Variations in the postnatal maternal environment in mice: effects on maternal behaviour and behavioural and endocrine responses in the adult offspring.

According to the maternal mediation hypothesis, brain and behavioural development in rodents is affected by environment-dependent variations in maternal care. Thus, it has been shown that early handling results in reduced behavioural and neuroendocrine responses to stressors and that these effects are associated with increased maternal care received during infancy. To investigate this further in mice, we chose a less artificial paradigm that is not confounded by human manipulation and reflects a more natural form of early environmental variation. We housed lactating C57BL/6 dams and their litters in cage systems composed of a nest cage (NC) and a foraging cage (FC) connected by a tunnel, and varied the dams' access to food by providing food either in the NC (NC dams) or FC (FC dams) until postnatal day 14. FC dams were more frequently observed in the FC than NC dams, and although the frequency of the dams being in physical contact with the pups did not differ between the two treatments, FC dams showed lower levels of active nursing than NC dams during the first week of lactation. These environment-dependent variations in maternal behaviour had sex-specific effects on the adult offspring's behavioural and HPA responses to stressors and altered their social behaviour in the home cage, with NC offspring showing higher levels of socio-positive behaviours than FC offspring. These results provide further independent evidence for the maternal mediation hypothesis and demonstrate that even subtle variations of the maternal environment can affect maternal care and induce persistent changes in offspring phenotype.