Neural mechanisms mediating the effects of expectation in visceral placebo analgesia: An fMRI study in healthy placebo responders and nonresponders

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PAIN 153 (2012) 382–390

www.elsevier.com/locate/pain

Neural mechanisms mediating the effects of expectation in visceral placebo analgesia: An fMRI study in healthy placebo responders and nonresponders Sigrid Elsenbruch a,⇑, Vassilios Kotsis a, Sven Benson a, Christina Rosenberger a,b, Daniel Reidick a, Manfred Schedlowski a, Ulrike Bingel c, Nina Theysohn d, Michael Forsting d, Elke R. Gizewski d,e a

Institute of Medical Psychology and Behavioral Immunobiology, University Hospital of Essen, University of Duisburg-Essen, Essen, Germany Institute of Medical Psychology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany Neuroimage Nord, Department of Neurology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany d Institute of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital of Essen, University of Duisburg-Essen, Essen, Germany e Department of Neuroradiology, Centre for Radiology, University Clinic of Gießen and Marburg, Justus-Liebig-University Gießen, Gießen, Germany b c

Sponsorships or competing interests that may be relevant to content are disclosed at the end of this article.

a r t i c l e

i n f o

Article history: Received 4 August 2011 Received in revised form 12 October 2011 Accepted 27 October 2011

Keywords: fMRI Placebo analgesia Nocebo effect Rectal distension Visceral pain

a b s t r a c t This functional magnetic resonance imaging study analysed the behavioural and neural responses during expectation-mediated placebo analgesia in a rectal pain model in healthy subjects. In N = 36 healthy subjects, the blood oxygen level–dependent (BOLD) response during cued anticipation and painful rectal stimulation was measured. Using a within-subject design, placebo analgesia was induced by changing expectations regarding the probability of receiving an analgesic drug to 0%, 50%, and 100%. Placebo responders were identified by median split based on pain reduction (0% to 100% conditions), and changes in neural activation correlating with pain reduction in the 0% and 100% conditions were assessed in a regions-of-interest analysis. Expectation of pain relief resulted in overall reductions in pain and urge to defecate, and this response was significantly more pronounced in responders. Within responders, pain reduction correlated with reduced activation of dorsolateral and ventrolateral prefrontal cortices, somatosensory cortex, and thalamus during cued anticipation (paired t tests on the contrast 0% > 100%); during painful stimulation, pain reduction correlated with reduced activation of the thalamus. Compared with nonresponders, responders demonstrated greater placebo-induced decreases in activation of dorsolateral prefrontal cortex during anticipation and in somatosensory cortex, posterior cingulate cortex, and thalamus during pain. In conclusion, the expectation of pain relief can substantially change perceived painfulness of visceral stimuli, which is associated with activity changes in the thalamus, prefrontal, and somatosensory cortices. Placebo analgesia constitutes a paradigm to elucidate psychological components of the pain response relevant to the pathophysiology and treatment of chronic abdominal pain. Ó 2011 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.

1. Introduction The search for effective treatment(s) of recurrent abdominal pain in functional gastrointestinal disorders such as irritable bowel syndrome (IBS) has proven difficult. This may at least in part be due to the fact that the pathophysiology of pain in IBS is complex and remains incompletely understood, calling for further studies on the mechanisms of abdominal pain not only in patients but also in healthy subjects. In addition, high placebo response rates in clin⇑ Corresponding author. Address: Institute of Medical Psychology and Behavioral Immunobiology, University Hospital of Essen, University of Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany. Tel.: +49 201 723 4502; fax: +49 201 723 5948. E-mail address: [email protected] (S. Elsenbruch).

ical trials have been felt to hamper progress in the identification of successful pharmacological treatment options for abdominal pain [10]. At the same time, there now exists compelling evidence that placebo interventions produce clinically relevant benefits in IBS [17,18] and other chronic pain conditions such as low back pain [15] and are commonly used by physicians in daily practice [36]. Hence, studies addressing the mechanisms mediating placebo and nocebo effects in the context of visceral pain are increasingly important and are highly relevant for the pathophysiology and treatment of IBS and other functional gastrointestinal disorders associated with recurrent pain [7]. Whereas the neural mechanisms of placebos in somatic pain models are increasingly well understood [10,11,45], little research has been conducted on the brain mechanisms mediating placebo analgesia in the context of visceral pain. Two imaging studies have

0304-3959/$36.00 Ó 2011 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2011.10.036

