Somatotopic activation of opioid systems by target-directed expectations of analgesia

Paradoxes in acupuncture research: strategies for moving forward

In November 2007, the Society for Acupuncture Research (SAR) held an international symposium to mark the 10th anniversary of the 1997 NIH Consensus Development Conference on Acupuncture. The symposium presentations revealed the considerable maturation of the field of acupuncture research, yet two provocative paradoxes emerged. First, a number of well-designed clinical trials have reported that true acupuncture is superior to usual care, but does not significantly outperform sham acupuncture, findings apparently at odds with traditional theories regarding acupuncture point specificity. Second, although many studies using animal and human experimental models have reported physiological effects that vary as a function of needling parameters (e.g., mode of stimulation) the extent to which these parameters influence therapeutic outcomes in clinical trials is unclear. This White Paper, collaboratively written by the SAR Board of Directors, identifies gaps in knowledge underlying the paradoxes and proposes strategies for their resolution through translational research. We recommend that acupuncture treatments should be studied (1) “top down” as multi-component “whole-system” interventions and (2) “bottom up” as mechanistic studies that focus on understanding how individual treatment components interact and translate into clinical and physiological outcomes. Such a strategy, incorporating considerations of efficacy, effectiveness and qualitative measures, will strengthen the evidence base for such complex interventions as acupuncture.

[Mechanisms of endogenous pain modulation illustrated by placebo analgesia : functional imaging findings]

Nociceptive information processing and related pain perception are subject to substantial pro- and antinociceptive modulation. Research on the involved circuitry and the implemented mechanisms is a major focus of contemporary neuroscientific studies in the field of pain and will provide new insights into the prevention and treatment of chronic pain states. Placebo analgesia is a powerful clinical example of the cognitive modulation of pain perception. In placebo analgesia the administration of an inert substance will produce an analgesic effect if the subject is convinced that the substance is a potent analgesic. Recent neuroimaging studies have started to characterize the neural circuitry supporting the placebo analgesic effect. The converging evidence from these studies supports the concept that during placebo analgesia cingulo-frontal regions interact with subcortical structures involved in endogenous antinociception to produce the placebo-induced reduction in pain perception. The subject's report of reduced pain during placebo analgesia coincides with decreased activity in the classic pain areas. This indicates that the altered pain experience during placebo analgesia results from active inhibition of nociceptive input. This cognitively triggered endogenous modulation of pain involves, at least in part, the endogenous opioid system. Most recently, functional magnetic resonance imaging data of the human spinal cord revealed that these mechanisms involve the inhibition of nociceptive processing at the level of the dorsal horn of the spinal cord. Here we discuss recent advances in pain imaging research focusing on cognitively triggered endogenous pain control mechanisms and respective implications for future research strategies.

Individual expectation: an overlooked, but pertinent, factor in the treatment of individuals experiencing musculoskeletal pain

Physical therapists consider many factors in the treatment of patients with musculoskeletal pain. The current literature suggests expectation is an influential component of clinical outcomes related to musculoskeletal pain for which physical therapists frequently do not account. The purpose of this clinical perspective is to highlight the potential role of expectation in the clinical outcomes associated with the rehabilitation of individuals experiencing musculoskeletal pain. The discussion focuses on the definition and measurement of expectation, the relationship between expectation and outcomes related to musculoskeletal pain conditions, the mechanisms through which expectation may alter musculoskeletal pain conditions, and suggested ways in which clinicians may integrate the current literature regarding expectation into clinical practice.

The silent healer: the role of communication in placebo effects

Placebo effects have an ambiguous reputation, as they are associated with sham treatment and deceit on the one hand and as interesting phenomena, which might be clinically relevant on the other. The goal of this paper is to demonstrate that placebo effects are relevant and can be used as an effective part of many treatments by using communication targeting placebo effect mechanisms. We examined the history of placebos and the placebo effect, addressing common misconceptions and disentangling ambiguities. We then reviewed whether the placebo effect can be robustly shown in the current literature, and zoomed in on the plausible mechanisms (conditioning, expectancies and affect manipulation) through which the placebo effect might be produced. Observing the link with the doctor-patient communication literature, and pleading for a better integration of the two research traditions we conclude by setting out a research agenda for testing the role of communication in placebo effects.

Deciphering the role of endogenous opioids in high-frequency TENS using low and high doses of naloxone

Previous human studies have shown that the analgesic effect of high-frequency TENS could not be reversed by low doses of naloxone. The aim of the present study was to reinvestigate the possible contribution of opioid receptors to high-frequency TENS analgesia by using low (0.02 mg/kg) and high (0.14 mg/kg) doses of naloxone. Naloxone (high and low doses) and saline were administered intravenously to young healthy adults using a triple-blind randomized cross-over design. For each visit, TENS (100 Hz, 60 μs) was applied for 25 min to the external surface of the left ankle. TENS intensity was adjusted to obtain strong but comfortable (innocuous) paresthesias. Experimental pain was evoked with a 1 cm(2) thermode applied on the lateral aspect of the left heel. Subjective pain scores were obtained before, during and after TENS. Because preliminary analyses showed that the order of presentation affected the pattern of results, only the first visit of every participant could be analyzed without fear of contamination from possible carry-over effects. These revealed that TENS maintained its analgesic properties following the injection of saline (p<.001) and the injection of a low dose of naloxone (p<.05). However, when a high dose of naloxone was administered, TENS analgesia was completely blocked (p=.20). These results suggest that high-frequency TENS involves opioid receptors. An insufficient amount of opioid antagonist likely prevented previous human studies from discovering the importance of opioid receptors in producing high-frequency TENS analgesia.

