The opioid placebo analgesia is mediated exclusively through μ-opioid receptor in rat

Behaviorally conditioned effects of psychoactive drugs in experimental animals: What we have learned from nearly a century of research and what remains to be learned

Continuous treatment with drugs is a crucial requirement for managing various clinical conditions, including chronic pain and neuropsychiatric disorders such as depression or schizophrenia. Associative learning processes, i.e. Pavlovian conditioning, can play an important role for the effects of drugs and could open new avenues for optimizing patient treatment. In this narrative literature review, we summarize available data in experimental animals regarding the behaviorally conditioned effects of psychostimulants such as d-amphetamine and cocaine, the dopamine receptor agonist apomorphine, the dopamine receptor antagonist haloperidol, morphine and antidepressant drugs. In each section, the drug under discussion is briefly introduced, followed by a detailed examination of conditioning features, including doses and dosing regimens, characteristics of the conditioning process such as test environments or specific conditioned stimuli, testing and conditioned response characteristics, possible extinction or reconditioning or reversal training, neural mechanisms, and finally, the potential clinical relevance of the research area related to the drug. We focus on key outcomes, delve into methodical issues, identify gaps in current knowledge, and suggest future research directions.

A novel investigation of placebo analgesia through social communication in mice

BACKGROUND AND AIMS

In rodents, placebo analgesia is often investigated through direct conditioning of stimuli, but humans can experience placebo analgesia through expectation without experience. In this study, we sought to determine whether placebo analgesia could be elicited through social communication.

METHODS

Male and female mice were housed in pairs (designated "Active" and "Bystander") and tested for thermal thresholds on a hot plate (53 °C). Food restriction (1 hr/day) was implemented. The Active mouse was taken to a new cage with food dusted with cocoa (COC) or cinnamon (CINN). The Bystander mice were given regular chow in the home cage. After feeding, the Active mice were given morphine (5 mg/kg, SC) or saline and tested on the hot plate. After 5 pairings of a flavor and treatment (counterbalanced), Active mice were tested following access to a flavored food. Bystander mice were given their first direct exposure to a flavored food and tested on the hot plate. The protocol was repeated with naloxone (10 mg/kg, IP) administered prior to testing. Finally, mice were tested in a two-choice test with both flavored foods available.

RESULTS

Active mice showed a conditioned analgesic response to the morphine-paired flavor that was reduced by naloxone. Bystander mice showed a placebo analgesic response to their cagemate's morphine-paired flavor that was not significantly impacted by naloxone. Bystander mice spent more time in the chamber associated with their cagemate's morphine-paired flavor.

CONCLUSIONS

To our knowledge, this is the first investigation of placebo analgesia without direct conditioning, instead relying on social communication between mice. The lack of effect with naloxone pretreatment suggests an opioid-independent effect in the Bystander mice. Placebo analgesia in mice may be possible without direct conditioning to better model the effect of expectation of a novel analgesic in humans.

Blinded Pain Cocktails: A Reliable and Safe Opioid Weaning Method

Weaning opioids in patients with noncancerous chronic pain often poses a challenge when psychosocial factors complicate the patient's chronic pain syndrome and opioid use. A blinded pain cocktail protocol used to wean opioid therapy has been described since the 1970s. At the Stanford Comprehensive Interdisciplinary Pain Program, a blinded pain cocktail remains a reliably effective medication-behavioral intervention. This review (1) outlines psychosocial factors that may complicate opioid weaning, (2) describes clinical goals and how to use blinded pain cocktails in opioid tapering, and (3) summarizes the mechanism of dose-extending placebos and ethical justification of its use in clinical practice.

