The ties that bond: neurochemistry of attachment in voles

Aggression Unleashed: Neural Circuits from Scent to Brain

Aggression is a fundamental behavior with essential roles in dominance assertion, resource acquisition, and self-defense across the animal kingdom. However, dysregulation of the aggression circuitry can have severe consequences in humans, leading to economic, emotional, and societal burdens. Previous inconsistencies in aggression research have been due to limitations in techniques for studying these neurons at a high spatial resolution, resulting in an incomplete understanding of the neural mechanisms underlying aggression. Recent advancements in optogenetics, pharmacogenetics, single-cell RNA sequencing, and in vivo electrophysiology have provided new insights into this complex circuitry. This review aims to explore the aggression-provoking stimuli and their detection in rodents, particularly through the olfactory systems. Additionally, we will examine the core regions associated with aggression, their interactions, and their connection with the prefrontal cortex. We will also discuss the significance of top-down cognitive control systems in regulating atypical expressions of aggressive behavior. While the focus will primarily be on rodent circuitry, we will briefly touch upon the modulation of aggression in humans through the prefrontal cortex and discuss emerging therapeutic interventions that may benefit individuals with aggression disorders. This comprehensive understanding of the neural substrates of aggression will pave the way for the development of novel therapeutic strategies and clinical interventions. This approach contrasts with the broader perspective on neural mechanisms of aggression across species, aiming for a more focused analysis of specific pathways and their implications for therapeutic interventions.

Maternal oxytocin treatment at birth increases epigenetic age in male offspring

Exogenous oxytocin (OT) is widely used to induce or augment labor with little understanding of the impact on offspring development. In rodent models, including the prairie vole (Microtus ochrogaster), it has been shown that oxytocin administered to mothers can affect the nervous system of the offspring with long lasting behavioral effects especially on sociality. Here, we examined the hypothesis that perinatal oxytocin exposure could have epigenetic and transcriptomic consequences. Prairie voles were exposed to exogenous oxytocin, through injections given to the mother just prior to birth, and were studied at the time of weaning. The outcome of this study revealed increased epigenetic age in oxytocin-exposed animals compared to the saline-exposed group. Oxytocin exposure led to 900 differentially methylated CpG sites (annotated to 589 genes), and 2 CpG sites (2 genes) remained significantly different after correction for multiple comparisons. Differentially methylated CpG sites were enriched in genes known to be involved in regulation of gene expression and neurodevelopment. Using RNA-sequencing we also found 217 nominally differentially expressed genes (p<0.05) in nucleus accumbens, a brain region involved in reward circuitry and social behavior; after corrections for multiple comparisons 6 genes remained significantly differentially expressed. Finally, we found that maternal oxytocin administration led to widespread alternative splicing in the nucleus accumbens. These results indicate that oxytocin exposure during birth may have long lasting epigenetic consequences. A need for further investigation of how oxytocin administration impacts development and behavior throughout the lifespan is supported by these outcomes.

Partner-seeking and limbic dopamine system are enhanced following social loss in male prairie voles (Microtus ochrogaster)

Death of a loved one is recognized as one of life's greatest stresses, and 10%-20% of bereaved individuals will experience a complicated or prolonged grieving period that is characterized by intense yearning for the deceased. The monogamous prairie vole (Microtus ochrogaster) is a rodent species that forms pair bonds between breeding partners and has been used to study the neurobiology of social behaviors and isolation. Male prairie voles do not display distress after isolation from a familiar, same-sex conspecific; however, separation from a bonded female partner increases emotional, stress-related, and proximity-seeking behaviors. Here, we tested the investigatory response of male voles to partner odor during a period of social loss. We found that males who lost their partner spent significantly more time investigating partner odor but not non-partner social odor or food odor. Bachelor males and males in intact pairings did not respond uniquely to any odor. Furthermore, we examined dopamine (DA) receptor mRNA expression in the anterior insula cortex (aIC), nucleus accumbens (NAc), and anterior cingulate (ACC), regions with higher activation in grieving humans. While we found some effects of relationship type on DRD1 and DRD2 expression in some of these regions, loss of a high-quality opposite-sex relationship had a significant effect on DA receptor expression, with pair-bonded/loss males having higher expression in the aIC and ACC compared with pair-bonded/intact and nonbonded/loss males. Together, these data suggest that both relationship type and relationship quality affect reunion-seeking behavior and motivational neurocircuits following social loss of a bonded partner.