Dynamic nature of the placebo response

Traditionally, placebo has been associated with using an inert substance, in part so the subsequent response could be attributed to the target treatment, controlling for the confound of a “placebo effect.” ^,, Placebo’s link with inert substances is so strong that “sham treatment” is a common synonym, and widespread placebo use is discouraged—even when there is supporting evidence for its effectiveness.^,, Recent research has helped to redefine placebo, and this editorial will highlight key information supporting a contemporary view of placebo.

Biological, clinical, and ethical advances of placebo effects

For many years, placebos have been defined by their inert content and their use as controls in clinical trials and treatments in clinical practice. Recent research shows that placebo effects are genuine psychobiological events attributable to the overall therapeutic context, and that these effects can be robust in both laboratory and clinical settings. There is also evidence that placebo effects can exist in clinical practice, even if no placebo is given. Further promotion and integration of laboratory and clinical research will allow advances in the ethical use of placebo mechanisms that are inherent in routine clinical care, and encourage the use of treatments that stimulate placebo effects.

Drug effects: agonistic and antagonistic processes

The research presented here has shown that tolerance to drugs can be accelerated by conditioning processes. Placebo effects may be considered the opposite of tolerance, and we have shown that placebo effects may be objectively recorded by physiological measures (electromyography, skin conductance responses, and event-related potentials), as well as by behavioral and subjective methods. The placebo response, or more precisely, the expectation of drug effects, can add to the effect of the drug. Drug antagonistic expectations can also reverse the effect of the drug. There is some evidence that placebo effects are strongest when expectations are reinforced by administration of an active drug. Expectations have graded effects and may affect symptoms to a smaller or larger degree. Although drug effects can be considered stimuli, the investigation of the role of classical conditioning in drug use and drug effects involves special issues that must be carefully considered.

Habituation to pain: further support for a central component

Habituation to repetitive painful stimulation may represent an important protection mechanism against the development of chronic pain states. However, the exact neurobiological mechanisms of this phenomenon remain unclear. In this study we (i) explore the somatotopic specificity of pain attenuation over time and (ii) investigate the role of the endogenous opioid system in its development. We investigated 24 healthy volunteers with a paradigm of daily painful stimulation of the left volar forearm for 1 week. Habituation was assessed by comparing pain-related responses (ratings and thresholds) between days 1 and 8. To test whether a repetition-dependent attenuation of pain is restricted to the site of stimulus application or induces additional systemic effects indicative of a central mechanism, we also measured pain-related responses at the contralateral arm and the left leg. To assess the role of the endogenous opioid system in this mechanism, we used the opioid-receptor antagonist naloxone in a double-blind design. Repetitive painful stimulation over several days resulted in a significant habituation to pain at the site of daily stimulation. In addition, we also observed significant pain attenuation at the non-stimulated limbs. This effect was less pronounced at the untreated arm compared to the treated arm and even weaker in the leg, displaying a significant Stimulation-Site x Time interaction. The development of pain habituation was unaffected by the opioid antagonist naloxone. Taken together, these results strongly support the role of central components in the mechanism of pain habituation that do not directly involve the endogenous opioid system.

Placebo interventions for all clinical conditions

BACKGROUND

Placebo interventions are often claimed to substantially improve patient-reported and observer-reported outcomes in many clinical conditions, but most reports on effects of placebos are based on studies that have not randomised patients to placebo or no treatment. Two previous versions of this review from 2001 and 2004 found that placebo interventions in general did not have clinically important effects, but that there were possible beneficial effects on patient-reported outcomes, especially pain. Since then several relevant trials have been published.

OBJECTIVES

Our primary aims were to assess the effect of placebo interventions in general across all clinical conditions, and to investigate the effects of placebo interventions on specific clinical conditions. Our secondary aims were to assess whether the effect of placebo treatments differed for patient-reported and observer-reported outcomes, and to explore other reasons for variations in effect.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library Issue 4, 2007), MEDLINE (1966 to March 2008), EMBASE (1980 to March 2008), PsycINFO (1887 to March 2008) and Biological Abstracts (1986 to March 2008). We contacted experts on placebo research, and read references in the included trials.

SELECTION CRITERIA

We included randomised placebo trials with a no-treatment control group investigating any health problem.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trial quality and extracted data. We contacted study authors for additional information. Trials with binary data were summarised using relative risk (a value of less than 1 indicates a beneficial effect of placebo), and trials with continuous outcomes were summarised using standardised mean difference (a negative value indicates a beneficial effect of placebo).

MAIN RESULTS

Outcome data were available in 202 out of 234 included trials, investigating 60 clinical conditions. We regarded the risk of bias as low in only 16 trials (8%), five of which had binary outcomes.In 44 studies with binary outcomes (6041 patients), there was moderate heterogeneity (P < 0.001; I(2) 45%) but no clear difference in effects between small and large trials (symmetrical funnel plot). The overall pooled effect of placebo was a relative risk of 0.93 (95% confidence interval (CI) 0.88 to 0.99). The pooled relative risk for patient-reported outcomes was 0.93 (95% CI 0.86 to 1.00) and for observer-reported outcomes 0.93 (95% CI 0.85 to 1.02). We found no statistically significant effect of placebo interventions in four clinical conditions that had been investigated in three trials or more: pain, nausea, smoking, and depression, but confidence intervals were wide. The effect on pain varied considerably, even among trials with low risk of bias.In 158 trials with continuous outcomes (10,525 patients), there was moderate heterogeneity (P < 0.001; I(2) 42%), and considerable variation in effects between small and large trials (asymmetrical funnel plot). It is therefore a questionable procedure to pool all the trials, and we did so mainly as a basis for exploring causes for heterogeneity. We found an overall effect of placebo treatments, standardised mean difference (SMD) -0.23 (95% CI -0.28 to -0.17). The SMD for patient-reported outcomes was -0.26 (95% CI -0.32 to -0.19), and for observer-reported outcomes, SMD -0.13 (95% CI -0.24 to -0.02). We found an effect on pain, SMD -0.28 (95% CI -0.36 to -0.19)); nausea, SMD -0.25 (-0.46 to -0.04)), asthma (-0.35 (-0.70 to -0.01)), and phobia (SMD -0.63 (95% CI -1.17 to -0.08)). The effect on pain was very variable, also among trials with low risk of bias. Four similarly-designed acupuncture trials conducted by an overlapping group of authors reported large effects (SMD -0.68 (-0.85 to -0.50)) whereas three other pain trials reported low or no effect (SMD -0.13 (-0.28 to 0.03)). The pooled effect on nausea was small, but consistent. The effects on phobia and asthma were very uncertain due to high risk of bias. There was no statistically significant effect of placebo interventions in the seven other clinical conditions investigated in three trials or more: smoking, dementia, depression, obesity, hypertension, insomnia and anxiety, but confidence intervals were wide.Meta-regression analyses showed that larger effects of placebo interventions were associated with physical placebo interventions (e.g. sham acupuncture), patient-involved outcomes (patient-reported outcomes and observer-reported outcomes involving patient cooperation), small trials, and trials with the explicit purpose of studying placebo. Larger effects of placebo were also found in trials that did not inform patients about the possible placebo intervention.

