Variations in maternal behavior among lactating rats associate with differences in estrogen-oxytocin interactions in the medial preoptic area (mPOA) and in dopamine levels in the nucleus accumbens (nAcc). Thus, stable, individual differences in pup licking/grooming (LG) are abolished by oxytocin receptor blockade or treatments that eliminate differences in the nAcc dopamine signal. We provide novel evidence for a direct effect of oxytocin at the level of the ventral tegmental area (VTA) in the regulation of nAcc dopamine levels. Mothers that exhibit consistently increased pup LG (i.e. high LG mothers) by comparison with low LG mothers show increased oxytocin expression in the mPOA and the paraventricular nucleus of the hypothalamus and increased projections of oxytocin-positive cells from both mPOA and paraventricular nucleus of the hypothalamus to the VTA. Direct infusion of oxytocin into the VTA increased the dopamine signal in the nAcc. Finally, high compared with low LG mothers show greater increases in dopamine signal in the nAcc during bouts of pup LG, and this difference is abolished with infusions of an oxytocin receptor antagonist directly into the VTA. These studies reveal a direct effect of oxytocin on dopamine release within the mesocorticolimbic dopamine system and are consistent with previous reports of oxytocin-dopamine interactions in the establishment and maintenance of social bonds.

Early life experiences shape an individual's physical and mental health across the lifespan. Not surprisingly, an upbringing that is associated with adversity can produce detrimental effects on health. A central theme that arises from studies in human and nonhuman species is that the effects of adversity are mediated by the interactions between a mother and her young. In this review we describe some of the long-term effects of maternal care on the offspring and we focus on the impact of naturally occurring variations in the behavior of female rats. Of particular interest are mothers that engage in high or low amounts of licking/grooming (LG) and arched-back nursing (ABN) of their pups, but do so within the normal range for this species. Such variations in LG-ABN can alter the function of the hypothalamic-pituitary-adrenal (HPA) axis, and cognitive and emotional development by directly affecting the underlying neural mechanisms. At the heart of these mechanisms is gene expression. By studying the hippocampal glucocorticoid receptor gene, we have identified that maternal care regulates its expression by changing two processes: the acetylation of histones H3-K9, and the methylation of the NGFI-A consensus sequence on the exon 1(7) promoter. Sustained "maternal effects" appear elsewhere in biology, including plants, insects, and lizards, and may have evolved to program advantages in the environments that the offspring will likely face as adults. Given the importance of early life and parent-child interactions to later behavior, prevention and intervention programs should target this critical phase of development.

Studies across multiple organisms reveal considerable phenotypic variation in reproductive tactics. In some species, this variation is associated with maternal effects in which variation in maternal investment results in stable individual differences in reproductive function. Recent studies with the rat suggest that maternal effects can alter the function of neuroendocrine systems associated with female sexual behaviour as well as maternal behaviour. These maternal effects appear to be mediated by epigenetic modifications at the promoter for oestrogen receptor alpha (ERα) and subsequent effects on gene expression. The tissue‐specific nature of such effects may underlie the co‐ordinated variation in multiple forms of reproductive function, resulting in distinct reproductive strategies.

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