The biochemical and neuroendocrine bases of the hyperalgesic nocebo effect

Interaction between cholecystokinin and the fibroblast growth factor system in the ventral tegmental area of selectively bred high- and low-responder rats

Individual differences in the locomotor response to novelty have been linked to basal differences in dopaminergic neurotransmission. Mesolimbic dopaminergic outputs are regulated by cholecystokinin (CCK), a neuropeptide implicated in anxiety. In turn, CCK expression is regulated by fibroblast growth factor-2 (FGF2), which has recently been identified as an endogenous regulator of anxiety. FGF2 binds to the high-affinity fibroblast growth factor receptor-1 (FGF-R1) to regulate the development and maintenance of dopamine neurons in the ventral tegmental area (VTA). However, the relationship between the FGF and CCK systems in the VTA is not well understood. Therefore, we utilized the selectively-bred low-responder (bLR; high-anxiety) and high-responder (bHR; low-anxiety) rats to examine the effects of repeated (21-day) FGF2 treatment on CCK and FGF-R1 mRNA in the rostral VTA (VTAr). In vehicle-treated controls, both CCK and FGF-R1 mRNA levels were increased in the VTAr of bLR rats relative to bHR rats. Following FGF2 treatment, however, bHR-bLR differences in CCK and FGF-R1 mRNA expression were eliminated, due to decreased CCK mRNA levels in the VTAr of bLR rats and increased FGF-R1 expression in bHR rats. Differences after FGF2 treatment may denote distinct interactions between the CCK and FGF systems in the VTAr of bHR vs. bLR rats. Indeed, significant correlations between CCK and FGF-R1 mRNA expression were found in bHR, but not bLR rats. Colocalization studies suggest that CCK and FGF-R1 are coexpressed in some VTAr neurons. Taken together, our findings suggest that the FGF system is poised to modulate both CCK and FGF-R1 expression in the VTAr, which may be associated with individual differences in mesolimbic pathways associated with anxiety-like behavior.

Mechanisms and therapeutic implications of the placebo effect in neurological and psychiatric conditions

The power of a placebo to effect clinically meaningful neurobiological change comparable to pharmacological therapies has been demonstrated, although the mechanisms are not fully understood. Predicting placebo responsiveness has only recently received more attention, but psychological disposition, contextual and biological factors are now known to dramatically affect a person's susceptibility to the placebo effect. The placebo effect depends upon expectancies that can be modified in a number of ways, including conditioning through explicit or implicit learned associations. Based on the dopaminergic response to anticipation of benefit in Parkinson's disease, it was suggested that the placebo effect can be seen as analogous to the expectation of reward. Dopaminergic pathways have since been implicated in the placebo response in pain and depression. Additionally, endogenous opioid release is known to mediate many forms of placebo analgesia. We provide an overview of the mechanisms and the therapeutic implications of the placebo effect in neurological and psychiatric conditions. We include evidence for detrimental effects arising from seemingly inert interventions, termed the 'nocebo effect.' Neuroimaging has critically advanced the study of the placebo effect and provides some of the strongest evidence for the mechanisms of this phenomenon prevalent across an array of human health-related circumstances. This review specifically focuses on mechanisms of the placebo effect in the three conditions that have most significantly demonstrated this effect and for which a plausible physiological basis can be identified: pain, PD and depression. Other neurological and psychiatric diseases reviewed include multiple sclerosis, Huntington's disease, Alzheimer's disease, schizophrenia and epilepsy.

Placebo and the new physiology of the doctor-patient relationship

Modern medicine has progressed in parallel with the advancement of biochemistry, anatomy, and physiology. By using the tools of modern medicine, the physician today can treat and prevent a number of diseases through pharmacology, genetics, and physical interventions. Besides this materia medica, the patient's mind, cognitions, and emotions play a central part as well in any therapeutic outcome, as investigated by disciplines such as psychoneuroendocrinoimmunology. This review describes recent findings that give scientific evidence to the old tenet that patients must be both cured and cared for. In fact, we are today in a good position to investigate complex psychological factors, like placebo effects and the doctor-patient relationship, by using a physiological and neuroscientific approach. These intricate psychological factors can be approached through biochemistry, anatomy, and physiology, thus eliminating the old dichotomy between biology and psychology. This is both a biomedical and a philosophical enterprise that is changing the way we approach and interpret medicine and human biology. In the first case, curing the disease only is not sufficient, and care of the patient is of tantamount importance. In the second case, the philosophical debate about the mind-body interaction can find some important answers in the study of placebo effects. Therefore, maybe paradoxically, the placebo effect and the doctor-patient relationship can be approached by using the same biochemical, cellular and physiological tools of the materia medica, which represents an epochal transition from general concepts such as suggestibility and power of mind to a true physiology of the doctor-patient interaction.

The nocebo effect: a clinicians guide

OBJECTIVE

This paper aims to provide an overview on the nocebo effect, focusing on recognition - its phenomenology, at-risk demographic profiles, clinical situations and personality factors, as well as discriminating somatic symptoms in the general population from treatment-related adverse effects. Lastly, the paper addresses available evidence-based strategies for management and minimisation of the nocebo effect.

