Ovarian hormones regulating sexual and social behaviors in female prairie voles, Microtus ochrogaster

Bonding against the odds: Male prairie vole response to the "widow effect" among females

Although pair bonding is the preferred mating tactic among socially monogamous prairie voles, naturalistic observations have demonstrated many males remain non-bonded. Moreover, although males readily re-bond after the loss of a partner, females do not (i.e., the "widow effect'). Few studies have attempted to address why so many males remain non-bonded or if a reluctance of re-bonding in females contributes to this outcome. We investigate how female bonding history impacts male pair bond formation. Specifically, we test two alternative hypotheses for how sexually naïve males will behave when paired with widow females. The fecundity hypothesis predicts males will avoid bonding with widow females and be more receptive to novel bond-naïve females. The preference to bond hypothesis predicts males will choose to bond and express a partner preference, irrespective of if a pair-mate is a widow or sexually naïve. Our results demonstrated that males expressed a partner preference for females regardless of their social history. These data support the preference to bond hypothesis and suggest natural variation in bonding may not be strongly due to males forgoing bonding opportunities.

Multi-Level Effects Driving Cognitive and Behavioral Variability among Prairie Voles: Insights into Reproductive Decision-Making from Biological Levels of Organization

Behavioral phenotypes play an active role in maximizing fitness and shaping the evolutionary trajectory of species by offsetting the ecological and social environmental factors individuals experience. How these phenotypes evolve and how they are expressed is still a major question in ethology today. In recent years, an increased focus on the mechanisms that regulate the interactions between an individual and its environment has offered novel insights into the expression of alternative phenotypes. In this review, we explore the proximate mechanisms driving the expression of alternative reproductive phenotypes in the male prairie vole (Microtus ochrogaster) as one example of how the interaction of an individual's social context and internal milieu has the potential to alter behavior, cognition, and reproductive decision-making. Ultimately, integrating the physiological and psychological mechanisms of behavior advances understanding into how variation in behavior arises. We take a "levels of biological organization" approach, with prime focus placed on the level of the organism to discuss how cognitive processes emerge as traits, and how they can be studied as important mechanisms driving the expression of behavior.

Hormonal Regulation of Mammalian Adult Neurogenesis: A Multifaceted Mechanism

Adult neurogenesis—resulting in adult-generated functioning, integrated neurons—is still one of the most captivating research areas of neuroplasticity. The addition of new neurons in adulthood follows a seemingly consistent multi-step process. These neurogenic stages include proliferation, differentiation, migration, maturation/survival, and integration of new neurons into the existing neuronal network. Most studies assessing the impact of exogenous (e.g., restraint stress) or endogenous (e.g., neurotrophins) factors on adult neurogenesis have focused on proliferation, survival, and neuronal differentiation. This review will discuss the multifaceted impact of hormones on these various stages of adult neurogenesis. Specifically, we will review the evidence for hormonal facilitation (via gonadal hormones), inhibition (via glucocorticoids), and neuroprotection (via recruitment of other neurochemicals such as neurotrophin and neuromodulators) on newly adult-generated neurons in the mammalian brain.

Towards a neurobiology of female aggression

Although many people think of aggression as a negative or undesirable emotion, it is a normal part of many species' repertoire of social behaviors. Purposeful and controlled aggression can be adaptive in that it warns other individuals of perceived breaches in social contracts with the goal of dispersing conflict before it escalates into violence. Aggression becomes maladaptive, however, when it escalates inappropriately or impulsively into violence. Despite ample data demonstrating that impulsive aggression and violence occurs in both men and women, aggression has historically been considered a uniquely masculine trait. As a result, the vast majority of studies attempting to model social aggression in animals, particularly those aimed at understanding the neural underpinnings of aggression, have been conducted in male rodents. In this review, we summarize the state of the literature on the neurobiology of social aggression in female rodents, including social context, hormonal regulation and neural sites of aggression regulation. Our goal is to put historical research in the context of new research, emphasizing studies using ecologically valid methods and modern sophisticated techniques. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity'.

