Hormonal priming and triggering of maternal behavior in the rat with special reference to the relations between estrogen receptor binding and ER mRNA in specific brain regions

Parental brain through time: The origin and development of the neural circuit of mammalian parenting

This review consolidates current knowledge on mammalian parental care, focusing on its neural mechanisms, evolutionary origins, and derivatives. Neurobiological studies have identified specific neurons in the medial preoptic area as crucial for parental care. Unexpectedly, these neurons are characterized by the expression of molecules signaling satiety, such as calcitonin receptor and BRS3, and overlap with neurons involved in the reproductive behaviors of males but not females. A synthesis of comparative ecology and paleontology suggests an evolutionary scenario for mammalian parental care, possibly stemming from male-biased guarding of offspring in basal vertebrates. The terrestrial transition of tetrapods led to prolonged egg retention in females and the emergence of amniotes, skewing care toward females. The nocturnal adaptation of Mesozoic mammalian ancestors reinforced maternal care for lactation and thermal regulation via endothermy, potentially introducing metabolic gate control in parenting neurons. The established maternal care may have served as the precursor for paternal and cooperative care in mammals and also fostered the development of group living, which may have further contributed to the emergence of empathy and altruism. These evolution-informed working hypotheses require empirical validation, yet they offer promising avenues to investigate the neural underpinnings of mammalian social behaviors.

Calcitonin receptor signaling in the medial preoptic area enables risk-taking maternal care

Maternal mammals exhibit heightened motivation to care for offspring, but the underlying neuromolecular mechanisms have yet to be clarified. Here, we report that the calcitonin receptor (Calcr) and its ligand amylin are expressed in distinct neuronal populations in the medial preoptic area (MPOA) and are upregulated in mothers. Calcr+ MPOA neurons activated by parental care project to somatomotor and monoaminergic brainstem nuclei. Retrograde monosynaptic tracing reveals that significant modification of afferents to Calcr+ neurons occurs in mothers. Knockdown of either Calcr or amylin gene expression hampers risk-taking maternal care, and specific silencing of Calcr+ MPOA neurons inhibits nurturing behaviors, while pharmacogenetic activation prevents infanticide in virgin males. These data indicate that Calcr+ MPOA neurons are required for both maternal and allomaternal nurturing behaviors and that upregulation of amylin-Calcr signaling in the MPOA at least partially mediates risk-taking maternal care, possibly via modified connectomics of Calcr+ neurons postpartum.

Alteration in oxytocin levels induced by early social environment affects maternal behavior and estrogen receptor alpha in mandarin voles (Microtus mandarinus)

Many studies have shown that the early social environment exerts long-term effects on the brain and also the parental behavior of adults. Oxytocin (OXT) is one of the most important neurotransmitters that regulate social behavior; howerve, whether the early social environment affects parental behavior via OXT remains unclear. Using socially monogamous adult mandarin voles (Microtus mandarinus), the present study found that 1) both paternal deprivation and early social deprivation significantly decreased OXT expression in both the paraventricular hypothalamic nucleus (PVN) and the supraoptic nucleus (SON) of F2 generation offspring; 2) systemic neonatal OXT injection in naïve animals promoted maternal but not paternal behavior in adult F2 offspring; 3) systemic neonatal OXT injection significantly increased ERα expression in both the medial preoptic area (MPOA) and the ventro medial hypothalamic nucleus (VMH) in female but not in male mandarin voles; 4) systemic neonatal administration of an OXT antagonist significantly reduced ERα expression in the bed nucleus of the stria terminalis (BNST), VMH, and the arcuate hypothalamic nucleus (Arc) in females and in all examined brain regions in males. In summary, the obtained data demonstrate that the early social environment could affect OXT level, which in turn leads to long-term effects on ERα expression in relevant brain regions, consequently affecting maternal behavior but not paternal behavior.

