Developmental changes in forebrain vasopressin receptor binding in prairie voles (Microtus ochrogaster) and montane voles (Microtus montanus)

Using Receptor Autoradiography to Visualize and Quantify Oxytocin and Vasopressin 1a Receptors in the Human and Nonhuman Primate Brain

Despite its development almost 40 years ago, receptor autoradiography remains a regular and reliable practice for the localization of oxytocin and vasopressin receptors in brain tissue sections. It is used across many laboratories, institutions, and animal species to characterize and quantify the distribution and density of these receptors at baseline and/or in response to experimental manipulations or lived experience. This powerful tool and the neuroanatomical receptor maps that it generates have allowed researchers to more accurately investigate and understand the neural substrates upon which oxytocin and vasopressin act to affect behavior. Researchers have used these maps to design site-specific pharmacological manipulations and electrophysiological recordings in animal studies to directly probe the underlying neural mechanisms in this system. This methods chapter describes the specific procedures by which a pharmacologically optimized, competitive binding modification to receptor autoradiography can be used to reliably localize oxytocin and vasopressin receptors in the human brain and in the brains of nonhuman primates. The ability to reliably perform receptor autoradiography for these targets in human brain tissue can finally inform our interpretation of past intranasal oxytocin neuroimaging studies and allows us to move past the reliance on transcriptomic studies using brain tissue homogenates so that we can directly investigate the involvement of oxytocin and vasopressin receptors in human behavior, physiology, and neuropsychiatric disease.

Specificity in sociogenomics: Identifying causal relationships between genes and behavior

There has been rapid growth in the use of transcriptomic analyses to study the interplay between gene expression and behavior. Experience can modify gene expression in the brain, leading to changes in internal state and behavioral displays, while gene expression variation between species is thought to specify many innate behavior differences. However, providing a causal association between a gene and a given behavior remains challenging as it is difficult to determine when and where a gene contributes to the function of a behaviorally-relevant neuronal population. Moreover, given that there are fewer genetic tools available for non-traditional model organisms, transcriptomic approaches have been largely limited to profiling of bulk tissue, which can obscure the contributions of subcortical brain regions implicated in multiple behaviors. Here, we discuss how emerging single cell technologies combined with methods offering additional spatial and connectivity information can give us insight about the genetic profile of specific cells involved in the neural circuit of target social behaviors. We also emphasize how these techniques are broadly adaptable to non-traditional model organisms. We propose that, ultimately, a combination of these approaches applied throughout development will be key to discerning how genes shape the formation of social behavior circuits.

Perinatal bisphenol A (BPA) exposure alters brain oxytocin receptor (OTR) expression in a sex- and region- specific manner: A CLARITY-BPA consortium follow-up study

Bisphenol A (BPA) is a well-characterized endocrine disrupting chemical (EDC) used in plastics, epoxy resins and other products. Neurodevelopmental effects of BPA exposure are a major concern with multiple rodent and human studies showing that early life BPA exposure may impact the developing brain and sexually dimorphic behaviors. The CLARITY-BPA (Consortium Linking Academic and Regulatory Insights on BPA Toxicity) program was established to assess multiple endpoints, including neural, across a wide dose range. Studies from our lab as part of (and prior to) CLARITY-BPA have shown that BPA disrupts estrogen receptor expression in the developing brain, and some evidence of oxytocin (OT) and oxytocin receptor (OTR) disruption in the hypothalamus and amygdala. While BPA disruption of steroid hormone function is well documented, less is known about its capacity to alter nonapeptide signals. In this CLARITY-BPA follow up study, we used remaining juvenile rat tissues to test the hypothesis that developmental BPA exposure affects OTR expression across the brain. Perinatal BPA exposure (2.5, 25, or 2500 μg/kg body weight (bw)/day) spanned gestation and lactation with dams gavaged from gestational day 6 until birth and then the offspring gavaged directly through weaning. Ethinyl estradiol (0.5 μg/kg bw/day) was used as a reference estrogen. Animals of both sexes were sacrificed as juveniles and OTR expression assessed by receptor binding. Our results demonstrate prenatal exposure to BPA can eliminate sex differences in OTR expression in three hypothalamic regions, and that male OTR expression may be more susceptible. Our data also identify a sub-region of the BNST with sexually dimorphic OTR expression not previously reported in juvenile rats that is also susceptible to BPA.