METHOD

Data for this paper were identified by searching PubMed using the search terms "nocebo" and "nocebo effect", augmented by a manual search of the references of the key papers and the related literature.

RESULTS

The nocebo effect refers to non-pharmacodynamic, harmful or undesirable effects occurring after inactive treatment, a phenomenon that also occurs in the context of active therapy. Known drivers include classical conditioning and negative expectations concerning treatment. Recent meta-analyses have reported a considerable prevalence, ranging from 18% in the symptomatic treatment of migraine, to more than 74% in multiple sclerosis. Recognition of the nocebo-driven adverse effects presents a challenge, especially because of its non-specific nature and the similarity to the active medication's expected profile. Traits such as neuroticism, pessimism and type A personalities may predispose individuals to this phenomenon. Clinical management of the nocebo effect includes awareness and recognition, changing the manner of disclosure of potential drug-related adverse effects, shaping patients' expectations and enhancing the treatment alliance.

CONCLUSION

The nocebo effect is a common, clinically significant, yet covert driver of clinical outcomes. Increased awareness of its features, as well as knowledge of strategies on how to manage it, are fundamental so that clinicians can mitigate its impact on clinical practice.

Negative expectations facilitate mechanical hyperalgesia after high-frequency electrical stimulation of human skin

BACKGROUND

High-frequency electrical stimulation (HFS) of human skin induces not only an increased pain sensitivity in the conditioning area but also an increased pain sensitivity to mechanical punctate stimuli in the non-conditioned surrounding skin area. The aim of the present study was to investigate whether this heterotopically increased mechanical pain sensitivity can be facilitated through the induction of negative expectations.

METHODS

In two independent conditions [a nocebo (n = 15) and control condition (n = 15)], we applied mechanical pain stimuli before, directly after, 10 min and 20 min after HFS in the skin area surrounding the conditioning area, and measured the reported pain intensity [visual analogue scale (VAS)]. All subjects (of both conditions) received a written instruction about the HFS protocol, but only the instruction in the nocebo condition was extended by the following text (in Dutch): 'After the HFS, your skin will become more sensitive to the pinprick stimulation'.

RESULTS

Our results clearly show that induced expectations of increased mechanical pain sensitivity after HFS facilitates the reported pain intensity after HFS more than when no information is given.

CONCLUSIONS

This study shows for the first time that brain mechanisms, via the induction of negative expectations, can facilitate heterotopic mechanical hyperalgesia after HFS of human skin.

Painful heat attenuates electrically induced muscle pain in men and women

Background and purpose

Women exhibit higher prevalence of most painful disorders. Several explanations have been proposed for this discrepancy, one being that endogenous pain modulatory pathways, which affect incoming nociceptive signals, act differently in men and women. A less efficient pain inhibitory system has been proposed as a contributing factor to explain why women exhibit higher prevalence of most painful disorders. The present study determined whether muscle pain, induced experimentally by electrical stimulation, is inhibited by a painful heat stimulus. This conditioned pain modulation (CPM) paradigm was used to determine whether women show signs of reduced inhibition compared to men.

Methods

Forty self-reported healthy individuals (20 female, 20 male) participated in a cross-over design with painful and non-painful heat as a conditioning stimulus. Test stimuli were painful intramuscular electrical stimulation of the tibialis anterior muscle at two intensities; low (1.1 × pain threshold) and high (1.6 × pain threshold). Painful conditioning was contact heat (45–49 ° C) to the contralateral forearm. Nonpainful conditioning was contact heat at 35 °C. Ten test stimuli were delivered in three blocks (before, during and after conditioning) in two sessions (painful and non-painful conditioning). The women were tested during days 12-14 of the menstrual cycle. This interval corresponds to the ovulatory phase of the menstrual cycle, the interval during which women are reported to show the largest inhibitory effects.

Results

Test stimuli were rated significantly lower during painful conditioning, compared with before conditioning. This was found for both low and high test stimulus intensities. Anonspecific attenuation was seen during non-painful conditioning for the low test stimulus intensity. Test stimuli were rated significantly lower also 3 min after conditioning, compared with before conditioning. The inhibitory effects were not different between men and women. Similar findings were obtained also if six non-CPM-responders (subjects rating test stimuli higher during conditioning than before conditioning) were excluded.

Conclusions and implications

The present findings indicate that painful contact heat inhibits electrically induced muscle pain and that inhibition was not different between men and women, when women were tested in the interval 12-14 days after their last menstruation. Some inhibition of muscle pain was seen during non-painful conditioning, indicating that nonspecific inhibitory effects were triggered. Also the nonspecific inhibitory effects were similar in men and women.

