The role of opioid receptor density in morphine tolerance

The effect of chronic opioid antagonist-induced functional supersensitivity and receptor upregulation on morphine tolerance was examined in Swiss Webster mice obtained from Taconic Farms and Charles River Laboratories. Mice were implanted s.c. with either naltrexone (NTX) or placebo pellets for 8 days. After 8 days, the pellets were removed, and 24 h later mice were either injected with morphine (50 or 100 mg/kg) or saline (acute tolerance protocol) or implanted with a 75-mg morphine or placebo pellet for 72 h (chronic tolerance protocol). Mice were tested for morphine analgesia 24 h after injections or 72 h after pellet implantations. Mice from Taconic Farms were more sensitive to morphine analgesia than Charles River mice, although the degree of tolerance in both strains was similar. Acute morphine produced a 1.7- and 1.9-fold increase in the ED50 for morphine analgesia in NTX and placebo pretreated mice, respectively. the chronic tolerance protocol produced the same shift (2.4-fold) in the ED50 in both NTX and placebo pretreated mice. In both tolerance protocols, NTX-pretreated mice were significantly more sensitive to the analgesic effects of morphine than placebo pretreated controls. Binding studies ([3H][D-Ala,2NMePhe4,Gly-ol5]enkephalin) indicated an approximately 40% increase in opioid receptor density with no significant alteration in affinity after chronic NTX treatment. These results indicate that acute and chronic tolerance to morphine develops comparably in control and upregulated, supersensitive mice. These findings suggest that new binding sites in upregulated mice mediate tolerance similarly to existing binding sites and that the degree of tolerance is unrelated to the density of opioid receptors.