Levels of estrogen receptor immunoreactivity are altered in behaviorally-relevant brain regions in female rats during pregnancy

Medial preoptic circuits governing instinctive social behaviors

The medial preoptic area (MPOA) has long been implicated in maternal and male sexual behavior. Modern neuroscience methods have begun to reveal the cellular networks responsible, while also implicating the MPOA in other social behaviors, affiliative social touch, and aggression. The social interactions rely on input from conspecifics whose most important modalities in rodents are olfaction and somatosensation. These inputs bypass the cerebral cortex to reach the MPOA to influence the social function. Hormonal inputs also directly act on MPOA neurons. In turn, the MPOA controls social responses via various projections for reward and motor output. The MPOA thus emerges as one of the major brain centers for instinctive social behavior. While key elements of MPOA circuits have been identified, a synthesis of these new data is now provided for further studies to reveal the mechanisms by which the area controls social interactions.

Electrophysiological characterization of medial preoptic neurons in lactating rats and its modulation by hypocretin-1

The medial preoptic area (mPOA) undergoes through neuroanatomical changes across the postpartum period, during which its neurons play a critical role in the regulation of maternal behavior. In addition, this area is also crucial for sleep-wake regulation. We have previously shown that hypocretins (HCRT) within the mPOA facilitate active maternal behaviors in postpartum rats, while the blockade of endogenous HCRT in this area promotes nursing and sleep. To explore the mechanisms behind these HCRT actions, we aimed to evaluate the effects of juxta-cellular HCRT-1 administration on mPOA neurons in urethane-anesthetized postpartum and virgin female rats. We recorded mPOA single units and the electroencephalogram (EEG) and applied HCRT-1 juxta-cellular by pressure pulses. Our main results show that the electrophysiological characteristics of the mPOA neurons and their relationship with the EEG of postpartum rats did not differ from virgin rats. Additionally, neurons that respond to HCRT-1 had a slower firing rate than those that did not. In addition, administration of HCRT increased the activity in one group of neurons while decreasing it in another, both in postpartum and virgin rats. The mechanisms by which HCRT modulate functions controlled by the mPOA involve different cell populations.

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.

Neural circuits of social behaviors: Innate yet flexible

Social behaviors, such as mating, fighting, and parenting, are fundamental for survival of any vertebrate species. All members of a species express social behaviors in a stereotypical and species-specific way without training because of developmentally hardwired neural circuits dedicated to these behaviors. Despite being innate, social behaviors are flexible. The readiness to interact with a social target or engage in specific social acts can vary widely based on reproductive state, social experience, and many other internal and external factors. Such high flexibility gives vertebrates the ability to release the relevant behavior at the right moment and toward the right target. This maximizes reproductive success while minimizing the cost and risk associated with behavioral expression. Decades of research have revealed the basic neural circuits underlying each innate social behavior. The neural mechanisms that support behavioral plasticity have also started to emerge. Here we provide an overview of these social behaviors and their underlying neural circuits and then discuss in detail recent findings regarding the neural processes that support the flexibility of innate social behaviors.

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.

Neuroanatomical and neurochemical basis of parenting: Dynamic coordination of motivational, affective and cognitive processes

This article is part of a Special Issue "Parental Care". Becoming a parent is arguably the most profound transforming experience in life. It is also inherently very emotionally and physically demanding, such that the reciprocal interaction with the young changes the brain and behavior of the parents. In this review, we examine the neurobiological mechanisms of parenting primarily discussing recent research findings in rodents and primates, especially humans. We argue that it is essential to consider parenting within a conceptual framework that recognizes the dynamics of the reciprocal mother-young relationship, including both the complexity and neuroplasticity of its underlying mechanisms. Converging research suggests that the concerted activity of a distributed network of subcortical and cortical brain structures regulates different key aspects of parenting, including the sensory analysis of infant stimuli as well as motivational, affective and cognitive processes. The interplay among these processes depends on the action of various neurotransmitters and hormones that modulate the timely and coordinated execution of caregiving responses of the maternal circuitry exquisitely attuned to the young's affect, needs and developmental stage. We conclude with a summary and a set of questions that may guide future research.

