The effect of dopamine on conditioned placebo analgesia in healthy individuals: a double-blind randomized trial

The Optimal Learning Cocktail for Placebo Analgesia: A Randomized Controlled Trial Comparing Individual and Combined Techniques

This study investigated for the first time the effects of individual and combined application of 3 learning techniques (verbal suggestions, classical conditioning, and observational learning) on placebo analgesia and extinction. Healthy participants (N = 206) were assigned to 8 different groups in which they were taught through either a verbal suggestion, a conditioning paradigm, a video observing someone, or any combination thereof that a placebo device (inactive transcutaneous electric nerve stimulation [TENS]) was capable of alleviating heat pain, whereas one group did not (control). Placebo analgesia was quantified as the within-group difference in experienced pain when the placebo device was (sham) 'activated' or 'inactivated' during equal pain stimuli, and compared between groups. Placebo analgesia was induced in groups with 2 or 3 learning techniques. Significantly stronger placebo analgesia was induced in the combination of all 3 learning techniques as compared to the individual learning techniques or control condition, underlining the additional contribution of 3 combined techniques. Extinction did not differ between groups. Furthermore, pain expectancies, but not state anxiety or trust, mediated placebo analgesia. Our findings emphasize the added value of combining 3 learning techniques to optimally shape expectancies that lead to placebo analgesia, which can be used in experimental and clinical settings. PERSPECTIVE: This unique experimental study compared the individual versus combined effects of 3 important ways of learning (verbal suggestions, classical conditioning, and observational learning) on expectation-based pain relief. The findings indicate that placebo effects occurring in clinical practice could be optimally strengthened if healthcare providers apply these techniques in combination.

Placebo effects on cutaneous pain and itch: a systematic review and meta-analysis of experimental results and methodology

ABSTRACT

Placebo effects, positive treatment outcomes that go beyond treatment processes, can alter sensations through learning mechanisms. Understanding how methodological factors contribute to the magnitude of placebo effects will help define the mechanisms by which these effects occur. We conducted a systematic review and meta-analysis of experimental placebo studies in cutaneous pain and itch in healthy samples, focused on how differences in methodology contribute to the resulting placebo effect magnitude. We conducted meta-analyses by learning mechanism and sensation, namely, for classical conditioning with verbal suggestion, verbal suggestion alone, and observational learning, separately for pain and itch. We conducted subgroup analyses and meta-regression on the type of sensory stimuli, placebo treatment, number of acquisition and evocation trials, differences in calibrated intensities for placebo and control stimuli during acquisition, age, and sex. We replicated findings showing that a combination of classical conditioning with verbal suggestion induced larger placebo effects on pain ( k = 68, g = 0 . 59) than verbal suggestion alone ( k = 39, g = 0.38) and found a smaller effect for itch with verbal suggestion alone ( k = 7, g = 0.14). Using sham electrodes as placebo treatments corresponded with larger placebo effects on pain than when topical gels were used. Other methodological and demographic factors did not significantly affect placebo magnitudes. Placebo effects on pain and itch reliably occur in experimental settings with varied methods, and conditioning with verbal suggestion produced the strongest effects. Although methods may shape the placebo effect to some extent, these effects appear robust overall, and their underlying learning mechanisms may be harnessed for applications outside the laboratory.

[Placebo effects in analgesia : Influence of expectations on the efficacy and tolerability of analgesic treatment]

Expectations of patients influence the perception and neuronal processing of acute and chronic pain and modulate the effectiveness of analgesic treatment. The expectation of treatment is not only the most important determinant of placebo analgesia. Expectations of treatment also influence the efficacy and tolerability of "active" pharmacological and non-pharmacological treatment of pain. Recent insights into the psychological and neurobiological mechanisms underlying the clinically relevant effects of treatment expectations enable and call for the systematic integration and modulation of treatment expectations into analgesic treatment concepts. Such a strategy promises to optimize analgesic treatment and to prevent or reduce the burden of unwanted side effects and the misuse of analgesics, particularly of opioids. This review highlights the current concepts, recent achievements and also challenges and key open research questions.

Acquisition learning is stronger for aversive than appetitive events

Appetitive and aversive learning are both key building blocks of adaptive behavior, yet knowledge regarding their differences is sparse. Using a capsaicin heat pain model in 36 healthy participants, this study directly compared the acquisition and extinction of conditioned stimuli (CS) predicting pain exacerbation and relief. Valence ratings show stronger acquisition during aversive compared to appetitive learning, but no differences in extinction. Skin conductance responses and contingency ratings confirmed these results. Findings were unrelated to individual differences in pain sensitivity or psychological factors. Our results support the notion of an evolutionarily hardwired preponderance to acquire aversive rather than appetitive cues as is protective for acute aversive states such as pain but may contribute to the development and maintenance of clinical conditions such as chronic pain, depression or anxiety disorders.

In this study, acquisition and extinction of conditioned stimuli are compared in a capsaicin heat pain paradigm in healthy human participants. The results demonstrate a preponderance to acquire aversive rather than appetitive cues, which could initially have a protective function but eventually contribute to the maintenance of chronic pain.

Orofacial Strength and Voice Quality as Outcome of Levodopa Challenge Test in Parkinson Disease

OBJECTIVE

To assess the usefulness of orofacial strength and voice quality as assessment of response to levodopa challenge test (LCT) used in the diagnosis of early idiopathic Parkinson disease (IPD).