Effects of alcohol and psychostimulants on the vasopressin system: behavioral implications

Drug addiction is a chronic brain disease characterized by a compulsion to seek drugs, a loss of control with respect to drug consumption, and negative emotional states, including increased anxiety and irritability during withdrawal. Central vasopressin (AVP) and its receptors are involved in controlling social behavior, anxiety and reward, all of which are altered by drugs of abuse. Hypothalamic AVP neurons influence the stress response by modulating the hypothalamic-pituitary-adrenal (HPA) axis. The extrahypothalamic AVP system, however, is commonly associated with social recognition, motivational and anxiety responses. The specific relationship between AVP and drugs of abuse has been rarely reviewed. Here, we provide an overview of the interaction between the brain AVP system and psychostimulants and alcohol. We focus on the effects of alcohol and psychostimulants on AVP regulation of the HPA axis, their effect on the brain AVP system and their behavioral implications, the influence of the AVP system on addictive behaviors, AVP's organizational effects on the brain and consequently on behavior, and we highlight clinical studies on the relation between the AVP system and drug addiction. Finally, we discuss the data to address areas that need further research to support clinical trials and prevent drug-related disorders. This article is protected by copyright. All rights reserved.

Selective localization of oxytocin receptors and vasopressin 1a receptors in the human brainstem

Intranasal oxytocin (OT) affects a suite of human social behaviors, including trust, eye contact, and emotion recognition. However, it is unclear where oxytocin receptors (OXTR) and the structurally related vasopressin 1a receptors (AVPR1a) are expressed in the human brain. We have previously described a reliable, pharmacologically informed receptor autoradiography protocol for visualizing these receptors in postmortem primate brain tissue. We used this technique in human brainstem tissue to identify the neural targets of OT and vasopressin. To determine binding selectivity of the OXTR radioligand and AVPR1a radioligand, sections were incubated in four conditions: radioligand alone, radioligand with the selective AVPR1a competitor SR49059, and radioligand with a low or high concentration of the selective OXTR competitor ALS-II-69. We found selective OXTR binding in the spinal trigeminal nucleus, a conserved region of OXTR expression in all primate species investigated to date. We found selective AVPR1a binding in the nucleus prepositus, an area implicated in eye gaze stabilization. The tissue's postmortem interval (PMI) was not correlated with either the specific or nonspecific binding of either radioligand, indicating that it will not likely be a factor in similar postmortem studies. This study provides critical data for future studies of OXTR and AVPR1a in human brain tissue.

Developmental perspectives on oxytocin and vasopressin

The related neuropeptides oxytocin and vasopressin are involved in species-typical behavior, including social recognition behavior, maternal behavior, social bonding, communication, and aggression. A wealth of evidence from animal models demonstrates significant modulation of adult social behavior by both of these neuropeptides and their receptors. Over the last decade, there has been a flood of studies in humans also implicating a role for these neuropeptides in human social behavior. Despite popular assumptions that oxytocin is a molecule of social bonding in the infant brain, less mechanistic research emphasis has been placed on the potential role of these neuropeptides in the developmental emergence of the neural substrates of behavior. This review summarizes what is known and assumed about the developmental influence of these neuropeptides and outlines the important unanswered questions and testable hypotheses. There is tremendous translational need to understand the functions of these neuropeptides in mammalian experience-dependent development of the social brain. The activity of oxytocin and vasopressin during development should inform our understanding of individual, sex, and species differences in social behavior later in life.

The relative contribution of proximal 5' flanking sequence and microsatellite variation on brain vasopressin 1a receptor (Avpr1a) gene expression and behavior

Certain genes exhibit notable diversity in their expression patterns both within and between species. One such gene is the vasopressin receptor 1a gene (Avpr1a), which exhibits striking differences in neural expression patterns that are responsible for mediating differences in vasopressin-mediated social behaviors. The genomic mechanisms that contribute to these remarkable differences in expression are not well understood. Previous work has suggested that both the proximal 5′ flanking region and a polymorphic microsatellite element within that region of the vole Avpr1a gene are associated with variation in V1a receptor (V1aR) distribution and behavior, but neither has been causally linked. Using homologous recombination in mice, we reveal the modest contribution of proximal 5′ flanking sequences to species differences in V1aR distribution, and confirm that variation in V1aR distribution impacts stress-coping in the forced swim test. We also demonstrate that the vole Avpr1a microsatellite structure contributes to Avpr1a expression in the amygdala, thalamus, and hippocampus, mirroring a subset of the inter- and intra-species differences observed in central V1aR patterns in voles. This is the first direct evidence that polymorphic microsatellite elements near behaviorally relevant genes can contribute to diversity in brain gene expression profiles, providing a mechanism for generating behavioral diversity both at the individual and species level. However, our results suggest that many features of species-specific expression patterns are mediated by elements outside of the immediate 5′ flanking region of the gene.