Psycho-Neuro-Endocrine-Immunological Basis of the Placebo Effect: Potential Applications beyond Pain Therapy

The placebo effect can be defined as the improvement of symptoms in a patient after the administration of an innocuous substance in a context that induces expectations regarding its effects. During recent years, it has been discovered that the placebo response not only has neurobiological functions on analgesia, but that it is also capable of generating effects on the immune and endocrine systems. The possible integration of changes in different systems of the organism could favor the well-being of the individuals and go hand in hand with conventional treatment for multiple diseases. In this sense, classic conditioning and setting expectations stand out as psychological mechanisms implicated in the placebo effect. Recent advances in neuroimaging studies suggest a relationship between the placebo response and the opioid, cannabinoid, and monoaminergic systems. Likewise, a possible immune response conditioned by the placebo effect has been reported. There is evidence of immune suppression conditioned through the insular cortex and the amygdala, with noradrenalin as the responsible neurotransmitter. Finally, a conditioned response in the secretion of different hormones has been determined in different studies; however, the molecular mechanisms involved are not entirely known. Beyond studies about its mechanism of action, the placebo effect has proved to be useful in the clinical setting with promising results in the management of neurological, psychiatric, and immunologic disorders. However, more research is needed to better characterize its potential use. This review integrates current knowledge about the psycho-neuro-endocrine-immune basis of the placebo effect and its possible clinical applications.

Engaging endogenous opioid circuits in pain affective processes

The pervasive use of opioid compounds for pain relief is rooted in their utility as one of the most effective therapeutic strategies for providing analgesia. While the detrimental side effects of these compounds have significantly contributed to the current opioid epidemic, opioids still provide millions of patients with reprieve from the relentless and agonizing experience of pain. The human experience of pain has long recognized the perceived unpleasantness entangled with a unique sensation that is immediate and identifiable from the first-person subjective vantage point as "painful." From this phenomenological perspective, how is it that opioids interfere with pain perception? Evidence from human lesion, neuroimaging, and preclinical functional neuroanatomy approaches is sculpting the view that opioids predominately alleviate the affective or inferential appraisal of nociceptive neural information. Thus, opioids weaken pain-associated unpleasantness rather than modulate perceived sensory qualities. Here, we discuss the historical theories of pain to demonstrate how modern neuroscience is revisiting these ideas to deconstruct the brain mechanisms driving the emergence of aversive pain perceptions. We further detail how targeting opioidergic signaling within affective or emotional brain circuits remains a strong avenue for developing targeted pharmacological and gene-therapy analgesic treatments that might reduce the dependence on current clinical opioid options.

Brainstem Mechanisms of Pain Modulation: A within-Subjects 7T fMRI Study of Placebo Analgesic and Nocebo Hyperalgesic Responses

Pain perception can be powerfully influenced by an individual's expectations and beliefs. Although the cortical circuitry responsible for pain modulation has been thoroughly investigated, the brainstem pathways involved in the modulatory phenomena of placebo analgesia and nocebo hyperalgesia remain to be directly addressed. This study used ultra-high-field 7 tesla functional MRI (fMRI) to accurately resolve differences in brainstem circuitry present during the generation of placebo analgesia and nocebo hyperalgesia in healthy human participants (N = 25, 12 male). Over 2 successive days, through blinded application of altered thermal stimuli, participants were deceptively conditioned to believe that two inert creams labeled lidocaine (placebo) and capsaicin (nocebo) were acting to modulate their pain relative to a third Vaseline (control) cream. In a subsequent test phase, fMRI image sets were collected while participants were given identical noxious stimuli to all three cream sites. Pain intensity ratings were collected and placebo and nocebo responses determined. Brainstem-specific fMRI analysis revealed altered activity in key pain modulatory nuclei, including a disparate recruitment of the periaqueductal gray (PAG)-rostral ventromedial medulla (RVM) pathway when both greater placebo and nocebo effects were observed. Additionally, we found that placebo and nocebo responses differentially activated the parabrachial nucleus but overlapped in engagement of the substantia nigra and locus coeruleus. These data reveal that placebo and nocebo effects are generated through differential engagement of the PAG-RVM pathway, which in concert with other brainstem sites likely influences the experience of pain by modulating activity at the level of the dorsal horn.SIGNIFICANCE STATEMENT Understanding endogenous pain modulatory mechanisms would support development of effective clinical treatment strategies for both acute and chronic pain. Specific brainstem nuclei have long been known to play a central role in nociceptive modulation; however, because of the small size and complex organization of the nuclei, previous neuroimaging efforts have been limited in directly identifying how these subcortical networks interact during the development of antinociceptive and pro-nociceptive effects. We used ultra-high-field fMRI to resolve brainstem structures and measure signal change during placebo analgesia and nocebo hyperalgesia. We define overlapping and disparate brainstem circuitry responsible for altering pain perception. These findings extend our understanding of the detailed organization and function of discrete brainstem nuclei involved in pain processing and modulation.

