Can tolerance to morphine be induced in arthritic rats?

The effects of acute injections of morphine (0.1-1 mg/kg i.v.) upon vocalization threshold elicited by pressure of the paw were analyzed in normal and arthritic rats, both groups being initially pretreated with calculated doses of morphine. The analgesic effects of morphine were greatly reduced in both groups of chronically morphine-treated rats. The lowest doses (0.1 mg/kg in normal rats; 0.1 and 0.3 mg/kg in arthritic animals) became totally ineffective, while the highest dose (1 mg/kg) elicited a threshold increase only equivalent to that induced by 0.3 mg/kg in non-tolerant chronically vehicle-treated rats. Tolerance to morphine can be induced in rats suffering from arthritis, and appeared to be more complete than in normal rats.