Sexual differentiation in prairie voles: the effects of corticosterone and testosterone

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.

Differential effects of steroid hormones on levels of broad-sense heritability in a wild bird: possible mechanism of environment × genetic variance interaction?

Genetic variation is one of the key concepts in evolutionary biology and an important prerequisite of evolutionary change. However, we know very little about processes that modulate its levels in wild populations. In particular, we still are to understand why genetic variances often depend on environmental conditions. One of possible environment-sensitive modulators of observed levels of genetic variance are maternal effects. In this study we attempt to experimentally test the hypothesis that maternally transmitted agents (e.g. hormones) may influence the expression of genetic variance in quantitative traits in the offspring. We manipulated the levels of steroid hormones (testosterone and corticosterone) in eggs laid by blue tits in a wild population. Our experimental setup allowed for full crossing of genetic and rearing effects with the experimental manipulation. We observed that birds treated with corticosterone exhibited a significant decrease in broad-sense genetic variance of tarsus length, and an increase in this component in body mass on the 2nd day post-hatching. Our study indicates, that maternally transmitted substances such as hormones may have measurable impact on the levels of genetic variance and hence, on the evolutionary potential of quantitative traits.

Endocrine Disruption of Vasopressin Systems and Related Behaviors

Endocrine disrupting chemicals (EDCs) are chemicals that interfere with the organizational or activational effects of hormones. Although the vast majority of the EDC literature focuses on steroid hormone signaling related impacts, growing evidence from a myriad of species reveals that the nonapeptide hormones vasopressin (AVP) and oxytocin (OT) may also be EDC targets. EDCs shown to alter pathways and behaviors coordinated by AVP and/or OT include the plastics component bisphenol A (BPA), the soy phytoestrogen genistein (GEN), and various flame retardants. Many effects are sex specific and likely involve action at nuclear estrogen receptors. Effects include the elimination or reversal of well-characterized sexually dimorphic aspects of the AVP system, including innervation of the lateral septum and other brain regions critical for social and other non-reproductive behaviors. Disruption of magnocellular AVP function has also been reported in rats, suggesting possible effects on hemodynamics and cardiovascular function.

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.

Circulating plasma testosterone during early neonatal life in the socially monogamous and biparental prairie vole (Microtus ochrogaster)

Adult male prairie voles (Microtus ochrogaster) are highly gregarious and socially monogamous, which is dissimilar to most other male rodents but reminiscent of many primates, including humans. This has resulted in prairie voles becoming a premier model in which to study the neural and hormonal basis of complex social behaviors, as well as the atypical development of these behaviors. This research is impeded by the complete lack of knowledge about the gonadal steroid environment during early development in this species. In many other animals, testicular hormones released during perinatal life permanently organize the neural substrates underlying later social behaviors in males, so knowledge about the presence or possibly even absence of testosterone in male prairie vole pups would provide important insight into their neurobehavioral development. In this study, male and female prairie vole pups were sacrificed 1, 2, 3, 4 or 5 days after birth and radioimmunoassay used to determine plasma levels of testosterone. We found that testosterone was detectable in both sexes on all days and that levels were significantly higher in males than females. Testosterone in neonatal males was sometimes as high as breeding males sampled one day after the birth of a litter. This study is the first to examine circulating testosterone in neonatal prairie voles, or any other species of Microtus, and the results indicate similarity to other rodents. This is surprising because some previous studies manipulating gonadal hormones in neonatal prairie voles have found limited effects on their neurobehavioral development, suggesting the existence of some unique, yet-to-be-revealed aspects of their neuroendocrine profile during early life.

Maternal corticosterone but not testosterone level is associated with the ratio of second-to-fourth digit length (2D:4D) in field vole offspring (Microtus agrestis)