Expectations modulate heterotopic noxious counter-stimulation analgesia

The present study examined the contribution of expectations to analgesia induced by heterotopic noxious counter-stimulation (HNCS) in healthy volunteers assigned to a control group or 1 of 3 experimental groups in which expectations were either assessed (a priori expectations) or manipulated using suggestions (hyperalgesia and analgesia). Acute shock-pain, the nociceptive flexion reflex (RIII-reflex), and shock-related anxiety were measured in response to electrical stimulations of the right sural nerve in the baseline, HNCS, and recovery periods. Counter-stimulation was applied on the contralateral forearm using a flexible cold pack. A priori expectations were strongly associated with the actual magnitude of the analgesia induced by HNCS. In comparison to the control condition, suggestions of hyperalgesia led to an increase in RIII-reflex amplitude and shock-pain, while suggestions of analgesia resulted in a greater decrease in RIII-reflex amplitude, which confirms that the analgesic process normally activated by HNCS can be blocked or enhanced by the verbal induction of expectations through suggestions. Changes in shock-anxiety induced by these suggestions were correlated to changes in shock-pain and RIII-reflex, but these changes did not emerge as a mediator of the association between manipulated expectations and HNCS analgesia. Overall, the results demonstrate that HNCS analgesia is modulated by expectations, either from a priori beliefs or suggestions, and this appears to be independent of anxiety processes.

PERSPECTIVE

This study demonstrates that a priori and manipulated expectations can enhance or block HNSC analgesia. Results also suggest that expectations might influence responses to analgesic treatments by altering descending modulation and contribute to observed deficit in pain inhibition processes of chronic pain patients.

Warning about side effects can increase their occurrence: an experimental model using placebo treatment for sleep difficulty

Patients in clinical practice and participants in clinical trials are warned about side effects that may result from their treatment. Such warnings could lead to placebo-induced side effects if they create an expectation of these effects. We used an experimental model to test this possibility. Undergraduates reporting sleep difficulty received placebo treatment disguised as a hypnotic for one week and were warned about either one or four bogus side effects. Placebo treatment significantly improved sleep difficulty relative to a no treatment control group, as indicated by self-report and by objective outcomes. At the end of the treatment week participants who had been warned about a single side effect showed better recall of this effect than those warned about four side effects. Most importantly, participants tended to report experiencing a side effect they had been warned about, with a trend towards a larger effect in participants warned about one side effect. This evidence for placebo-induced side effects may need to be considered when interpreting data on side effects from clinical trials.

How positive and negative expectations shape the experience of visceral pain: an experimental pilot study in healthy women

BACKGROUND

In order to elucidate placebo and nocebo effects in visceral pain, we analyzed the effects of positive and negative expectations on rectal pain perception, rectal pain thresholds, state anxiety and cortisol responses in healthy women.

METHODS

Painful rectal distensions were delivered at baseline, following application of an inert substance combined with either positive instructions of pain relief (placebo group, N = 15), negative instructions of pain increase (nocebo group, N = 17), or neutral instructions (control, N = 15). Perceived pain intensity, unpleasantness/aversion and urge-to-defecate, state anxiety and serum cortisol were determined at baseline, immediately following group-specific instructions and on a second study day after the same instructions (test day). Rectal pain thresholds were determined at baseline and on the test day.

KEY RESULTS

Whereas perceived pain intensity was significantly decreased in the placebo group, the nocebo group revealed significantly increased pain intensity ratings, along with significantly greater anticipatory anxiety on the test day (all P < 0.05 vs controls). Cortisol concentrations were significantly increased in the nocebo group following treatment but not on the test day.

CONCLUSIONS & INFERENCES

The experience of abdominal pain can be experimentally increased or decreased by inducing positive or negative expectations. Nocebo effects involve a psychological stress response, characterized by increased anticipatory anxiety. These findings further underscore the role of cognitive and emotional factors in the experience of visceral pain, which has implications for the pathophysiology and treatment of patients with chronic abdominal complaints.

Modulation of pain sensation by stress-related testosterone and cortisol

Stress increases cortisol and decreases testosterone. It is not known whether pain is affected by stress-related testosterone. Therefore, we investigated whether stress can affect pain perception by decreasing testosterone and increasing cortisol. Pain thresholds, pain and anxiety ratings and salivary testosterone and cortisol levels were measured in 46 healthy men during resting and stressful conditions. Pain was induced by electrical stimulation. Stress was induced by having participants perform a medical test. Stress significantly increased anxiety ratings and salivary cortisol levels, but decreased salivary testosterone levels. Stress also increased pain ratings and decreased pain thresholds. During stress, cortisol levels were negatively correlated with pain thresholds and testosterone levels were positively correlated with pain thresholds. Results indicated that testosterone can decrease and cortisol can increase pain induced by electrical stimulation, suggesting that acute clinical pain may be relieved by controlling stress and managing consequent stress-related testosterone and cortisol.

Nocebo phenomena in medicine: their relevance in everyday clinical practice

BACKGROUND

Nocebo phenomena are common in clinical practice and have recently become a popular topic of research and discussion among basic scientists, clinicians, and ethicists.

METHODS

We selectively searched the PubMed database for articles published up to December 2011 that contained the key words "nocebo" or "nocebo effect."

RESULTS

By definition, a nocebo effect is the induction of a symptom perceived as negative by sham treatment and/or by the suggestion of negative expectations. A nocebo response is a negative symptom induced by the patient's own negative expectations and/or by negative suggestions from clinical staff in the absence of any treatment. The underlying mechanisms include learning by Pavlovian conditioning and reaction to expectations induced by verbal information or suggestion. Nocebo responses may come about through unintentional negative suggestion on the part of physicians and nurses. Information about possible complications and negative expectations on the patient's part increases the likelihood of adverse effects. Adverse events under treatment with medications sometimes come about by a nocebo effect.

