The behavioral neuroendocrinology of maternal behavior: Past accomplishments and future directions.

Research on the neuroendocrine-endocrine-neural regulation of maternal behavior has made significant progress the past 50 years. In this mini-review progress during this period has been divided into five stages. These stages consist of advances in the identification of endocrine factors that mediate maternal care, the characterization of the neural basis of maternal behavior with reference to endocrine actions, the impact of developmental and experiential states on maternal care, the dynamic neuroplastic maternal brain, and genes and motherhood. A final section concludes with a discussion of future directions in the field of the neurobiology/neuroendocrinology of motherhood.

Neuroendocrine regulation of maternal behavior.

The expression of maternal behavior in mammals is regulated by the developmental and experiential events over a female's lifetime. In this review the relationships between the endocrine and neural systems that play key roles in these developmental and experiential processes that affect both the establishment and maintenance of maternal care are presented. The involvement of the hormones estrogen, progesterone, and lactogens are discussed in the context of ligand, receptor, and gene activity in rodents and to a lesser extent in higher mammals. The roles of neuroendocrine factors, including oxytocin, vasopressin, classical neurotransmitters, and other neural gene products that regulate aspects of maternal care are set forth, and the interactions of hormones with central nervous system mediators of maternal behavior are discussed. The impact of prior developmental factors, including epigenetic events, and maternal experience on subsequent maternal care are assessed over the course of the female's lifespan. It is proposed that common neuroendocrine mechanisms underlie the regulation of maternal care in mammals.

Long-term alterations in neural and endocrine processes induced by motherhood in mammals.

This article is part of a Special Issue "Parental Care". The reproductive experience of pregnancy, lactation and motherhood can significantly remodel the female's biological state, affecting endocrine, neuroendocrine, neural, and immunological processes. The brain, pituitary gland, liver, thymus, and mammary tissue are among the structures that are modified by reproductive experience. The present review that focuses on rodent research, but also includes pertinent studies in sheep and other species, identifies specific changes in these processes brought about by the biological states of pregnancy, parturition, and lactation and how the components of reproductive experience contribute to the remodeling of the maternal brain and organ systems. Findings indicate that prior parity alters key circulating hormone levels and neural receptor gene expression. Moreover, reproductive experience results in modifications in neural processes and glial support. The possible role of pregnancy-induced neurogenesis is considered in the context of neuroplasticity and behavior, and the effects of reproductive experience on maternal memory, i.e. the retention of maternal behavior, together with anxiety and learning are presented. Together, these sets of findings support the concept that the neural and biological state of the adult female is significantly and dramatically altered on a long-term basis by the experiences of parity and motherhood. Remodeling of the maternal brain and other biological systems is posited to help facilitate adaptations to environmental/ecological challenges as the female raises young and ages.

The effects of bromocriptine treatment during early pregnancy on postpartum maternal behaviors in rats.

Prolactin, a hormone of the anterior pituitary, is involved in initiating maternal behavior, alleviating postpartum anxiety, and stimulating lactogenesis. Bromocriptine, a dopamine D2 receptor agonist, inhibits prolactin secretion. Bromocriptine administration represses postpartum maternal behaviors (pup retrieval) in mice, and causes elevated anxiety in the elevated plus maze [Larsen & Grattan (2010). Endocrinology 151(8): 3805-3814]. Whether similar effects exist in other species is unknown. The present study examined the possible involvement of prolactin during early gestation on maternal behavior and anxiety in rats. Bromocriptine given on days 2-4 of pregnancy resulted in impaired postpartum maternal behaviors in a novel environment during early lactation. However, compared to controls, bromocriptine-treated subjects did not exhibit increased postpartum anxiety in the elevated plus maze. These findings support work in mice that bromocriptine treatment during early gestation impedes postpartum maternal care, and indicate that early gestational hormonal status affects postpartum behavior more broadly in other mammals.

Reproductive experience modifies the effects of estrogen receptor alpha activity on anxiety-like behavior and corticotropin releasing hormone mRNA expression.

