ventral aIns, pgACC,
dorsal PAG, caudate, and right PFC [DLPFC, superior frontal
sulcus (SFS), and inferior frontal junction, (IFJ)]. Notable is the
dissociation between rACC, reported in Wager et al. (26), and
pgACC, reported in Zubieta et al. (27): rACC responded specifi-
cally during pain, whereas pgACC responded specifically during
anticipatory warm stimulation. Right vs. left amygdala showed a
similar distinction. Among these regions, negative correlations
between (CH ? PH) and reported analgesia were found in right
ventral aINS (r ? 0.67; SI Fig. 8).
Detailed Analysis of Midbrain. Results of exploratory analysis (P ?
0.05, two-tailed) in midbrain nuclei are shown in SI Fig. 10.
found in the dorsal PAG and overlying superior colliculus. Placebo
of the PAG and most strongly in the nucleus cuneiformis (NCF)
and dorsal raphe nucleus (DRN) (inferior to the red nucleus),
extending into the ventral tegmental area (VTA). The tempera-
ture ? placebo interaction (Fig. 2 Inset and SI Fig. 10) was
significant and positive in both dorsal and midventral PAG, ex-
tending into DRN as well. These results are consistent with
mechanisms nos. 1 and 3.
Multivariate Connectivity Analysis. Portions of ROIs that showed
placebo effects in one or more contrasts, 32 contiguous regions in
all were selected for network analysis. We analyzed average opioid
BP across conditions for each participant, computing Spearman’s ?
values between pairs of regions. These interregion correlations can
reveal whether individual differences in opioid binding are consis-
tent across the brain or whether there are functional subsystems
in opioid responses that show different patterns of individual
decomposed with nonmetric multidimensional scaling (NMDS)
(eight-dimensional solution; SI Fig. 11 A and B and SI Text), and
hierarchical cluster analysis with average linkage was used on
component scores to group regions into sets (‘‘subsystems’’). Non-
parametric tests were used to determine the number of subsystems
and to establish significance. We found that the best solution was
hypothesis of no functional subsystems (P ? 0.0001, Z ? 4.0) (for
details, see SI Fig. 11 C and D and SI Text).
space of the first two components. Each circle represents a unique
Regions closer together on the graph were on average more highly
correlated, and the marker color indicates subsystem membership.
Lines show pairs of regions with significant bivariate ?’s [gray, P ?
0.05; black, false discovery rate (FDR) (37) corrected P ? 0.05].
The size of each region indicates its centrality in the overall
network; regions with larger circles are more centrally connected
Fig. 4 shows that OFC regions (yellow) have high connectivity
two separable prefrontal subsystems (red and magenta). PAG–
rACC (green) and amydgala–pgACC subsystems (light blue) also
Regions showing placebo-induced opioid increases
during heat (CH ? PH). Color coding is as in Fig. 1. (B)
regions (x axis). R, right; L, left. Other labels are as in
Fig. 2. (C) Regions showing placebo-induced anticipa-
tory opioid decreases (PW ? CW). (D) Bar graphs for
effects in C. Amy, amygdala; aOFC, anterior OFC; Cau,
caudate; IFJ, inferior frontal junction; lOFC, lateral
OFC/inferior frontal border; mOFC, medial OFC; thal,
Placebo effects in heat vs. anticipation. (A)
www.pnas.org?cgi?doi?10.1073?pnas.0702413104Wager et al.
A large, interconnected subsystem of limbic regions (dark blue)
includes NAC, insula, lateral OFC, thalamus, and some frontal and
Averaging ?-opioid BP within subsystems, we tested whether
connectivity between networks was stronger with painful than with
warm stimulation and stronger with placebo than with control
during heat (ref. 38; see SI Fig. 12 and SI Text). This analysis can
test whether individual differences are more coherent across brain
subsystems with heat (analysis 1) and with placebo (analysis 2).
Increased connectivity with placebo would indicate that placebo
induces coherent opioid activity across multiple regions that varies
individual differences in placebo responses. We found that heat
increased integration of OFC with DLPFC and the limbic sub-
system (statistics in SI Table 3). Correlations between the PAG–
than with control treatment (?placebo? 0.67 vs. ?control? 0.09, Z ?
2.25, P ? 0.02, two-tailed). Placebo treatment also increased
connectivity between the PAG and the rACC themselves,
(?placebo? 0.71 vs. ?control? 0.06, Z ? 2.31, P ? 0.02). Scatter plots
are shown in Fig. 5.
