Frank A. Beach Award. Oxytocin and vasopressin receptors and species-typical social behaviors

Romantic love: a mammalian brain system for mate choice

Mammals and birds regularly express mate preferences and make mate choices. Data on mate choice among mammals suggest that this behavioural 'attraction system' is associated with dopaminergic reward pathways in the brain. It has been proposed that intense romantic love, a human cross-cultural universal, is a developed form of this attraction system. To begin to determine the neural mechanisms associated with romantic attraction in humans, we used functional magnetic resonance imaging (fMRI) to study 17 people who were intensely 'in love'. Activation specific to the beloved occurred in the brainstem right ventral tegmental area and right postero-dorsal body of the caudate nucleus. These and other results suggest that dopaminergic reward and motivation pathways contribute to aspects of romantic love. We also used fMRI to study 15 men and women who had just been rejected in love. Preliminary analysis showed activity specific to the beloved in related regions of the reward system associated with monetary gambling for uncertain large gains and losses, and in regions of the lateral orbitofrontal cortex associated with theory of mind, obsessive/compulsive behaviours and controlling anger. These data contribute to our view that romantic love is one of the three primary brain systems that evolved in avian and mammalian species to direct reproduction. The sex drive evolved to motivate individuals to seek a range of mating partners; attraction evolved to motivate individuals to prefer and pursue specific partners; and attachment evolved to motivate individuals to remain together long enough to complete species-specific parenting duties. These three behavioural repertoires appear to be based on brain systems that are largely distinct yet interrelated, and they interact in specific ways to orchestrate reproduction, using both hormones and monoamines. Romantic attraction in humans and its antecedent in other mammalian species play a primary role: this neural mechanism motivates individuals to focus their courtship energy on specific others, thereby conserving valuable time and metabolic energy, and facilitating mate choice.

Gene Regulation as a Modulator of Social Preference in Voles

Most mammalian species are nonmonogamous: the female alone cares for the young and males and females do not share nest sites. Within the genus Microtus, there exists ample diversity in social structure for neuroethological and neurobiological investigation. Prairie voles (Microtus ochrogaster) are socially monogamous: both the males and females contribute to care of the young within a shared nest site as a breeding pair through multiple breeding seasons. Closely related species such as the montane (M. montanus) and meadow (M. pennsylvanicus) voles do not typically show these behaviors. Over a decade of research has demonstrated that species differences in neuropeptide systems play significant roles in the behavioral divergence of these species. In particular, species differences in regional gene expression patterns of neuropeptide receptors in the brain mediate some of the behavioral traits associated with the divergence in social structure. Differences in gene expression patterns of a key gene in mediating social behavior, the arginine vasopressin 1a receptor (avpr1a), appear to be due to species divergence in a repeat locus in the 5' regulatory region of avpr1a. This highly repetitive locus is prone to expansion and contraction over relatively short evolutionary timescales and may give rise to the rapid evolution of sociobehavioral traits.

Oxytocin, vasopressin and pair bonding: implications for autism

Understanding the neurobiological substrates regulating normal social behaviours may provide valuable insights in human behaviour, including developmental disorders such as autism that are characterized by pervasive deficits in social behaviour. Here, we review the literature which suggests that the neuropeptides oxytocin and vasopressin play critical roles in modulating social behaviours, with a focus on their role in the regulation of social bonding in monogamous rodents. Oxytocin and vasopressin contribute to a wide variety of social behaviours, including social recognition, communication, parental care, territorial aggression and social bonding. The effects of these two neuropeptides are species-specific and depend on species-specific receptor distributions in the brain. Comparative studies in voles with divergent social structures have revealed some of the neural and genetic mechanisms of social-bonding behaviour. Prairie voles are socially monogamous; males and females form long-term pair bonds, establish a nest site and rear their offspring together. In contrast, montane and meadow voles do not form a bond with a mate and only the females take part in rearing the young. Species differences in the density of receptors for oxytocin and vasopressin in ventral forebrain reward circuitry differentially reinforce social-bonding behaviour in the two species. High levels of oxytocin receptor (OTR) in the nucleus accumbens and high levels of vasopressin 1a receptor (V1aR) in the ventral pallidum contribute to monogamous social structure in the prairie vole. While little is known about the genetic factors contributing to species-differences in OTR distribution, the species-specific distribution pattern of the V1aR is determined in part by a species-specific repetitive element, or 'microsatellite', in the 5' regulatory region of the gene encoding V1aR (avpr1a). This microsatellite is highly expanded in the prairie vole (as well as the monogamous pine vole) compared to a very short version in the promiscuous montane and meadow voles. These species differences in microsatellite sequence are sufficient to change gene expression in cell culture. Within the prairie vole species, intraspecific variation in the microsatellite also modulates gene expression in vitro as well as receptor distribution patterns in vivo and influences the probability of social approach and bonding behaviour. Similar genetic variation in the human AVPR1A may contribute to variations in human social behaviour, including extremes outside the normal range of behaviour and those found in autism spectrum disorders. In sum, comparative studies in pair-bonding rodents have revealed neural and genetic mechanisms contributing to social-bonding behaviour. These studies have generated testable hypotheses regarding the motivational systems and underlying molecular neurobiology involved in social engagement and social bond formation that may have important implications for the core social deficits characterizing autism spectrum disorders.