Nuclear receptors in transgenerational epigenetic inheritance

Nuclear Receptors are ligand-activated transcription factors that translate information about the lipid environment into specific genetic programs, a property that renders them good candidates to be mediators of rapid adaptation changes of a species. Lipid-based morphogens, endocrine hormones, fatty acids and xenobiotics might act through this class of transcription factors making them regulators able to fine-tune physiological processes. Here we review the basic concepts and current knowledge on the process whereby small molecules act through nuclear receptors and contribute to transgenerational changes. Several molecules shown to cause transgenerational changes like phthalates, BPA, nicotine, tributylin bind and activate nuclear receptors like ERs, androgen receptors, glucocorticoid receptors or PPARγ. A specific subset of observations involving nuclear receptors has focused on the effects of environmental stress or maternal behaviour on the development of transgenerational traits. While these effects do not involve environmental ligands, they change the expression levels of Estrogen and glucocorticoid receptors of the second generation and consequently initiate an altered genetic program in the second generation. In this review we summarize the available literature about the role of nuclear receptors in transgenerational inheritance.

Rat dams exposed repeatedly to a daily brief separation from the pups exhibit increased maternal behavior, decreased anxiety and altered levels of receptors for estrogens (ERα, ERβ), oxytocin and serotonin (5-HT1A) in their brain

In the present study we investigated the neurobiological mechanisms underlying expression of maternal behavior. Increased maternal behavior was experimentally induced by a brief 15-min separation between the mother and the pups during postnatal days 1 to 22. On postnatal days (PND) 12 and 22, we determined in experimental and control dams levels of anxiety in the elevated plus maze (EPM) as well as the levels of receptors for estrogens (ERα, ERβ), oxytocin (OTR) and serotonin (5-HT1AR) in areas of the limbic system (prefrontal cortex-PFC, hippocampus, lateral septum-SL, medial preoptic area-MPOA, shell of nucleus accumbens-nAc-Sh, central-CeA and basolateral-BLA amygdala), involved in the regulation of maternal behavior. Experimental dams, which showed increased maternal behavior towards their offspring, displayed reduced anxiety in the EPM on both PND12 and PND22. These behavioral differences could be attributed to neurochemical alterations in their brain: On both PND12 and PND22, experimental mothers had higher levels of ERα and OTRs in the PFC, hippocampus, CeA, SL, MPOA and nAc-Sh. The experimental manipulation-induced increase in ERβ levels was less widespread, being localized in PFC, the hippocampal CA2 area, MPOA and nAc-Sh. In addition, 5-HT1ARs were reduced in the PFC, hippocampus, CeA, MPOA and nAc-Sh of the experimental mothers. Our results show that the experience of the daily repeated brief separation from the pups results in increased brain ERs and OTRs, as well as decreased 5-HT1ARs in the dam's brain; these neurochemical changes could underlie the observed increase in maternal behavior and the reduction of anxiety.

Neuroendocrinology of pregnancy and parturition

During pregnancy, the maternal brain drives a series of adaptive mechanisms that are fundamental for allowing fetal growth and development, protecting both mother and fetus from adverse programming and timing of parturition. This neuroendocrine concept is even more complex as fetal brain and placenta also participate as regulators of maternal-placental-fetal physiology. The placenta is now seen as a neuroendocrine organ, acting as a source of several neuroactive factors that may exert their biologic effects either locally or by entering maternal and fetal circulation, thus acting in an autocrine, paracrine, and endocrine manner. A variety of hypothalamic neurohormones (GnRH, GHRH, somatostatin, CRH, oxytocin) are expressed in the placenta. When stress occurs during pregnancy, the maternal, fetal, and placental hypothalamic-pituitary-adrenal (HPA) axes are activated to stimulate a series of responses contributing to maintain physiologic conditions while at the same time avoiding the adverse effects of stress on the mother and offspring. However, when stress is excessive, a number of obstetric complications may occur, such as preterm birth, pre-eclampsia and intrauterine growth restriction, related to an impairment of the placental adaptive response.

