Estrogen receptor immunoreactivity in specific brain areas of the prairie vole (Microtus ochrogaster) is altered by sexual receptivity and genetic sex

Comparative analysis of gonadal hormone receptor expression in the postnatal house mouse, meadow vole, and prairie vole brain

The socially monogamous prairie vole (Microtus ochrogaster) and promiscuous meadow vole (Microtus pennsylvanicus) are closely related, but only prairie voles display long-lasting pair bonds, biparental care, and selective aggression towards unfamiliar individuals after pair bonding. These social behaviors in mammals are largely mediated by steroid hormone signaling in the social behavior network (SBN) of the brain. Hormone receptors are reproducible markers of sex differences that can provide more information than anatomy alone and can even be at odds with anatomical dimorphisms. We reasoned that behaviors associated with social monogamy in prairie voles may emerge in part from unique expression patterns of steroid hormone receptors in this species, and that these expression patterns would be more similar across males and females in prairie than in meadow voles or the laboratory mouse. To obtain insight into steroid hormone signaling in the developing prairie vole brain, we assessed expression of estrogen receptor alpha (Esr1), estrogen receptor beta (Esr2), and androgen receptor (Ar) within the SBN, using in situ hybridization at postnatal day 14 in mice, meadow, and prairie voles. We found species-specific patterns of hormone receptor expression in the hippocampus and ventromedial hypothalamus, as well as species differences in the sex bias of these markers in the principal nucleus of the bed nucleus of the stria terminalis. These findings suggest the observed differences in gonadal hormone receptor expression may underlie species differences in the display of social behaviors.

Inhibiting ERα expression in the medial amygdala increases prosocial behavior in male meadow voles (Microtus pennsylvanicus)

This study tested the hypothesis that site-specific estrogen receptor alpha (ERα) expression is a critical factor in the expression of male prosocial behavior and aggression. Previous studies have shown that in the socially monogamous prairie vole (Microtus ochrogaster) low levels of ERα expression, in the medial amygdala (MeA), play an essential role in the expression of high levels of male prosocial behavior and that increasing ERα expression reduced male prosocial behavior. We used an shRNA adeno-associated viral vector to knock down/inhibit ERα in the MeA of the polygynous male meadow vole (M. pennsylvanicus), which displays significantly higher levels of ERα in the MeA than its monogamous relative. Control males were transfected with a luciferase expressing AAV vector. After treatment males participated in three social behavior tests, a same-sex dyadic encounter, an opposite-sex social preference test and an alloparental test. We predicted that decreasing MeA ERα would increase male meadow vole's prosocial behavior and reduce aggression. The results generally supported the hypothesis. Specifically, MeA knockdown males displayed lower levels of defensive aggression during dyadic encounters and increased levels of overall side-x-side physical contact with females during the social preference test, eliminating the partner preference observed in controls. There was no effect on pup interactions, with both treatments expressing low levels of alloparental behavior. Behaviors affected were similar to those in male prairie voles with increased ERα in the BST rather than the MeA, suggesting that relative changes of expression within these nuclei may play a critical role in regulating prosocial behavior.

Estrogen Receptor Alpha Distribution and Expression in the Social Neural Network of Monogamous and Polygynous Peromyscus

In microtine and dwarf hamsters low levels of estrogen receptor alpha (ERα) in the bed nucleus of the stria terminalis (BST) and medial amygdala (MeA) play a critical role in the expression of social monogamy in males, which is characterized by high levels of affiliation and low levels of aggression. In contrast, monogamous Peromyscus males display high levels of aggression and affiliative behavior with high levels of testosterone and aromatase activity. Suggesting the hypothesis that in Peromyscus ERα expression will be positively correlated with high levels of male prosocial behavior and aggression. ERα expression was compared within the social neural network, including the posterior medial BST, MeA posterodorsal, medial preoptic area (MPOA), ventromedial hypothalamus (VMH), and arcuate nucleus in two monogamous species, P. californicus and P. polionotus, and two polygynous species, P. leucopus and P. maniculatus. The results supported the prediction, with male P. polionotus and P. californicus expressing higher levels of ERα in the BST than their polygynous counter parts, and ERα expression was sexually dimorphic in the polygynous species, with females expressing significantly more than males in the BST in both polygynous species and in the MeA in P. leucopus. Peromyscus ERα expression also differed from rats, mice and microtines as in neither the MPOA nor the VMH was ERα sexually dimorphic. The results supported the hypothesis that higher levels of ERα are associated with monogamy in Peromyscus and that differential expression of ERα occurs in the same regions of the brains regardless of whether high or low expression is associated with social monogamy. Also discussed are possible mechanisms regulating this differential relationship.

Developmental expression of estrogen receptor beta in the brain of prairie voles (Microtus ochrogaster)

Here, for the first time, the expression of estrogen receptor beta (ERβ) is characterized in the brains of the highly prosocial prairie vole (Microtus ochrogaster). ERβ immunoreactivity was compared in weanlings (postnatal Day 21) and adult males and females. The results indicate several major findings. First, unlike ERα, ERβ expression is not sexually dimorphic. Second, the adult pattern of ERβ-IR is established at the time of weaning, as there were no age-dependent effects on distribution. Finally, ERβ does not appear to be as widely distributed in voles compared with rats and mice. High levels of ERβ-IR were observed in several regions/nuclei within the medial pre-optic area, ventrolateral pre-optic nuclei, and in the hypothalamus, especially in the paraventricular and supraoptic nuclei. The visualization of ERβ in prairie voles is important as the socially monogamous prairie vole functions as a human relevant model system for studying the expression of social behavior and social deficit disorders. Future studies will now be able to determine the effect of treatments on the expression and/or development of ERβ in this highly social species.

Hypothalamic expression of oestrogen receptor α and androgen receptor is sex-, age- and region-dependent in mice

Sex steroid hormones act on developing neural circuits regulating the hypothalamic-pituitary-gonadal axis and are involved in hormone-sensitive behaviours. These hormones act mainly via nuclear receptors, such as oestrogen receptor (ER)-α and androgen receptor (AR). By using immunohistochemistry, we analysed the expression level of ERα and AR throughout perinatal life [at embryonic (E) day 19 and postnatal (P) days 5, 15 and 25] and in adulthood in several hypothalamic nuclei controlling reproduction in both wild-type and aromatase knockout (ArKO) (i.e. which cannot convert testosterone into oestradiol) mice to determine whether there are sex differences in hypothalamic ERα and AR expression and, if so, whether these are established by the action of oestradiol. As early as E19, ERα immunoreactivity (-IR) was observed at same expression levels in both sexes in the anteroventral periventricular nucleus (AVPv), the medial preoptic area (MPOA), the bed nucleus of the stria terminalis (BnST), the ventrolateral part of the ventromedial hypothalamic nucleus and the arcuate nucleus (ARC). Sex differences (female > male) in ERα-IR were observed not only during the prepubertal period in the BnST (P5 to P25) and the MPOA (P15), but also in adulthood in these two brain regions. Sex differences in AR-IR (male > female) were observed at P5 in the AVPv and ARC, and at P25 in the MPOA and ARC, as well as in adulthood in all hypothalamic regions analysed. In adulthood, gonadectomy and hormonal treatment (oestradiol or dihydrotestosterone) also strongly modulated ERα-IR and AR, respectively. Taken together, sex differences in ERα-IR and AR-IR were observed in all hypothalamic regions analysed, although they most likely do not reflect the action of oestradiol because ArKO mice of both sexes showed expression levels very similar to wild-type mice throughout perinatal development.