Peer Social Environment Impacts Behavior and Dopamine D1 Receptor Density in Prairie Voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are socially monogamous rodents that form selective, long-lasting relationships with mates and with same-sex peers. It is unknown to what extent mechanisms supporting 'peer relationships' are similar to those involved in mate relationships. The formation of pair bonds is dependent on dopamine neurotransmission, whereas the formation of peer relationships is not, providing evidence of relationship type-specificity. The current study assessed endogenous structural changes in dopamine D1 receptor density in male and female voles across different social environments, including long-term same-sex partnerships, new same-sex partnerships, social isolation, and group housing. We also related dopamine D1 receptor density and social environment to behavior in social interaction and partner preference tests. Unlike prior findings in mate pairs, voles paired with new same-sex partners did not exhibit upregulated D1 binding in the nucleus accumbens (NAcc) relative to controls paired from weaning. This is consistent with differences in relationship type: D1 upregulation in pair bonds aids in maintaining exclusive relationships through selective aggression, and we found that formation of new peer relationships did not enhance aggression. Isolation led to increases in NAcc D1 binding, and even across socially housed voles, individuals with higher D1 binding exhibited increased social avoidance. These findings suggest that elevated D1 binding may be both a cause and a consequence of reduced prosociality. These results highlight the neural and behavioral consequences of different non-reproductive social environments and contribute to growing evidence that the mechanisms underlying reproductive and non-reproductive relationship formation are distinct. Elucidation of the latter is necessary to understand mechanisms underlying social behavior beyond a mating context.

Is Oxytocin "Nature's Medicine"?

Oxytocin is a pleiotropic, peptide hormone with broad implications for general health, adaptation, development, reproduction, and social behavior. Endogenous oxytocin and stimulation of the oxytocin receptor support patterns of growth, resilience, and healing. Oxytocin can function as a stress-coping molecule, an anti-inflammatory, and an antioxidant, with protective effects especially in the face of adversity or trauma. Oxytocin influences the autonomic nervous system and the immune system. These properties of oxytocin may help explain the benefits of positive social experiences and have drawn attention to this molecule as a possible therapeutic in a host of disorders. However, as detailed here, the unique chemical properties of oxytocin, including active disulfide bonds, and its capacity to shift chemical forms and bind to other molecules make this molecule difficult to work with and to measure. The effects of oxytocin also are context-dependent, sexually dimorphic, and altered by experience. In part, this is because many of the actions of oxytocin rely on its capacity to interact with the more ancient peptide molecule, vasopressin, and the vasopressin receptors. In addition, oxytocin receptor(s) are epigenetically tuned by experience, especially in early life. Stimulation of G-protein–coupled receptors triggers subcellular cascades allowing these neuropeptides to have multiple functions. The adaptive properties of oxytocin make this ancient molecule of special importance to human evolution as well as modern medicine and health; these same characteristics also present challenges to the use of oxytocin-like molecules as drugs that are only now being recognized.

The role of dopamine signaling in prairie vole peer relationships

Dopamine signaling mediates the formation of some types of social relationships, including reproductive pair bonds in the socially monogamous prairie vole (Microtus ochrogaster). In addition to these pair bonds with mates, prairie voles demonstrate selective preferences for familiar same-sex peers. The dependence of peer relationships on dopamine signaling has not been tested, and the mechanisms supporting these relationships may differ from those underlying pair bonds. We examined the effects of pharmacological manipulations of dopamine signaling on peer partner preference and socially conditioned place preference in female prairie voles. Haloperidol blockade of dopamine receptors at multiple doses did not alter selective preferences for familiar same-sex partners, suggesting that dopamine neurotransmission is not necessary for the formation of prairie vole peer relationships, unlike mate relationships. Dopamine receptor agonist apomorphine facilitated peer partner preferences under conditions normally insufficient for partner preference formation; however, in the absence of effects from blockade, it is difficult to distinguish between a role for dopamine in partner preference formation and the generally rewarding properties of a dopamine agonist. Prairie voles exhibited socially conditioned place preferences for new but not long-term same-sex peers, and these preferences were not blocked by haloperidol. These results suggest that prairie vole peer relationships are less dependent on dopamine signaling than pair bonds, while still being rewarding. The data support distinct roles of dopamine and motivation in prairie vole peer relationships relative to mate relationships, suggesting that reproductive bonds are mediated differently from non-reproductive ones.

