l-Dopa effect on frequency-dependent depression of the H-reflex in adult rats with complete spinal cord transection

This study investigated whether l-dopa (DOPA), locomotor-like passive exercise (Ex) using a motorized bicycle exercise trainer (MBET), or their combination in adult rats with complete spinal cord transection (Tx) preserves and restores low frequency-dependent depression (FDD) of the H-reflex. Adult Sprague-Dawley rats (n = 56) transected at T8-9 had one of five treatments beginning 7 days after transection: Tx (transection only), Tx + Ex, Tx + DOPA, Tx + Ex + DOPA, and control (Ctl, no treatment) groups. After 30 days of treatment, FDD of the H-reflex was tested. Stimulation of the tibial nerve at 0.2, 1, 5, and 10 Hz evoked an H-reflex that was recorded from plantar muscles of the hind paw. No significant differences were found at the stimulation rate of 1 Hz. However, at 5 Hz, FDD of the H-reflex in the Tx + Ex, Tx + DOPA and Ctl groups was significantly different from the Tx group (p < 0.01). At 10 Hz, all of the treatment groups were significantly different from the Tx group (p < 0.01). No significant difference was identified between the Ctl and any of the treatment groups. These results suggest that DOPA significantly preserved and restored FDD after transection as effectively as exercise alone or exercise in combination with DOPA. Thus, there was no additive benefit when DOPA was combined with exercise.