Sex hormones and expression pattern of cytoskeletal proteins in the rat brain throughout pregnancy

Pregnancy involves diverse changes in brain function that implicate a re-organization in neuronal cytoskeleton. In this physiological state, the brain is in contact with several hormones that it has never been exposed, as well as with very high levels of hormones that the brain has been in touch throughout life. Among the latter hormones are progesterone and estradiol which regulate several brain functions, including learning, memory, neuroprotection, and the display of sexual and maternal behavior. These functions involve changes in the structure and organization of neurons and glial cells that require the participation of cytoskeletal proteins whose expression and activity is regulated by estradiol and progesterone. We have found that the expression pattern of Microtubule Associated Protein 2, Tau, and Glial Fibrillary Acidic Protein changes in a tissue-specific manner in the brain of the rat throughout gestation and the start of lactation, suggesting that these proteins participate in the plastic changes observed in the brain during pregnancy. This article is part of a Special Issue entitled 'Pregnancy and Steroids'.

Prefrontal activity during response inhibition decreases over time in the postpartum period

The postpartum period is characterized by complex hormonal changes, but human imaging studies in the postpartum period have thus far predominantly focused on the neural correlates of maternal behavior or postpartum depression, whereas longitudinal studies on neural correlates of cognitive function across the postpartum period in healthy women are lacking. The aim of this study was to longitudinally examine response inhibition, as a measure of executive function, during the postpartum period and its neural correlates in healthy postpartum women and non-postpartum controls. Thirteen healthy postpartum women underwent event-related functional magnetic resonance imaging while performing a Go/NoGo task. The first assessment was made within 48 h of delivery, and the second at 4-7 weeks postpartum. In addition, 13 healthy women examined twice during the menstrual cycle were included as non-postpartum controls. In postpartum women region of interest analyses revealed task-related decreased activations in the right inferior frontal gyrus, right anterior cingulate, and bilateral precentral gyri at the late postpartum assessment. Generally, postpartum women displayed lower activity during response inhibition in the bilateral inferior frontal gyri and precentral gyri compared to non-postpartum controls. No differences in performance on the Go/NoGo task were found between time-points or between groups. In conclusion, this study has discovered that brain activity in prefrontal areas during a response inhibition task decreases throughout the course of the first postpartum weeks and is lower than in non-postpartum controls. Further studies on the normal adaptive brain activity changes that occur during the postpartum period are warranted.

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

Oestrogen receptor (ER)alpha immunoreactivity in three brain regions relevant to maternal behaviour (medial preoptic area, bed nucleus of the stria terminalis and medial amygdala) was measured in two species of dwarf hamster that both mate during a postpartum oestrous but differ in expression of paternal behaviour. Male and female Phodopus campbelli and Phodopus sungorus were sampled as sexually naive adults, following mating to satiety, and as new parents. In all brain regions, females expressed higher levels of ER alpha than males. Species did not have an effect on ER alpha distribution except in the medial amygdala, where P. sungorus females had higher expression levels than all other groups. Behavioural status was not associated with altered ER alpha expression. These results were not expected for females and suggest that a primary activational role for oestrogen, acting through ER alpha in these regions, does not generalize to maternal behaviour in Phodopus. In males, these results are consistent with previous manipulations of the ER alpha ligand, oestrogen, and suggest that paternal behaviour in P. campbelli is likely to be regulated by developmental effects of oestrogen on the brain during early life (similar to Microtus ochrogaster), rather than through activation by oestrogen at the time of fatherhood (similar to Peromyscus californicus).

