Effects of variation in postnatal maternal environment on maternal behaviour and fear and stress responses in rats

Abstract

Postnatal manipulation studies involving human handling suggest that rats adjust the hypothalamic-pituitary-adrenal (HPA) system in response to environmentally induced variations in maternal behaviour but adverse environmental conditions can constrain this potentially adaptive mechanism. We studied whether other variations in postnatal maternal environment have similar effects on maternal behaviour and the offspring's behavioural and HPA responses to stressors. We housed Lister hooded dams and their litters in systems made of a nest cage (NC) and an exploration cage (EC) connected by a tunnel and varied access to food either spatially or temporally until postnatal day 8. Dams received food ad libitum in the NC (NC-fed dams) or EC (EC-fed dams) or intermittently during nine 30-min periods each day (IEC-fed dams). Although EC-fed dams spent more time in the EC and less time at the nest site than NC-fed dams, EC-fed dams nursed in longer bouts which may indicate a more active nursing style. Intermittent feeding resulted in reduced food intake, but did not affect maternal behaviour. Compared to NC-fed offspring, EC-fed offspring showed reduced HPA reactivity and fearfulness, whereas IEC-fed offspring showed increased HPA reactivity compared to EC-fed offspring. These findings support the view that moderately challenging maternal environments may down-regulate the offspring's HPA system, possibly by altering maternal behaviour, while adverse environmental conditions (e.g. food restriction) may exert opposing effects, independent of maternal behaviour.