Thirty Years of Neuroscientific Investigation of Placebo and Nocebo: The Interesting, the Good, and the Bad

Over the past 30 years there has been a surge of research on the placebo effect using a neuroscientific approach. The interesting aspects of this effort are related to the identification of several biological mechanisms of both the placebo and nocebo effects, the latter of which is defined as a negative placebo effect. Some important translational implications have emerged both in the setting of clinical trials and in routine medical practice. One of the principal contributions of neuroscience has been to draw the attention of the scientific and medical communities to the important role of psychobiological factors in therapeutic outcomes, be they drug related or not. Indeed, many biological mechanisms triggered by placebos and nocebos resemble those modulated by drugs, suggesting a possible interaction between psychological factors and drug action. Unfortunately, this new knowledge regarding placebos has the potential of being dangerously exploited by pseudoscience. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Bridging the Translational Divide in Pain Research: Biological, Psychological and Social Considerations

A gap exists between translating basic science research into effective pain therapies in humans. While preclinical pain research has primarily used animal models to understand biological processes, a lesser focus has been toward using animal models to fully consider other components of the pain experience, such as psychological and social influences. Herein, we provide an overview of translational studies within pain research by breaking them down into purely biological, psychological and social influences using a framework derived from the biopsychosocial model. We draw from a wide landscape of studies to illustrate that the pain experience is highly intricate, and every attempt must be made to address its multiple components and interactors to aid in fully understanding its complexity. We highlight our work where we have developed animal models to assess the cognitive and social effects on pain modulation while conducting parallel experiments in people that provide proof-of-importance for human pain modulation. In some instances, human pain research has sparked the development of novel animal models, with these animal models used to better understand the complexity of phenomena considered to be uniquely human such as placebo responses and empathy.

Morphine-Conditioned Placebo Analgesia in Female and Male Rats with Chronic Neuropathic Pain: c-Fos Expression in the Rostral Ventromedial Medulla

Placebo analgesia has great potential to overcome the inadequacies of current drug therapies to treat conditions of chronic pain. The rostral ventromedial medulla (RVM) has been implicated as a critical relay in the antinociceptive pathway underpinning placebo analgesia in humans. We developed a model of opiate-conditioned placebo analgesia in rats with neuropathic injury to identify medullary nuclei active during placebo analgesia. Using female and male rats the degree of thermal allodynia was first determined following nerve injury, and a pharmacological conditioning procedure, pairing contextual cues with the experience of morphine-induced analgesia, was used to elicit placebo analgesic reactions. This protocol revealed clear subpopulations of placebo reactors (36% of males, 25% of females) and non-reactors in proportions similar to those reported in human studies. We detected injury-specific c-Fos expression in the gracile nucleus and morphine-specific c-Fos expression in the serotonergic midline raphe nuclei and the caudal nuclei of the solitary tract. However, c-Fos expression did not differ between placebo reactors and non-reactors in either serotonergic or non-serotonergic neurons of the RVM. Despite a subpopulation of rats demonstrating placebo reactions, we found no evidence for enhanced activity in the nuclei from which the classical RVM → spinal cord descending analgesic pathways emerge.

Classical conditioning of antidepressant placebo effects in mice

INTRODUCTION

Placebo effects in human clinical trials for depression treatment are robust and often comparable to drug effects. Placebo effects are traditionally difficult to study in rodents due to the slow-onset action of classical antidepressant drugs. We hypothesized that the rapid antidepressant actions of ketamine would allow modeling antidepressant placebo effects in rodents.

