Steroid-associated osteonecrosis animal model in rats

Zheng, Li-Zhen; Wang, Jia-Li; Kong, Ling; Huang, Le; Tian, Li; Pang, Qian-Qian; Wang, Xin-Luan; Qin, Ling

Successfully established SAON rabbit model has been reported, with a clinically orientated induction protocol composed of a single injection of low dose of lipopolysaccharide (LPS) and subsequently three injections of high dose of methylprednisolone (MPS) with high incidence of SAON and low incidence of mortality [3] . For studying molecular mechanism of such metabolic diseases and drug prevention or treatment effects, rats are more suitable as they have been regarded as cost-effective animal models. So far, there are several reports on SAON rat models, but the protocols to induce SAON are different [4e8] . LPS could increase the incidence of SAON in animal models [5] . The dose of LPS in the published protocols ranged from 10 mg/kg to 2 mg/kg to induce SAON in rats [8, 9] . The dose of LPS used in the rabbit SAON model is 10 mg/kg [3] , but because rats are resistant to LPS, whereas rabbits are sensitive to LPS [10] , we should use higher dose of LPS to induce SAON in the rat model than that used in the rabbit model. A preliminary study showed that the injection of 1 mg/kg LPS could induce 50% mortality in 24-week-old male SpragueeDawley rats [11] ; therefore, in this study, we tested the dose of LPS at 0.2 mg/kg that might reduce the mortality. In a clinical situation, high dose of pulsed MPS intravascularly (30 mg/kg) is prescribed to severe inflammation patients to rescue their life [12] . As smaller animals have higher metabolic rates and require larger drug dose per body weight, we calculated dose conversion between rat and human being by multiple clinical higher dose based on a published work [13] . To mimic the clinical situation, we tested the experimental protocol to induce SAON in rats with a single injection of high dose of LPS (0.2 mg/kg) and followed subsequently by pulsed injections of high dose of MPS (100 mg/kg). As SAON may get repaired spontaneously in quadrupedal animals, it is essential to inhibit repairing of the osteonecrosis. Accordingly, we continually gave rats sustained low dose of MPS (40 mg/kg) from week 2 to week 6 before harvesting specimens for analysis. For determining the age of rats used in this study, unlike humans and rabbits, it is problematic to determine adulthood using skeletal maturity because there is no closed growth plate in long bones in rats. For SpragueeDawley rats, the skeletal growth tapers off at approximately 7e8 months [14] ; therefore, we used 24week-old young adult rats for the present study as this was the age where rat femoral head matured [15] .