Central but not peripheral oxytocin administration reduces risk-based decision-making in male rats

The hormone oxytocin has long been associated with social behaviors, but recent evidence suggests that it may also affect reward processing in non-social contexts. Decisions are an integral component of many social and reward-based behavioral paradigms. Thus, a broad role for oxytocin in decision-making may explain the wide variety of effects that have been previously observed and resolve controversies in the literature about its role. To determine if oxytocin can selectively modulate decision-making in male rats, we assessed the dose-dependent effects of central (intracerebroventricular) or peripheral (intraperitoneal) administration of oxytocin on probability and delay discounting, two commonly used decision-making tasks that are free of social contexts. Our results showed that central administration of oxytocin dose-dependently reduced preference for risky outcomes in the probability discounting task, but had no impact on delay discounting or reward sensitivity. This effect was blocked by the co-administration of an oxytocin antagonist. Additionally, we found no effect of peripheral oxytocin administration on any task. To identify potential cognitive mechanisms of central oxytocin's effect on decision-making, we determined if central or peripheral oxytocin affects reward sensitivity using an intracranial self-stimulation task, and motivation using a progressive ratio task. These results showed that at the dosage that affects decision-making, central oxytocin had a mild and short-lasting effect on motivation, but no observable effect on reward sensitivity. This pattern of results suggests that oxytocin may selectively reduce risky decisions in male rats, even at dosages that have no major effects on reward processing and motivation. These findings highlight a potentially novel role for oxytocin in non-social cognitive processes and expand our understanding of the mechanism by which oxytocin may regulate social behavior.

Oxytocin receptor antagonist reverses the blunting effect of pair bonding on fear learning in monogamous prairie voles

Social relationships among spouses, family members, and friends are known to affect physical and mental health. In particular, long-lasting bonds between socio-sexual partners have profound effects on cognitive, social, emotional, and physical well-being. We have previously reported that pair bonding in monogamous prairie voles (Microtus ochrogaster) is prevented by a single prolonged stress (SPS) paradigm, which causes behavioral and endocrine symptoms resembling post-traumatic stress disorder (PTSD) patients in rats (Arai et al., 2016). Since fear memory function is crucial for anxiety-related disorders such as PTSD, we investigated the effects of pair bonding on fear learning in prairie voles. We applied an SPS paradigm to male prairie voles after the cohabitation with a male (cage-mate group) or female (pair-bonded group). The cage-mate group, but not the pair-bonded group, showed enhanced fear response in a contextual fear conditioning test following the SPS treatment. Immunohistochemical analyses revealed that cFos-positive cells in the central amygdala were increased in the pair-bonded group after the contextual fear conditioning test and that oxytocin immunoreactivity in the paraventricular nucleus of the hypothalamus was significantly higher in the pair-bonded group than the cage-mate group. This pair-bonding dependent blunting of fear memory response was confirmed by a passive avoidance test, another fear-based learning test. Interestingly, intracerebroventricular injection of an oxytocin receptor antagonist 30 min before the passive avoidance test blocked the blunting effect of pair bonding on fear learning. Thus, pair bonding between socio-sexual partners results in social buffering in the absence of the partner, blunting fear learning, which may be mediated by oxytocin signaling.

Regulation of defeat-induced social avoidance by medial amygdala DRD1 in male and female prairie voles

Social interaction with unfamiliar individuals is necessary for species-preserving behaviors such as finding mates and establishing social groups. However, social conflict is a potential negative outcome to interaction with a stranger that can be distressing enough to cause an individual to later avoid interactions with other unfamiliar conspecifics. Unfortunately, stress research using a prominent model of social conflict, social defeat stress, has largely omitted female subjects. This has left a void in the literature regarding social strain on female stress biology and adequate comparison of the effect of sex in stress pathways. The prairie vole (Microtus ochrogaster) exhibits aggressive behavior in both sexes, making voles an attractive candidate to model social defeat in both sexes. This study sought to establish a model of social defeat stress in both male and female prairie voles, characterize behavioral changes in response to this stressor, and investigate the role of dopamine signaling in the response to social defeat stress. Defeated male and female prairie voles displayed social avoidance as well as an increase in the level of dopamine receptor D1 (DRD1) in the medial amygdala (MeA). Pharmacological manipulation of DRD1 signaling in the MeA revealed that increased DRD1 signaling is sufficient to induce a social avoidant state, and could be a necessary component in the defeat-induced social avoidance response. These findings provide the prairie vole as a model of social defeat in both sexes, and implicate the MeA in avoidance of unfamiliar conspecifics after a distressing social encounter.

