Variability in "spontaneous" maternal behavior is associated with anxiety-like behavior and affiliation in naïve juvenile and adult female prairie voles (Microtus ochrogaster)

Maladaptive cardiac and behavioral reactivity to repeated vicarious stress exposure in socially bonded male prairie vole siblings

Behaviors, emotions, and cardiovascular functions are influenced by stress. But these detrimental effects are not exclusive to an individual that directly experiences stress. Stress is also experienced vicariously through observation of another individual undergoing stress. The current study used the strong social bonds in socially monogamous prairie voles to determine effects of repeated vicarious stress on cardiac and behavioral outcomes. Male prairie voles were exposed to either a 5-minute open field chamber alone [separate (control)] or while concurrently witnessing their sibling undergo a tail-suspension stressor [concurrent (experimental)], repeated across 4 sessions. Cardiac responses in animals in the open field were evaluated for heart rate and heart rate variability prior to, during, and after each test session, and behaviors were evaluated for motion, exploration, stress reactivity, and anxiety-relevant behaviors during each test session. The concurrent condition (versus separate) displayed increased heart rate and reduced heart rate variability during repeated test sessions, and impaired recovery of these parameters following the test sessions. The pattern of disturbances suggests that both increased sympathetic and reduced parasympathetic influence contributed to the cardiac responses. Animals in the concurrent condition (versus separate) displayed disrupted rearing, grooming, and motion; reduced duration of center section exploration; and increased freezing responses across repeated test sessions. Collectively, cardiac and behavioral stress reactivity are increased as a function of vicarious stress in prairie voles, which are evident across repeated experiences of stress. These results inform our understanding of the experience of vicarious stress in social species, including humans.

Effects of oxytocin ablation on pup rescue, nursing behaviors and response to pup separation in early-to-mid postpartum mice

Oxytocin, a neuropeptide hormone, is indispensable for milk ejection during nursing and is important for uterine contractions during parturition. The exact functions of oxytocin in postpartum maternal behaviors and motivations require further investigation. To this end, we characterized the role of oxytocin in components of maternal motivations during the mid-postpartum period, which has not been previously studied. To maintain suckling stimuli, postpartum oxytocin knockout (Oxt^-/- ) and heterozygous (Oxt^+/- ) littermates were co-housed with a wild-type lactating mother and its litter, and were examined for their ability to retrieve pups under standard or high-risk conditions, nursing behavior, maternal aggression towards an unfamiliar intruder, and motivation to regain contact with separated pups. One-third of Oxt^-/- mothers exhibited prolonged parturition but were otherwise grossly healthy. Despite their inability to eject milk, Oxt^-/- mothers displayed nursing behaviors for similar durations to Oxt^+/- mothers during the second postpartum week. In addition, Oxt^-/- mothers were essentially intact for pup retrieval under standard conditions and were motivated to stay close to pups, although they showed a mild decrease in maternal care under high-risk conditions and increased anxiety-like behaviors in pup-related contexts. The present findings indicate that oxytocin is dispensable for nursing behavior and maternal motivations, yet suggest that oxytocin may be relevant for stress resilience in the postpartum period.

Behavioral trajectories of aging prairie voles (Microtus ochrogaster): Adapting behavior to social context wanes with advanced age

Several studies using mice have examined the effects of aging on cognitive tasks, as well as sensory and motor functions. However, few studies have examined the influence of aging on social behavior. Prairie voles (Microtus ochrogaster) are a socially monogamous and biparental rodent that live in small family groups and are now among the most popular rodent models for studies examining social behavior. Although the social behavioral trajectories of early-life development in prairie voles have been well-studied, how social behavior may change throughout adulthood remains unknown. Here we examined behavior in virgin male and female prairie voles in four different age groups: postnatal day (PND) 60–80, 140–160, 220–240, and 300–320. All animals underwent testing in a novel object task, a dominance test, a resident-intruder test, and several iterations of social approach and social interaction tests with varying types of social stimuli (i.e., novel same-sex conspecific, novel opposite-sex conspecific, familiar same-sex sibling/cagemate, small group of novel same-sex conspecifics). We found that age influenced neophobia and dominance, but not social approach behavior. Further, we found that young adult, but not older adult, prairie voles adapt prosocial and aggressive behavior relative to social context, and that selective aggression occurs in relation to age even in the absence of a pair bond. Our results suggest that prairie voles calibrate social phenotype in a context-dependent manner in young adulthood and stop adjusting behavior to social context in advanced age, demonstrating that social behavior is plastic not only throughout early development, but also well into adulthood. Together, this study provides insight into age-related changes in social behavior in prairie voles and shows that prairie voles may be a viable model for studying the cognitive and physiological benefits of social relationships and social engagement in advanced age.

Individual and Combined Effects of Paternal Deprivation and Developmental Exposure to Firemaster 550 on Socio-Emotional Behavior in Prairie Voles

The prevalence of neurodevelopmental disorders (NDDs) is rapidly rising, suggesting a confluence of environmental factors that are likely contributing, including developmental exposure to environmental contaminants. Unfortunately, chemical exposures and social stressors frequently occur simultaneously in many communities, yet very few studies have sought to establish the combined effects on neurodevelopment or behavior. Social deficits are common to many NDDs, and we and others have shown that exposure to the chemical flame retardant mixture, Firemaster 550 (FM 550), or paternal deprivation impairs social behavior and neural function. Here, we used a spontaneously prosocial animal model, the prairie vole (Microtus ochrogaster), to explore the effects of perinatal chemical (FM 550) exposure alone or in combination with an early life stressor (paternal absence) on prosocial behavior. Dams were exposed to vehicle (sesame oil) or 1000 µg FM 550 orally via food treats from conception through weaning and the paternal absence groups were generated by removing the sires the day after birth. Adult offspring of both sexes were then subjected to open-field, sociability, and a partner preference test. Paternal deprivation (PD)-related effects included increased anxiety, decreased sociability, and impaired pair-bonding in both sexes. FM 550 effects include heightened anxiety and partner preference in females but reduced partner preference in males. The combination of FM 550 exposure and PD did not exacerbate any behaviors in either sex except for distance traveled by females in the partner preference test and, to a lesser extent, time spent with, and the number of visits to the non-social stimulus by males in the sociability test. FM 550 ameliorated the impacts of parental deprivation on partner preference behaviors in both sexes. This study is significant because it provides evidence that chemical and social stressors can have unique behavioral effects that differ by sex but may not produce worse outcomes in combination.