Previous studies have demonstrated that prior reproductive experience can influence anxiety-like behaviors, although neural mechanisms underlying this shift remain unknown. Studies in virgin females suggest that activation of the two estrogen receptor subtypes, ERα and ERß, have differing effects on anxiety. Specifically, ERß activation has been shown to reduce anxiety-like behaviors, while ERα activation has no significant effect. The purpose of the present study was to examine the possible roles of ERα and ERß subtypes in parity-induced alterations in anxiety-like behavior, as tested on the elevated plus maze (EPM). Groups of ovariectomized, age-matched, nulliparous and primiparous females were tested on the EPM following administration of the ERα agonist 4,4',4''-(4-Propyl-{1H}-pyrazole-1,3,5-tryl)trisphenol (PPT; 1 mg/kg), the ERß agonist Diarylpropionitrile (DPN; 1 mg/kg) or vehicle (DMSO). All drugs were administered once daily for 4 days prior to testing as this dosing paradigm has previously been used to demonstrate anxiolytic effects of DPN in virgin rats. In addition, as exposure to the EPM is a psychological stressor, physiological markers of the stress response were measured in both plasma (corticosterone) and brain (corticotropin releasing hormone; CRH) post-EPM testing. Unexpectedly, the ERα agonist PPT selectively increased the time spent exploring the open arms of the EPM in non-lactating, primiparous females, with no significant effects of DPN observed in either nulliparous or primiparous subjects. All females administered PPT and tested on the EPM demonstrated significantly reduced corticosterone secretion when compared to vehicle-treated controls. In addition, significant effects of both reproductive experience and PPT administration on CRH mRNA expression were observed in both the paraventricular nucleus and amygdala using qPCR. These findings indicate that reproductive experience modulates the effects of ERα activation on both EPM behavior related to anxiety and CRH gene expression.

Reproductive experience alters prolactin receptor expression in mammary and hepatic tissues in female rats.

Recent studies have reported that reproductive experience in female rats alters prolactin (PRL) receptor gene expression in the brain as well as neural sensitivity to PRL. Given PRL's actions in nonneural tissues, that is, mammary tissue and liver, it was asked whether reproductive experience may also alter prolactin receptor (Prlr) gene expression in these tissues. Groups of age-matched female rats were generated with varying reproductive histories. Separate groups of primiparous (first lactation) and multiparous (second lactation) had mammary tissue and liver samples collected on Day 3 or 10 of lactation. A fifth group raised one litter to weaning and then resumed estrous cyclicity. This group and a final group of age-matched, virgin controls were killed on diestrus. Tissue was processed by quantitative PCR for expression rates of the long and short forms of Prlr mRNA as well as casein beta mRNA (mammary tissue only). Western blots were performed to quantify receptor protein content. Multiple lactations as well as lactation itself resulted in alterations in Prlr expression. Prlr gene expression in mammary tissue was increased in primiparous mothers compared with that in multiparous dams, whereas in the liver, Prlr expression was reduced during an initial lactation. In contrast, PRLR protein levels declined during lactation in mammary, but not hepatic, tissues. Overall, the results demonstrate that the prolactin receptor system is altered in nonneural tissues as a result of the female's reproductive history. The findings are discussed in the context of milk and bile production and PRL's possible role in breast cancer.

Effects of chronic social stress during lactation on maternal behavior and growth in rats.

Maternal mood disorders such as depression and chronic anxiety can negatively affect the lives of not only mothers, but also of partners, offspring, and future generations. Chronic exposure to psychosocial stress is common in postpartum mothers, and one of the strongest predictors of postpartum depression is social conflict. The objective of the current study was to evaluate the effects of chronic social stress (CSS) during lactation on the maternal behavior (which consists of maternal care and aggression toward a novel conspecific) of lactating rats, as well as on the growth of the dams and their offspring. It was hypothesized that chronic daily exposure to a novel male intruder would alter the display of maternal behavior and impair growth in both the dam and offspring during lactation due to the potentially disruptive effects on maternal behavior and/or lactation. The data indicate that CSS during lactation attenuates maternal care and the growth of both dams and pups, and increases self-grooming and maternal aggression toward a novel male intruder. These results support the use of CSS as a relevant model for disorders that impair maternal behavior and attenuate growth of the offspring, such as postpartum depression and anxiety.

