Maladaptive effects of learning with the less-affected forelimb after focal cortical infarcts in rats

Abstract

It is common following stroke to focus early rehabilitation efforts on developing compensatory use of the less-affected body side. Here we used a rat model of focal cortical infarct to examine how motor skill acquisition with the less-affected ("intact") forelimb influences sensorimotor function of the infarct-impaired forelimb and neural activity in peri-infarct cortex. Rats proficient in skilled reaching with one forelimb were given focal ischemic lesions in the contralateral sensorimotor cortex (SMC). Recovery in this forelimb was tested following a period of reach training focused on the intact forelimb or control procedures. Quantitative measures of the cumulatively expressed transcription factor, FosB/DeltaFosB, were used to assay intact forelimb training effects on neuronal activity in remaining SMC of the infarcted hemisphere. Intact forelimb training worsened behavioral recovery in the impaired forelimb following unilateral focal ischemia. Furthermore, it decreased neuronal FosB/DeltaFosB expression in layer II/III of peri-infarct SMC. These effects were not found in sham-operated rats trained sequentially with both forelimbs or in animals receiving bilateral forelimb training after unilateral infarcts. Thus, focused use of the intact forelimb has detrimental effects on recovery of impaired forelimb function following a focal ischemic injury and this is linked to reduced neuronal activation in remaining cortex. These results suggest that peri-infarct cortex becomes vulnerable to early post-stroke experience with the less-affected forelimb and that this experience may drive neural plasticity here in a direction that is maladaptive for functional outcome.

Fig 1

Summary of Experimental Designs. Experiment 1 was designed to test the hypothesis that training the intact forelimb worsens functional recovery in the infarct-impaired forelimb. After training to proficiency with the preferred forelimb on a skilled reaching task, the single pellet retrieval task, rats underwent either an ischemic sensorimotor cortex (SMC) lesion or a sham operation in the hemisphere contralateral to this limb. Beginning on postoperative day 5 (D5), rats received either 15 days of training (60 trials/day) with their intact/non-preferred forelimb (IntactT and Sham NPrefT) or control procedures (Cont, Sham Cont). All animals were then tested for 13 days (30 trials/day) with their impaired and/or preferred forelimb and then sacrificed to assay neuronal activation in peri-infarct cortex (n’s = 10/group). Experiment 2 was designed to test whether focused intact forelimb training is necessary to influence recovery of impaired forelimb function. Animals were trained pre-operatively as in Experiment 1. All animals then received an SMC lesion followed by 12 days of training (30 trials/day) with the intact forelimb (IntactT) both forelimbs (BiT) or control procedures (Cont). All rats were then tested with their impaired forelimb as in Experiment 1 followed by 5 days of intact forelimb retesting (n’s = 9/group).

Fig 2

Training the intact forelimb in the skilled reaching task early after unilateral SMC lesions worsened subsequent performance in the impaired forelimb. A: Intact training period of Experiment 1. Lesion rats trained with their intact forelimb (IntactT) performed significantly better than sham (Sham NPrefT) rats, p < .01. B: Impaired testing period. Sham animals performed significantly better than lesion animals (p < .01). IntactT rats performed significantly worse with their impaired forelimb compared to Cont rats (p < .05). The shaded forelimb indicates the trained/tested forelimb. Data are means ± SEM.

Fig 3

Training both forelimbs in skilled reaching early after the lesions did not reproduce the effects of focused intact limb training. IntactT rats performed significantly worse with their impaired forelimb compared to BiT rats (p < .01) and performed worse compared to Cont dependent on day (p < .01). Training with both forelimbs did not significantly change impaired forelimb performance compared to control animals (p > .05). These results are from Experiment 2. Data are means ± SEM. Note the difference in scale between Figure 2 and Figure 3.

Fig 4

Reach training focused on the intact forelimb increased reliance on this limb in a measure of coordinated forelimb placement during locomotion (the footfault test). Unilateral lesions resulted in impairments in the contralateral forelimb on the footfault test which recovered by Day 31. IntactT rats had increased slips with their impaired forelimb during the lidocaine challenge test compared to Cont (*p < .05). Data are means ± SEM.

Fig. 5

Schematic coronal sections throughout the sensorimotor cortex showing representative lesions in Cont and IntactT animals (from Experiment 1).

Fig 6

FosB/ΔFosB representative photomicrographs from layer II/III peri-infarct cortex (A,B, scale bars = 10 μm) and quantitative results (C) from Experiment 1. Unilateral lesions increased FosB/ΔFosB neuronal density in remaining SMC compared to sham-operates. Intact forelimb training significantly decreased FosB/ΔFosB neuronal density in layer II/III compared to Cont rats. A representative lesion is shown in panel A. Scale bar = 500μm. Data are means ± SEM. *p < .05.