Differential control of sex differences in estrogen receptor α in the bed nucleus of the stria terminalis and anteroventral periventricular nucleus

The principal nucleus of the bed nucleus of the stria terminalis (BNSTp) and anteroventral periventricular nucleus of the hypothalamus (AVPV) are sexually dimorphic, hormone-sensitive forebrain regions. Here we report a profound sex difference in estrogen receptor-α (ERα) immunoreactivity (IR) in the BNSTp, with robust ERα IR in females and the near absence of labeling in males. This sex difference is due to the suppression of ERα IR by testicular hormones in adulthood: it was not present at birth and was not altered by neonatal treatment of females with estradiol; gonadectomy of adult males increased ERα IR to that of females, whereas gonadectomy of adult females had no effect. Treating gonadally intact males with an aromatase inhibitor partially feminized ERα IR in the BNSTp, suggesting that testicular suppression required aromatization. By contrast, in AVPV we found a modest sex difference in ERα IR that was relatively insensitive to steroid manipulations in adulthood. ERα IR in AVPV was, however, masculinized in females treated with estradiol at birth, suggesting that the sex difference is due to organizational effects of estrogens. The difference in ERα IR in the BNSTp of males and females appears to be at least in part due to greater expression of mRNA of the ERα gene (Esr1) in females. The sex difference in message is smaller than the difference in immunoreactivity, however, suggesting that posttranscriptional mechanisms also contribute to the pronounced suppression of ERα IR and presumably to functions mediated by ERα in the male BNSTp.

Dendritic arbor of neurons in the hypothalamic ventromedial nucleus in female prairie voles (Microtus ochrogaster)

Female mating behavior in rats is associated with hormone-induced changes in the dendritic arbor of neurons in the ventromedial nucleus of the hypothalamus (VMH), particularly the ventrolateral portion. Regulation of mating behavior in female prairie voles differs substantially from that in rats; therefore, we examined the dendritic morphology of VMH neurons in this species. Sexually naïve adult female prairie voles were housed with a male to activate the females' reproductive endocrine system. Following 48 h of cohabitation, females were tested for evidence of reproductive activation by assessing the level of male sexual interest, after which their brains were processed using Golgi impregnation, which allowed ventrolateral VMH neurons to be visualized and analyzed. Dendritic arborization in the female prairie vole VMH neurons was strikingly similar to that of female rats. The key difference was that in the prairie voles the long primary dendrites extended considerably further than those observed in rats. Although most female voles paired with males exhibited sexual activation, some females did not. These two groups displayed specific differences in their VMH dendrites. In particular, the long primary dendrites were longer in the reproductively active females compared with those in the non-activated females. Overall, dendrite lengths were positively correlated with plasma estradiol levels in females exposed to males, but not in unpaired females. Although causal relationships between the neuroendocrine events, dendrite length, and the outward, behavioral manifestation of reproductive activation cannot be determined from this study, these results suggest an association between ventrolateral VMH dendrite morphology and female mating behavior in prairie voles, akin to what has been observed in female rats.

Expression of oestrogen receptor α in the brain of Brandt's voles (Lasiopodomys brandtii ): sex differences and variations during ovarian cycles

Oestrogen receptor (ER) α plays an important role in a variety of cognitive and behavioural functions. It has been shown that ERα expression in the brain is sexually dimorphic and is influenced by circulating oestrogen. In the present study, we mapped ERα-immunoreactive (-ir) cells in the forebrain of Brandt's voles (Lasiopodomys brandtii) to examine differences in ERα-ir expression between males and females and to reveal variations of ERα-ir expression during ovarian cycles in females. ERα-ir cells were found in many forebrain regions, including the lateral septum, bed nucleus of the stria terminalis, medial preoptic area (MPOA), anterior, arcuate and ventral medial (VMH) nuclei of the hypothalamus, as well as medial (MeA) and anterior cortical nuclei of the amygdala. Females had more ERα-ir cells in the VMH than males. Females during ovarian oestrus, but not di-oestrus or pro-oestrus, also had more ERα-ir cells in the MPOA than males. Furthermore, females in ovarian di-oestrus or oestrus had more ERα-ir cells in the MeA than males. Together, these data indicate that ERα expression in the brain of Brandt's voles is sexually dimorphic in specific brain areas. In addition, variations in the levels of circulating oestrogen during ovarian cycles can affect ERα expression in the female brain in a region-specific manner.

The neurobiology of pair bonding: insights from a socially monogamous rodent

The formation of enduring relationships between adult mates (i.e., pair bonds) is an integral aspect of human social behavior and has been implicated in both physical and psychological health. However, due to the inherent complexity of these bonds and the relative rarity with which they are formed in other mammalian species, we know surprisingly little about their underlying neurobiology. Over the past few decades, the prairie vole (Microtus ochrogaster) has emerged as an animal model of pair bonding. Research in this socially monogamous rodent has provided valuable insight into the neurobiological mechanisms that regulate pair bonding behaviors. Here, we review these studies and discuss the neural regulation of three behaviors inherent to pair bonding: the formation of partner preferences, the subsequent development of selective aggression toward unfamiliar conspecifics, and the bi-parental care of young. We focus on the role of vasopressin, oxytocin, and dopamine in the regulation of these behaviors, but also discuss the involvement of other neuropeptides, neurotransmitters, and hormones. These studies may not only contribute to the understanding of pair bonding in our own species, but may also offer insight into the underlying causes of social deficits noted in several mental health disorders.