Fecal microbiota in the female prairie vole (Microtus ochrogaster)

We examined the fecal microbiota of female prairie voles. This species is socially and, likely, sexually monogamous, and thus serves as a valuable model in which to examine the interaction between the microbiota-gut-brain axis and social behavior. At present, little is known about the gastrointestinal microbiota of prairie voles; therefore, we performed a first characterization of the fecal microbiota using 16S rRNA gene amplicon sequencing. Semiconductor sequencing technology on an Ion Torrent PGM platform was used to assess the composition of fecal microbiotas from twelve female prairie voles. Following quality filtering, 1,017,756 sequencing reads were classified from phylum to genus level. At the phylum level, Firmicutes, Bacteroidetes, and Saccharibacteria were the predominant taxa, while the Bacteriodales, Erysipelotrichaceae, Ruminococcaceae, and Lachnospiraceae contributed the most dominant microbial groups and genera. Microbial community membership was most similar between vole sibling pairs, but consideration of taxon abundances weakened these associations. The interdependence of host factors such as genetics and behavior with the gastrointestinal microbiota is likely to be particularly pronounced in prairie voles. Our pilot characterization of the prairie vole intestinal microbiota revealed a microbial community composition remarkably consistent with the monogastric alimentary system of these rodents and their diet rich in complex plant carbohydrates. The highly social nature of these animals poses specific challenges to microbiome analyses that nonetheless are valuable for advancing research on the microbiota-gut-brain-behavior axis. Our study provides an important basis for future microbiome research in this emerging model organism for studying social behavior.

Mating and social exposure induces an opioid-dependent conditioned place preference in male but not in female prairie voles (Microtus ochrogaster)

In rodents, sexual stimulation induces a positive affective state that is evaluated by the conditioned place preference (CPP) test. Opioids are released during sexual behavior and modulate the rewarding properties of this behavior. Prairie voles (Microtus ochrogaster) are a socially monogamous species, in which copulation with cohabitation for 6h induces a pair bond. However, the mating-induced reward state that could contribute to the establishment of the long-term pair bond has not been evaluated in this species. The present study aimed to determine whether one ejaculation or cohabitation with mating for 6h is rewarding for voles. We also evaluated whether this state is opioid dependent. Our results demonstrate that mating with one ejaculation and social cohabitation with mating for 6h induce a CPP in males, while exposure to a sexually receptive female without mating did not induce CPP. In the female vole, mating until one ejaculation, social cohabitation with mating, or exposure to a male without physical interaction for 6h did not induce CPP. To evaluate whether the rewarding state in males is opioid dependent, the antagonist naloxone was injected i.p. The administration of naloxone blocked the rewarding state induced by one ejaculation and by social cohabitation with mating. Our results demonstrate that in the prairie vole, on the basis of the CPP in the testing conditions used here, the stimulation received with one ejaculation and the mating conditions that lead to pair bonding formation may be rewarding for males, and this reward state is opioid dependent.

Species differences in behavior and cell proliferation/survival in the adult brains of female meadow and prairie voles

Microtine rodents display diverse patterns of social organization and behaviors, and thus provide a useful model for studying the effects of the social environment on physiology and behavior. The current study compared the species differences and the effects of oxytocin (OT) on anxiety-like, social affiliation, and social recognition behaviors in female meadow voles (Microtus pennsylvanicus) and prairie voles (Microtus ochrogaster). Furthermore, cell proliferation and survival in the brains of adult female meadow and prairie voles were compared. We found that female meadow voles displayed a higher level of anxiety-like behavior but lower levels of social affiliation and social recognition compared to female prairie voles. In addition, meadow voles showed lower levels of cell proliferation (measured by Ki67 staining) and cell survival (measured by BrdU staining) in the ventromedial hypothalamus (VMH) and amygdala (AMY), but not the dentate gyrus of the hippocampus (DG), than prairie voles. Interestingly, the numbers of new cells in the VMH and AMY, but not DG, also correlated with anxiety-like, social affiliation, and social recognition behaviors in a brain region-specific manner. Finally, central OT treatment (200 ng/kg, icv) did not lead to changes in behavior or cell proliferation/survival in the brain. Together, these data indicate a potential role of cell proliferation/survival in selected brain areas on different behaviors between vole species with distinct life strategies.

Social bonding: regulation by neuropeptides

Affiliative social relationships (e.g., among spouses, family members, and friends) play an essential role in human society. These relationships affect psychological, physiological, and behavioral functions. As positive and enduring bonds are critical for the overall well-being of humans, it is not surprising that considerable effort has been made to study the neurobiological mechanisms that underlie social bonding behaviors. The present review details the involvement of the nonapeptides, oxytocin (OT), and arginine vasopressin (AVP), in the regulation of social bonding in mammals including humans. In particular, we will discuss the role of OT and AVP in the formation of social bonds between partners of a mating pair as well as between parents and their offspring. Furthermore, the role of OT and AVP in the formation of interpersonal bonding involving trust is also discussed.