Physiological and behavioral responses to observing a sibling experience a direct stressor in prairie voles

Uncontrollable stress precipitates negative mental and physical health outcomes. Furthermore, the vicarious experience of stress (e.g. observing another individual experience a direct stressor) can mimic the effects of directly experiencing the stressor. The current experiment examined the behavioral and physiological effects of the vicarious experience of stress using the socially monogamous prairie vole. Male prairie voles were exposed to either an empty open field chamber, or a chamber in which the animal observed a sibling undergoing a concurrent direct physical stressor (tail suspension test) for five minutes. Exploratory and anxiety-like behaviors were recorded in all observers during the test session. Cardiac indices of heart rate and heart rate variability were recorded in a subset of observers prior to, during, and following the test session. Corticosterone levels were measured in all observers and siblings following the test session. When compared to animals exposed to an empty open field chamber, animals that observed a sibling undergo a direct physical stressor exhibited increased heart rate and circulating corticosterone, and decreased heart rate variability. These physiological stress indicators were supported by behavioral changes, including increased freezing followed immediately by orienting of the head toward the center of the apparatus, and decreased locomotion, grooming, and rearing. These preliminary results suggest that prairie voles experience stress vicariously, and provide a foundation for additional studies focused on the underlying mechanisms of vicarious stress. The use of this model may inform our understanding of the social transmission of stress among social species, including humans.LAY SUMMARYThe experience of stress, including observing stress in a loved one, has negative consequences on mental and physical health. This study used a social rodent (prairie voles) to demonstrate that stress transfers among social individuals, consequently producing an increased physiological and behavioral stress response in prairie voles observing their siblings experience stress. This research informs our understanding of the interactions of social experiences and stress in humans.

Sex-specific effects of perinatal FireMaster® 550 (FM 550) exposure on socioemotional behavior in prairie voles

The rapidly rising incidence of neurodevelopmental disorders with social deficits is raising concern that developmental exposure to environmental contaminants may be contributory. Firemaster 550 (FM 550) is one of the most prevalent flame-retardant (FR) mixtures used in foam-based furniture and baby products and contains both brominated and organophosphate components. We and others have published evidence of developmental neurotoxicity and sex specific effects of FM 550 on anxiety-like and exploratory behaviors. Using a prosocial animal model, we investigated the impact of perinatal FM 550 exposure on a range of socioemotional behaviors including anxiety, attachment, and memory. Virtually unknown to toxicologists, but widely used in the behavioral neurosciences, the prairie vole (Microtus ochrogaster) is a uniquely valuable model organism for examining environmental factors on sociality because this species is spontaneously prosocial, biparental, and displays attachment behaviors including pair bonding. Dams were exposed to 0, 500, 1000, or 2000 μg of FM 550 via subcutaneous (sc) injections throughout gestation, and pups were directly exposed beginning the day after birth until weaning. Adult offspring of both sexes were then subjected to multiple tasks including open field, novel object recognition, and partner preference. Effects were dose responsive and sex-specific, with females more greatly affected. Exposure-related outcomes in females included elevated anxiety, decreased social interaction, decreased exploratory motivation, and aversion to novelty. Exposed males also had social deficits, with males in all three dose groups failing to show a partner preference. Our studies demonstrate the utility of the prairie vole for investigating the impact of chemical exposures on social behavior and support the hypothesis that developmental FR exposure impacts the social brain. Future studies will probe the possible mechanisms by which these effects arise.

The ventromedial hypothalamic circuitry and male alloparental behaviour in a socially monogamous rodent species

As prairie voles (Microtus ochrogaster) display spontaneous biparental care, and the ventromedial hypothalamus (VMH) has been implicated in reproductive behaviour, we conducted experiments to test the hypothesis that the VMH neurochemical circuitry is involved in alloparental behaviours in male prairie voles. We compared alloparental behaviours of adult, sexually naïve male and female voles-both displayed licking/grooming, huddling and retrieving behaviours towards conspecific pups. We also stained for the immediate-early gene encoded early growth protein Egr-1 in the vole brain. The pup-exposed animals showed levels of Egr-1 staining that was higher in the VMH but lower in the amygdala compared to animals exposed to a pup-sized piece of plastic (control). A retrograde tracer, Fluoro-Gold (FG), was injected into the VMH of male voles that were subsequently tested in the pup exposure or control condition. More FG/Egr-1 cells were detected for glutamatergic (GLU) staining in the ventral bed nucleus of the stria terminalis (BNSTv) and medial amygdala (MeA), whereas less FG/Egr-1 cells were stained for gamma-aminobutyric acid (GABA) in the MeA of the pup-exposed group compared to the control group. Further, the ratio of GLU:GABA expression in FG/Egr-1 projection neurons from both the BNSTv and MeA to the VMH was increased following pup exposure. Finally, pharmacological blockade of either dopamine D1 receptor or oxytocin receptor in the VMH impaired the onset of male alloparental behaviour. Together, these data suggest that the VMH may be involved in the onset of alloparental care and play a role in regulating social approach in male prairie voles.

