No evidence for opioid-mediated analgesia induced by phobic fear

The influence of anxiety on acute pain sensation was investigated, studying the relative contribution of endogenous opioids and attentional mechanisms. Thirty-six spider phobics received mildly painful electrical stimulation, while anxiety and focus of attention were manipulated within subjects. The opioid antagonist naloxone or placebo was administered between subjects to examine an analgesia owing to anxiety-induced endorphinergic activity. In contrast to earlier findings, attention towards pain failed to increase pain as opposed to distraction from pain, probably owing to a less effective attention manipulation. Furthermore, despite high levels of anxiety, subjective pain ratings were not influenced by anxiety, although heart rate responses were slightly inhibited. Accordingly, there was no increase in subjective or physiological pain responses as a result of naloxone, nor did beta-endorphin plasma levels rise during anxiety. The results suggest that phobic anxiety does not induce an opioid-mediated analgesia. Curiously, naloxone itself effected a dose-dependent analgesia compared to placebo during both high and low anxiety, which is compatible with the assumption of agonist properties of naloxone in the absence of opioid activity.