Introduction
pain
Facial expressions of pain play an important part in social interactions as well as in clinical settings (Craig et al. In contrast, there is a substantial lack of knowledge with regard to neurotransmitter systems being involved in facial expressions of pain. An interesting candidate in this context might be the serotonergic system. First of all, serotonin (5-HT) plays an important role in nociceptive processing; among others, via descending inhibitory serotonergic projections from the nucleus raphe magnus to the spinal cord (Basbaum & Fields Second, besides its role in pain processing, 5-HT also seems to affect behavioral inhibition and impulse control (Cools et al. Taken together, 5-HT has the potential to modulate facial expressions of pain at least via two mechanisms, namely via changes in nociceptive processing as well as via changes in behavioral inhibition. The aim of the present study was to use ATD to investigate the role of 5-HT in facial expressions of pain in response to experimental pain. We hypothesized that ATD and thus, low levels of central 5-HT, will result in increased facial expressions of pain because of both a lowering of nociceptive inhibition and a reduction of behavioral inhibition. For control, ATD related changes in pain ratings and in affective states were also assessed.