AUTHORS' CONCLUSIONS

We did not find that placebo interventions have important clinical effects in general. However, in certain settings placebo interventions can influence patient-reported outcomes, especially pain and nausea, though it is difficult to distinguish patient-reported effects of placebo from biased reporting. The effect on pain varied, even among trials with low risk of bias, from negligible to clinically important. Variations in the effect of placebo were partly explained by variations in how trials were conducted and how patients were informed.

Placebo conditioning and placebo analgesia modulate a common brain network during pain anticipation and perception

The neural mechanisms whereby placebo conditioning leads to placebo analgesia remain unclear. In this study we aimed to identify the brain structures activated during placebo conditioning and subsequent placebo analgesia. We induced placebo analgesia by associating a sham treatment with pain reduction and used fMRI to measure brain activity associated with three stages of the placebo response: before, during and after the sham treatment, while participants anticipated and experienced brief laser pain. In the control session participants were explicitly told that the treatment was inactive. The sham treatment group reported a significant reduction in pain rating (p=0.012). Anticipatory brain activity was modulated during placebo conditioning in a fronto-cingulate network involving the left dorsolateral prefrontal cortex (DLPFC), medial frontal cortex and the anterior mid-cingulate cortex (aMCC). Identical areas were modulated during anticipation in the placebo analgesia phase with the addition of the orbitofrontal cortex (OFC). However, during altered pain experience only aMCC, post-central gyrus and posterior cingulate demonstrated altered activity. The common frontal cortical areas modulated during anticipation in both the placebo conditioning and placebo analgesia phases have previously been implicated in placebo analgesia. Our results suggest that the main effect of placebo arises from the reduction of anticipation of pain during placebo conditioning that is subsequently maintained during placebo analgesia.

An fMRI study on the interaction and dissociation between expectation of pain relief and acupuncture treatment

It is well established that expectation can significantly modulate pain perception. In this study, we combined an expectancy manipulation model and fMRI to investigate how expectation can modulate acupuncture treatment. Forty-eight subjects completed the study. The analysis on two verum acupuncture groups with different expectancy levels indicates that expectancy can significantly influence acupuncture analgesia for experimental pain. Conditioning positive expectation can amplify acupuncture analgesia as detected by subjective pain sensory rating changes and objective fMRI signal changes in response to calibrated noxious stimuli. Diminished positive expectation appeared to inhibit acupuncture analgesia. This modulation effect is spatially specific, inducing analgesia exclusively in regions of the body where expectation is focused. Thus, expectation should be used as an important covariate in future studies evaluating acupuncture efficacy. In addition, we also observed dissociation between subjective reported analgesia and objective fMRI signal changes to calibrated pain in the analysis across all four groups. We hypothesize that as a peripheral-central modulation, acupuncture needle stimulation may inhibit incoming noxious stimuli; while as a top-down modulation, expectancy (placebo) may work through the emotional circuit.

Effects of placebo interventions on gastric motility and general autonomic activity

OBJECTIVE

The study aimed to investigate placebo effects on gastric motility and to examine possible autonomic mediating mechanisms.

METHODS

Eighteen healthy volunteers received a placebo pill on three occasions together with the verbal suggestion that it would stimulate, relax, or not affect gastric activity. Electrogastrogram, electrocardiogram, and electrodermal activity recordings were conducted for 30 min prior to and following intervention.

RESULTS

Dominant frequency of the gastric slow wave decreased in the stimulant condition, and increased in the relaxant condition, the difference among conditions being significant. No differential effects of the interventions on cardiac interbeat intervals, heart rate variability, and skin conductance levels were observed.

CONCLUSION

Stomach relaxant and stimulant placebo interventions modulated gastric motility independently from changes in general autonomic activity.