METHODS

Male and female CD-1 mice received either ketamine or saline injections with concomitant exposure to specific environmental conditioning stimuli, for a total of three drug/conditioning sessions each 2 weeks apart. Two weeks later, during an evocation phase, mice were exposed to the drug-paired conditioning stimuli or no conditioned stimuli followed by testing for motor stimulatory actions and antidepressant-like effects using the forced swim test. Negative (no ketamine administration at any time) and positive (acute ketamine administration prior to evocation testing) control groups were included as comparators.

RESULTS

Both male and female mice exhibited increased locomotor activity following ketamine administration during the conditioning phase, which was not observed following exposure to the conditioning stimuli. Exposure to the conditioning stimuli previously paired with ketamine, similar to an acute ketamine administration, reduced immobility time in the forced swim test both 1 and 24 h after administration in male, but not female, mice.

CONCLUSIONS

These results represent the first evidence of antidepressant-like placebo-conditioned effects in an animal model. The developed approach can be used as a model to explore the neurobiological mechanisms of placebo effects, their possible sexually dimorphic effects, and relevance to mechanisms underlying antidepressant action.

Failure of Placebo Analgesia Model in Rats with Inflammatory Pain

With the shifting role of placebos, there is a need to develop animal models of placebo analgesia and elucidate the mechanisms underlying the effect. In the present study, male Sprague-Dawley rats with chronic inflammatory pain caused by complete Freund's adjuvant (CFA) underwent a series of conditioning procedures, in which morphine was associated with different cues, but they failed to induce placebo analgesia. Then, conditioning with the conditioned place preference apparatus successfully induced analgesic expectancy and placebo analgesia in naïve rats but only induced analgesic expectancy and no analgesic effect in CFA rats. Subsequently, we found enhanced c-fos expression in the nucleus accumbens and reduced expression in the anterior cingulate cortex in naïve rats while c-fos expression in the anterior cingulate cortex in CFA rats was not altered. In summary, the behavioral conditioning model demonstrated the difficulty of establishing a placebo analgesia model in rats with a pathological condition.

In search of a rodent model of placebo analgesia in chronic orofacial neuropathic pain

All treatments are given in a context, suggesting that conditioning cues may significantly influence therapeutic outcomes. We tested the hypothesis that context affects placebo analgesia in rodents. To produce neuropathic pain in rats, we performed chronic constriction injury of the infraorbital nerve. We then treated the rats daily, over a seven day period, with injections of either fentanyl or saline, with or without associated conditioning cues; a fourth group received no treatment. On the eighth day, we replaced fentanyl with saline to test for conditioned placebo analgesia. We tested the effects of treatment by measuring sensitivity to mechanical stimuli and grimace scale scores. We found no significant differences in either of these outcomes among the four experimental groups. These findings suggest that chronic, neuropathic pain in rats may not be susceptible to placebo analgesia.

The Placebo Effect in Pain Therapies

Pharmacological strategies for pain management have primarily focused on dampening ascending neurotransmission and on opioid receptor-mediated therapies. Little is known about the contribution of endogenous descending modulatory systems to clinical pain outcomes and why some patients are mildly affected while others suffer debilitating pain-induced dysfunctions. Placebo effects that arise from patients' positive expectancies and the underlying endogenous modulatory mechanisms may in part account for the variability in pain experience and severity, adherence to treatment, distinct coping strategies, and chronicity. Expectancy-induced analgesia and placebo effects in general have emerged as useful models to assess individual endogenous pain modulatory systems. Different systems and mechanisms trigger placebo effects that highly impact pain processing, clinical outcomes, and sense of well-being. This review illustrates critical elements of placebo mechanisms that inform the methodology of clinical trials, the discovery of new therapeutic targets, and the advancement of personalized pain management.

