In Search of a Molecular Switch for Parenting

Infants elicit three basic behavioral responses from adult conspecifics: aggression, indifference, and care. The response elicited depends on both intrinsic (sex, hormones) and extrinsic (experience, context) factors. Not surprisingly, then, responses are not static but change based on the above factors. For instance, while virgin rats tend to initially ignore or avoid infants prior to giving birth, following the hormonal events of pregnancy and parturition, postpartum rats are so motivated to care for infants that they will learn to press a lever to gain access to pups, traverse a novel context to retrieve pups back to the nest, and even choose a chamber previously associated with pups rather than cocaine when given the choice. In the absence of hormonal stimulation, experience with infants can also increase caregiving behaviors. For example, virgin rats can overcome their initial aversion and care for infants following days of interaction with them. Virgin B6 mice, in contrast to rats, readily care for infants in a familiar environment and therefore do not need hormones to reduce fearful responses towards infants. However, virgin mice are still not as motivated as hormone-primed mice and thus fearful responses towards infants can emerge under novel and stressful conditions. Increased experience with infants (2h/4d) eliminates this fearful response in a novel context and enables virgin mice to show similar levels of maternal motivation as postpartum mice even one month after the initial experience. Thus, experience with pups seems to stabilize care over other responses, at least in females, regardless of the context in which pups are presented.Despite the fact that B6 mice are not a bi-parental species, male B6 mice can also display caregiving behaviors under certain conditions. For instance, whereas a large percentage of sexually inexperience male mice by default are aggressive or indifferent towards pups, a small percentage spontaneously care for infants. Further, even in aggressive males, caregiving behaviors can emerge following sexual experience and subsequent cohabitation with a female and pups. Therefore, not unlike females, certain factors can enable males to also provide care for infants. Much is known about the neural circuitry regulating these behavioral responses towards infants from functional neuroanatomical work done in rats, though there is evidence to suggest that this circuit is conserved across species and sex. For instance, the medial preoptic area (MPOA) of the anterior hypothalamus is the critical site for the regulation of maternal care and coordinates hormonal and experiential inputs. The MPOA, however, does not work in isolation. The MPOA is thought to promote caregiving behaviors through its connections with the mesolimbic dopamine system. The mesolimbic dopamine system plays an important role in regulating the motivation to care for infants, as dopamine is released into the nucleus accumbens (NA) of maternal female rats during licking and grooming of infants and stimulation of the dopamine system by injecting a dopamine agonist into the NA promotes maternal behavior in female rats whose pregnancies have been terminated. Dopamine release in the NA is mediated by the VTA, whereas the NA in turn is thought to inhibit the ventral pallidum (VP). The VP is released from inhibition by disinhibition to ultimately promote caregiving behaviors. In contrast, The MPOA is thought to have a predominantly inhibitory influence on a hypothalamic circuit that has been implicated in the suppression of maternal care. For instance, the anterior hypothalamic nucleus (AHN) and the ventromedial nucleus of the hypothalamus (VMN), two regions that comprise the hypothalamic aggressive area, induce maternal behavior when lesioned. There is strong evidence suggesting that the AHN/VMN receive projections from the medial amygdala (MeA) to suppress caregiving behaviors, as unilateral lesions of the MeA reduce pup-induced cFos expression in the AHN/VMN in non-maternal rats. The AHN/VMN projects to the periaqueductal gray (PAG), a region in the brainstem thought to be one of the final output regions involved in regulating avoidant responses towards infants. Although this circuit has been well delineated in female rats and male mice, who by default are respectively avoidant or aggressive towards pups, there is evidence to suggest that a latent aversion circuit also emerges in female mice under challenging conditions. For example, recall that our previous work shows that although female virgin mice are spontaneously maternal, fearful and indifferent responses emerge under stressful circumstances. Further, lesions of the central MPOA induce infanticide in female mice, which suggests that the MPOA must suppress a circuit that regulates negative responses towards infants. Hormones and experience are thought to act on these conserved neural systems through the MPOA to decrease activation of the pathways responsible for fear and avoidance responses towards infants and increase activation in the pathways involved in regulating parental motivation. For some animals, such as female rats and male mice, the onset of care requires a reduction in fear or aggression. For animals that are not afraid or aggressive by default, such as female mice, these avoidance responses can emerge under some conditions, in which case a reduction in fear is also necessary for caregiving behaviors to be displayed. Thus, an important question is how does increased experience allow for the central motivation circuit to be continuously selected over the central aversion circuit to mediate long-term changes in parental motivation? One mechanism through which experience with infants may promote caregiving behaviors and motivation circuit selection is by increasing histone acetylation within gene promoters. In support of this idea, our lab has previously found that administration of the histone deacetylase inhibitor (HDACi) drug sodium butyrate, which allows for increased histone acetylation and gene expression, to virgin female mice increases maternal responses compared to control treated virgin females. Additionally, histone acetylation may also mediate experience-dependent gene expression changes because treatment of the HDACi increases the expression of several genes in the MPOA known to be involved in maternal behavior. These data suggest that experience-dependent transcriptional changes, at least in the MPOA, may act to permanently alter the way central aversion and motivation circuits respond to infant stimuli. Thus, the present set of studies will begin to investigate the molecular mechanisms by which experience alters behavior and associated immediate early gene (IEG) expression in neural circuits that govern motivation and aversion in both virgin female and male mice. Specifically, Chapter 1 will examine the extent to which the facilitatory effects of HDACi administration on behavior in virgin female mice are associated with the stable activation of a neural circuit that regulates motivation to care for infants and a stable inhibition of a latent aversion circuit that becomes activated under challenging circumstances. The fact that caregiving behaviors can be turned off under certain conditions in female mice led us to follow up the findings from Chapter 1 by asking whether parenting behaviors can also be turned on under certain conditions in male mice. Thus, Chapter 2 will investigate whether HDACi treatment can increase caregiving behaviors in sexually naïve male mice, who are generally aggressive or avoidant towards infants, and whether this behavioral change is associated with altered transcriptional programming in the central motivation and aversion pathways. We hypothesize that experience prevents pup cues from activating the central aversion circuit, while strengthening the connection between pup-responsive cells in the MPOA and cells within the central motivation circuit. Thus, the MPOA and other nodes within the central motivation circuit may be critical sites where maternal experiences are consolidated. However, until recently, current methods have only allowed us to compare separate groups of inexperienced and experienced mice, making it presently unclear whether experience-dependent molecular changes occur in cells that were always responsive to pups. Thus, a critical unanswered question is whether cells that initially encode pup cues in regions within maternal neural circuits are reactivated during maternal memory retrieval. In the third and final chapter, we will use Targeted Recombination in Active Populations (TRAP) to first ask how central motivation and aversive circuits differentially respond to pups across experience within the same animal. We will then investigate whether cells across the central motivation and aversive circuits that are initially responsive to pups are the same cells in which maternal memories may be stored