Complex patterns of dopamine-related gene expression in the ventral tegmental area of male zebra finches relate to dyadic interactions with long-term female partners

Dopaminergic projections from the ventral tegmental area (VTA) to multiple efferent targets are implicated in pair bonding, yet the role of the VTA in the maintenance of long-term pair bonds is not well characterized. Complex interactions between numerous neuromodulators modify activity in the VTA, suggesting that individual differences in patterns of gene expression in this region may explain individual differences in long-term social interactions in bonded pairs. To test this hypothesis we used RNA-seq to measure expression of over 8000 annotated genes in male zebra finches in established male-female pairs. Weighted gene co-expression network analysis identified a gene module that contained numerous dopamine-related genes with TH found to be the most connected gene of the module. Genes in this module related to male agonistic behaviors as well as bonding-related behaviors produced by female partners. Unsupervised learning approaches identified two groups of males that differed with respect to expression of numerous genes. Enrichment analyses showed that many dopamine-related genes and modulators differed between these groups, including dopamine receptors, synthetic and degradative enzymes, the avian dopamine transporter and several GABA- and glutamate-related genes. Many of the bonding-related behaviors closely associated with VTA gene expression in the two male groups were produced by the male's partner, rather than the male himself. Collectively, results highlight numerous candidate genes in the VTA that can be explored in future studies and raise the possibility that the molecular/genetic organization of the VTA may be strongly shaped by a social partner and/or the strength of the pair bond.

Aggressive behavior and brain neuronal activation in sexually naïve male Mongolian gerbils

Aggressive behavior plays an important role in animal's survival and reproductive success. Although there has been growing interests in studying neural mechanisms underlying aggressive behavior using traditional laboratory animal models, little is known about mechanisms controlling naturally occurring aggression in sexually naïve animals. In the present study, we characterized aggressive behavior displayed by sexually naïve male Mongolian gerbils (Meriones unguiculatus) and examined the subsequent neuronal activation in the brain measured by Fos-immunoreactive (Fos-ir) staining. We found that resident males initiated attacks and showed intense levels of aggression (including chase, bite, offensive sideway, lunge and on-top) towards a conspecific male intruder. Furthermore, attacks from the resident males towards the intruder produced a nonrandom distribution of bites, with the most on the rump, flank, back and tail and few on the limbs, ventrum and head. In contrast, control males that were exposed to a woodblock (control for novelty) never attacked the woodblock and showed higher levels of object/environmental investigation. Male gerbils exposed to an intruder had significantly higher levels of Fos-ir density in the medial (MeA) and anterior cortical (ACo) subnuclei of the amygdala, principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the bed nucleus of the stria terminalis, ventrolateral subdivision of the ventromedial hypothalamus (VMHvl), and paraventricular nucleus of the hypothalamus (PVN), compared to control males. Together, our results indicate that sexually naïve, group housed male gerbils naturally display aggression towards conspecific strangers, and such aggressive behavior is associated with special patterns of neuronal activation in the brain.

Mental Effects of Excess Parathyroid Hormone in Hemodialysis Patients: A Possible Role for Parathyroid 2 Hormone Receptor?

Depression as measured by the kidney disease quality of life (KDQOL) form is known to be an independent risk factor for mortality dialysis patients. Excess parathyroid hormone (PTH) has long been associated with neuropsychiatric disturbances. Those psychiatric complications are currently attributed to hypercalcemia with very little evidence; however, with the discovery of the parathyroid hormone 2 receptor (PTH2R) in the brain which can be activated by PTH, PTH2R might indicate a direct effect of PTH. As secondary and tertiary hyperparathyroidism is common in dialysis patients where the serum calcium is low or normal, we chose to investigate a possible relationship between PTH levels and depression in dialysis patients. This was a matched pair observational study with 10 patients with intact PTH values above 1000 pg/mL who were matched with 10 patients who had PTH values less than 400 pg/mL for the presence of diabetes, years on dialysis, duration of dialysis time, Kt/V, hemoglobin, and 25 OH vitamin D levels, as well as intravenous iron and erythropoietin administration. The Kidney Disease Quality of Life questionnaire (KDQOL-36) scores and patient Health Questionnaire scores were analyzed during that time. All variables underwent tests for normality and matched pair t-test. All subscales of the KDQOL-36 were worse in the high PTH group with the effect on daily life reaching P = 0.01 and the burden of disease and symptoms both reaching P = 0.02. PTH and PTH2R may be appropriate targets for investigations to improve the quality of life in hemodialysis patients.

