Background Osteoporotic patients with no evidence of fractures sometimes experience vague low back pain. However, there have been few reports regarding the correlation between osteoporosis and pain-related behavior.
Objectives The objective of the current study was to investigate pain-related behavior in osteoporotic mouse.
Methods Female ddY mice (8-week-old) were either ovariectomized bilaterally under anesthesia with pentobarbital sodium intraperitoneally (OVX group; n=6). In sham-operated mice, the ovaries were exteriorized but not removed (SHAM group; n=7). Mechanical sensitivity was tested using the von-Frey filaments 4 weeks after surgery. The frequency of withdrawal response and the withdrawal threshold to the application of von-Frey filaments to the planter surface of hindpaws was examined. The animals were placed into a plastic box over a wire mesh floor and allowed to acclimate for at least 30 min before beginning experiments. To evaluate the frequency of withdrawal response, three von Frey filaments with forces of 0.4, 0.6, 1.0 g were applied 10 times each in ascending order of force and the number and intensity of withdrawal responses noted. Results were expressed as the percent response frequency of paw withdrawals. To evaluate the withdrawal threshold, each von Frey filament was applied once, starting with 0.008g and increasing until a withdrawal response was reached which was considered a positive response. The lowest force producing a response was considered as the withdrawal threshold. The filaments were applied for 3s with an inter stimulus interval of 5s. At the end of the experiment, the mice were killed with pentobarbital sodium, and the right and left femora were removed. The femoral distal metaphysis was analyzed three-dimensionally by use of micro-computed tomography (μCT).
Results The paw-withdrawal-frequency stimulated by von-Frey filaments with strength of 0.4, 0.6, 1.0 g was significantly increased in OVX group as compared with SHAM group respectively (Fig. 1). The withdrawal threshold tended to be lower in OVX group as compared with SHAM group, but there was no significant difference between each group (data not shown). μCT analysis of femoral distal metaphysis showed that bone volume/tissue volume (BV/TV) (Fig. 2) and trabecular thickness (Tb.Th) (data not shown) and trabecular number (Tb.N) (data not shown) in OVX group was significantly less than in SHAM group, whereas trabecular separation (Tb.Sp) (data not shown) in OVX group was significantly greater than in SHAM group.
Conclusions In this study, ovariectomy induced mechanical hyperalgesia and bone loss in hindlimb. The results suggest that hyperalgesic state is one of the causes of postmenoposal osteoporotic pain.
Disclosure of Interest None Declared