Chemosensory cues affect amygdaloid neurogenesis and alter behaviors in the socially monogamous prairie vole

The current study examined the effects of pheromonal exposure on adult neurogenesis and revealed the role of the olfactory pathways on adult neurogenesis and behavior in the socially monogamous prairie vole (Microtus ochrogaster). Subjects were injected with a cell proliferation marker [5-bromo-2'-deoxyuridine (BrdU)] and then exposed to their own soiled bedding or bedding soiled by a same- or opposite-sex conspecific. Exposure to opposite-sex bedding increased BrdU labeling in the amygdala (AMY), but not the dentate gyrus (DG), of female, but not male, voles, indicating a sex-, stimulus-, and brain region-specific effect. The removal of the main olfactory bulbs or lesioning of the vomeronasal organ (VNOX) in females reduced BrdU labeling in the AMY and DG, and inhibited the male bedding-induced BrdU labeling in the AMY, revealing the importance of an intact olfactory pathway for amygdaloid neurogenesis. VNOX increased anxiety-like behavior and altered social preference, but it did not affect social recognition memory in female voles. VNOX also reduced the percentage of BrdU-labeled cells that co-expressed the neuronal marker TuJ1 in the AMY, but not the DG. Together, our data indicate the importance of the olfactory pathway in mediating brain plasticity in the limbic system as well as its role in behavior.

Estrous phase alters social behavior in a polygynous but not a monogamous Peromyscus species

The social organization of rodent species determines behavioral patterns for both affiliative and agonistic encounters. The neuropeptide oxytocin has been implicated in the mediation of social behavior; however, variability in both neuropeptide expression and social behavior within a single species indicates an additional mediating factor. The purpose of the present comparative study was to investigate social behaviors in naïve mixed-sex pairs of monogamous Peromyscus californicus and polygynous Peromyscus leucopus. We identified substantial inter- and intra-specific variability in the expression of affiliative and agonistic behaviors. Although all P. californicus tested engaged in frequent and prolonged intervals of social contact and rarely engaged in aggressive behaviors, P. leucopus exhibited significant variability in both measures of social behaviors. The naturally occurring differences in social behavior displayed by P. leucopus vary across the estrous cycle, and correspond to hypothalamic oxytocin, as well as circulating oxytocin and glucocorticoid concentrations. These results provide evidence for a rhythm in social behavior across the estrous cycle in polygynous, but not monogamous, Peromyscus species.

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.

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.

The Neurobiology of Trust

This is the first report that endogenous oxytocin in humans is related to social behaviors, which is consistent with a large animal literature. Subjects are put into a social dilemma in which absent communication, cooperative behavior can benefit both parties randomly assigned to a dyad. The dilemma arises because one participant must make a monetary sacrifice to signal the degree of trust in the other before the other's behavioral response is known. We show that receipt of a signal of trust is associated with a higher level of peripheral oxytocin than that in subjects receiving a random monetary transfer of the same average amount. Oxytocin levels were also related to trustworthy behavior (sharing a greater proportion of the monetary gains). We conclude that oxytocin may be part of the human physiology that motivates cooperation.

Forebrain c-fos expression under conditions conducive to pair bonding in female prairie voles (Microtus ochrogaster)

Repeated mating over a period of 6 h facilitates pair-bond formation in monogamous prairie voles. Using this paradigm, we examined fos expression in brain areas implicated in social behavior in voles. We hypothesized that the presence of the fos protein after a period of time sufficient for pair bonding to occur may indicate brain areas that are especially important in pair bond formation. We found elevated levels of fos immunoreactivity in the medial and cortical amygdala, medial preoptic area (MPOA), and bed nucleus of the stria terminalis (BNST) in females that mated several times over a 6-h period as compared to a variety of unmated controls. No treatment effects were found in the central amygdala, nucleus accumbens (NAcc), or lateral septum (LS). We suggest that areas that show evidence of fos expression after sufficient time for pair bonding to occur may be important in the formation of associations between the partner and mating stimuli.

Neuronal activation in the caudal brainstem associated with mating by voles

The expression of c-fos, a marker of neuronal activation, was examined in the gracile nucleus (GN) and nucleus of the solitary tract (NTS) after social interactions, including mating, between male and female prairie voles. In GN, mating, but not non-sexual interactions, induced similar significant increases in c-fos immunoreactivity in both males and females. The increased immunoreactivity was concentrated in medial and dorsal GN suggesting that expression was driven by stimulation of reproductive organs. In contrast, in NTS, mating-induced increases in c-fos expression occurred only in males. These results suggest that both GN and NTS comprise different functional components of mating circuitry and may contribute to pair bonding in monogamous voles.