The salient characteristics of the central effects of acupuncture needling: limbic-paralimbic-neocortical network modulation

Human and animal studies suggest that acupuncture produces many beneficial effects through the central nervous system. However, the neural substrates of acupuncture actions are not completely clear to date. fMRI studies at Hegu (LI4) and Zusanli (ST36) indicated that the limbic system may play an important role for acupuncture effects. To test if this finding applies to other major classical acupoints, fMRI was performed on 10 healthy adults during manual acupuncture at Taichong (LV3), Xingjian (LV2), Neiting (ST44), and a sham point on the dorsum of the left foot. Although certain differences could be observed between real and sham points, the hemodynamic response (BOLD signal changes) and psychophysical response (sensory experience) to acupuncture were generally similar for all four points. Acupuncture produced extensive deactivation of the limbic-paralimbic-neocortical system. Clusters of deactivated regions were seen in the medial prefrontal cortex (frontal pole, pregenual cingulate), the temporal lobe (amygdala, hippocampus, and parahippocampus) and the posterior medial cortex (precuneus, posterior cingulate). The sensorimotor cortices (somatosensory cortices, supplementary motor cortex), thalamus and occasional paralimbic structures such as the insula and anterior middle cingulate cortex showed activation. Our results provide additional evidence in support of previous reports that acupuncture modulates the limbic-paralimbic-neocortical network. We hypothesize that acupuncture may mediate its antipain, antianxiety, and other therapeutic effects via this intrinsic neural circuit that plays a central role in the affective and cognitive dimensions of pain as well as in the regulation and integration of emotion, memory processing, autonomic, endocrine, immunological, and sensorimotor functions.

The rebirth of neuroscience in psychosomatic medicine, Part II: clinical applications and implications for research

During the second half of the last century, biopsychosocial research in psychosomatic medicine largely ignored the brain. Neuroscience has started to make a comeback in psychosomatic medicine research and promises to advance the field in important ways. In this paper we briefly review select brain imaging research findings in psychosomatic medicine in four key areas: cardiovascular regulation, visceral pain in the context of functional gastrointestinal disorders, acute and chronic somatic pain and placebo. In each area, there is a growing literature that is beginning to define a network of brain areas that participate in the functions in question. Evidence to date suggests that cortical and subcortical areas that are involved in emotion and emotion regulation play an important role in each domain. Neuroscientific research is therefore validating findings from previous psychosomatic research and has the potential to extend knowledge by delineating the biological mechanisms that link mind and body more completely and with greater specificity. We conclude with a discussion of the implications of this work for how research in psychosomatic medicine is conducted, the ways in which neuroscientific advances can lead to new clinical applications in psychosomatic contexts, the implications of this work for the field of medicine more generally, and the priorities for research in the next 5 to 10 years.

Expectancy and treatment interactions: a dissociation between acupuncture analgesia and expectancy evoked placebo analgesia

Recent advances in placebo research have demonstrated the mind's power to alter physiology. In this study, we combined an expectancy manipulation model with both verum and sham acupuncture treatments to address: 1) how and to what extent treatment and expectancy effects - including both subjective pain intensity levels (pain sensory ratings) and objective physiological activations (fMRI) - interact; and 2) if the underlying mechanism of expectancy remains the same whether placebo treatment is given alone or in conjunction with active treatment. The results indicate that although verum acupuncture+high expectation and sham acupuncture+high expectation induced subjective reports of analgesia of equal magnitude, fMRI analysis showed that verum acupuncture produced greater fMRI signal decrease in pain related brain regions during application of calibrated heat pain stimuli on the right arm. We believe our study provides brain imaging evidence for the existence of different mechanisms underlying acupuncture analgesia and expectancy evoked placebo analgesia. Our results also suggest that the brain network involved in expectancy may vary under different treatment situations (verum and sham acupuncture treatment).

Imaging CNS Modulation of Pain in Humans

Pain is a highly complex and subjective experience that is not linearly related to the nociceptive input. What is clear from anecdotal reports over the centuries and more recently from animal and human experimentation is that nociceptive information processing and consequent pain perception is subject to significant pro- and anti-nociceptive modulations. These modulations can be initiated reflexively or by contextual manipulations of the pain experience including cognitive and emotional factors. This provides a necessary survival function since it allows the pain experience to be altered according to the situation rather than having pain always dominate. The so-called descending pain modulatory network involving predominantly medial and frontal cortical areas, in combination with specific subcortical and brain stem nuclei appears to be one key system for the endogenous modulation of pain. Furthermore, recent findings from functional and anatomical neuroimaging support the notion that an altered interaction of pro- and anti-nociceptive mechanisms may contribute to the development or maintenance of chronic pain states. Research on the involved circuitry and implemented mechanisms is a major focus of contemporary neuroscientific research in the field of pain and should provide new insights to prevent and treat chronic pain states.

[Clinical significance of the placebo effect]

Placebo controlled studies examining clinical problems, e.g. in pain therapy, are considered the "gold standard" for evidence-based medicine. In these studies the placebo effect itself is not the main focus of interest, but serves more as a control for the specificity of the effect of a certain treatment. What physicians in this context often do not realize is that the placebo effect itself represents a true measurable correlate of an organism's psycho-neurobiological response and, thereby, influences the healing process, e.g. the pain relief. Placebo is, therefore, not equivalent to "no treatment". The number of placebo responders, the degree and the duration of the placebo effect is not fixed, but are subject to a much greater variability then hitherto believed. The myth that placebo responders have a certain personality has not been proven correct; instead, the relationships between physicians and patients as well as sociocultural factors have a considerable impact on the placebo effect. Psychological theories explain that classical conditioning, enhanced expectation and motivation of the patient determine the degree of the placebo effect. These directly influence neurobiological systems such as the endogenous opioids which according to modern brain imaging are predominantly activated in pain-relevant areas and contribute to the effect of placebo analgesia. Placebo effects that should be deliberately excluded in controlled clinical trials, can be desirable in clinical practice to optimize the total therapeutic effect. This should mean that the context effect of each therapeutic intervention is maximized towards an improved therapeutic effect, as outlined in the recent AWMF guidelines for postoperative pain therapy, but should not include the administration of an inert substance. The latter is controlled by rigorous ethical guidelines and is only permitted in the context of ethically approved controlled clinical trials. A possible alternative is suggested by Benedetti et al. in which the hidden administration of an active substance identifies the specific response in contrast to the open application of the same substance characterizing the specific plus the placebo effect, after which the pure placebo effect can be determined.