Zebrafish orthopedia (otp) is required for isotocin cell development

Several behavioral and physiological processes such as social, sexual, and maternal behaviors, learning and memory, and parturition are influenced by the neurohypophysial peptide oxytocin. Studies in knockout mice have identified four transcriptional regulatory genes that are required for oxytocin neuronal development in the hypothalamus. These are the basic helix-loop-helix PAS genes Single-minded 1 (Sim1) and Arylhydrocarbon receptor nuclear translocator 2 (Arnt2), the POU homeobox gene Pou3f2, and the paired homeobox gene Orthopedia (Otp). Overall, however, the molecular control of oxytocin cell development is poorly understood. Studies in zebrafish provide a complementary view to mouse knockout experiments and facilitate understanding of neuroendocrine cell development. Isotocin, which is orthologous to oxytocin, is expressed early in the developing zebrafish brain. In this paper we show that zebrafish otp mRNA expression in the embryonic forebrain is dynamic and complex, and that it overlaps with isotocin expression in the dorsal preoptic area. Additionally, these studies demonstrate that otp is required for isotocin cell development. Evidence is also provided that otp and sim1 function in parallel to direct the differentiation of isotocin cells, and that otp is unlikely to affect brain patterning. Overall, these studies support the hypothesis that the role of otp in zebrafish neuroendocrine cell development is evolutionarily conserved with that of mammals.

The zebrafish bHLH PAS transcriptional regulator, single-minded 1 (sim1), is required for isotocin cell development

A wide range of physiological and behavioral processes, such as social, sexual, and maternal behaviors, learning and memory, and osmotic homeostasis are influenced by the neurohypophysial peptides oxytocin and vasopressin. Disruptions of these hormone systems have been linked to several neurobehavioral disorders, including autism, Prader-Willi syndrome, affective disorders, and obsessive-compulsive disorder. Studies in zebrafish promise to reveal the complex network of regulatory genes and signaling pathways that direct the development of oxytocin- and vasopressin-like neurons, and provide insight into factors involved in brain disorders associated with disruption of these systems. Isotocin, which is homologous to oxytocin, is expressed early, in a simple pattern in the developing zebrafish brain. Single-minded 1 (sim1), a member of the bHLH-PAS family of transcriptional regulatory genes, is required for terminal differentiation of mammalian oxytocin cells and is a master regulator of neurogenesis in Drosophila. Here we show that sim1 is expressed in the zebrafish forebrain and is required for isotocin cell development. The expression pattern of sim1 mRNA in the embryonic forebrain is dynamic and complex, and overlaps with isotocin expression in the preoptic area. We provide evidence that the role of sim1 in zebrafish neuroendocrine cell development is evolutionarily conserved with that of mammals.

Stress Hormone Is Implicated in Satellite‐Caller Associations and Sexual Selection in the Great Plains Toad

The effects of androgens on male-typical traits suggest that variation among males in circulating levels can play a major role in sexual selection. We examined whether variation in vocal attractiveness is attributable to differences in androgen levels among Great Plains toads (Bufo cognatus). We found that noncalling "satellite" males practicing an alternative mating tactic were more likely to associate with males producing long calls. However, callers with satellites did not have higher androgen levels than callers without satellites. Rather, callers with satellites had significantly lower corticosterone (CORT) levels than callers without satellites. A CORT manipulation experiment suggested that differences in calls for males with and without satellites were related to differences in CORT levels. Furthermore, there was a negative correlation between CORT level and call duration within most nights of chorus activity. However, the correlation was weak for the pooled data (across all nights), suggesting that local environmental and/or social factors also affect call duration. Last, we show that females preferred broadcast calls of longer duration, characteristic of males with satellites and low CORT. These results imply that satellites optimize their reproductive success by associating with males producing long calls. However, this association should negatively affect the fitness of attractive callers.

