Vasopressin and Pain Perception in the Brain

The purpose of this research is to investigate how vasopressin (AVP) influences the neural mechanisms associated with observationally-induced analgesia. More specifically, the investigators will determine how observing another person experiencing analgesia shapes subsequent behavioral and neural responses to painful stimulations.

Aim 1: To investigate how AVP influences the neural mechanisms associated with observationally-induced analgesia.

Hypothesis 1: The investigators expect that AVP will boost activation in brain regions involved in social cognition.

The overall objective is to determine the effect of AVP on brain mechanisms associated with observationally-induced analgesia. This experiment has a between-subject study design. Participants will be randomized into an AVP group and a control group. The experimenter as well as the participant will be blinded regarding the allocation of the participant. Participants in the AVP group will receive intranasal AVP and the participants in the control group will receive intranasal saline before the fMRI experiment. Participants have 50% chance of being placed in either group. The experiment in the fMRI scanner is divided into two phases: an observational phase, in which participants will observe a video of a demonstrator experiencing analgesia, and a testing phase, in which study participants will receive heat pain to investigate how pain perception was influenced. During both phases there will be a placebo condition (pain with the expectation of having received a treatment) and a control condition (pain without the expectation of having received a treatment). All participants will complete both phases, including the observational and testing phase.

Expectancy of analgesia related to the act of receiving a treatment (e.g. a painkiller) can reduce pain perception, even if this treatment is in fact an inert substance (e.g. placebo). Expectancies about analgesic treatments can be acquired in several different ways, including direct experience of analgesia (i.e. the patient learns that after taking a specific drug, pain will decrease), verbal instructions (i.e. the doctor tells the patient that a specific drug will reduce pain) or social observation (i.e. the patient observes pain relief in another patient after this patient took a specific drug).

The direct experience of pain relief in the context of treatment cues (i.e. conditioning) reduces subsequent pain perception when the same treatment cues are present. The neural underpinnings of how treatment expectancies are acquired during conditioning have been investigated previously. These studies indicate that the prefrontal cortex is involved in learning treatment expectations in the context of conditioned analgesic effects.

However, these expectancies can also be acquired by observing others. Our group was the first to demonstrate that analgesia can be triggered by observing another person that experiences analgesia. This finding has been corroborated with additional studies by our group and other groups. In these experiments, participants acquire expectancies of analgesia not by experiencing pain relief themselves, but rather by observing another person (the demonstrator) experiencing analgesia after receiving a certain analgesic treatment (actually a sham treatment). These placebo manipulations generate expectancies which lead to placebo effects of similar sizes that those shown through direct experience via conditioning paradigms.

Even though previous research has focused on the direct experience of pain relief within a treatment context, social psychology suggests that most human behaviors are in fact modulated by sociality and learned by observing others. The investigators recently established a paradigm to investigate neural mechanisms of observational learning in placebo analgesia. The preliminary data analysis (unpublished data) suggests that while participants are observing someone else experiencing less pain due to an analgesic treatment, brain regions associated with mentalizing processes such as left and right temporoparietal junction (TPJ) and medial prefrontal cortex (mPFC) show increased activation. However, the underlying neurotransmitter systems are unknown. Here, the investigators aim to investigate how AVP modulates this network of brain regions. AVP is a likely candidate system, because recently it has been associated with placebo analgesia by my current mentor's lab, is involved in controlling a wide variety of social behaviors, and has been shown to critically modulate TPJ activity. The investigators expect that AVP will lead to increased activation of the mentalizing network during observational learning, and therefore, to increased placebo analgesia as a result of the observational learning. In order to investigate this, the investigators will perform a similar observational learning study in a group receiving intranasal AVP and a control group receiving intranasal saline.

Background: Clinical outcomes are not just related to pharmacological substances, but also to the context in which a treatment is given as well as expectancies, fears, desires and beliefs of the patient. The beneficial effects on health related outcome changes due to the treatment context and not due to specific actions of a drug are known as placebo effects. In the field of pain, the reduction of pain perception due to placebo effects is called placebo analgesia or expectancy-induced analgesia.

Previous research shows that in placebo analgesia, informational cues of the treatment context generate the expectancy of pain relief due to a treatment. These expectations can be acquired through several ways, including learning through direct experience (i.e. conditioning), verbal instruction or observation of others.

Several studies investigated the influence of direct experience of analgesia using conditioning paradigms on placebo effects. These studies show that conditioning creates more robust placebo effects than verbal suggestions alone and that the magnitude of experienced pain relief and the duration influence subsequent placebo effects.

On the neural level, placebo effects on pain perception are mediated by the descending pain modulatory system. Endogenous opioids are involved in the pain descending modulation systems, and placebo analgesia can be substantially reduced by opioid antagonists. Several studies implicate functional connectivity between the rostral anterior cingulate and the periaqueductal gray, a region critical for descending pain modulation, and placebo analgesia. Additionally, there is considerable evidence that prefrontal regions, especially the dorsolateral prefrontal cortex (DLPFC), are critically involved in placebo analgesia. The prefrontal cortex consistently shows higher activations related to the anticipation of analgesia and experience of pain relief induced by a placebo manipulation, and is involved in the acquisition of expectancies during conditioning of placebo analgesia.

Therefore the current understanding is that the prefrontal cortex maintains and updates expectancies regarding pain, and that these prefrontal regions influence the experience of pain by activating the descending pain modulatory system.

The influence of social learning on placebo analgesia has been investigated to a lesser degree. Recent research suggests that placebo analgesia can also be induced by observational learning, however, the neural neurotransmitter systems underpinning observationally-induced placebo analgesia have not yet been investigated.

Rationale: To harness the placebo effects in clinical contexts, it is important to understand how placebo effects arise and are maintained. Previous neuroscience research has primarily focused on conditioning paradigms, however human behaviors are affected by social learning. Social learning refers to learning about the environment due to information gained by observing others. Our overall hypothesis is that AVP will increase neural activation in regions related to social cognition during observational learning. Our objective is to determine the interplay of AVP and observationally-induced analgesia using an fMRI approach. fMRI is a noninvasive technique to measure changes in blood oxygenation in the brain enabling us to draw inferences about the localization and extend of neural activation associated with specific events and cues including the perception of experimental painful stimulations and modulation. The investigators designed an experiment to be performed behaviorally and with fMRI measurements in order to determine the interplay of AVP and observationally-induced analgesia.

Significance: This experiment will advance our understanding of endogenous processes associated with observationally-induced analgesia and the factors that influences pain processing. A better understanding of how treatment expectancies, formed through social observation, influence the individual experience of pain is significant in several ways. First, it will allow a better understanding of the contextual factors shaping pain and responses to treatments in clinical settings. Second, knowledge about the neural processes associated with endogenous pain relief might lead to novel developments in pain therapeutic strategies. Third, the investigators anticipate generating findings that will advance our knowledge of how cognitive processes (i.e. expectancy) are represented in the brain and how these factors influence human social behaviors.