Developmental estrogen exposures and disruptions to maternal behavior and brain: Effects of ethinyl estradiol, a common positive control

Due of its structural similarity to the endogenous estrogen 17β-estradiol (E2), the synthetic estrogen 17α-ethinyl estradiol (EE2) is widely used to study the effects of estrogenic substances on sensitive organs at multiple stages of development. Here, we investigated the effects of EE2 on maternal behavior and the maternal brain in females exposed during gestation and the perinatal period. We assessed several components of maternal behavior including nesting behavior and pup retrieval; characterized the expression of estrogen receptor (ER)α in the medial preoptic area (MPOA), a brain region critical for the display of maternal behavior; and measured expression of tyrosine hydroxylase, a marker for dopaminergic cells, in the ventral tegmental area (VTA), a brain region important in maternal motivation. We found that developmental exposure to EE2 induces subtle effects on several aspects of maternal behavior including time building the nest and time spent engaged in self-care. Developmental exposure to EE2 also altered ERα expression in the central MPOA during both early and late lactation and led to significantly reduced tyrosine hydroxylase immunoreactivity in the VTA. Our results demonstrate both dose- and postpartum stage-related effects of developmental exposure to EE2 on behavior and brain that manifest later in adulthood, during the maternal period. These findings provide further evidence for effects of exposure to exogenous estrogenic compounds during the critical periods of fetal and perinatal development.

Bisphenol S (BPS) Alters Maternal Behavior and Brain in Mice Exposed During Pregnancy/Lactation and Their Daughters

Estrogenic endocrine disrupting chemicals have been shown to disrupt maternal behavior in rodents. We investigated the effects of an emerging xenoestrogen, bisphenol S (BPS), on maternal behavior and brain in CD-1 mice exposed during pregnancy and lactation (F0 generation) and in female offspring exposed during gestation and perinatal development (F1 generation). We observed different effects in F0 and F1 dams for a number of components of maternal behavior, including time on the nest, time spent on nest building, latency to retrieve pups, and latency to retrieve the entire litter. We also characterized expression of estrogen receptor α in the medial preoptic area (MPOA) and quantified tyrosine hydroxylase immunoreactive cells in the ventral tegmental area, 2 brain regions critical for maternal care. BPS-treated females in the F0 generation had a statistically significant increase in estrogen receptor α expression in the caudal subregion of the central MPOA in a dose-dependent manner. In contrast, there were no statistically significant effects of BPS on the MPOA in F1 dams or the ventral tegmental area in either generation. This work demonstrates that BPS affects maternal behavior and brain with outcomes depending on generation, dose, and postpartum period. Many studies examining effects of endocrine disrupting chemicals view the mother as a means by which offspring can be exposed during critical periods of development. Here, we demonstrate that pregnancy and lactation are vulnerable periods for the mother. We also show that developmental BPS exposure alters maternal behavior later in adulthood. Both findings have potential public health implications.

The neurobiology of mammalian parenting and the biosocial context of human caregiving

This article is part of a Special Issue "Parental Care". Research on the neurobiology of attachment, pioneered by scholars in the generation that followed the discovery of social bonding, examined the biological basis of mammalian parenting through systematic experiments in animal models and their application to theories on human attachment. This paper argues for the need to construct a theory on the neurobiology of human attachment that integrates findings in animal models with human neuroscience research to formulate concepts based on experimental, not only extrapolative data. Rosenblatt's (2003) three characteristics of mammalian parenting - rapid formation of attachment, behavioral synchrony, and mother-offspring attachment as basis of social organization - are used to guide discussion on mammalian-general versus human-specific attributes of parental care. These highlight specific components of attachment in rodents, primates, and humans that chart the evolution from promiscuous, nest-bound, olfactory-based bonds to exclusive, multi-sensory, and representation-based attachments. Following, three continua are outlined in parental behavior, hormones, and brain, each detailing the evolution from rodents to humans. Parental behavior is defined as a process of trophallaxis - the reciprocal multisensory exchange that supports approach orientation and enables collaboration in social species - and includes human-specific features that enable behavioral synchrony independent of tactile contact. The oxytocin system incorporates conserved and human-specific components and is marked by pulsatile activity and dendritic release that reorganize neural networks on the basis of species-specific attachment experiences. Finally, the subcortical limbic circuit underpinning mammalian mothering extends in humans to include multiple cortical networks implicated in empathy, mentalizing, and emotion regulation that enable flexible, goal-directed caregiving. I conclude by presenting a philosophical continuum from Hobbes to Lorenz, which illustrates how research on the neurobiology of attachment can put in the forefront the social-collaborative elements in human nature and afford a new perspective on the mind-brain polarity.