The dynamic mechanisms of placebo induced analgesia: Evidence of sustained and transient regional involvement

Previously, we demonstrated that placebo analgesia (PA) accompanies reductions in neural activity during painful stimulation. This study investigated areas of the brain where the neural activity was increased during PA. The literature has associated PA with two potential mechanisms of action; one sustained (e.g., engaged for the duration of PA), the other, transitory (e.g., a feedback mechanism). We propose that PA results from the engagement of two complementary pain-modulation mechanisms that are identified with fMRI data as a main effect for condition or a time *condition interaction. The mechanism with sustained activity should activate the emotional regulation circuitry needed for memory formation of the event. The mechanism with transient activity should process cognitive and evaluative information of the stimuli in the context of the placebo suggestion to confirm the expectations set by it. To identify regions involved with these mechanisms, we re-analyzed fMRI data from two conditions: baseline (B) and PA. Results support the presence of both mechanisms, identified as two neural-networks with different temporal characteristics. Regions with sustained activity primarily involved the temporal and parahippocampal cortices. Conversely, brain regions with transient activity included linguistic centers in the left hemisphere and frontal regions of the right hemisphere generally associated with executive functioning. Together, these mechanisms likely engage analgesic processes and then simply monitor the system for unexpected stimuli, effectively liberating resources for other processes. Identifying brain regions associated with pain-modulation with different temporal profiles is consistent with the multidimensionality of PA and highlights the need for continued investigation of this construct.

The importance of placebo in headache research

The best way to appreciate the efficacy of drug and behavioural therapy in the acute and prophylactic treatment of headache is to perform placebo-controlled randomized trials. In order to plan and conduct these studies in the most appropriate way, it is desirable to know which factors influence the placebo response. This paper reviews factors which influence the placebo response in clinical trials, such as expectation, blinding, route of application of drugs and age, gender and geographical distribution. Response rates of placebo in the treatment of acute headache episodes are higher than in headache prophylaxis. Invasive procedures such as injections have a higher placebo response compared with oral drugs. Variables known to influence the placebo response have to be taken into consideration to calculate properly the power of planned randomized trials.

Experimentally manipulating perceptions regarding acupuncture elicits different responses to the identical acupuncture stimulation

Excessive expectancy or an aversion to acupuncture makes it difficult to evaluate the efficacy of acupuncture during clinical trials. We investigated whether experimental manipulations of the perception of acupuncture could elicit different responses to identical acupuncture stimulation. Seventeen participants were assessed with the acupuncture belief scale (ABS) and by measuring tactile and pain sensitivity. Identical acupuncture-related pictures and either positive (positive group; n=9) or negative (negative group, n=8) statements related to treatment were presented, and participants used the self-assessment manikin (SAM) to rate each acupuncture-related image. Participants were stimulated with the same intensity for 5 min and then evaluated for acupuncture-induced, self-reported pain. Heart rate variability (HRV) was measured before and after the acupuncture stimulation. Participants in the negative group were less valenced and more aroused in response to the same acupuncture picture compared to those in the positive group. Negative cognition regarding acupuncture modality resulted in a change of the relationship between the pre-experimental expectancy of acupuncture and self-reported pain. The negative group produced an increased low-frequency component of HRV after acupuncture, whereas the positive group did not. Subjective and sympathetic responses to acupuncture can be modified by perception. Our findings may help to understand the psychological factors related to acupuncture modality.

Imaging the placebo response: a neurofunctional review

An emerging literature has started to document the neuronal changes associated with the placebo phenomenon. This has altered placebo from being considered a nuisance factor in clinical research to a target of scientific investigation per se. This paper reviews the neuroimaging literature on the placebo effect, and illustrates how imaging tools can improve current understanding of brain mechanisms underlying the placebo response. Imaging studies provide evidence of specific, predictable and replicable patterns of neural changes associated with placebo administration. In general, placebo responses seem mediated by "top-down" processes dependent on frontal cortical areas that generate and maintain cognitive expectancies. Dopaminergic reward pathways may underlie these expectancies. Placebo-induced clinical benefits also involve disorder-specific neuronal responses, yielding neurofunctional or neurochemical alterations similar to those produced by pharmacological treatments.

Learning potentiates neurophysiological and behavioral placebo analgesic responses

Expectation and conditioning are supposed to be the two main psychological mechanisms for inducing a placebo response. Here, we further investigate the effects of both expectation, which was induced by verbal suggestion alone, and conditioning at the level of N1 and N2-P2 components of CO2 laser-evoked potentials (LEPs) and subjective pain reports. Forty-four healthy volunteers were pseudorandomly assigned to one of three experimental groups: Group 1 was tested with verbal suggestion alone, Group 2 was tested with a conditioning procedure, whereby the intensity of painful stimulation was reduced surreptitiously, so as to make the volunteers believe that the treatment was effective, Group 3 was a control group that allowed us to rule out phenomena of sensitization and/or habituation. Pain perception was assessed according to a Numerical Rating Scale (NRS) ranging from 0=no pain sensation to 10=maximum imaginable pain. Both verbal suggestions (Group 1) and conditioning (Group 2) modified the N2-P2 complex, but not the N1 component of LEPs. However, the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2-P2 amplitude changes that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. Compared to natural history, conditioning produced more robust reductions of LEP amplitudes than verbal suggestions alone. Overall, these findings indicate that prior positive experience plays a key role in maximizing both behavioral and neurophysiological placebo responses, emphasizing that the placebo effect is a learning phenomenon which affects the early central nociceptive processing.