Gene expression profiling of pulmonary fibrosis identifies Twist1 as an antiapoptotic molecular "rectifier" of growth factor signaling.

Idiopathic pulmonary fibrosis (IPF) is a progressive and typically fatal lung disease. To gain insight into IPF pathogenesis, we performed gene expression profiling of IPF lungs. Twist1, a basic helix-loop-helix protein, was found among the most consistently and highly up-regulated genes and was expressed in nuclei of type II epithelial cells, macrophages, and fibroblasts in IPF lungs. We studied the function of Twist1 in fibroblasts further, because they are the major effector cells in this disease and persist despite an ambient proapoptotic environment. Twist1 was induced by the profibrotic growth factors (GFs) basic fibroblast growth factor, platelet-derived growth factor, and epidermal growth factor in primary rat lung fibroblasts (RLFs). Suppression of Twist1 expression resulted in decreased RLF accumulation due to increased apoptosis, whereas Twist1 overexpression protected RLFs against several apoptotic stimuli. Addition of platelet-derived growth factor in combination with other GFs led to an increase in proliferation. When Twist1 was depleted, GFs continued to act as mitogens but caused a marked increase in cell death. The increase in apoptosis under basal or growth factor-stimulated conditions was partly mediated by up-regulation of the proapoptotic Bcl-2 family members, Bim and PUMA. These findings indicate that Twist1 promotes survival and accumulation of fibroblasts by shaping their responsiveness to growth factor stimulation. We propose that Twist1 represents one of the factors that promotes pathogenic accumulation of fibroblasts in fibrotic lung disease.

The role of the estrogen receptor-α gene, Esr1, in maternal-like behavior in juvenile female and male rats.

The estrogen receptor-alpha (ER-α) is an important ligand activated transcription factor that works to control gene transcription in many species. Previous studies have shown estrogen to be an important hormone in the regulation of maternal behavior. Like adult female rats, both male and female juvenile rats exhibit increased level of maternal-like behavior when exposed to pups. The aim of this study was to determine whether ER-α is critical for the expression of maternal-like behavior in juvenile male and female rats. ER-α knock-out and wildtype (WT) juvenile male and female rats were generated and tested for maternal behaviors. Latencies to display maternal-like behaviors that included retrieval, grouping and crouching responses, revealed no genotype differences between KO and WT subjects. Male juvenile rats exhibited slightly shorter latencies than WT juvenile female rats indicating a sex difference in the latency to display these responses. Additionally, ER-α KO females exhibited a delay in onset of vaginal opening compared to WT females, indicating a role for ER-α in sexual maturation. The behavioral findings indicate that ER-α is not obligatory for the expression of full maternal-like behavior in male and female juvenile rats. Understanding this neurobiological system will help to elucidate the developmental involvement of the endocrine and brain networks in the regulation of maternal behaviors in mammals.

30 years after: CNS actions of prolactin: Sources, mechanisms and physiological significance.

Our understanding of the neural actions of prolactin (PRL) and its biochemical basis has expanded greatly over the past three decades. During this time, major progress has been made, including clarification of how PRL accesses the brain, identification of the PRL receptor and the sites where it is expressed within the brain, determination of the neurochemical mechanism of action of PRL and its effect on genomic expression in neurones, identification of the neural sites where PRL acts to stimulate maternal behaviour and related affective states, and exploration of how life experiences impact neural PRL receptor activity and actions. The next 30 years promise to reveal a myriad of basic and clinical findings regarding new roles for PRL and a greater indepth understanding of how and where PRL affects physiological and behavioural processes.

Nursing stimulation is more than tactile sensation: It is a multisensory experience.

Novel sensory experiences, particularly those associated with epochal developmental events like nursing alter cortical representation, affecting memory, perception and behavior. Functional MRI was used here to test whether the sensoricortical map of the ventrum is modified during lactation. Three stimuli were used to drive cortical activation in primiparous rats: natural, artificial suckling stimulation and general mechanical rubbing of the skin of the ventrum. These stimuli significantly activated the somatosensory cortex of dams. Of the three stimuli, artificial and pup suckling robustly activated much of the cerebrum, most notably the visual, auditory and olfactory cortices. Surprisingly, activation occurred even in the absence of pups, with artificial suckling. This finding suggests that incoming information from a single modality was sufficient to drive activity of others. Enhanced sensitivity across the cortical mantle during nursing may help the dam to perceive, process, and remember stimuli critical to the care and protection of her young.