Beyond a means of exposure: a new view of the mother in toxicology research

Toxicological studies generally view pregnant animals as a conduit through which gestational exposure of offspring to chemicals can be achieved, allowing for the study of developmental toxicity.

Regulation of Estrogen Receptor α Expression in the Hypothalamus by Sex Steroids: Implication in the Regulation of Energy Homeostasis

Sex differences exist in the complex regulation of energy homeostasis that utilizes central and peripheral systems. It is widely accepted that sex steroids, especially estrogens, are important physiological and pathological components in this sex-specific regulation. Estrogens exert their biological functions via estrogen receptors (ERs). ERα, a classic nuclear receptor, contributes to metabolic regulation and sexual behavior more than other ER subtypes. Physiological and molecular studies have identified multiple ERα-rich nuclei in the hypothalamus of the central nervous system (CNS) as sites of actions that mediate effects of estrogens. Much of our understanding of ERα regulation has been obtained using transgenic models such as ERα global or nuclei-specific knockout mice. A fundamental question concerning how ERα is regulated in wild-type animals, including humans, in response to alterations in steroid hormone levels, due to experimental manipulation (i.e., castration and hormone replacement) or physiological stages (i.e., puberty, pregnancy, and menopause), lacks consistent answers. This review discusses how different sex hormones affect ERα expression in the hypothalamus. This information will contribute to the knowledge of estrogen action in the CNS, further our understanding of discrepancies in correlation of altered sex hormone levels with metabolic disturbances when comparing both sexes, and improve health issues in postmenopausal women.

Developmental timing of the effects of maternal care on gene expression and epigenetic regulation of hormone receptor levels in female rats

Maternal care experienced during postnatal development has enduring effects on neuroendocrine function and behavior. Previous studies in rats have illustrated the effect of maternal licking/grooming (LG) on hormone receptors and maternal behavior of adult female offspring associated with altered DNA methylation. However, the developmental timing of these effects, which provide insight into the cellular and molecular pathways through which early experience alters later behavior, had not been explored. Here, we demonstrate the developmental emergence of these outcomes and use cross-fostering to identify sensitive periods for these effects. Estrogen receptor (ER)α and ERβ mRNA levels within the medial preoptic area (MPOA) of the hypothalamus were increased by postnatal day (PN)21 in female offspring of high LG dams; LG-associated increases in oxytocin receptor mRNA levels were observed beyond the weaning period. Quantification of ERα-immunoreactivity indicated a high degree of neuroanatomical specificity of LG effects within the MPOA that were observed by PN6. Reduced DNA methylation and histone 3 lysine 9 tri-methylation and increased histone 3 lysine 4 tri-methylation at the ERα gene promoter (Esr1) were detected at PN21 in high LG female offspring. Latency to engage in maternal behavior toward donor pups was significantly shorter among high LG females. Cross-fostering revealed that maternal sensitization and MPOA ERα levels are sensitive to maternal care experienced before but not after PN10. Differential windows of plasticity were identified for ERβ and oxytocin receptor mRNA levels. These studies contribute significantly to our understanding of the molecular, neurobiological, and behavioral pathways through which variation in maternal behavior is transmitted from one generation to the next.

Wired for behaviors: from development to function of innate limbic system circuitry

The limbic system of the brain regulates a number of behaviors that are essential for the survival of all vertebrate species including humans. The limbic system predominantly controls appropriate responses to stimuli with social, emotional, or motivational salience, which includes innate behaviors such as mating, aggression, and defense. Activation of circuits regulating these innate behaviors begins in the periphery with sensory stimulation (primarily via the olfactory system in rodents), and is then processed in the brain by a set of delineated structures that primarily includes the amygdala and hypothalamus. While the basic neuroanatomy of these connections is well-established, much remains unknown about how information is processed within innate circuits and how genetic hierarchies regulate development and function of these circuits. Utilizing innovative technologies including channel rhodopsin-based circuit manipulation and genetic manipulation in rodents, recent studies have begun to answer these central questions. In this article we review the current understanding of how limbic circuits regulate sexually dimorphic behaviors and how these circuits are established and shaped during pre- and post-natal development. We also discuss how understanding developmental processes of innate circuit formation may inform behavioral alterations observed in neurodevelopmental disorders, such as autism spectrum disorders, which are characterized by limbic system dysfunction.

