Age-Dependent Alterations of Functional Inhibition in a Rat Model of Cortical Lesions

Cortical lesions in the rat brain induce changes in neuronal activity like reduced functional inhibition. Here we investigated post-lesional effects of a focal lesion on cortical field-potentials in young, middle-aged and old rats. Focal cortical lesions were induced at the border of parietal and occipital areas by injection of the photosensitive dye rose bengal and illumination of the skull. The surgery was performed on 3-, 12- and 24-month-old rats. Cortical field potentials were recorded 7 days after lesion induction on coronal brain slices using the paired pulse protocol. A bipolar stimulation electrode was placed in layer VI and field potentials recorded in layer II/III. GABA-ergic paired-pulse inhibition was investigated through the application of double pulses of 50µs duration with 20 ms inter-stimulus intervals. In young rats with a photothrombotic lesion the ratio of the second versus the first response was significantly increased over wide regions of the neocortex, both in the ipsilateral and contralateral hemispheres. This pattern of alterations between sham-operated and lesioned animals was not observed in older rats. Furthermore, we found significant age-dependent alteration in the functional inhibition of the parietal cortex. These results suggest that neocortical infarcts induce different changes in inhibition, depending on the animal age. This supports the hypothesis of differential plasticity and functional reorganization in juvenile versus aging brains.