FORMALIN-INDUCED PAIN ANTAGONIZES THE DEVELOPMENT OF OPIATE DEPENDENCE IN THE RAT

Clinical studies have suggested that patients who take morphine for pa in relief do not show a high degree of dependence. The present study e xamined the development of naloxone-precipitated withdrawal in rats re ceiving morphine in the presence or absence of formalin-induced pain. Morphine (10 mg/kg, i.p.) or saline was administered for 4 consecutive days 10 min after a subcutaneous injection of 50 mul of 2.5% formalin or saline into the hind-paw. On the 5th day, rats were injected with naloxone (1 mg/kg, i.p.) and observed for signs of precipitated withdr awal (ptosis, teeth chattering and excretion/diarrhea). Naloxone-preci pitated withdrawal symptoms were significantly greater in rats that re ceived morphine in the absence of pain than in rats that received morp hine in the presence of pain.