Effects of the synthetic enkephalin analogue FK 33-824 on pain threshold and pain tolerance in man.

Natural enkephalins exert weak and transitory analgesic effects. The synthetic enkephalin, FK 33-824 (FK), is less susceptible to metabolic breakdown and produces long-lasting analgesia in animals. The present studies examined the effects of FK on threshold and tolerance of electrically evoked pain in man under double blind conditions. 1.0 mg FK given intramuscularly (saline control) increased tolerance significantly without affecting the pain threshold, but also produced vasodilatation and feelings of oppression and heaviness (study I). In study II, where 50 mg betazole was employed as "placebo" because of its vasodilatatory effects, 1.0 mg FK increased pain tolerance significantly more than 0.25 mg FK while the threshold remained unchanged. Self-ratings of activation and well-being decreased; those of oppression increased, as did reaction time, equally after 0.25 and 1.0 mg FK but were not altered by betazole. In conclusion, 1.0 mg FK i.m. increases tolerance but not perception of pain, thus mimicking the analgesic effects of morphine.