Activation and circuitry of uterine-cervix-related neurons in the lumbosacral dorsal root ganglia and spinal cord at parturition

Stimulation of the uterine cervix at parturition activates neural circuits involving primary sensory nerves and supraspinally projecting neurons of the lumbosacral spinal cord, resulting in output of hypothalamic neurohormones. Dorsal root ganglia (DRG) and spinal neurons of these circuits are not well-characterized. The objectives of this study were to detail the activation of DRG and spinal neurons of the L6/S1 levels that are stimulated at late pregnancy, verify hypothalamic projections of activated spinal neurons, and determine whether activated neurons express estrogen receptor-alpha (ERalpha). Expression of phosphorylated cyclic-AMP response element-binding protein (PCREB) and Fos immunohistochemistry were used to "mark" activated DRG and spinal neurons, respectively. Retrograde tracing identified uterine-cervix-related and spinohypothalamic neurons. Baseline PCREB expression in the DRG increased during pregnancy and peaked during the last trimester. Some PCREB-expressing neurons contained retrograde tracer identifying them as cervix-related neurons. Fos-expressing neurons were few in spinal cords of nonpregnant and day 22 pregnant rats but were numerous in parturient animals. Some Fos-expressing neurons located in the dorsal half of the spinal cord contained retrograde tracer identifying them as spinohypothalamic neurons. Some DRG neurons expressing PCREB also expressed ERalpha, and some spinal neurons activated at parturition projected axons to the hypothalamus and expressed ERalpha. These results indicate that DRG and spinal cord neurons are activated at parturition; that those in the spinal cord are present in areas involved in autonomic and sensory processing; that some spinal neurons project axons to the hypothalamus, ostensibly part of a neuroendocrine reflex; and that sensory and spinal neurons can respond to estrogens. Moreover, some activated sensory neurons may be involved in the animal's perception of labor pain.

Neural steroid hormone receptor gene expression in pregnant rats

Estrogen and progesterone play important roles during pregnancy in stimulating the onset of maternal behavior at parturition. The status of receptor expression of these hormones during pregnancy in neural regions that regulate maternal behavior is unclear. The objective of the present study is to characterize changes in neural gene expression of the estrogen receptors alpha and beta (ERalpha and ERbeta) and the progesterone receptor (PR) during the latter part of pregnancy. Brains from primigravid Sprague-Dawley rats were collected on days 15 and 21 of pregnancy. Micropunches of the olfactory bulb (OB), medial preoptic area (MPOA), bed nucleus of the stria terminalis (BnST), hypothalamus (HYP), medial amygdala (MeA), and the temporal cortex (TCx) were analyzed by real-time RT-PCR (Taqmantrade mark) for levels of gene expression. No changes in either ERalpha or ERbeta mRNA levels were detected in any brain region between days 15 and 21 of pregnancy: however, the MPOA had higher levels of both ERalpha and ERbeta than other brain regions. Progesterone receptor mRNA levels, in contrast, declined significantly in the MPOA, HYP, and TCx, between days 15 and 21 of pregnancy (P < 0.05). In addition, the levels of PR mRNA were significantly higher in the HYP and TCx compared to both the OB and MeA. These data indicate that there is a downregulation of PR prepartum and suggest that this decrease may play a role in the disinhibition of maternal behavior at parturition.

Estradiol receptor-alpha expression in hypothalamic and limbic regions of ewes is influenced by physiological state and maternal experience

The influence of estrus, pregnancy, parturition, and maternal experience on the expression of estrogen receptor-alpha (ERalpha) was investigated in hypothalamic and limbic regions of the sheep brain, using immunocytochemistry. Four days before parturition, previous maternal experience was associated with a higher density of ERalpha-labeled neurons in the paraventricular and supraoptic nuclei, the medial preoptic area, and the medial amygdala, but not in the mediobasal hypothalamus. Furthermore, an interaction was found between physiological state and experience in the peripartum period as the effect of experience existing 4 days prepartum was not found at parturition, when densities were lowest both in primiparous and in multiparous ewes. An additional effect of physiological state was also observed between parturition and estrus, densities being significantly lower at parturition than at estrus in the SON, PVN, and MPOA, but not in the medial amygdala. These results indicate that in sheep ERalpha expression is influenced by previous physiological and/or maternal experience at specific times of the reproductive cycle. They are also congruent with the higher ability of multiparous than nulliparous ewes to show maternal behavior several days prepartum.