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been conducted within patients with IBS [23,30], but thus far no knowledge regarding the neural mechanisms of visceral placebo analgesia for rectal pain exists in healthy subjects. Therefore, we conducted a functional magnetic resonance imaging (fMRI) experiment addressing behavioral and neural mechanisms mediating placebo analgesia for painful rectal stimuli in a sample of healthy subjects, which was subsequently divided into groups of placebo responders and nonresponders. Using a deceptive study design, painful rectal stimuli were delivered in 3 conditions designed to vary the level of expectancy regarding a potent analgesic drug to 0%, 50%, and 100%, which in reality was a placebo (sodium chloride solution) in all conditions. The following hypotheses were tested: (1) At the behavioral level, expectation of a potent analgesic drug effectively reduces perceived painfulness of visceral stimuli. (2) These behavioral effects correlate with activity changes in the activation of brain regions mediating visceral pain, including anterior cingulate cortex and insular cortex, as well as areas mediating cognitive expectancies and descending pain inhibition (i.e., the dorsolateral and ventrolateral prefrontal cortices; dorsal pons/periaqueductal grey). Given previous evidence supporting the role of anticipatory attentional and arousal brain networks in visceral pain modulation [3,22], we assessed neural activation not only during the delivery of painful rectal distensions but also during a cued pain anticipation phase. Given the clinical and scientific relevance of identifying and characterizing placebo responders [11], we first conducted analyses within placebo responders, and subsequently compared responders and nonresponders on the behavioral and neural levels. 2. Methods 2.1. Participants Healthy adult subjects, N = 36, were recruited by local advertisement and carefully screened for a number of exclusion criteria. Exclusion criteria included age <18 years and >45 years, body mass index <18 or P27, any concurrent medical condition, including gastrointestinal, neurological, psychiatric, cardiovascular, immunological, and endocrine conditions, and evidence of structural brain abnormality upon structural MRI scan. Additional MRI-specific exclusion criteria included phobic anxiety, claustrophobia, and ferromagnetic implantations. Only women on oral contraceptives were studied to reduce potential confounding by menstrual cycle phase. All participants were evaluated digitally for anal tissue damage (e.g., painful hemorrhoids), which may interfere with balloon placement. A history of psychological conditions (based on self-report) or presently increased scores on the Hospital Anxiety and Depression Inventory (HADS) were also exclusionary. Righthandedness was ensured using a validated questionnaire (for questionnaire references, see later). Frequency and severity of gastrointestinal complaints suggestive of any functional or organic gastrointestinal condition were assessed in a structured telephone interview and subsequently with a standardized questionnaire. Pregnancy was excluded by urinary test. The study protocol was approved by the local ethics committee. All participants gave written informed consent, and were paid for their participation. 2.2. Study design On the first study day, rectal perceptual and pain thresholds were determined and a structural MRI scan was completed. On the subsequent day, visceral pain-induced brain activation was measured with fMRI in a within-subject, repeated-measures design with 3 expectation conditions (i.e., 3 separate scanning sessions, separated by 30-minute pauses) using deceptive instructions. To manipulate the expectation of analgesia, participants were informed that they

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would receive an intravenous infusion containing either a highly potent analgesic drug (100% expectation); possibly the analgesic drug or placebo because administration was being accomplished in a double-blind fashion (50% expectation), or an inert substance, i.e., sodium chloride (0% expectation). In reality, participants received sodium chloride (intravenous drip) in all 3 conditions (for more details, see also Placebo Instructions and Procedures). The order of conditions was randomized (i.e., subjects were randomized to 1 of a total of 6 different orders: 0-50-100, 0-100-50, 50-0-100, 50-100-0, 1000-50, 100-50-0). Between sessions, a 30-minute waiting period was accomplished as a wash-out period. The effectiveness of this procedure was tested in a pilot study [21]. In all conditions, subjects received rectal distensions at individual pressures determined on the first study day to establish similar perceptual intensities across individuals and between groups. Visual analogue scales (VAS) (0 to 100 mm; ends defined as 0: none to 100: very much) were completed after each session to quantify subjective pain, urge to defecate, and discomfort. Classification of responders and nonresponders was accomplished post hoc based on pain reduction computed using VAS scores in the 0% versus the 100% expectation conditions. 2.3. Placebo instructions and procedures All instructions were standardized and provided in writing prior to the experiments following the screening procedure (as part of informed consent). Instructions included detailed information on the supposed potent pain killer, i.e., the commercially available spasmolytic drug butylscopolaminiumbromid, which subjects were told would be added to an intravenous saline drip during the experiments. Information materials included several statements on previous clinical use of the substance as well as on known side effects. During the experiments, pertinent aspects of the instructions were repeated orally by the investigator who carried out the different treatments (0%, 50%, 100%). Specifically: In the 0% condition, subjects were informed that an inert substance, i.e., sodium chloride, was administered via an intravenous drip into the forearm vein. They were told that the purpose of this condition was to control for all possible factors present in the complex MRI environment that could potentially influence neural responses to painful distensions, such as somatosensory responses to the intravenous drip. The container used for the infusion drip was clearly marked as ‘‘sodium chloride’’. In the 50% condition, subjects were informed that this was the double-blind part of the study, i.e., that they had a 50% chance of receiving a potent pain killer or sodium chloride as an inert substance. During the experiment, a syringe containing a clear fluid with a label that showed a medical code was squirted into the container used for the infusion drip. Subjects were told that only the University Clinic’s pharmacy had the coding information, which would not be decoded until after the completion of the entire study. In the 100% condition, subjects were informed that they received the actual painkiller in order for us to document the effects of this substance on neural responses to rectal pain. The syringe that was injected into the container used for the continuous infusion drip was clearly labeled with the drug name. This was again carried out in full view of the subjects. 2.4. Rectal distensions Rectal distensions were carried out with a pressure-controlled barostat system (modified ISOBAR 3 device, G & J Electronics, Ontario, Canada), as previously described [8,9,33]. Briefly, perception