Consequences of prenatal exposure to valproic acid in the socially monogamous prairie voles

Environmental risk factors contribute to autism spectrum disorders (ASD) etiology. In particular, prenatal exposure to the highly teratogenic anticonvulsant valproic acid (VPA) significantly increases ASD prevalence. Although significant discoveries on the embryopathology of VPA have been reported, its effects on the ability to form enduring social attachment—characteristic of ASD but uncommonly displayed by rats and mice—remains unknown. We aimed to examine the effects of prenatal VPA exposure in the social, monogamous prairie voles (Microtus ochrogaster). Compared to prenatal vehicle-exposed controls, prenatal VPA-exposed prairie voles had lower body weight throughout postnatal development, engaged in fewer social affiliative behaviors in a familial context, exhibited less social interactions with novel conspecifics, and showed enhanced anxiety-like behavior. Along these behavioral deficits, prenatal VPA exposure downregulated prefrontal cortex vasopressin receptor (V1aR) and methyl CpG-binding protein 2 (MeCP2) mRNA expression, but did not alter spine density in adults. Remarkably, adult social bonding behaviors, such as partner preference formation and selective aggression, were not disrupted by prenatal VPA exposure. Collectively, these studies suggest that, in this animal model, VPA alters only certain behavioral domains such as sex-naive anxiety and affiliative behaviors, but does not alter other domains such as social bonding with opposite sex individuals.

Investigation of social, affective, and locomotor behavior of adolescent Brattleboro rats reveals a link between vasopressin's actions on arousal and social behavior

Arginine vasopressin (AVP) has recently been implicated in juvenile and adolescent social development. How AVP influences social development, however, is not understood. Adolescent homozygous Brattleboro rats (Hom), which lack AVP due to a mutation in the Avp gene, exhibit fewer active social behaviors (e.g., social play) but more passive social behaviors (e.g., huddling) than their wild type and heterozygous (Het) littermates, raising the possibility that AVP impacts social development through an arousal mechanism. Here, we test whether the atypical social phenotype of adolescent Hom rats is associated with altered behavioral arousal, social approach, or affective behaviors and whether Brattleboro mothers impact these behavioral phenotypes. Male and female Het and Hom adolescents born to Het or Hom mothers were tested in social interaction, open field, novelty-seeking, social approach, and marble burying tests. As reported previously, Hom rats played less and emitted fewer 50 kHz ultrasonic vocalizations while huddling more than their Het littermates. No genotype differences were detected in novelty seeking or social approach, nor were consistent differences found between offspring from Het and Hom mothers. However, Hom rats were less active in the open field and buried fewer marbles than Het rats indicating a hypoaroused, low anxiety phenotype. Open field activity correlated with levels of social play indicating that the effects of the Brattleboro mutation on arousal and social behavior are linked. These data demonstrate that chronic AVP deficiency impacts behavioral arousal during adolescence and support the hypothesis that AVP influences adolescent social development, in part, through its regulation of arousal.

Developmental trajectories and influences of environmental complexity on oxytocin receptor and vasopressin 1A receptor expression in male and female prairie voles

Nonapeptide receptors, like oxytocin receptor (OTR) and vasopressin 1a receptor (V1aR), modulate a variety of functions across taxa, and mediate phenotypic variation within and between species. Despite the popularity of studying nonapeptides in adults, developmental perspectives on properties of OTR and V1aR expression are lacking. Study of prairie voles (Microtus ochrogaster) has facilitated an understanding of mechanisms of social behavior and provides great potential to inform how early life experiences alter phenotype. We provide the first comprehensive profiling of OTR and V1aR in male and female prairie voles across postnatal development and into adulthood. Differences in receptor densities across the forebrain were region- and sex-specific. Postnatal changes in receptor expression fell into four themes: (a) constant over time, (b) increasing with age, (c) decreasing with age, or (d) peaking during late pre-weaning (postnatal day 15-21). We also examined the influence of post-weaning social and spatial enrichment (i.e., environmental complexity) on OTR and V1aR. Environmental complexity appeared to promote expression of OTR in males and females, and reduced expression of V1aR across several brain regions in males. Our results show that nonapeptide receptor profiles are plastic over development and suggest that different patterns of expression might represent functional differences in sensitivity to nonapeptide activation over a period when social environments are dynamic. Our results on environmental complexity suggest that nonapeptide sensitivity responds flexibly to different environmental contexts during development. Understanding the developmental trajectories of nonapeptide receptors provides a better understanding of the dynamic nature of social behavior and the underlying mechanisms.