CONCLUSION

Physicians face an ethical dilemma, as they are required not just to inform patients of the potential complications of treatment, but also to minimize the likelihood of these complications, i.e., to avoid inducing them through the potential nocebo effect of thorough patient information. Possible ways out of the dilemma include emphasizing the fact that the proposed treatment is usually well tolerated, or else getting the patient's permission to inform less than fully about its possible side effects. Communication training in medical school, residency training, and continuing medical education would be desirable so that physicians can better exploit the power of words to patients' benefit, rather than their detriment.

Induced fear reduces the effectiveness of a placebo intervention on pain

Fear was induced by the anticipation of electric shock in order to investigate whether fear reduced the effectiveness of a placebo intervention on reported pain and the acoustic startle reflex. Thirty-three subjects participated in a 3 Condition (Natural History [NH], Placebo [P], Placebo+Fear [PF])×3 Test (Pretest, Posttest 1, Posttest 2) within-subject design, tested on 3 separate days. Measures of fear were fear of pain (FOP), measured by the Fear of Pain Questionnaire (FPQ-III); fear-potentiated startle; and a self-report measure that assessed the effectiveness of the fear induction procedure. In the pain intensity data, there was a trend towards a placebo effect. This trend was abolished by induced fear, and was most pronounced in subjects who were highest in measures of fear. The placebo manipulation also caused a reduction in startle reflex amplitude. This effect was abolished by induced fear, and was strongest amongst high FOP subjects. In conclusion, induced fear abolished placebo analgesia, and this effect was strongest in subjects who had high scores on measures of fear.

Placebo analgesia: cognitive influences on therapeutic outcome

The therapeutic response to a drug treatment is a mixture of direct pharmacological action and placebo effect. Therefore, harnessing the positive aspects of the placebo effect and reducing the negative ones could potentially benefit the patient. This article is aimed at providing an overview for clinicians of the importance of contextual psychosocial variables in determining treatment response, and the specific focus is on determinants of the placebo response. A better understanding of the physiological, psychological, and social mechanisms of placebo may aid in predicting which contexts have the greatest potential for inducing positive treatment responses. We examine the evidence for the role of psychological traits, including optimism, pessimism, and the effect of patient expectations on therapeutic outcome. We discuss the importance of the patient-practitioner relationship and how this can be used to enhance the placebo effect, and we consider the ethical challenges of using placebos in clinical practice.

The placebo effect: advances from different methodological approaches

There is accumulating evidence from different methodological approaches that the placebo effect is a neurobiological phenomenon. Behavioral, psychophysiological, and neuroimaging results have largely contributed to accepting the placebo response as real. A major aspect of recent and future advances in placebo research is to demonstrate linkages between behavior, brain, and bodily responses. This article provides an overview of the processes involved in the formation of placebo responses by combining research findings from behavioral, psychophysiological, and neuroimaging methods. The integration of these different methodological approaches is a key objective, motivating our scientific pursuits toward a placebo research that can inform and guide important future scientific knowledge.

To tell the truth, the whole truth, may do patients harm: the problem of the nocebo effect for informed consent

The principle of informed consent obligates physicians to explain possible side effects when prescribing medications. This disclosure may itself induce adverse effects through expectancy mechanisms known as nocebo effects, contradicting the principle of nonmaleficence. Rigorous research suggests that providing patients with a detailed enumeration of every possible adverse event-especially subjective self-appraised symptoms-can actually increase side effects. Describing one version of what might happen (clinical "facts") may actually create outcomes that are different from what would have happened without this information (another version of "facts"). This essay argues that the perceived tension between balancing informed consent with nonmaleficence might be resolved by recognizing that adverse effects have no clear black or white "truth." This essay suggests a pragmatic approach for providers to minimize nocebo responses while still maintaining patient autonomy through "contextualized informed consent," which takes into account possible side effects, the patient being treated, and the particular diagnosis involved.

Cognitive manipulation targeted at decreasing the conditioning pain perception reduces the efficacy of conditioned pain modulation

Although painfulness of the conditioning stimulus (CS) is required for the activation of conditioned pain modulation (CPM), it is still unclear whether CPM expression depends on the objective physical intensity of the CS or the subjective perception of its pain. Accordingly, we cognitively manipulated the perceived CS pain, rendering the physical aspects of the CPM paradigm untouched. Baseline CPM was measured among 48 young healthy male subjects using the parallel paradigm with contact heat as test pain and hand immersion in hot water as CS. Subjects were then randomized into 4 groups, all of which were cognitively manipulated as to the CS-induced pain: group 1, placebo (CS less painful); group 2, nocebo (CS more painful); and groups 3 and 4, the informed control groups for groups 1 and 2, respectively. CPM was reassessed after the manipulation. Comparing the groups by MANCOVA (multivariate analysis of covariance) revealed that placebo exerted decreased CS pain and consequent attenuation of CPM magnitudes, while nocebo elicited increased CS pain, but without CPM elevation (P<.0001). Within the placebo group, the reduction in CS pain was associated with diminished CPM responses (r=0.767; P=.001); however, no such relationship characterized the nocebo group. Pain inhibition under CPM seems to depend on the perceived level of the CS pain rather than solely its physical intensity. Cognitively decreasing the perceived CS pain attenuates CPM magnitude, although a ceiling effect may limit CPM enhancement after cognitively increased CS pain. These findings emphasize the relevance of cognitive mechanisms in determining endogenous analgesia processes in humans.