Exposure to chronic pregnancy stress reverses peripartum-associated adaptations: implications for postpartum anxiety and mood disorders

Maternal adaptations, such as decreased anxiety and attenuated stress responsiveness, are necessary to enable successful postnatal development of the offspring. However, there is growing evidence that they are also required to protect the mental health of the mother and that exposure to chronic stress during pregnancy may prevent such adaptations. Overcrowding stress (24 h) and restraint stress (2 × 1 h) were employed on alternate days between pregnancy d 4-16 to examine the impact of chronic pregnancy stress on relevant behavioral, neuroendocrine, and neuronal peripartum adaptations. To determine whether the chronic stress-induced alterations were specific to the peripartum period, we included virgins as controls. Validating the stress procedure, we demonstrated decreased body-weight gain and increased adrenal weight in stressed dams, relative to their nonstressed controls. Chronic stress prevented a number of peripartum adaptations, including basal plasma hypercorticosterone levels, increased oxytocin mRNA expression in the hypothalamic paraventricular nucleus, and anxiolysis. However, chronic stress did not prevent the peripartum-associated decrease in CRH mRNA expression or attenuate corticosterone response to an acute stressor, nor did it affect hypothalamic vasopressin mRNA expression. Illustrating the specificity of these stress-induced changes to the peripartum period, none of these parameters were affected in stressed virgins. Although chronic stress did not alter depression-related behavior, it reversed the response to acute imipramine treatment and increased active maternal behavior in lactation. Thus, prevention of the peripartum-associated increases in basal corticosterone and oxytocin system activity by pregnancy stress reveal two alterations that may increase the risk of postpartum psychiatric disorders, particularly anxiety.

Neuromolecular basis of parental behavior in laboratory mice and rats: with special emphasis on technical issues of using mouse genetics

To support the well-being of the parent-infant relationship, the neuromolecular mechanisms of parental behaviors should be clarified. From neuroanatomical analyses in laboratory rats, the medial preoptic area (MPOA) has been shown to be of critical importance in parental retrieving behavior. More recently, various gene-targeted mouse strains have been found to be defective in different aspects of parental behaviors, contributing to the identification of molecules and signaling pathways required for the behavior. Therefore, the neuromolecular basis of "mother love" is now a fully approachable research field in modern molecular neuroscience. In this review, we will provide a summary of the required brain areas and gene for parental behavior in laboratory mice (Mus musculus) and rats (Rattus norvegicus). Basic protocols and technical considerations on studying the mechanism of parental behavior using genetically-engineered mouse strains will also be presented.

Impact of gestational cocaine treatment or prenatal cocaine exposure on early postpartum oxytocin mRNA levels and receptor binding in the rat

Prior research reported decreased oxytocin levels in specific brain regions correlated with disruptions in maternal care following gestational cocaine treatment in rats. Similarly, prenatal exposure to cocaine impaired subsequent maternal behavior in adulthood, but behavioral alterations were not associated with decreases in oxytocin levels in the same brain regions as were found in their cocaine-treated rat dams. To determine if other aspects of the oxytocin system are disrupted by cocaine treatment or prenatal exposure to cocaine during critical time points associated with maternal care, oxytocin mRNA transcription and receptor binding were examined on postpartum day two in relevant brain regions following gestational treatment with, or prenatal exposure to, either cocaine or saline. We hypothesized that oxytocin mRNA levels and receptor binding would be differentially affected by cocaine in the early postpartum period of dams and their offspring. Our findings indicate that gestational cocaine treatment resulted in significant increases in oxytocin mRNA levels in only the paraventricular nucleus of cocaine-treated dams, with almost significant increases in both generations in the supraoptic nucleus, but no significant effects of cocaine on receptor binding in either generation of dams. These findings indicate that in addition to oxytocin levels, cocaine treatment or prenatal exposure primarily affects oxytocin mRNA synthesis, with little effect on receptor binding in specific brain regions associated with maternal behavior in the early postpartum period of the rat.

