Rapid nonapeptide synthesis during a critical period of development in the prairie vole: plasticity of the paraventricular nucleus of the hypothalamus

Structural connectivity of the fore- and mid-brain in prairie voles

Mammals live in complex social systems that require higher order cognition to process and display complex social behaviors. It is suggested that brain networks, such as the social decision-making network (SDMN), have evolved to process such information. Recent functional connectivity studies of the SDMN have revealed distinct network dynamics during different social events across several species. However, the structural mapping of this network is incomplete which limits structural-functional modeling. Here, we assess the structural connectivity of an extended SDMN as well as the fore- and mid-brain afferent projections with the use of cholera toxin subunit-B retrograde tracers and the prairie vole (Microtus ochrogaster), a socially monogamous rodent that displays complex social behaviors. This work greatly expands upon the limited structural connectivity of the vole social brain and highlights important regions within the SDMN and other highly innervated regions that may serve as information hubs.

Aging leads to sex-dependent effects on pair bonding and increased number of oxytocin-producing neurons in monogamous prairie voles

Pair bonds powerfully modulate health, which becomes particularly important when facing the detrimental effects of aging. To examine the impact of aging on relationship formation and response to loss, we examined behavior in naive 6-, 12-, and 18-month male and female prairie voles, a monogamous species that forms mating-based pair bonds. We found that older males (18-months) bonded quicker than younger voles, while similarly aged female voles increased partner directed affiliative behaviors. Supporting sex differences in bonding behaviors, we found that males were more likely to sample both partner and stranger voles while females were more likely to display partner preference during the initial 20 min of the test. We also found that male voles of all ages show enduring bonding behavior despite four weeks of partner separation while females show an overall decrease in partner-directed affiliation, including an erosion of partner preference in 12-month females. Finally, we found that the number of oxytocin, but not vasopressin, cells in the paraventricular hypothalamus increased at 18 months of age. These results establish prairie voles as a novel model to study the effects of normal and abnormal aging on pair bonding.

Egalitarian cooperation linked to central oxytocin levels in communal breeding house mice

Relationships between adult females are fundamental to understanding diversity in animal social systems. While cooperative relationships between kin are known to promote fitness benefits, the proximate mechanisms underlying this are not well understood. Here we show that when related female house mice (Mus musculus domesticus) cooperate to rear young communally, those with higher endogenous oxytocin levels have more egalitarian and successful cooperative relationships. Sisters with higher oxytocin concentrations in the paraventricular nucleus (PVN) of the hypothalamus weaned significantly more offspring, had lower reproductive skew and spent more equal proportions of time in the nest. By contrast, PVN oxytocin was unrelated to the number of weaned offspring produced in the absence of cooperation, and did not vary in response to manipulation of nest site availability or social cues of outgroup competition. By linking fitness consequences of cooperation with oxytocin, our findings have broad implications for understanding the evolution of egalitarian social relationships.

Hypothalamic vasopressin neural densities are higher in male Mongolian gerbils after separation from a pair bond partner and may facilitate behavior to form a new bond

Although pair bonding has been studied for several decades, only somewhat recently have researchers began studying the neural consequences of separation from a pair bond partner. Here we examined the impact of partner separation on the socially monogamous Mongolian gerbil. Using a within-subjects design, we assessed nonsocial, nonreproductive, and reproductive behavior in male gerbils pre- and post- either 4 weeks of cohabitation with or separation from a pair bond partner. We then conducted an immediate early gene study to examine the influence of partner separation on hypothalamic oxytocin and vasopressin neural responses to interactions with a novel, opposite-sex conspecific.