Vasopressin and pair-bond formation: genes to brain to behavior

Microtine rodents provide an excellent model for the study of the neurobiology of social bonds. In this review, we discuss how the presence of a microsatellite sequence in the prairie vole vasopressin receptor gene may determine vasopressin receptor binding patterns in the brain and how these patterns may in turn affect social behavior.

Mammalian monogamy is not controlled by a single gene

Complex social behavior in Microtus voles and other mammals has been postulated to be under the direct genetic control of a single locus: the arginine vasopressin 1a receptor (avpr1a) gene. Using a phylogenetic approach, we show that a repetitive element in the promoter region of avpr1a, which reportedly causes social monogamy, is actually widespread in nonmonogamous Microtus and other rodents. There was no evidence for intraspecific polymorphism in regard to the presence or absence of the repetitive element. Among 25 rodent species studied, the element was absent in only two closely related nonmonogamous species, indicating that this absence is certainly the result of an evolutionarily recent loss. Our analyses further demonstrate that the repetitive structures upstream of the avpr1a gene in humans and primates, which have been associated with social bonding, are evolutionarily distinct from those in rodents. Our evolutionary approach reveals that monogamy in rodents is not controlled by a single polymorphism in the promoter region of the avpr1a gene. We thus resolve the contradiction between the claims for an evolutionarily conserved genetic programming of social behavior in mammals and the vast evidence for highly complex and flexible mating systems.

Distribution of corticotropin-releasing factor and urocortin 1 in the vole brain

Brain receptor patterns for the corticotropin-releasing factor (CRF) receptors, CRF1 and CRF2, are dramatically different between monogamous and promiscuous vole species, and CRF physiologically regulates pair bonding behavior in the monogamous prairie vole. However, it is uncertain whether species differences also exist in the neuroanatomical distribution of the endogenous ligands for the CRF1 and CRF2 receptors, such as CRF and urocortin-1 (Ucn1). We compared the expression of CRF and Ucn1 in four vole species, monogamous prairie and pine voles, and promiscuous meadow and montane voles, using in situ hybridization of CRF and Ucn1 mRNA. Our results reveal that CRF mRNA expression patterns in all four vole species appear highly conserved throughout the brain, including the olfactory bulb, nucleus accumbens, bed nucleus of the stria terminalis, medial preoptic area, central amygdala, hippocampus, posterior thalamus, and cerebellum. Similarly, Ucn1 mRNA primarily localized to the Edinger-Westphal nucleus in all four vole species. Immunocytochemistry in prairie and meadow voles confirmed localization of CRF and Ucn1 protein to these previously identified brain regions. These data demonstrate a striking dichotomy between the extraordinary species diversity of brain receptor patterns when compared to the highly conserved brain distributions of their respective ligands. Our findings generate novel hypotheses regarding the evolutionary mechanisms underlying the neural circuitry of species-typical social behaviors.

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

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

Social and sexual motivation in the mouse

Does a sexual encounter have reward value for a learned operant response? Ovariectomized female mice with or without estradiol replacement were trained to perform a bar-contact operant response for either male or female targets. Response rates of females with estradiol replacement did not differ from those of females without estradiol replacement or females responding for access to females. Reflexive receptive sexual behavior remained responsive to estradiol replacement. Experiment 2 demonstrated that socially isolated females would respond faster for access to a female target than when group housed. Finally, the oxytocin blocker, atosiban, reduced both operant and reflexive social behavior. These results converge on the conclusion that the operant reward value of social and sexual contact is primarily social.