Comparison of sociability, parental care and central estrogen receptor α expression between two populations of mandarin voles (Microtus mandarinus)

The socially monogamous mandarin vole (Microtus mandarinus) shows significant behavioral plasticity. We examined whether levels of sociability, parental care and central expression of estrogen receptor alpha differed between two populations with different ecologies. Our results show that males from the Chengcun population display significantly more amicable and less aggressive behaviors towards novel same-sex individuals compared to males from the second population of Xinzheng. Chengcun voles directed more licking behavior towards neonatal pups than did Xinzheng voles. Differences were also found in the number of estrogen receptor alpha-immunoreactive neurons. For example, Xinzheng males displayed significantly higher immunoreactivity than Chengcun males in the medial amygdala, medial preoptic area and ventromedial nucleus of the hypothalamus. Xinzheng females expressed higher levels of estrogen receptor alpha-immunoreactivity than Chengcun females in the medial preoptic area. Chengcun females exhibited significantly more estrogen receptor alpha expression than Xinzheng females in the bed nucleus of the stria terminalis. Our results indicate that mandarin voles from the Chengcun site possess monogamous traits, and animals from Xinzheng possess polygamous traits. It also appears that different social behavior and levels of parental care in these two populations may be associated with differences in estrogen receptor alpha-immunoreactive neurons.

Reproduction: a new venue for studying function of adult neurogenesis?

Adult neurogenesis has been a focus within the past few years because it is a newly recognized form of neuroplasticity that may play significant roles in behaviors and recovery process after disease. Mammalian adult neurogenesis could be found in two brain regions: hippocampus and subventricular zone (SVZ). While it is well established that hippocampal neurogenesis participates in memory formation and anxiety, the physiological function of SVZ neurogenesis is still under intense investigation. Recent studies disclose that SVZ neurogenesis is under regulation of reproductive cues like pheromones. Reciprocally, the newborn neurons may exert their effect on reproductive and maternal behaviors. This review discusses recent understanding of the interrelationship between neurogenesis and reproduction. The studies highlighted in this review illustrate the potential importance of neurogenesis in reproductive function and will provide new insights for the significance of adult neurogenesis.

Estrogen receptor-alpha in the bed nucleus of the stria terminalis regulates social affiliation in male prairie voles (Microtus ochrogaster)

Estrogen receptor alpha (ERalpha) typically masculinizes male behavior, while low levels of ERalpha in the medial amygdala (MeA) and the bed nucleus of the stria terminalis (BST) are associated with high levels of male prosocial behavior. In the males of the highly social prairie vole (Microtus ochrogaster), increasing ERalpha in the MeA inhibited the expression of spontaneous alloparental behavior and produced a preference for novel females. To test for the effects of increased ERalpha in the BST, a viral vector was used to enhance ERalpha expression in the BST of adult male prairie voles. Following treatment, adult males were tested for alloparental behavior with 1-3-day-old pups, and for heterosexual social preference and affiliation. Treatment did not affect alloparental behavior as 73% of ERalpha-BST males and 62.5% of control males were alloparental. Increasing ERalpha in the BST affected heterosexual affiliation, with ERalpha-BST males spending significantly less total time in side-by-side contact with females relative to time spent with control males. ERalpha-BST males did not show a preference for either the familiar or novel female. These findings differed significantly from those reported in ERalpha-MeA enhanced males, where ERalpha inhibited alloparental behavior and produced a preference for a novel female. The findings from this study suggest two things: first, that increased ERalpha in the BST decreases social affiliation and second, that altering ERalpha in different regions of the social neural circuit differentially impacts the expression of social behavior.

A niche for adult neurogenesis in social behavior

The structural and functional changes occurring into the brain is the hallmark of its tremendous capacity for dealing with the complexity that we are facing throughout life. It is also the hallmark of what neuroscientists refer as neuroplasticity. The continuous generation of cohorts of new neurons in some discrete regions of the adult brain, including the olfactory system, is a newly recognized form of neuroplasticity that has been recently the focus of neuroscience studies. Several lines of evidence indicate that this recruitment of newly-generated neurons is extremely sensitive to the overall neuronal activity of the host circuits. Therefore, adult neurogenesis represents, not only a constitutive replacement mechanism for lost neurons, but also a process supporting a capacity of neural plasticity in response to specific experience throughout life. The remarkable complexity of the social life offers a host of daily challenges that require a diversity of brain mechanism to make sense of the ever-changing social world. This review describes some recent findings which have begun to define reciprocal relationships between the production and integration of newborn neurons in the adult brain and social behavior. These studies demonstrate how this domain of research has the potential to address issues in the functional contribution of adult neurogenesis in the expression of some social traits as well in the role of some social contexts to finely regulate the production, survival and integration of adult newborn neurons.

Estrogen receptor immunoreactivity in late-gestation fetal lambs

Prenatal androgens masculinize postnatal reproductive neuroendocrine function and behavior in sheep. Testosterone treatment of pregnant ewes during midgestation masculinizes sexual behavior and luteinizing hormone secretion in female lambs, presumably in part via aromatization and estrogen receptor (ESR) binding in the brain. We hypothesized that male and female sheep also differ in the number and distribution of ESR-containing neurons. If so, ESR expression should be sensitive to prenatal hormones delivered exogenously or in situ. ESR alpha (ESR1) was compared by immunocytochemistry in male and female lambs at the end of gestation, as well as in fetal females exposed prenatally to testosterone or dihydrotestosterone. ESR1-positive neurons were abundant in the posteromedial bed nucleus of the stria terminalis (BSTpm), medial preoptic area (MPOA), posterior medial amygdaloid nucleus (MeP), amygdalohippocampal area (AHi), ventromedial hypothalamic nuclei (VMH), and arcuate hypothalamic nuclei (ARC). In females, the ARC had the largest number of stained cells (mean +/- SEM, 475.6 +/- 57.4 cells/0.173 mm(2)), while staining intensity was greatest in the MPOA (mean +/- SEM gray level, 31.3 +/- 5.3). The mean +/- SEM integrated gray level (IGL) was high in the ARC (0.63 +/- 0.13) and in the MPOA (0.51 +/- 0.08). The mean +/- SEM IGL was low in the MeP (0.31 +/- 0.10) and in the BSTpm (0.21 +/- 0.06), while it was intermediate in the AHi (0.36 +/- 0.10) and in the VMH (0.37 +/- 0.07). ESR immunostaining was not significantly different in male and female fetal lambs, nor in females fetuses exposed prenatally to androgens (P > 0.05). However, ESR1 staining was significantly increased in the ARC, MPOA, and AHi of adult rams vs. adult ewes. These results suggest that brain ESR immunoreactivity in fetal lambs is unlikely to account for postnatal sex differences in reproductive function. Instead, sex differences in ESR emerge postnatally.