The central oxytocinergic system of the prairie vole

Oxytocin (OXT) is a peptide hormone and a neuropeptide that regulates various peripheral physiological processes and modulates behavioral responses in the central nervous system. While the humoral release occurs from the axons arriving at the median eminence, the neuropeptide is also released from oxytocinergic cell axons in various brain structures that contain its receptor, and from their dendrites in hypothalamic nuclei and potentially into the cerebrospinal fluid (CSF). Understanding oxytocin's complex functions requires the knowledge on patterns of oxytocinergic projections in relationship to its receptor (OXTR). This study provides the first comprehensive examination of the oxytocinergic system in the prairie vole (Microtus ochrogaster), an animal exhibiting social behaviors that mirror human social behaviors linked to oxytocinergic functioning. Using light and electron microscopy, we characterized the neuroanatomy of the oxytocinergic system in this species. OXT+ cell bodies were found primarily in the hypothalamus, and axons were densest in subcortical regions. Examination of the OXT+ fibers and their relationship to oxytocin receptor transcripts (Oxtr) revealed that except for some subcortical structures, the presence of axons was not correlated with the amount of Oxtr across the brain. Of particular interest, the cerebral cortex that had high expression of Oxtr transcripts contained little to no fibers. Electron microscopy is used to quantify dense cored vesicles (DCV) in OXT+ axons and to identify potential axonal release sites. The ependymal cells that line the ventricles were frequently permissive of DCV-containing OXT+ dendrites reaching the third ventricle. Our results highlight a mechanism in which oxytocin is released directly into the ventricles and circulates throughout the ventricular system, may serve as the primary source for oxytocin that binds to OXTR in the cerebral cortex.

Paternal absence and increased caregiving independently and interactively shape the development of male prairie voles at subadult and adult life stages

The influence of maternal caregiving is a powerful force on offspring development. The absence of a father during early life in biparental species also has profound implications for offspring development, although it is far less studied than maternal influences. Moreover, we have limited understanding of the interactive forces that maternal and paternal caregiving impart on offspring. We investigated if behaviorally upregulating maternal care compensates for paternal absence on prairie vole (Microtus ochrogaster) pup development. We used an established handling manipulation to increase levels of caregiving in father-absent and biparental families, and later measured male offspring behavioral outcomes at sub-adulthood and adulthood. Male offspring raised without fathers were more prosocial (or possibly less socially anxious) than those raised biparentally. Defensive behavior and responses to contextual novelty were also influenced by the absence of fathers, but only in adulthood. Offensive aggression and movement in the open field test changed as a function of life-stage but not parental exposure. Notably, adult pair bonding was not impacted by our manipulations. Boosting parental care produced males that moved more in the open field test. Parental handling also increased oxytocin immunoreactive cells within the supraoptic nucleus of the hypothalamus (SON), and in the paraventricular nucleus (PVN) of biparentally-reared males. We found no differences in vasopressinergic cell groups. We conclude that male prairie voles are contextually sensitive to the absence of fathers and caregiving intensity. Our study highlights the importance of considering the ways early experiences synergistically shape offspring behavioral and neural phenotypes across the lifespan.

Aging leads to sex-dependent effects on pair bonding and increased number of oxytocin-producing neurons in monogamous prairie voles

Pair bonds powerfully modulate health, which becomes particularly important when facing the detrimental effects of aging. To examine the impact of aging on relationship formation and response to loss, we examined behavior in 6-, 12-, and 18-month male and female prairie voles, a monogamous species that forms mating-based pair bonds. We found that older males (18-months) bonded quicker than younger voles, while similarly aged female voles increased partner directed affiliative behaviors. Supporting sex differences in bonding behaviors, we found that males were more likely to sample both partner and novel voles while females were more likely to display partner preference during the initial 20 minutes of the test. Using partner separation to study loss, we observed an erosion of partner preference only in 12-month females, but an overall decrease in partner-directed affiliation in females across all groups, but not in males. Finally, we found that the number of oxytocin, but not vasopressin, cells in the paraventricular hypothalamus increased during aging. These results establish prairie voles as a novel model to study the effects of normal and abnormal aging on pair bonding.

Highlights

18-month male voles demonstrate accelerated bond formation18-month female voles increase partner-directed huddling after 2 wksBonds erode faster in 12-month female voles after partner separationFemale behavior from partner preference tests is reflected in free interactionThe number of paraventricular hypothalamus oxytocin cells increase during aging.

Leveraging the unique social organization of California mice to study circuit-specific effects of oxytocin on behavior

Oxytocin is a versatile neuropeptide that modulates many different forms of social behavior. Recent hypotheses pose that oxytocin enhances the salience of rewarding and aversive social experiences, and the field has been working to identify mechanisms that allow oxytocin to have diverse effects on behavior. Here we review studies conducted on the California mouse (Peromyscus californicus) that shed light on how oxytocin modulates social behavior following stressful experiences. In this species, both males and females exhibit high levels of aggression, which has facilitated the study of how social stress impacts both sexes. We review findings of short- and long-term effects of social stress on the reactivity of oxytocin neurons. We also consider the results of pharmacological studies which show that oxytocin receptors in the bed nucleus of the stria terminalis and nucleus accumbens have distinct but overlapping effects on social approach behaviors. These findings help explain how social stress can have different behavioral effects in males and females, and how oxytocin can have such divergent effects on behavior. Finally, we consider how new technological developments and innovative research programs take advantage of the unique social organization of California mice to address questions that can be difficult to study in conventional rodent model species. These new methods and questions have opened new avenues for studying the neurobiology of social behavior.