The relation of emotions to placebo responses

The hypothesis put forth is that expectations of treatment effects reduce negative emotions and thereby reduce symptoms, e.g. pain. Negative emotions increase pain, and it is hypothesized that placebos reduce pain by reducing negative emotions, i.e. feelings of nervousness, fear and anxiety. Placebo analgesia has been shown to be mediated via opioid activity, and relaxation increases opioid activity. The placebo acquires its relaxing effect due to verbal information that pain will be reduced, or due to associations between the placebo and the reduction in pain after effective treatment. Thus, the placebo signals that unpleasantness will be less after administration of the placebo. This involves negative reinforcement which is due to activation of a dopaminergic system that has been found to be activated during placebo analgesia and is involved in positive emotions. The nocebo effect of increased pain is, consistent with this model, because of increased fear and anxiety. The new aspect of the presented model is the hypothesis that expectations reduce negative emotions, and that negative reinforcement that involves the dopaminergic reinforcement system should be a contributor to placebo responses.

Placebo mechanisms across different conditions: from the clinical setting to physical performance

Although the great increase in interest in the placebo phenomenon was spurred by the clinical implications of its use, the progressive elucidation of the neurobiological and pharmacological mechanisms underlying the placebo effect also helps cast new light on the relationship between mind (and brain) and body, a topic of foremost philosophical importance but also a major medical issue in light of the complex interactions between the brain on the one hand and body functions on the other. While the concept of placebo can be a general one, with a broad definition generally applicable to many different contexts, the description of the cerebral processes called into action in specific situations can vary widely. In this paper, examples will be given where physiological or pathological conditions are altered following the administration of an inert substance or verbal instructions tailored to induce expectation of a change, and explanations will be offered with details on neurotransmitter changes and neural pathways activated. As an instance of how placebo effects can extend beyond the clinical setting, data in the physical performance domain and implications for sport competitions will also be presented and discussed.

The nocebo effect and its relevance for clinical practice

Negative expectations deriving from the clinical encounter can produce negative outcomes, known as nocebo effects. Specifically, research on the nocebo effect indicates that information disclosure about potential side effects can itself contribute to producing adverse effects. Neurobiological processes play a role in the nocebo effect, and this article provides a selective review of mechanistic research on the nocebo effect. Comparatively little attention has been directed to clinical studies and their implications for daily clinical practice. The nocebo response is influenced by the content and the way information is presented to patients in clinical trials in both the placebo and active treatment conditions. Nocebo effects adversely influence quality of life and therapy adherence, emphasizing the need for minimizing these responses to the extent possible. Evidence further indicates that the informed consent process in clinical trials may induce nocebo effects. This article concludes with ethical directions for future patient-oriented research and routine practice.

Variability in placebo analgesia and the role of fear of pain--an ERP study

Fear of pain (FOP) and its effect on placebo analgesia was investigated. It was hypothesized that FOP should interfere with placebo-mediated pain inhibition and result in weaker placebo responding in pain intensity, pain unpleasantness, stress, and event-related potentials to contact heat pain. Thirty-three subjects participated in a balanced 2 condition (natural history, placebo)×3 test (pretest, posttest 1, posttest 2) within-subject design, tested on 2 separate days. FOP was measured by the Fear of Pain Questionnaire and subjective stress by the Short Adjective Check List. Placebo effects were found on reported pain unpleasantness and N2 and P2 amplitudes. FOP was related to reduced placebo responding in pain unpleasantness, but this was only evident for the subjects who received the placebo condition on day 1. Subjects who received the placebo condition on day 1 experienced more pretest stress than those who received the placebo condition on day 2 (ie, reversed condition order), and this explained the interaction effect on placebo responding. FOP was related to reduced placebo responding on P2 amplitude, whereas placebo responding on N2 amplitude was unaffected by FOP. Higher placebo responses on N2 and P2 amplitudes were both related to higher placebo analgesic magnitude in pain unpleasantness. In conclusion, increased FOP was found to reduce subjective and electrophysiological placebo analgesic responses.

Review of neuroimaging studies related to pain modulation

Background and purpose: A noxious stimulus does not necessarily cause pain. Nociceptive signals arising from a noxious stimulus are subject to modulation via endogenous inhibitory and facilitatory mechanisms as they travel from the periphery to the dorsal horn or brainstem and on to higher brain sites. Research on the neural structures underlying endogenous pain modulation has largely been restricted to animal research due to the invasiveness of such studies (e.g., spinal cord transection, brain lesioning, brain site stimulation). Neuroimaging techniques (e.g., magnetoencephalography (MEG), positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)) provide non-invasive means to study neural structures in humans. The aim is to provide a narrative review of neuroimaging studies related to human pain control mechanisms.