The steroid environment encountered by a foetus can strongly affect its post-natal physiology and behaviour. It has been proposed that steroid concentrations experienced in utero could be estimated from adults by measuring their second-to-fourth digit length ratio (2D:4D). However, there is still little direct evidence that intra-uterine steroid levels affect individual 2D:4D. We examined whether maternal pre-pregnancy testosterone and corticosterone levels (as estimates of intra-uterine testosterone and corticosterone exposure) affected the 2D:4D of pups in non-domesticated field voles (Microtus agrestis), measured by X-rays at the age of weaning (21 days). Furthermore, for the first time in a non-human species, we studied whether testosterone and corticosterone levels correlated with 2D:4D in adult females. We found that the maternal pre-pregnancy level of testosterone was not associated with offspring 2D:4D in either the left or the right paw. Instead, maternal pre-pregnancy corticosterone level was positively correlated with offspring 2D:4D in the right paw, but unrelated to 2D:4D in the left paw. In addition, the 2D:4D of adult females was not associated with either their circulating testosterone or corticosterone levels. Our results suggest that in field voles maternally administered testosterone is not a major determinant of offspring 2D:4D, whereas maternal stress appears to account for some of the variation in the 2D:4D of their offspring.

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.

Age-related variation in the adrenocortical response to stress in nestling white storks (Ciconia ciconia) supports the developmental hypothesis

The post-natal development of the adrenocortical response to stress was investigated in European white storks. Sixty wild nestlings aged 24-59 days old were subjected to a standardized capture and restraint protocol, and the time-course pattern of the response to stress was assessed through determination of circulating corticosterone in blood samples collected at five fixed times during the 45-min period following capture. The time course of the response was best fit to a third-order function of handling time, and showed a strong effect of age. Although age did not affect baseline titers and all birds showed a positive post-capture increase in circulating corticosterone, age had a positive effect on the relative increase from baseline titer, the recorded time to reach maximum level, and the acute concentration after 10 min following capture and restraint. While young nestlings displayed very little response to capture, the response near fledging resembled the typical adrenocortical pattern widely reported in fully developed birds. Our results concur with those found in altricial and semi-altricial species, and suggest that non-precocial birds follow a similar mode of development of the hypothalamic-pituitary-adrenal (HPA) axis. The fact that HPA sensitivity to stress is functional suggests that young storks gradually develop emergency responses of adaptive value and are able to overcome acute perturbations in spite of their parental dependence, at least during the last two-thirds of post-natal development. According to the Developmental Hypothesis, such gradual changes would allow nestlings to respond to perturbations as a function of the specific behavioral and physiological abilities of their age. The potential sources of stress that nestlings have to face during development (i.e., weather conditions, dietary restrictions, and social competition) are discussed according to developmental changes in behavioral and physiological abilities.

Tyrosine hydroxylase-synthesizing cells in the hypothalamus of prairie voles (Microtus ochrogaster): sex differences in the anteroventral periventricular preoptic area and effects of adult gonadectomy or neonatal gonadal hormones

The vertebrate hypothalamus and surrounding region contain a large population of cells expressing tyrosine hydroxylase (TH), the rate limiting enzyme for synthesis of dopamine and other catecholamines. Some of these populations are sexually dimorphic in rats. We here examined sex differences in TH-immunoreactive populations in the forebrain of gonadally intact and gonadectomized prairie voles (Microtus ochrogaster), a species that sometimes shows unusual sexual differentiation of brain and behavior. A sex difference was found in the anteroventral periventricular preoptic area (AVPV; likely analogous to the rat rostral A14) only in gonadectomized subjects, which was due to a 50% reduction in the number of TH-immunoreactive cells after castration in males. There was no significant sex difference or effects of gonadectomy on the number of TH-immunoreactive cells in the anteroventral preoptic area (AVP), periventricular anterior hypothalamus (caudal A14), arcuate nucleus (A12), zona incerta (A13), or posterodorsal hypothalamus (A11). In a second experiment, testosterone propionate (TP; 500 microg), diethylstilbestrol (DES; 1 microg), or estradiol benzoate (EB; 30 microg) injected daily during the first week after birth each significantly reduced later TH expression in the AVPV of females by approximately 40-65% compared to oil-treated controls. Unlike rats, therefore, a sex difference in TH expression in the prairie vole AVPV is found only after removal of circulating gonadal hormones in males. Furthermore, unlike our previous findings on the generation of sex differences in extra-hypothalamic arginine-vasopressin expression in prairie voles, TH expression in the AVPV of female prairie voles can be highly masculinized by neonatal exposure to either aromatizable androgens or estrogens.