Variation in social systems within Chaetodon butterflyfishes, with special reference to pair bonding

For many animals, affiliative relationships such as pair bonds form the foundation of society and are highly adaptive. Animal systems amenable for comparatively studying pair bonding are important for identifying underlying biological mechanisms, but mostly exist in mammals. Better establishing fish systems will enable comparison of pair bonding mechanisms across taxonomically distant lineages that may reveal general underlying mechanistic principles. We examined the utility of wild butterflyfishes (f: Chaetodontidae; g: Chaetodon) for comparatively studying pair bonding. Using stochastic character mapping, we provide the first analysis of the evolutionary history of butterflyfish sociality, revealing that pairing is ancestral, with at least seven independent transitions to gregarious grouping and solitary behavior since the late Miocene. We then formally verified social systems in six sympatric and wide-spread species representing a clade with one ancestrally reconstructed transition from paired to solitary grouping at Lizard Island, Australia. In situ observations of the size, selective affiliation and aggression, fidelity, and sex composition of social groups confirmed that Chaetodon baronessa, C. lunulatus, and C. vagabundus are predominantly pair bonding, whereas C. rainfordi, C. plebeius, and C. trifascialis are predominantly solitary. Even in the predominantly pair bonding species, C. lunulatus, a proportion of adults (15%) are solitary. Importantly, inter- and intra-specific differences in social systems do not co-vary with other previously established attributes, including parental care. Hence, the proposed butterflyfish populations are promising for inter- and intra-species comparative analyses of pair bonding and its mechanistic underpinnings. Avenues for further developing the system are proposed, including determining whether the aforementioned utility of these species applies across their geographic disruptions.

Mechanistic substrates of a life history transition in male prairie voles: Developmental plasticity in affiliation and aggression corresponds to nonapeptide neuronal function

Although prairie vole (Microtus ochrogaster) social behavior is well-characterized in adults, surprisingly little is known about the development of social behavior in voles. Further, the overwhelming majority of studies in prairie voles examine social behavior in a reproductive context. Here, we examine developmental plasticity in affiliation and aggression and their underlying neural correlates. Using sexually naïve males, we characterized interactions with an age-matched, novel, same-sex conspecific in four different age groups that span pre-weaning to adulthood. We found that prosocial behavior decreased and aggression increased as males matured. Additionally, pre-weaning males were more prosocial than nonsocial, whereas post-weaning males were more nonsocial than prosocial. We also examined nonapeptide neural activity in response to a novel conspecific in brain regions important for promoting sociality and aggression using the immediate early gene cFos. Assessment of developmental changes in neural activity showed that vasopressin neurons in the medial bed nucleus of the stria terminalis exhibit functional plasticity, providing a potential functional mechanism that contributes to this change in sociality as prairie voles mature. This behavioral shift corresponds to the transition from a period of allopatric cohabitation with siblings to a period of time when voles disperse and presumably attempt to establish and defend territories. Taken together our data provide a putative mechanism by which brain and behavior prepare for the opportunity to pairbond (characterized by selective affiliation with a partner and aggression toward unfamiliar conspecifics) by undergoing changes away from general affiliation and toward selective aggression, accounting for this important life history event.

Central and peripheral roles of vasopressin in the circadian defense of body hydration

Vasopressin is a neuropeptide synthesized by specific subsets of neurons within the eye and brain. Studies in rats and mice have shown that vasopressin produced by magnocellular neurosecretory cells (MNCs) that project to the neurohypophysis is released into the blood circulation where it serves as an antidiuretic hormone to promote water reabsorption from the kidney. Moreover vasopressin is a neurotransmitter and neuromodulator that contributes to time-keeping within the master circadian clock (i.e. the suprachiasmatic nucleus, SCN) and is also used as an output signal by SCN neurons to direct centrally mediated circadian rhythms. In this chapter, we review recent cellular and network level studies in rodents that have provided insight into how circadian rhythms in vasopressin mediate changes in water intake behavior and renal water conservation that protect the body against dehydration during sleep.