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and pain thresholds were determined using staircase distensions with random pressure increments of 2 to 10 mm Hg. Subjects were prompted to rate the sensation as follows: 1 = no perception; 2 = doubtful perception; 3 = sure perception; 4 = little discomfort; 5 = severe discomfort, still tolerable; 6 = pain, not tolerable. The threshold for first perception was defined as the pressure when the rating changed from 2 to 3; the pain threshold as the pressure at which the rating changed from 5 to 6. For repeated distensions in the scanner, the pressure corresponding to a rating of 5 was chosen. The maximal distension pressure was 50 mm Hg. Distensions in the scanner were preceded by a brief auditory cue (‘‘warning signal’’) in the form of a short beep delivered at pseudorandomized intervals (3 seconds, 6 seconds, 9 seconds, 12 seconds) before initiation of balloon distension. At the end of each distension, a distinctly different short beep was implemented to inform subjects about the completion of the stimulation interval. 2.5. Questionnaires Right-handedness was assessed with a validated questionnaire [32]. Symptoms of anxiety and depression were assessed with the German version of the HADS [16], chronic stress with the German version of the Perceived Stress Questionnaire [12], personality traits with the German version of the NEO-Personality-InventoryRevised [26], and emotional distress with the Symptom Checklist 90 Revised [13]. 2.6. Statistical analysis of questionnaire data VAS scales were analyzed with repeated-measures analysis of variance with expectation condition as the repeated factor (0%, 50%, 100%), followed up by t tests to compare individual means. For responders analyses, the entire sample was separated into responders (N = 18) and nonresponders (N = 18) based on a median-split of VAS pain reduction, i.e., difference between the 0% and the 100% expectation conditions. For comparisons of responders and nonresponders, independent samples t tests or v2 tests (for dichotomous variables) were computed. In all analyses, the alpha level for significance was set at 0.05 and results are shown as mean ± SEM. 2.7. fMRI imaging and analyses fMRI was used to measure the blood oxygen level–dependent (BOLD) response during cued pain anticipation and pain. A total of 8 distensions (duration each 31 seconds) were accomplished in each condition/session, i.e., a total of 24 distensions were delivered during fMRI scanning. All MR images were acquired using a 1.5-T scanner (Sonata, Siemens, Erlangen, Germany) with a standard head coil. A 3-dimensional fast low angle shot (FLASH) FLASH sequence [repetition time (TR) (TR 10 ms, time to techo (TE) TE 4.5 ms, flip angle 30°, field of view (FOV) FOV 240 mm, matrix 512, slice thickness 1.5 mm)] was acquired. BOLD contrast images were acquired using an echo-planar technique (TR 3100 ms, TE 50 ms, flip angle 90°, FOV 240 mm, and matrix 64) with 34 transversal slices angulated in direction of the corpus callosum with a thickness of 3 mm and a 0.3-mm slice gap. For analysis, SPM 05 software (Wellcome Trust Centre for Neuroimaging, London, UK) was used. Prior to statistical analysis, images were realigned to the mean, normalized to a standard echo-planar imaging (EPI) template as implemented in SPM 05 and finally smoothed with an isotropic Gaussian kernel of 9 mm. Data were also subjected to high-pass filtering (cutoff period: 120 seconds), low-pass filtering with the hemodynamic response function (hrf), and correction for temporal autocorrelations (based on a first-order autoregressive model).

Data analysis was performed using a general linear model approach. For each subject, the first-level design matrix included a 2  3 factorial design with the factors ‘‘expectation level’’ (0%, 50%, 100%) and ‘‘stimulation condition’’ (anticipation of pain, painful stimulation). All regressors were obtained by convolving a boxcar function of the event duration with the canonical hemodynamic response function implemented in SPM. Specific effects were tested with appropriate linear contrasts of the parameter estimates for the different regressors resulting in a t statistic for each voxel. After model estimation, the ensuing first-level contrast images (0% > 100%, 0% < 100% expectation) from each subject were used for second-level analysis treating individual subjects as a random factor and including nonsphericity correction. Data from the 50% will be reported elsewhere. We performed one-sample t tests as a first step to describe neural activation in our regions of interest (specified later) in all subjects in the 0% expectation condition. To specifically analyze placebo response–correlated changes in activation of specific regions of interest (ROI), we computed paired t tests (0% > 100%, 100% > 0%) within the subgroup of placebo responders using pain reduction as a covariate of interest. Finally, we used two-sample t tests to compare responders and nonresponders with regard to placebo-induced changes in neural activation, i.e., we compared the groups with respect to difference contrasts [responders0%>100% > nonresponders0%>100%]. A supplementary linear regression analysis including all subjects correlating change in pain rating (0% to 100% condition) with change in BOLD response was also conducted (see Supplementary Table). Given our specific a priori hypotheses, our main approach was comprised of ROI analyses. We used small-volume correction with familywise error (FWE) correction for multiple comparisons in specific ROIs at a level of P < .05. ROIs were chosen based on previous findings in placebo analgesia fMRI studies (4,5,24,25,30,41-43) and included the thalamus, somatosensory cortex, cingulate cortex, insula, and relevant prefrontal areas, i.e., dorsolateral prefrontal cortex (DLPFC), the ventrolateral prefrontal cortex (VLPFC), and the orbitofrontal cortex, and the periaqueductal grey. Small-volume correction was performed with templates constructed from the automated anatomical labeling toolbox in SPM [38], except for the periaqueductal grey, for which the seed voxel from [4] was taken given that no template exists for this region. All results are reported at P < .05 corrected for multiple comparisons unless indicated otherwise. We additionally performed exploratory analyses using a more liberal threshold of P < .001 (uncorrected), which are given in table legends. All results are given as Montral Neurological Institute (MNI) coordinates. 2.8. Supplementary analyses of possible treatment order/sequence effects Although the order of conditions was randomized to control for possible order/sequence effects, we conducted the following supplementary analyses in light of the postexperimental median split of the sample and the putative relevance of learning or conditioning effects that could be taking place in this within-subject design: 1. Using a v2 test, we assessed the distribution of responders and nonresponders in the different treatment orders (i.e., 0% condition before 100% condition, 0% condition after 100% condition). Twenty subjects underwent the 0% condition before the 100% condition, and 16 subjects underwent the 0% condition after the 100% condition. To allow for adequate group sizes for these analyses, we ignored the position of the 50% condition as this was again randomly distributed. 2. Within responders and nonresponders, respectively, we assessed effects of treatment order on the behavioral (VAS pain ratings) and neural (BOLD change) placebo response with treatment order as a newly created group variable (i.e., 0% before 100% vs

