Neonatal frontal cortical lesions in rats alter cortical structure and connectivity

Rats were given frontal cortical lesions at day 1 or 10 of life. Later, as adults, they were either: (1) processed with Golgi-Cox in order to analyze cortical dendritic arborization; (2) given injections of True Blue into the parietal or visual cortex, or (3) given injections of [3H]leucine into the substantia nigra. An additional group of normal rats were given injections of fluorescent dyes into the cortex on day 4 or 10 of life. The main findings were that (1) adult hemispheres with day 10 lesions had greater dendritic arbor than normal hemispheres, (2) adult hemispheres with day 1 lesions had reduced dendritic branching relative to normal hemispheres, (3) adult rats with day 10 lesions had no obvious abnormalities in cortical connections, (4) adult rats with day 1 lesions had abnormal thalamo-cortical, amygdalo-cortical, and nigro-cortical connections, and (5) many of these abnormal connections were present in the brains of 4-day-old normal rats. Since the 'abnormal' connections in the very early frontal operates were present in day 4 animals, it appears that they result from the failure of exuberant connections to retract after the lesions. The increased dendritic growth in day 10 operates does not appear related to qualitative changes in cortical afferents or efferents and may related to increased intrinsic cortical connectivity. Since rats with day 10 lesions have previously been shown to exhibit significant recovery of function, it is possible that the increased dendritic arborization is supporting the functional restitution.