STUDY DESIGN

Controlled Prospective Study.

METHODS

From January 2014 to April 2019, patients with early IPD and healthy individuals were recruited and evaluated for clinical findings (Hoehn and Yahr scale; Unified Parkinson's Disease Rating Scale); Voice Handicap Index (VHI); grade of dysphonia, roughness, breathiness, asthenia, and strain and instability (GRBASI); maximal phonation time; phonation quotient; acoustic parameters; and orofacial muscle strength Oral Performance Instrument (IOPI; IOPI Medical, Woodinville, WA, USA) t) at baseline and 45 minutes after the levodopa intake (LCT).

RESULTS

A total of 32 IPD patients and 20 healthy individuals completed the study. Healthy individuals exhibited better VHI, grade of dysphonia, breathiness, asthenia, strain, instability, and acoustic measurements (noise-related, tremor, F0 short- and mid-term and intensity short-term parameters) than healthy subjects. The mean values of muscle strength of lips, cheeks, fundamental frequency (F0), highest F0, and shimmer significantly improved from pre- to post-LCT in IPD patients. Healthy individuals did not exhibit significant changes of orofacial strength and voice quality assessment from pre- to post-LCT. Significant associations were found between clinical, orofacial strength, and some aerodynamic and acoustic measurements.

CONCLUSION

Orofacial strength and acoustic voice quality measurements may be used as objective outcomes of the LCT responsiveness in patients with early IPD.

LEVEL OF EVIDENCE

3A. Laryngoscope, 2020.

Effects of Oxytocin on Placebo and Nocebo Effects in a Pain Conditioning Paradigm: A Randomized Controlled Trial

Oxytocin has been shown to increase trust, decrease anxiety, and affect learning as has been observed in conditioning paradigms. Trust, anxiety, and learning are important factors that influence placebo effects. In this study, we investigated whether oxytocin can increase placebo analgesia, decrease nocebo hyperalgesia, and influence extinction processes of both. Eighty male volunteers were assigned to a 40 IU of oxytocin nasal spray group, or to a placebo control group. Placebo analgesia and nocebo hyperalgesia were induced by a conditioning procedure in combination with verbal suggestions. The results demonstrate that the conditioning procedure successfully elicited significant placebo analgesia and nocebo hyperalgesia responses (P < .001). Furthermore, extinction was observed (P < .001), although placebo and nocebo responses did not return to baseline and remained significant. Oxytocin did not influence placebo analgesia or nocebo hyperalgesia and had no effect on extinction. This study provides support against the placebo-boosting effects of oxytocin and was the first one to demonstrate that it also did not influence nocebo effects or extinction processes, however, these results pertain to only a male sample. As managing placebo and nocebo effects has widespread clinical implications, further research should investigate other neurobiological or behavioral pathways to boost placebo and decrease nocebo effects. PERSPECTIVE: The present study demonstrated that placebo analgesia and nocebo hyperalgesia can be successfully induced by conditioning and verbal suggestions. We could not confirm the hypothesis that oxytocin affects either of these phenomena. Other pharmacological agents and behavioral manipulations for increasing placebo and decreasing nocebo effects should be investigated.

Mesocorticolimbic Pathways Encode Cue-Based Expectancy Effects on Pain

Expectation interacting with nociceptive input can shape the perception of pain. It has been suggested that reward-related expectations are associated with the activation of the ventral tegmental area (VTA), which projects to the striatum (e.g., nucleus accumbens [NAc]) and prefrontal cortex (e.g., rostral anterior cingulate cortex [rACC]). However, the role of these projection pathways in encoding expectancy effects on pain remains unclear. In this study, we leveraged a visual cue conditioning paradigm with a long pain anticipation period and collected magnetic resonance imaging (MRI) data from 30 healthy human subjects (14 females). At the within-subject level, whole-brain functional connectivity (FC) analyses showed that the mesocortical pathway (VTA-rACC FC) and the mesolimbic pathway (VTA-NAc FC) were enhanced with positive expectation but inhibited with negative expectation during pain anticipation period. Mediation analyses revealed that cue-based expectancy effects on pain were mainly mediated by the VTA-NAc FC, and structural equation modeling showed that VTA-based FC influenced pain perception by modulating pain-evoked brain responses. At the between-subject level, multivariate pattern analyses demonstrated that gray matter volumes in the VTA, NAc, and rACC were able to predict the magnitudes of conditioned pain responses associated with positive and/or negative expectations across subjects. Our results therefore advance the current understanding of how the reward system is linked to the interaction between expectation and pain. Furthermore, they provide precise functional and structural information on mesocorticolimibic pathways that encode within-subject and between-subject variability of expectancy effects on pain.SIGNIFICANCE STATEMENT Studies have suggested that reward-related expectation is associated with the activation of the VTA, which projects to the striatum and prefrontal cortex. However, the role of these projection pathways in encoding expectancy effects on pain remains unclear. Using multimodality MRI and a visual cue conditioning paradigm, we found that the functional connectivity and gray matter volumes in key regions (the VTA, NAc, and rostral ACC) within the mesocorticolimbic pathways encoded expectancy effects on pain. Our results advance the current understanding of how the reward system is linked to the interaction between expectation and pain, and provide precise functional and structural information on mesocorticolimbic pathways that encode expectancy effects on pain.