Background:  Long-term sequential expression of receptor activator of NF-κB ligand (RANKL) and osteoprotegrin (OPG) in lipopolysaccharide (LPS)-induced rat periapical lesions has not been studied.

Materials:  Seventy-two 4-week-old Wistar rats were divided into eight experimental groups and one control group (eight animals in each).

Methods:  Lipopolysaccharide-induced periapical lesions were produced in rats by occlusal exposure of the pulp of their lower first molars in all experimental groups but not the control group. The extent of periapical destruction was measured by radiographic imaging. RANKL and OPG mRNA were measured in all tissue sections containing the periapical lesions as well as the control group every week from week 1 to week 8 by real-time quantitative reverse transcription polymerase chain reaction. RANKL and OPG protein were determined by immunohistochemistry. Osteoclasts were identified by enzyme histochemistry.

Results:  The sequential changes in the mRNA and protein expression of RANKL and OPG were largely compatible with the occurrence of osteoclasts histologically and enzymes histochemically, as well as the mean areas of the periapical lesions radiographically during long-term observation of the LPS-induced rat periapical lesions.

Conclusion:  This study may be the first to demonstrate the long-term RANKL and OPG expression every week from week 1 to week 8 using LPS to produce periapical infection in a Wistar rat model. The long-term findings of high expressions of RANKL and OPG further extend the potential application of the Wistar rat model for future experimental trials using RANKL inhibitor to evaluate the treatment outcome for LPS-induced rat periapical lesions.