Social factors regulate female-female aggression and affiliation in prairie voles

Although patterns of aggression and affiliation may play a major role in social organization, the mechanisms underlying these behaviors are not well understood. The purpose of this study was to examine the effects of social and hormonal experience on female-female aggression and affiliation in prairie voles, Microtus ochrogaster. Prairie voles exhibit the traits of social monogamy and tend to live in communal families structured around a male-female pair. It is rare for two unrelated females within a family to successfully reproduce. In this study, the social and/or hormonal experiences of female prairie voles were varied and female-female aggression and affiliation were measured during dyadic encounters with unfamiliar, nonaggressive females. An increase in aggression and decline in affiliative behaviors toward a stimulus female was observed during pregnancy and following male-cohabitation, with or without mating. Pairing with another female did not result in changes in either aggressive or affiliative behaviors toward the stimulus female. Female-female aggression increased and affiliative behaviors declined, with a maximal effect following approximately 8-12 days of male cohabitation. Similar patterns of change were seen in both ovariectomized and gonadally intact females, and treatment with estradiol and subsequent sexual experience did not significantly alter the tendency of females to show aggression or affiliative contact. Social experiences associated with prolonged cohabitation with a male facilitate the induction of female-female aggression; however, ovarian hormones, pregnancy or mating are not essential to induce aggression.

The effects of social environment on adult neurogenesis in the female prairie vole

In the mammalian brain, adult neurogenesis has been found to occur primarily in the subventricular zone (SVZ) and dentate gyrus of the hippocampus (DG) and to be influenced by both exogenous and endogenous factors. In the present study, we examined the effects of male exposure or social isolation on neurogenesis in adult female prairie voles (Microtus ochrogaster). Newly proliferated cells labeled by a cell proliferation marker, 5-bromo-2'-deoxyuridine (BrdU), were found in the SVZ and DG, as well as in other brain areas, such as the amygdala, hypothalamus, neocortex, and caudate/putamen. Two days of male exposure significantly increased the number of BrdU-labeled cells in the amygdala and hypothalamus in comparison to social isolation. Three weeks later, group differences in BrdU labeling generally persisted in the amygdala, whereas in the hypothalamus, the male-exposed animals had more BrdU-labeled cells than did the female-exposed animals. In the SVZ, 2 days of social isolation increased the number of BrdU-labeled cells compared to female exposure, but this difference was no longer present 3 weeks later. We have also found that the vast majority of the BrdU-labeled cells contained a neuronal marker, indicating neuronal phenotypes. Finally, group differences in the number of cells undergoing apoptosis were subtle and did not seem to account for the observed differences in BrdU labeling. Together, our data indicate that social environment affects neuron proliferation in a stimulus- and site-specific manner in adult female prairie voles.

Parental responsiveness is feminized after neonatal castration in virgin male prairie voles, but is not masculinized by perinatal testosterone in virgin females

We previously found a large sex difference in the parental responsiveness of adult virgin prairie voles (Microtus ochrogaster) such that most males are spontaneously parental, whereas most females are not. Because this sex difference is independent of the gonadal hormones normally circulating in adult virgin voles, the present study examined whether perinatal hormones influence the development of this sex difference. Males were treated prenatally (via their pregnant dam) with both the androgen receptor blocker flutamide (5 mg/day/dam) and the aromatase inhibitor ATD (1 mg/day/dam), or oil, for the last 2 weeks of gestation. Half of the subjects from each group were castrated on the day of birth and the other half received a sham surgery. As adults, intact males were castrated and all males received a silastic capsule filled with testosterone. Prenatal treatment with flutamide and ATD had no effect on males' behavior toward pups, but neonatal castration significantly reduced the percentage of males acting parentally. In a second experiment, females were exposed to testosterone propionate (TP; 50 microg/day/dam) or oil via their dam during the last 2 weeks of gestation. For the first neonatal week, half of the females from each group were injected with TP (1 mg/day) and the other half oil. As adults, females were ovariectomized and half from each group received a testosterone-filled capsule and the other half received an empty capsule. None of the perinatal TP treatments increased females' parental responsiveness, although females from all groups that received testosterone capsules as adults were highly parental. Therefore, although postnatal testicular hormones are necessary for high parental responsiveness in males, the behavior of females is not influenced by perinatal exposure to testosterone.

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).