Finally, we tested for placebo-induced increases in functional
integration among regions as a whole. We compared interregion
correlations under PH with those under CH, and counted the
number of increased vs. decreased correlations (SI Fig. 12). We
found 36 increased pairwise correlations under placebo compared
with 5 decreased (difference ? 26, P ? 0.0001 by using a permu-
tation test). All significant placebo effects on pairwise correlations
left DLPFC–amygdala are in SI Fig. 12 Inset 3).
Understanding expectancy effects on endogenous opioid neuro-
transmission is an important step in explaining how cognitive
processes modulate perception and affect, with implications for
healthy endogenous regulation and clinical pain syndromes (39).
Consistent with behavioral studies (7, 11, 40), the results indicate
that placebo treatment induces increases in endogenous opioid
activity in ?-opioid-rich limbic and paralimbic regions, including
PAG, NCF, DRN, OFC, amygdala, two dissociable regions of the
effects in pain or pharmacological administration of opioids. NCF
and DRN are and have been reported in recent fMRI studies of
central pain modulation (41–43).
The results suggest that placebo may affect opioid responses
[OFC, rACC, NAC, NCF, DRN, ventral tegmental area (VTA),
right amygdala, and thalamus] by potentiating the opioid release
caused by noxious stimulation (mechanism no. 1 in Fig. 1A). In
another set of regions (lateral prefrontal cortex, aIns, pgACC,
and left amygdala), placebo treatment appears to reduce antic-
ipatory opioid activity, perhaps by reducing anticipatory threat
responses (mechanism no. 3 in Fig. 1A). These regions may be
involved in preparing for upcoming pain. Connectivity analyses
supported this distinction, as the regions showing each pattern of
responses belonged to different, separable subsystems. Although
these results suggest a prominent role for endogenous opioids in
placebo analgesia, placebo analgesia does not appear to be
opioid-mediated in all conditions; for example, after condition-
ing with a nonopioid analgesic (11) and in a recent study of
irritable bowel syndrome (44).
Placebo Effects in the Midbrain.ThefindingofPAGopioidincreases
in placebo is important because it is perhaps the key region linking
of Bingel et al. (35) on placebo-dependent connectivity between
rACC and PAG in fMRI. Interestingly, however, the strongest
midbrain placebo effects during noxious heat were found in and
antinociceptive pathways (16, 45) that have been reported in recent
imaging studies to show expectancy-related effects (41–43).
Functional Integration. We assessed interregional connectivity
across participants (rather than on dynamic measurements over
33, 35, 36). Such analyses are informative about the pattern of
individual differences in brain activity. Connectivity analyses sug-
gested that there are multiple subsystems in the human ?-opioid
show placebo opioid responses. (B) Nonmetric multidi-
mensional scaling (NMDS) connectivity graph of the re-
gions in A. See text for explanation. L, left; R, right; amy,
amygdala; aofc, anterior OFC; cau, caudate; ifj, inferior
mofc, medial OFC; thal, thalamus.
Connectivity analysis of opioid binding poten-
Fig. 5.Placebo-modulated connectivity between PAG and rACC.
Wager et al.
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system, as evidenced by the findings that opioid BP levels were not
correlated across ROIs. Rather, opioid BP levels were correlated
regions (identified by cluster analysis). For example, our findings
suggest that one subsystem is composed of left and right lateral and
midlateral OFC. Opioid BP levels among these regions was corre-
each other one) and correlations with other regions are lower.
Other subsystems included dissociable superior and mid-
dorsolateral subsystems, a limbic subsystem, a midline (rACC and
amygdala, caudate, and one prefrontal region). These are shown in
Fig. 4. These findings argue against the notion of a unitary ‘‘opioid
In addition to increasing the PAG–rACC connectivity hypoth-
esized a priori, placebo treatment increased functional connectivity
among the distinct subsystems (SI Fig. 12), particularly among
subcortical regions, including NAC, thalamus, amygdala, and in-
sula, and between these regions and DLPFC and OFC regions in
other subsystems. The most straightforward interpretation of in-
creased opioid–BP connectivity with placebo is that individuals
an individual’s response magnitude is consistent across ‘‘con-
the same as those in other regions, implying a central common
mechanism for placebo-induced opioid release that varies across
individuals. These results suggest that it may be meaningful to
placebo responders,’’ in future studies.