Effects of lateral olfactory tract stimulation on Fos immunoreactivity in vasopressin neurones of the rat piriform cortex

In the main olfactory system, odours are registered at the main olfactory epithelium and are then processed at the main olfactory bulb (MOB) and, subsequently, by the anterior olfactory nucleus (AON), the piriform cortex (PC) and the cortical amygdala. Previously, we reported populations of vasopressin neurones in different areas of the rat olfactory system, including the MOB, accessory olfactory bulb (AOB) and the AON and showed that these are involved in the coding of social odour information. Utilising immunohistochemistry and a transgenic rat in which an enhanced green fluorescent protein reporter gene is expressed in vasopressin neurones (eGFP-vasopressin), we now show a population of vasopressin neurones in the PC. The vasopressin neurones are predominantly located in the layer II of the PC and the majority co-express the excitatory transmitter glutamate. Furthermore, there is no sex difference in the number of neurones expressing vasopressin. Electrical stimulation of the lateral olfactory tract leads to a significant increase in the number of Fos-positive nuclei in the PC, MOB, AOB, dorsal AON and supraoptic nucleus (SON). However, there was only a significant increase in Fos expression in vasopressin cells of the PC and SON. Thus, functionally distinct populations of vasopressin cells are implicated in olfactory processing at multiple stages of the olfactory pathway.

Development of a radioligand for imaging V1a vasopressin receptors with PET

A series of vasopressin receptor V_1a ligands have been synthesized for positron emission tomography (PET) imaging. The lead compound (1S,5R)-1 ((4-(1H-indol-3-yl)-3-methoxyphenyl) ((1S,5R)-1,3,3-trimethyl-6-azabicyclo[3.2.1]octan-6-yl)methanone) and its F-ethyl analog 6c exhibited the best combination of high binding affinity and optimal lipophilicity within the series. (1S,5R)-1 was radiolabeled with ¹¹C for PET studies. [¹¹CH₃](1S,5R)-1 readily entered the mouse (4.7% ID/g tissue) and prairie vole brains (∼2% ID/g tissue) and specifically (30-34%) labeled V_1a receptor. The common animal anesthetic Propofol significantly blocked the brain uptake of [¹¹CH₃](1S,5R)-1 in the mouse brain, whereas anesthetics Ketamine and Saffan increased the uptake variability. Future PET imaging studies with V_1a radiotracers in non-human primates should be performed in awake animals or using anesthetics that do not affect the V_1a receptor.

Imaging, Behavior and Endocrine Analysis of "Jealousy" in a Monogamous Primate

Understanding the neurobiology of social bonding in non-human primates is a critical step in understanding the evolution of monogamy, as well as understanding the neural substrates for emotion and behavior. Coppery titi monkeys (Callicebus cupreus) form strong pair bonds, characterized by selective preference for their pair mate, mate-guarding, physiological and behavioral agitation upon separation, and social buffering. Mate-guarding, or the "maintenance" phase of pair bonding, is relatively under-studied in primates. In the current study, we used functional imaging to examine how male titi monkeys viewing their pair mate in close proximity to a stranger male would change regional cerebral glucose metabolism. We predicted that this situation would challenge the pair bond and induce "jealousy" in the males. Animals were injected with [¹⁸F]-fluorodeoxyglucose (FDG), returned to their cage for 30 min of conscious uptake, placed under anesthesia, and then scanned for 1 hour on a microPET P4 scanner. During the FDG uptake, males (n=8) had a view of either their female pair mate next to a stranger male ("jealousy" condition) or a stranger female next to a stranger male (control condition). Blood and cerebrospinal fluid samples were collected and assayed for testosterone, cortisol, oxytocin, and vasopressin. Positron emission tomography (PET) was co-registered with structural magnetic resonance imaging (MRI), and region of interest analysis was carried out. Bayesian multivariate multilevel analyses found that the right lateral septum (Pr(b>0)=93%), left posterior cingulate cortex (Pr(b>0)=99%), and left anterior cingulate (Pr(b>0)=96%) showed higher FDG uptake in the jealousy condition compared to the control condition, while the right medial amygdala (Pr(b>0)=85%) showed lower FDG uptake. Plasma testosterone and cortisol concentrations were higher during the jealousy condition. During the jealousy condition, duration of time spent looking across at the pair mate next to a stranger male was associated with higher plasma cortisol concentrations. The lateral septum has been shown to be involved in mate-guarding and mating-induced aggression in monogamous rodents, while the cingulate cortex has been linked to territoriality. These neural and physiological changes may underpin the emotion of jealousy, which can act in a monogamous species to preserve the long-term integrity of the pair.

