Background Osteoporosis may cause not only fractures but also chronic back pain in elderly women. Iwamoto et al reported that alendronate (ALN) increases bone mineral density of the lumbar spine and improves chronic back pain, with suppression of bone resorption in postmenopausal women with osteoporosis1). However, there have been no reports regarding the effect of ALN on pain in animal models of osteoporosis.
Objectives In this study, we investigated the effect of ALN on pain-related behavior and markers in ovariectomized mice.
Methods 8-week-old female ddY mice were ovariectomized (OVX) and assigned to 3groups; SHAM-operated mice treated with vehicle (SHAM), OVX mice treated with vehicle (OVX-V), OVX mice treated with ALN (OVX-ALN). Starting immediately after surgery, vehicle or 40μg/kg ALN was injected subcutaneously twice a week for 4 weeks. The bilateral proximal tibial metaphyses were analyzed three-dimensionally by μCT 4 weeks after surgery (each group; 16 hindlimbs). Mechanical sensitivity was tested using von Frey filaments 4 weeks after surgery. The frequency of the withdrawal response and the withdrawal threshold to the application of von Frey filaments to the planter surface of the hindpaws was examined. To evaluate the frequency, three von Frey filaments with forces of 0.4, 0.6 and 1.4 g were applied 10 times each. Results were expressed as the percent response frequency of paw withdrawals. To evaluate the threshold, each von Frey filament was applied once, starting with 0.008g and increasing until a response was reached, which was considered a positive response. The lowest force producing a response was considered the threshold. TRPV1 and CGRP expression, which were neuropeptide markers of pain, in L5 DRG neurons were examined 4 weeks after surgery using immunohistochemistry (each group; 16 DRGs). The ratio of TRPV1-, CGRP-immunoreactive (ir) cells to total DRG neurons was counted and averaged for each DRG.
Results μCT analysis of the proximal tibial metaphysis (Fig. 1) showed that bone volume/tissue volume (BV/TV) and trabecular number (Tb.N) were significantly less in the OVX-V group than in the SHAM group, whereas trabecular separation (Tb.Sp) was significantly greater in the OVX-V group than in the SHAM group. In the OVX-ALN group, BV/TV and Tb.N were significantly greater than in the OVX-V group, whereas Tb.Sp was significantly less than in the OVX-V group. The paw-withdrawal-frequency stimulated by von Frey filaments with strength of 0.4, 0.6 and 1.4 g was significantly higher in the OVX group than in the SHAM group, whereas it was significantly lower in the OVX-ALN group than in the OVX-V group. The withdrawal threshold was significantly lower in the OVX group than in the SHAM group, whereas it was significantly higher in the OVX-ALN group than in the OVX-V group (Fig. 2). Immunohistochemical analysis showed that the ratio of TRPV1-, CGRP-ir DRG neurons in the OVX-V group was significantly higher than in the SHAM group, whereas it was significantly lower in the OVX-ALN group than in the OVX-V group (Fig. 3).
Conclusions ALN prevented ovariectomy-induced bone loss and mechanical hyperalgesia in hindlimbs, and it suppressed TRPV1 and CGRP expression in DRG. The results suggest that bone resorption with upregulation of TRPV1 and CGRP expression is one of the causes of postmenopausal osteoporotic pain.
Iwamoto J et al. Clin Rheumatol. 2004.
Disclosure of Interest None declared