Correlations Between Reported Placebo and Opioid Activation. Opi-
oid activity increases in a substantial proportion of ROIs was
in most regions, suggesting that greater reported reduction in pain
is associated with more modest opioid increases. One reason may
be that placebo responders show nonspecific opioid activity across
both placebo and control conditions and thus smaller (CH ? PH)
differences. There was evidence for this effect in a number of
regions; a representative example in the right lateral OFC is shown
in SI Fig. 9 A–C. Strong placebo responders showed evidence of
higher opioid activation (lower BP values) averaged across task
conditions (r ? 0.56, P ? 0.03; SI Fig. 9B) as well as evidence of
higher opioid activation (lower BP) in the CH condition (r ? 0.72,
P ? 0.003). SI Fig. 9D shows whole-brain maps of correlations
between reported placebo and BP in the CH and CW conditions.
The widespread negative correlations reinforce the notion that
placebo responders show relatively lower BP in many opioid-rich
regions. This could occur for two reasons: Placebo responders may
release more endogenous opioids in response to the experimental
context or to the manipulation phase preceding PET, or placebo
responders may have higher receptor-binding affinity, which would
lead to lower overall BP levels for these participants in the context
of an existing opioid tone. In the former case, inclusion of the
expectancy manipulation may be an important experimental dif-
ference from previous work (27), because it may elicit opioid
release by itself. In the later case, affinity differences may help
analgesic responses (11). More detail and discussion of additional
alternatives is provided in SI Text.
Placebo-induced expectancies of pain relief have been shown to
is known about the central brain mechanisms of opioid release
during placebo treatment. This study examined placebo effects on
endogenous opioid neurotransmission with PET and the ?-opioid
receptor-selective radiotracer [11C]carfentanil in relation to several
alternative mechanisms: Placebo treatment may potentiate either
anticipatory or pain-related endogenous opioid release or elicit
conditions differed only in contextual instructions (temperatures
were equivalent). Increases in endogenous opioid neurotransmis-
sion (decreases in ?-opioid receptor availability in vivo) were found
of placebo and cognitive regulation, including the PAG, amygdala,
OFC, insula, rACC, and lateral PFC. Two distinct patterns were
found: First, placebo-induced increases in opioid activity specific to
heat in OFC, right amygdala, and rACC suggests that placebo
potentiates pain-related opioid release; opioid activity in many of
these regions was correlated with reported placebo analgesia.
Second, placebo-induced decreases in opioid activity specific to
anticipation in pregenual cingulate, insula, and PFC suggests that
anticipatory anxiety may result in opioid release, and placebo may
block this release (although this is speculative); these anticipatory
opioid responses were not correlated with reported placebo anal-
and cluster analysis revealed distinct clusters of regions with similar
opioid activity across participants, suggesting that coherent sub-
systems within the endogenous opioid system can be identified.
Painful heat (vs. warm stimulation) increased the functional inte-
gration of opioid activity in OFC with those in other areas. Placebo
treatment increased connectivity between the PAG and rACC and
effects on endogenous opioid activity in cortical and subcortical
based control of pain.
Materials and Methods
Experimental Design. Fifteen healthy volunteers (details in SI Text)
underwent two 90-min scans with PET and [11C]carfentanil (Fig.
1B). Each scan consisted of 30 warm trials (one per minute) and 30
noxious heat trials (one per minute), in that order. We stimulated
the placebo-treated forearm region during one scan and the
control-treated region in the other, with order randomized, coun-
per scan. These doses occupy ?0.5% of the available ?-opioid
receptors and are devoid of physiological effects.
During each trial, a 1-s warning tone was followed by 4 s of
anticipation. Next, 24.5 s of thermal stimulation (3 s ramp up, 17 s
at target, 4.5 s ramp down) was applied using a Medoc TSA 2001
(Medoc Ltd., Chapel Hill, NC). Next, a warning tone cued partic-
ipants to rate the stimulus intensity on a 0–10 visual analogue scale
with the following verbal anchors: 0 was ‘‘no sensation’’; 1 was
‘‘detectable sensation’’; 2 was ‘‘nonpainful warmth’’; 3 was ‘‘just
painful’’; and 10 was ‘‘unbearable.’’ The interval between stimula-
scanning, warm and noxious temperatures were chosen on an
(44–46.5°C) were chosen at calibration level 2. Noxious tempera-
tures (48–49.5°C) were chosen at level 8. After calibration, placebo
and control treatments were applied to two regions of the volar
forearm of each volunteer. As in previous work, before scanning,
involves surreptitiously presenting reduced temperatures during
a minimum of 20 min after the manipulation phase. See SI Text for
to the forearm. During scanning, each block of warm or noxious
www.pnas.org?cgi?doi?10.1073?pnas.0702413104Wager et al.
of Logan plots (47) used for BP quantification (see SI Text). The Download full-text
study was designed to maximize the sensitivity of the placebo vs.
control comparison with noxious heat, which began 40 min after
injection in each condition. We were also interested in the placebo
vs. control comparison during warm stimulation (providing infor-
mation relevant for mechanisms no. 2 and no. 3) and the pain
(hot–warm) ? placebo interaction (relevant for mechanism no. 1).