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0% after 100%). We computed two-sample t tests to compare the change in VAS pain ratings in responders and nonresponders, respectively, who underwent the 0% condition before vs after the 100% condition. For analysis of the placebo-induced change in BOLD responses during anticipation and pain in our a priori defined ROIS, we used two-sample t tests within responders and nonresponders, respectively. Given the exploratory nature of these analyses and the relatively small groups, we report uncorrected as well as FWE-corrected P values. 3. Results 3.1. Participants The sample consisted of N = 36 healthy subjects (21 female, 15 male) with a mean age of 25.89 ± 1.22 years and mean body mass index of 23.45 ± 0.58. All participants (100%, N = 36) had a high school degree, 58.8% (N = 21) were currently university students, and the rest of the sample was employed either full time (22.2%, N = 8) or part time (19.4%, N = 7). Of the sample, a quarter (25%, N = 9) were smokers who reportedly smoked 10 ± 2.2 cigarettes per day on average. Self-perceived health was either good (26.5, N = 9) or very good (73.5%, N = 25). Regarding family status, approximately half of the sample (44.5%, N = 16) was in a steady relationship or married, the remainder of subjects were either single (50%, N = 18) or divorced (5.6%, N = 2). HADS depression and anxiety scores were well within the normal range (depression: 1.08 ± 0.21, anxiety: 3.28 ± 0.36). Average rectal threshold for first sensation was 16.06 ± 0.69 mm Hg and for pain was 39.00 ± 1.14 mm Hg. 3.2. Brain activation during anticipation and visceral pain in the 0% expectation condition To initially describe the pattern of neural activation in this paradigm, we initially performed one-sample t tests in the 0% expectation condition on data from the whole sample of N = 36 subjects. We observed activation of bilateral insular cortices, somatosensory cortices (bilateral postcentral gyri and inferior parietal lobules), left anterior midcingulate cortex, and inferior frontal cortex (VLPFC, DLPFC) during rectal distensions (Table 1). No significant activations within the ROIs were observed during cued pain anticipation. 3.3. Behavioral responses to placebo Changing expectations regarding the likelihood of receiving an analgesic drug resulted in significantly reduced pain (analysis of variance [ANOVA] condition effect: P < .01) and urge to defecate (ANOVA condition effect: P < .05) in the whole sample of N = 36 subjects (Table 2). The placebo response for pain, computed as pain reduction from the 0% to the 100% expectation condition, correlated significantly with the change in urge to defecate and overall distension-related discomfort (urge to defecate: Pearson r(degrees of freedom: 35) = .63; discomfort: Pearson r(degrees of freedom: 35) = .80, both P < .001). Therefore, we subsequently focused on VAS pain ratings as primary outcome measure to quantify the placebo response at the behavioral level and to define placebo responders for subsequent fMRI analyses. 3.4. Identification of placebo responders based on changes in subjective pain ratings Large interindividual variations in the change in pain ratings between the 0% and 100% conditions were observed (Fig. 1A). Postexperimental division of the sample into responders and nonresponders by method of median split resulted in 2 subgroups (N = 18, N = 18) that expectedly differed substantially in their

Table 1 One-sample t test over all subjects (N = 36) in the 0% expectation condition. Brain region of interest

Coordinates H

X

y

z

t value

8 10 6 4 26 22 40 36 46

12.54 8.95 5.32 7.88 7.58 7.16 7.38 7.37 4.43

Pain Insula Thalamus Inferior frontal gyrus (VLPFC, DLPFC) Posterior lobe (SI, SII) Inferior parietal lobule (SII) Cingulate cortex (aMCC)

R L R L R L R L L

34 36 18 40 52 58 54 58 10

12 6 10 16 24 22 36 40 16

All P < .05 based on region-of-interest analysis using small-volume correction with familywise error correction (no significant activations were found for anticipation of pain). Additional one-sample t test results of whole-brain statistics at P < .001 uncorrected during cued pain anticipation: right insula (x = 36, y = 22, z = 6, t = 3.98); during pain left posterior midcingulate cortex (x = 10, y = 4, z = 48, t = 4.01); left medial frontal gyrus (x = 6, y = 24, z = 52, t = 4.19); bilateral cerebellum (x = 14, y = 72, z = 48, t = 5.20; x = 24, y = 66, z = 28, t = 5.24). H = hemisphere with activation; R = right asymmetrical activation; L = left asymmetrical activation; aMCC = anterior midcingulate cortex, DLPFC = dorsolateral prefrontal cortex; VLPFC = ventrolateral prefrontal cortex.

Table 2 Behavioural responses to painful stimuli in the 0%, 50%, and 100% expectation conditions in the entire sample of N = 36 subjects. Visual analogue scale

Pain Discomfort Urge to defecate

F, P*

Expectation condition 0%

50%

100%

56 ± 5a 76 ± 4a 73 ± 4a,b

52 ± 5b 70 ± 3 66 ± 4b

45 ± 5a,b 68 ± 3a 65 ± 4a

F = 6.92, P < .001 F = 2.75, P = .072 F = 3.33, P < .05

Visual analogue scale ratings of rectal distensions (visual analogue scale: 0 to 100 mm; ends defined as 0: none to 100: very much) across expectation conditions. Results are shown as mean ± SEM. aa,bb Equal letters indicate significant differences between means (post hoc comparisons with paired t tests, all P < .05). * F and P values are results or repeated-measures analyses of variance.