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

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

Association of vasopressin 1a receptor levels with a regulatory microsatellite and behavior

Vasopressin regulates complex behaviors such as anxiety, parenting, social engagement and attachment and aggression in a species-specific manner. The capacity of vasopressin to modulate these behaviors is thought to depend on the species-specific distribution patterns of vasopressin 1a receptors (V1aRs) in the brain. There is considerable individual variation in the pattern of V1aR binding in the brains of the prairie vole species, Microtus ochrogaster. We hypothesize that this individual variability in V1aR expression levels is associated with individual variation in a polymorphic microsatellite in the 5' regulatory region of the prairie vole v1ar gene. Additionally, we hypothesize that individual variation in V1aR expression contributes to individual variation in vasopressin-dependent behaviors. To test these hypotheses, we first screened 20 adult male prairie voles for behavioral variation using tests that measure anxiety-related and social behaviors. We then assessed the brains of those animals for V1aR variability with receptor autoradiography and used polymerase chain reaction to genotype the same animals for the length of their 5' microsatellite polymorphism in the v1ar gene. In this report, we describe the results of this discovery-based experimental approach to identify potential gene, brain and behavior interrelationships. The analysis reveals that V1aR levels, in some but not all brain regions, are associated with microsatellite length and that V1aR levels in those and other brain regions correlate with anxiety-related and social behaviors. These results generate novel hypotheses regarding neural control of anxiety-related and social behaviors and yield insight into potential mechanisms by which non-coding gene polymorphisms may influence behavioral traits.

Genes, brains and mammalian social bonds

Recent studies of monogamous species have revealed the role of the neuropeptides oxytocin and vasopressin in activating reward mechanisms of the brain that are involved in establishing partner recognition and selective 'bonding'. The evolutionary history of these findings resides, at a mechanistic level, in the reciprocal bonding between mother and infant that is common to all mammals. However, in Old World primates, where mother and infant alone would not survive, living in large social groups brings extended family relationships and provides for alloparenting. This has required the emancipation of parenting behaviour from the constraints of hormonal state and the evolution of large brains for decision making that was previously restricted and determined by hormonal state. How this has been achieved, what conserved mechanisms underpin social bonding, and what genetic and mechanistic changes have occurred in the evolution of social bonds are the issues addressed here.

APeg3, a novel paternally expressed gene 3 antisense RNA transcript specifically expressed in vasopressinergic magnocellular neurons in the rat supraoptic nucleus

Vasopressin (VP) and oxytocin (OT) play critical roles in the regulation of salt and water balance, lactation, and various behaviors and are expressed at very high levels in specific magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS). In addition to the cell-specific expression of the VP and OT genes in these cells, there are other transcripts that are preferentially expressed in the VP or OT MCNs. One such gene, paternally expressed gene 3 (Peg3), is an imprinted gene expressed exclusively from the paternal allele that encodes a Kruppel-type zinc finger-containing protein involved in maternal behavior and is abundantly expressed in the VP-MCNs. We report here the robust expression in the VP-MCNs of an RNA, which we designate APeg3 that is transcribed in the antisense direction to the 3' untranslated region of the Peg3 gene. The APeg3 mRNA is about 1 kb in size, and the full-length sequence of APeg3, as determined by 5' and 3' RACE, contains an open reading frame that predicts a protein of 93 amino acids and is predominantly expressed in VP-MCNs. Both Peg3 and APeg3 gene expression in the VP-MCNs increase during systemic hyperosmolality in vivo, demonstrating that both of these genes are osmoregulated.

Vasopressin-dependent neural circuits underlying pair bond formation in the monogamous prairie vole

Arginine vasopressin and its V1a receptor subtype (V1aR) are critical for pair bond formation between adult prairie voles. However, it is unclear which brain circuits are involved in this vasopressin-mediated facilitation of pair bond formation. Here, we examined mating-induced Fos expression in several brain regions involved in sociosexual and reward circuitry in male prairie voles. Consistent with studies in other species, Fos expression was induced in several regions known to be involved in sociosexual behavior, namely, the medial amygdala, bed nucleus of the stria terminalis, and medial preoptic area. Fos induction also occurred in limbic and reward regions, including the ventral pallidum, nucleus accumbens, and mediodorsal thalamus (MDthal). Next, we infused a selective V1aR antagonist into three candidate brain regions that seemed most likely involved in vasopressin-mediated pair bond formation: the ventral pallidum, medial amygdala, and MDthal. Blockade of V1aR in the ventral pallidum, but not in the medial amygdala or MDthal, prevented partner preference formation. Lastly, we demonstrated that the mating-induced Fos activation in the ventral pallidum was vasopressin-dependent, since over-expression of V1aR using viral vector gene transfer resulted in a proportionate increase in mating-induced Fos in the same region. This is the first study to show that vasopressin neurotransmission occurs in the ventral pallidum during mating, and that V1aR activation in this region is necessary for pair bond formation in male prairie voles. The results from this study have profound implications for the neural circuitry underlying social attachment and generate novel hypotheses regarding the neural control of social behavior.