Neuroendocrine regulation of GnRH release in induced ovulators

GnRH is the key neuropeptide controlling reproductive function in all vertebrate species. Two different neuroendocrine mechanisms have evolved among female mammals to regulate the mediobasal hypothalamic (MBH) release of GnRH leading to the preovulatory secretion of LH by the anterior pituitary gland. In females of spontaneously ovulating species, including rats, mice, guinea pigs, sheep, monkeys, and women, ovarian steroids secreted by maturing ovarian follicles induce a pulsatile pattern of GnRH release in the median eminence that, in turn, stimulates a preovulatory LH surge. In females of induced ovulating species, including rabbits, ferrets, cats, and camels, the preovulatory release of GnRH, and the resultant preovulatory LH surge, is induced by the receipt of genital somatosensory stimuli during mating. Induced ovulators generally do not show "spontaneous" steroid-induced LH surges during their reproductive cycles, suggesting that the positive feedback actions of steroid hormones on GnRH release are reduced or absent in these species. By contrast, mating-induced preovulatory surges occasionally occur in some spontaneously ovulating species. Most research in the field of GnRH neurobiology has been performed using spontaneous ovulators including rat, guinea pig, sheep, and rhesus monkey. This review summarizes the literature concerning the neuroendocrine mechanisms controlling GnRH biosynthesis and release in females of several induced ovulating species, and whenever possible it contrasts the results with those obtained for spontaneously ovulating species. It also considers the adaptive, evolutionary benefits and disadvantages of each type of ovulatory control mechanism. In females of induced ovulating species estradiol acts in the brain to induce aspects of proceptive and receptive sexual behavior. The primary mechanism involved in the preovulatory release of GnRH among induced ovulators involves the activation of midbrain and brainstem noradrenergic neurons in response to genital-somatosensory signals generated by receipt of an intromission from a male during mating. These noradrenergic neurons project to the MBH and, when activated, promote the release of GnRH from nerve terminals in the median eminence. In contrast to spontaneous ovulators, there is little evidence that endogenous opioid peptides normally inhibit MBH GnRH release among induced ovulators. Instead, the neural signals that induce a preovulatory LH surge in these species seem to be primarily excitatory. A complete understanding of the neuroendocrine control of ovulation will only be achieved in the future by comparative studies of several animal model systems in which mating-induced as well as spontaneous, hormonally stimulated activation of GnRH neurons drives the preovulatory LH surge.

Hypothalamic vasopressin gene expression increases in both males and females postpartum in a biparental rodent

In previous studies, the closely related neuropeptide hormones oxytocin and vasopressin have been implicated in the central mediation of parental behaviour. Several studies in rats and sheep have demonstrated a role for oxytocin in the initiation of maternal behaviour. Recently, a few studies in a biparental species, the prairie vole (Microxytocinus ochrogaster) have suggested that vasopressin is important for paternal care. The present study investigated this latter possibility by measuring changes in vasopressin and oxytocin hypothalamic gene expression 1 day and 6 days following parturition in prairie voles which show paternal care and in montane voles (M. montanus) which lack paternal care. In prairie voles, vasopressin gene expression increased in both males and females postpartum, relative to sexually naive controls. In the non-paternal montane vole, no change in vasopressin gene expression was observed in either sex. In contrast to this species difference in vasopressin gene expression, hypothalamic oxytocin gene expression increased in both prairie and montane vole females, but not in males of either species. To augment measures of gene expression, we assessed vasopressin (V1a) and oxytocin receptor binding in both species. Although forebrain vasopressin V1a receptor binding was not altered following parturition in either species, oxytocin receptor binding increased in the ventromedial nucleus of the hypothalamus in females, but not males, in both prairie and montane voles. In summary, vasopressin gene expression increases in both males and females postpartum in a biparental species and oxytocin gene expression and receptor binding increase selectively in females. These results are consistent with earlier reports of a role for vasopressin in paternal care and for oxytocin in maternal behaviour.

Prior exposure To oxytocin mimics the effects Of social contact and facilitates sexual behaviour In females

The purpose of this study was to determine whether pretreatment with oxytocin could mimic the effects of social contact and enhance sexual receptivity in female prairie voles. Female prairie voles require prolonged exposure to males to become sexually active and oxytocin has been shown to play a major role in the establishment of social bonds between males and females. Therefore, we hypothesized that prior exposure to exogenous oxytocin, in the absence of males, would enhance sexual activity in females. Two experiments were conducted to test this hypothesis. Experiment 1 examined the capacity of oxytocin to enhance sexual behaviour in females undergoing natural oestrus. Sexually naive female prairie voles received a daily subcutaneous injection of 20 microg oxytocin or isotonic saline for 5 days before being placed with a sexually experienced male for 48 h. Females treated with oxytocin were significantly more likely to mate during this period than saline-treated females. In experiment 2 the ability of oxytocin to increase subsequent sensitivity of sexually naive females to oestradiol was tested. Females that received oxytocin pretreatment, as in experiment 1, followed by oestradiol displayed a significant increase in sexual receptivity when compared to females treated with saline and oestradiol or oestradiol only. The results supported the hypothesis that prior exposure to oxytocin can mimic the effects of social contact, and can facilitate sexual receptivity by increasing the sensitivity of females to very low doses of oestradiol.