Progesterone receptor in the forebrain of female gray short-tailed opossums: effects of exposure to male stimuli

Progesterone receptor immunoreactivity (PRir) in brain areas involved in reproductive behavior in eutherian species was examined for the first time in a female marsupial, the gray short-tailed opossum (Monodelphis domestica, hereinafter, opossum). PRir in nuclei of neurons, measured as area covered by stained nuclei, was seen in the arcuate nucleus (Arc); anteroventral periventricular nucleus (AVPv); bed nucleus of the stria terminalis (BST); medial preoptic area (MPOA), and ventromedial hypothalamus (VMH), but not in control areas adjacent to the hypothalamus or cortex. Female opossums are induced into cytological, urogenital sinus (UGS), estrus by male pheromones and into behavioral estrus, i.e., receptivity, by pairing with a male, and both estradiol (E) and progesterone (P) are involved in induction of receptivity in intact and ovariectomized females. PRir in the AVPv, MPOA, and VMH was very low in females that had never been exposed to males or their scent marks, i.e., naïve anestrous (NVA) females, and either previous or current exposure to males or their scent marks was associated with elevated PRir. PRir was significantly higher in the AVPv and MPOA of anestrous females with previous but no current exposure to males and their scent marks, i.e., experienced anestrous (EXPA) females, than in NVA females, but PRir was significantly lower in the MPOA and VMH of EXPA females than in females that were behaviorally receptive and had recently copulated, i.e., behavioral receptive estrous (BRE) females. PRir was higher in the VMH of both UGS estrous (UGSE) and BRE females compared to that in EXPA animals, but PRir did not differ between UGSE and BRE females in any of the 3 brain areas examined, including the MPOA These results provide evidence that pheromonal induction of estrus and sexual receptivity in opossums is associated with elevation of PRir in the VMH and MPOA and that prior exposure to males or their pheromones, even in the absence of current male stimuli, is associated with persistent elevation of PRir in the AVPv and MPOA.

Estrogen receptors in the medial amygdala inhibit the expression of male prosocial behavior

Studies using estrogen receptor alpha (ERalpha) knock-out mice indicate that ERalpha masculinizes male behavior. Recent studies of ERalpha and male prosocial behavior have shown an inverse relationship between ERalpha expression in regions of the brain that regulate social behavior, including the medial amygdala (MeA), and the expression of male prosocial behavior. These studies have lead to the hypothesis that low levels of ERalpha are necessary to "permit" the expression of high levels of male prosocial behavior. To test this, viral vectors were used to enhance ERalpha in male prairie voles (Microtus ochrogaster), which display high levels of prosocial behavior and low levels of MeA ERalpha. Adult male prairie voles were transfected with ERalpha in the MeA (MeA-ERalpha) or the caudate-putamen (ERalpha control) or luciferase (MeA-site-specific control), and 3 weeks later tested for spontaneous alloparental behavior and partner preference. Enhancing ERalpha in the MeA altered/reduced male prosocial behavior. Only one-third of MeA-ERalpha males, compared with all control males, were alloparental. MeA-ERalpha males also displayed a significant preference for a novel female. This is a critical finding because the manipulations of neuropeptides, oxytocin and vasopressin, can inhibit the formation of a partner preference, but do not lead to the formation of a preference for a novel female. The results support the hypothesis that low levels of ERalpha are necessary for high levels of male prosocial behavior, and provide the first direct evidence that site-specific ERalpha expression plays a critical role in the expression of male prosocial behavior.

Sex differences and effects of neonatal aromatase inhibition on masculine and feminine copulatory potentials in prairie voles

Copulatory behaviors in most rodents are highly sexually dimorphic, even when circulating hormones are equated between the sexes. Prairie voles (Microtus ochrogaster) are monomorphic in their display of some social behaviors, including partner preferences and parenting, but differences between the sexes in their masculine and feminine copulatory behavior potentials have not been studied in detail. Furthermore, the role of neonatal aromatization of testosterone to estradiol on the development of prairie vole sexual behavior potentials or their brain is unknown. To address these issues, prairie vole pups were injected daily for the first week after birth with 0.5 mg of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) or oil. Masculine and feminine copulatory behaviors in response to testosterone or estradiol were later examined in both sexes. Males and females showed high mounting and thrusting in response to testosterone, but only males reliably showed ejaculatory behavior. Conversely, males never showed feminine copulatory behaviors in response to estradiol. Sex differences in these behaviors were not affected by neonatal ATD, but ATD-treated females received fewer mounts and thrusts than controls, possibly indicating reduced attractiveness to males. In other groups of subjects, neonatal ATD demasculinized males' tyrosine hydroxylase expression in the anteroventral periventricular preoptic area, and estrogen receptor alpha expression in the medial preoptic area. Thus, although sexual behavior in both sexes of prairie voles is highly masculinized, aromatase during neonatal life is necessary only for females' femininity. Furthermore, copulatory behavior potentials and at least some aspects of brain development in male prairie voles are dissociable by their requirement for neonatal aromatase.

The organizational effects of oxytocin on the central expression of estrogen receptor alpha and oxytocin in adulthood

Background

Previous studies have demonstrated that neonatal manipulation of oxytocin (OT) has effects on the expression of estrogen receptor α (ERα) and the central production of oxytocin observed in juveniles (at weaning, 21 days of age). The goal of this study was to determine whether the effects of neonatal manipulation of OT last into adulthood, and if the effects differ from those observed during the early postnatal period. On the first day of life, prairie voles (Microtus ochrogaster) received one of three doses of OT (High, 3 μg; Med, 0.3 μg; Low, 0.03 μg), an OT antagonist, or isotonic saline. Another group was handled, but not injected. Then as adults, brains were collected, sectioned, and stained for ERα or OT using immunocytochemistry.

Results

In females, treatment with OT increased the expression of ERα immunoreactivity in the ventral lateral septum (0.03 μg) and the ventromedial nucleus of the hypothalamus and central amygdala (0.3 μg). In males, OT antagonist increased ERα expression in the bed nucleus of the stria terminalis. There was no apparent effect of OT on the number of cells producing OT in the paraventricular nucleus of the hypothalamus.

Conclusion

The current results suggest that neonatal manipulation of OT has long-term organizational effects on the expression of ERα in both males and females. The lack of effect on OT neurons in the paraventricular nucleus suggests that some developmental effects of OT previously observed in weanlings do not persist into adulthood. Developmental effects of OT on ERα patterns were sexually dimorphic, dose-dependent, and site-specific.

Estrogen and adult neurogenesis in the amygdala and hypothalamus

In mammals, adult neurogenesis has been extensively studied in the dentate gyrus of the hippocampus and subventricular zone. However, newly proliferated neurons have also been documented in other brain regions, including the amygdala and hypothalamus. In this review, we will examine the evidence for new neurons in the adult amygdala and hypothalamus and then discuss how environmental influences can alter cell proliferation. As some of these environmental effects may be attributed to changes in the levels of circulating hormones, we will provide evidence for estrogen-mediated cell proliferation among different species and between sexes. Finally, we will review recent data suggesting that new neurons may become functionally significant in adulthood.