changes in pain ratings across experimental conditions (Fig. 1A and B). Responders demonstrated significant pain reduction (repeated measures ANOVA: F = 22.58, P < .001; post hoc paired t tests: 0% vs 50%: P < .01; 50% vs 100%, P < .01), which was paralleled by significant decreases in discomfort and urge to defecate (for discomfort: F = 10,829, P < .001; for urge to defecate: P < .01, data not shown). In contrast, no change in any VAS rating was observed in the group of nonresponders (Fig. 1B). For further group comparisons, see section ‘‘Comparison of Responders and Nonresponders’’. 3.5. Placebo-induced changes in brain activation in responders The BOLD response during cued anticipation and pain was assessed within responders by computing paired t tests, using the behavioral placebo response (pain reduction measured by VAS) as a covariate of interest. For the contrast 0% > 100%, these analyses revealed significant differences in the activation of several ROIs during pain anticipation, including the superior, medial, middle, and inferior frontal gyri comprising the DLPFC and VLPFC, the superior parietal lobule/postcentral gyrus comprising parts of secondary somatosensory cortex, and the thalamus (all P FWE-corrected < .05, Table 3, Fig. 2A). During pain, the same contrast showed a significant difference for the thalamus (Table 3, Fig. 2B). No significant activations were observed for the opposite contrast (100% > 0%). For visualization purposes, we selected the thalamus (as the only region that showed a response during both cued anticipation

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Fig. 1. (A) Individual changes in pain rating from the 0% to the 100% expectation condition for each of the N = 36 subjects. (B) Placebo analgesia response across the 0%, 50%, and 100% expectation conditions in N = 18 responders and N = 18 nonresponders, classified using a postexperimental median split based on change in pain rating from the 0% to the 100% condition (median = 9.00 mm). Although responders showed significant pain reduction already in the 50% condition and most pronounced in the 100% condition (analysis of variance condition effect: P < .001; post hoc comparisons of means: 0% vs 50%, 50% vs 100%, both P < .01, paired t tests), no change was observed in nonresponders. In addition, responders demonstrated significantly greater pain ratings in the 0% condition when compared with nonresponders (independent samples t test, P < .05). All pain ratings were accomplished using 100-mm visual analogue scales on perceived pain. Data are shown as mean ± SEM.

Table 3 Paired t tests (0% > 100%) within placebo responders testing placebo modulation of cortical activation during cued pain anticipation and pain correlating with behavioural placebo response in specific regions of interest. Brain region of interest

Coordinates H

X

y

Thalamus

Anticipation L 2 L 40 R 56 L 32 R 36 L 12

Thalamus

Pain L

Medial frontal gyrus, DLPFC Middle frontal gyrus, DLPFC Inferior frontal gyrus, VLPFC Postcentral gyrus, SII

20

z

t value

50 22 26 56 58 14

32 32 10 42 54 6

6.42 6.01 5.04 6.33 7.02 4.30

28

6

5.13

Results of paired t tests on the contrast 0% > 100% expectation within N = 18 placebo responders using behavioral placebo response (reduction in visual analogue scale pain score) as a covariate of interest. All P < .05 based on region-of-interest analysis using small-volume correction with familywise error correction. Additional paired t test results of whole-brain statistics at P < .001 uncorrected during pain anticipation: right superior frontal gyrus (x = 2, y = 34, z = 52, t value = 5.57), right postcentral gyrus (SII, x = 36, y = 56, z = 54, t value = 5.06), right cerebellum (x = 38, y = 60, z = 40, t value = 4.97), right putamen (x = 28, y = 14, z = 8, t value = 4.06), right thalamus (x = 14, y = 22, z = 14, t value = 4.20); no additional findings were revealed with whole-brain statistics for the pain condition. H = hemisphere with activation; R = right asymmetrical activation; L = left asymmetrical activation; DLPFC = dorsolateral prefrontal cortex; VLPFC = ventrolateral prefrontal cortex.

and pain), extracted peak voxel activity for the group of responders, and plotted averaged parameter estimates of anticipation-related (Fig. 3A) and pain-related (Fig. 3B) BOLD responses in the 0% and 100% expectation conditions. These plots support that within responders thalamic activation was upregulated in the 0% condition and downregulated in the 100% expectation condition. 3.6. Comparisons of placebo responders and nonresponders There were no significant differences between responders and nonresponders in any sociodemographic or psychological variable

(data not shown) or in rectal thresholds (sensory thresholds: 16.1 ± 0.6 mm Hg for responders versus 16.1 ± 1.3 mm Hg for nonresponders; pain thresholds: 37.4 ± 1.6 mm Hg for responders vs 40.6 ± 1.6 mm Hg for nonresponders). Interestingly, responders demonstrated significantly higher pain ratings in the 0% expectation condition when compared with nonresponders (independent-sample t test: p < .05, Fig. 1B). To compare the groups with regard to expectation-induced changes in neural activation during cued anticipation and pain, one-sample t tests were computed on first-level difference contrasts [responders0%>100% > nonresponders0%>100%]. These analyses revealed significant effects for the DLPFC during pain anticipation, and for somatosensory cortex (gyrus angularis), posterior cingulate cortex, and thalamus during pain (all P FWE-corrected <.05, Table 4), indicating a significantly greater placebo-induced change in activation in responders compared with nonresponders in these areas. The reverse contrast [responders0%>100% < nonresponders0%>100%] revealed no significant effects. Given that responders demonstrated greater pain ratings in the 0% expectation condition, we further conducted exploratory whole-brain analysis of fMRI data at a more liberal threshold of P < .001 uncorrected. A two-sample t test on the contrast responder > nonresponder in the 0% expectation condition revealed greater activation in bilateral medial prefrontal cortices during cued pain anticipation (Fig. 4). 3.7. Supplementary analyses of possible treatment order/sequence effects We first assessed the percentage of responders and nonresponders in the 2 different treatment orders (i.e., 0% before 100% condition, 0% after 100% condition). Of the N = 20 subjects who underwent the 0% condition before the 100% condition, N = 10 (50%) were responders and N = 10 (50%) were nonresponders. Of the 16 subjects who underwent the 0% condition after the 100% condition, N = 8 (50%) were responders and N = 8 (50%) were nonresponders. Given equal percentages, the v2 test was nonsignificant, supporting that the distribution of responders and nonresponders did not differ based