Oxytocin receptor knockout prairie voles generated by CRISPR/Cas9 editing show reduced preference for social novelty and exaggerated repetitive behaviors

Behavioral neuroendocrinology has benefited tremendously from the use of a wide range of model organisms that are ideally suited for particular questions. However, in recent years the ability to manipulate the genomes of laboratory strains of mice has led to rapid advances in our understanding of the role of specific genes, circuits and neural populations in regulating behavior. While genome manipulation in mice has been a boon for behavioral neuroscience, the intensive focus on the mouse restricts the diversity in behavioral questions that can be investigated using state-of-the-art techniques. The CRISPR/Cas9 system has great potential for efficiently generating mutants in non-traditional animal models and consequently to reinvigorate comparative behavioral neuroendocrinology. Here we describe the efficient generation of oxytocin receptor (Oxtr) mutant prairie voles (Microtus ochrogaster) using the CRISPR/Cas9 system, and describe initial behavioral phenotyping focusing on behaviors relevant to autism. Oxtr mutant male voles show no disruption in pup ultrasonic vocalization, anxiety as measured by the open field test, alloparental behavior, or sociability in the three chamber test. Mutants did however show a modest elevation in repetitive behavior in the marble burying test, and an impairment in preference for social novelty. The ability to efficiently generate targeted mutations in the prairie vole genome will greatly expand the utility of this model organism for discovering the genetic and circuit mechanisms underlying complex social behaviors, and serves as a proof of principle for expanding this approach to other non-traditional model organisms.

Exploration in a dispersal task: Effects of early experience and correlation with other behaviors in prairie voles (Microtus ochrogaster)

Socially monogamous prairie voles (Microtus ochrogaster) display remarkable individual variation in social behaviors, which has been associated with differences in early life experience and neuropeptide receptor densities. These differences are also seen in the wild, where approximately 70% of young voles remain in their natal group as non-breeding alloparents, while the other 30% disperse. We investigated whether natural variation in early parental care could contribute to offspring's willingness to "disperse" (willingness to explore) in a laboratory context. Behavioral differences between dispersers and residents could also provide a way to interpret individual variation in other behaviors commonly observed under laboratory conditions. Breeder pairs ranked as high, medium or low-contact, according to the amount of early parental care they provided to offspring, were used to produce and rear experimental subjects. Effects of early parental care on the offspring's willingness to disperse were seen at post-natal day 21, with high-contact offspring spending more time in the start cage and low-contact offspring spending more time exploring. Variations in parental care were also associated with differences in juvenile and adult behaviors that could potentially encourage philopatry or dispersal behavior in the wild. High-contact offspring displayed less anxiety-like behavior compared to low-contact animals. Low-contact offspring displayed the lowest amount of alloparental care. High-contact offspring spent more time in side-by-side contact with a potential partner compared to medium and low-contact offspring. These results suggest that variations in early parental care can impact weanlings' exploratory behavior, but that philopatry is not driven by high anxiety.

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

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

Comparative analysis of oxytocin receptor density in the nucleus accumbens: an adaptation for female and male alloparental care?

Parental behavior is commonly displayed by progenitors. However, other individuals, genetically related (e.g. siblings, aunts, uncles) or not with the newborns, also display parental behavior (commonly called alloparental, or adoptive behavior). I hypothesize that species that live in family or social groups where other non-reproductive members (males and females) take care of infants, have brain adaptations to promote or facilitate that behavioral response. The present work revises the evidence supporting the hypothesis that high density of oxytocin receptors (OXTR) in the nucleus accumbens (NA) is one of those adaptations. All species known to have high NA OXTR show not only female, but also male alloparental care. Therefore, I predict that high NA OXTR could be present in all species in which juvenile and adult male alloparental behavior have been observed. Strategies to test this and other alternative working hypothesis and its predictions are presented.

Intergenerational transmission of alloparental behavior and oxytocin and vasopressin receptor distribution in the prairie vole

Variation in the early environment has the potential to permanently alter offspring behavior and development. We have previously shown that naturally occurring variation in biparental care of offspring in the prairie vole is related to differences in social behavior of the offspring. It was not, however, clear whether the behavioral differences seen between offspring receiving high compared to low amounts of parental care were the result of different care experiences or were due to shared genetics with their high-contact or low-contact parents. Here we use cross-fostering methods to determine the mode of transmission of alloparental behavior and oxytocin receptor (OTR) and vasopressin V1a receptor (V1aR) binding from parent to offspring. Offspring were cross-fostered or in-fostered on postnatal day 1 and parental care received was quantified in the first week postpartum. At weaning, offspring underwent an alloparental care test and brains were then collected from all parents and offspring to examine OTR and V1aR binding. Results indicate that alloparental behavior of offspring was predicted by the parental behavior of their rearing parents. Receptor binding for both OTR and V1aR tended to be predicted by the genetic mothers for female offspring and by the genetic fathers for male offspring. These findings suggest a different, sex-dependent, role of early experience and genetics in shaping behavior compared to receptor distribution and support the notion of sex-dependent outcomes.

RNAi knockdown of oxytocin receptor in the nucleus accumbens inhibits social attachment and parental care in monogamous female prairie voles

Oxytocin modulates many aspects of social cognition and behaviors, including maternal nurturing, social recognition and bonding. Natural variation in oxytocin receptor (OXTR) density in the nucleus accumbens (NAcc) is associated with variation in alloparental behavior, and artificially enhancing OXTR expression in the NAcc enhances alloparental behavior and pair bonding in socially monogamous prairie voles. Furthermore, infusion of an OXTR antagonist into the NAcc inhibits alloparental behavior and partner preference formation. However, antagonists can promiscuously interact with other neuropeptide receptors. To directly examine the role of OXTR signaling in social bonding, we used RNA interference to selectively knockdown, but not eliminate, OXTR in the NAcc of female prairie voles and examined the impact on social behaviors. Using an adeno-associated viral vector expressing a short hairpin RNA (shRNA) targeting Oxtr mRNA, we reduced accumbal OXTR density in female prairie voles from juvenile age through adulthood. Females receiving the shRNA vector displayed a significant reduction in alloparental behavior and disrupted partner preference formation. These are the first direct demonstrations that OXTR plays a critical role in alloparental behavior and adult social attachment, and suggest that natural variation in OXTR expression in this region alone can create variation in social behavior.

