BDNF-exercise interactions in the recovery of symmetrical stepping after a cervical hemisection in rats

Abstract

Clinical evidence indicates that motor training facilitates functional recovery after a spinal cord injury (SCI). Brain-derived neurotrophic factor (BDNF) is a powerful synaptic facilitator and likely plays a key role in motor and sensory functions. Spinal cord hemisection decreases the levels of BDNF below the injury site, and exercise can counteract this decrease [Ying Z, Roy RR, Edgerton VR, Gomez-Pinilla F (2005) Exercise restores levels of neurotrophins and synaptic plasticity following spinal cord injury. Exp Neurol 193:411-419]. It is not clear, however, whether the exercise-induced increases in BDNF play a role in mediating the recovery of locomotion after a SCI. We performed a lateral cervical ( approximately C4) hemisection in adult rats. Seven days after hemisection, the BDNF inhibitor trkB IgG was injected into the cervical spinal cord below the lesion ( approximately C5-C6). Half of the rats were exposed to voluntary running wheels for 14 days. Locomotor ability was assessed by determining the symmetry between the contralateral (unaffected) vs. the ipsilateral (affected) forelimb at the most optimum treadmill speed for each rat. Sedentary and exercised rats with BDNF inhibition showed a higher level of asymmetry during the treadmill locomotion test than rats not treated with the BDNF inhibitor. In hemisected rats, exercise normalized the levels of molecules important for synaptic function, such as cyclic AMP response element binding protein (CREB) and synapsin I, in the ipsilateral cervical enlargement, whereas the BDNF blocker lessened these exercise-associated effects. The results indicate that BDNF levels play an important role in shaping the synaptic plasticity and in defining the level of recovery of locomotor performance after a SCI.

Figure 1

Examples of the trajectory of the ipsilateral (right) and contralateral (left) forelimbs during stepping on a treadmill at the maximum speed for a representative rat in each group are shown. Note that the highest level of asymmetry is present in the rats in the two groups receiving the BDNF blocker. Stick figures of the swing phase of the shoulder (S), elbow (E) and wrist (W) of the left and right forelimb for one-step cycle for one rat from each group is illustrated in the middle columns of the figure. The step cycle represented in the stick figure from the multiple steps illustrated as wrist trajectories is marked with a bold line. The displacement values represent absolute positions in space, but the multiple trajectories were transposed so that the beginning of the stance phase was positioned consistently relative to the other step cycles. Therefore, the absolute vertical distance of the trajectory only approximates the relative distance of the wrist to the surface of the treadmill belt. Group abbreviations are the same as in Fig. 1.

Figure 2

The measures of locomotor performance are shown for each hemisected group: (A) maximum speed of treadmill locomotion; (B) ratio of percent stance per step cycle; (C) ratio of oscillating steps (weight-bearing plus non-weight bearing steps); (D) ratio of weight-bearing steps; (E) proportion of weight-bearing steps on the hemisected side (right side); and (F) the distance that the wrist was extended beyond the vertical line through the shoulder during the swing phase of the step cycle for both the right and left forelimbs. All ratios are expressed as right limb (R, ipsilateral to the hemisection)/left limb (L, contralateral to the hemisection). Sed, sedentary; Sal, saline, Ex, exercise: IgG, BDNF blocker. Values are mean ± SEM. * and +, significant group difference and right-left difference, respectively, at P < 0.05.

Figure 3

Relative levels of mRNAs for BDNF (A), synapsin I (B), NT-3 (C), CREB (D), and GAP-43 (E) in the cervical region of the ipsilateral hemicord in sedentary (Sed/Sal, Sed/IgG) and exercised (Ex/Sal, Ex/IgG) hemisected rats that received saline or trkB IgG treatment. Data are presented as a percent of sedentary intact control rats. mRNA levels were measured using realtime RT-PCR and corrected for equivalent levels of total mRNA using a GAPDH mRNA probe in the same assay solution. Values are means ± SEM for 5-8 rats/group. * P < 0.05.

Figure 4

The relationship between BDNF mRNA levels and total wheel revolutions for each rat in the Ex/Sal and Ex/IgG groups is shown in (A). The relationships between the levels of Synapsin I, CREB, GAP-43, BDNF and NT-3 mRNA levels and the ratio (ipsilateral/contralateral to the hemisection) of weight-bearing steps are shown in (B), (C), (D), (E), and (F), respectively. Group abbreviations are the same as in Fig. 1. * p < 0.05.