Male pheromones initiate prolactin-induced neurogenesis and advance maternal behavior in female mice

Prolactin is required for rapid onset of maternal behavior after parturition, inducing adaptive changes in the maternal brain including enhanced neurogenesis in the subventricular zone during pregnancy. The resultant increase in olfactory interneurons may be required for altered processing of olfactory cues during the establishment of maternal behavior. Pheromones act through olfactory pathways to exert powerful effects on behavior in rodents and also affect prolactin secretion. Hence, this study aimed to investigate the effect of male pheromones on neurogenesis and maternal behavior in female mice. Virgin female mice were housed individually or in split-cages where they had pheromonal but not physical contact with a male. Maternal behavior was assessed in a foster pup retrieval paradigm. Some mice were injected with bromodeoxyuridine, and the labeled cells visualized using immunohistochemistry. The data show that exposure to male pheromones, for a duration equivalent to a murine pregnancy, advanced maternal behavior in both virgin and postpartum female mice. The pheromone action was dependent on prolactin and ovarian steroids, and was associated with increased cell proliferation in the subventricular zone and subsequent increases in new neurons in the olfactory bulb. Moreover, the effect of pheromones on both cell proliferation and maternal behavior could be induced solely through administration of exogenous prolactin to mimic the pheromone-induced changes in prolactin secretion. The data suggest that male pheromones induce a prolactin-mediated increase in neurogenesis in female mice, resulting in advanced maternal behavior.

Individual differences in novelty-seeking and emotional reactivity correlate with variation in maternal behavior

Numerous studies have demonstrated that Sprague-Dawley rats exhibit a wide range of locomotor reactivity when placed in a novel environment. High Responder (HR) rats show exaggerated locomotor response to novelty, enhanced neuroendocrine stress reactivity, decreased anxiety-like behavior, and propensity to self-administer psychostimulants, compared to the less active Low Responder (LR) animals. Few studies have explored the early environmental factors which may underlie the HR-LR differences in emotional reactivity. Considering the enormous impact of maternal care on rodent neurodevelopment, we sought to examine maternal behavior in HR-LR dams to determine whether they exhibit differences which could contribute to their offspring's differential temperaments. Females, like males, can be classified as HR versus LR, showing marked differences in novelty-induced locomotor activity and anxiety-like behavior. HR-LR mothers behaved differently with their pups during the first two postpartum weeks. LR dams spent greater time licking and nursing their pups compared to HR dams, with the most prominent differences occurring during the second postpartum week. By contrast, when non-lactating HR-LR females were presented with orphaned pups, the pattern of maternal response was reversed. HR females were more responsive and showed greater maternal care of the novel pups compared to LR females, which were probably inhibited due to fear of the unfamiliar pups. This underscores the critical interplay between the female's emotional phenotype, her hormonal status and her familiarity with the pup as key factors in determining maternal behavior. Future work should explore neural and hormonal mechanisms which drive these HR-LR differences in maternal behavior and their impact on the development of the offspring.

Evidence for non-genomic transmission of ecological information via maternal behavior in female rats

Maternal behavior is flexible and programs offspring development. Using a novel manipulation, we demonstrate that rat maternal behavior is sensitive to ecologically relevant stimuli. Long-Evans hooded rat dams (F0) and pups were exposed to a predator condition (cat odor) or a control condition (no odor) for 1 h on the day of parturition. Predator-exposed F0 dams displayed significantly more maternal behavior (licking/grooming, arched-back nursing) relative to control-exposed dams across five subsequent observation days. Female offspring (F1) were raised to adulthood, bred and maternal behavior was observed. F1 dams reared by a predator-exposed F0 dam displayed significantly higher maternal behavior relative to F1 dams reared by a control-exposed F0 dam across 5 days of observation. Increased levels of maternal behavior in predator-reared (PR) F1 dams were evident even in F1 females that had been cross-fostered (CF) from a control-exposed F0 dam, suggesting a non-genomic transmission of increased levels of maternal behavior. Lactating PR F1 dams had significantly elevated estrogen receptor alpha and beta mRNA in the medial preoptic area relative to control-reared (CR) F1 dams. Furthermore, among CR F1 dams, there was no significant difference between those dams that had been CF from predator-exposed F0 dams and those that had been sham CF. These results support the hypothesis that flexible rat maternal behavior can shape offspring development according to current environmental conditions. The results also suggest that estrogen signaling may be part of an epigenetic mechanism by which changes in maternal behavior are passed from F0 to F1 dams.

