General mechanisms of placebo and nocebo effects

Different explanatory mechanisms have been proposed for both placebo and nocebo effects, each supported by experimental evidence. They need not be mutually exclusive and can actually work simultaneously (04).

The theory of classical conditioning proposes that the placebo or nocebo effect is the result of Pavlovian conditioning. In this process, the repeated co-occurrence of an unconditioned response to an unconditioned stimulus (eg, salivation after the sight of food) with a conditioned stimulus (eg, a bell ringing) induces a conditioned response (ie, salivation that is induced by bell ringing alone). Likewise, aspects of the clinical setting (eg, taste, color, or shape of a pill as well as white coats or the peculiar hospital smell) can also act as conditioned stimuli, eliciting a therapeutic response in the absence of an active principle, just because they have been paired with it in the past. In the same way, the conditioned response can be a negative outcome, as in the case of nausea elicited by the sight of the environment where chemotherapy has been administered in the past. Classical conditioning seems to work best where unconscious processes are at play, as in placebo or nocebo effects involving endocrine or immune systems, but it has also been documented in clinical and experimental placebo analgesia and nocebo hyperalgesia.

The theory of expectation conceives the placebo effect as the product of cognitive engagement, with the patient consciously foreseeing a positive or negative outcome based on factors as diverse as verbal instructions, environmental clues, previous experience, emotional arousal, and interactions with care providers. This anticipation of the future outcome, in turn, triggers internal changes resulting in specific experiences (eg, analgesia or hyperalgesia). Desire, self-efficacy, and self-reinforcing feedback all interact with expectation, potentiating its effects. Desire is the experiential dimension of wanting something to happen or wanting to avoid something happening, whereas self-efficacy is the belief to be able to manage the disease, performing the right actions to induce positive changes (eg, to withstand and lessen pain). Self-reinforcing feedback is a positive loop whereby the subject attends selectively to signs of improvement, taking them as evidence that the placebo treatment has worked. This is also called the somatic focus (ie, the degree to which individuals focus on their symptoms). A related proposed mechanism posits that anxiety reduction also has a role in placebo responses because the subject interpretation of ambiguous sensations is changed from noxious and menacing to benign and unworthy of attention.

Mechanisms across different conditions

Pain

The last three decades have witnessed the beginning of clarification of neurochemical and pharmacological details of placebo analgesia (04). In 1978, a pioneering study by Levine and colleagues showed that the opiate antagonist naloxone was able to reduce the placebo response in dental postoperative pain (41). That was the first indication that endogenous opioids were involved in placebo analgesia. Subsequent experiments provided even more compelling evidence that the secretion of endogenous opioids in the brain was the key event in placebo pain modulation. Placebo responders had levels of beta-endorphin in the cerebrospinal fluid that were more than double those of nonresponders; opioids released by a placebo procedure displayed the same side effects as exogenous opiates; naloxone-sensitive cardiac effects could be observed during placebo-induced expectation of analgesia. Indirect support also came from the placebo-potentiating role of the CCK antagonist proglumide. In fact, the CCK system effects counteracted those of opioids, delineating a picture where the placebo effect seems to be under the opposing influence of facilitating opioids and inhibiting CCK. In some situations, however, a placebo effect can still occur after blockade of opioid mechanisms by naloxone, indicating that systems other than opioids are also implicated. For example, with a morphine conditioning or expectation-inducing protocol, naloxone was able to completely reverse placebo analgesia induced in experimental ischemic arm pain. Conversely, with the use of ketorolac (a nonopioid analgesic) in the same protocol, the cannabinoid antagonist rimonabant was capable of reversing placebo analgesia, thus, suggesting that at least two different biochemical pathways could be involved: endogenous opioids and endocannabinoids (04).

The advent of neuroimaging techniques and their use for experimental purposes added anatomic and temporal details to the neurochemical information (52). The first positron emission tomography study to investigate placebo analgesia was conducted in 2002 (44). It showed overlapping in the brain activation pattern generated by opioid-induced analgesia (by the μ-agonist remifentanil) and by placebo-induced analgesia. Common activated areas included the rostral anterior cingulate cortex and the orbitofrontal cortex. In the following years, despite some discrepancies likely explained by methodological and procedural differences, PET, fMRI, and MEG studies all suggested placebo activation of the descending pain control system, with modulation of activity in areas such as the dorsolateral prefrontal cortex, ACC, orbitofrontal cortex, periaqueductal gray, hypothalamus, central nucleus of the amygdala, parabrachial nuclei, and ventromedial medulla. Notably, a direct demonstration of endogenous opioid release was obtained through [11C] carfentanil displacement by the activation of opioid neurotransmission, with the decrease in binding correlating with placebo reduction of pain intensity reports (53). Moreover, naloxone was observed to block placebo-induced responses in pain-modulatory cortical structures and in key structures of the descending pain control system (23).