Plasticity in parental behavior and vasopressin: responses to co-parenting, pup age, and an acute stressor are experience-dependent

Introduction

The impact of variation in parental caregiving has lasting implications for the development of offspring. However, the ways in which parents impact each other in the context of caregiving is comparatively less understood, but can account for much of the variation observed in the postnatal environment. Prairie voles (Microtus ochrogaster) demonstrate a range of postnatal social groups, including pups raised by biparental pairs and by their mothers alone. In addition to the challenges of providing parental care, prairie vole parents often experience acute natural stressors (e.g., predation, foraging demands, and thermoregulation) that could alter the way co-parents interact.

Methods

We investigated how variation in the experience of raising offspring impacts parental behavior and neurobiology by administering an acute handling stressor on prairie vole families of single mothers and biparental parents over the course of offspring postnatal development.

Results

Mothers and fathers exhibited robust behavioral plasticity in response to the age of their pups, but in sex-dependent ways. Pup-directed care from mothers did not vary as a function of their partner's presence, but did covary with the number of hypothalamic vasopressin neurons in experience-dependent ways. The relationship between vasopressin neuron numbers and fathers' behaviors was also contingent upon the stress handling manipulation, suggesting that brain-behavior associations exhibit stress-induced plasticity.

Conclusion

These results demonstrate that the behavioral and neuroendocrine profiles of adults are sensitive to distinct and interacting experiences as a parent, and extend our knowledge of the neural mechanisms that may facilitate parental behavioral plasticity.

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.

Species-typical group size differentially influences social reward neural circuitry during nonreproductive social interactions

We investigated whether nonreproductive social interactions may be rewarding for colonial but not non-colonial species. We found that the colonial spiny mouse (Acomys cahirinus) is significantly more gregarious, more prosocial, and less aggressive than its non-colonial relative, the Mongolian gerbil (Meriones unguiculatus). In an immediate-early gene study, we examined oxytocin (OT) and tyrosine hydroxylase (TH) neural responses to interactions with a novel, same-sex conspecific or a novel object. The paraventricular nucleus of the hypothalamus (PVN) OT cell group was more responsive to interactions with a conspecific compared to a novel object in both species. However, the ventral tegmental area (VTA) TH cell group showed differential responses only in spiny mice. Further, PVN OT and VTA TH neural responses positively correlated in spiny mice, suggesting functional connectivity. These results suggest that colonial species may have evolved neural mechanisms associated with reward in novel, nonreproductive social contexts to promote large group-living.

Support for the parental practice hypothesis: Subadult prairie voles exhibit similar behavioral and neural profiles when alloparenting kin and non-kin

Parental care is critical for offspring survival in altricial species. Although parents are the most common caregivers, other individuals (e.g., older siblings) can also provide alloparental care. Some have argued that animals engage in alloparental behavior to practice providing care for their eventual offspring, whereas others have argued that alloparental behavior enhances indirect fitness. Proximate measures have the potential to test ultimate functions of behavior. A focus on neural expression of oxytocin and vasopressin (two neuropeptides modulating alloparental care) or neural activation following exposure to related and unrelated individuals could reveal whether practice or investment in indirect fitness explains alloparental behavior. This study examined alloparental behaviors and neural responses in prairie voles (Microtus ochrogaster), a species that engages in alloparental behavior. Subadult (independent, yet sexually immature) male prairie voles were exposed to one of four stimuli: same-age sibling, neonatal sibling, unrelated neonate, or inanimate neonate-sized object. We assessed alloparental behaviors and quantified cFos protein expression in oxytocin and vasopressin neuronal populations of the paraventricular nucleus of the hypothalamus and the supraoptic nucleus of the hypothalamus in response to stimulus exposure. We detected no differences in cFos and nonapeptide co-localization among stimulus groups. Subjects performed similar amounts of alloparental care toward related and unrelated neonates, but not other subadults or inanimate objects. Notably, caregiving did not differ based on kin-status. The lack of difference in alloparenting toward related and non-related neonates suggests that alloparental care in prairie voles primarily serves to provide subadults with parental practice.