Enhanced partner preference in a promiscuous species by manipulating the expression of a single gene

The molecular mechanisms underlying the evolution of complex behaviour are poorly understood. The mammalian genus Microtus provides an excellent model for investigating the evolution of social behaviour. Prairie voles (Microtus ochrogaster) exhibit a monogamous social structure in nature, whereas closely related meadow voles (Microtus pennsylvanicus) are solitary and polygamous. In male prairie voles, both vasopressin and dopamine act in the ventral forebrain to regulate selective affiliation between adult mates, known as pair bond formation, as assessed by partner preference in the laboratory. The vasopressin V1a receptor (V1aR) is expressed at higher levels in the ventral forebrain of monogamous than in promiscuous vole species, whereas dopamine receptor distribution is relatively conserved between species. Here we substantially increase partner preference formation in the socially promiscuous meadow vole by using viral vector V1aR gene transfer into the ventral forebrain. We show that a change in the expression of a single gene in the larger context of pre-existing genetic and neural circuits can profoundly alter social behaviour, providing a potential molecular mechanism for the rapid evolution of complex social behaviour.

Effects of central vasotocin and mesotocin manipulations on social behavior in male and female zebra finches

Male and female zebra finches (Taeniopygia guttata; total n = 40) were fitted with chronic guide cannulae directed at the lateral ventricle and were tested for aggression, affiliation, and partner preference following infusions of mesotocin (MT), vasotocin (VT), their antagonists, and vehicle control. Aggressive behavior was tested in a mate competition paradigm and tests of intersexual affiliation and partner preference were conducted following 1 day of cohabitation with an opposite-sex individual. These tests also provided data on male courtship singing. The results demonstrate a modest dose-dependent facilitation of aggression by VT, but not MT, in both male and female finches. However, only males were sensitive to infusions of a vasopressin antagonist, suggesting that endogenous VT is more important for behavioral modulation in males. Peptide effects were specific to aggression, as no treatments influenced intersexual affiliation, partner preference, or male courtship singing. Thus, in contrast to rodents, partner preference is not readily induced by VT or MT in this species. However, the potential necessity of endogenous VT and MT for natural pair-bond formation remains to be tested.

The effects of neonatal castration on the subsequent behavioural response to centrally administered arginine vasopressin and the expression of V1a receptors in adult male prairie voles

Centrally administered arginine vasopressin induces the formation of partner preferences in male prairie voles (Microtus ochrogaster). The expression of many vasopressin-regulated behaviours is testosterone dependent. In this study, we tested the hypothesis that early exposure to gonadal steroids are necessary to establish the typical response of adult male prairie voles to exogenous vasopressin, predicting that adult males which were castrated neonatally would not form partner preferences in response to centrally administered vasopressin. We also examined the effect of neonatal castration on the expression of vasopressin (V1a) receptors. Voles were castrated on the day of birth (NEOCAST), sham-castrated on the day of birth (NEOSHAM) or castrated as adults (ADULTCAST). With the exception of one group of neonatal sham males (NEOSHAM CON), which served as a control for the effects of vasopressin, as adults, all males received a 1- micro l intracerebroventricular injection of vasopressin (100 ng) in artificial cerebrospinal fluid. In addition, 2 weeks before testing, one group of neonatally castrated males received an implant of testosterone propionate (NEOCAST + TP). Between 60 and 90 days of age, an internal cannula was placed in the lateral cerebral ventricle and, 24 h later, males were injected with vasopressin. Subsequently, after an additional 15 min, males were cohabitated with a female 'partner' for 1 h. Immediately following cohabitation, males were placed in a Y-shaped partner preference test apparatus for 3 h, in which the male had access to the 'partner' and a novel female, 'stranger.' Time spent with the partner versus the stranger was compared within and between treatments. The results were found to support our hypothesis as the NEOSHAM and ADULTCAST males formed partner preferences, spending more time with the partner, and they spent significantly more time with their partner than did NEOSHAM CON, NEOCAST or NEOCAST + TP males. Replacement of testosterone in neonatally castrated males did not restore partner preference formation in response to vasopressin in adult males. Finally, neonatal castration did not affect the distribution of V1a receptors.