Arginine vasopressin V1a receptor antagonist impairs maternal memory in rats.

Primiparous female rats rapidly respond to foster pups following an extended separation from pups after an initial maternal experience. This consolidation of maternal behavior has been referred to as maternal memory. The neurochemical regulation of maternal memory is not clearly understood. One neuropeptide that may mediate maternal memory is arginine vasopressin (AVP), a neuropeptide which is modulated around the time of parturition and has an established role in learning and memory processes. Thus, the present studies examine the possible involvement of AVP in the establishment of maternal memory in female rats. Pregnant rats were implanted with chronic cannulae connected to subcutaneous osmotic minipumps filled with a V1a receptor antagonist [d(CH2)5Tyr(Me)AVP, 0.1-12.5 ng/h] or saline vehicle which were chronically infused either into the lateral ventricles or bilaterally into the medial amygdala beginning on day 18 of gestation. Both the osmotic pumps and the newborn pups were removed 24 h following parturition. The effects of the V1a antagonist treatments on social recognition and maternal behavior were measured following parturition and maternal memory was assessed following a ten day separation from pups. Whereas none of the AVP treatments affected the initial establishment of maternal behavior postpartum, maternal memory was impaired in rats infused into the amygdala with the AVP antagonist (1.25 and 12.5 ng/h). Social recognition was not impaired by intracerebroventricular infusion of either the 0.1 or 1.0 ng/h dose of the V1a antagonist. The present results suggest a role for medial amygdaloid V1a receptors in the establishment of maternal memory.

Central actions of arginine vasopressin and a V1a receptor antagonist on maternal aggression, maternal behavior, and grooming in lactating rats.

Maternal aggression is a robust type of aggression displayed by lactating female rats. Although arginine vasopressin (AVP) has been implicated in the control of male aggression, its involvement in maternal aggression has not been thoroughly investigated. Previous neuroanatomical studies suggest that AVP may mediate the display of aggression during lactation. In the current study, AVP and an AVP V1a receptor antagonist were centrally administered to primiparous rats on days 5 and 15 of lactation, and aggression, maternal behavior, and grooming were recorded. Although AVP did not affect the number of attacks or duration of aggression, it increased the latency to initiate aggression on day 5, in addition to decreasing maternal behavior and increasing grooming. Conversely, V1a antagonist treatment increased maternal aggression on both days of lactation, decreased maternal behavior on day 15, and decreased grooming on day 5. Thus, it appears that central AVP activity modulates maternal aggression, as well as maternal behavior and grooming behavior during lactation.

The progesterone receptor and parental behavior in juvenile rats.

Juvenile rats exhibit enhanced parental behavior responses to foster pups from 18 to 25 days of age, compared to virgin adults. Previous studies in adult rats and mice suggest that progesterone can inhibit the display of parental care towards offspring. The present study investigated the role of progesterone in juvenile rat parental behavior. It is hypothesized that the decrease in parental responsiveness as juveniles age is due a progesterone-dependent inhibitory mechanism. Groups of male and female juveniles were subcutaneously implanted with the progesterone antagonist RU486, progesterone, or control implants. All juveniles were then tested for parental behaviors. No significant effects of either RU486 or progesterone treatments on parental behavior latencies were detected in male or female juveniles. The current data support the concept that the mechanisms controlling juvenile and adult parental behavior are divergent. Progesterone does not seem to be involved in the display of parental behavior in juvenile rats.

Sensorimotor gating and dopamine function in postpartum rats.