Methods: The approach taken is to summarise specific pain modulation mechanisms within the somatosensory (diffuse noxious inhibitory controls, acupuncture, movement), affective (depression, anxiety, catastrophizing, stress) and cognitive (anticipation/placebo, attention/distraction, hypnosis)domains with emphasis on the contribution of neuroimaging studies.

Results and conclusions: Findings from imaging studies are complex reflecting activation or deactivation in numerous brain areas. Despite this, neuroimaging techniques have clarified supraspinal sites involved in a number of pain control mechanisms. The periaqueductal grey (PAG) is one area that has consistently been shown to be activated across the majority of pain mechanisms. Activity in the rostral ventromedial medulla known to relay descending modulation from the PAG, has also been observed both during acupuncture analgesia and anxiety-induced hyperalgesia. Other brain areas that appear to be involved in a number of mechanisms are the anterior cingulate cortex, prefrontal cortex, orbitofrontal cortex and nucleus accumbens, but their exact role is less clear.

Implications: Neuroimaging studies have provided essential information about the pain modulatory pathways under normal conditions, but much is still to be determined. Understanding the mechanisms of pain control is important for understanding the mechanisms that contribute to failed pain control in chronic pain. Applying fMRI outside the brain, such as in the trigeminal nucleus caudalis of the spinotrigeminal pathway and in the dorsal horn of the spinal cord, and coupling brain activity with activity at these sites may help improve our understanding of the function of brain sites and shed light on functional connectivity in the pain pathway.

© 2011 Scandinavian Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.

How placebo responses are formed: a learning perspective

Despite growing scientific interest in the placebo effect and increasing understanding of neurobiological mechanisms, theoretical conceptualization of the placebo effect remains poorly developed. Substantial mechanistic research on this phenomenon has proceeded with little guidance by any systematic theoretical paradigm. This review seeks to present a theoretical perspective on the formation of placebo responses. We focus on information processing, and argue that different kinds of learning along with individuals' genetic make-up evolved as the proximate cause for triggering behavioural and neural mechanisms that enable the formation of individual expectations and placebo responses. Conceptualizing the placebo effect in terms of learning offers the opportunity for facilitating scientific investigation with a significant impact on medical care.

Harnessing the placebo effect: the need for translational research

Laboratory research recently has greatly enhanced the understanding of placebo and nocebo effects by identifying specific neuromodulators and brain areas associated with them. However, little progress has been made in translating this knowledge into improved patient care. Here, we discuss the limitations in our knowledge about placebo (and nocebo) effects and the need for translational research with the aim of guiding physicians in maximizing placebo effects and minimizing nocebo effects in their routine clinical practice. We suggest some strategies for how, when and why interventions to promote beneficial placebo responses might be administered in the clinical setting.

Pain and emotion: a biopsychosocial review of recent research

OBJECTIVE AND METHOD

Research on emotion and pain has burgeoned. We review the last decade's literature, focusing on links between emotional processes and persistent pain.

RESULTS

Neurobiological research documents the neural processes that distinguish affective from sensory pain dimensions, link emotion and pain, and generate central nervous system pain sensitization. Psychological research demonstrates that greater pain is related to emotional stress and limited emotional awareness, expression, and processing. Social research shows the potential importance of emotional communication, empathy, attachment, and rejection.

CONCLUSIONS

Emotions are integral to the conceptualization, assessment, and treatment of persistent pain. Research should clarify when to eliminate or attenuate negative emotions, and when to access, experience, and express them. Theory and practice should integrate emotion into cognitive-behavioral models of persistent pain.

The effect of treatment expectation on drug efficacy: imaging the analgesic benefit of the opioid remifentanil

Evidence from behavioral and self-reported data suggests that the patients' beliefs and expectations can shape both therapeutic and adverse effects of any given drug. We investigated how divergent expectancies alter the analgesic efficacy of a potent opioid in healthy volunteers by using brain imaging. The effect of a fixed concentration of the μ-opioid agonist remifentanil on constant heat pain was assessed under three experimental conditions using a within-subject design: with no expectation of analgesia, with expectancy of a positive analgesic effect, and with negative expectancy of analgesia (that is, expectation of hyperalgesia or exacerbation of pain). We used functional magnetic resonance imaging to record brain activity to corroborate the effects of expectations on the analgesic efficacy of the opioid and to elucidate the underlying neural mechanisms. Positive treatment expectancy substantially enhanced (doubled) the analgesic benefit of remifentanil. In contrast, negative treatment expectancy abolished remifentanil analgesia. These subjective effects were substantiated by significant changes in the neural activity in brain regions involved with the coding of pain intensity. The positive expectancy effects were associated with activity in the endogenous pain modulatory system, and the negative expectancy effects with activity in the hippocampus. On the basis of subjective and objective evidence, we contend that an individual's expectation of a drug's effect critically influences its therapeutic efficacy and that regulatory brain mechanisms differ as a function of expectancy. We propose that it may be necessary to integrate patients' beliefs and expectations into drug treatment regimes alongside traditional considerations in order to optimize treatment outcomes.