Sex and species differences in tyrosine hydroxylase-synthesizing cells of the rodent olfactory extended amygdala

The bed nucleus of the stria terminalis (BST) and the medial amygdala (MeA) are anatomically connected sites necessary for chemosensory regulation of social behaviors in rodents. Prairie voles (Microtus ochrogaster) are a valuable model for studying the neural regulation of social behaviors because, unlike many other rodents, they are gregarious, pair bond after copulating, and are biparental. We herein describe sex and species differences in immunoreactivity for tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine synthesis, in the BST and MeA. Virgin male prairie voles had a large number of TH-immunoreactive cells in areas analogous to the rat principal nucleus of the BST (pBST) and the posterodorsal medial amygdala (MeAPd). Virgin female prairie voles had far fewer TH-immunoreactive cells in these sites ( approximately 17% of the number of cells as males in the pBST, approximately 35% of the number of cells in the MeAPd). A few TH-immunoreactive cells were found in the BST of male and female hamsters and meadow voles, but not in rats. The MeApd also contained a few TH-immunoreactive cells in male and female hamsters and male meadow voles, but not rats. Castration greatly reduced the number of TH-immunoreactive cells in the male prairie vole pBST and MeAPd, an effect that could be reversed with testosterone. Furthermore, treating ovariectomized females with testosterone substantially increased TH-immunoreactive cells in both sites. Therefore, a species-specific sex difference in TH expression is found in a chemosensory pathway in prairie voles. Expression of TH in these sites is influenced by circulating gonadal hormones in adults, which may be related to changes in their display of social behaviors across the reproductive cycle.

Parental regulation of central patterns of estrogen receptor α

Reduced levels of estrogen receptor alpha (ERalpha) in the medial amygdala (MeA) and bed nucleus of stria terminalis (BST) have been hypothesized to play a significant role in the expression of male behaviors associated with monogamy. Therefore, the regulation of ERalpha could be a critical factor in determining male behavior and the evolution of monogamy. Central expression of ERalpha immunoreactivity was compared in hybrid offspring from crosses between two phenotypically distinct populations of prairie voles (Microtus ochrogaster). Illinois voles (IL) are socially monogamous and display low levels of ERalpha, while Kansas voles (KN) display some characteristics associated with polygyny and have higher levels of ERalpha. In offspring from hybrid crosses, the pattern of ERalpha expression was dependent upon parentage; the two types of hybrid crosses did not produce the same ERalpha pattern in the offspring. In the BST and MeA, hybrid males expressed ERalpha patterns consistent with those of males from their mother's population, while hybrid females had ERalpha patterns typical of females belonging to their father's population. The parental-specific patterns of ERalpha expression are suggestive of genomic imprinting, therefore, the vole ERalpha (Esr1) gene was cloned and sequenced, and examined for allele-specific expression. Results from this study indicate that while maternal factors may play a major role the expression of ERalpha in their male offspring, genomic imprinting is unlikely to be involved, suggesting another mechanism is responsible.

Estrogen receptor-alpha distribution in male rodents is associated with social organization

It has been hypothesized that site-specific reduction of estrogen receptor-alpha (ERalpha) is associated with the expression of male prosocial behaviors. Specifically, highly social males are predicted to express significantly lower levels of ERalpha than females and less social males in brain regions associated with prosocial behavior including the bed nucleus of the stria terminalis (BST) and the medial amygdala (MeA). This hypothesis was tested by comparing ERalpha immunoreactivity (IR) in three species of microtines, the polygynous montane (Microtus montanus) and meadow (M. pennsylvanicus) voles and the monogamous pine vole (M. pinetorum), and two species of cricetines that differ in the extent of social pair-bond formation, Siberian (Phodopus sungorus) and Djungarian (P. campbelli) hamsters. As predicted, ERalpha-IR was sexually dimorphic in the BST and MeA of the highly social species, with females expressing more ERalpha-IR cells than males. Male and female montane voles did not differ. Male and female meadow voles differed in the ventromedial hypothalamus, with females expressing more ERalpha-IR cells. Male pine voles expressed lower levels of ERalpha-IR in the MeA than male montane and meadow voles and in the BST relative to montane males. Male Djungarian hamsters, which show higher levels of parental care, had fewer ERalpha-IR cells in the BST than male Siberian hamsters. Results indicate that the distribution of ERalpha differs relative to the continuum of species-typical affiliative behavior and supports the hypothesis that ERalpha has a significant role in regulating species-specific social organization.

Preference for cocaine- versus pup-associated cues differentially activates neurons expressing either Fos or cocaine- and amphetamine-regulated transcript in lactating, maternal rodents

We studied the neuronal basis of the motivational response to two powerful but radically different rewards-cocaine and maternal nurturing of pups in the postpartum rat (dam) which is in a unique motivational state. We used a place preference method designed to offer a choice between cues associated with a natural reinforcer (pups) and those associated with a pharmacologic reinforcer (cocaine). Using c-Fos or cocaine- and amphetamine-regulated transcript (CART) immunocytochemistry, we identified the neuronal groups that are activated when the dams expressed a preference for either cues-associated with pups or cues-associated with cocaine. Dams that preferred the cocaine-associated cues had more c-Fos positive neurons in medial prefrontal cortex, nucleus accumbens, and basolateral nucleus of amygdala than pup-associated cue preferring dams or control. Except for the accumbens, there was activation of neurons in these same regions with the pup-associated cue preference. In the nucleus accumbens only CART-immunoreactive (not c-Fos) neurons were activated with pup-cue preference. Notably, the medial preoptic area was the single area where greater activation of neurons was seen with a preference for pup-associated versus cocaine-associated cues. These responses were identified in the absence of the stimuli (cocaine or pups) and are proposed to be, in part, activation of these neurons related to motivational processing. Neither the distribution of neurons responding to pup-associated cue preference nor the demonstration that CART-expressing neurons are responsive to reward-associated cue preference has been previously reported. We hypothesize that the expression of preference for cocaine versus pup-associated cues is made possible by the concerted activity of these regionally distributed networks of neurons that are in part specific to the preference response.