The mushroom body D1 dopamine receptor controls innate courtship drive

Mating is critical for species survival and is profoundly regulated by neuromodulators and neurohormones to accommodate internal states and external factors. To identify the underlying neuromodulatory mechanisms, we investigated the roles of dopamine receptors in various aspects of courtship behavior in Drosophila. Here, we report that the D1 dopamine receptor dDA1 regulates courtship drive in naïve males. The wild‐type naïve males actively courted females regardless their appearance or mating status. On the contrary, the dDA1 mutant (dumb) males exhibited substantially reduced courtship toward less appealing females including decapitated, leg‐less and mated females. The dumb male's reduced courtship activity was due to delay in courtship initiation and prolonged intervals between courtship bouts. The dampened courtship drive of dumb males was rescued by reinstated dDA1 expression in the mushroom body α/β and γ neurons but not α/β or γ neurons alone, which is distinct from the previously characterized dDA1 functions in experience‐dependent courtship or other learning and memory processes. We also found that the dopamine receptors dDA1, DAMB and dD2R are dispensable for associative memory formation and short‐term memory of conditioned courtship, thus courtship motivation and associative courtship learning and memory are regulated by distinct neuromodulatory mechanisms. Taken together, our study narrows the gap in the knowledge of the mechanism that dopamine regulates male courtship behavior.

Dynamic modulation of sociality and aggression: an examination of plasticity within endocrine and neuroendocrine systems

Endocrine and neuroendocrine systems are key mediators of behavioural plasticity and allow for the ability to shift social behaviour across dynamic contexts. These systems operate across timescales, modulating both rapid responses to environmental changes and developmental plasticity in behavioural phenotypes. Thus, not only do endocrine systems mediate behavioural plasticity, but also the systems themselves exhibit plasticity in functional capabilities. This flexibility at both the mechanistic and behavioural levels can be crucial for reproduction and survival. Here, we discuss how plasticity in nonapeptide and steroid actions may influence the expression of, and allow rapid shifts between, sociality and aggression-behavioural shifts that can be particularly important for social interactions. Recent findings of overlap in the mechanisms that modulate social and aggressive behaviour suggest the potential for a mechanistic continuum between these behaviours. We briefly discuss the potential for a sociality-aggression continuum and novel techniques that will enable probing of the functional connectivity of social behaviours. From an evolutionary perspective, we suggest that plasticity in endocrine and neuroendocrine mechanisms of behaviour may be important targets of selection, and discuss the conditions under which we would predict selection to have resulted in differences in endocrine plasticity across species that differ in social organization.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

A single prolonged stress paradigm produces enduring impairments in social bonding in monogamous prairie voles

Traumatic events such as natural disasters, violent crimes, tragic accidents, and war, can have devastating impacts on social relationships, including marital partnerships. We developed a single prolonged stress (SPS) paradigm, which consisted of restraint, forced swimming, and ether anesthesia, to establish an animal model relevant to post-traumatic stress disorder. We applied a SPS paradigm to a monogamous rodent, the prairie vole (Microtus ochrogaster) in order to determine whether a traumatic event affects the establishment of pair bonds. We did not detect effects of the SPS treatment on anhedonic or anxiety-like behavior. Sham-treated male voles huddled with their partner females, following a 6day cohabitation, for a longer duration than with a novel female, indicative of a pair bond. In contrast, SPS-treated voles indiscriminately huddled with the novel and partner females. Interestingly, the impairment of pair bonding was rescued by oral administration of paroxetine, a selective serotonin reuptake inhibitor (SSRI), after the SPS treatment. Immunohistochemical analyses revealed that oxytocin immunoreactivity (IR) was significantly decreased in the supraoptic nucleus (SON), but not in the paraventricular nucleus (PVN), 7days after SPS treatment, and recovered 14days after SPS treatment. After the presentation of a partner female, oxytocin neurons labeled with Fos IR was significantly increased in SPS-treated voles compared with sham-treated voles regardless of paroxetine administration. Our results suggest that traumatic events disturb the formation of pair bond possibly through an interaction with the serotonergic system, and that SSRIs are candidates for the treatment of social problems caused by traumatic events. Further, a vole SPS model may be useful for understanding mechanisms underlying the impairment of social bonding by traumatic events.