There is much speculation regarding the effects of estrogen withdrawal at the end of pregnancy on forebrain dopamine, however, few studies have directly examine changes in this system postpartum. The present work sought to determine what changes in forebrain dopamine function occur in the postpartum rat. Specifically, prepulse inhibition of the acoustic startle response (PPI) was measured in primiparous female rats on postpartum day 2 (PPD2) or 14 (PPD14) following treatment with saline or the dopamine D2 agonist, quinpirole. Diestrus (DI) females served as controls. Dopamine content and turnover as well as cyclic AMP (cAMP) accumulation were determined within the nucleus accumbens and dorsal striatum in these same females. In addition, circulating levels of plasma corticosterone, estradiol and progesterone were measured. PPI was significantly disrupted in both postpartum groups. This effect was associated with decreased cAMP content within the nucleus accumbens. Quinpirole treatment (0.1 and 0.5 mg/kg) dose-dependently disrupted PPI in DI controls while PPD2 and PPD14 animals demonstrated reduced sensitivity to the D2 agonist. PPD14 animals demonstrated increased startle amplitude, an effect that was attenuated by quinpirole treatment. PPD14 females were also less sensitive to quinpirole-mediated reductions in DA turnover within the nucleus accumbens and both PPD2 and PPD14 females had an attenuated response to the stimulatory effects of quinpirole on corticosterone secretion. Collectively these findings suggest that the postpartum period is associated with reduced sensorimotor gating and altered forebrain DA systems, which may be related to shifts in circulating hormones.

Reproductive experience reduces circulating 17beta-estradiol and prolactin levels during proestrus and alters estrogen sensitivity in female rats.

The reproductive experiences of pregnancy, parturition, and lactation affect a range of neural and endocrine processes after the end of lactation. In women, previous parity results in reduced circulating prolactin (PRL) and androgen levels years after giving birth. Reductions in PRL secretion also occur in reproductively experienced, female rats. In the present study we examined the status and regulation of estradiol (E(2)) and PRL during the reproductive cycle after reproductive experience. These hormones regulate one another and have been implicated in a number of disease and aging processes. Using a rat model, the patterns of E(2) and PRL secretion, pituitary PRL content, and estrogen receptor alpha expression were characterized from 1200-1800 h on proestrus in age-matched, primiparous and nulliparous animals. The possible effect of parity on estrogen sensitivity was then examined by challenging nonlactating, ovariectomized, age-matched, multiparous and nulliparous rats with estradiol benzoate (EB; 0, 1, 5, 25, and 125 microg/kg) and measuring PRL responses 24 and 48 h later. Previous parity resulted in modest, yet significant, reductions in E(2) and PRL levels on proestrus, a limited increase in pituitary estrogen receptor alpha expression, and a significant shift in estrogen sensitivity, as measured by EB-induced PRL secretion. Nulliparous animals were more sensitive than multiparous rats to the two lower doses of EB, whereas multiparous animals were more responsive to the highest EB dose. These unique parity-induced alterations in the female's endocrine state that persist beyond lactation may impact a multitude of estrogen-mediated processes over the female's adult life span.

Reproductive experience increases prolactin responsiveness in the medial preoptic area and arcuate nucleus of female rats.

The experience of pregnancy plus lactation produces long-term enhancements in maternal behavior as well as reduced secretion of prolactin, a key hormone for the initial establishment of maternal care. Given that prolactin acts centrally to induce maternal care as well as regulate its own secretion, we tested whether prolactin receptors in brain regions known to regulate behavioral and neuroendocrine processes were up-regulated and more responsive to prolactin in reproductively experienced females. Diestrous primiparous (8 wk after weaning) and age-matched virgin rats were treated with 250 microg ovine prolactin sc or vehicle and the brains collected 2 h later for measurement of mRNA for genes involved in prolactin signaling. Reproductively experienced rats had lower serum prolactin concentrations, compared with virgin rats, suggesting enhanced prolactin feedback on the arcuate neurons regulating prolactin secretion. In the medial preoptic area and arcuate nucleus (regions involved in regulating maternal behavior and prolactin secretion, respectively), the level of long-form prolactin receptor mRNA was higher in primiparous rats, and prolactin treatment induced a further increase in receptor expression in these animals. In the same regions, suppressors of cytokine signaling-1 and -3 mRNA levels were also markedly increased after prolactin treatment in reproductively experienced but not virgin rats. These results support the idea that reproductive experience increases central prolactin responsiveness. The induction of prolactin receptors and enhanced prolactin responsiveness as a result of pregnancy and lactation may help account for the retention of maternal behavior and shifts in prolactin secretion in reproductively experienced females.