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

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

Transgenesis and the study of expression, cellular targeting and function of oxytocin, vasopressin and their receptors

The neuropeptides oxytocin and vasopressin and the neurons in the hypothalamus that synthesize them have been a rich source for the exploration and understanding of both the brain and the endocrine system. Because of their large size and compact nuclear organization the magnocellular neurons of the hypothalamoneurohypophysial system have traditionally attracted scientists using state-of-the-art techniques, including the subject of this review, transgenesis. We discuss the role of transgenics in deciphering gene elements necessary for the appropriate expression of oxytocin and vasopressin and to deliver exogenous genes, such as green fluorescent protein, selectively to secretory granules in the neurons in the hypothalamoneurohypophysial system. Finally, we review the studies of mice whose genes for oxytocin and, most recently, for the oxytocin and vasopressin receptors have been knocked out through homologous recombination.

Developmental consequences of oxytocin

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

Putative isotocin distributions in sonic fish: relation to vasotocin and vocal-acoustic circuitry

Recent neurophysiological evidence in the plainfin midshipman fish (Porichthys notatus) demonstrated that isotocin (IT) and arginine vasotocin (AVT) modulate fictive vocalizations divergently between three reproductive morphs. To provide an anatomical framework for the modulation of vocalization by IT and to foster comparisons with the distributions of the IT homologues mesotocin (MT) and oxytocin (OT) in other vertebrate groups, we describe putative IT distributions in the midshipman and the closely related gulf toadfish, Opsanus beta. Double-label fluorescent histochemistry was used for IT and AVT (by using antibodies for MT, OT, and the mammalian AVT homologue, arginine vasopressin [AVP]). MT/OT-like immunoreactive (MT/OT-lir) cell groups were found in the anterior parvocellular, posterior parvocellular, and magnocellular preoptic nuclei. MT/OT-lir fibers and putative terminals densely innervated the ventral telencephalon and numerous areas in the hypothalamus and brainstem. These distributions included all sites of vocal-acoustic integration recently identified for the forebrain and midbrain and diencephalic components of the ascending auditory pathway. Results were qualitatively comparable across morphs, species, and seasons. In contrast to the widespread distribution of MT/OT-lir, AVP-lir somata, fibers, and putative terminals were almost completely restricted to vocal-acoustic regions. These data parallel previous descriptions of AVT immunoreactivity in these species, although the present methods showed a previously undescribed, seasonally variable AVP-lir cell group in the anterior tuberal hypothalamus, a vocally active site and a component of the ascending auditory pathway. These findings provided anatomic support for the role of IT and AVT in the modulation of vocal behavior at multiple levels of the central vocal-acoustic circuitry.

Vasopressin and oxytocin decrease excitatory amino acid release in adult rat supraoptic nucleus

Oxytocin and vasopressin reduce the amplitude of excitatory postsynaptic responses in magnocellular neuroendocrine cells of the supraoptic nucleus (SON). To test whether synaptic glutamate release is modulated by these neuropeptides, we examined the combined effect of vasopressin and oxytocin on depolarization-induced glutamate and aspartate release from acutely dissected rat SON or fronto-parietal cortex punches. Glutamate release was stimulated with 60 mm K+ for 5-10 min and measured using ion exchange chromatography or high-performance liquid chromatography. During depolarization with high K+, extracellular glutamate levels increased, on average, to 204% of control values. In the presence of vasopressin/oxytocin, K+-stimulated glutamate and aspartate release were significantly reduced by 34% and 62%, respectively, in the SON. Treatment with the aminopeptidase inhibitor amastatin did not mimic the effects of exogenous vasopressin/oxytocin on glutamate or aspartate release, suggesting that, under the conditions tested here, amastatin treatment may produce more complex effects. The effects of exogenous neuropeptides are likely mediated by oxytocin and/or vasopressin receptors, as the oxytocin- and V1a-receptor antagonist, Manning Compound (10-100 micro m), partially reversed the effects of vasopressin/oxytocin on SON glutamate release. In contrast, in cortical punches, glutamate release was enhanced by high K+, but vasopressin/oxytocin did not significantly reduce glutamate/aspartate release, consistent with the relatively sparse distribution of vasopressin/oxytocin receptors in fronto-parietal cortex. These findings suggest that locally released oxytocin and vasopressin may autoregulate SON magnocellular neuroendocrine cell activity in part by modulating the release of excitatory amino acids from afferent terminals targeting these cells and/or from other cellular sources.

