Interaction of magnesium chloride treatment and testing parameters on recovery from sensorimotor cortical lesions in the rat

Abstract

Many of the behavioral impairments that follow cortical injury may be caused by secondary subcortical degeneration or metabolic depression. Administration of MgCl$\sb2$ after injury can prevent subcortical degeneration and facilitate recovery of behavioral impairments. The purpose of the present study was to examine the effects of delaying post-operative testing following cortical lesions and to determine if delays affect the efficacy of MgCl$\sb2$ treatment. Rats received unilateral electrolytic lesions of the somatic sensorimotor cortex and a regimen of MgCl$\sb2$ (1 mmol/kg) starting at specific post-operative intervals (15 min, 8 hr, 24 hr) or saline. Several sensorimotor tests were initiated post-operatively (starting Day 2, Day 8, or Day 14) after injury. In addition, degeneration was assessed in the thalamus and cortex to determine if drug treatments limited subcortical and cortical degeneration. The results indicated that MgCl$\sb2$ treatment at 15 min or 8 hrs after injury facilitated recovery on all behavioral tests assessed when testing was initiated on Day 2. Treatment at 24 hrs facilitated recovery on a subset of these behaviors when testing was initiated on Day 2. Reduction of thalamic degeneration was observed following the 15 min treatments. It was also found that treatment at 15 min and 8 hrs limited forced-use deterioration of the cortex surrounding the lesion. Delaying the onset of behavioral testing (i.e., Day 8 or Day 14 onset) diminished the effectiveness of drug treatments. The results from this study suggest that MgCl$\sb2$ can be administered up to 24 hrs post-injury and still improve behavioral outcome if testing is begun on Day 2. However, delaying the onset of behavioral testing may diminish the effectiveness of MgCl$\sb2$. These data suggest that onset of testing is an important factor to consider when evaluating the effectiveness of drug treatments in facilitating recovery of function.