Mechanical loading of a long bone induces plasticity in sensory input to the central nervous system

Although the skeleton is extensively innervated by sensory nerves, the importance of this innervation to skeletal physiology is unclear. Neuronal connectivity between limbs is little studied and likely underestimated. In this study, we examined the effect of bone loading on spinal plasticity in young male Sprague-Dawley rats, using end-loading of the ulna and transynaptic tracing with the Bartha pseudorabies virus (PRV). PRV was inoculated onto the periosteum of the right ulna after 10 days of adaptation to a single period of cyclic loading of the right ulna (1,500 cycles of load at 4 Hz, initial peak strain of -3,750 micro epsilon). We found that neuronal circuits connect the sensory innervation of right thoracic limb to all other limbs, as PRV was detectable in the dorsal root ganglia (DRG) of left and right brachial and lumbosacral intumescences. We also found that mechanical loading of the right ulna induced plasticity in the spinal cord, with significant augmentation of the connectivity between limbs, as measured by PRV translocation. Within the spinal cord, PRV was predominantly found adjacent to the central canal and in the dorsal horns, suggesting that plasticity in cross-talk between limbs is likely a consequence of dendritic growth, and enhanced connectivity of propriospinal interneurons. In conclusion, the data clearly demonstrate that the innervation of the skeleton exhibits plasticity in response to loading events, suggesting the existence of a dynamic control system that may be of regulatory importance during functional skeletal adaptation.