Ventral striatopallidal oxytocin and vasopressin V1a receptors in the monogamous prairie vole (Microtus ochrogaster)

Oxytocin receptors (OTR) and vasopressin V1a receptors (V1aR) in the ventral forebrain play critical roles in the formation of pair bonds in the monogamous prairie vole. Previous reports have been inconsistent in the identification of the specific brain regions in the ventral forebrain that express these receptors. To delineate more clearly the neuroanatomical boundaries of the OTR and V1aR fields in this species, we compared OTR and V1aR binding in adjacent brain sections and also with markers that delineate neuroanatomical boundaries in the ventral forebrain. OTR binding displayed an overlapping distribution with substance P mRNA and preproenkephalin mRNA, both markers for the shell and core of the nucleus accumbens. V1aR binding was nonoverlapping with each of these markers but colocalized with iron accumulation as shown by Perls' iron stain as well as leucine-enkephalin immunoreactivity, both markers for the ventral pallidum. OTR and V1aR mRNA were also restricted within the nucleus accumbens and ventral pallidum, respectively. Furthermore, destruction of ventral striatal dopaminergic terminals with 6-hydroxydopamine infusions into the nucleus accumbens did not alter OTR binding. Immunocytochemical analysis of oxytocin and vasopressin in the ventral forebrain demonstrated the presence of oxytocin-immunoreactive fibers in the nucleus accumbens and vasopressin-immunoreactive fibers in the ventral pallidum, with males showing a greater density of vasopressin fibers than females, but there was no such sex difference in the oxytocin system. Based on these results, we discuss potential neural mechanisms by which receptors in these brain regions mediate pair bond formation in this monogamous species. J. Comp. Neurol. 468:555-570, 2004.

Variation in the vasopressin V1a receptor promoter and expression: implications for inter- and intraspecific variation in social behaviour

Instability in highly repetitive microsatellite DNA located in the regulatory regions of genes may be a major factor producing diversity in both region-specific gene expression and the resulting phenotypes. Polymorphisms in promoter regions affecting expression of genes involved in regulating behaviour may play a role in generating individual variation in behaviour, including psychopathologies in humans, and probably are also important for the evolution of behaviour. Here we discuss the prairie vole vasopressin V1a receptor gene as a model that may be useful for understanding the evolution of promoter sequences and the relationship between gene sequence, expression and behavioural phenotype.

Social experience and social context alter the behavioral response to centrally administered oxytocin in female Syrian hamsters

The type of social behavior displayed by an individual is profoundly influenced by its immediate social environment or context and its prior social experience. Although oxytocin is important in the expression of social behavior in several species, it is not known if social factors alter the ability of oxytocin to influence behavior. The purpose of the present study was to test the hypothesis that social experience and social context alter the ability of oxytocin to regulate flank marking (a form of scent marking) in female Syrian hamsters. Oxytocin was microinjected into the medial preoptic anterior hypothalamic continuum (MPOA-AH) of socially experienced, dominant female hamsters which were then tested with either a subordinate partner, with a novel partner, or alone. Oxytocin induced flank marking in a dose-dependent manner but only when the experienced dominant hamsters were tested with their familiar, subordinate partners. Oxytocin did not induce flank marking when injected into socially naive female hamsters that were tested with an opponent or alone. In males, by contrast, oxytocin induced flank marking in dominant hamsters when they were tested with their subordinate partner or alone. These data support the hypothesis that social experience and social context interact to regulate the ability of oxytocin to stimulate flank marking by its actions in the MPOA-AH in female hamsters.

