Effects of hindlimb unloading and reloading on c-fos expression of spinal cord evoked by vibration of rat Achille...

To determine whether the neuronal activity of the spindle cord in muscle spindle afferent pathways is altered after a period of hindlimb unloading (Hu), and after recovery, we focused on c-Fos-immunoreactivity in the spinal cord evoked by excitation of the muscle spindle in normal gravity and after 3, 7, and 14 days of Hu, and after 14 days of Hu with an additional 2, 5, and 9 days of unrestricted cage activity. High frequency sinusoidal vibration (HFV) applied to the Achilles tendon was used for activation of the muscle spindle. Results showed that c-Fos-immunoreactive neurons evoked by HFV were mainly concentrated in lamina IV-VII of the ipsilateral spinal cord. After 14 days of Hu, the total number of labeled neurons in the spinal cord was significantly increased (P<0.05). The number of c-Fos-immunoreactive neurons in lamina IV and VI-VII were also significantly higher than that in normal rats (P<0.05). After 9 days of reloading, the total number of c-Fos-immunoreactive neurons and the pattern of their lamellar distribution were both recovered to pre-experimental levels. Our findings suggest that simulated weightlessness induces a large increase in neuronal excitation of the spinal cord in muscle spindle afferent pathways. Weight bearing immediately after Hu could induce completed recovery of excitability of the spinal cord in muscle spindle afferent pathways.