Persistent beneficial impact of H-reflex conditioning in spinal cord-injured rats

Abstract

Operant conditioning of a spinal cord reflex can improve locomotion in rats and humans with incomplete spinal cord injury. This study examined the persistence of its beneficial effects. In rats in which a right lateral column contusion injury had produced asymmetric locomotion, up-conditioning of the right soleus H-reflex eliminated the asymmetry while down-conditioning had no effect. After the 50-day conditioning period ended, the H-reflex was monitored for 100 [±9 (SD)] (range 79-108) more days and locomotion was then reevaluated. After conditioning ended in up-conditioned rats, the H-reflex continued to increase, and locomotion continued to improve. In down-conditioned rats, the H-reflex decrease gradually disappeared after conditioning ended, and locomotion at the end of data collection remained as impaired as it had been before and immediately after down-conditioning. The persistence (and further progression) of H-reflex increase but not H-reflex decrease in these spinal cord-injured rats is consistent with the fact that up-conditioning improved their locomotion while down-conditioning did not. That is, even after up-conditioning ended, the up-conditioned H-reflex pathway remained adaptive because it improved locomotion. The persistence and further enhancement of the locomotor improvement indicates that spinal reflex conditioning protocols might supplement current therapies and enhance neurorehabilitation. They may be especially useful when significant spinal cord regeneration becomes possible and precise methods for retraining the regenerated spinal cord are needed.

Keywords: H-reflex conditioning; learning; locomotion; memory; motor control; rehabilitation; spinal cord injury; spinal cord plasticity.

Copyright © 2014 the American Physiological Society.

Figures

Fig. 1.

Study protocol. After learning to walk on the treadmill, each rat was subjected under anesthesia to a midthoracic contusion injury of the right lateral column (LC) at T9 and was implanted with soleus EMG electrodes and nerve cuff stimulating electrodes 20 days later. At least 56 days after the injury, each rat was exposed to the control mode (i.e., the right soleus H-reflex was simply measured) for 20 days; the H-reflex up- or down-conditioning mode [i.e., reward occurred when the H-reflex was greater (up-conditioning) or smaller (down-conditioning) than a criterion] for 50 days; and the control mode for 79–108 more days. Soleus locomotor EMG, H-reflexes, and kinematics were assessed before and immediately after conditioning and at the end of data collection. The data from days 60–70, 110–120, and 210–220 (indicated by heavy lines) were used to determine initial (i.e., control) H-reflex size, H-reflex size at the end of up- or down-conditioning, and H-reflex size at the end of data collection (i.e., final H-reflex size), respectively.

Fig. 2.

A: average (±SE) values of protocol H-reflex (HR) size for HR up-conditioned (HRup) and HR down-conditioned (HRdown) rats (▲ and ▼, respectively) for each 5-day period during the final 10 days of the control-mode period, the 50 days of the up- or down-conditioning period, and 100 days of the second control-mode period. The average (±SE) values of the locomotor reflexes of HRup and HRdown rats (△ and ▽, respectively) for the 3 locomotor sessions are also shown. After conditioning ends, the H-reflex continues to increase in HRup rats, while the H-reflex decrease in HRdown rats gradually disappears. The protocol and locomotor H-reflexes behave similarly. B: data from representative HRup and HRdown rats: average poststimulus EMG activity elicited in the conditioning protocol (top) or during locomotion (middle) and average soleus locomotor burst (bottom) on a day during the initial control-mode period (solid line), a day at the end of conditioning (dashed line), and a day at the end of the second control-mode period (dotted line).

Fig. 3.

A: impact of up- or down-conditioning on right and left hip heights: average left and right hip heights (±SE) before and immediately after conditioning and at the end of data collection. Up-conditioning produced a persistent increase in right hip height (**P < 0.01 vs. before, Tukey test). Down-conditioning did not significantly decrease right hip height or increase the difference between left and right hip heights. B: average right and left leg positions during the stance phase of locomotion before and immediately after up-conditioning and 105 days later in a representative HRup rat. Up-conditioning increases right ankle angle and right (and left) hip height, and these changes persist. (The rat is walking toward the right; A, K, and H indicate ankle, knee, and hip angles, respectively.)

Fig. 4.

A: impact of H-reflex conditioning on step cycle length and step symmetry. A: average step-cycle length (left) and right/left step cycle symmetry (right) (±SE) before and immediately after up- or down-conditioning and at the end of data collection. Up-conditioning produced persistent improvements in step cycle length and right/left step cycle symmetry (*P < 0.05, **P < 0.01 vs. before; Tukey test). Down-conditioning had no significant effect on step cycle length or on right/left symmetry. B: data from a representative HRup rat illustrating the persistent impact of up-conditioning on step cycle length and symmetry. ● and ○, Onsets of right and left stance, respectively. Dashed vertical lines are halfway between right stance onsets, when left stance onset should occur. Prior to conditioning, right stance is not adequately maintained and left stance onset occurs too early (i.e., the rat limps). After up-conditioning, the steps are longer and left stance onset occurs on time. These improvements persist 100 days later.