Efficacy of rehabilitative experience declines with time after focal ischemic brain injury

Abstract

To maximize the effectiveness of rehabilitative therapies after stroke, it is critical to determine when the brain is most responsive (i.e., plastic) to sensorimotor experience after injury and to focus such efforts within this period. Here, we compared the efficacy of 5 weeks of enriched rehabilitation (ER) initiated at 5 d (ER5), ER14, or ER30 after focal ischemia, as judged by functional outcome and neuromorphological change. ER5 provided marked improvement in skilled forelimb reaching ability and ladder-rung- and narrow-beam-walking tasks and attenuated the stroke-induced reliance on the unaffected forepaw for postural support. ER14 provided improvement to a somewhat lesser extent, whereas recovery was diminished after ER30 such that motor function did not differ from ischemic animals exposed to social housing. To examine potential neural substrates of the improved function, we examined dendritic morphology in the undamaged motor cortex because our previous work (Biernaskie and Corbett, 2001) suggested that recovery was associated with enhanced dendritic growth in this region. ER5 increased the number of branches and complexity of layer V neurons compared with both social housing and control animals. Dendritic arbor after ER14 (although increased) and ER30 did not differ from those exposed to social housing. These data suggest that the poststroke brain displays heightened sensitivity to rehabilitative experience early after the stroke but declines with time. These findings have important implications for rehabilitation of stroke patients, many of whom experience considerable delays before therapy is initiated.

Figure 2.

Single-pellet reaching test. A, Sequential frames illustrating a rat retrieving a pellet in the pellet reaching test. B, Focal ischemia resulted in a severe and persistent impairment in the ability to retrieve pellets. Although all animals showed slight improvement after 14 d of therapy, by 35 d, ER5 and ER14 animals retrieved significantly more pellets relative to socially housed animals. C, Treatment efficacy was discriminated further by reaching accuracy (which accounts for the number of attempted reaches for each pellet), demonstrating that ER5 animals were improved relative to both ER30 and socially housed animals. ER14 animals showed moderate improvement but did not attain statistical significance (p = 0.09). ER5, n = 8; ER14, n = 7; ER30, n = 7; social housing, n = 9; controls, n = 8. *p < 0.05 from socially housed animals; ^†p < 0.001 from controls.

Figure 1.

Typical areas of infarction resulting from endothelin-1-induced middle cerebral artery occlusion. Coordinates are relative to bregma (in millimeters). Adapted from Paxinos and Watson (1997).

Figure 3.

Staircase reaching test. The mean number of pellets eaten from the staircase below the impaired limb is shown. Because rats were able to lick pellets from the top two steps, reaching scores only included the bottom five steps of the staircase. All ischemic groups were severely impaired on days 3 and 4 after ischemia (before treatment; p < 0.001) relative to controls. Although there was no difference between ischemic groups before treatment or after 14 d of respective treatment (p > 0.05), ER5 animals were able to retrieve more pellets than socially housed animals after 35 d of treatment (p < 0.05). ER5 animals also showed a strong trend for improvement relative to ER30 animals (p = 0.07). ER5, n = 8; ER14, n = 7; ER30, n = 7; social housing, n = 6; controls, n = 9. *p < 0.05 different from all other groups; ^†p < 0.05 different from social housing.

Figure 4.

Narrow-beam-walking task. Four days after ischemia (before treatment), all groups made a greater number of foot slips relative to controls (p < 0.01). ER provided a marked reduction in the number of foot faults when initiated at either 5 or 14 d after the stroke. After 14 d, ER(regardless of start time) reduced foot slips compared with social housing, and this improvement persisted after 35 d in ER5 and ER14 animals. ER5, n = 8; ER14, n = 7; ER30, n = 7; social housing, n = 9; controls, n = 9. *p < 0.01, different from controls; ^†p < 0.05, different from ER5 and ER14; ^#p < 0.01, different from ER14.

Figure 5.

Cylinder test of forelimb use during upright postural support. A, Ischemia resulted in an increased preference for ipsilateral limb use that persisted for the duration of the experiment in all groups (p < 0.05). Interestingly, ER5 reduced ipsilateral limb preference to control levels, which was not observed in ER14, ER30, or social housing. B, Bilateral limb use was reduced after ischemia. Surprisingly, ER5 animals returned to control levels after 35 d of treatment. In contrast, ER14, ER30, and socially housed animals persistently used fewer bilateral contacts for upright weight support. ER5, n = 8; ER14, n = 7; ER30, n = 7; social housing, n = 9; controls, n = 9. *p < 0.05 from controls; ^†p < 0.05 from ER5; ^§p < 0.05 from ER14.

Figure 6.

Ladder-rung walking test. A, ER5 animals showed a significant reduction in errors per step while traversing an irregular pattern of rungs compared with both ER30 and social-housing animals. ER30 resulted in a delayed improvement that reached control levels by 35 d after treatment. A persistent impairment was observed in socially housed animals. B, Foot placement errors were significantly elevated at 14 d after treatment. After 35 d of therapy, all ischemic groups made a greater number of placement errors compared with controls, although this did not reach significance. ER5, n = 5; ER30, n = 5; social housing, n = 5; controls, n = 4. *p < 0.05 relative to controls; ^†p < 0.05 different from ER5; ^§p < 0.05 relative to ER30.

Figure 7.

Dendritic branching in the undamaged motor cortex. A, Complexity of the basilar dendritic arbor for layer V pyramidal cells was analyzed by branch order, an indicator of increasing numbers of branch bifurcations from the cell body. ER5 elevated the number of higher-order branches relative to all other groups. ER5 resulted in significant elevations at fourth-order, fifth-order, and sixth-order branches compared with controls. Also, early rehabilitation enhanced the dendritic branch number relative to delayed ER; ER5 was different from ER14 (at fifth- and sixth-order branches) and ER30 (at fourth-, fifth-, and sixth-order branches). B, Total dendritic length per cell in ER5, ER14, and ER30 animals was increased relative to controls; however, there was no change in length resulting from the ischemic insult. ER5, n = 5; ER14, n = 7; ER30, n = 7; social housing, n = 8; controls, n = 7. *p < 0.05 from controls; ^#p < 0.05 from ER14; ^§p < 0.05 from ER30; ^†p < 0.05 from social housing.

Figure 8.

Size of infarction correlates with dendritic length in the undamaged motor cortex. Decreasing amounts of remaining cortical (A) or total (B) tissue (cortical and striatal) after ischemic injury resulted in greater dendritic length of layer V pyramidal cells in the undamaged motor cortex. Cortical tissue, R² = 0.209, p = 0.0192; total remaining tissue, R² = 0.212, p = 0.0054.