In three brain regions central to maternal behaviour, neither male nor female Phodopus dwarf hamsters show changes in oestrogen receptor alpha distribution with mating or parenthood

Hormonal Actions in the Medial Preoptic Area Governing Parental Behavior: Novel Insights From New Tools

The importance of hormones in mediating a behavioral transition in mammals from a virgin or nonparenting state to parental state was established around 50 years ago. Extensive research has since revealed a highly conserved neural circuit that underlies parental behavior both between sexes and between mammalian species. Within this circuit, hormonal action in the medial preoptic area of the hypothalamus (MPOA) has been shown to be key in timing the onset of parental behavior with the birth of offspring. However, the mechanism underlying how hormones act in the MPOA to facilitate this change in behavior has been unclear. Technical advances in neuroscience, including single cell sequencing, novel transgenic approaches, calcium imaging, and optogenetics, have recently been harnessed to reveal new insights into maternal behavior. This review aims to highlight how the use of these tools has shaped our understanding about which aspects of maternal behavior are regulated by specific hormone activity within the MPOA, how hormone-sensitive MPOA neurons integrate within the wider neural circuit that governs maternal behavior, and how maternal hormones drive changes in MPOA neuronal function during different reproductive states. Finally, we review our current understanding of hormonal modulation of MPOA-mediated paternal behavior in males.

Parental Behavior in Rodents

Members of the order Rodentia are among the best-studied mammals for understanding the patterns, outcomes, and biological determinants of maternal and paternal caregiving. This research has provided a wealth of information but has historically focused on just a few rodents, mostly members of the two Myomorpha families that easily breed and can be studied within a laboratory setting (including laboratory rats, mice, hamsters, voles, gerbils). It is unclear how well this small collection of animals represents the over 2000 species of extant rodents. This chapter provides an overview of the hormonal and neurobiological systems involved in parental care in rodents, with a purposeful eye on providing information known or could be gleaned about parenting in various less-traditional members of Rodentia. We conclude from this analysis that the few commonly studied rodents are not necessarily even representative of the highly diverse members of Myomorpha, let alone other rodent suborders, and that additional laboratory and field studies of members of this order more broadly would surely provide invaluable information toward revealing a more representative picture of the rich diversity in rodent parenting.

Plasticity of the paternal brain: Effects of fatherhood on neural structure and function

Care of infants is a hallmark of mammals. Whereas parental care by mothers is obligatory for offspring survival in virtually all mammals, fathers provide care for their offspring in only an estimated 5%-10% of genera. In these species, the transition into fatherhood is often accompanied by pronounced changes in males' behavioral responses to young, including a reduction in aggression toward infants and an increase in nurturant behavior. The onset of fatherhood can also be associated with sensory, affective, and cognitive changes. The neuroplasticity that mediates these changes is not well understood; however, fatherhood can alter the production and survival of new neurons; function and structure of existing neurons; morphology of brain structures; and neuroendocrine signaling systems. Although these changes are thought to promote infant care by fathers, very little evidence exists to support this hypothesis; in most cases, neither the mechanisms underlying neuroplasticity in fathers nor its functional significance is known. In this paper, we review the available data on the neuroplasticity that occurs during the transition into fatherhood. We highlight gaps in our knowledge and future directions that will provide key insights into how and why fatherhood alters the structure and functioning of the male brain.

Estradiol and estrogen receptor α in the mPOA and MeA in dwarf hamster (Phodopus campbelli) fathers

E₂ and its alpha receptor (ERα) have an essential role in the regulation of maternal behavior. In dwarf hamster (Phodopus campbelli), E₂ facilitates the display of paternal care, and it is possible that ERα is part of the neuroendocrine mechanisms that regulate this behavior. The aim of this study was to analyze the influence of copulation, cohabitation with the pregnant mate and the presence of the pups on paternal behavior, circulating E₂ levels and the presence of ERα in the medial preoptic area (mPOA) and medial amygdala (MeA) in dwarf hamsters. Eight males were mated with intact females (IFs), 8 with tubally ligated females (TLFs) and 8 with ovariectomized females (OFs). In males mated with IFs, paternal behavior tests were performed after copulation, halfway through pregnancy and 24 h after the birth of their pups. Males mated with TLFs were subjected to paternal behavior tests at equivalent periods as the males mated with IFs. In males mated with OFs, paternal behavior tests were performed on days 1, 5 and 10 of cohabitation. After the last paternal behavior tests, blood samples were taken for quantification of E₂ by radioimmunoassay (RIA), and the brains were dissected to determine ERα immunoreactivity (ir) in the mPOA and MeA. Fathers mated with IFs had higher serum E₂ concentrations and more ERα-ir cells in the mPOA than those of males mated with TLFs and OFs. These results suggest that E₂ and its ERα may be associated with paternity in the dwarf hamster.