Evolutionary change in the endocrinology of behavioral receptivity: divergent roles for progesterone and prolactin within the genus Phodopus

Siberian (Phodopus sungorus) and Djungarian (P. campbelli) hamsters are phenotypically similar and were long considered subspecies. Progesterone (P4) and prolactin (PRL) changes (determined by repeated sampling of individuals) during the behavioral receptivity of both an ovulatory cycle and a postpartum mating, as well as the hormonal requirements for behavioral receptivity, were determined. Changes in P. sungorus were similar to well-described hormonal changes in rats, mice, and golden hamsters, suggesting that previously described differences between P. campbelli and those species had evolved recently. Specifically, 1) P4 facilitated behavioral receptivity at low priming doses of estradiol in P. sungorus but was not needed in P. campbelli; 2) in P. sungorus, P4 increases were synchronous across females and of similar amplitude during each estrus, whereas in P. campbelli, P4 increases were less synchronous across females and were reduced in amplitude postpartum; and 3) PRL profiles were similar (high average PRL levels, few high surges detected) in each species on Day 18, but on proestrus, cyclic P. sungorus had elevated PRL levels and frequent surges while cyclic P. campbelli had lower PRL levels and rare surges. As the endocrinology of P. campbelli also differs from known laboratory rodents in other ways, additional within-genus divergence is predicted.

Intraspecific variation in the induction of female sexual receptivity in prairie voles

Prairie voles (Microtus ochrogaster) are monogamous New World rodents which show geographic variation in social behavior. In this study, parameters of female reproduction which might be related to mating system were compared in prairie voles from eastern Kansas (KAN) versus central Illinois (ILL). KAN females showed a more rapid onset of natural estrus following exposure to a male and were more likely to respond to injections of a low dose (0.5 microg) of exogenous estradiol benzoate than ILL females. Neither mating duration nor pregnancy success after mate removal differed in KAN versus ILL females. These results suggest that ILL voles are less sensitive than KAN voles to the estrus-inducing effects of either endogenous or exogenous estrogen, supporting the hypothesis that variations in reproductive strategy occur among geographically discrete populations of prairie voles.

Effects of estradiol on sexual receptivity, wheel-running behavior, and vaginal estrus in virgin prairie voles

This study examined the effects of estradiol on sexual receptivity and vaginal estrus in experienced and virgin female prairie voles, and running wheel activity in virgin females. Because prairie voles undergo induced ovulation, we predicted that exogenous estradiol would not affect activity patterns nor stimulate lordosis in virgins. Females were SC injected with estradiol benzoate (ED) and placed with males, and sexual receptivity was monitored. In experienced females 0.5 micrograms of EB resulted in 100% of the females displaying lordosis after 48 h. In contrast, only 20% of the females receiving 0.05 micrograms displayed lordosis. Less than 20% of virgin females receiving EB displayed lordosis. Virgins displayed no change in wheel-running activity after EB treatments, indicating that behaviors that are influenced by estradiol and regulated by the brain may require previous exposure to males to develop. Although all females receiving the 0.5 micrograms of EB displayed vaginal estrus, virgin females achieved vaginal estrus later than experienced females, suggesting that exposure to males has a priming effect resulting in a reduced period to vaginal estrus.

Estradiol concentration and the regulation of locomotor activity

We predicted that female prairie voles (Microtus ochrogaster) would not increase in locomotor activity during "induced" proestrus. We developed and tested two alternative a priori hypotheses to explain this predicted lack of activity. The Non-Response Hypothesis in which voles cannot, physiologically, increase activity in response to estradiol and, the Threshold Effect Hypothesis in which a minimal concentration of estradiol is necessary to achieve estrus, while higher concentrations influence other behavioral parameters. The non-response hypothesis predicts that hormone concentration will not affect locomotor activity, while the threshold effects hypothesis predicts that voles achieve estrus at low concentrations, while higher concentrations should trigger increasing locomotor activity. Initial results using running wheels indicated that females decreased activity during induced proestrus. Radioimmunoassay revealed that induced proestrus was achieved at relatively low concentrations of estradiol. Results from a dose response experiment allowed for rejection of the nonresponse hypothesis and supported the threshold effects hypothesis as females decreased activity at low concentrations of estradiol, showed no relative change at moderate concentrations, and increased activity at higher concentrations.

