Critical periods and the effects of prenatal heat stress on the learning and brain growth of mature guinea pigs

The exposure of pregnant guinea pigs to hyperthermia for 1 hr daily on 5 consecutive days produces learning deficits and reduced brain weight in the mature offspring, depending on the gestational age at which exposure took place. The offspring of those treated approximately one-third of the way through pregnancy (20–24 days) show significant deficits in learning performance, and reductions in whole and part brain weights. Those treated about two-thirds of the way through pregnancy (40–44 days) produce offspring which have lesser impairments in learning and a restricted reduction of brain weight. Hyperthermia in late pregnancy, (56–60 days) does not affect the learning or brain weights of the offspring. Several possible explanations for these findings have been examined; and the conclusions are that reduced brain weight is due to a blocking of mitosis at a period of rapid cell proliferation.