The role of opioid systems on orthodontic tooth movement in cholestatic rats

Endogenous opioids have been reported to accumulate in the plasma of cholestatic subjects. Another report showed that human osteoblast-like cells, MG-63, express 3 types of opioid receptors. In our laboratory we noticed that orthodontic tooth movement (OTM) is enhanced in cholestatic rats. Therefore, we suggest a possible role of opioid systems in bone remodeling and raising the rate of OTM in cholestatic conditions. To investigate this hypothesis, rat models were established and divided into 5 study groups. An orthodontic appliance, consisting of a 5 mm nickel-titanium closed coil spring, was ligated between the maxillary right incisor and first molar of each rat to deliver an initial force of 60 g. The bile duct ligated (BDL) group underwent a bile duct ligation operation and received orthodontic appliance 7 days after surgery. Another group underwent a sham operation and orthodontic appliances were inserted just as in the BDL group protocol. Surgery was performed the BDL + naltrexone group and orthodontic appliances were inserted 7 days after surgery. This group received daily subcutaneous injections of naltrexone HCI (an opioid antagonist) at 20 mg/kg at 24-hour intervals from the day of force application until the end of the study period. Another group, the naltrexone group, received naltrexone injections like the BDL + naltrexone group. A fifth control group neither underwent surgery nor received injections. Orthodontic tooth movement was measured 14 days after appliance insertion. The bile duct ligated group showed significantly increased OTM compared to all other study groups (P < .001). The difference between the OTM in the BDL + naltrexone and control groups was insignificant. This study suggests a role for opioid systems in OTM in cholestasis conditions.