The differences in the status of methylation of the exon 17 GR develop between the two groups emerges between postnatal day 1 and 6, which is precisely the period when differences in the maternal behavior of high- and low-LG dams are apparent. There are no differences in maternal LG between high- and low-LG mothers beyond day 8.65,69 By postnatal day 6, the 5' CpG dinucleotide of the NGFIA response element is demethylated in the highLG, but not in the low-LG group. These findings are consistent with data from the cross-fostering experiment, which illustrates that the differences between the two groups developed following birth in response to maternal behavior. The group difference in CpG dinucleotide methylation then remains consistent through to adulthood. Our findings suggest that the group difference in DNA methylation occurs as a function of a maternal behavior over the first week of life. The results of earlier studies indicated that the first week of postnatal life is indeed a critical period for the effects of early experience on hippocampal GR expression.121