Physiological substrates of mammalian monogamy: the prairie vole model

Prairie voles (Microtus ochrogaster) are described here as a model system in which it is possible to examine, within the context of natural history, the proximate processes regulating the social and reproductive behaviors that characterize a monogamous social system. Neuropeptides, including oxytocin and vasopressin, and the adrenal glucocorticoid, corticosterone, have been implicated in the neural regulation of partner preferences, and in the male, vasopressin has been implicated in the induction of selective aggression toward strangers. We hypothesize here that interactions among oxytocin, vasopressin and glucocorticoids could provide substrates for dynamic changes in social and agonistic behaviors, including those required in the development and expression of monogamy. Results from research with voles suggest that the behaviors characteristics of monogamy, including social attachments and biparental care, may be modified by hormones during development and may be regulated by different mechanisms in males and females.

Oxytocin receptor binding in female prairie voles: endogenous and exogenous oestradiol stimulation

Abstract Previous studies have demonstrated that oxytocin receptors in specific nuclei of rat forebrain are regulated by gonadal steroids. The current study used in vitro receptor autoradiography to investigate the distribution and regulation of oxytocin receptors in the forebrain of the female prairie vole (Microtus ochrogaster). In contrast to rats, in female prairie voles gonadal steroid secretion and oestrus behaviour result from male chemosignal stimulation and ovulation is induced by mating. Thus, the prairie vole brain provides an opportunity for investigating links between environmental stimuli, gonadal steroids and oxytocin receptors. Using a selective oxytocin receptor ligand [(125)l]d(CH(2))(5)[Tyr(Me)(2),Tyr-NH(2) (9)]ornithine vasotocin ([(125)I]OTA), specific binding was found in several regions including the anterior olfactory nucleus, the ventromedial nucleus of the hypothalamus, the bed nucleus of the stria terminalis, the amygdala and several cortical areas. Following ovariectomy, oestradiol benzoate (10 mug) administration increased oxytocin receptor binding 100% in the anterior olfactory nucleus, but did not affect receptors in other regions. Gonadallyintact females, exposed to male chemosignals, had significant increases in both endogenous oestradiol levels and anterior olfactory nucleus oxytocin receptor binding relative to gonadally-intact females unexposed to male chemosignals. Following prolonged exposure to males with ad libitum mating, [(125)I]OTA receptor binding decreased to the levels found in unstimulated females. These results demonstrate that increases in oestrogen levels, of either exogenous or endogenous origin, can modulate oxytocin receptors in the brains of female prairie voles. In contrast to rats, oestrogen in female prairie voles appears to affect receptors in the anterior olfactory nucleus rather than the hypothalamus. We suggest that the species differences in oxytocin receptor distribution and gonadal steroid responsiveness reflect variations in reproductive physiology and possibly behaviour.

Central and peripheral effects of oxytocin administration in prairie voles (Microtus ochrogaster)

The present study examined the hypothesis that oxytocin (OT) may influence female sexual behavior in prairie voles (Microtus ochrogaster). The effectiveness of OT to induce sexual behavior was tested in ovariectomized females that were injected daily with estradiol benzoate (EB, 0.02 micrograms, twice), a dose insufficient for estrus induction. On the third day females received intracerebroventricular (ICV) injections of OT (1, 300, or 1000 ng) or saline vehicle. In the presence of minimal estrogen stimulation, OT did not induce sexual receptivity, or influence autogrooming or other social interactions. The behavioral effects of OT were examined in another group of ovariectomized females that received daily oil or EB injections (10 micrograms, twice) followed on the third day by either ICV (1, 300, or 1000 ng) or intraperitoneal (IP) (1, 3, or 10 micrograms) injections of OT. Among EB-treated females, only those in confirmed estrus, prior to ICV or IP injection, were included in these studies. There was a dose-related decrease in the percentage of females that remained in behavioral estrus after ICV OT. In those females that continued to show sexual behavior, lordosis frequencies and durations were unaffected by ICV OT. Nonsexual behavior did not differ between mated females and those exhibiting OT-inhibited sexual behavior. In females that were EB-treated, autogrooming and side-by-side behavior increased after ICV OT, while there was a decline in aggression. Female sexual and nonsexual behaviors were not significantly affected by IP OT.(ABSTRACT TRUNCATED AT 250 WORDS)

