Male and female rodents receive differential maternal care with male pups receiving more LG than their female littermates (Moore, 1984, 1985; Moore and Chadwick-Dias, 1986; Moore and Morelli, 1979). Therefore, manipulating the sex composition of litters has implications for maternal behavior. Consistent with work from the Meaney laboratory showing a regulatory role of quantity of LG behavior on GR methylation, adolescent female offspring raised in a single-sex litter (i.e. all female pups) were found to have increased methylation of the GR promoter in the nucleus accumbens relative to females reared in a mixed-litter (Kosten et al., 2014). Methylation of Oprm1, which is important in maternal attachment, was increased in the hippocampus in adolescent rats reared in single-sex litters (Hao et al., 2011). Taken together, these data show that altering the quality of maternal care a rodent received in infancy has long-term effects on the methylation status of behaviorally-relevant genes.