Further clarification of the functional neuroanatomy of placebo analgesia comes from a more recent meta-analysis to assess whether the neurologic pain signature, which represents the brain regions involved in pain processing, is affected specifically by placebos (54). In this analysis, Medline (PubMed) was searched from inception to May 2015 and involved studies of functional neuroimaging of the human brain with evoked pain delivered under stimulus intensity-matched placebo and control conditions. Data were obtained from 20 of 28 identified eligible studies, resulting in a total sample size of 603 healthy individuals. The neurologic pain signature responses to painful stimulation compared with baseline conditions were positive in 575 participants (95.4%). Placebo treatments showed significant behavioral outcomes on pain ratings in 17 of 20 studies (85%) and also moderate analgesic effects on pain reports compared with matched control conditions. However, placebo effects on the neurologic pain signature response were significant in only 3 of 20 studies (15%) and were small. Similarly, analyses restricted to studies with a low risk of bias indicated small effects, as did analyses of just placebo responders. These findings show that placebo treatments have moderate analgesic effects on pain reports and that the small effects on neurologic pain signature are probably attributable to the fact that placebos affect pain via brain mechanisms largely independent of effects on bottom-up nociceptive processing. In other words, placebos may act on regions other than the neurologic pain signature.

The fact that knowledge of placebo analgesia can be gained by focusing on changes in brain activity that occur with modulation of expectation alone is also of interest. In fact, expectation of benefit can induce a placebo effect even without the physical administration of a placebo. Because no placebo is actually given, these effects may be more appropriately called “placebo-like” effects. Thus, activity in pain areas following a constant painful stimulus can be modulated, at least in part, just by varying the subject’s expectation of the level of stimulation: the higher the expected level of the stimulus, the stronger the activity in ACC and other areas implicated in the activation of the descending inhibitory pathway. Taken together, these studies show how the same result (ie, the activation of the same receptors in the brain) can be obtained by a pharmacologic (drug) or a psychologic (placebo) means. In his book, Benedetti comprehensively described the studies mentioned here (04).

Compared to placebo effect research, the investigation of the nocebo effect raises more ethical difficulties, especially in the clinical setting. However, a few experimental studies have begun to shed light on this phenomenon, focusing mainly on the model of nocebo hyperalgesia (03; 04; 11; 18). In the protocols used, an inert treatment is given along with verbal suggestions of pain worsening, resulting in exacerbation of pain. It has been suggested that the anticipatory anxiety about the impending pain, brought about by negative expectations, triggers the activation of CCK, which in turn facilitates pain transmission and results in hyperalgesia. Accordingly, this hyperalgesia can be blocked by proglumide, a nonspecific CCK-1 and CCK-2 antagonist, in a dose-dependent manner. The proglumide block is related specifically to nocebo- or anxiety-induced hyperalgesia rather than to the more general process of nocebo-induced anxiety, as it is selectively exerted on nocebo hyperalgesia but not on the concurrent stress-induced hypothalamic-pituitary-adrenal axis hyperactivity.

Proglumide also exhibits placebo-potentiating effects, raising the question of how the two endogenous systems, CCK and opioids, may interact to produce negative or positive outcomes. It can be speculated that the placebo or nocebo phenomenon is a continuum, with opioid and CCK-ergic systems acting as the mediators of opposing effects. More recent experimental evidence also suggests that nocebo modulation of hypobaric hypoxia headache involves the cyclooxygenase pathway as well (10).

As for placebo analgesia, neuroimaging techniques have also brought important contributions to the knowledge of nocebo hyperalgesia. Here again, expectations without the physical administration of a nocebo treatment have been exploited (“nocebo-like” effects). Inducing negative expectations resulted in both amplified unpleasantness of innocuous stimuli as assessed by psychophysical pain measures (subject report) and increased fMRI responses in ACC, insula, hypothalamus, secondary somatosensory areas, and prefrontal cortex. From these studies, it appears that the circuitry underlying nocebo hyperalgesia largely involves, with opposite modulation, the same areas engaged by placebo analgesia (04).

The effects of nocebo on the spinal cord have also been found for complex psychological factors, such as value information about a drug, eg, the price tag. Tinnermann and colleagues discovered that labeling an inert treatment as an expensive medication led to stronger nocebo hyperalgesia than labeling it as a cheap medication (51). This effect was mediated by the activity in the prefrontal cortex and coupling between prefrontal areas, brainstem, and spinal cord. This might represent a mechanism through which higher cognitive representations, such as value, can modulate early pain processing.

Parkinson disease

As occurs for pain, also in motor disorders, such as Parkinson disease, a placebo procedure can be followed by the release of endogenous substances. In particular, the high placebo response rate in Parkinson disease clinical trials provided the impetus for investigating the underlying mechanisms.

