Regulatory role of prolactin in paternal behavior in male parents: A narrative review

Prenatal prolactin predicts postnatal parenting attitudes and brain structure remodeling in first-time fathers

Despite the important contributions that fathers make to parenting, the neurobiological underpinnings of men's adaptation to parenthood are still not well understood. The current study focuses on prolactin, a hormone that has been extensively linked with reproduction, lactation, and parental behavior in mothers. There is preliminary evidence that prolactin may also reflect the transition to sensitive fatherhood. We sampled prolactin in 91 first-time expectant fathers who participated in a laboratory visit along with their pregnant partners. Fathers' prolactin levels were correlated with their partners' prolactin levels. Men's prolactin levels during their partner's pregnancy were associated with their self-reported antenatal bonding to the unborn infant. Prenatal prolactin levels in fathers also predicted more positive attitudes toward fatherhood at three months postpartum, including lower parenting stress, greater enjoyment of the infant, and a more attunement-oriented parenting style. Within a smaller sample of 32 men who participated in MRI scanning before and after their child's birth, prenatal prolactin also predicted greater reductions in grey matter volume in the left posterior cingulate, left insula, and left nucleus accumbens. In conclusion, men's prenatal prolactin may reflect their perceptions of fatherhood and changes to their perinatal brain structure.

Mother-Young Bonding: Neurobiological Aspects and Maternal Biochemical Signaling in Altricial Domesticated Mammals

Mother-young bonding is a type of early learning where the female and their newborn recognize each other through a series of neurobiological mechanisms and neurotransmitters that establish a behavioral preference for filial individuals. This process is essential to promote their welfare by providing maternal care, particularly in altricial species, animals that require extended parental care due to their limited neurodevelopment at birth. Olfactory, auditory, tactile, and visual stimuli trigger the neural integration of multimodal sensory and conditioned affective associations in mammals. This review aims to discuss the neurobiological aspects of bonding processes in altricial mammals, with a focus on the brain structures and neurotransmitters involved and how these influence the signaling during the first days of the life of newborns.

The influence of manual semen collection in male trained dogs (Canis familiaris), in the presence or absence of a female in estrus, on the concentrations of cortisol, oxytocin, prolactin and testosterone

Sex pheromones are chemical substances secreted into the environment that affect the physiology and behavior of recipients. Females use these compounds during oestrus to attract males, which leads to attempts of mating. This study evaluates the influence of manual semen collection in male dogs, in the presence or absence of a female in estrus, on the blood concentrations of cortisol (CRT), oxytocin (OXT), prolactin (PRL) and testosterone (T), as hormones involved both in the physiology of reproduction and stress. Ten male dogs were used in Experiment 1 to measure the serum and plasma concentrations of the aforementioned hormones in the absence of semen collection. Subsequently in the same animals, the concentrations of these hormones were evaluated before and after semen collection in the presence (Exp. 2) or in absence of a female in estrus (Exp. 3). No significant changes in hormone concentration caused by the semen collection were found, either with, or without the presence of female in estrus. Obtained results suggest that the procedure of manual semen collection in dogs, probably due to its passive character, does not stimulate endocrine glands to secrete hormones, and the process of ejaculation is probably controlled by neural pathway. The lack of effect of semiochemical stimulation to the CRT, PRL, OXT and T level, could be caused by a short contact with female during semen collection. Further studies on involvement of the hormones during the process of natural mating, especially preceded by long courtships, similar to that observed under natural conditions, should shed a light on the physiology of mating and the connection between the endocrine system and semiochemical stimulation in dogs.

Prolactin promotes parental responses and alters reproductive axis gene expression, but not courtship behaviors, in both sexes of a biparental bird

Prolactin, a hormone involved in vertebrate parental care, is hypothesized to inhibit reproductive hypothalamic-pituitary-gonadal (HPG) axis activity during parenting, thus maintaining investment in the current brood as opposed to new reproductive efforts. While prolactin underlies many parental behaviors in birds, its effects on other reproductive behaviors, such as courtship, remain unstudied. How prolactin affects neuropeptide and hormone receptor expression across the avian HPG axis also remains unknown. To address these questions, we administered ovine prolactin (oPRL) or a vehicle control to both sexes in experienced pairs of the biparental rock dove (Columba livia), after nest removal at the end of incubation. We found that oPRL promoted parental responses to novel chicks and stimulated crop growth compared to controls, consistent with other studies. However, we found that neither courtship behaviors, copulation rates nor pair maintenance differed with oPRL treatment. Across the HPG, we found oPRL had little effect on gene expression in hypothalamic nuclei, but increased expression of FSHB and hypothalamic hormone receptor genes in the pituitary. In the gonads, oPRL increased testes size and gonadotropin receptor expression, but did not affect ovarian state or small white follicle gene expression. However, the oviducts of oPRL-treated females were smaller and had lower estrogen receptor expression compared with controls. Our results highlight that some species, especially those that show multiple brooding, may continue to express mating behavior despite elevated prolactin. Thus, mechanisms may exist for prolactin to promote investment in parental care without concurrent inhibition of reproductive function or HPG axis activity.