Reproductive experience reduces the sedative, but not anxiolytic effects of diazepam.

Benzodiazepines are frequently prescribed to women for both their anxiolytic and hypnotic effects. Previous studies in rodents have demonstrated reproductive experience, i.e. pregnancy and lactation, can alter sensitivity to certain drugs, such as morphine. The purpose of the present study was to determine whether reproductive experience alters sensitivity to the benzodiazepine, diazepam. Two groups of subjects were generated, a primparous group (pregnancy+21 days of lactation) and an age-matched, nulliparous group. All subjects were injected with diazepam (0.0, 0.5, 2.0 or 2.5 mg/kg) at least 6 weeks after primiparous females weaned their litters. Twenty minutes post-injection, subjects were place in an activity chamber and locomotor behavior was measured. Thirty minutes post-injection, subjects were tested on an automated elevated plus maze. In addition to behavioral testing, diazepam's effects on corticosterone levels were measured. Overall, diazepam's sedative effects on locomotor activity were significantly reduced in primiparous females when compared to nulliparous controls as determined both in the activity chamber and on the elevated plus maze. There was, however, no significant effect of reproductive experience on the anxiolytic effects of diazepam in the elevated plus maze. Finally, while diazepam increased corticosterone in both groups, primiparous females were less sensitive to the effects of the drug on corticosterone secretion. These results indicate that the effects of diazepam on locomotor activity and corticosterone secretion are attenuated following reproductive experience.

Steroid-induced alterations in mRNA expression of the long form of the prolactin receptor in the medial preoptic area of female rats: Effects of exposure to a pregnancy-like regimen of progesterone and estradiol.

It is firmly established that the onset of maternal behavior in the female rat is stimulated by a combination of hormones that include prolactin (PRL), estradiol (E2), and progesterone (P(4)). Specifically, nulliparous rats display short latencies to respond to foster young when primed with Silastic capsules filled with P4 and E2 and then administered PRL centrally to the medial preoptic area (MPOA), an area integrally involved in the expression of maternal behavior in this species. PRL or P4 treatments alone are ineffective in stimulating the expression of maternal care. Since the actions of PRL in the MPOA appear to be mediated by PRL receptors, it was of interest to determine whether and how treatment with P4 and E2 together or separately might alter mRNA expression of the long form of the PRL receptor (PRL-R(L)) in the MPOA. Using in situ hybridization histochemistry (ISHH), mRNA expression of the PRL-R(L) was measured in the MPOA of ovariectomized, nulliparous rats treated with various combinations of P4 and E2. Treatment of animals with P4 alone for 10 days or with P4 followed by E2 for 1 or 4 days resulted in reductions in PRL receptor mRNA expression in the MPOA when compared with the expression in animals treated with E2 alone or blank capsules. The actions of P4 on mRNA expression of the PRL-R(L) were more pronounced in the dorsal MPOA. Circulating PRL levels collected at the time of sacrifice were elevated in all groups treated with E2, but no association between PRL levels and receptor mRNA expression within the MPOA was evident. These findings indicate that the dorsal MPOA may be one site of progesterone's action in facilitating prolactin-mediated maternal behavior.

Gestation-induced cell proliferation in the rat brain.

The states of pregnancy and lactation bring about a range of physiological and behavioral changes in the adult mammal that prepare the mother to care for her young. In the present study, the effects of pregnancy on cell proliferation in the subventricular zone (SVZ) in female rats were examined. This region has been identified as a site of neurogenesis in adult rodents. To examine the proliferating cell population, control and pregnant rats were injected with bromodeoxyuridine (BrdU, 90 mg/kg, i.v.) and euthanized 1 h following injection. BrdU was visualized using diaminobenzidine. Changes in BrdU-labeled cells occurred in the female rat brain over pregnancy with significant increases found in the SVZ on day 21 when compared with cycling diestrous females. No changes were evident in the dentate gyrus of the hippocampus as a function of gestation. These data demonstrate that, in the rat, as in mice, gestation stimulates an increase in the number of new cells in selected regions of the central nervous system.