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Fig. 2. Placebo modulation of rectal distension-induced cortical activation during cued pain anticipation (A) and pain (B) within N = 18 placebo responders. Results of paired t tests using pain reduction as a covariate of interest on the contrast 0% > 100% expectation overlaid on a structural T1-weighted magnetic resonance imaging used for spatial normalization. Significant placebo modulation is revealed during cued pain anticipation in (A) left medial and middle frontal gyri (dorsolateral prefrontal cortex, x = 2; y = 50, z = 32; x = 40; y = 22; z = 32), right inferior frontal gyrus (ventrolateral prefrontal cortex, (x = 56, y = 26, z = 10), left superior parietal lobule/postcentral gyrus (SII, x = 32, y = 56; z = 42), and left thalamus (x = 12, y = 14, z = 6) during cued pain anticipation and (B) in left thalamus (x = 20, y = 28, z = 6) during pain. All P familywise error–corrected <.05; for more details, see Table 1.

Table 4 Two-sample t tests assessing group differences between responders and nonresponders in the placebo modulation of the neural response during cued pain anticipation and pain in specific regions of interest. Brain region of interest

Coordinates H

Fig. 3. Parameter estimates of anticipation-related (A) and pain-related (B) blood oxygen level–dependent responses in the 0% and 100% expectation conditions. Individual peak voxel activity within the thalamus was extracted for the group of responders, averaged, and plotted for visualization purposes. a.u. = arbitrary units.

on treatment order. Subsequently, we assessed possible effects of treatment order on the behavioral level, i.e., change in VAS pain ratings within responders and nonresponders, respectively. Within responders, there was no difference in the behavioral placebo response, assessed as change in VAS pain ratings, between treatment orders (VAS pain change: 26.70 ± 4.8 mm for 0% before 100%, 22.75 ± 2.7 mm for 0% after 100%). Similarly, no evidence for an order effect was observed within nonresponders (VAS pain change score: 0.4 ± 2.8 mm for 0% before 100%, 7.50 ± 6.7 mm for 0% after 100%, all nonsignificant). Finally, we assessed possible order effects at the neural level by computing a two-sample t test on the placebo-induced change in BOLD responses for anticipation and pain in our predefined ROIs. These analyses revealed no order effects within nonresponders. However, within responders, we observed significantly greater placebo-induced changes in neural activation for the order 0% before 100% in somatosensory cortex (x = 38, y = 34, z = 64, t = 5.19, P < .001 uncorrected, PFWE-corrected = .08) and middle

X

Middle frontal gyrus, DLPFC Inferior frontal gyrus, DLPFC

Anticipation L 42 L 50

Gyrus angularis, SII Posterior cingulate cortex (vPCC) Thalamus

Pain R L L

42 12 18

y

z

T value

22 26

30 22

5.46 4.67

24 26 4

6.17 4.34 5.62

52 46 30

Results of two-sample t tests on difference contrasts [responders0%>100% > nonresponders0%>100%]. All P < .05 based on region-of-interest analysis using small-volume correction with familywise error correction. Additional findings of two-sample t test results of whole-brain statistics at P < .001 uncorrected during cued pain anticipation: right middle frontal gyrus (DLPFC, x = 40, y = 32, z = 26, t value = 3.73), bilateral superior frontal gyrus (x = 28, y = 46, z = 10, t value = 5.37; x = 32, y = 48, z = 8, t value = 3.82); bilateral inferior parietal lobule/postcentral gyrus (SII, x = 34, y = 56, z = 42, t value = 4.26; x = 30, y = 6, z = 4, t value = 3.62); putamen (x = 28, y = 6, z = 8, t value = 3.63), cerebellum (x = 34, y = 64, z = 40, t value = 4.32); brainstem (x = 6, y = 12, z = 8, t value = 3.54); during pain: left precuneus (x = 12, y = 50, z = 50, t = 4.10), left middle frontal gyrus (x = 36, y = 20, z = 30, t = 4.06). H = hemisphere with activation; R = right asymmetrical activation; L = left asymmetrical activation; DLPFC = dorsolateral prefrontal cortex; VLPFC = ventrolateral prefrontal cortex, vPCC = ventral posterior cingulate cortex.

frontal gyrus (x = 34, y = 30, z = 48, t = 4.71, P < .001 uncorrected, PFWE-corrected > .1) during anticipation and in middle frontal gyrus (x = 42, y = 24, z = 46, t = 4.98, PFWE-corrected > .1) during pain. 4. Discussion The expectation to receive a potent analgesic drug significantly reduced perceived painfulness and urge to defecate in a rectal

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Fig. 4. Comparison of cortical activation during cued pain anticipation in responders > nonresponders in the 0% expectation condition (2-sample t test, whole-brain analysis, P < .001 uncorrected, overlaid on a structural T1-weighted magnetic resonance images used for spatial normalization). Responders revealed greater activation in bilateral medial prefrontal cortices during cued pain anticipation in the 0% expectation condition.

