Long-term changes in postnatal susceptibility to pilocarpine-induced seizures in rats exposed to gamma radiation at different stages of prenatal development.

To determine whether brains irradiated at different stages of prenatal development also have different postnatal susceptibility to seizures evoked by pilocarpine. Pregnant Wistar rats were exposed to a single 1.0-Gy dose of gamma rays on gestation days 13, 15, 17, or 19 (E13, E15, E17, and E19, respectively). On postnatal day 60, their offspring received i.p. pilocarpine injections to evoke status epilepticus. Behavior of the animals was observed continuously for 6 h after the injection, and motor manifestations of seizure activity were rated, and survival times recorded. After 7-day survival, the animals were killed, and their brains were weighed. The average brain weight of animals exposed to irradiation at earlier prenatal stages (E13 or E15) was significantly lower than that after irradiation on E17 or E19. However, effects of the irradiation on the susceptibility to pilocarpine-induced seizures were quite opposite. The intensity of status epilepticus evoked in rats irradiated on E13 or E15 was significantly lower than that in nonirradiated controls or in those irradiated on E17 or E19. Moreover, after irradiation on E13 or E15, survival of the animals was significantly higher in relation not only to other irradiated groups but also to the controls. The results suggest than the extent of neuronal deficit, even if relatively greater, cannot always lead to higher susceptibility of the dysplastic brain to seizures. Functional consequences of the deficit, even if its magnitude is relatively smaller but involving specific brain areas, appear to be critical for the epileptogenesis.