Reduced gravitational loading does not account for the skeletal effect of botulinum toxin-induced muscle inhibition suggesting a direct effect of muscle on bone

Abstract

Intramuscular injection of botulinum toxin (botox) into rodent hindlimbs has developed as a useful model for exploring muscle-bone interactions. Botox-induced muscle inhibition rapidly induces muscle atrophy and subsequent bone loss, with the latter hypothesized to result from reduced muscular loading of the skeleton. However, botox-induced muscle inhibition also reduces gravitational loading (as evident by reduced ground reaction forces during gait) which may account for its negative skeletal effects. The aim of this study was to investigate the skeletal effect of botox-induced muscle inhibition in cage control and tail suspended mice, with tail suspension being used to control for the reduced gravitational loading associated with botox. Female C57BL/ 6J mice were injected unilaterally with botox and contralaterally with vehicle, and subsequently exposed to tail suspension or normal cage activities for 6 weeks. Botox-induced muscle inhibition combined with tail suspension had the largest detrimental effect on the skeleton, causing the least gains in midshaft tibial bone mass, cortical area and cortical thickness, greatest gains in midshaft tibial medullary area, and lowest proximal tibial trabecular bone volume fraction. These data indicate botox-induced muscle inhibition has skeletal effects over and above any effect it has in altering gravitational loading, suggesting that muscle has a direct effect on bone. This effect may be relevant in the development of strategies targeting musculoskeletal health.

Mechanical loading effect to the functional bone adaptation

“…In a study conducted with forty virgin female mice up to 14 weeks of age which of each animal was injected with Botox in the right posterior leg and with vehicle in the left posterior leg, after muscle inactivation, animals were randomly separated into two groups: 1) cage control and 2) tail suspended for six weeks and it was found that the skeletal effects of Botox-provoked muscle inactivation are not only due to the diminution of gravitational forces 20 . The retraction of gravitational forces of the posterior legs through tail suspension caused a loss of bone mass as a result of declined periosteal bone formation and increased endosteal bone resorption 20 .…”

“…The retraction of gravitational forces of the posterior legs through tail suspension caused a loss of bone mass as a result of declined periosteal bone formation and increased endosteal bone resorption 20 . The increase in endosteal bone resorption was noticeable by a significant increase in medullary area and a decline in cortical area and cortical thickness 20 .…”

“…The retraction of gravitational forces of the posterior legs through tail suspension caused a loss of bone mass as a result of declined periosteal bone formation and increased endosteal bone resorption 20 . The increase in endosteal bone resorption was noticeable by a significant increase in medullary area and a decline in cortical area and cortical thickness 20 . Botox-provoked muscle inactivation on tail suspension worsened these skeletal modifications with both of retraction of gravitational forces and muscle inactivation had the biggest harmful influence on the skeleton, caused the least benefits in midshaft tibial bone mass, cortical area and cortical thickness, greatest benefits in midshaft tibial medullary area and lowest proximal tibial trabecular bone volume fraction 20 .…”

“…Botox-provoked muscle inactivation on tail suspension worsened these skeletal modifications with both of retraction of gravitational forces and muscle inactivation had the biggest harmful influence on the skeleton, caused the least benefits in midshaft tibial bone mass, cortical area and cortical thickness, greatest benefits in midshaft tibial medullary area and lowest proximal tibial trabecular bone volume fraction 20 . These data demonstrate Botox-provoked muscle inactivation causes skeletal reactions further those associated with removed gravitational forces 20 . These results show that muscle has a direct influence on bone.…”

Human Musculoskeletal Models of Disuse

“…BOTOX®), which is an extremely powerful agent to block neuromuscular transmission. They found decreases in bone strength and mass in mice 23 . Basically the studies suggested that BOTOX® had a deleterious effect in animal bones' especially trabecular bone 24 .…”