The onset of maternal behavior in the pregnant female rat is coincident with a sharp decrease in progesterone and an increase in estrogen and prolactin. These changes in hormones appear to be both necessary and sufficient for the onset of maternal behavior because, after pregnancy termination, estrogen facilitates and progesterone delays the onset of maternal behavior. How does estrogen prime the brain to influence behavior? It is now clear that the effects of estradiol on behavior may be due to its actions on the estrogen receptors, which are hormone-dependent transcription factors. Thus, estrogen is best considered as an early step in a cascade of cellular events that can ultimately lead to neuronal activation, recruitment of a neural system, and ultimately behavioral change.
The most important genes that are targets of estrogen action are likely to be oxytocin and prolactin and their respective receptors. Not only are these hormones important for parturition and lactation, but receptors for these hormones within the brain appear to support the onset of maternal behavior. Oxytocin given to estrogen-primed females can induce maternal care within 30 minutes. Prolactin had similar effects, although the onset takes place over days rather than minutes.
Although there is not a single brain area that represents the neuroanatomical locus of maternal behavior, lesions of the medial preoptic area (MPOA) in the most anterior aspect of the hypothalamus impair nest building and retrieval of pups, although females show no retrieval deficits for nonpup stimuli. The MPOA is not the only region involved in maternal behavior, but it is of particular interest because of its potential role in the hormonal regulation of the onset of maternal care. The MPOA is rich in estrogen and progesterone receptors as well as oxytocin and pro-lactin receptors. All of these receptors increase in number during gestation, and estrogen implanted directly in the MPOA facilitates the onset of pup retrieval.
In primates, including humans, it is generally assumed that hormonal factors are less important than experiential factors. In marked contrast to rats, maternal behavior in primates is generally not restricted to the postpartum period but can be observed in females of any endocrine status. Other factors such as social rank or environmental stress may be more important for primate maternal behavior.
Parental care provides an extraordinary opportunity to explore how genes and hormones influence the brain to modify behavior. In particular, the diversity of parental care, while discouraging simple generalizations across species, offers several remarkable experiments of nature, which, properly analyzed, should yield important insights into the mechanisms by which parental care evolved.
Thomas r. Insel
National Institute of Mental Health
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