In patients affected by Parkinson disease, the placebo procedure consists of administering an inert substance (placebo) along with the information that it is an anti-parkinsonian drug that produces an improvement in motor performance. In 2001, the first brain imaging study of the placebo effect using PET was conducted (22). In this study, patients were aware that they would receive an injection of either active drug (apomorphine, a dopamine receptor agonist) or placebo (an inert substance that the patient believed to be apomorphine), according to classic clinical trials methodology. The authors assessed the competition between endogenous dopamine and [11C]-raclopride for D2/D3 receptors, a method that allows identification of endogenous dopamine release. After placebo administration, a release of dopamine was found in the striatum (both dorsal and ventral), corresponding to a change of 200% or more in extracellular dopamine concentration. However, although dopamine release in the dorsal striatum was greater in those patients who reported clinical improvement, its release in the ventral striatum (nucleus accumbens) was associated with patient expectation of improvement in symptoms (22; 21).

The placebo response in Parkinson disease is associated with changes in the activity of neurons in the subthalamic nucleus, substantia nigra pars reticulata, and motor thalamus (09; 13; 28). Indeed, the possibility of recording from single neurons during the implantation of electrodes for deep brain stimulation makes Parkinson disease an excellent model for investigating the placebo mechanisms occurring when patients expect a therapeutic benefit. In particular, placebo administration in patients with Parkinson disease affects the activity of the neurons in the subthalamic nucleus, a brain region belonging to the basal ganglia circuitry and whose activity is increased in Parkinson disease (12; 29).

Depression and social anxiety

Both EEG and metabolic changes have been observed in the brain of depressed patients who receive a placebo treatment. EEG changes were found in the prefrontal cortex of patients with major depression (37; 39). As to the changes in brain glucose metabolism, they were documented by using PET in subjects with unipolar depression who were treated for 6 weeks with either placebo or the selective serotonin reuptake inhibitor fluoxetine (43). The authors showed that both placebo and fluoxetine treatment induced metabolic increases in the prefrontal, anterior cingulate, premotor, parietal, and posterior cingulate cortices, and posterior insula, together with metabolic decreases in the subgenual cingulate, thalamus, and parahippocampus. The magnitude of regional fluoxetine changes was generally greater than those induced by placebo. However, fluoxetine responses were associated with additional changes in the brainstem, striatum, anterior insula, and hippocampus. Therefore, because the brain changes associated with the placebo response most closely match those of the SSRI fluoxetine, a possible role for serotonin in placebo-induced antidepressant effects is suggested. Interestingly, ventral striatal (nucleus accumbens) and orbital frontal changes were found in both placebo and drug responders well before clinical benefit, namely at 1 week of treatment. These changes are, thus, associated with expectation and anticipation of the clinical benefit rather than with clinical response.

In social anxiety disorder, PET imaging has been used to assess regional cerebral blood flow during an anxiogenic public speaking task, before and after 6 to 8 weeks of treatment with selective serotonin reuptake inhibitors under double-blind conditions (26; 25). The authors found that both SSRIs and placebo responders show a common attenuation of regional cerebral blood flow in the basolateral and ventrolateral amygdala and common projections to the dorsolateral prefrontal cortex and rostral anterior cingulate cortex. This pattern correlates with behavioral measures of reduced anxiety and indicates that in this specific clinical condition, drugs and placebos act on common amygdala targets and amygdala-frontal connections.

Immune and endocrine responses

The mechanisms of the placebo effect in the immune and endocrine systems are related to conditioning (49; 50; 33). If the active drug is replaced by a placebo after being administered repeatedly, the placebo can evoke immune or hormonal responses comparable to those obtained by the previously administered drug. For example, immunosuppressive placebo responses can be induced in humans by repeated administration of cyclosporine A (unconditioned stimulus) associated with a flavored drink (conditioned stimulus), as assessed by interleukin-2 and interferon-gamma mRNA expression, in vitro release of interleukin-2 and interferon-gamma, and lymphocyte proliferation (31). Furthermore, if a placebo is given after repeated administrations of sumatriptan, a serotonin agonist of the 5-HT1B/1D receptors that stimulates growth hormone and inhibits cortisol (glucocorticoids) secretion, a placebo growth hormone increase and a placebo cortisol decrease can be found (15). These hormonal and immune responses represent the best examples of unconscious placebo effects, that is, placebo effects that occur without a conscious cognitive process. Conditioning of the immune response in association with immunomodulatory or chemotherapy may allow dose extension or dose-skipping strategies that would save cost and side effects (32).