DNA sequence variation underlies many differences both within and between species. In this paper, we investigate a specific DNA sequence that is thought to influence expression of a gene that modulates behavior, the vasopressin V1a receptor gene (Avpr1a). Specifically, differences in the expression of V1a receptor in the brain have been causally tied to social behavior differences, but the genetic basis of these differences is not understood. Using transgenic mice, we investigate the role of DNA sequences upstream of this gene in generating species-specific and individual variation in Avpr1a expression. We find that, contrary to our expectation, this region has only a modest influence on differences in expression patterns across rodent species. This indicates that DNA elements outside of this region play a larger role in species-level differences in expression. We confirm that variation in Avpr1a expression mediated by this upstream region translates to differences in behavior. We also find that variable DNA sequences associated with repetitive motifs within this region subtly influence gene expression. Together these findings highlight the complexity of genetic mechanisms that influence diversity in brain receptor patterns and support the idea that variable repetitive elements can influence both species and individual differences in gene expression patterns.

Vasopressin eliminates the expression of familiar odor bias in neonatal female mice through V1aR

V1aR has a well established role in the neural regulation of adult mammalian social behavior. The role of V1aR in developmentally emerging social behavior is less well understood. We mapped V1aR at post-natal day 8 (P8) and demonstrate developmentally-specific expression in the neocortex and hippocampus. We tested the ability of male and female C57BL/6J mice to show orienting bias to a familiar odor at this age. We demonstrate that females, but not males, show an orienting bias for odors previously paired with the mother, which is eliminated by V1aR signaling.

Modulation of parvalbumin interneuron number by developmentally transient neocortical vasopressin receptor 1a (V1aR)

Arginine-vasopressin (AVP) and the vasopressin 1a receptor (V1aR) modulate social behavior and learning and memory in adult animals. Both functions depend upon the normal emergence of the balance of excitation and inhibition (E/I balance) in the neocortex. Here, we tested the hypothesis that V1aR signaling and E/I balance converge through the influence of the neuropeptide on interneuron number achieved in the neocortex. Postnatal mapping of forebrain V1aR binding in male and female mice revealed a transient expression of high levels of receptor in the neocortex and hippocampus in the second and third post-natal weeks. Receptor binding levels in these cortical structures fell dramatically in the adult, maintaining high levels of expression subcortically. Surprisingly, we observed sex differences in the number of calbindin interneurons, and a contribution of V1aR to the number of parvalbumin-immunoreactive neurons in the adult mouse neocortex. These data suggest that individual differences in developmentally transient V1aR signaling and even sex may alter the development of E/I balance in the neocortex, with long-lasting influence on information processing.

Monogamous and promiscuous rodent species exhibit discrete variation in the size of the medial prefrontal cortex

Limbic-associated cortical areas, such as the medial prefrontal and retrosplenial cortex (mPFC and RS, respectively), are involved in the processing of emotion, motivation, and various aspects of working memory and have been implicated in mating behavior. To determine whether the independent evolution of mating systems is associated with a convergence in cortical mechanisms, we compared the size of mPFC and RS between the monogamous prairie vole (Microtus ochrogaster) and the promiscuous meadow vole (Microtus pennsylvanicus), and between the monogamous California mouse (Peromyscus californicus) and the promiscuous white-footed mouse (Peromyscus leucopus). For both promiscuous mice and voles, the mPFC occupied a significantly larger percentage of total cortex than in the monogamous species. No significant differences were observed for the RS or overall cortex size with respect to mating system, supporting the convergent evolution of mPFC size, specifically. Individual differences in the mating behavior of male prairie voles (wandering versus pair-bonding), presumably facultative tactics, were not reflected in the relative size of the mPFC, which is likely a heritable trait. Given the importance of the mPFC for complex working memory, particularly object-place and temporal order memory, we hypothesize that the relatively greater size of the mPFC in promiscuous species reflects a greater need to remember multiple individuals and the times and locations in which they have been encountered in the home range.