Unexpected effects of perinatal gonadal hormone manipulations on sexual differentiation of the extrahypothalamic arginine-vasopressin system in prairie voles

The sexually dimorphic extrahypothalamic arginine-vasopressin (AVP) projections from the bed nucleus of the stria terminalis to the lateral septum (LS) and lateral habenula (LHb) are denser in males than females and, in rats, require males' perinatal exposure to gonadal hormones but the absence of such exposure in females. We examined perinatal hormone effects on development of this sex difference in prairie voles (Microtus ochrogaster), which show atypical effects of hormones on sexual differentiation of some reproductive behaviors. Neonatal castration reduced the number of AVP mRNA-expressing cells in the bed nucleus of the stria terminalis and AVP immunoreactivity (ir) in the LS and LHb. Surprisingly, daily injections of 1000 microg of testosterone propionate (TP) during the first postnatal week did not maintain high levels of AVP-ir in neonatally castrated males. Furthermore, perinatal treatments with TP (75, 500, or 1000 microg), testosterone (100 microg), or dihydrotestosterone (200 microg) did not masculinize AVP-ir in the female LS or LHb. In fact, 1000 microg TP reduced it in some cases. However, 1000 microg TP lengthened anogenital distance, indicating that TP was biologically active. Neonatal estrogen receptor antagonism with tamoxifen reduced AVP-ir in the male LS, whereas treating neonatal females with the synthetic estrogen diethylstilbestrol increased septal AVP-ir. Tamoxifen and diethylstilbestrol had no effects in the LHb. Similar to rats, therefore, postnatal estrogen influences some components of the extrahypothalamic AVP system in prairie voles, but this developing system appears to be insensitive to exogenous androgens, including aromatizable androgens. Such insensitivity is atypical for a sexually dimorphic neural system in a rodent and may reflect the unusual effects of hormones on sexual differentiation of some behaviors in prairie voles.

Extraordinary diversity in vasopressin (V1a) receptor distributions among wild prairie voles (Microtus ochrogaster): Patterns of variation and covariation

The vasopressin V1a receptor is a gene known to be central to species differences in social behavior, including differences between the monogamous prairie vole and its promiscuous congeners. To examine how individual differences compare with species differences, we characterize variability in the expression of the vasopressin V1a receptor (V1aR) in a large sample of wild prairie voles. We find a surprising degree of intraspecific variation in V1aR binding that does not seem attributable to experimental sources. Most brain regions exhibit differences between upper and lower quartiles that are comparable to differences between species in this genus. Regions that are less variable have been implicated previously in regulating monogamous behaviors, suggesting that the lack of variation at these sites could reflect natural selection on mating system. Many brain regions covary strongly. The overall pattern of covariation reflects the developmental origins of brain regions. This finding suggests that shared mechanisms of transcriptional regulation may limit the patterns of gene expression. Such biases may shape both the efficacy of selection and the pattern of individual and species differences. Overall, our data indicate that the prairie vole would be a useful model for exploring how individual differences in gene expression influence complex social behaviors.

Effects of prenatal hydrocortisone acetate exposure on fertility and sexual behavior in male rats

The aim of the present study was to investigate the effects of hydrocortisone during the prenatal period and its later repercussions on the fertility and sexual behavior of male rats. Pregnant rats were treated (s.c.) with hydrocortisone acetate, at 1.5 mg/day on the 17th, 18th, and 19th days of gestation. Decreased body weight and no alteration in anogenital distance were observed in male offspring. Adulthood, presented reductions of body weight, plasma testosterone levels, and seminal-vesicle wet weight without secretion as well as no alteration in the wet weights of the testes, epididymis, and seminal vesicle with secretion in the treated group. Males exposed to hydrocortisone during the prenatal period were able to mate with normal females, which became pregnant but exhibited an increased number of post-implantation losses. In spite of this, these treated males exhibited decreased male sexual behavior and the appearance of female sexual behavior after these male rats were castrated and pretreated with exogenous estrogen. These results indicate that exposure to hydrocortisone in the later stages of pregnancy may have a long-term effect on the fertility and sexual behavior of male rats, suggesting an incomplete masculinization and defeminization of the central nervous system.