The elusive role of prolactin in the sociality of the naked mole-rat

Despite decades of research into the evolutionary drivers of sociality, we know relatively little about the underlying proximate mechanisms. Here we investigate the potential role of prolactin in the highly social naked mole-rat. Naked mole-rats live in large social groups but, only a small number of individuals reproduce. The remaining non-breeders are reproductively suppressed and contribute to burrow maintenance, foraging, and allo-parental care. Prolactin has well-documented links with reproductive timing and parental behaviour, and the discovery that non-breeding naked mole-rats have unusually high prolactin levels has led to the suggestion that prolactin may help maintain naked mole-rat sociality. To test this idea, we investigated whether urinary prolactin was correlated with cooperative behaviour and aggression. We then administered the prolactin-suppressing drug Cabergoline to eight female non-breeders for eight weeks and assessed the physiology and behaviour of the animals relative to controls. Contrary to the mammalian norm, and supporting previous findings for plasma, we found non-breeders had elevated urinary prolactin concentrations that were similar to breeding females. Further, prolactin levels were higher in heavier, socially dominant non-breeders. Urinary prolactin concentrations did not explain variation in working behaviour or patterns of aggression. Furthermore, females receiving Cabergoline did not show any behavioural or hormonal (progesterone) differences, and urinary prolactin did not appear to be suppressed in individuals receiving Cabergoline. While the results add to the relatively limited literature experimentally manipulating prolactin to investigate its role in reproduction and behaviour, they fail to explain why prolactin levels are high in non-breeding naked mole-rats, or how female non-breeding phenotypes are maintained.

Neurobiological Correlates of Fatherhood During the Postpartum Period: A Scoping Review

During the postpartum period, the paternal brain suffers extensive and complex neurobiological alterations, through the experience of father-infant interactions. Although the impact of such experience in the mother has been increasingly studied over the past years, less is known about the neurobiological correlates of fatherhood-that is, the alterations in the brain and other physiological systems associated with the experience of fatherhood. With the present study, we aimed to perform a scoping review of the available literature on the genetic, neuroendocrine, and brain correlates of fatherhood and identify the main gaps in the current knowledge. PubMed, Scopus, and Web of Science electronic databases were searched for eligible studies on paternal neuroplasticity during the postpartum period, over the past 15 years. Reference lists of relevant key studies and reviews were also hand-searched. The research team independently screened the identified studies based on the established inclusion criteria. Extracted data were analyzed using tables and descriptive synthesis. Among the 29 studies that met our inclusion criteria, the vast majority pertained to neuroendocrine correlates of fatherhood (n = 19), followed by brain activity or connectivity (n = 7), association studies of candidate genes (n = 2), and brain structure correlates (n = 1). Collectively, studies published during the past 15 years suggest the existence of significant endocrine (testosterone, oxytocin, prolactin, and cortisol levels) and neurofunctional alterations (changed activity in several brain networks related to empathy and approach motivation, emotional processing and mentalizing, emotion regulation, dorsal attention, and default mode networks) as a result of fatherhood, as well as preliminary evidence of genetic variability accounting for individual differences during the postpartum period in fathers. No studies were so far published evaluating epigenetic mechanisms associated with the paternal brain, something that was also the focus of the current review. We highlight the need for further research that examines neuroplasticity during the experience of fatherhood and that considers both the interplay between hormones and simultaneous assessment of the different biomarkers (e.g., associations between hormones and neural activity); data collection protocols and assessment times should also be refined.

