Chronic opioid antagonist treatment dose-dependently regulates mu-opioid receptors and trafficking proteins in vivo.

Chronic opioid antagonist treatment increases the density of mu-opioid receptors (muOR) in many model systems. In previous studies, naltrexone treatment produced an increase in muOR density accompanied by decreases in GRK-2 and DYN-2 protein abundance. To examine the relationship between changes in receptor density and proteins involved in receptor trafficking, the dose-dependent effect of chronic naloxone infusion was determined. Dose-dependent antagonism of morphine analgesia was also examined. Mice were infused with naloxone (0.1, 1.0, 5.0 mg/kg/day sc) for 7 days via osmotic pump. Controls were treated with placebo pellets. On the 7th day, morphine dose-response studies were determined using the tail flick. Other mice were sacrificed at the end of the treatment and spinal cords were collected for determination of muOR density and GRK-2 and DYN-2 protein abundance. Naloxone infusion dose-dependently increased spinal muOR density with no change in affinity. The increases in mu-receptor density were proportional to dose-dependent decreases in GRK-2 and DYN-2 protein levels. Furthermore, naloxone dose-dependently antagonized morphine. These data suggest that opioid antagonist-induced muOR up-regulation in mouse spinal cord is associated with regulation of proteins involved in receptor trafficking and support suggestions that opioid antagonist-induced receptor up-regulation is due to reduced constitutive internalization of opioid receptors.