Mild prenatal stress enhances learning performance in the non-adopted rat offspring.

The present study was designed to investigate whether mild stress during pregnancy affects offspring behaviors, including learning performance. Prenatal stress was induced by short-lasting, mild restraint stress, which had previously been shown to facilitate the morphological development of fetal brain neurons. Adult offspring whose dams had been restrained in a small cage for 30min daily from gestation day 15 to 17 showed enhanced active avoidance and radial maze learning performance. In addition, the prenatally stressed rats showed weaker emotional responses than unstressed control, as indicated by decreases both in ambulation upon initial exposure to an open field and in Fos expression in the amygdala induced by physical stress. The observed effects of prenatal stress on learning performance and emotional behavior were attenuated by foster rearing by unstressed dams. Fos expression in the hypothalamic paraventricular nucleus following physical stress and corticosterone secretion during physical and psychological stress did not differ between the prenatally stressed and unstressed control rats. From these results we suggest that mild prenatal stress facilitates learning performance in the adult offspring. The enhancement of learning performance appears to be accompanied by reduced emotionality, but not by any apparent alterations in hypothalamic-pituitary-adrenal responses. In addition, the observation of differential behaviors in the adopted and non-adopted animals supports the notion that the postnatal environment modifies the behavioral effects of prenatal stress.