The effects of environmental enrichment on depressive and anxiety-relevant behaviors in socially isolated prairie voles

OBJECTIVES

Social isolation is associated with depression, anxiety, and negative health outcomes. Environmental enrichment, including environmental and cognitive stimulation with inanimate objects and opportunities for physical exercise, may be an effective strategy to include in treatment paradigms for affective disorders as a function of social isolation. In a rodent model-the socially monogamous prairie vole-we investigated the hypothesis that depression- and anxiety-related behaviors after social isolation would be prevented and remediated with environmental enrichment.

METHODS

Experiment 1 investigated the preventive effects of environmental enrichment on negative affective behaviors when administered concurrently with social isolation. Experiment 2 investigated the remediating effects of enrichment on negative affective behaviors when administered after a period of isolation. Behaviors were measured in three operational tests: open field, forced swim test (FST), and elevated plus maze.

RESULTS

In isolated prairie voles, enrichment prevented depression-relevant (immobility in FST, group × housing interaction, p = .049) and anxiety-relevant behaviors (exploration in open field, group × housing interaction, p = .036; exploration in elevated plus maze, group × housing interaction, p = .049). Delayed enrichment also remediated these behaviors in isolated animals (immobility in FST, main effect of housing, p = .001; exploration in open field, main effect of housing, p = .047; exploration in elevated plus maze, main effect of housing, p = .001) and was slightly more effective than physical exercise alone in remediating anxiety-relevant behaviors.

CONCLUSIONS

These findings provide insight into the beneficial effects of an enriched environment on depression- and anxiety-relevant behaviors using a translational rodent model of social isolation.

Chronic intranasal oxytocin causes long-term impairments in partner preference formation in male prairie voles

BACKGROUND

Oxytocin (OT) is a hormone shown to be involved in social bonding in animal models. Intranasal OT is currently in clinical trials for use in disorders such as autism and schizophrenia. We examined long-term effects of intranasal OT given developmentally in the prairie vole (Microtus ochrogaster), a socially monogamous rodent, often used as an animal model to screen drugs that have therapeutic potential for social disorders.

METHODS

We treated voles with one of three dosages of intranasal OT, or saline, from day 21 (weaning) through day 42 (sexual maturity). We examined both social behavior immediately following administration, as well as long-term changes in social and anxiety behavior after treatment ceased. Group sizes varied from 8 to 15 voles (n = 89 voles total).

RESULTS

Treatment with OT resulted in acute increases in social behavior in male voles with familiar partners, as seen in humans. However, long-term developmental treatment with low doses of intranasal OT resulted in a deficit in partner preference behavior (a reduction of contact with a familiar opposite-sex partner, used to index pair-bond formation) by male voles.

CONCLUSIONS

Long-term developmental treatment with OT may show results different to those predicted by short-term studies, as well as significant sex differences and dosage effects. Further animal study is crucial to determining safe and effective strategies for use of chronic intranasal OT, especially during development.

New theoretical and experimental approaches on maternal motivation in mammals

Maternal behavior is expressed in different modalities, physiological conditions, and contexts. It is the result of a highly motivated brain, that allows the female to flexibily adapt her caring activities to different situations and social demands. To understand how mothers coordinate maternal and other motivated behaviors we discuss the limitations of current theoretical approaches to study maternal motivation (e.g. distinction between appetitive and consummatory behaviors), and propose a different approach (i.e. motorically active vs. passive motivations) and a distinction between maternal motivated state and maternal motivated behaviors. We review the evidence supporting dopamine mediation of maternal motivation and describe how different phases of the dopaminergic response - basal, tonic, and phasic release in the nucleus accumbens - relate to increased salience, invigorating behavior, and behavioral switching. The existing and new experimental paradigms to investigate maternal motivation, and its coexpression and coordination with other social or non-social motivations are also analyzed. An example of how specificity of motivational systems (e.g. maternal and sexual behavior at postpartum estrus) could be processed at the neural level is also provided. This revision offers new theoretical and experimental approaches to address the fundamental question of how mothers flexibly adapt and coordinate the different components of maternal behavior with other motivated behaviors, also critical for the survival of the species.

Flexibility and adaptation of the neural substrate that supports maternal behavior in mammals

Maternal behavior is species-specific and expressed under different physiological conditions, and contexts. It is the result of neural processes that support different forms (e.g. postpartum, cycling sensitized and spontaneous maternal behavior) and modalities of mother-offspring interaction (e.g. maternal interaction with altricial/precocious young; selective/non-selective bond). To understand how the brain adapts to and regulates maternal behavior in different species, and physiological and social conditions we propose new neural models to explain different forms of maternal expression (e.g. sensitized and spontaneous maternal behavior) and the behavioral changes that occur across the postpartum period. We emphasize the changing role of the medial preoptic area in the neural circuitry that supports maternal behavior and the cortical regulation and adjustment of ongoing behavioral performance. Finally, we discuss how our accumulated knowledge about the psychobiology of mothering in animal models supports the validity of animal studies to guide our understanding of human mothering and to improve human welfare and health.

Natural variation in early parental care correlates with social behaviors in adolescent prairie voles (Microtus ochrogaster)

Natural variation in early parental care may contribute to long-term changes in behavior in the offspring. Here we investigate the role of variable early care in biparental prairie voles (Microtus ochrogaster). Total amounts of parental care were initially quantified for 24 breeder pairs and pairs were ranked in relation to one another based on total contact. Consistency in key components of care suggested a trait-like quality to parental care. Based on this ranking, breeder pairs from the top (high-contact) and bottom (low-contact) quartiles were selected to produce high- and low-contact offspring to investigate adolescent behavior after varying early care. Parental care of subject offspring was again observed postnatally. Offspring of high-contact parents spent more time passively nursing and received more paternal non-huddling contact while low-contact offspring spent more time actively nursing and received more paternal huddling and pseudohuddling in the first postnatal days (PNDs). Low-contact offspring also displayed faster rates of development on a number of physical markers. Post-weaning, offspring were evaluated on anxiety-like behavior, social behavior and pre-pulse inhibition (PPI) to a tactile and an acoustic startle. High-contact offspring spent more time sniffing a juvenile and less time autogrooming. With an infant, high-contact offspring spent more time in non-huddling contact and less time autogrooming and retrieving than did low-contact offspring. Considering sexes separately, high-contact females spent more time sniffing a novel juvenile than low-contact females. High-contact males spent more time in non-huddling contact with an infant than low-contact males; while low-contact females retrieved infants more than high-contact females. In both measures of social behavior, high-contact males spent less time autogrooming than low-contact males. These results suggest a relationship between early-life care and differences in social behavior in adolescence.