Administration of 17alpha-ethinylestradiol during pregnancy elicits modifications of maternal behavior and emotional alteration of the offspring in the rat

Intraperitoneal administration of 17alpha-ethinylestradiol (15 microg.kg(-1)) in pregnant rats, every day from day 9 to day 14 of pregnancy, elicited a high percentage of abortions. Quantification of maternal behavior showed that treated dams took better care of their pups than control dams, injected with the vehicle only, did. Postnatal reflexes, which reflect maturational rate, were established more promptly in the offspring of treated dams than in the offspring of control dams. However, when adult, the rats born from treated dams developed anxiety- and depressive-like behaviors. All these results are explained by the effects of the exogenous estrogen on the developing brain of the fetuses.

Lactogenic hormone regulation of maternal behavior

Biological factors can profoundly affect a mother's response to her young. For example, it is well known that the hormones of pregnancy act on the maternal brain to stimulate the spontaneous onset of maternal behavior at parturition. Studies in the rat have provided an excellent model to investigate maternal behavior in mammals, since maternal behavior in rats is easily observable and readily quantifiable and it is well-documented that the endocrine state of gestation helps to bring about the onset of maternal behavior around the time of birth. The same response in virgin animals requires a number of days of constant exposure to pups before maternal-like behaviors emerge. To date, research has established that the steroid hormones, estradiol and progesterone, and the lactogenic hormones, prolactin and the placental lactogens, act in concert to stimulate maternal behavior in the pregnant female. Treatment of adult, virgin rats with these hormones can stimulate a rapid onset of maternal care. In the present chapter experiments are described that demonstrate key roles for prolactin and placental lactogens in the onset of maternal behavior. Central sites of action of prolactin and placental lactogens, including the medial preoptic area, appear to be involved in stimulating the onset of maternal care. Other studies are discussed which support the involvement of the prolactin receptor in the endocrine regulation of maternal behavior using prolactin receptor antagonist and 'knock-out' models in rats and mice, respectively. Overall, these studies indicate that during pregnancy the endocrine system primes the mother's brain so that the new mother displays appropriate and successful behaviors toward her newborn at parturition.

Brain preparations for maternity--adaptive changes in behavioral and neuroendocrine systems during pregnancy and lactation. An overview

Pregnancy, parturition and lactation comprise a continuum of adaptive changes necessary for the development and maintenance of the offspring. The endocrine changes that are driven by the conceptus and are essential for the maintenance of pregnancy and are involved in the preparations for motherhood are outlined. These changes include large increases in the secretion of sex steroid hormones, and the secretion of peptide hormones that are unique to pregnancy. The ability of these pregnancy hormones to alter several aspects of brain function in pregnancy is considered, and the adaptive importance of some of these changes is discussed, for example in metabolic and body fluid adjustments, and the induction of maternal behavior. The importance of sex steroids in determining the timing of the various adaptive changes in preparing for parturition and maternal behavior is emphasized, and the concept that the actions of prolactin and oxytocin, quintessential mammalian motherhood neuropeptides, can serve to coordinate a spectrum of adaptive changes is discussed. The part played by oxytocin neurons and their regulatory mechanisms is reviewed to illustrate how neural systems involved in maternity are prepared in pregnancy via changes in phenotype, synaptic organization and in the relative importance of their different inputs, to function optimally when needed. For oxytocin neurons secreting from the posterior pituitary, important in parturition and essential in lactation, these changes include mechanisms to restrain their premature activation, and adaptations to support synchronized burst firing for pulsatile oxytocin secretion in response to stimulation via afferents from the birth canal, olfactory system or suckled nipples. Within the brain, expression of oxytocin receptors permits centrally released oxytocin to facilitate the expression of maternal behavior. Changes in other neuroendocrine systems are similarly extensive, leading to lactation, suppression of ovulation, reduced stress responses and increased appetite; these changes in lactation are driven by the suckling stimulus. The possible link between these adaptations and changes in cognition and mood in pregnancy and post partum are considered, as well as the dysfunctions that lead to common problems of depression and puerperal psychoses.

Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: a potential circuit for the initiation of male sexual behavior

The present study used anterograde and retrograde tract tracing techniques to examine the organization of the medial preoptic-periaqueductal gray-nucleus paragigantocellularis pathway in the male rat. The location of neurons containing estrogen (alpha subtype; ER alpha) and androgen receptors (AR) were also examined. We report here that injection of the anterograde tracer biotinylated dextran amine (BDA) into the medial preoptic (MPO) produced dense labeling within the periaqueductal gray (PAG); anterogradely labeled fibers terminated in close juxtaposition to neurons retrogradely labeled from the nucleus paragigantocellularis (nPGi). Dual immunostaining for Fluoro-Gold (FG) and ER alpha or FG and AR showed that over one-third of MPO efferents to the PAG contain receptors for either estrogen or androgen. In addition, approximately 50% of PAG neurons retrogradely labeled from the nPGi were immunoreactive for either ER alpha or AR. These results are the first to establish an MPO-->PAG-->nPGi circuit and further indicate that gonadal steroids can influence neuronal synaptic activity within these sites. We reported previously that nPGi reticulospinal neurons terminate preferentially within the motoneuronal pools of the lumbosacral spinal cord that innervate the pelvic viscera. Together, we propose that the MPO-->PAG-->nPGi circuit forms the final common pathway whereby MPO neural output results in the initiation and maintenance of male copulatory reflexes.

Comparison of two positive reinforcing stimuli: pups and cocaine throughout the postpartum period

This set of experiments investigated the appetitive or motivational processes underlying the performance of maternal behavior. The place preference paradigm was adapted to simultaneously investigate the reinforcing properties of cocaine and pups for maternal, lactating dams. These modifications allowed the authors to assess which stimulus, either a 10 mg/kg s.c. injection of cocaine or 3 pups, had the strongest reinforcing value. At Postpartum Days 10 and 16, the dams preferred the cocaine cue-associated chamber, whereas the dams tested at Postpartum Day 8 preferred the pup cue-associated chamber. Overall, the data revealed an interaction between the postpartum period at testing and the exhibited preference for cocaine or pups. Further testing will investigate the neural circuitry underlying the appetitive processes of each stimulus.

Food availability affects behavior but not circulating gonadal hormones in maternal Belding's ground squirrels

We tested predictions of hypotheses suggesting that the steroid hormones, testosterone (T), progesterone (P), and estradiol (E2), contribute to the energetic regulation of behaviors associated with rearing young in free-living female Belding's ground squirrels (Spermophilus beldingi). We provisioned some female S. beldingi with food rich in fat and calories, and used unprovisioned females as controls. We observed the behavior of females throughout the reproductive cycle, and regularly collected blood samples to measure plasma hormone concentrations. Circulating concentrations of T, P, and E2 were similar in provisioned and unprovisioned females, as were temporal patterns of variation in these hormones. Peaks in rates of nest maintenance and aggressive behavior occurred during gestation and were associated with elevated concentrations of circulating T, P, and E2, raising the possibility that one or more of these hormones mediates behaviors that help females establish maternal nest sites and territories after mating. Temporal patterns of variation in behavior were similar among provisioned and unprovisioned females; however, rates of resting, vigilance, and aggression were higher among provisioned females, whereas unprovisioned females devoted significantly more time to feeding and locomotion. Thus, our data suggest that in maternal S. beldingi, gonadal steroids play a role in mediating behavior associated with raising offspring, but do not facilitate changes in rates of behavior associated with increased energy availability.

Variation in steroid hormones associated with infant care behaviour and experience in male marmosets (Callithrix kuhlii)

We describe temporal patterns of change in paternal behaviour and urinary concentrations of the steroid hormones testosterone (T) and oestradiol (E(2)) in male black tufted-ear marmosets, Callithrix kuhlii, relative to the birth of their young, and test predictions of the hypotheses that (1) high levels of T are incompatible with paternal care and (2) levels of T and E(2)vary with a father's prior experience in his family group. After young were born, levels of urinary T and E(2)remained near prepartum concentrations and rates at which fathers carried infants were below peak levels until the approximate time that postpartum mating ordinarily occurs, suggesting a possible trade-off between readiness to mate and paternal behaviour in C. kuhlii. Infant-carrying behaviour of fathers occurred at its highest rate 3-4 weeks after parturition and coincided with significant declines in urinary levels of T and E(2), providing preliminary support for the hypothesis that these hormones are antagonistic to paternal behaviour. Urinary T and E(2)declined among fathers regardless of whether their young survived to weaning or died at birth, indicating that variation in these hormones after parturition occurs even in the absence of continued stimuli from infants. When adjusted for declines ordinarily associated with aging, urinary T tended to be lower among fathers with a great deal of prior experience caring for young compared with fathers having little or no experience, suggesting that either experience affects T levels of fathers, or that T levels influence fathers' chances of successfully rearing infants. Overall, our results suggest that in male C. kuhlii, T, and possibly E(2), play an important role in balancing the expression of paternal care with that of other reproductive behaviours. Copyright 2000 The Association for the Study of Animal Behaviour.