A comprehensive review of the placebo effect: recent advances and current thought

Our understanding and conceptualization of the placebo effect has shifted in emphasis from a focus on the inert content of a physical placebo agent to the overall simulation of a therapeutic intervention. Research has identified many types of placebo responses driven by different mechanisms depending on the particular context wherein the placebo is given. Some placebo responses, such as analgesia, are initiated and maintained by expectations of symptom change and changes in motivation/emotions. Placebo factors have neurobiological underpinnings and actual effects on the brain and body. They are not just response biases. Other placebo responses result from less conscious processes, such as classical conditioning in the case of immune, hormonal, and respiratory functions. The demonstration of the involvement of placebo mechanisms in clinical trials and routine clinical practice has highlighted interesting considerations for clinical trial design and opened up opportunities for ethical enhancement of these mechanisms in clinical practice.

Mechanisms of placebo and placebo-related effects across diseases and treatments

The placebo effect has evolved from being thought of as a nuisance in clinical and pharmacological research to a biological phenomenon worthy of scientific investigation in its own right. It is now clear that the term placebo effect is too restrictive and, in fact, many placebo-related effects have recently been investigated. A placebo effect differs from a placebo-like effect in that the former follows the administration of a placebo, whereas in the latter no placebo is administered. However, in both cases, the psychosocial context around the treatment plays a key role. In recent years, placebo and placebo-related effects have been analyzed with sophisticated biological tools that have uncovered specific mechanisms at both the biochemical and cellular level. This recent research has revealed that these psychosocial-induced biochemical changes in a patient's brain and body in turn may affect the course of a disease and the response to a therapy.

The influence of expectation on spinal manipulation induced hypoalgesia: an experimental study in normal subjects

Background

The mechanisms thorough which spinal manipulative therapy (SMT) exerts clinical effects are not established. A prior study has suggested a dorsal horn modulated effect; however, the role of subject expectation was not considered. The purpose of the current study was to determine the effect of subject expectation on hypoalgesia associated with SMT.

Methods

Sixty healthy subjects agreed to participate and underwent quantitative sensory testing (QST) to their leg and low back. Next, participants were randomly assigned to receive a positive, negative, or neutral expectation instructional set regarding the effects of a specific SMT technique on pain perception. Following the instructional set, all subjects received SMT and underwent repeat QST.

Results

No interaction (p = 0.38) between group assignment and pain response was present in the lower extremity following SMT; however, a main effect (p < 0.01) for hypoalgesia was present. A significant interaction was present between change in pain perception and group assignment in the low back (p = 0.01) with participants receiving a negative expectation instructional set demonstrating significant hyperalgesia (p < 0.01).

Conclusion

The current study replicates prior findings of c- fiber mediated hypoalgesia in the lower extremity following SMT and this occurred regardless of expectation. A significant increase in pain perception occurred following SMT in the low back of participants receiving negative expectation suggesting a potential influence of expectation on SMT induced hypoalgesia in the body area to which the expectation is directed.

Imaging pain modulation in health and disease

PURPOSE OF REVIEW

In this review, we discuss recent advances in pain imaging research. We focus on the involvement of endogenous pain control mechanisms in the healthy central nervous system and the potential contribution of failure within this system for chronic pain states.

RECENT FINDINGS

Nociceptive information processing and related pain perception is subject to substantial pro and antinociceptive modulation. Recent studies demonstrate that this modulation can take place at any stage of ascending information processing. A network of cortical, predominantly mesial and frontal areas, in combination with specific brainstem nuclei, appear to be the key players in the context of endogenous pain modulation. Recent findings from functional and anatomical neuroimaging support the notion that an altered interaction of pro and antinociceptive mechanisms may contribute to the development or maintenance of chronic pain states. The additional use of pharmacological intervention in pain imaging research provides an alternative tool for investigating mechanisms of pain modulation.

SUMMARY

Top-down pain modulation relies on both cortical and subcortical structures. Research on the involved circuitry, including the implemented mechanisms, is a major focus of contemporary neuroscientific research in the field of pain and will provide new insights into the prevention and treatment of chronic pain states.

Neuropathic pain: multiple mechanisms at multiple sites

Damage to nerves at various levels of the peripheral and central nervous systems will lead to sensory loss, but in a significant number of patients this is accompanied by a series of distressing painful signs and symptoms. Although animal models and clinical studies have shed much needed light on the underlying mechanisms that produce this maladaptive plasticity, the presently available drugs do not always fully control the pain. This review covers some of the important mechanisms that include ion channels, central processing through excitatory amino acid and neuropeptide receptors and, finally, the role of monoamine systems that originate in the brain and descend to alter spinal events. The targets for presently licensed and potential novel drugs are covered in this context, as are perspectives on future research priorities.

Low intensity permanent magnets in the treatment of chronic lumbar radicular pain

We assessed the pain-relieving efficacy of static magnetic fields produced by 200 Gauss (G) magnets compared with 50G magnets in a double-blind, randomized, two-phase crossover study in patients with chronic lumbar radicular pain. The surface field strengths of the magnets were 200 and 50G. Phase I included four random periods of two-week duration: two periods with 200G, one period with 50G, and one period of "no treatment." The magnets were positioned either vertically or horizontally in standard lumbosacral elastic corsets. Phase II consisted of two five-week periods with the most effective magnet from Phase I and its corresponding 50 or 200G device. The primary outcome was average daily leg pain score (0-10 scale) in each period of Phase II. Thirty-eight of 40 randomized patients completed Phase I, and 28 of 31 Phase II participants completed the study. In Phase I, pain scores did not differ significantly between 200 and 50G magnets. Phase II average leg pain scores tended to be lower with 200 vs. 50G magnets (3.2+/-2.1 for 200G vs. 3.9+/-2.2 for 50G magnets [P=0.08]) after excluding one unblinded patient. The relative treatment effect of the 200G magnets appeared to increase throughout the five-week period. Although these data cannot rule out a chance effect, the positive trends suggest that larger, longer-duration, sham-controlled trials with 200G magnets be considered in patients with chronic lumbar radicular pain.

