Paradoxical hyperalgesic effect of exceedingly low doses of systemic morphine in an animal model of persistent pain (Freund's adjuvant-induced arthritic rats)

The effects of exceedingly low doses of morphine (3-50 micrograms/kg i.v.) were studied upon the vocalization threshold induced by paw pressure in rats with Freund's adjuvant-induced arthritis. The highest dose used (50 micrograms/kg i.v.) clearly induced an analgesic effect. No significant modification of the vocalization threshold was observed with 30 micrograms/kg. By contrast, a significant hyperalgesic effect resulted with doses of 10 down to 3 micrograms/kg. Maximum hyperalgesia was observed with 6 micrograms/kg.