Developmental consequences of oxytocin

This paper examines the developmental effects of the mammalian neuropeptide, oxytocin (OT). In adults, OT is the most abundant neuropeptide in the hypothalamus and serves integrative functions, coordinating behavioral and physiological processes. For example, OT has been implicated in parturition, lactation, maternal behavior and pair bond formation. In addition, OT is capable of moderating behavioral responses to various stressors as well as the reactivity of the hypothalamic-pituitary-adrenal (HPA) axis. Neonates may be exposed to hormones of maternal origin, possibly including peptides administered to the mother in the perinatal period to hasten or delay birth and in milk; however, whether peptide hormones from the mother influence the developing infant remains to be determined. In rodents, endogenous OT is first synthesized during the early postnatal period, although its functions at this time are not well known. Experiments in neonatal prairie voles have documented the capacity of OT and OT receptor antagonists to have immediate and lifelong consequences for social behaviors, including adult pair bonding and parental behaviors, as well as the reactivity of the HPA axis; most of these effects are sexually dimorphic. Possible mechanisms for such effects, including long-lasting changes in OT and vasopressin, are summarized.

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.

Influence of gonadal hormones on the development of parental behavior in adult virgin prairie voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are a socially monogamous species and both sexes are parental after the birth of pups. In contrast, sexually inexperienced adult prairie voles differ in their behavior towards pups such that virgin males are paternal whereas virgin females are often infanticidal. To test whether there exists a discrete perinatal 'sensitive period' during which gonadal hormones influence this behavior, and to distinguish between the relative contributions of estrogenic and androgenic mechanisms to this influence, prairie voles were exposed to testosterone propionate (TP), the anti-androgen flutamide, or the aromatase inhibitor 1,4,6-androstatriene-3,17-doine (ATD) either prenatally via their pregnant dam for the last 15-19 days of the 22-day gestational period or postnatally on days 1-7. None of the treatments altered the high paternal responsiveness of males or the high infanticide rate in females when compared with controls. Females exposed prenatally to ATD, however, had levels of parental behavior that were significantly higher than the lowest levels observed in prenatally TP-treated females. These results suggest that sex differences in the parental behavior of adult virgin prairie voles are not generated exclusively by androgenic or estrogenic mechanisms during a restricted prenatal or early postnatal 'sensitive period' and that the parental behavior of virgin females may be more susceptible to any influence of gonadal hormones during development than males.

Developmental changes and among-sibling variation of corticosterone levels in an altricial avian species

The postnatal development of the activity of the brain-pituitary-adrenal axis was investigated in an altricial bird species by measurements of plasma corticosterone levels in nestling and fledgling canaries, Serinus canaria. Corticosterone was detectable (>2.6 ng/ml) in 30% of 5-day-old, 67% of 10-day-old, 72% of 15-day-old, and 88% of 23-day-old birds. When detectable, the corticosterone levels of 5-day-old nestlings were comparable to the baseline levels of adult birds. Levels were higher in 10- and 15- than in 5-day-old nestlings. The levels of 23-day-old fledglings (about 6 to 7 days after fledgling) were significantly higher than those of 15-day-old nestlings. They were intermediate between adult baseline and stress-induced levels. Sex did not influence this general profile, but levels varied with the order of hatching within broods. At the age of 15 and 23 days first hatched chicks had higher corticosterone levels than last hatched chicks, while second hatched chicks had intermediate levels. These differences were not correlated with body mass. The results suggest that (1) the brain-pituitary-adrenal axis of this altricial bird becomes fully functional after hatching and (2) birth order within broods influences corticosterone secretion during subsequent stages of development. It is unlikely that the brain-pituitary-adrenal axis matures at different rates in first and later hatched chicks or that the different levels of first and later hatched chicks were caused by capture and handling stress. Rather, they may result from such maternal effects as hatching asynchrony or differential concentrations of yolk steroids among the eggs in a clutch. Further studies will have to show whether this systematic variation of corticosterone levels among siblings during early life persists into adulthood and how it is related to behavior and fitness.