Placebo Analgesia in Rodents: Current and Future Research

The investigation of placebo effects in animal pain models has received less attention than human research. This may be related to a number of difficulties, including the fact that animals lack the ability to use language and establish expectancies verbally, that animals cannot report and rate the extent to which they experience pain, and the inadequacy of current models of pain. Here, we describe the relatively small number of studies that have been published, communicating the opportunities and excitement of this research. We critically discuss pitfalls and limitations with the hope that this will advance future animal placebo-related research.

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Novel designs and paradigms to study the placebo response in gastroenterology

The investigation of the placebo and the nocebo effect and their mechanisms has a rather short history of less than 20 years, especially in gastroenterology, and only the last 5 years have resulted in substantial improvement of understanding. Placebo refers to symptom improvement following a treatment, nocebo to the opposite, symptom worsening. Among the factors driving this progress are traditional psychological models derived from learning (conditioning) theory bridging into clinical science, new animal models to investigate the pharmacology of placebo analgesia, and novel study designs to overcome limitations of traditional randomized and placebo-controlled study designs in drug testing. These are explored here for their implementation and application in gastroenterology, with a focus on visceral pain and nausea.

Nature of the placebo and nocebo effect in relation to functional neurologic disorders

Placebos have long been considered a nuisance in clinical research, for they have always been used as comparators for the validation of new treatments. By contrast, today they represent an active field of research, and, due to 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. There is not a single placebo effect, but many, with different mechanisms across different medical conditions and therapeutic interventions. Expectation, anxiety, and reward are all involved, as well as a variety of learning phenomena and genetic variants. The most productive models to better understand the neurobiology of the placebo effect are pain and Parkinson's disease. In these medical conditions, several neurotransmitters have been identified, such as endogenous opioids, cholecystokinin, dopamine, as well as lipidic mediators, for example, endocannabinoids and prostaglandins. Since the placebo effect is basically a psychosocial context effect, these data indicate that different social stimuli, such as words and therapeutic rituals, may change the chemistry of the patient's brain, and these effects are similar to those induced by drugs.

Relieving pain using dose-extending placebos: a scoping review

Placebos are often used by clinicians, usually deceptively and with little rationale or evidence of benefit, making their use ethically problematic. In contrast with their typical current use, a provocative line of research suggests that placebos can be intentionally exploited to extend analgesic therapeutic effects. Is it possible to extend the effects of drug treatments by interspersing placebos? We reviewed a database of placebo studies, searching for studies that indicate that placebos given after repeated administration of active treatments acquire medication-like effects. We found a total of 22 studies in both animals and humans hinting of evidence that placebos may work as a sort of dose extender of active painkillers. Wherever effective in relieving clinical pain, such placebo use would offer several advantages. First, extending the effects of a painkiller through the use of placebos may reduce total drug intake and side effects. Second, dose-extending placebos may decrease patient dependence. Third, using placebos along with active medication, for part of the course of treatment, should limit dose escalation and lower costs. Provided that nondisclosure is preauthorized in the informed consent process and that robust evidence indicates therapeutic benefit comparable to that of standard full-dose therapeutic regimens, introducing dose-extending placebos into the clinical arsenal should be considered. This novel prospect of placebo use has the potential to change our general thinking about painkiller treatments, the typical regimens of painkiller applications, and the ways in which treatments are evaluated.

Different contexts, different pains, different experiences

Pain is an ambiguous perception: the same pain stimulation can be perceived differently in different contexts, producing different experiences, ranging from mild to unbearable pain. It can be even experienced as a rewarding sensation within the appropriate context. Overall, placebo and nocebo effects appear to be very good models to understand how the psychosocial context modulates the experience of pain. In this review, we examine the effects of different contexts on pain, with a specific focus on the neurobiological mechanisms. Positive and rewarding contexts inform the patients that an effective treatment is being delivered and are capable of producing pain relief through the activation of specific systems such as opioids, cannabinoids and dopamine. Conversely, a negative context can produce pain exacerbation and clinical worsening through the modulation of different systems, such as the activation of cholecystokinin and the deactivation of opioids and dopamine. In addition, when a therapy is delivered unbeknownst to the patient, its effects are reduced. A better understanding of the neurobiological underpinnings of the context-pain interaction is a challenge both for future pain research and for good clinical practice.

