Ectopic neurons in the hippocampus may be a cause of learning disability after prenatal exposure to X-rays in rats.

The relationship between an impairment of spatial navigation and an incidence of ectopic neurons in the dorsal hippocampus was investigated in adult rats that were prenatally exposed to X-ray irradiation. Adult rats which had received 1.5 Gy X-rays at embryonic day 15 (E15) showed significant learning disability in the water-maze task. According to the mean value of the swimming time, we categorized the irradiated adult rats into the following three groups: slightly damaged group, mildly damaged group and severely damaged group. No significant difference in the brain weight was found between the three categorized groups. Ectopic neurons appearing at abnormal places were prominently observed in the dorsal hippocampus of the severely damaged group with a remarkable learning disturbance, while no ectopia in the hippocampus was observed in the slightly damaged group. This may suggest that the cognitive dysfunction induced by prenatal exposure to X-ray irradiation may be, at least in part, attributable to ectopic neurons of the hippocampus.