Social isolation stress during the third week of life has age-dependent effects on spatial learning in rats

Despite extensive research on the relationship between acute stress and hippocampal function in adults, little is known about the short- and long-term effects of prolonged juvenile stress on learning, memory, and other hippocampal functions. This experiment investigated whether spatial learning would be altered in juvenile and adult rats previously exposed to a chronic stressor: 6 h of social isolation (SI) daily at 15-21 days of age. SI was found to increase circulating plasma levels of corticosterone (CORT) and allopregnanolone (3-alpha,5-alpha-pregnan-20-one; 3,5-THP) at 1 h after separation on the fourth day, indicating that the isolation was an effective stressor. When tested as juveniles (post-natal (PN) 22-24), spatial learning was impaired on the Morris water maze in the previously isolated subjects compared to non-isolated controls. However, when tested as adults (PN 92-94), subjects previously exposed to SI during the third week of life demonstrated more rapid learning of the task than controls. These results are discussed in light of research on the effects of CORT on the developing hippocampus.