A new animal model of placebo analgesia: involvement of the dopaminergic system in reward learning

We suggest a new placebo analgesia animal model and investigated the role of the dopamine and opioid systems in placebo analgesia. Before and after the conditioning, we conducted a conditioned place preference (CPP) test to measure preferences for the cues (Rooms 1 and 2), and a hot plate test (HPT) to measure the pain responses to high level-pain after the cues. In addition, we quantified the expression of tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) and c-Fos in the anterior cingulate cortex (ACC) as a response to reward learning and pain response. We found an enhanced preference for the low level-pain paired cue and enhanced TH expression in the VTA of the Placebo and Placebo + Naloxone groups. Haloperidol, a dopamine antagonist, blocked these effects in the Placebo + Haloperidol group. An increased pain threshold to high-heat pain and reduced c-Fos expression in the ACC were observed in the Placebo group only. Haloperidol blocked the place preference effect, and naloxone and haloperidol blocked the placebo analgesia. Cue preference is mediated by reward learning via the dopamine system, whereas the expression of placebo analgesia is mediated by the dopamine and opioid systems.

Neuron-restrictive silencer factor in periaqueductal gray contributes to remifentanil-induced postoperative hyperalgesia via repression of the mu-opioid receptor

BACKGROUND

The ultra-short-acting mu-opioid receptor (MOR) agonist remifentanil induces postoperative hyperalgesia both in preclinical and clinical research studies. However, the precise mechanisms remain unclear, although changes in opioid receptor expression might be a correlative feature. Neuron-restrictive silencer factor (NRSF) functions as a crucial regulator of MOR expression in specific neuronal cells. Using a mouse model of incisional postoperative pain, we assessed the expression of MOR and NRSF and investigated whether disruption of NRSF expression could prevent the postoperative nociceptive sensitization induced by surgical incision and subcutaneous infusion of remifentanil.

METHODS

Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were independently used to assess mechanical allodynia and thermal hyperalgesia after surgery and cerebral ventricle injection of NRSF antisense oligonucleotide. Western blotting analyses were preformed to assess the expression levels of MOR and NRSF.

RESULTS

NRSF expression levels were enhanced after intraoperative infusion of remifentanil, resulting in repression of MOR expression in the periaqueductal gray (PAG). NRSF blockade with an NRSF antisense oligonucleotide significantly enhanced the expression levels of MOR and alleviated mechanical allodynia and thermal hyperalgesia induced by intraoperative infusion of remifentanil.

CONCLUSION

NRSF functions as a negative regulator of MOR in PAG and contributes to remifentanil-induced postoperative hyperalgesia. NRSF in PAG may be a potential target for this pain therapy.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Placebo effects: from the neurobiological paradigm to translational implications

Today we are witnessing a new science of placebo, a complex discipline that encompasses several experimental approaches and translational implications. Modern neurobiological tools have been used to answer important questions in placebo research, such as the top-down modulation of sensory and motor systems as well as the influence of cognition, emotions, and learning on symptoms, diseases, and responses to treatments. What we have learned is that there is not one single placebo effect, but many. This review highlights the translational implications of this new knowledge, ranging from clinical trial design to medical practice to social and ethical issues.

Strain differences in the expression of endocannabinoid genes and in cannabinoid receptor binding in the brain of Lewis and Fischer 344 rats

The Lewis (LEW) and Fischer 344 (F344) rat strains have been proposed as a model to study certain genetic influences on drug use. These strains differ in terms of the self-administration of several drugs, and in their expression of various components of the dopaminergic, glutamatergic, GABAergic and endogenous opioid neurotransmitter systems. As the endocannabinoid system is linked to these systems, we investigated whether these two strains exhibit differences in cannabinoid receptor binding and in the expression of cannabinoid-related genes. Quantitative autoradiography of [(3)H]-CP 55,940 binding levels and real-time PCR assays were used. F344 rats displayed higher levels of cannabinoid receptor binding in the lateral globus pallidus and weaker CNR1 gene expression in the prefrontal cortex (PFc) than LEW rats. Moreover, the N-acyl phosphatidylethanolamine-specific phospholipase D/fatty acid amide hydrolase ratio was greater in the PFc and NAcc of F344 rats. Our results suggest that the endocannabinoid system may be a mediator of the individual differences that exist in the susceptibility to the rewarding effects of drugs of abuse.

