Effects of an opioid antagonist on pain intensity and withdrawal reflexes during induction of hypnotic analgesia in high- and low-hypnotizable volunteers

Abstract

The aim of this investigation was to study the effect of suggestions of hypnotic analgesia on spinal pain transmission and processing. Pain intensity and amplitude of nociceptive withdrawal reflexes to electrical stimuli were measured in 10 high- and 10 low-hypnotizable subjects during two sessions taking place at least 24 h apart under five conditions of. (1) pre-hypnosis; (2) neutral hypnotic relaxation; (3) suggestions of hypnotic analgesia; (4) suggestions of hypnotic analgesia after injections of either naloxone (1 ml, 1 mg/ml) or saline (1 ml) under double-blinded conditions; and (5) post-hypnosis. The conditions of naloxone or saline were allocated at random to either Day 1 or Day 2 in a double-blinded fashion. Results showed significant reductions of pain intensity during hypnotic analgesia, and a significant reduction in nociceptive reflexes during hypnotic analgesia on Day 1 in the highly hypnotizable group. No differences were found for low-hypnotizable subjects. The results support previous findings that pain intensity as well as the nociceptive reflex can be modulated by suggestions of hypnotic analgesia. While no effect of naloxone on pain intensity was found during hypnotic analgesia, naloxone significantly reversed the suppressive effect of suggestions of hypnotic analgesia on reflexes in high-hypnotizable subjects. Subsequent analysis showed that the effect of naloxone was associated with the intensity of the stimulus needed to elicit a reflex, and was unrelated to hypnotic susceptibility when controlling for stimulus intensity. These results suggest that the effect of naloxone was related to the greater stimulus intensities needed to elicit a reflex in the high-hypnotizable group, rather than to hypnosis or hypnotic susceptibility in itself. It is unclear why greater stimulus intensities were needed in high-hypnotizable subjects and further studies are needed.