Abstract
Seeing another person in pain can cause one to experience pain-like sensation, referred to as empathy for pain. It has been suggested that the pain matrix, including the anterior insula and anterior cingulate cortex, is activated by observing another person in pain, as well as physical pain stimuli. Although drugs acting on the central nervous system have the potential to modulate the empathy, most studies have assessed the effect of drugs on empathy by means of subjective self-ratings. Tramadol is an analgesic drug used to treat acute or chronic pain by affecting the central nervous system through μ-opioid receptor activation and monoamine transporter inhibition. Here, we investigated how tramadol mediates the brain activity associated with empathy for pain using functional magnetic resonance imaging (fMRI).
A double-blind, placebo-controlled, randomized cross-over trial was conducted for healthy adults under placebo or tramadol treatment. Blood oxygenation level-dependent (BOLD) signals in fMRI were examined while performing a task for empathic pain. Participants were shown a series of images of hands and feet in situations that seemed painful and a matched set of control images without any painful events. During the MRI scanning, the participants were asked to rate how much pain the persons in the pictures of the tasks felt and how unpleasant the participants felt about the pictures. For analysis of the empathic pain-related brain activity, contrasts were created by subtracting the non-pain conditions from the pain conditions.
The BOLD signal in the left insula was decreased under tramadol treatment compared to that under placebo treatment. There was no significant difference in own unpleasant feeling between tramadol and placebo treatments, while there was a tendency to reduce the rating of others' pain intensity under the tramadol treatment. Therefore, it is suggested that tramadol modulates empathic pain as well as physically-evoked pain. This effect might contribute to pain-relieving action of tramadol by alleviating anxiety caused by excessive empathy for others' pain.