The elusive rat model of conditioned placebo analgesia

Recent research on human placebo analgesia has suggested the need for rodent models to further elucidate the neural substrates of the placebo effect. This series of 3 experiments therefore was performed in an attempt to develop a model of placebo analgesia in rats. In each study, female Sprague-Dawley rats received an L5 spinal nerve ligation to induce a neuropathic pain condition. Each rat then underwent a 4-day conditioning procedure in which an active analgesic drug or its vehicle (unconditioned stimulus) was associated with the following cues (conditioned stimuli): novel testing room (environmental), vanilla scent cue (olfactory), dim incandescent lighting (visual), restraint procedure/injection (tactile), and time of day and injection-test latency (temporal). The analgesics for each experiment were as follows: Experiment 1 used 90 mg/kg gabapentin, experiment 2 used 3mg/kg loperamide hydrochloride, and experiment 3 used 6 mg/kg morphine sulfate. On the following test day, half of the animals received the opposite treatment, resulting in 4 conditioning manipulations: drug/drug, drug/vehicle, vehicle/drug, and vehicle/vehicle. Nociceptive thresholds were assessed with the mechanical paw withdrawal threshold test each day after the conditioning procedure. In all 3 experiments, no significant differences were detected on test day between control and placebo groups, indicating a lack of a conditioned placebo analgesic response. Our results contrast with prior research that implies the existence of a reliable and robust response to placebo treatment. We conclude that placebo analgesia in rats is not particularly robust and that it is difficult to achieve using conventional procedures and proper experimental design.

Pain and the context

Pain is a sensory and emotional experience that is substantially modulated by psychological, social and contextual factors. Research now indicates that the influence of these factors is even more powerful than expected and involves the therapeutic response to analgesic drugs as well as the pain experience itself, which in some circumstances can even be a form of reward. Different experimental approaches and models, both in the laboratory and in the clinical setting, have been used to better characterize and understand the complex neurobiology of pain modulation. These approaches include placebo analgesia, nocebo hyperalgesia, hidden administration of analgesics, and the manipulation of the pain-reward relationship. Overall, these studies show that different neurochemical systems are activated in different positive and negative contexts. Moreover, pain can activate reward mechanisms when experienced within contexts that have special positive meaning. Because routine medical practice usually takes place in contexts that use different rituals, these neurobiological insights might have profound clinical implications.

Placebo, nocebo, and learning mechanisms

Recent substantial laboratory and theoretical research hints for different learning mechanisms regulating the formation of placebo and nocebo responses. Moreover, psychological and biological variants may play a role as modulators of learning mechanisms underlying placebo and nocebo responses. In this chapter, we present pioneering and recent human and nonhuman research that has impressively increased our knowledge of learning mechanisms in the context of placebo and nocebo effects across different physiological processes and pathological conditions.

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.

Cognitive and emotional control of pain and its disruption in chronic pain

Chronic pain is one of the most prevalent health problems in our modern world, with millions of people debilitated by conditions such as back pain, headache and arthritis. To address this growing problem, many people are turning to mind-body therapies, including meditation, yoga and cognitive behavioural therapy. This article will review the neural mechanisms underlying the modulation of pain by cognitive and emotional states - important components of mind-body therapies. It will also examine the accumulating evidence that chronic pain itself alters brain circuitry, including that involved in endogenous pain control, suggesting that controlling pain becomes increasingly difficult as pain becomes chronic.