Bisphenol-A exposure during pregnancy and lactation affects maternal behavior in rats

In mammals, endogenous estrogens are crucial for sexual differentiation during the perinatal period, and the modulation in adulthood of many neuroendocrine and behavioral functions involved in reproduction. In rats, the estrogenic environment during pregnancy and lactation affects directly maternal behavior. This experiment was aimed to test whether the exposure to the estrogenic compound bisphenol-A (BPA; 0.040 mg/kg/die, orally) of adult female rats, from mating to weaning of the pups, could alter maternal behavior. An appropriate methodology was applied to reveal differences in the behavior of dams directed to male and female pups, testing the dams on postnatal days 3-4 and 8-9. Results show different maternal behavioral patterns towards male and female pups of control mothers, with more ano-genital licking to males than to females. Exposure of mothers to BPA modified their behavior, reducing specific components of maternal behavior, both active and passive, irrespective of the sex of pups and the period of observation. This experiment shows that maternal behavior is affected by a prolonged exposure to a low dose of BPA during pregnancy and lactation, thus suggesting an effect on neural circuits in adulthood.

Estrogen receptor beta messenger ribonucleic acid expression in the forebrain of proestrous, pregnant, and lactating female rats

Estrogen receptor (ER)beta is present in hypothalamic and limbic neurons of female rat brains, but little is known about its regulation under physiological conditions. To determine whether ER beta expression varies during physiological conditions in which sex steroid hormone profiles are significantly different, we used in situ hybridization to assess ER beta mRNA expression in the periventricular preoptic area, bed nucleus of stria terminalis, paraventricular nucleus, supraoptic nucleus, and the posterodorsal medial amygdala of female rats on proestrus, on d 22 of pregnancy, or on d 10 of lactation (L10). In the periventricular preoptic area, d-22 pregnant females had fewer ER beta-mRNA-expressing cells than did females at proestrus, but the level of ER beta mRNA expression per cell in pregnant females was higher than in the two other groups. In the paraventricular nucleus, no changes in ER beta mRNA expression were observed; whereas in the supraoptic nucleus, proestrous females had fewer ER beta-mRNA-expressing cells than L10 females. In the posterodorsal medial amygdala, proestrous females had a greater number of ER beta-mRNA-expressing cells than did L10 females. These results demonstrate that ER beta mRNA expression is differentially regulated in a brain-region-specific and temporal manner under physiological conditions and suggest that ER beta may participate in the regulation of estrogen-sensitive reproductive functions in female rats.

Intracellular preoptic and striatal monoamines in pregnant and lactating rats: possible role in maternal behavior

In many mammals, hormonal fluctuations during pregnancy and parturition produce neurochemical events that are necessary for the transition from a non-maternal state to a maternal state that occurs when infants are born. However, the nature of these events is mostly unknown. We investigated whether changes in dopamine (DA) and serotonin (5-HT) activity within the preoptic area (POA) and striatum, neural sites important for some maternal behaviors, could be part of this process. Female rats were sacrificed as either diestrus virgins, on pregnancy day 10 or 20, on the day of parturition, or on day 7 or 17 of lactation. Bilateral tissue punches from the POA, dorsolateral striatum (ST(dl)), and nucleus accumbens (NA) were obtained and levels of intracellular DA and 5-HT analyzed with high-performance liquid chromatography with electrochemical detection (HPLC-EC). In the POA, DA was high in virgins and during early pregnancy, lowest on the day of parturition, and very high during lactation. Although there were no changes in the DOPAC to DA ratio (i.e., turnover), DOPAC levels also followed this pattern. 5-HT turnover in the POA was lower in virgins compared to other groups. In the ST(dl), DA turnover was highest during late pregnancy and on the day of parturition, while no changes in 5-HT measures were found. No significant effects were found in the NA. Therefore, decreased DAergic activity in the POA and increased DAergic activity in the ST(dl) occurs around parturition, the time when maternal behavior emerges, and may influence its onset.