Neonatal manipulation of oxytocin affects expression of estrogen receptor alpha

In adult females many of the effects of the neuropeptide oxytocin are steroid, and especially estrogen dependent. Here we demonstrate for the first time that neonatal manipulation of oxytocin can affect the expression of estrogen receptor alpha. On the first day of postnatal life male and female prairie voles (Microtus ochrogaster) were randomly assigned to receive one of four treatments; (a) 50 microl i.p. injection of 3 microg oxytocin (approximately 1 microg/g), (b) 0.3 microg of an oxytocin antagonist (approximately 0.1 microg/g), or (c) isotonic saline. A fourth group was handled, but not injected. On postnatal day 8 or 21, brain tissue was collected, fixed and sectioned. Free-floating sections were stained for estrogen receptor alpha using immunocytochemistry, and estrogen receptor alpha immunoreactive neurons were compared by age, treatment, and sex. To compare the temporal expression of estrogen receptor alpha an additional set of brains was collected from untreated males and females on the day of birth. The effects of oxytocin manipulations were age dependent, sexually dimorphic, and site-specific. While there were no significant treatment effects on postnatal day 8, by postnatal day 21 females that received oxytocin showed a significant increase in the number of cells expressing estrogen receptor alpha-immunoreactivity in the ventromedial nucleus of the hypothalamus. Treatment with oxytocin antagonist resulted in a significant decrease in estrogen receptor alpha-immunoreactivity in the medial preoptic area in postnatal day 21 females. While there were no significant effects in males, males treated with oxytocin antagonist trended toward a reduction in estrogen receptor alpha-immunoreactivity in the medial amygdala. The results indicate that oxytocin can have organizational effects on the expression of estrogen receptor alpha, that these effects are sexually dimorphic, and finally that during the preweaning period the development of estrogen receptor alpha is sexually dimorphic.

Arginine vasopressin and vasoactive intestinal polypeptide fibers make appositions with gonadotropin-releasing hormone and estrogen receptor cells in the diurnal rodent Arvicanthis niloticus

Diurnal and nocturnal animals differ with respect to the timing of a host of behavioral and physiological events including those associated with estrus, but the neural bases of these differences have not been elucidated. We investigated this issue by examining the distribution of cells containing gonadotropin-releasing hormone (GnRH) as well as estrogen receptors (ERs) in relation to fibers containing peptides present in the suprachiasmatic nucleus (SCN) in a diurnal animal, Arvicanthis niloticus (the unstriped Nile grass rat). We found that fibers containing two peptides found in SCN cells, arginine vasopressin and vasoactive intestinal polypeptide appeared to be in contact with GnRH and ER positive cells. These data suggest that temporal information is carried along the same direct pathways from the SCN to GnRH and ER neurons in day- and night-active species.

Estrogen regulation of cell proliferation and distribution of estrogen receptor-alpha in the brains of adult female prairie and meadow voles

Adult female prairie (Microtus ochrogaster) and meadow (M. pennsylvanicus) voles were compared to examine neural cell proliferation and the effects of estrogen manipulation on cell proliferation in the amygdala, ventromedial hypothalamus (VMH), and dentate gyrus of the hippocampus (DG). Unlike prior studies, our study focused on the amygdala and VMH, because they are involved in social behaviors and may underlie behavioral differences between the species. Meadow voles had a higher density of cells labeled with the cell proliferation marker 5-bromo-2'-deoxyuridine (BrdU) in the amygdala and DG than did prairie voles. Treatment with estradiol benzoate (EB) for 3 days increased the density of BrdU-labeled cells in the amygdala, particularly in the posterior cortical (pCorA) and medial (pMeA) nuclei, in meadow, but not prairie, voles. Furthermore, the majority of the BrdU-labeled cells in the pCorA and pMeA displayed either a neuronal or a glial progenitor phenotype, but no species or treatment differences were found in the percentage of neuronal or glial progenitor cells. To understand better estrogen's effects on adult neurogenesis, we also examined estrogen receptor-alpha (ERalpha) distribution. Meadow voles had more ERalpha-labeled cells in the pCorA and VMH, but not in the pMeA or DG, than did prairie voles. In addition, more than one-half of the BrdU-labeled cells in the amygdala of both species coexpressed ERalpha labeling. Together, these data indicate that estrogen alters cell proliferation in a species- and region-specific manner, and some of these effects may lie in the specific localization of estrogen receptors in the adult vole brain.

Intraspecific variation in estrogen receptor alpha and the expression of male sociosexual behavior in two populations of prairie voles

Estrogen (E) regulates a variety of male sociosexual behaviors. We hypothesize that there is a relationship between the distribution of estrogen receptor alpha (ERalpha) and the degree of male social behavior. To test this hypothesis, ERalpha immunoreactivity (IR) was compared in prairie voles (Microtus ochrogaster) from Illinois (IL), which are highly social, and Kansas (KN), which are less social. The expression of androgen receptors (AR) in males also was compared between populations. The expression of ERalpha and AR were compared in brains from KN and IL males and females using immunocytochemistry (ICC). There were significant intrapopulational differences, with males expressing less ERalpha-IR than females in the medial preoptic area, ventromedial nucleus, ventrolateral portion of the hypothalamus, and bed nucleus of the stria terminalis (BST). IL males also displayed less ERalpha-IR in the medial amygdala (MeA) than IL females. While IL males expressed significantly less ERalpha-IR in the BST and MeA than KN males, there was no difference in AR-IR. Differences in the pattern of ERalpha-IR between KN and IL males were behaviorally relevant, as low levels of testosterone (T) were more effective in restoring sexual activity in castrated KN males than IL males. The lack of difference in AR combined with lower expression of ERalpha-IR in IL males suggests that behavioral differences in response to T are associated with aromatization of T to E and that reduced sensitivity to E may facilitate prosocial behavior in males.

Newly proliferated cells in the adult male amygdala are affected by gonadal steroid hormones

Gonadal steroid hormones play an important role in the proliferation, survival, and activation of neurons. The present study was performed to examine the effects of testosterone and its metabolites on newly proliferated cells in the amygdala of adult male meadow voles (Microtus pennsylvanicus). Treatment with testosterone propionate (TP) in castrated males resulted in plasma testosterone levels similar to males following mating. TP-treated males displayed a significant increase in the density of cells labeled with a cell proliferation marker (BrdU) in the amygdala. Treatment with estradiol benzoate (EB) exerted a similar effect as TP on the density of BrdU-labeled cells, whereas 5alpha-dihydrotestosterone (DHT) was ineffective. A larger proportion (approximately 44%) of the BrdU-labeled cells in the amygdala displayed a neuronal phenotype, and a lesser percentage (approximately 35%) displayed a glial progenitor phenotype; however, treatment effects were not found in either population of cells. Hormonal effects appeared to be site-specific as no group differences were found in the dentate gyrus of the hippocampus or ventromedial hypothalamus. Finally, a time course study indicated that BrdU-labeled cells in the amygdala are present as early as 30 min following an acute injection of BrdU. Together, these data suggest that gonadal steroid hormones influence the number of newly proliferated cells in the amygdala, most likely by acting through an estrogenic mechanism, and these effects may be exerted on locally proliferating progenitors within the amygdala.

