Combined pre- and postnatal environmental enrichment programs the HPA axis differentially in male and female rats

Experimental environmental enrichment (EE) is usually applied in adulthood or immediately after weaning, with robust effects on physiology and behaviour. To investigate the effects of EE earlier in life, female rats were maintained under moderate enrichment during pregnancy and, together with their pups, during lactation until weaning. A separate group of dams housed under standard conditions during pregnancy and lactation served as controls. Dams housed under EE exhibited fewer nursing episodes and were off the nest more often, but the frequency of pup licking was not affected on postnatal days 3-5. EE effects on hypothalamus-pituitary-adrenal (HPA) axis responses to an acute stressor were determined in adult male and female offspring with and without previous exposure to the chronic stressor of constant light. In female offspring, chronic stress significantly increased basal corticosterone (CORT) levels, but not if rats had been exposed to early EE. Furthermore, while control females exposed to chronic stress showed a greatly reduced adrenocorticotropin (ACTH) response to an acute stressor, EE females did not display this desensitization. There was no significant effect of EE on basal ACTH and CORT levels in adult male offspring, nor did it alter their response to acute stress. Maternal licking frequency was moderately but significantly correlated with net corticosterone increases in response to acute stress, the direction of the correlation crucially depending on the offspring's sex and stress conditions. This study shows that EE during pregnancy and lactation has long-lasting effects on reactivity to acute and chronic stress in offspring and that these effects are dependent on the offspring's sex but not greatly on early postpartum maternal behaviour.