gms | German Medical Science

Objectives: Previously, we reported impaired biomechanical bone properties and inferior bone matrix quality in tachykinin1 (Tac1)-deficient mice lacking the sensory neuropeptide substance P (SP). Additionally, fracture callus development is affected by the absence of SP indicating a critical effect of sensory nerve fibers on bone health and regeneration. For α-calcitonin gene-related peptide (α-CGRP)-deficient mice, a profound distortion of bone microarchitecture has also been described. We hypothesize that SP and α-CGRP modulate inflammatory as well as pain-related processes and positively affect bone regeneration during impaired fracture healing under osteoporotic conditions. Therefore, this study investigates the effects of SP and α-CGRP on fracture healing and fracture-related pain processes under conditions of experimental osteoporosis using SP- and α-CGRP-deficient mice and WT controls.

Methods: We ovariectomized female WT, Tac1-/- and α-CGRP-/- mice (age 10 weeks; background C57Bl/6J) and set intramedullary fixed femoral fractures in the left femora 28 days later. We analyzed pain threshold (Dynamic Plantar Aesthesiometer Test) and locomotion (recorded at day and night, each for 1 h; EthoVision®XT/Noldus) at 5, 9, 13, 16 and 21 days after fracture. At each time point, fractured femora were prepared for histochemical analysis of callus tissue composition (alcian blue/sirius red staining).

Results and conclusion: Pain threshold is significantly higher in Tac1-/- mice 13 days after fracture and tends to be higher after 21 days compared to WT controls. In contrast, touch sensibility was similar in α-CGRP-/- mice and WT controls but compared to Tac1-/- mice pain threshold was significantly lower in α-CGRP-/- mice 13 and 16 days and tends to be lower 21 days after fracture. Locomotion of Tac1-/- mice during daylight was by trend higher 9 days after fracture and significantly higher 16 days after fracture whereas nightly locomotion is reduced compared to WT mice. Analysis of locomotion during daylight or night revealed no differences between α-CGRP-/- and WT mice. During early fracture healing phase, 5 and 9 days after fracture, transition of mesenchymal to cartilaginous callus tissue tends to be faster in Tac1-/- mice compared to WT controls whereas no difference was observed during late stage of fracture healing, 13, 16 and 21 days after fracture. In contrast, callus tissue maturation seems to be similar in α-CGRP-/- and WT mice.

Our data indicate different effects of SP and α-CGRP on fracture healing under conditions of experimental osteoporosis as a model for impaired bone tissue. Lack of α-CGRP seems to have minor effects, but loss of SP affects locomotion throughout osteoporotic fracture healing and fracture-related pain processes during late phases of osteoporotic fracture healing. This indicates a modified role of SP during fracture healing under impaired versus healthy conditions, where SP changed early fracture-related pain processes and had no influence on callus tissue composition.