Importantly, however, the design is not suited for a direct compar-
ison between hot vs. warm stimulation, because these conditions
were not randomized, and BP calculation bias may differ between
early and late scanning periods. The comparison between control
and pain states has been the subject of previous work (28, 48) from
which we draw inferences about the effects of noxious stimulation
on opioid binding.
Image Acquisition and Analysis. [11C]carfentanil PET distribution
volume ratio (DVR) [equal to BP ? 1 or (Bmax/Kd) ? 1] images for
each condition were acquired, reconstructed, and corrected for
attenuation by using a modified Logan plot analysis (27). High-
resolution anatomical MRI images were warped into Montreal
Neurologic Institute (MNI) standard space, and warping parame-
ters were applied to DVR images (details in SI Text). DVR images
opioid effects were localized with reference to the group mean
warped anatomical MRI image, which was used as the anatomical
underlay in all figures.
‘‘activation’’ as the relative decrease ?-opioid receptor BP between
conditions, i.e., a positive value for (CH ? PH) in DVR indicates
that placebo increases in endogenous opioid activity during heat.
Similarly, positive (CW ? PW) values indicate that placebo in-
creases during anticipatory warm stimulation, and positive inter-
action values, [(CH ? PH) ? (CW ? PW)] indicate that placebo-
induced opioid increases greater during heat. Contrast values were
entered into a random effects, multiple regression model at the
second level with mean-centered reported placebo during heat
(reported pain in CH ? PH) and administration order (contrast
coded) entered as covariates. Robust model estimation was per-
formed using iteratively reweighted least squares (IRLS) in Matlab
software (Mathworks, Natick, MA), which produces valid P values
while minimizing the influence of outliers and violations of nor-
mality (49). See SI Text for details.
basis of at least one activation in placebo studies of fMRI or
?-opioid receptor BP; (ii) high BP in the current sample (? 1.1,
reflecting specific binding; see SI Fig. 6); and (iii) activation in at
least two previous studies of either placebo, opiate administration,
or emotion regulation, from ref. 50 (SI Fig. 6). In some cases, ROIs
Eleven ROIs of primary interest included PAG, rACC, aINS,
pgACC, medial orbital sulcus (MOS), lateral OFC/inferior frontal
gyrus (LOFC), amygdala (Amy), and nucleus accumbens (NAC).
Each of these regions has been shown to be important for placebo
effects in fMRI and/or opioid-binding studies. Additional ROIs in
the thalamus (6), DLPFC, medial OFC, and dorsal caudate were
also identified as described above, making a complete set of 27
regions. Correction for multiple comparisons across regions was
performed with a permutation test that compared the number of
significant SVC ROIs with the number expected under the null
hypothesis (see SI Text). For ROI sets that show a significant
number of activated regions, we report SVC-corrected regions
(yellow in all figures).
Because of the importance of brainstem pain modulation path-
ways, additional exploratory ROIs included the NCF (41, 43),
lateral to PAG in the midbrain, the midline DRN (e.g., ref. 42; SI
Fig. 10), and dopaminergic ventral tegmental area (VTA) due to
are heavily interconnected with PAG and the rostral ventral
medulla and play key roles in spinal afferent inhibition and sensi-
tization (45). Because too few imaging studies have examined these
of midbrain placebo effects with approximate locations of these
nuclei as defined in refs. 41–43 and in the Duvernoy atlas (53).
We thank Drs. Ed Smith and Ken Casey for helpful comments and
discussion; Raza Zaidi and Alex Sokolik for help with data collection;
Matthew Davidson and Brent Hughes for technical assistance; the PET
technologist at the University of Michigan PET Center for assistance;
and the authors of Statistical Parametric Mapping and the Montreal
Neurological Institute. Support was provided by the Mind, Brain, Body,
Foundation (T.D.W.), and National Science Foundation Grants 0631637
(to T.D.W.) and R01 AT 001415 (to J.K.Z.).
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