Continuous morphine produces more tolerance than intermittent or acute treatment.

Dosing protocol and analgesic efficacy have been proposed to be important determinants of the magnitude of opioid tolerance. The present study examined the effect of acute, intermittent and continuous treatment with the low analgesic efficacy agonist morphine on analgesic tolerance. Mice were implanted s.c. with a 25 mg morphine pellet for 1-7 days. Other mice were implanted s.c. with two 25 mg, or one 75 mg morphine pellet for 7 days. The release of morphine from subcutaneous implanted pellets was quantitated using a spectrophotometric assay. In other studies, mice were injected with morphine once (18.5-185 mg/kg/day; approximately 10-100 times ED(50) for morphine analgesia) or once/day for 7 days. Controls were implanted with a placebo pellet or injected with saline. Analysis of drug release from a 25 mg pellet indicated that release was greatest during the first 24 h, declined and then remained relatively constant. The amount of morphine released over 7 days by a 75 mg pellet (23.9 mg) was more than that of a single 25 mg pellet (15.4 mg) but less than two 25 mg pellets (30.8 mg). Following treatment, morphine cumulative dose-response studies were conducted (tail flick). Continuous treatment with morphine using pellet implantation produced a dose-dependent shift in the morphine ED(50) by 3.3, 5.8 and 8.5 fold for one 25 mg pellet, one 75 mg pellet and two 25 mg pellets, respectively. Acute and intermittent morphine administration produced substantially less analgesic tolerance than continuous release of morphine by implant pellets. The maximum shift in the ED(50) was 1.6 for acute treatment and 2.7 for 7 day intermittent treatment; despite a larger total daily dose. The present results indicate that continuous treatment with morphine results in greater analgesic tolerance than acute or intermittent morphine treatment even at comparable daily doses. These results are consistent with the suggestion that intermittent dosing has reduced risk of producing opioid tolerance.