Developmental exposure to vasopressin increases aggression in adult prairie voles

Although the biological roots of aggression have been the source of intense debate, the precise physiological mechanisms responsible for aggression remain poorly understood. In most species, aggression is more common in males than females; thus, gonadal hormones have been a focal point for research in this field. Although gonadal hormones have been shown to influence the expression of aggression, in many cases aggression can continue after castration, indicating that testicular steroids are not completely essential for the expression of aggression. Recently, the mammalian neuropeptide arginine vasopressin (AVP) has been implicated in aggression. AVP plays a particularly important role in social behavior in monogamous mammals, such as prairie voles (Microtus ochrogaster). In turn, the effects of social experiences may be mediated by neuropeptides, including AVP. For example, sexually naïve prairie voles are rarely aggressive. However, 24 h after the onset of mating, males of this species become significantly aggressive toward strangers. Likewise, in adult male prairie voles, central (intracerebroventricular) injections of AVP can significantly increase intermale aggression, suggesting a role for AVP in the expression of postcopulatory aggression in adult male prairie voles. In this paper, we demonstrate that early postnatal exposure to AVP can have long-lasting effects on the tendency to show aggression, producing levels of aggression in sexually naïve, adult male prairie voles that are comparable to those levels observed after mating. Females showed less aggression and were less responsive to exogenous AVP, but the capacity of an AVP V(1a) receptor antagonist to block female aggression also implicates AVP in the development of female aggression.

Castration does not inhibit aggressive behavior in adult male prairie voles (Microtus ochrogaster)

The relationship between castration and reduced male aggression is well established. However, anecdotal observations of male prairie voles (Microtus ochrogaster) suggest that castration does not reduce aggressive behavior. To investigate the role of testicular androgens on aggressive behavior, castrated or gonadally intact male prairie voles were paired in a neutral arena with a gonadally intact vole. Castration did not reduce the frequency of intermale aggression. In Experiment 2, aggressive behavior was examined further using resident-intruder, grouped aggression, and aggression against a lactating female models. Again, castration did not affect the frequency of aggression in male prairie voles. Taken together, the results of this study suggest that aggressive behavior may be independent of gonadal steroid hormones in adult male prairie voles.

Neonatal androgen affects copulatory behavior in the female musk shrew

The role of neonatal testosterone in the development of copulatory behavior was examined in an insectivore, the musk shrew (Suncus murinus). Female musk shrews were treated with testosterone propionate (TP) for the first 5 days of life and then tested in adulthood for either female or male-like copulatory behavior. Early TP had a masculinizing effect; neonatally treated animals mounted a stimulus female more frequently, and with shorter latencies, in response to adult testosterone treatment than did control females. Neonatally androgenized females also showed deficits in female sexual behavior; few received ejaculations from stud males. This difference was likely caused by increased aggression exhibited by the neonatally TP-treated females toward males. In turn, female aggression decreased efficiency of male partners' intromission attempts. Early TP treatments also caused structural abnormalities in the ovaries, but did not effect their capacity to ovulate in response to either gonadotropin-releasing hormone or human chorionic gonadotropin injection. In sum, exposure to TP during development augmented display of male-like behavior in females and had subtle deleterious effects on expression of feminine behavior.

Neuroendocrine perspectives on social attachment and love

The purpose of this paper is to review existing behavioral and neuroendocrine perspectives on social attachment and love. Both love and social attachments function to facilitate reproduction, provide a sense of safety, and reduce anxiety or stress. Because social attachment is an essential component of love, understanding attachment formation is an important step toward identifying the neurobiological substrates of love. Studies of pair bonding in monogamous rodents, such as prairie voles, and maternal attachment in precocial ungulates offer the most accessible animal models for the study of mechanisms underlying selective social attachments and the propensity to develop social bonds. Parental behavior and sexual behavior, even in the absence of selective social behaviors, are associated with the concept of love; the analysis of reproductive behaviors, which is far more extensive than our understanding of social attachment, also suggests neuroendocrine substrates for love. A review of these literatures reveals a recurrent association between high levels of activity in the hypothalamic pituitary adrenal (HPA) axis and the subsequent expression of social behaviors and attachments. Positive social behaviors, including social bonds, may reduce HPA axis activity, while in some cases negative social interactions can have the opposite effect. Central neuropeptides, and especially oxytocin and vasopressin have been implicated both in social bonding and in the central control of the HPA axis. In prairie voles, which show clear evidence of pair bonds, oxytocin is capable of increasing positive social behaviors and both oxytocin and social interactions reduce activity in the HPA axis. Social interactions and attachment involve endocrine systems capable of decreasing HPA reactivity and modulating the autonomic nervous system, perhaps accounting for health benefits that are attributed to loving relationships.