Protective effects of fetal neocortical transplants on cognitive function and neuron size in rats with congenital micrencephaly.

The rat with micrencephaly, produced by prenatal exposure to methylazoxymethanol, provides a useful model to study neurobehavioral abnormalities associated with congenital brain defects. The micrencephalic animals have a life-long learning impairment. As they age, their already impaired learning competence deteriorates further. To determine whether the aging-associated functional deterioration could be ameliorated by a neural transplant, micrencephalic rats bearing solid transplants of normal fetal neocortical tissue since infancy were evaluated on a visual pattern discrimination learning at 15 months and a spatial navigation test at 24 months of age. The transplant-bearing rats learned both tasks significantly better than the micrencephalic rats without transplants. Morphometric analyses revealed that cortical pyramidal neurons were larger in the transplant-bearing rats than in micrencephalic rats without transplants. The life-long presence of a transplant appeared to have protected the micrencephalic brain against aging-associated deterioration. This is the first demonstration that a neural transplant, placed in a congenitally defective infant brain, can ameliorate aging-associated cognitive deficits.