Variation in vasopressin receptor (Avpr1a) expression creates diversity in behaviors related to monogamy in prairie voles

Polymorphisms in noncoding regions of the vasopressin 1a receptor gene (Avpr1a) are associated with a variety of socioemotional characteristics in humans, chimpanzees, and voles, and may impact behavior through a site-specific variation in gene expression. The socially monogamous prairie vole offers a unique opportunity to study such neurobiological control of individual differences in complex behavior. Vasopressin 1a receptor (V1aR) signaling is necessary for the formation of the pair bond in males, and prairie voles exhibit greater V1aR binding in the reward-processing ventral pallidum than do asocial voles of the same genus. Diversity in social behavior within prairie voles has been correlated to natural variation in neuropeptide receptor expression in specific brain regions. Here we use RNA interference to examine the causal relationship between intraspecific variation in V1aR and behavioral outcomes, by approximating the degree of naturalistic variation in V1aR expression. Juvenile male prairie voles were injected with viral vectors expressing shRNA sequences targeting Avpr1a mRNA into the ventral pallidum. Down-regulation of pallidal V1aR density resulted in a significant impairment in the preference for a mated female partner and a reduction in anxiety-like behavior in adulthood. No effect on alloparenting was detected. These data demonstrate that within-species naturalistic-like variation in V1aR expression has a profound effect on individual differences in social attachment and emotionality. RNA interference may prove to be a useful technique to unite the fields of behavioral ecology and neurogenetics to perform ethologically relevant studies of the control of individual variation and offer insight into the evolutionary mechanisms leading to behavioral diversity.

Neuroendocrine and behavioural responses to exposure to an infant in male prairie voles

Paternal behaviour and pair-bond formation are defining characteristics of social monogamy. However, in comparison to pair-bonding, the endocrine factors associated with the male care of young are not well studied. In the present study, plasma concentrations of oxytocin, vasopressin and corticosterone (CORT) were measured in reproductively naïve male prairie voles as a function of exposure to an infant or control manipulations (i.e. handling or exposure to a wooden dowel). Plasma oxytocin concentrations were transiently elevated within 10 min of pup exposure. Although plasma CORT concentration typically increases after handling, after 10 min of pup exposure, the concentration of plasma CORT was not increased, suggesting an attenuation of CORT release by pup exposure. Group differences in the concentrations of plasma hormones were no longer detected at 20 or 60 min after treatment. These patterns of rapid change in the concentrations of plasma oxytocin and CORT were observed in both juvenile and adult males but not detected after control procedures. Plasma vasopressin, assessed only in adult males, did not vary as a function of pup exposure or other manipulations. In the paraventricular nucleus of the hypothalamus, pup exposure also increased activation (as assessed by the measurement of c-Fos) of neurones that stained for either oxytocin or vasopressin, whereas it decreased c-Fos expression in neurones stained for corticotrophin-releasing hormone. In addition, brief pup exposure (20 min) facilitated subsequent partner preference formation when alloparental males and pup attackers were considered as a group. In the context of other studies, these data support the hypothesis that neuroendocrine changes associated with male alloparental behaviour are related to those implicated in pair-bonding.

Maternal care in the African striped mouse Rhabdomys pumilio: a behaviorally flexible phenotype that is modified by experience

The development of maternal care in mammals can be influenced by the type and quality of maternal care received. Using biparental striped mice Rhabdomys pumilio, we investigated whether development of maternal care is influenced by the mother during early rearing and by an adult female's experience and that of her mate. Offspring were raised in one of three treatments, by: both parents; mothers alone; and mothers separated from the father with a barrier. Since female striped mice increase their care when raising litters alone, which influences expression of parental care of their adult sons, we expected daughters to respond like sons. However, there was no treatment effect in the development of maternal care in daughters. In subsequent experiments, experienced and inexperienced females decreased care when raising their offspring with experienced but not inexperienced males. Therefore, maternal care in striped mice is modulated in response to prevailing environmental and social conditions.

Increasing oxytocin receptor expression in the nucleus accumbens of pre-pubertal female prairie voles enhances alloparental responsiveness and partner preference formation as adults

Oxytocin receptors (OXTR) in the nucleus accumbens (NAcc) promote alloparental behavior and partner preference formation in female prairie voles. Within the NAcc there is significant individual variation in OXTR binding and virgin juvenile and adult females with a high density of OXTR in the NAcc display an elevated propensity to engage in alloparental behavior toward novel pups. Over-expression of OXTR in the NAcc of adult female prairie voles using viral vector gene transfer facilitates partner preference formation, but has no effect on alloparental behavior, even though OXTR antagonists infused into the NAcc blocks both behaviors. We therefore hypothesized that long-term increases in OXTR signaling during development may underlie the relationship between adult OXTR density in the NAcc and alloparental behavior. To test this hypothesis, we used viral vector gene transfer to increase OXTR density in the NAcc of prepubertal, 21day old female prairie voles and tested for both alloparental behavior and partner preference formation as adults. Consistent with a developmental impact of OXTR signaling, adults over-expressing OXTR from weaning display both increased alloparental behavior and partner preference formation. Thus, the relatively acute impact of elevated OXTR signaling in the NAcc on partner preference formation previously reported appears to be dissociable from the effects of longer term, developmentally relevant OXTR signaling necessary for modulating alloparental behavior. These results are consistent with the hypothesis that oxytocin can have both long-term "organizational" effects as well as acute "activational" effects on affiliative behaviors.

