Altered bone development in a mouse model of peripheral sensory nerve inactivation

Abstract

Objectives: The present study sought to determine the effects of decreased peripheral sensory nerve function on skeletal development and bone metabolism in mice.

Methods: C57BL/6 neonatal mice were treated with capsaicin to induce peripheral sensory nerve degeneration, and compared to vehicle-treated controls at 4, 8 and 12 weeks of age. Changes in bone structure were assessed using micro-computed tomography, mechanical properties and fracture resistance were assessed using three-point bending of radii, and bone turnover was assessed using dynamic histomorphometry and serum biomarkers.

Results: Capsaicin treatment resulted in small but significant decreases in bone structure, particularly affecting trabecular bone. Capsaicin-treated mice exhibited lower trabecular thickness at the femoral metaphysis and L5 vertebral body compared with vehicle-treated mice. However, capsaicin- and vehicle-treated mice had similar mechanical properties and bone turnover rates.

Conclusion: Neonatal capsaicin treatment affected trabecular bone during development; however these small changes may not be meaningful with respect to bone strength under normal loading conditions. It is possible that capsaicin-sensitive neurons may be more important for bone under stress conditions such as increased mechanical loading or injury. Future studies will investigate this potential role of peripheral sensory nerves in bone adaptation.

Fig. 1

Body weights of treated and untreated mice were recorded from weaning until 12 weeks of age. Female mice treated with capsaicin had lower body weight than vehicle-treated female mice from weaning until day 47. Body weights of male mice were unaffected at all time points. * Capsaicin vs. vehicle; p < 0.05.

Fig. 2

Hot-plate analgesia testing of treated and untreated mice was performed to verify decreased peripheral sensory nerve function. Treatment and age were main effects for latency time (p < 0.0001, p = 0.0052). Capsaicin-treated mice had significantly longer latency times than vehicle-treated mice at all time points when exposed to a constant 55° C thermal stimulus. * Capsaicin vs. vehicle; p < 0.05.

Fig. 3

Micro-computed tomography of femoral cortical and trabecular bone. (A) Femur length and total cross-sectional area (Tt.Ar) of the femoral diaphysis were significantly affected in capsaicin-treated female mice. (B) Capsaicin treatment did not significantly affect trabecular bone volume fraction (BV/TV) at the femoral metaphysis but reduced trabecular thickness (Tb.Th) at this location in female mice. (C) Capsaicin treatment significantly affected BV/TV at the femoral epiphysis in female mice, but did not alter Tb.Th at this location. * Capsaicin vs. vehicle; p < 0.05.

Fig. 4

Mechanical properties of radii determined using three-point bending. A micro-computed tomography reconstruction of a representative radius (A) showing the mechanical testing setup. At 12 weeks of age, the moment of inertia (I) (B) and yield force (C) were significantly lower in capsaicin-treated male mice. However, modulus of elasticity (D) was not significantly different between treated and untreated mice. Age was a main effect for I, yield force and modulus. * Capsaicin vs. vehicle; p < 0.05.

Fig. 5

Dynamic histomorphometry was used to identify changes in bone formation rates in capsaicin- and vehicle-treated mice. Fluorescent images (A) show cortical bone from the tibias of 8 week old female vehicle- and capsaicin-treated mice. Mineral apposition rate (MAR), percent mineralizing surface (MS/BS), and bone formation rate (BFR/BS) were quantified for the endosteal (B) and periosteal surfaces (C). Age was a significant main effect for MAR, MS/BS and BFR at the endosteal and periosteal surfaces. * Capsaicin vs. vehicle; p < 0.05.

Fig. 6

Capsaicin treatment did not significantly affect serum concentrations of CTX-I or P1NP, which were measured using ELISAs. Sex was a main effect for serum concentrations of CTX-I, while P1NP concentrations did not vary significantly by sex or age. * Capsaicin vs. vehicle; p < 0.05.