Neural Regulation of Paternal Behavior in Mammals: Sensory, Neuroendocrine, and Experiential Influences on the Paternal Brain

Across the animal kingdom, parents in many species devote extraordinary effort toward caring for offspring, often risking their lives and exhausting limited resources. Understanding how the brain orchestrates parental care, biasing effort over the many competing demands, is an important topic in social neuroscience. In mammals, maternal care is necessary for offspring survival and is largely mediated by changes in hormones and neuropeptides that fluctuate massively during pregnancy, parturition, and lactation (e.g., progesterone, estradiol, oxytocin, and prolactin). In the relatively small number of mammalian species in which parental care by fathers enhances offspring survival and development, males also undergo endocrine changes concurrent with birth of their offspring, but on a smaller scale than females. Thus, fathers additionally rely on sensory signals from their mates, environment, and/or offspring to orchestrate paternal behavior. Males can engage in a variety of infant-directed behaviors that range from infanticide to avoidance to care; in many species, males can display all three behaviors in their lifetime. The neural plasticity that underlies such stark changes in behavior is not well understood. In this chapter we summarize current data on the neural circuitry that has been proposed to underlie paternal care in mammals, as well as sensory, neuroendocrine, and experiential influences on paternal behavior and on the underlying circuitry. We highlight some of the gaps in our current knowledge of this system and propose future directions that will enable the development of a more comprehensive understanding of the proximate control of parenting by fathers.

Hormonal stimulation and paternal experience influence responsiveness to infant distress vocalizations by adult male common marmosets, Callithrix jacchus

Parental experience and hormones play a large role in the common marmoset (Callithrix jacchus) father's care of their offspring. We tested the effect of exogenous estradiol or testosterone on the responsiveness of common marmosets to respond to infant distress vocalizations and whether males who haven't become fathers yet (paired males) would have increased responsiveness to infant distress calls with either steroid or whether parental experience is the most important component for the onset of paternal care. Sixteen male marmosets (8 fathers, 8 paired males) received a vehicle, low dose or high dose of estradiol and additional 16 males were tested with testosterone at three doses for their response either to a vocal control or a recording of an infant distress call for 10min. Without steroid stimulation fathers were significantly more likely to respond to the infant distress stimulus than paired males. Low dose estradiol stimulation resulted in a significant increase in fathers' behavioral response towards the infant distress stimulus but not in paired males. Fathers also showed a significant increase in infant responsiveness from the vehicle dose to the estradiol low dose treatment, but not to the estradiol high dose treatment. Testosterone treatment did not show significant differences between infant responsiveness at either dose and between fathers and paired males. We suggest that neither steroid is involved in the onset of paternal care behaviors in the marmoset but that estradiol may be involved in facilitating paternal motivation in experienced fathers.

Fathering in rodents: Neurobiological substrates and consequences for offspring

This article is part of a Special Issue "Parental Care". Paternal care, though rare among mammals, is routinely displayed by several species of rodents. Here we review the neuroanatomical and hormonal bases of paternal behavior, as well as the behavioral and neuroendocrine consequences of paternal behavior for offspring. Fathering behavior is subserved by many of the same neural substrates which are also involved in maternal behavior (for example, the medial preoptic area of the hypothalamus). While gonadal hormones such as testosterone, estrogen, and progesterone, as well as hypothalamic neuropeptides such as oxytocin and vasopressin, and the pituitary hormone prolactin, are implicated in the activation of paternal behavior, there are significant gaps in our knowledge of their actions, as well as pronounced differences between species. Removal of the father in biparental species has long-lasting effects on behavior, as well as on these same neuroendocrine systems, in offspring. Finally, individual differences in paternal behavior can have similarly long-lasting, if more subtle, effects on offspring behavior. Future studies should examine similar outcome measures in multiple species, including both biparental species and closely related uniparental species. Careful phylogenetic analyses of the neuroendocrine systems presumably important to male parenting, as well as their patterns of gene expression, will also be important in establishing the next generation of hypotheses regarding the regulation of male parenting behavior.

Functional significance of hormonal changes in mammalian fathers

In the 5-10% of mammals in which both parents routinely provide infant care, fathers as well as mothers undergo systematic endocrine changes as they transition into parenthood. Although fatherhood-associated changes in such hormones and neuropeptides as prolactin, testosterone, glucocorticoids, vasopressin and oxytocin have been characterised in only a small number of biparental rodents and primates, they appear to be more variable than corresponding changes in mothers, and experimental studies typically have not provided strong or consistent evidence that these endocrine shifts play causal roles in the activation of paternal care. Consequently, their functional significance remains unclear. We propose that endocrine changes in mammalian fathers may enable males to meet the species-specific demands of fatherhood by influencing diverse aspects of their behaviour and physiology, similar to many effects of hormones and neuropeptides in mothers. We review the evidence for such effects, focusing on recent studies investigating whether mammalian fathers in biparental species undergo systematic changes in (i) energetics and body composition; (ii) neural plasticity, cognition and sensory physiology; and (iii) stress responsiveness and emotionality, all of which may be mediated by endocrine changes. The few published studies, based on a small number of rodent and primate species, suggest that hormonal and neuropeptide alterations in mammalian fathers might mediate shifts in paternal energy balance, body composition and neural plasticity, although they do not appear to have major effects on stress responsiveness or emotionality. Further research is needed on a wider variety of biparental mammals, under more naturalistic conditions, to more fully determine the functional significance of hormone and neuropeptide profiles of mammalian fatherhood and to clarify how fatherhood may trade off with (or perhaps enhance) aspects of organismal function in biparental mammals.