Neuroanatomical projections of the species-specific tyrosine hydroxylase-immunoreactive cells of the male prairie vole bed nucleus of the stria terminalis and medial amygdala

The principal nucleus of the bed nucleus of the stria terminalis (BSTpr) and posterodorsal part of the medial amygdalar nucleus (MEApd) are densely interconnected sites transmitting olfactory information to brain areas mediating sociosexual behaviors. In male prairie voles (Microtus ochrogaster), the BSTpr and MEApd contain hundreds of cells densely immunoreactive for tyrosine hydroxylase (TH). Such tremendous numbers of TH-immunoreactive (TH-ir) cells do not exist in other rodents examined, and studies from our laboratory suggest these cells may be part of a unique chemical network necessary for monogamous behaviors in prairie voles. To obtain information about how these TH-ir cells communicate with other sites involved in social behaviors, we first used biotinylated dextran amine (BDA) to determine sites that receive BSTpr efferents and also contain TH-ir fibers. Only in the medial preoptic area (MPO) and MEApd did we find considerable comingling of BDA-containing and TH-ir fibers. To examine if these sites receive input specifically from BSTpr TH-ir cells, the retrograde tracer Fluorogold was infused into the MPO or MEApd. Almost 80% of TH-ir projections to the MPO originated from the BSTpr or MEApd, involving about 40% of all TH-ir cells in these sites. In contrast, the MEApd received almost no input from TH-ir cells in the BSTpr, and received it primarily from the ventral tegmental area. Retrograde tracing from the BSTpr itself revealed substantial input from MEApd TH-ir cells. Thus, the male prairie vole brain contains a species-specific TH-ir network involving the BSTpr, MEApd, and MPO. By connecting brain sites involved in olfaction, sociality and motivation, this network may be essential for monogamous behaviors in this species.

Normal maternal behavior, but increased pup mortality, in conditional oxytocin receptor knockout females

Oxytocin (Oxt) and the Oxt receptor (Oxtr) are implicated in the onset of maternal behavior in a variety of species. Recently, we developed two Oxtr knockout lines: a total body knockout (Oxtr-/-) and a conditional Oxtr knockout (OxtrFB/FB) in which the Oxtr is lacking only in regions of the forebrain, allowing knockout females to potentially nurse and care for their biological offspring. In the current study, we assessed maternal behavior of postpartum OxtrFB/FB females toward their own pups and maternal behavior of virgin Oxtr-/- females toward foster pups and compared knockouts of both lines to wildtype (Oxtr+/+) littermates. We found that both Oxtr-/- and OxtrFB/FB females appear to have largely normal maternal behaviors. However, with first litters, approximately 40% of the OxtrFB/FB knockout dams experienced high pup mortality, compared to fewer than 10% of the Oxtr+/+ dams. We then went on to test whether or not this phenotype occurred in subsequent litters or when the dams were exposed to an environmental disturbance. We found that regardless of the degree of external disturbance, OxtrFB/FB females lost more pups on their first and second litters compared to wildtype females. Possible reasons for higher pup mortality in OxtrFB/FB females are discussed.

Autism and oxytocin: new developments in translational approaches to therapeutics

Autism is a neurodevelopmental disorder characterized by dysfunction in three core symptom domains: speech and communication deficits, repetitive or compulsive behaviors with restricted interests, and social impairment. The neuropeptide oxytocin, along with the structurally similar peptide arginine vasopressin, may play a role in the etiology of autism, and especially in the social impairment domain. Oxytocin is a nonapeptide (i.e., it has nine amino acids). It is synthesized in magnocellular neurons in the paraventricular nucleus and the supraoptic nucleus of the hypothalamus and is released into the bloodstream by way of axon terminals in the posterior pituitary. Oxytocin is released both peripherally, where it is involved in milk letdown and the facilitation of uterine contractions, and centrally, where it acts as a neuromodulator along with arginine vasopressin. Here, we discuss relevant translational research pertaining to the role of oxytocin in social and repetitive behaviors and consider clinical implications. We also discuss current research limitations, review recent preliminary findings from studies involving oxytocin in autism spectrum disorder patient populations, and point to possible directions for future research.

D2 antagonist during development decreases anxiety and infanticidal behavior in adult female prairie voles (Microtus ochrogaster)

On postnatal day 8, prairie vole pups were randomly assigned a treatment of 1mg/kg SKF38393 (D1 agonist), quinpirole (D2 agonist), SCH23390 (D1 antagonist), eticlopride (D2 antagonist), or saline vehicle. As adults, females treated with eticlopride exhibited reduced anxiety-like behavior in an elevated plus maze and a reduction in infanticidal behavior. These behavioral effects were not seen in males. These data demonstrate that a single exposure to a D2 antagonist during development can have persistent, sex-specific effects on behavior into adulthood.

Alloparenting experience affects future parental behavior and reproductive success in prairie voles (Microtus ochrogaster)

Various hypotheses have been proposed to explain the function of alloparental behavior in cooperatively breeding species. We examined whether alloparental experience as juveniles enhanced later parental care and reproductive success in the prairie vole (Microtus ochrogaster), a cooperatively breeding rodent. Juveniles cared for one litter of siblings (1EX), two litters of siblings (2EX) or no siblings (0EX). As adults, these individuals were mated to other 0EX, 1EX or 2EX voles, yielding seven different pair combinations, and we recorded measures of parental behaviors, reproductive success, and pup development. As juveniles, individuals caring for siblings for the first time were more alloparental; and as adults, 0EX females paired with 0EX males spent more time in the nest with their pups. Taken together, these results suggest that inexperienced animals spend more time in infant care. As parents, 1EX males spent more time licking their pups than 2EX and 0EX males. Pups with either a 1EX or 2EX parent gained weight faster than pups with 0EX parents during certain developmental periods. While inexperienced animals may spend more time in pup care, long-term benefits of alloparenting may become apparent in the display of certain, particularly important parental behaviors such as licking pups, and in faster weight gain of offspring.

