LAUSR

EditordThe endogenous opioid system is involved in modulation of both pain and pleasure. The opioid system also contributes to mood regulation. In short-term use, exogenous opioids have been suggested to increase positive mood and reduce negative feelings such as fear and anger. The effects of opioids on emotional responses to external stimuli are not fully understood. Opioids have been shown to reduce perception of anger but leave perception of happiness in facial expressions unaffected, or to have no effects on responses to emotional stimuli. We examined the effects of remifentanil and naloxone on subjective emotional responses evoked by pleasant and unpleasant film clips. Our hypothesis was that opioid administration either dampens the emotional responses in general, or shifts them towards pleasure, whereas opioid antagonism diminishes the perceived valence of the stimuli. Male volunteers aged 20e35 yr (n1⁄431) completed a set of subjective emotional rating questionnaires and then received three i.v. infusions in the same order: remifentanil, placebo, and naloxone. The participants were blinded to the sequence of the infusions. Remifentanil was administered at an effectsite concentration of 1 ng ml 1 using a target-controlled infusion pump. The dose was chosen from a previous study that showed an analgesic effect of this dose without excessive sedation or desaturation. Naloxone was administered as a 5 mg kg 1 i.v. bolus over 2 min followed by an infusion at 40 mg kg 1 h , providing an estimated naloxone plasma concentration of 10 ngml . A saline bolus identical in volume to the naloxone bolus was administered during the 2 min preceding the remifentanil and saline infusions. Each infusion lasted 23 min and was followed by a 10-min washout period. During each infusion, participants saw 10 film clips in a random order and rated their experience of emotional valence (pleasure) and arousal. The stimulus material consisted of 30 brief film clips with varying affective valence (strong pleasureestrong displeasure) selected from a validated