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

The effect of male proximity on vaginal patency, estrous cycle length and feed intake of female grasscutters

This study was conducted to investigate the influence of male proximity on vaginal patency, estrous cycle length and feed intake of nulliparous and pluriparous grasscutter (Thryonomys swinderianus) does. Twelve non-pregnant and non-lactating adult female (six nulliparous and six pluriparous) grasscutters and six proven male grasscutters were used for the study in a 2 × 2 factorial design. The factors were the type of contact between the sexes (no visual contact v. visual contact) and parity (nulliparous v. pluriparous). The condition of the vaginal membrane, which was monitored daily, was used as an indicator of the reproductive status of the experimental females. The findings of the study showed significant effect (P < 0.001) of male presence on feed intake. Visual contact between breeding pairs significantly (P < 0.05) increased the rate of vaginal opening (11 ± 3.3 % v. 22 ± 3.3 %) and significantly (P < 0.001) reduced the length of estrous cycling in the grasscutters (58 ± 5 days v. 20 ± 5 days). However, the duration of vaginal patency was not (P > 0.05) affected by the contact type. Parity had a significant (P < 0.05) effect on the duration of vaginal patency with pluriparous does recording longer duration compared to nulliparous does (76 ± 12 h v. 38 ± 12 h), but not on feed intake (P > 0.05). The study has revealed that housing breeding grasscutters such that females have visual contact with males could increase rate of vaginal patency and reduce the length of the estrous cycle in both pluriparous and nulliparous grasscutter does. This practice however compromises feed intake of the females.

A novel model for neuroendocrine toxicology: neurobehavioral effects of BPA exposure in a prosocial species, the prairie vole (Microtus ochrogaster)

Impacts on brain and behavior have been reported in laboratory rodents after developmental exposure to bisphenol A (BPA), raising concerns about possible human effects. Epidemiological data suggest links between prenatal BPA exposure and altered affective behaviors in children, but potential mechanisms are unclear. Disruption of mesolimbic oxytocin (OT)/vasopressin (AVP) pathways have been proposed, but supporting evidence is minimal. To address these data gaps, we employed a novel animal model for neuroendocrine toxicology: the prairie vole (Microtus ochrogaster), which are more prosocial than lab rats or mice. Male and female prairie vole pups were orally exposed to 5-μg/kg body weight (bw)/d, 50-μg/kg bw/d, or 50-mg/kg bw/d BPA or vehicle over postnatal days 8-14. Subjects were tested as juveniles in open field and novel social tests and for partner preference as adults. Brains were then collected and assessed for immunoreactive (ir) tyrosine hydroxylase (TH) (a dopamine marker) neurons in the principal bed nucleus of the stria terminalis (pBNST) and TH-ir, OT-ir, and AVP-ir neurons in the paraventricular nucleus of the hypothalamus (PVN). Female open field activity indicated hyperactivity at the lowest dose and anxiety at the highest dose. Effects on social interactions were also observed, and partner preference formation was mildly inhibited at all dose levels. BPA masculinized principal bed nucleus of the stria terminalis TH-ir neuron numbers in females. Additionally, 50-mg/kg bw BPA-exposed females had more AVP-ir neurons in the anterior PVN and fewer OT-ir neurons in the posterior PVN. At the 2 lowest doses, BPA eliminated sex differences in PVN TH-ir neuron numbers and reversed this sex difference at the highest dose. Minimal behavioral effects were observed in BPA-exposed males. These data support the hypothesis that BPA alters affective behaviors, potentially via disruption of OT/AVP pathways.

Sex hormones' regulation of rodent physical activity: a review

There is a large body of emerging literature suggesting that physical activity is regulated to a varying extent by biological factors. Available animal data strongly suggests that there is a differential regulation of physical activity by sex and that the majority of this differential regulation is mediated by estrogen/testosterone pathways with females in many animal species having higher daily activity levels than males. The purpose of this manuscript is to review the mechanisms by which estrogen, progesterone, and testosterone affect the regulation of physical daily activity. This review lays the foundation for future investigations in humans as well as discussions about relative disease risk mediated by differential biological regulation of physical activity by sex.

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.

Neonatal manipulation of oxytocin influences female reproductive behavior and success

During early neonatal development, oxytocin (OT) may influence the expression of adult behavior and physiology. Here we test the prediction that early postnatal exposure to OT or an oxytocin antagonist (OTA) can affect the subsequent expression of sexual receptivity and reproductive success of females. To test this hypothesis, female prairie voles (Microtus ochrogaster) received one of four treatments within 24 h of birth. Three groups received an intraperitoneal injection of OT, OTA, or isotonic saline. A fourth group was handled, but not injected. Around 75 days of age, females were paired with sexually experienced males for 72 h and sexual activity was recorded. Treatment had no effect on the probability of mating. Injection, regardless of treatment, reduced latency to mate compared with handled controls. OT and OTA treatment decreased mating bout frequency compared to saline and handled controls, while OTA treatment increased reproductive success, probability of successfully producing a litter. The results suggest that neonatally OT, both endogenous and exogenous, can affect the expression of adult female reproductive activity and that blocking the effects of endogenous OT during neonatal development can affect female reproductive success. Finally, the results suggest that a number of aspects of reproduction are regulated by OT during the postnatal period, but that the mechanism of action may differ depending upon the reproductive activity.

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.

Effects of the phytoestrogen coumestrol on locomotor and fear-related behaviors in female mice

Effects of the estrogenic plant isoflavonoid coumestrol (COUM) on locomotor activity and fear-related behaviors in both the absence and the presence of estradiol benzoate (EB) were examined in adult ovariectomized Swiss-Webster mice. In a running wheel paradigm conducted over 10 days, with treatment beginning 12 days prior to testing, daily subcutaneous (sc) injections of 10 microg of COUM did not influence locomotor activity, whereas even the low dose of EB supplied by sc Silastic implant (2.5 microg lasting 5 weeks) produced a significant increase in locomotor activity over animals receiving vehicle alone. In animals receiving both COUM and EB, locomotor activity was significantly diminished compared to the activity observed in animals receiving EB alone. The same animals were also tested in three behavioral paradigms having known activity and fear components (open field, dark/light transition, elevated plus-maze). COUM did not produce significant effects in these assays. However, in an associative fear learning paradigm (fear conditioning), COUM produced significantly less freezing ("fearfulness") in the conditioned fear task than with animals receiving vehicle or a low dose of EB, which did not differ from each other. Unlike EB, which has comparable affinities for estrogen receptor (ER) alpha and ER beta, COUM has a higher affinity for ER beta than for ER alpha. COUM might disrupt estrogen-enhanced locomotor activity by altering the dynamic by which EB acts on the brain through these two ER isoforms. The conditioned fear result suggests that COUM may produce additional effects through alternative pathways.

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.

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.

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.