Oxytocin and the neural mechanisms regulating social cognition and affiliative behavior

Oxytocin is produced in the hypothalamus and released into the circulation through the neurohypophyseal system. Peripherally released oxytocin facilitates parturition and milk ejection during nursing. Centrally released oxytocin coordinates the onset of maternal nurturing behavior at parturition and plays a role in mother-infant bonding. More recent studies have revealed a more general role for oxytocin in modulating affiliative behavior in both sexes. Oxytocin regulates alloparental care and pair bonding in female monogamous prairie voles. Social recognition in male and female mice is also modulated by oxytocin. In humans, oxytocin increases gaze to the eye region of human faces and enhances interpersonal trust and the ability to infer the emotions of others from facial cues. While the neurohypopheseal oxytocin system has been well characterized, less is known regarding the nature of oxytocin release within the brain. Here we review the role of oxytocin in the regulation of prosocial interactions, and discuss the neuroanatomy of the central oxytocin system.

The impact of early life family structure on adult social attachment, alloparental behavior, and the neuropeptide systems regulating affiliative behaviors in the monogamous prairie vole (microtus ochrogaster)

Early social attachments lie at the heart of emotional and social development in many mammals, including humans. In nature, monogamous prairie voles (Microtus ochrogaster) experience considerable natural variation in early social attachment opportunities due to differences in family structure [e.g., single-mothers (SM), solitary breeding pairs, and communal groups]. We exploited some of this natural variation in family structure to examine the influence of early social environment on the development of adult social behavior. First, we characterized the parental care received by pups reared biparentally (BP) or by SM in the laboratory. Second, we examined whether BP- and SM-reared offspring differed in adult nurturing, bonding, and emotional behaviors. Finally, we investigated the effects of rearing condition on neuropeptide systems that regulate adult social behavior [oxytocin (OT), vasopressin, and corticotropin-releasing factor, (CRF)]. Observations revealed that SM-reared pups were exposed more frequently (P < 0.01), licked and groomed less (P < 0.01), and matured more slowly (P < 0.01) than BP-reared pups. In adulthood, there were striking socio-behavioral differences: SM-reared females showed low spontaneous, pup-directed alloparental behavior (P < 0.01) and both males and females from the SM-reared condition showed delayed partner preference formation. While rearing did not impact neuropeptide receptor densities in the ventral forebrain as we predicted, SM-reared animals, particularly females, had increased OT content (P < 0.01) and greater dorsal raphe CRF2 densities (P < 0.05) and both measures correlated with licking and grooming experienced during the first 10 days of life. These results suggest that naturalistic variation in social rearing conditions can introduce diversity into adult nurturing and attachment behaviors.

Oxytocin: the great facilitator of life

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

Post-weaning social isolation alters anxiety-related behavior and neurochemical gene expression in the brain of male prairie voles

Stressful social experiences early in life, such as maternal separation and social isolation, have enduring effects on the development of the brain and behavior. In the present study in socially monogamous male prairie voles (Microtus ochrogaster), we found that following 6 weeks of social isolation after weaning males spent more time in the closed arms and less time in the open arms during an elevated plus maze (EPM) test, moved more frequently from central to peripheral squares in an open field test, and diminished their preferences for the empty chamber during a two-chamber affiliation test, compared to control males that were housed with siblings. This increased behavioral anxiety in socially isolated males was also associated with enhanced mRNA expression for vasopressin (AVP), oxytocin (OT), corticotrophin releasing factor (CRF), and tyrosine hydroxylase (TH) in the paraventricular nucleus of the hypothalamus (PVN). Together, these data illustrate the importance of the post-weaning social environment on anxiety-related behavior and suggest a potential role of neurochemical systems in the PVN in the regulation of this behavior.

Variation in oxytocin receptor density in the nucleus accumbens has differential effects on affiliative behaviors in monogamous and polygamous voles

Oxytocin receptors in the nucleus accumbens have been implicated in the regulation of alloparental behavior and pair bond formation in the socially monogamous prairie vole. Oxytocin receptor density in the nucleus accumbens is positively correlated with alloparenting in juvenile and adult female prairie voles, and oxytocin receptor antagonist infused into the nucleus accumbens blocks this behavior. Furthermore, prairie voles have higher densities of oxytocin receptors in the accumbens than nonmonogamous rodent species, and blocking accumbal oxytocin receptors prevents mating-induced partner preference formation. Here we used adeno-associated viral vector gene transfer to examine the functional relationship between accumbal oxytocin receptor density and social behavior in prairie and meadow voles. Adult female prairie voles that overexpress oxytocin receptor in the nucleus accumbens displayed accelerated partner preference formation after cohabitation with a male, but did not display enhanced alloparental behavior. However, partner preference was not facilitated in nonmonogamous meadow voles by introducing oxytocin receptor into the nucleus accumbens. These data confirm a role for oxytocin receptor in the accumbens in the regulation of partner preferences in female prairie voles, and suggest that oxytocin receptor expression in the accumbens is not sufficient to promote partner preferences in nonmonogamous species. These data are the first to demonstrate a direct relationship between oxytocin receptor density in the nucleus accumbens and variation in social attachment behaviors. Thus, individual variation in oxytocin receptor expression in the striatum may contribute to natural diversity in social behaviors.

Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress

BACKGROUND

Early-life emotional stress may be associated with affective and cognitive disorders later in life, yet satisfactory animal models for studying the underlying mechanisms are limited. Because maternal presence and behavior critically influence molecular and behavioral stress responses in offspring, we sought to create a model of dysfunctional, fragmented maternal nurturing behavior that would, in turn, provoke chronic early-life stress in the offspring.