Period gene expression in mouse endocrine tissues

Circadian rhythms are generated by the oscillating expression of the Per1 and Per2 genes, which are expressed not only in the central brain pacemaker but also in peripheral tissues. Hormones are likely to coordinate physiological function in time. We performed in situ hybridization to localize mPer1 and mPer2 mRNA to particular cell types and tissue compartments in adrenal, thyroid, and testis. BALB/c mice maintained in a 12:12-h light-dark cycle expressed mPer1 in adrenal medulla, particularly in late afternoon and early night. mPer2 mRNA was more intensely expressed in adrenal cortex, especially in afternoon and evening. mPer1 mRNA was detected in thyroid. mPer1 was found in some but not all seminiferous tubules of each mouse at all times of day. Quantitation in C57BL/6 mice revealed a significant increase in the number of heavily labeled seminiferous tubules early in the night. Consistent with in situ hybridization, immunocytochemistry showed PER1 protein in spermatocytes and spermatids (spermatogenic stages VII-XII). Staining in spermatogonia and interstitial cells was inconsistent. Double labeling with 5'-bromodeoxyuridine showed PER1 expression first occurring 5 days after DNA replication. We conclude that mPeriod genes are expressed in peripheral endocrine glands. Central regulation, adenohypophyseal control, and functional importance of expression and phase remain to be elucidated.

Sex differences in GABA(A)ergic system in rat substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR) is involved in the control of movement disorders including seizures through its GABAergic neurons. Microinfusions of muscimol (a GABA(A) receptor agonist) produce specific effects on seizures depending on sex, infusion site (SNR(anterior) or SNR(posterior)) and age. To assess whether these effects are due to sex differences in GABAergic indices within the SNR we analyzed the expression of alpha(1) subunit mRNA of the GABA(A) receptor and the levels of GABA immunoreactivity (IR) of male and female rats at postnatal day 15 (PN15) and PN30. In each age, within the same SNR region, expression of alpha(1) subunit mRNA and intensity of GABA IR per neuron was higher in females compared to males. At PN15, in both sexes, there were no regional differences in expression of alpha(1) subunit mRNA and intensity of GABA IR. However, at PN30 in both sexes, expression of alpha(1) subunit mRNA and intensity of GABA IR per cell was higher in SNR(anterior) than in SNR(posterior). These results demonstrate that expression of alpha(1) subunit mRNA for GABA(A) receptor and levels of GABA IR in the SNR are sex- and site-specific, which may contribute to sex-, regional- and age-related differences in the expression of movement disorders and seizures.

Reproductive status influences cell proliferation and cell survival in the dentate gyrus of adult female meadow voles: a possible regulatory role for estradiol

Galea and McEwen [Galea and McEwan (1999) Neuroscience 89, 955-964] found that cell proliferation was suppressed in female meadow voles trapped during the breeding season relative to females trapped during the non-breeding season. We investigated the effect of reproductive status and estradiol level on cell proliferation and cell survival in adult laboratory-reared female meadow voles to control for the variables of age, experience and pregnancy that could confound the results derived from a wild sample. Voles were housed in either a long- or short-photoperiod to simulate season and a male or female cage partner was introduced to influence reproductive status. Because females are reflex ovulators, exposure to a male rapidly induces behavioural estrous and high levels of estradiol. Forty-eight hours after introducing a cage partner, we injected either bromodeoxyuridine or [3H]thymidine to mark cell synthesis and then examined labelled cells 2h (cell proliferation) or five weeks (cell survival) later, respectively. To determine whether estradiol mimicked the effect of reproductive status, groups of reproductively inactive females were given a single injection of estradiol benzoate (10 microg) either four or 48h prior to bromodeoxyuridine labelling. The density of proliferating cells in the granule cell layer and the hilus was elevated in reproductively inactive females compared to reproductively active females and was correlated negatively with serum estradiol level. Exposure to estradiol benzoate initially increased cell proliferation (within 4h) but subsequently suppressed cell proliferation (within 48h). In addition, the density of surviving cells was greater in reproductively inactive females relative to reproductively active females but reproductively active females had a greater rate of cell survival than did reproductively inactive females. Reproductive status did not influence the number of pyknotic cells in the dentate gyrus (at either 2h or five weeks).We conclude that reproductive status regulates cell proliferation in adult female meadow voles, possibly via an estradiol-regulated mechanism. The results from the present study showed that reproductively active female meadow voles have suppressed rates of cell proliferation in the dentate gyrus relative compared with reproductively inactive female meadow voles. Administering estradiol initially (within 4h) elevates the cell proliferation within the dentate gyrus of adult females but subsequently (within 48h) suppresses cell proliferation. However, more new cells survived in females with high endogenous levels of estradiol (reproductively active females). In conclusion, reproductive status regulates the level of cell proliferation and survival through a complex estradiol regulated mechanism(s).

Increased number of BrdU-labeled neurons in the rostral migratory stream of the estrous prairie vole

In the mammalian forebrain, most neurons originate from proliferating cells in the ventricular zone lining the lateral ventricles, including a discrete area of the subventricular zone in which neurogenesis continues into adulthood. The majority of the cells generated in the anterior portion of the subventricular zone (SVZa) are neuronal precursors with progeny that migrate to the olfactory bulb (OB) along a pathway known as the rostral migratory stream (RMS). The list of factors that influence the proliferation and survival of neurons in the adult brain remains incomplete, but previous studies have implicated neurotrophins in mammals and estrogen in birds. This study examined the effect of estrus induction on the proliferation of SVZa neurons in female prairie voles. Prairie voles, unlike many other rodents, are induced into estrus by chemosensory cues from a male. This olfactory-mediated process results in an increase in serum estrogen levels and the consequent induction of behavioral estrus (sexual receptivity). Female prairie voles induced into estrus by male exposure had a 92% increase in BrdU-labeled cells in the SVZa compared to females exposed to a female. Double-label immunocytochemical studies demonstrated that 80% of the BrdU-labeled cells in the RMS displayed a neuronal phenotype. Ovariectomized females exposed to a male did not show an increase in serum estrogen or BrdU labeling in the RMS. Conversely, ovariectomized females injected with estrogen were sexually receptive and had more BrdU-labeled cells in the RMS than oil-injected females. These data suggest that, in female prairie voles, estrus induction is associated with increased numbers of dividing cells in the RMS, possibly via an estrogen-mediated process.