Resident and Non-resident Father Involvement, Coparenting, and the Development of Children's Self-Regulation Among Families Facing Economic Hardship

Self-regulation, or the ability to effectively manage emotions and behavior, is a critical skill to develop in early childhood. Children living in a context of economic hardship are at an increased risk for developing self-regulation difficulties. However, few studies have comprehensively examined how multiple aspects of the caregiving environment, including fathers' parenting and coparenting quality, may contribute to child self-regulation. Thus, this study applied a family systems perspective to examine whether coparenting and resident and non-resident fathers' reports of parenting quantity and quality were associated with observations of children's self-regulation. Participants were drawn from the Embedded Developmental Study (n = 257) of the Three-City Study, a longitudinal study of children and families facing economic hardship. At Wave 1, when children were 2-4 years old, reports of parenting (i.e., quantity and quality) and coparenting (i.e., support) were obtained. At Wave 2, when children were 3-6 years old, children participated in a snack delay and gift wrap task, which assessed their self-regulation. Multi-group path analyses indicated that resident fathers' harsh parenting at Wave 1 predicted decreased levels of self-regulation at Wave 2. Non-resident fathers' reported hours of involvement at Wave 1 predicted greater levels of self-regulation at Wave 2. Additionally, supportive coparenting among families with a non-resident father predicted greater self-regulation. Supportive coparenting was not associated with child self-regulation in families with a resident father. The implications for research focused on facilitating positive father-child relationships in diverse family contexts are discussed.

Hormonal and neural correlates of care in active versus observing poison frog parents

The occasional reversal of sex-typical behavior suggests that many of the neural circuits underlying behavior are conserved between males and females and can be activated in response to the appropriate social condition or stimulus. Most poison frog species (Family Dendrobatidae) exhibit male uniparental care, but flexible compensation has been observed in some species, where females will take over parental care duties when males disappear. We investigated hormonal and neural correlates of sex-typical and sex-reversed parental care in a typically male uniparental species, the Dyeing Poison Frog (Dendrobates tinctorius). We first characterized hormone levels and whole brain gene expression across parental care stages during sex-typical care. Surprisingly, hormonal changes and brain gene expression differences associated with active parental behavior in males were mirrored in their non-caregiving female partners. To further explore the disconnect between neuroendocrine patterns and behavior, we characterized hormone levels and neural activity patterns in females performing sex-reversed parental care. In contrast to hormone and gene expression patterns, we found that patterns of neural activity were linked to the active performance of parental behavior, with sex-reversed tadpole transporting females exhibiting neural activity patterns more similar to those of transporting males than non-caregiving females. We suggest that parallels in hormones and brain gene expression in active and observing parents are related to females' ability to flexibly take over parental care in the absence of their male partners.

Preparation for fatherhood: A role for olfactory communication during human pregnancy?

There is evidence across a range of bi-parental species that physiological changes may occur in partnered males prior to the birth of an infant. It has been hypothesised that these hormonal changes might facilitate care-giving behaviours, which could augment infant survival. The mechanism that induces these changes has not been identified, but evidence from several species suggests that odour may play a role. The current study investigated this in humans by recording testosterone and psychological measures related to infant interest and care in men (n = 91) both before and after exposure to odours from either pregnant women or non-pregnant control women. We found no evidence for an effect of odour cues of pregnancy on psychological measures including self-reported sociosexual orientation and social dominance scores, ratings of infant or adult faces, or testosterone levels. However, we found that brief exposure to post-partum odours significantly increased the reward value of infant faces. Our study is the first to show that the odour of peri-partum women may lead to upregulation of men's interest in infants.

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.

Paternal Care in Biparental Rodents: Intra- and Inter-individual Variation

Parental care by fathers, although rare among mmmals, can be essential for the survival and normal development of offspring in biparental species. A growing body of research on biparental rodents has identified several developmental and experiential influences on paternal responsiveness. Some of these factors, such as pubertal maturation, interactions with pups, and cues from a pregnant mate, contribute to pronounced changes in paternal responsiveness across the course of the lifetime in individual males. Others, particularly intrauterine position during gestation and parental care received during postnatal development, can have long-term effects on paternal behavior and contribute to stable differences among individuals within a species. Focusing on five well-studied, biparental rodent species, we review the developmental and experiential factors that have been shown to influence paternal responsiveness, and consider their roles in generating both intra- and inter-individual variation. We also review hormones and neuropeptides that have been shown to modulate paternal care and discuss their potential contributions to behavioral differences within and between males. Finally, we discuss the possibility that vasopressinergic and possibly oxytocinergic signaling within the brain, modulated by gonadal steroid hormones, may represent the "final common pathway" mediating effects of developmental and experiential variables on intra- and inter-individual variation in paternal care.