METHODS

Sprague-Dawley rat dams' nursing and nurturing behaviors were altered by limiting their ability to create satisfactory nests during postpartum days 2-9. Maternal behavior was observed throughout the diurnal cycle, and the frequency and duration of nurturing behaviors were scored. In addition, potential stress and anxiety of the dams were assessed using behavioral, molecular and hormonal measures.

RESULTS

Both the quantity and the quality of dams' care of their pups were profoundly influenced by restriction of nesting materials in their cages: licking/grooming activities decreased and the frequency of leaving the pups increased, resulting in fragmented interactions between the dams and pups. The abnormal activity patterns of the dams were accompanied by increased anxiety-like behavior in the open field, but not in the elevated plus maze tests. Additionally, dams' plasma corticosterone levels and adrenal weights were augmented, suggesting chronic stress of these dams. By the end of the limited-nesting, stress-inducing period, hypothalamic corticotropin releasing hormone (CRH) mRNA expression was reduced in the limited-nesting dams, while arginine-vasopressin (AVP) mRNA levels were not significantly affected.

CONCLUSION

Limiting dams' ability to construct a nest for their pups leads to an abnormal repertoire of nurturing behaviors, possibly as a result of chronic stress and mild anxiety of the dams. Because the fragmented and aberrant maternal behavior provoked chronic stress in the pups, the limited-nesting paradigm provides a useful tool for studying the mechanisms and consequences of such early-life stress experience in the offspring.

Role of pregnancy and parturition in induction of maternal behavior in prairie voles (Microtus ochrogaster)

In prairie voles (Microtus ochrogaster), most virgin females are infanticidal. To determine the onset of maternal responsiveness, female prairie voles were tested for maternal behavior as virgins and at different times throughout pregnancy. Female voles that were infanticidal as virgins by and large remained infanticidal throughout pregnancy. In contrast, about 30% of voles that were maternal as virgins became infanticidal during pregnancy. To test whether events associated with parturition facilitate the onset of maternal behavior, females had their litters delivered by Caesarean section within a day of expected delivery or were allowed to give birth naturally with sham surgery occurring shortly before or after birth. Females that gave birth naturally were fully maternal and did not attack unrelated pups, but females subjected to artificial delivery remained infanticidal. This suggests that events closely related to parturition are crucial for full development of maternal behavior in female prairie voles.

Species and individual differences in juvenile female alloparental care are associated with oxytocin receptor density in the striatum and the lateral septum

The neuropeptide oxytocin has been implicated in the regulation of affiliative behavior and maternal responsiveness in several mammalian species. Rodent species vary considerably in the expression of juvenile alloparental behavior. For example, alloparental behavior is spontaneous in juvenile female prairie voles (approximately 20 days of age), takes 1-3 days of pup exposure to develop in juvenile rats, and is nearly absent in juvenile mice and meadow voles. Here, we tested the hypothesis that species differences in pup responsiveness in juvenile rodents are associated with oxytocin receptor (OTR) density in specific brain regions. We found that OTR density in the nucleus accumbens (NA) is highest in juvenile prairie voles, intermediate in juvenile rats, and lowest in juvenile mice and meadow voles. In the caudate putamen (CP), OTR binding was highest in prairie voles, intermediate in rats and meadow voles, and lowest in mice. In contrast, the lateral septum (LS) shows an opposite pattern, with OTR binding being high in mice and meadow voles and low in prairie voles and rats. Thus, alloparental responsiveness in juvenile rodents is positively correlated with OTR density in the NA and CP and negatively correlated with OTR density in the LS. We then investigated whether a similar receptor-behavior relationship exists among juvenile female prairie voles by correlating individual variation in alloparental behavior with variation in OTR density. The time spent adopting crouching postures, the most distinctive component of alloparental behavior in juveniles, was positively correlated with OTR density in the NA (r = 0.47) and CP (r = 0.45) and negatively correlated with OTR density in the lateral septum (r = -0.53). Thus, variation in OTR density in the NA, CP, and LS may underlie both species and individual differences in alloparental care in rodents.

Oxytocin receptors in the nucleus accumbens facilitate “spontaneous” maternal behavior in adult female prairie voles

Oxytocin and the nucleus accumbens have been extensively implicated in the regulation of maternal behavior, and the processing of pup-related stimuli relevant for this behavior. Oxytocin receptor density in the nucleus accumbens is highly variable in virgin female prairie voles, as is their behavioral response to pups, ranging from neglecting and infanticidal to full maternal behavior. We hypothesized that oxytocin receptor in the nucleus accumbens facilitates the expression of "spontaneous" maternal behavior in prairie voles. Forty sexually-naive adult females were exposed to pups for the first time and tested for maternal behavior. Oxytocin receptor binding in the nucleus accumbens and other brain regions was later determined using autoradiography. Females that showed maternal behavior (lick and groom the pups and hover over them for at least 30 s, n=24) had higher oxytocin receptor density in the nucleus accumbens (shell subregion) (P<0.05) than females that did not show maternal behavior or attacked the pups (n=16). No differences were found in other brain regions (medial preoptic area, septum, prelimbic cortex). In a second experiment, we tested whether infusions of the oxytocin receptor antagonist (d(CH2)5(1),Tyr(Me)2,Orn8)-AVT into the nucleus accumbens would block "spontaneous" maternal behavior. As a control region, oxytocin receptor antagonist was also infused into the caudate putamen. Ten females were infused bilaterally into the nucleus accumbens or caudate putamen with either 2 ng/0.5 microl of oxytocin receptor antagonist or CSF (vehicle). While five of 10 nucleus accumbens CSF-infused animals showed maternal behavior, none of the nucleus accumbens oxytocin receptor antagonist-infused subjects did (0/10; chi2, P<0.01). Nucleus accumbens oxytocin receptor antagonist-infused females recovered the next day and were not different from controls. Animals infused with CSF or oxytocin receptor antagonist into the caudate putamen did not differ (four/10, four/10). This is the first study to show that the nucleus accumbens is involved in the regulation of "spontaneous" maternal behavior and that oxytocin receptor in this brain region facilitates maternal responses.