Modes of estrus induction as a factor in studies of the reproductive behavior of rodents

The choice of a mode of estrus for use in studies related to reproductive processes often is made arbitrarily. Females may be in hormone-induced estrus (HIE), cycling estrus (CE), male-induced estrus (MIE), or postpartum estrus (PPE). Although mode of estrus affects various aspects of copulatory behavior in different species in different ways, males mating with PPE females generally require more intromissions to reach ejaculation than with females in HIE or CE; males mating with females in CE generally attain more ejaculations than with PPE females. The duration of receptivity generally is shorter in PPE than in CE. Females in CE and PPE have different stimulus thresholds for pregnancy initiation, with the direction of the difference varying with the species. Among the other phenomena affected by mode of estrus are sperm competition, mate choice, and the Bruce effect. Because mode of estrus can be an important determinant of results, selection of a mode of estrus for any experiment should not be simply a matter of convenience. Further study of effects of mode of estrus is warranted.

Hormonal correlates of sexual behavior and ovulation in male-induced and postpartum estrus in female prairie voles

The purpose of the present study was a description of hormonal profiles in female prairie voles (Microtus ochrogaster) in estrus that was induced by male exposure versus postpartum estrus. Hormonal profiles are reported in sexually naive females and in sexually experienced females, as a function of varying amounts of coital stimulation and as a function of time since male exposure. Ovarian estradiol levels, uterine weights and uterine protein levels increased in virgin females after exposure to a male, were highest in females that showed lordosis, declined slowly when estrous females were isolated from males and decreased sharply following mating. Ovarian progesterone levels increased more rapidly following mating in females in male-induced estrus than in females in postpartum estrus. Serum progesterone levels did not increase significantly within 24 hr following mating, but were elevated by 72 hr after mating. These findings are discussed as they relate to the hormonal control of female sexual behavior.

Effects of hormonal, sexual, and social history on mating and pair bonding in prairie voles

The interactive effects of hormones, sexual history and cohabitation on sexual and social behaviors were examined in pairs of ovariectomized female and sexually experienced male prairie voles (Microtus ochrogaster). Monitoring with time lapse video tape revealed that females in estradiol benzoate (EB)-induced estrus, and their male partners engaged in high levels of sexual activity which continued intermittently for at least 3 days (until observations were arbitrarily terminated). In conjunction with other studies, these results indicate that the hormonal condition of the female at the time of testing is a major determinant of sexual activity. Prior hormonal, copulatory, or cohabitation experience did not significantly influence sexual responses between females and unfamiliar male partners. However, affiliative behaviors, such as side by side contact, were higher in pairs that were familiar due to prior sexual and cohabitational experience. These results indicate that social and sexual behaviors are independently regulated. Other behaviors, including nasogenital investigation and autogrooming were influenced by the hormonal and sexual history of the female. The implications of these behavioral patterns for reproductive activation, pair bonding, and incest avoidance are discussed.

Mating-induced inhibition of receptivity in the female golden hamster. I. Short-term and long-term effects

Female golden hamsters (Mesocricetus auratus) which have actively terminated mating with one male will respond in succession to two or more novel males. However, mating with the first male results in a reduction in receptivity shown toward a second male, with the size of the decrease being proportional to the amount of copulatory stimuli provided by the first male. Furthermore, the decreased receptivity shown toward the second male results in his achieving fewer ejaculations. These effects are more pronounced when the second mating occurs 5 min or 8 h after the first mating than when it occurs 1 or 4 h later, suggesting both short-term and long-term inhibitory effects. Copulation with a single male also effects a 40% shortening of behavioral receptivity. In a promiscuous species such as the hamster, prolonged copulation by the male may enhance the short-term inhibition of female receptivity, thereby reducing the risk of subsequent sperm displacement. The long-term effect occurs too late to affect sperm competition and may, instead, function to reduce the risk of predation as a result of unnecessary further mating.

Physiology of sexual behavior. Embryologic organization and adult activation

Recent studies suggest that sexual behavioral function should be integrated into the framework of sexual physiology as a neuroendocrine phenomenon. Evidence of embryonic neuronal induction and subsequent adult activation and functioning of sex specific circuitry is reviewed.

Neural gonadal steroid actions

Neurons sensitive to gonadal steroids are located strategically within neural circuits that mediate behaviors broadly related to the reproductive process. Some neuronal events and properties are regulated by these hormones. Variability in the occurrence and distribution of particular neural hormonal sensitivities across species may be related to variations in the hormonal requirements for sexual differentiation and for activation of reproductive behaviors.