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.