Differential fate between oxytocin and vasopressin cells in the developing mouse brain

Oxytocin (OXT) and arginine vasopressin (AVP), two neuropeptides involved in socio-emotional behaviors have been anatomically defined in the adult brain. Yet their spatial organization during postnatal development is not clearly defined. We built a developmental atlas using 3D imaging of cleared immunolabeled tissue over four early postnatal (P) stages, from birth (P0, P3, P7, P14) to young adulthood (≥P56). Our atlas-based mapping revealed that the number of OXT neurons doubles according to unique temporal dynamics in selective hypothalamic regions, namely, the periventricular and paraventricular nuclei, and in a novel location we named the antero-lateral preoptic. In the paraventricular nucleus, single-cell densities and fluorescence analysis demonstrated selective expansion of OXT cells in the antero-ventral division, whereas the postero-dorsal division contained cells present at birth. No changes were observed for AVP neurons. Our findings show the coexisting of innate and plastic OXT/AVP brain circuits probably triggered by environmental adaptation of the social brain.

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The OXT system continues to mature during early development but not the AVP system

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OXT hypothalamic nuclei emerge at different rates after birth

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PVN cells gradually acquire an oxytocinergic phenotype

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OXT cells are organized into innate and adaptive neural networks

Distribution of Vasopressin and Oxytocin Neurons in the Basal Forebrain and Midbrain of Spiny Mice (Acomys cahirinus)

The nonapeptides vasopressin (VP) and oxytocin (OT) are present in some form in most vertebrates. VP and OT play critical roles in modulating physiology and are well-studied for their influences on a variety of social behaviors, ranging from affiliation to aggression. Their anatomical distributions have been mapped for numerous species across taxa, demonstrating relatively strong evolutionary conservation in distributions throughout the basal forebrain and midbrain. Here we examined the distribution of VP-immunoreactive (-ir) and OT-ir neurons in a gregarious, cooperatively breeding rodent species, the spiny mouse (Acomys cahirinus), for which nonapeptide mapping does not yet exist. Immunohistochemical techniques revealed VP-ir and OT-ir neuronal populations throughout the hypothalamus and amygdala of males and females that are consistent with those of other rodents. However, a novel population of OT-ir neurons was observed in the median preoptic nucleus of both sexes, located dorsally to the anterior commissure. Furthermore, we found widespread sex differences in OT neuronal populations, with males having significantly more OT-ir neurons than females. However, we observed a sex difference in only one VP cell group - that of the bed nucleus of the stria terminalis (BST), a VP neuronal population that exhibits a phylogenetically widespread sexual dimorphism. These findings provide mapping distributions of VP and OT neurons in Acomys cahirinus. Spiny mice lend themselves to the study of mammalian cooperation and sociality, and the nonapeptide neuronal mapping presented here can serve as a basic foundation for the study of nonapeptide-mediated behavior in a group of highly social rodents.

Oxytocin, Dopamine, and Opioid Interactions Underlying Pair Bonding: Highlighting a Potential Role for Microglia

Pair bonds represent some of the strongest attachments we form as humans. These relationships positively modulate health and well-being. Conversely, the loss of a spouse is an emotionally painful event that leads to numerous deleterious physiological effects, including increased risk for cardiac dysfunction and mental illness. Much of our understanding of the neuroendocrine basis of pair bonding has come from studies of monogamous prairie voles (Microtus ochrogaster), laboratory-amenable rodents that, unlike laboratory mice and rats, form lifelong pair bonds. Specifically, research using prairie voles has delineated a role for multiple neuromodulatory and neuroendocrine systems in the formation and maintenance of pair bonds, including the oxytocinergic, dopaminergic, and opioidergic systems. However, while these studies have contributed to our understanding of selective attachment, few studies have examined how interactions among these 3 systems may be essential for expression of complex social behaviors, such as pair bonding. Therefore, in this review, we focus on how the social neuropeptide, oxytocin, interacts with classical reward system modulators, including dopamine and endogenous opioids, during bond formation and maintenance. We argue that an understanding of these interactions has important clinical implications and is required to understand the evolution and encoding of complex social behaviors more generally. Finally, we provide a brief consideration of future directions, including a discussion of the possible roles that glia, specifically microglia, may have in modulating social behavior by acting as a functional regulator of these 3 neuromodulatory systems.