Habituation to painful stimulation involves the antinociceptive system

The perception of pain results from an interaction between nociceptive and antinociceptive mechanisms. A better understanding of the neural circuitry underlying these physiological interactions provides an important opportunity to develop better treatment strategies for and ultimately even prevent pain. Here, we investigated how repeated painful stimulation over several days is processed, perceived and finally modulated in the healthy human brain. Twenty healthy subjects were stimulated daily with a 20min pain paradigm for 8 consecutive days, and functional MRI performed on days 1, 8 and 22. Repeated painful stimulation over several days resulted in substantially decreased pain ratings to identical painful stimuli. The decreased perception of pain over time is reflected in decreased BOLD responses to nociceptive stimuli in classical pain areas, including thalamus, insula, SII and the putamen. In contrast to this finding, we found that pain-related responses in the rACC, specifically the subgenual anterior cingulate cortex (sgACC), significantly increased over time. Given this area's predominant role in endogenous pain control, this response pattern suggests that habituation to pain is at least in part mediated by increased antinociceptive activity.

Placebo analgesia is not due to compliance or habituation: EEG and behavioural evidence

This study was designed to resolve whether experimental placebo responses are due to either increased compliance or habituation. We stimulated both forearms and recorded laser-evoked potentials from 18 healthy volunteers treated on one arm with a sham analgesic cream and an inactive cream on the other (treatment group), and 13 volunteers with an inactive cream on both arms (controls). The treatment group showed a significant reduction in the pain ratings and laser-evoked potentials with both the sham and inactive creams. The control group showed no evidence of habituation to the laser stimulus. The results indicate that the reduction in pain during experimental placebo response is unlikely to be due to sensory habituation or compliance with the experimental instructions.

New insights into placebo analgesia

PURPOSE OF REVIEW

The placebo effect is a widespread phenomenon in medicine, both in clinical trials and in routine medical practice. Most of our knowledge about the underlying psychological and physiological mechanisms comes from the study of placebo analgesia.

RECENT FINDINGS

When the correct methodological approach is used, striking placebo effects can be detected and these can be mediated by conscious anticipatory processes or unconscious conditioning mechanisms. However, it should be stressed that many improvements observed after the administration of a placebo are not real placebo effects, but different phenomena such as spontaneous remission, regression to the mean and symptom detection ambiguity. Both neuropharmacological studies and brain imaging investigations show that placebo analgesia is mediated by endogenous opioids. Moreover, we also know that during placebo analgesia other systems change their functions, like the respiratory centres and the cardiovascular system. The placebo effect has also been approached from a different perspective by administering analgesics covertly. The results show that hidden medical treatments are less effective than open ones.

SUMMARY

The understanding of the placebo effect may lead to better design of clinical trials and better medical practice. For example, it can be used in therapeutic protocols aimed at reducing drug intake.

Placebo effects on human mu-opioid activity during pain

Placebo-induced expectancies have been shown to decrease pain in 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 [(11)C]carfentanil, which measures regional mu-opioid receptor availability in vivo. Noxious thermal stimulation was applied at the same temperature for placebo and control conditions. Placebo treatment affected endogenous opioid activity in a number of predicted mu-opioid receptor-rich regions that play central roles in pain and affect, including periaqueductal gray and nearby dorsal raphe and nucleus cuneiformis, 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 connectivity 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.

When words are painful: Unraveling the mechanisms of the nocebo effect

The nocebo effect is a phenomenon that is opposite to the placebo effect, whereby expectation of a negative outcome may lead to the worsening of a symptom. Thus far, its study has been limited by ethical constraints, particularly in patients, as a nocebo procedure is per se stressful and anxiogenic. It basically consists in delivering verbal suggestions of negative outcomes so that the subject expects clinical worsening. Although some natural nocebo situations do exist, such as the impact of negative diagnoses upon the patient and the patient's distrust in a therapy, the neurobiological mechanisms have been understood in the experimental setting under strictly controlled conditions. As for the placebo counterpart, the study of pain has been fruitful in recent years to understand both the neuroanatomical and the neurochemical bases of the nocebo effect. Recent experimental evidence indicates that negative verbal suggestions induce anticipatory anxiety about the impending pain increase, and this verbally-induced anxiety triggers the activation of cholecystokinin (CCK) which, in turn, facilitates pain transmission. CCK-antagonists have been found to block this anxiety-induced hyperalgesia, thus opening up the possibility of new therapeutic strategies whenever pain has an important anxiety component. Other conditions, such as Parkinson's disease, although less studied, have been found to be affected by nocebo suggestions as well. All these findings underscore the important role of cognition in the therapeutic outcome, and suggest that nocebo and nocebo-related effects might represent a point of vulnerability both in the course of a disease and in the response to a therapy.

Placebo theory and its implications for research and clinical practice: a review of the recent literature

Although placebo effect is a common phenomenon in medicine and research, its mechanisms are not well understood. With the advent of modern medicine, placebo became a symbol for an outdated, morally questionable practice implying deceit and paternalism. However, in recent years, there has been an increasing amount of rigorous research into the mechanisms of placebo response and placebo analgesia with most studies coming from the field of pain medicine. New theories on placebo mechanisms have shown that placebo represents the psychosocial aspect of every treatment and the study of placebo is essentially the study of psychosocial context that surrounds the patient. Therefore, its understanding is essential for researchers and all medical practitioners, particularly those dealing with patients suffering from pain, depression, and motor disorders. In this article, we review the theories on placebo mechanisms and discuss their implications for clinical practice and the design of clinical trials.