Hypothalamic involvement in the regulation of maternal behaviour in the rat: inhibitory roles for the ventromedial hypothalamus and the dorsal/anterior hypothalamic areas

The present report examines the possible involvement of the ventromedial hypothalamus (VMH), the dorsal hypothalamus (DH), and the anterior hypothalamic area (AHA) in the regulation of maternal behaviour in the female rat. In a series of experiments it was found that either infusions of saline or lowering cannulas into the VMH stimulated a rapid onset of maternal behaviour in progesterone plus oestrogen-primed, nulliparous rats. The stimulatory effect of cannula lowering into the VMH on maternal behaviour was shown to be steroid-dependent. Next, the involvement of cell bodies located in the DH/AHA in maternal behaviour was examined after bilateral lesions of these regions with the neurotoxin, N-methyl-D-aspartic acid (NMA). NMA lesions of the DH/AHA stimulated a rapid onset of maternal behaviour in oestrogen-treated, nulliparous rats, while NMA lesions in non-steroid-treated animals or vehicle infusions in steroid or non-steroid-treated rats failed to induce a rapid onset of behaviour. In a final study the effects of NMA lesions of the VMH were evaluated. As in AHA lesioned rats, NMA lesions of the VMH stimulated a fast onset of maternal behaviour in steroid-primed females. These findings indicate that the VMH as well as the DH/AHA exert chronic steroid-dependent inhibitory influences on the induction of maternal behaviour.

Estrogen implants in the medial preoptic area stimulate maternal behavior in male rats

The present study investigated whether the medial preoptic area (MPOA) mediates estrogen stimulation of maternal behavior in the male as it does in the female. Previous studies have shown that lesions of the medial preoptic area prevent sensitization of maternal behavior in male rats and that in gonadectomized, hormonally primed males, systemically administered estradiol benzoate stimulates short-latency maternal behavior. These findings are similar to those found in females. In the present study adult males were gonadectomized and hormonally primed with subcutaneously implanted capsules of estradiol (Days 1-16) and progesterone (Days 3-15) and then were stereotaxically implanted bilaterally in the MPOA with implants containing 10% estradiol. Tests with young pups were started 48 h later and continued for 10 days (11 tests). Control groups were implanted in the MPOA with cholesterol or were injected subcutaneously with estradiol benzoate (100 microg/kg). Estradiol implanted males had shorter latencies for maternal behavior (retrieving, crouching, licking pups) than cholesterol implanted males, but their latencies were slightly longer than those of estradiol benzoate injected males. The medial preoptic area, therefore, mediates estrogen stimulation of maternal behavior in males as it does in females.

Differential expression of estrogen receptor mRNA and protein in the female rat preoptic area

The expression of estrogen receptor (ER) mRNA and ER protein in the medial preoptic area of ovariectomized rat was investigated at both cellular and regional levels using non-isotopic in situ hybridization and immunohistochemistry. ER mRNA was localized in the cytoplasm, while both liganded and unliganded forms of the ER protein were confined to the nucleus. Furthermore, ER mRNA containing cells were evenly distributed throughout the medial preoptic area, showing a homogeneous staining pattern compared to that of ER protein. ER immunoreactive cells were highly distributed in the medial, moderately in the lateral aspect of the medial preoptic area, showing a heterogeneous staining pattern with strongly and weakly labeled cells. These results suggest that ER protein levels are controlled by cellular posttranscriptional mechanisms.

Comparative neuroendocrinology of steroid receptor gene expression and regulation: Relationship to physiology and behavior

Great diversity exists among vertebrates in reproductive behaviors and the neuroendocrine mechanisms underlying these behaviors. Comparisons of species with different hormone-brain-behavior relationships reveal three factors which may explain species differences in endocrine physiology and behavior: (a) sensitivity to sex steroid hormones, (b) hormone-dependent regulation of sex steroid hormone receptor gene expression, and (c) neuroanatomical distribution of steroid receptor gene expression, especially in nonlimbic structures.