Role of central neurophysiological systems in placebo analgesia and their relationships with cognitive processes mediating placebo responding

The harnessing of the positive aspects of the placebo effect in clinical practice is a major clinical and ethical challenge, and requires better understanding of placebo mechanisms. In this article, we present an explanatory cognitive model of placebo analgesia, centered on expectation of pain relief, and present direct and indirect evidence for the psychological and physiological drivers and downstream mediators of the effects of expectation on reduction in pain. The endogenous opioid system is involved in expectation-mediated analgesia, but it is not known whether this system is required for the generation or downstream effects of expectation. There is indirect evidence to support the role of other neurotransmitter systems, such as the serotonergic and dopamine systems, and a possible role of the hypothalamic–pituitary–adrenal stress axis. The future challenge is the identification of the causal role of these systems in placebo analgesia, which would provide an empirical basis for exploring new pain therapies.

Disruption of opioid-induced placebo responses by activation of cholecystokinin type-2 receptors

RATIONALE

Placebos are known to induce analgesia through the activation of μ-opioid receptors in some circumstances, such as after morphine pre-conditioning, an effect that is blocked by opioid antagonists.

OBJECTIVES

On the basis of the anti-opioid action of cholecystokinin, here we tested whether the activation of the cholecystokinin type-2 receptors abolishes opioid-induced placebo responses.

METHODS

The activation of the cholecystokinin type-2 receptors was performed by means of the agonist pentagastrin, and placebo responses were obtained after morphine pre-conditioning in an experimental human model of pain (tourniquet technique).

RESULTS

Opioid-induced placebo responses were completely disrupted by pentagastrin administration. In addition, a high correlation between the response to morphine and the response to placebo was found, and this correlation was completely abolished by pentagastrin.

CONCLUSION

These results show that the cholecystokinin-2 receptor agonist, pentagastrin, has the same effect as the μ-opioid receptor antagonist, naloxone, on placebo analgesia induced by morphine pre-conditioning, which suggests that the balance between cholecystokinergic and opioidergic systems is crucial in placebo responsiveness in pain. These findings also suggest that cholecystokinin type-2 receptor hyperactivity might be present in placebo non-responders.

Nocebo is the enemy, not placebo. A meta-analysis of reported side effects after placebo treatment in headaches

The aim was to determine the magnitude of the nocebo (adverse effects following placebo administration) in clinical trials for primary headache disorders. We reviewed randomized, placebo-controlled studies for migraine, tension-type headache (TTH), and cluster headache treatments published between 1998 and 2009. The frequency of nocebo was estimated by the percentage of placebo-treated patients reporting at least one adverse side effect. The dropout frequency was estimated by the percentage of placebo-treated patients who discontinued the treatment due to intolerance. In studies of symptomatic treatment for migraine, the nocebo and dropout frequencies were 18.45% and 0.33%, but rose to 42.78% and 4.75% in preventative treatment studies. In trials for prevention of TTH, nocebo and dropout frequencies were 23.99% and 5.44%. For symptomatic treatment of cluster headache, the nocebo frequency was 18.67%. Nocebo is prevalent in clinical trials for primary headaches, particularly in preventive treatment studies. Dropouts due to nocebo effect may confound the interpretation of many clinical trials.

How placebos change the patient's brain

Although placebos have long been considered a nuisance in clinical research, today they represent an active and productive field of research and, because of the involvement of many mechanisms, the study of the placebo effect can actually be viewed as a melting pot of concepts and ideas for neuroscience. Indeed, there exists not a single but many placebo effects, with different mechanisms and in different systems, medical conditions, and therapeutic interventions. For example, brain mechanisms of expectation, anxiety, and reward are all involved, as well as a variety of learning phenomena, such as Pavlovian conditioning, cognitive, and social learning. There is also some experimental evidence of different genetic variants in placebo responsiveness. The most productive models to better understand the neurobiology of the placebo effect are pain and Parkinson's disease. In these medical conditions, the neural networks that are involved have been identified: that is, the opioidergic-cholecystokinergic-dopaminergic modulatory network in pain and part of the basal ganglia circuitry in Parkinson's disease. Important clinical implications emerge from these recent advances in placebo research. First, as the placebo effect is basically a psychosocial context effect, these data indicate that different social stimuli, such as words and rituals of the therapeutic act, may change the chemistry and circuitry of the patient's brain. Second, the mechanisms that are activated by placebos are the same as those activated by drugs, which suggests a cognitive/affective interference with drug action. Third, if prefrontal functioning is impaired, placebo responses are reduced or totally lacking, as occurs in dementia of the Alzheimer's type.

Getting the pain you expect: mechanisms of placebo, nocebo and reappraisal effects in humans

The perception of pain is subject to powerful influences. Understanding how these are mediated at a neuroanatomical and neurobiological level provides us with valuable information that has a direct impact on our ability to harness positive and minimize negative effects therapeutically, as well as optimize clinical trial designs when developing new analgesics. This is particularly relevant for placebo and nocebo effects. New research findings have directly contributed to an increased understanding of how placebo and nocebo effects are produced and what biological and psychological factors influence variances in the magnitude of the effect. The findings have relevance for chronic pain states and other disorders, where abnormal functioning of crucial brain regions might affect analgesic outcome even in the normal therapeutic setting.