Genetics

Although genetic studies of placebo are still at the beginning, different polymorphisms have been associated with low or high placebo responsiveness, and the analysis of genetic variants has been centered around different systems, such as the dopamine, opioid, serotonin, and endocannabinoid systems (35; 20). For example, patients affected by social anxiety disorder were genotyped for the serotonin transporter-linked polymorphic region (5-HTTLPR) and the G-703T polymorphism in the tryptophan hydroxylase-2 gene promoter (30). The fMRI analysis showed that both robust placebo responses and reduced activity in the amygdala only occurred in those patients who were homozygous for the long allele of the 5-HTTLPR or the G variant of the TPH2 G-703T polymorphism. In another study, catabolic enzymes catechol-O-methyltransferase (COMT) and monoamine oxidase A polymorphisms were examined in patients affected by major depressive disorder (38). Small placebo responses were found in those patients with monoamine oxidase A polymorphisms coding for the highest activity form of the enzyme and in those patients with the COMT polymorphisms coding for a lower-activity form of the enzyme. Interestingly, COMT val158met polymorphisms have also been associated with placebo responsiveness in irritable bowel syndrome (34).

Disruption of placebo effects

Hidden administration of therapies has provided compelling evidence that expectation is a key element in the therapeutic outcome (19; 08). If the patient is unaware that a treatment is being performed and has no expectations about any clinical improvement, the therapy is less effective. For example, the effectiveness of the benzodiazepine diazepam is reduced or completely abolished when it is administered unbeknownst to the patient (19; 08).

The same effects are present in other conditions, such as pain and Parkinson disease (19; 08). In postoperative pain following the extraction of the third molar (42; 40), a hidden intravenous injection of 6 to 8 mg morphine corresponds to an open intravenous injection of saline solution in full view of the patient (placebo). In other words, telling the patient that a painkiller is being injected (with what is actually a saline solution) is as potent as 6 to 8 mg of morphine. This holds true for a variety of painkillers, such as morphine, buprenorphine, tramadol, ketorolac, metamizole, and remifentanil (01; 19; 16).

A natural situation in which hidden therapies are delivered is represented by impaired cognition. Cognitively impaired patients do not have expectations about therapeutic benefits, so the psychological (placebo) component of a treatment is likely to be absent. Based on these considerations, Benedetti and colleagues studied patients with Alzheimer disease at the initial stage of the disease and after 1 year to see whether the placebo component of the therapy was affected by the disease (05). The placebo component of an analgesic therapy was correlated with both cognitive status and functional connectivity among different brain regions according to the rule that “the more impaired the prefrontal connectivity, the smaller the placebo response” (05). To support this view, several studies indicate that placebo responses are reduced when prefrontal functioning is impaired. First, the individual placebo analgesic effect is correlated with white matter integrity indexed by fractional anisotropy, as assessed through diffusion tensor magnetic resonance imaging (DT-MRI); stronger placebo analgesic responses are associated with increased mean fractional anisotropy values within white matter tracts connecting the periaqueductal gray with the rostral anterior cingulate cortex and the dorsolateral prefrontal cortex (48). Second, inactivation of the frontal cortex with repetitive transcranial magnetic stimulation completely blocks the analgesic placebo response (36). Third, the opioid antagonist naloxone blocks placebo analgesia, along with a reduction in the activation of the dorsolateral prefrontal cortex, suggesting that a prefrontal opioidergic mechanism is crucial in the placebo analgesic response (23). Therefore, both magnetic and pharmacological inactivation of the prefrontal lobes have the same effects as those observed in prefrontal degeneration in Alzheimer disease and reduced integrity of prefrontal white matter.

Conclusions and future directions

We strongly believe that placebo research needs to be considered a priority in biomedical research, both basic and clinical. The research work should aim to provide a mechanistic explanation for where, when, and how placebos and nocebos work and to understand to what extent psychosocial factors can challenge biology. This goal, which requires mapping placebo and nocebo responses across many conditions, from neurology to psychiatry and many other disciplines, is certainly challenging and will require intensive work in the coming years.

Besides the understanding of placebo and nocebo effects as psychologic and neuroscientific phenomena, the substantial neurobiological advances in this research area are essential for disentangling drug effects from placebo effects. This knowledge may allow clinicians to develop new strategies to better design and interpret clinical trials and to maximize neurotherapeutic outcomes in routine medical practice. Accordingly, we endorse (1) measuring patients’ treatment expectations from baseline to the end of a clinical trial to identify those participants susceptible to a placebo effect and (2) identifying biomarkers of placebo responses, such as genetic polymorphisms and neuroimaging, to improve the interpretability of clinical trials by enabling adjusted effect sizes or stratification of the study population. It should finally be emphasized that all the approaches presented in this perspective article must take place within the context of a patient-provider relationship, whereby the relational aspect of the placebo effect represents one of the most important elements in routine medical practice.