Estrogen Receptors in the Spinal Cord, Sensory Ganglia, and Pelvic Autonomic Ganglia

Until relatively recently, most studies of the effects of estradiol in the nervous system focused on hypothalamic, limbic, and other brain centers involved in reproductive hormone output, feedback, and behaviors. Almost no studies addressed estradiol effects at the spinal cord or peripheral nervous system level. Prior to the mid-1960s-1970s, few studies examined neural components of reproductive endocrine organs (e.g., ovary or testis) or the genital organs (e.g., uterus or penis) because available data supported endocrine regulation of these structures. Over the last two decades interest in and studies on the innervation of the genital organs have burgeoned. Because of the responsiveness of genital organs to sex steroid hormones, these neural studies seeded interest in whether or not autonomic and sensory neurons that innervate these organs, along with their attendant spinal cord circuits, also are responsive to sex hormones. From the mid-1980s there has been a steady growth of interest in, and studies of the neuroanatomy, neurochemistry, neural connectivity, and neural functional aspects in reproductive organs and the response of these parameters to sex steroids. Thus, with the growth of probes and techniques, has come studies of anatomy, neurochemistry, and circuitry of sex hormone-responsive neurons and circuits in the spinal cord and peripheral nervous system. This review focuses on estrogen receptors in sensory, autonomic, and spinal cord neurons in locales that are associated with innervation of female reproductive organs.

Estrogen receptor-alpha and neural circuits to the spinal cord during pregnancy

Estrogen receptor immunoreactivity and mRNAs are present in spinal cord neurons in locations that are associated with sensory and autonomic innervation of female reproductive organs. The present study was undertaken to examine the expression of estrogen receptor-alpha in the spinal cord during different stages of pregnancy and to determine whether estrogen receptor-alpha-expressing neurons are related to uterine afferent nerves bringing information to the spinal cord at parturition. Immunohistochemistry showed estrogen receptor-alpha-immunoreactive neurons in the dorsal one-half of the spinal cord, i.e., dorsal horn, dorsal intermediate gray areas (dorsal commissural nucleus), and around the central canal and sacral parasympathetic autonomic nucleus of the lumbosacral spinal cord. Neurons in these areas corresponded topographically to the distribution of central processes of visceral primary afferent neurons (e.g., containing calcitonin gene-related peptide and substance P) that innervate and activate second-order spinal cord neurons (evidenced by their expression of Fos) at parturition. Western blots showed that estrogen receptor-alpha increases in the spinal cord, with a peak at day 20 of gestation, followed by a slight decrease by 2 days postpartum. These studies show that estrogen receptor-alpha is expressed by neurons in autonomic and sensory areas of the lumbosacral spinal cord that have connections with the female reproductive system and that the level of estrogen receptor-alpha changes over the course of pregnancy, which may follow profiles of steroid hormones. Many of these neurons may be involved in processing information related to reproductive organ function, changes during pregnancy, and relays to other CNS centers.

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.