distension model in healthy subjects. These findings confirm and complement previous findings in IBS patients [30,39,40], as well as results of a recent esophageal placebo analgesia study in healthy subjects [24]. Our sample was large enough to allow the selection of a group of responders who demonstrated increased pain while expecting no pain relief (0% expectation condition), and substantial reductions in perceived pain during possible (50% condition) and certain (100% condition) expectation of pain relief. Interestingly, the magnitude of change in pain ratings was proportional to the degree of certainty regarding the probability of receiving a potent analgesic drug in line with previous studies using somatic pain stimuli in healthy subjects [27] and in patients with postoperative pain [29]. Extending these findings, it has recently been demonstrated in healthy subjects that positive treatment expectancy substantially enhanced the analgesic effect of a l-opioid agonist, whereas negative treatment expectancy completely abolished the analgesic efficacy [5]. Hence, the expectation of a drug’s effect may shape both therapeutic and adverse effects in clinical settings. Our study did not include a nocebo condition, i.e., our instructions were not designed to induce negative treatment expectations. Nevertheless, we observed that responders not only were characterized by a pronounced placebo response, but also revealed significantly higher pain ratings in the 0% expectation condition along with increased medial prefrontal cortex activation. This could suggest that in this within-subject study design, the expectation that painful stimuli are delivered explicitly without pain relief may have in fact induced negative expectations of greater pain, in line with a nocebo response, within responders. Clearly, within 1 individual, cognitions and emotions mediating positive (i.e., placebo) and negative (i.e., nocebo) expectancies can dynamically change. Our complex finding underscores that a saline (or no treatment) condition can also induce expectations, at least in some individuals, which should be taken into account in future study designs aimed at disentangling the mechanisms of placebo and nocebo effects. At the level of the brain, we hypothesized that behavioral placebo effects would be correlated with activity changes in the activation of brain regions mediating sensory-affective aspects of visceral pain as well as areas mediating cognitive expectancies and descending pain inhibition. We found that during pain anticipation, subjective pain reduction correlated with changes in activation of the thalamus, secondary somatosensory cortex, and prefrontal areas including DLPFC and VLPFC within placebo responders. During both anticipation and pain, activation of the thalamus was upregulated when no pain relief was expected (0% condition) and reduced during certain pain relief (100% condition).

Placebo-induced changes in the activation of the thalamus during the experience of pain have also been reported in the hallmark IBS study by Price et al. [30] as well as in the esophageal study by Lu et al. [24]. Our study extends these findings by suggesting that thalamic activation can upregulate and downregulate based on cognitive expectancies during pain anticipation and actual painful stimulation. Given the sensory gatekeeper function of the thalamus, these findings further support the conclusion that placebo/ nocebo effects do not simply reflect rating biases but rather an actual change in central nociceptive processing [31]. This conclusion is supported in our study by evidence that the expectation condition also changed the activation of secondary somatosensory regions, and is further substantiated by results showing a reduction of nociceptive processing at the level of the spinal cord in a somatic placebo analgesia model [6]. Together, our findings support the conclusion that activation in pain-processing brain regions reflects a combination of afferent nociceptive input and top-down mechanisms related to expectation [2]. There is converging evidence to support that the influence of cognitions and emotions induced by placebos and nocebo paradigms is at least in part mediated by prefrontal areas. The VLPFC and DLPFC are 2 highly interconnected prefrontal regions known to be involved in cognitive modulation of pain, including attention, evaluation, and appraisal/reappraisal processes linking the expectation with the experience of pain. In our study, effects of expectation on neural activation in prefrontal cortices correlating with the behavioral response were observed during pain anticipation, with greater activation in the 0% condition compared with the 100% condition. Existing visceral placebo analgesia studies have not assessed the brain response during cued pain anticipation [23,24], but a number of somatic placebo analgesia studies also support the role of prefrontal activity changes during cued anticipation [25,41–43]. In contrast to our finding, the majority of these studies have documented increased (rather than decreased) prefrontal activation in the placebo condition. There are several possible explanations for this disparity, including marked differences in study designs and pain modalities, and possible effects of negative expectations and sequence effects in our study. Importantly, prefrontal regions mediate not only descending pain inhibition but also facilitation of transmission of nociceptive information [37,44], e.g., during nocebo hyperalgesia [20]. Given high variability in placebo responses across participants in our study as well as in other studies [28,42,46], the ability to predict whether an individual is highly susceptible to placebo effects would have numerous clinical as well as scientific implications. Therefore,

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we compared responders and nonresponders with regard to psychosocial characteristics as well as behavioral and neural responses. Of note, we could ascertain that order effects did not constitute a confound in the postexperimental classification into responders and nonresponders. However, we also acknowledge that our chosen method of median split to separate responders from nonresponders has some disadvantages. Further, the present group of nonresponders was heterogeneous with respect to changes in pain ratings. Both aspects may constitute important limitations that limit the generalizability of our findings. Our data revealed no differences between responders and nonresponders in the distribution of male and female subjects, in rectal pain thresholds, and in a variety of state and trait variables. These negative findings are in accordance with the larger placebo literature, which thus far has provided no or very few reliable predictors in psychosocial measures [1,14,19]. On the other hand, there likely exist interindividual variations in brain neurochemistry, e.g., in the endogenous opioidergic and/or dopaminergic systems, relevant to placebo and nocebo effects that we did not assess in our study [34,35,46]. Further, a recent reanalysis of fMRI data suggested that the pattern of activation in specific brain networks during the anticipation and experience of pain are indeed predictors of individual differences in placebo responses [41]. Our analysis of fMRI data with regard to differences between responders and nonresponders confirms that behavioral placebo responses are paralleled by differences in brain activation. 4.1. Study limitations and future directions Subjects had no prior experience with either the rectal distension paradigm or the supposed analgesic drug. This was intentionally done to exclude possible effects of prior experience and/or conditioning processes because our specific focus was to assess effects of expectation. However, learning or conditioning processes constitute an important mechanism mediating placebo and nocebo effects [10,11,45]. Such effects may also play a role in within-subjects study designs such as the one utilized in our study involving repeated assessments of the dependent variable in question (in this case, pain responses) under different conditions. Although we could confirm that order effects did not constitute a confound in the classification of subjects into responders and nonresponders, we found preliminary evidence suggesting that within responders, order effects may be present at the neural level. Specifically, we found that within responders, placebo-induced changes in neural activation were more pronounced in 2 relevant brain regions (i.e., somatosensory and prefrontal areas) in those subjects who received the 0% before the 100% expectation condition. This may reflect a learning process, but could also be attributable to other uncontrolled factors. Despite this possible limitation, we argue that it would be premature to conclude that future studies should rather employ between-group designs (which would require much larger Ns) to avoid possible order/sequence effects. Future studies are needed to assess placebo and nocebo responses in healthy subjects and patients in order to discern the putative interaction of expectation and conditioning processes, as has recently been elegantly accomplished in a somatic placebo analgesia model [25]. In light of the difficulties and resources needed to study larger clinical samples, we think that weighing the advantages and disadvantages of within- and between-study designs should be part of future research efforts in the placebo field. 4.2. Conclusions Taken together, our behavioral findings in healthy subjects confirm the pivotal role of expectations in shaping the experience of visceral pain at both the behavioral and the neural levels. These findings in a clinically relevant rectal pain model extend previous