Understanding the Placebo Effect: Contributions from Neuroimaging

Neuroimaging studies have provided a major contribution to our understanding of the mechanisms of the placebo effect in neurological and psychiatric disorders. Expectation of symptom improvement has long been believed to play a critical role in the placebo effect, and is associated with increased endogenous striatal dopamine release in Parkinson's disease and increased endogenous opioid transmission in placebo analgesia. Evidence from positron emission tomography and functional magnetic resonance imaging studies suggests that expectations of symptom improvement are driven by frontal cortical areas, particularly the dorsolateral prefrontal, orbitofrontal, and anterior cingulate cortices. The ventral striatum is involved in the expectation of rewarding stimuli and, together with the prefrontal cortex, has also been shown to play an important role in the placebo-induced expectation of therapeutic benefit. Understanding the mechanisms of the placebo effect has important implications for treatment of several medical conditions, including depression, pain, and Parkinson's disease.

Placebo and endogenous mechanisms of analgesia

The discovery of the endogenous systems of analgesia has produced a large amount of research aimed at investigating their biochemical and neurophysiological mechanisms and their neuroanatomical localization. Nevertheless, the neurobiological acquisitions on these mechanisms have not been paralleled by behavioural correlates in humans--in other words, by the understanding of when and how these endogenous mechanisms of analgesia are activated. Until recent times one of the most studied behavioural correlates of endogenous analgesia was stress-induced analgesia, in which the activation of endogenous opioid systems is known to be involved. By contrast, today the placebo analgesic effect represents one of the best-described situations in which this endogenous opioid network is naturally activated in humans. Therefore, not only is placebo research helpful towards improving clinical trial design and medical practice, but it also provides us with a better understanding of the endogenous mechanisms of analgesia.

Serotonin receptor ligands: treatments of acute migraine and cluster headache

Fuelled by the development of the serotonin 5-HT(1B/1D) receptor agonists, the triptans, the last 15 years has seen an explosion of interest in the treatment of acute migraine and cluster headache. Sumatriptan was the first of these agonists, and it launched a wave of therapeutic advances. These medicines are effective and safe. Triptans were developed as cranial vasoconstrictors to mimic the desirable effects of serotonin, while avoiding its side-effects. It has subsequently been shown that the triptans' major action is neuronal, with both peripheral and central trigeminal inhibitory effects, as well as actions in the thalamus and at central modulatory sites, such as the periaqueductal grey matter. Further refinements may be possible as the 5-HT(1D) and 5-HT(1F) receptor agonists are explored. Serotonin receptor pharmacology has contributed much to the better management of patients with primary headache disorders.

Placebo analgesia: Friend or foe?

The magnitude of placebo analgesia is influenced by environmental and perceptual factors. Environmental factors include past exposure to effective analgesic agents and verbal suggestions and cues that foster a perception of being given an effective treatment. Environmental factors, in turn, influence the proximate psychologic mediators of placebo analgesia, which include decreased desire for and increased expectations of pain relief. Strategies to maximize placebo analgesic effects in clinical practice could focus on using verbal suggestions and external cues to increase expectations of pain relief and/or decrease the perceived need for pain reduction. Placebo analgesic effects could be minimized in clinical trials by avoiding these same suggestions and cues.

Classical conditioning and expectancy in placebo hypoalgesia: a randomized controlled study in patients with atopic dermatitis and persons with healthy skin

The effectiveness of placebos is unchallenged. However, it is still not clear on which mechanisms the placebo effect is based. Besides expectancy theories, classical conditioning is discussed as a major explanatory model. In an experimental conditioning design we tested 96 participants, 48 with atopic dermatitis (24 male, 24 female) and 48 with healthy skin (24 male and 24 female). All of them received a neutral ointment with a different briefing ("pain-reducing ointment" versus "neutral ointment"). Electrical pain stimuli were subsequently applied, which selectively induce a painful sensation. In the case of the learning condition (classical conditioning) and unbeknown to the participants, the intensity of the pain stimulus was reduced by 50% after the ointment had been applied. The study addressed the question whether the pain experienced by the patients with atopic dermatitis could be reduced through a placebo effect and whether the placebo effect was achieved through expectancy or through a process of classical conditioning or both. The results indicate that a placebo effect is achieved via expectancy and classical conditioning. However, conditioning processes seem to be necessary for a longer lasting effect. The extent of this effect seemed to be greater in atopics than in healthy controls. Expectancy, achieved through verbal instruction, might also be seen as a conditioned stimulus that reactivates earlier stimulus associations.

Focused hypnotic analgesia: Local and remote effects

Suggestion for hypnotic analgesia aimed at a specific body area is termed "focused hypnotic analgesia". It is not clear, however, whether this analgesia is limited to a specific body location or spread all over the body. Focused hypnotic analgesia was studied, in response to ascending electrical stimuli, when analgesia and stimulation were applied to the same area (local), and when analgesia was applied to one location and stimulation was delivered to a different area (remote). The face or leg served alternately as the local or remote areas, and the effect was tested in 12 high-hypnotizable (HH) and 13 low-hypnotizable (LH) subjects. Hypnotic analgesia in the local site produced a significant pain reduction compared to the remote site in HH subjects (P<0.0001) but not in LH subjects (P=0.68). As stimuli increased in intensity the reduction in pain as a result of hypnosis was larger both in HH and LH subjects (P<0.0001). Nevertheless, significant analgesia occurred in the 3 highest intensities in the local and remote location of HH subjects, but only in 2 highest intensities in the local and 1 in the remote of LH subjects. We conclude that in HH subjects focused hypnotic analgesia is mostly confined to the area aimed at, but some spread of analgesia to remote areas cannot be dismissed all together. Alternatively, this "spread" of analgesia could be due to a placebo effect in the remote area. Focused hypnotic analgesia requires increased attention to the body area aimed at, unlike analgesia achieved by distraction of attention.