Using c-Fos immunocytochemistry to identify forebrain regions that may inhibit maternal behavior in rats

Evidence indicates there is a neural system that inhibits maternal behavior in virgin rats. It has been suggested that pregnancy hormones promote the onset of maternal behavior by reducing the behavioral influence of this system. The authors used c-Fos immunocytochemistry to identify brain regions more activated by pup exposure in nonmaternal rats than in maternal rats. Previous experiments indicated that some of these regions, such as the posterodorsal medial amygdala and several medial hypothalamic sites, inhibit maternal behavior. For others, such as the ventral lateral septum, dorsal premammillary nucleus, and principal bed nucleus of the stria terminalis, this is the first indication that they could also inhibit maternal responding. These regions have previously been implicated in promoting defensive behaviors, consistent with the finding that nonmaternal rats actively avoid pups. These findings suggest the existence of a neural circuit through which pup exposure could promote defensive responses in virgin rats, and how pregnancy hormones could reduce such activity to stimulate maternal behavior.

Appetitive and consummatory sexual behaviors of female rats in bilevel chambers. II. Patterns of estrus termination following vaginocervical stimulation

Copulation with intromission or manual vaginocervical stimulation (VCS) shortens the duration that intact female rats maintain lordosis responding during estrus. The present study examined whether VCS could shorten the duration of both appetitive and consummatory measures of female sexual behavior, and whether these effects occur differentially in time and across different hormone priming intervals. Ovariectomized, sexually experienced female rats were administered subcutaneous injections of estradiol benzoate 48 h and progesterone 4 h, before receiving 50 manual VCSs with a lubricated glass rod distributed over 1 h. Control females received sham VCSs distributed over the same time. The females were then tested for sexual behavior in bilevel chambers with two sexually vigorous males (to one ejaculatory series or 10 min with each male, separated by 5 min) 12, 16, and 20 h after VCS. Prior to the final hormone treatment, different groups of females had been given the same hormone treatment either 28, 14, 7, or 4 days before. In females tested at 28- and 14-day hormone intervals, VCS induced both active and passive rejection responses at 12, 16, and 20 h. In contrast, females that received sham VCS displayed relatively normal sexual behavior at 12 h, although by 16 and 20 h these females displayed active and passive rejection. Females tested at 7- or 4-day intervals displayed normal levels of lordosis at all testing times, regardless of VCS treatment. These data indicate that VCS facilitates rejection responses that precede the decrease in lordosis responsiveness. However, the effects of VCS are dependent on the frequency of hormone priming, suggesting that hormone treatment may block some of the long-term inhibitory effects of VCS on female sexual behavior.

Atlas of the neurons that express mRNA for the long form of the prolactin receptor in the forebrain of the female rat

Prolactin has a variety of important physiological effects on peripheral tissue and on the brain. The behavioral effects of prolactin include the induction of maternal behavior and increased food intake. Prolactin acts via its cognate receptors which have two forms, a short and a long form. The long form of the receptor is predominant in the preoptic area-hypothalamus and is positioned to support maternal behavior since this form is regulated across pregnancy and lactation (Nagano and Kelly [1994] J. Biol. Chem. 269:13337-13345; Sugiyama et al. [1994] J. Endocrinol. 141:325-333). By using in situ hybridization with [33P] labelled cRNA probe specific for the long form of the receptor mRNA(L-PRL mRNA) we have mapped, in brains from 2- and 21-day-old pregnant females, the neuroanatomical distribution of neurons expressing the long form of the receptor. Many neurons with high expression of L-PRL mRNA were located in the anteroventral periventricular nucleus, the medial preoptic area (MPO), specific subdivisions of the paraventricular and supraoptic nuclei, and in the arcuate and ventromedial nuclei. Labelled neurons were also found in limbic system structures such as the bed nucleus of stria terminalis (BST) and the medial nucleus of the amygdala, in a few thalamic nuclei, and in the central gray. All cells throughout the choroid plexus expressed high levels of L-PRL mRNA. The levels of L-PRL mRNA were higher in females on day 21 of pregnancy in the MPO and in the choroid plexus, than in females on day 2 of pregnancy; levels in the ventromedial nucleus of the hypothalamus (VMH) were unchanged across pregnancy. The neuroanatomical distribution of neurons expressing L-PRL mRNA may have special relevance for the mediation of maternal behavior, lactation, sexual behavior, and feeding.