Developmental experiences and the oxytocin receptor system

The long-term effects of developmental experiences on social behavior, and the neuropeptide systems such as oxytocin which subserve the behavior, are still little understood. In this article, we review various types of early experience, including normal development, knockout models, pharmacological exposures, and early social experiences. We consider the processes by which experience can affect oxytocin receptor binding, and what is known about the directionality of experience effects on oxytocin receptors. Finally, we attempt to synthesize the literature into a predictive model as to the direction of early experience effects on oxytocin receptor binding potential, and whether these changes have functional significance. These predictions are relevant to current human health practice, given proposals to use chronic intranasal oxytocin to treat developmental disorders including autism and schizophrenia. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

The neurobiology of pair bonding: insights from a socially monogamous rodent

The formation of enduring relationships between adult mates (i.e., pair bonds) is an integral aspect of human social behavior and has been implicated in both physical and psychological health. However, due to the inherent complexity of these bonds and the relative rarity with which they are formed in other mammalian species, we know surprisingly little about their underlying neurobiology. Over the past few decades, the prairie vole (Microtus ochrogaster) has emerged as an animal model of pair bonding. Research in this socially monogamous rodent has provided valuable insight into the neurobiological mechanisms that regulate pair bonding behaviors. Here, we review these studies and discuss the neural regulation of three behaviors inherent to pair bonding: the formation of partner preferences, the subsequent development of selective aggression toward unfamiliar conspecifics, and the bi-parental care of young. We focus on the role of vasopressin, oxytocin, and dopamine in the regulation of these behaviors, but also discuss the involvement of other neuropeptides, neurotransmitters, and hormones. These studies may not only contribute to the understanding of pair bonding in our own species, but may also offer insight into the underlying causes of social deficits noted in several mental health disorders.

Genetics of aggression in voles

Prairie voles (Microtus ochrogaster) are socially monogamous rodents that form pair bonds-a behavior composed of several social interactions including attachment with a familiar mate and aggression toward conspecific strangers. Therefore, this species has provided an excellent opportunity for the study of pair bonding behavior and its underlying neural mechanisms. In this chapter, we discuss the utility of this unique animal model in the study of aggression and review recent findings illustrating the neurochemical mechanisms underlying pair bonding-induced aggression. Implications of this research for our understanding of the neurobiology of human violence are also discussed.

Oxytocin and vasopressin immunoreactive staining in the brains of Brandt's voles (Lasiopodomys brandtii) and greater long-tailed hamsters (Tscherskia triton)

Immunoreactive (ir) staining of the neuropeptides oxytocin (OT) and vasopressin (AVP) was performed in the brains of Brandt's voles (Lasiopodomys brandtii) and greater long-tailed hamsters (Tscherskia triton)-two species that differ remarkably in social behaviors. Social Brandt's voles had higher densities of OT-ir cells in the medial preoptic area (MPOA) and medial amygdala (MeA) as well as higher densities of AVP-ir cells in the lateral hypothalamus (LH) compared to solitary greater long-tailed hamsters. In contrast, the hamsters had higher densities of OT-ir cells in the anterior hypothalamus (AH) and LH and higher densities of AVP-ir cells in the MPOA than the voles. OT-ir and AVP-ir fibers were also found in many forebrain areas with subtle species differences. Given the roles of OT and AVP in the regulation of social behaviors in other rodent species, our data support the hypothesis that species-specific patterns of central OT and AVP pathways may underlie species differences in social behaviors. However, despite a higher density of OT-ir cells in the paraventricular nucleus of the hypothalamus (PVN) in females than in males in both species, no other sex differences were found in OT-ir or AVP-ir staining. These data failed to support our prediction that a sexually dimorphic pattern of neuropeptide staining in the brain is more apparent in Brandt's voles than in greater long-tailed hamsters.

The neurobiology of social attachment: A comparative approach to behavioral, neuroanatomical, and neurochemical studies

The formation and maintenance of social bonds in adulthood is an essential component of human health. However studies investigating the underlying neurobiology of such behaviors have been scarce. Microtine rodents offer a unique comparative animal model to explore the neural processes responsible for pair bonding and its associated behaviors. Studies using monogamous prairie voles and other related species have recently offered insight into the neuroanatomical, neurobiological, and neurochemical underpinnings of social attachment. In this review, we will discuss the utility of the microtine rodents in comparative studies by exploring their natural history and social behavior in the laboratory. We will then summarize the data implicating vasopressin, oxytocin, and dopamine in the regulation of pair bonding. Finally, we will discuss the ways in which these neurochemical systems may interact to mediate this complex behavior.