[Mechanisms for pain inhibiton in the central nervous system]

BACKGROUND

Several endogenous factors regulate the perception of pain. Understanding of pain-alleviating mechanisms is increasing, which is useful both for doctors who treat pain-ridden patients and for researchers interested in the physiology of pain. This article provides an overview of such mechanisms.

MATERIAL AND METHODS

This review article is based on literature identified through a non-systematic search in PubMed.

RESULTS

Endogenous pain-alleviating mechanisms are mainly controlled by different parts of the reticular substance, and are normally activated by painful stimulation. Expectation of pain reduction (placebo analgesic effect), painful stimulation in other sites and high blood pressure are examples of factors which may increase the body's endogenous pain-alleviating mechanisms. Opioid-sensitive cells in the brain stem are important for endogenous pain alleviation. Reduced endogenous pain alleviation is found in a number of painful conditions, but it has not been clarified whether reduced endogenous pain inhibition is a cause of or a result of chronic pain.

INTERPRETATION

Strengthening of the body's own mechanisms for pain alleviation is possible and potentially useful in treatment of pain-ridden patients.

Nocebo effects in multiple sclerosis trials: a meta-analysis

Objective: To estimate the incidence and severity of nocebo responses in trials of symptomatic treatments (STs) and disease-modifying treatments (DMTs) for multiple sclerosis (MS).

Methods: We conducted a systematic Medline search for all randomised, placebo-controlled MS trials published between 1989 and 2009. Meta-analysis of the incidence of nocebo responses was performed by pooling the percentage of placebo-treated patients that exhibited adverse events. Nocebo severity was calculated from the percentage of placebo-treated patients that dropped-out due to drug-related adverse events.

Results: Data were extracted from 56 DMT and 44 ST eligible trials. The pooled incidence of nocebo responses was 74.4% (95% CI: 69.92—88.30) in DMT trials and 25.3% (95% CI: 15.24—36.90) in ST trials and was significantly higher in the former ( p < 0.0001). The pooled nocebo severity was 2.1% (95% CI: 1.6—2.67) in DMT and 2.34% (95% CI: 1.54—3.29) in ST trials. Meta-regression analysis revealed a higher nocebo incidence in parallel design ST studies compared to crossover ones ( p = 0.013) and a higher nocebo severity in phase II ST studies compared to phase III ones ( p = 0.0001). Nocebo severity in DMT trials exhibited an association with the year of study publication ( p = 0.011) and the frequency of drug administration ( p = 0.0082).

Conclusions: Nocebo responses in MS trials are substantial and appear to have increased significantly in recent years with important implications for both trial design and clinical practice. Furthermore, nocebo responses exhibit an association with medication and trial-related factors.

Is fear of pain related to placebo analgesia?

OBJECTIVE

Verbal information that a painkiller has been administered generates an expectation of pain relief which in turn decreases pain. This expectation-based pain reduction is termed placebo analgesia. We hypothesized that fear of pain would be related to higher stress and pain intensity and to reduced placebo analgesia.

METHODS

Sixty-three students (30 females) participated in a Two-Condition (placebo, natural history)xFive-Test (one pretest, four post-tests) within-subjects design. Heat pain was induced by a 30x30-mm contact thermode to the medial volar forearm. Each pain test lasted for 4 min at a temperature of 46 degrees C. Stress, arousal, and pain intensity and pain unpleasantness were rated on 100-mm visual analogue scales.

RESULTS

Fear of pain was related to higher anticipatory stress and to higher stress and pain intensity during pain. Fear of pain was also related to reduced placebo analgesic responding.

CONCLUSION

Fear of pain was positively related to stress both during pain and in the anticipation of pain, and negatively related to placebo analgesia. Previous research has indicated a role for increased stress in the nocebo response, and the present findings suggest that decreased stress may strengthen the placebo response.

The effects of psychopathology on subjective experience of side effects of antipsychotics in patients with schizophrenia

This study was conducted to delineate the relationship between self-reported side effects and psychopathology in schizophrenia patients. Patients with schizophrenia completed the Liverpool University Neuroleptic Side Effects Rating Scale for subjective side effects and were evaluated with the Positive and Negative Syndrome Scale for their psychopathology. Based on a series of multiple linear regression analyses, we derived a model accounting for the relationships among the specific domains of psychopathology and red herring (RH) items of the Liverpool University Neuroleptic Side Effects Rating Scale in predicting subjective side effects. The model with anxiety/depressive symptoms and RH serving as mediators between positive symptoms and side effects was found to show good fit. Positive symptoms caused mostly anxiety symptoms and tendency to report RH items, whereby resulting in over-generalized reporting of subjective side effects. However, a large proportion of variance of side effects was explained by RH, which was only partially explained by positive symptoms alone. Therefore, patients with severe levels of positive and anxiety/depressive symptoms may be prone to nocebo-like effects of antipsychotics. Studies that include acute stage patients presenting severe levels of these symptoms should not rely only on the subjective report of side effects but also apply objective measures.