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studies on the relevance of cognitions and emotions in the neural processing of painful stimuli, and provide a framework for the conceptualization of how thoughts and emotions may contribute to the pathophysiology of visceral hyperalgesia in patients with chronic abdominal pain. Acknowledgements This project was funded by a research grant to S.E., M.F., and E.R.G. from the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) (DFG EL 236/8-1). The authors thank Armin de Greiff and Luciana Besedovsky for excellent technical support in the planning and execution of the experimental paradigms. All authors declare no conflict of interest. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.pain.2011.10.036. References [1] Aslaksen PM, Bystad M, Vambheim SM, Flaten MA. Gender differences in placebo analgesia: event-related potentials and emotional modulation. Psychosom Med 2011;73:193–9. [2] Atlas LY, Bolger N, Lindquist MA, Wager TD. Brain mediators of predictive cue effects on perceived pain. J Neurosci 2010;30:12964–77. [3] Berman SM, Naliboff BD, Suyenobu B, Labus JS, Stains J, Ohning G, Kilpatrick L, Bueller JA, Ruby K, Jarcho J, Mayer EA. Reduced brainstem inhibition during anticipated pelvic visceral pain correlates with enhanced brain response to the visceral stimulus in women with irritable bowel syndrome. J Neurosci 2008;28:349–59. [4] Bingel U, Lorenz J, Schoell E, Weiller C, Büchel C. Mechanisms of placebo analgesia: rACC recruitment of a subcortical antinociceptive network. Pain 2006;120:8–15. [5] Bingel U, Wanigasekera V, Wiech K, Ni Mhuircheartaigh R, Lee MC, Ploner M, Tracey I. The effect of treatment expectation on drug efficacy: imaging the analgesic benefit of the opioid remifentanil. Sci Transl Med 2011;3:70ra14. [6] Eippert F, Finsterbusch J, Bingel U, Büchel C. Direct evidence for spinal cord involvement in placebo analgesia. Science 2009;326:404. [7] Elsenbruch S. Abdominal pain in irritable bowel syndrome: a review of putative psychological, neural and neuro-immune mechanisms. Brain Behav Immun 2011;25:386–94. [8] Elsenbruch S, Rosenberger C, Bingel U, Forsting M, Schedlowski M, Gizewski ER. Patients with irritable bowel syndrome have altered emotional modulation of neural responses to visceral stimuli. Gastroenterology 2010;139:1310–9. [9] Elsenbruch S, Rosenberger C, Enck P, Forsting M, Schedlowski M, Gizewski ER. Affective disturbances modulate the neural processing of visceral pain stimuli in irritable bowel syndrome: an fMRI study. Gut 2010;59:489–95. [10] Enck P, Benedetti F, Schedlowski M. New insights into the placebo and nocebo responses. Neuron 2008;59:195–206. [11] Finniss DG, Kaptchuk TJ, Miller F, Benedetti F. Biological, clinical, and ethical advances of placebo effects. Lancet 2010;375:686–95. [12] Fliege H, Rose M, Arck P, Walter OB, Kocalevent RD, Weber C, Klapp BF. The Perceived Stress Questionnaire (PSQ) reconsidered: validation and reference values from different clinical and healthy adult samples. Psychosom Med 2005;67:78–88. [13] Franke G. SCL-90R. Symptom-Checkliste von L.R. Degoratis—Deutsche Version. Goettingen: Beltz; 1995. [14] Geers AL, Wellman JA, Fowler SL, Helfer SG, France CR. Dispositional optimism predicts placebo analgesia. J Pain 2010;11:1165–71. [15] Haake M, Müller HH, Schade-Brittinger C, Basler HD, Schäfer H, Maier C, Endres HG, Trampisch HJ, Molsberger A. German Acupuncture Trials (GERAC) for chronic low back pain: randomized, multicenter, blinded, parallel-group trial with 3 groups. Arch Intern Med 2007;167:1892–8. [16] Herrmann-Lingen C, Buss U, Snaith RP. Hospital Anxiety and Depression Scale (HADS)—Deutsche Version (2. Auflage). Bern: Hans Huber; 2005. [17] Kaptchuk TJ, Friedlander E, Kelley JM, Sanchez MN, Kokkotou E, Singer JP, Kowalczykowski M, Miller FG, Kirsch I, Lembo AJ. Placebos without deception: a randomized controlled trial in irritable bowel syndrome. PLoS One 2010;5:e15591. [18] Kaptchuk TJ, Kelley JM, Conboy LA, Davis RB, Kerr CE, Jacobson EE, Kirsch I, Schyner RN, Nam BH, Nguyen LT, Park M, Rivers AL, McManus C, Kokkotou E, Drossman DA, Goldman P, Lembo AJ. Components of placebo effect: randomised controlled trial in patients with irritable bowel syndrome. BMJ 2008;336:999–1003. [19] Klosterhalfen S, Kellermann S, Braun S, Kowalski A, Schrauth M, Zipfel S, Enck P. Gender and the nocebo response following conditioning and expectancy. J Psychosom Res 2009;66:323–8.

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