Paternal deprivation impairs social behavior putatively via epigenetic modification to lateral septum vasopressin receptor

Although it is well appreciated that the early-life social environment asserts subsequent long-term consequences on offspring brain and behavior, the specific mechanisms that account for this relationship remain poorly understood. Using a novel assay that forced biparental pairs or single mothers to prioritize caring for offspring or themselves, we investigated the impact of parental variation on adult expression of nonapeptide-modulated behaviors in prairie voles. We demonstrated that single mothers compensate for the lack of a co-parent. Moreover, mothers choose to invest in offspring over themselves when faced with a tradeoff, whereas fathers choose to invest in themselves. Furthermore, our study suggests a pathway whereby variation in parental behavior (specifically paternal care) may lead to alterations in DNA methylation within the vasopressin receptor 1a gene and gene expression in the lateral septum. These differences are concomitant with changes in social approach, a behavior closely associated with septal vasopressin receptor function.

Oxytocin receptor gene loss influences expression of the oxytocin gene in C57BL/6J mice in a sex- and age-dependent manner

Parental care and sensory stimulation are critical environmental factors that influence oxytocin (OXT) and its receptor (OXTR). Because developmental Oxt mRNA expression is enhanced by sensory-rich early life experience and reduced by sensory deprivation, we predicted that compared to wild-type (WT) littermates, mice with congenital loss of OXTR (OXTR KO), as a genetically induced deprivation, would show impaired Oxt mRNA expression in the offspring hypothalamus during development. Oxt mRNA levels of male and female OXTR KO mice were not different from WT littermates from postnatal day (P)0 to P6, although, by P8, OXTR KO showed significantly decreased Oxt mRNA expression in the hypothalamus compared to WT littermates. At P14, male and female OXTR KO mice had significantly decreased Oxt mRNA expression specifically in the paraventricular nucleus (PVN), but not the supraoptic nucleus (SON), compared to WT littermates. We investigated whether this effect persisted in adulthood (P90) and found a significant genotype by sex interaction where male OXTR KO mice displayed a reduction in Oxt expression specific to the PVN compared to male WT littermates. By contrast, male and female OXTR KO adults had increased Oxt mRNA levels in the SON. These findings suggest that OXTR plays a role in developmental Oxt mRNA expression with sex by genotype interactions apparent at adulthood. We then measured OXT and neural activation in the PVN and SON at P14. We observed more OXT-immunoreactive cells in the PVN of OXTR KO mice but significantly fewer c-Fos immunoreactive cells. There were no genotype differences in immunoreactivity for OXT and no c-Fos activity in the SON at P14. Combined, these data suggest that OXTR WT P14 mice have more PVN activity and are more likely to release OXT than OXTR KO mice. Future experiments are warranted to understand which OXTR-expressing neural circuits modulate the development of the PVN oxytocin system.

Age-specific and context-specific responses of the medial extended amygdala in the developing prairie vole

The social needs of organisms change as they mature. Yet, little is known about the mechanisms that subserve processing social interactions or how these systems develop. The medial extended amygdala (meEA) is comprised of the medial bed nucleus of the stria terminalis (BSTm) and the medial amygdala (MeA). This neural complex holds great promise for understanding how the social brain processes information. We assessed expression of the immediate early gene cFos and the enzyme tyrosine hydroxylase (TH) at three developmental time-points (postnatal day [PND] 2, 9, and 21) to determine how developing prairie voles process familial social contact, separation, and reunion. We demonstrate that (1) BSTm cFos responses were sensitive to separation from family units at PND 9 and PND 21, but not at PND 2; (2) MeA cFos responses were sensitive to reunion with the family, but only in PND 21 pups; (3) BSTm TH neurons did not exhibit differential responses to social condition at any age; and (4) MeA TH neurons responded strongly to social contact (remaining with family or following reunion), but only at PND 21. Our results suggest that the sub-units of the meEA become functionally responsive at different developmental time points, and are differentially activated in response to distinct social contexts. Overall, our results support the notion that interconnected regions of the meEA follow divergent developmental timelines and are sensitive to distinct properties of social contexts.