Effects Of Glucocorticoids On Bone Mass, Bone Microarchitecture And The Expression Of Cannabinoid Receptor 1 In Rats

Objective:To investigate the effects of glucocorticoids(GC)on bone mass,microarchitecture of trabeculae and the expression of cannabinoid receptor 1(CB1R)in bone tissues of rats model of glucocorticoid-induced osteoporosis(GIOP)in order to explore the mechanisms of GIOP.Methods:Fifty two 3.5-month-old female SD rats were randomly divided into baseline group,GC-treated group(GCT)and control group. The rats in GCT group were given a daily subcutaneous injection with methylprednisolone at dosage of 3.5mg/kg and the rats in control group with physiological saline of the same volume in the same way.The rats were killed at the beginning of the experiment in baseline group,at the first week and ninth week post-experiment in GCT and control groups respectively.Whole body bone mineral density(BMD)in vivo,the BMD of femur and femoral regions of interest(FROI)were all measured by dual energy X-ray absorptiometry(DXA)using QDR-4500A equipment. A high resolution micro-CT was used to identify the densimetric and microarchitectural properties of trabeculae in the proximal metaphysis of the right tibiae and in the sixth lumbar vertebrae in vitro. Immunohistochemistry were used to detect the expression of CB1R in the bone samples of left tibiae and the fifth lumbar vertebrae in all rats. Results:①Both at the first and ninth week after administration of GC,there was a decrease in whole body BMD in vivo,the femoral BMD in vitro and FROI(at the first week post-experiment,except FROI-1), compared with the control rats at each time-point,but whole body BMD in vivo,the femoral BMD in vitro and FROI-5 in GCT group at the ninth week post-experiment was significantly lower than that in the control group(P＜0.05).At the ninth week post-experiment,the bone loss rate was listed in order in GCT group as compared with the control group: FROI-2(-11.43%)＞FROI-5(-9.83%)＞FROI-7(-9.21%)＞FROI-1(-8.71%)＞femoral BMD in vitro(-8.33%)＞FROI-3(-8.12%)＞FROI-4(-6.10%)＞whole body BMD in vivo(-5.77%)＞FROI-6 (-5.18%).Whole body BMD in vivo,the BMD of femur and femoral regions of interest(FROI)increased significantly in the control group with aging,but there was no significant change in GCT group.②Compared with the control rats at each time-point,there was a decrease in the volumetric BMD(vBMD),tissue BMD(tBMD),bone volume fraction (BVF),trabecular number(Tb.N),degree of anisotropy(DA)and trabecular connectivity(Conn.D)in the trabecular bone of tibia of the GCT rats,the decrease was more in the ninth GCT rats(P＜0.05).In addition,there was an increase in the trabecular thickness(Tb.Th), trabecular separation(Tb.Sp)and structure model index(SMI)in GCT rats.A time-related analysis within the GCT groups showed there was a declination in BVF,Conn.D,Tb.N and DA with administration time,but Tb.Th and Tb.Sp were increased significantly(P＜0.05).③CB1R was expressed in osteoblasts,osteoclasts and bone marrow stromal cells in the left proximal tibiae as well as the fifth lumbar vertebrae of all rats. Compared with control group,the expression of CB1R in osteoblasts and osteoclasts was increased significantly in GCT group at the ninth week(P＜0.01).④The expressional ratio of CB1R in osteoblasts and osteoclasts of left tibiae and the fifth lumbar vertebrae was significantly correlated with partially microarchitectural parameters of trabeculae of right tibiae and the sixth lumbar vertebrae(P＜0.05～0.01).Conclusion:①Rats model of GIOP can be successfully reproduced by subcutaneous injection with methylprednisolone at dosage of 3.5mg/kg per day for nine weeks.②The administration of GC is associated with a decrease in BMD and disturbance in mineralization and deterioration in microarchitecture of trabecular bone in rats,and the regions of interest which are rich in trabecular bone are affected most markedly,at the same time,the remaining trabeculae seem thicken to increase their strength as compensation.③That GC can upregulate the expression of CB1R protein in osteoblasts and osteoclasts may be one of the molecular mechanisms of GIOP.④The other pathway by which CB1R regulates bone metabolism may exist except central regulation.