A stereotaxic atlas of the forebrain of the bank vole (Clethrionomys glareolus)

In this article part of the forebrain of the bank vole (Clethrionomys glareolus) is presented in stereotaxic coordinates. The stereotaxic procedure was performed as follows. With the vole's head mounted in a stereotaxic adaptor, internal reference tracks were made with a 0.5-mm diameter microdialysis cannula and India ink, 2 mm in front and 2.6 mm behind the skull landmark bregma. Brains were fixed for 72 h in 4% commercial formaldehyde in sodiumcacodylate buffer containing 1% CaCl2. To determine shrinkage they were weighed before and after fixation. After embedding in paraffin they were sectioned at 25 microm and stained with Nissl. Photomicrographs were taken from the brain of one animal while its frontal (antero-posterior) coordinates of five neural structures were compared with those of 12 other voles. Variability was also checked in lateral and vertical directions at frontal level -1.0 mm (relative to bregma). The results show that the distance between the two skull landmarks bregma and lambda correlates significantly and negatively with the antero-posterior position of each of the brain areas. On the basis of these results an equation is proposed to improve accuracy in locating neural structures that deviate due to biological variability.

Androgenic regulation of hypothalamic aromatase activity in prepubertal and postpubertal male golden hamsters

Doses of testosterone that fully activate male reproductive behavior in castrated adult male hamsters fail to elicit mounting and intromissions in prepubertal castrates, even when circulating levels of testosterone are equivalent in the two age groups. We hypothesize that this differential responsiveness to testosterone is mediated at least in part by the efficacy with which testosterone in the hypothalamus is aromatized to estradiol, an important hormone mediating male sexual behavior. Therefore, hypothalamic aromatase activity, as measured by the conversion of [3H]testosterone to [3H]estradiol in tissue homogenates, was assessed in four separate experiments: 1) intact prepubertal and adult male golden hamsters, 2 and 3) castrated adult or prepubertal males that received either a 0- or 2.5-mg dose of testosterone, and 4) castrated adult and prepubertal males treated with the 2.5-mg dose oftestosterone. These studies demonstrate that hypothalamic aromatase activity is significantly higher in adult males compared with prepubertal males, and that hypothalamic aromatase activity is increased by testosterone to the same extent in both the adult and prepubertal male hamster. Therefore, the failure of testosterone-treated castrated prepubertal male hamsters to engage in the full suite of male reproductive behaviors is not due to the inability of testosterone to be converted into estradiol in the hypothalamus. Differences in the ability of testosterone to increase aromatase activity in other brain regions, or differences in the action of testosterone and/or estradiol on other cellular processes must account for the inability of testosterone to facilitate male reproductive behavior in juvenile males.

Effects of photoperiod and androgen on proopiomelanocortin gene expression in the arcuate nucleus of golden hamsters

In golden hamsters, seasonal changes in day length act via a pineal-dependent mechanism to regulate feedback and behavioral effects of androgen. Endogenous opiates participate in photoperiodically regulated neuroendocrine functions, but the effects of androgen on expression of the gene encoding POMC, the precursor of beta-endorphin, have been controversial. We used quantitative in situ hybridization to examine regulation of POMC messenger RNA (mRNA) by testosterone and to test the hypothesis that short day lengths act through the pineal gland to amplify POMC mRNA expression. We studied intact hamsters and castrates with or without androgen treatment held in long (14 h of light, 10 h of darkness) or short (5 h of light, 19 h of darkness) days for 10 weeks. POMC gene expression differed with rostral-caudal plane, photoperiod, and surgical treatment (castration and testosterone administration). Testosterone increased the number of silver grains in labeled cells throughout the arcuate nucleus, and short day castrates given androgen consistently had more silver grains per labeled cell than did their long day counterparts. Testosterone exerted an inhibitory effect, however, on the number of POMC mRNA-positive cells, and more POMC mRNA-labeled cells were found in the arcuate nucleus of long than short day castrates treated with testosterone. Photoperiod had no significant influence in castrates not receiving androgen. Testosterone treatment had generally similar effects whether it was begun at the time of castration or 5 weeks later. Pinealectomy blocked the influence of photoperiod on both the mean number of silver grains per labeled cell and the number of labeled cells. The results indicate that day length regulates POMC gene expression when androgen levels are held constant, but that androgen is necessary for photoperiod effects to be expressed.

Exposure to male siblings facilitates the response to estradiol in sexually naive female prairie voles

Female prairie voles undergo induced estrus, and require both physical contact with males and exposure to male urine to become reproductively active. This study attempted to determine if physical contact with males enhanced female response to estradiol. Two groups of sexually naive females were tested. One was reared without any exposure to males after weaning, and the other was reared with sibling males to 60 days of age. Sibling males were used because females avoid direct contact with the urine of related males, allowing for the establishment of a group of females that experienced physical contact in the relative absence of exposure to male pheromones associated with urine. Females were then subcutaneously injected with 0.5 microg estradiol benzoate once a day for 7 days. Sexual receptivity was tested with novel adult males 48 h and 168 h after the first injection. There was a significant difference between the treatment groups, with 10% of sexually naive females reared without sibling males displaying lordosis compared to 70% of females raised with sibling males. The results indicate that exposure to sibling males significantly increased a female's behavioral response to estradiol.

Molecular aspects of monogamy

Comparative studies of monogamous and nonmonogamous voles demonstrate species differences in the regional expression of oxytocin (OT) receptors in the brain. These species differences have not been observed with other neurotransmitter receptors (except vasopressin). Species differences for OT receptor distribution were also observed in other microtine and murine species selected as monogamous or promiscuous. These chemical neuroanatomic differences appear to be functionally relevant, as treatments with selective OT agonists and antagonists influence those behaviors that appear critical to pair bonding in the monogamous prairie vole. To investigate the mechanism controlling tissue-specific expression of OT receptors, we sequenced the OT receptor gene in both prairie voles and montane voles. The findings are inconclusive. Although both species differ markedly from rat and human in their regulatory (but not their coding) sequences, the species show very subtle differences from each other. Ongoing studies are investigating the consequences of these subtle differences between prairie and montane voles. At the same time, several transactivating factors that might influence OT receptor expression need to be explored. NOTE ADDED IN PROOF: The rat oxytocin receptor gene sequence, cited in FIGURES 4 and 5, was based on an error published in ref. 22. The corrected sequence has now been published (Rosen et al. 1996. Proc. Natl. Acad. Sci USA 93: 12501). The correct sequence shows greater homology with the vole oxytocin receptor gene sequences, but the remaining differences support the argument made herein for species differences in regional receptor expression.

Estrogen receptor immunoreactivity in ferret brain is regulated by estradiol in a region-specific manner

The effects of estrogen on estrogen receptor (ER) immunoreactivity in the male ferret brain were examined. Estrogen treatment reduced the mean number of ER-immunopositive (ER+) cells/unit area in periventricular preoptic area but increased the mean number of ER+ cells/unit area in the medial division of the ventromedial hypothalamic nucleus, while having no effect on the number of ER+ cells/unit area in the lateral VMH and arcuate nucleus. Thus, estrogen regulates brain ER immunoreactivity in male ferrets and the direction and magnitude of this regulation are brain region-specific.