The neurobiology of social recognition, approach, and avoidance

Rodent models of social behavior provide powerful experimental tools for elucidating the molecular, cellular, and neurobiological mechanisms regulating social behavior. Here I discuss several rodent models that have been particularly useful in understanding the neurobiology of the discrimination of social verses nonsocial stimuli, affiliative behavior, and social avoidance. The oxytocin knockout mouse model has been useful for understanding how, in the context of social recognition, the brain may process social stimuli differently from nonsocial stimuli. Vole species that are either highly social and monogamous or solitary and promiscuous have provided a model for investigating the brain mechanisms involved in promoting social interactions. Comparative studies in these species strongly implicate the neuropeptides oxytocin and vasopressin in the regulation of affiliative behavior as well as social attachment. A conditioned defeat model in hamsters may provide a useful model to understand how adverse social experiences may facilitate social avoidance. These models have yielded valuable insights into the regulation of social behaviors, and the findings of these studies may prove useful in understanding the neural mechanisms that underlie individual differences in human personality traits.

Oxytocin: who needs it?

The neuropeptide oxytocin has been implicated in the initiation of maternal behavior, based on studies in rats and sheep. Females in both of these species naturally avoid infants until parturition when they begin to show an intense interest in maternal care. Oxytocin pathways in the brain appear to be important for this transition from avoidance to approach of the young. Recent studies in mice with a null mutation of the oxytocin gene suggest a different scenario. These mice, which completely lack oxytocin, exhibit full maternal and reproductive behavior, except for a deficit in milk ejection. Apparently, oxytocin is not essential for maternal behavior in this species. Consistent with the role of oxytocin for the transition from avoidance to approach in rats and sheep, nulliparous mice show full maternal behavior and therefore do not require the peptide for the initiation of maternal care. The species differences in the behavioral effects of oxytocin are associated with profound species differences in the location of oxytocin receptors in the brain. Recent transgenic studies suggest that these species differences in the neuroanatomical distribution of oxytocin receptors may be a function of inter-species variation in the flanking region of the oxytocin receptor gene. So, who needs oxytocin? For maternal care, not mice and (possibly) other species, like primates, with promiscuous parental care. Most important, in considering the behavioral or cognitive functions of oxytocin, one cannot accurately extrapolate across species unless one knows the species have the same neuroanatomical location of oxytocin receptors.

Facilitation of affiliation and pair-bond formation by vasopressin receptor gene transfer into the ventral forebrain of a monogamous vole

Behaviors associated with monogamy, including pair-bond formation, are facilitated by the neuropeptide vasopressin and are prevented by a vasopressin receptor [V1a receptor (V1aR)] antagonist in the male prairie vole. The neuroanatomical distribution of V1aR dramatically differs between monogamous and nonmonogamous species. V1aR binding is denser in the ventral pallidal region of several unrelated monogamous species compared with nonmonogamous species. Because the ventral pallidum is involved in reinforcement and addiction, we hypothesize that V1aR activation in this region promotes pair-bond formation via a mechanism similar to conditioning. Using an adeno-associated viral vector to deliver the V1aR gene, we increased the density of V1aR binding in the ventral pallial region of male prairie voles. These males exhibited increased levels of both anxiety and affiliative behavior compared with control males. In addition, males overexpressing the V1aR in the ventral pallidal region, but not control males, formed strong partner preferences after an overnight cohabitation, without mating, with a female. These data demonstrate a role for ventral pallidal V1aR in affiliation and social attachment and provide a potential molecular mechanism for species differences in social organization.

Hormonal changes in mammalian fathers

Known and hypothesized relationships between steroid (estradiol, testosterone, and cortisol) and peptide (oxytocin, vasopressin, and prolactin) hormones and the expression of mammalian paternal behavior are reviewed. Emphasis is placed on newly emerging animal models, including nonhuman primates and men, with elaborate paternal behavior repertoires. Currently available data are broadly consistent with a working hypothesis that the expression of parental behavior will involve homologous neuroendocrine circuits in male and females. Understanding the neuroendocrinology of paternal behavior is an emerging research opportunity in behavioral neuroscience.

Cellular mechanisms of social attachment

Pharmacological studies in prairie voles have suggested that the neuropeptides oxytocin and vasopressin play important roles in behaviors associated with monogamy, including affiliation, paternal care, and pair bonding. Our laboratory has investigated the cellular and neuroendocrine mechanisms by which these peptides influence affiliative behavior and social attachment in prairie voles. Monogamous prairie voles have a higher density of oxytocin receptors in the nucleus accumbens than do nonmonogamous vole species; blockade of these receptors by site-specific injection of antagonist in the female prairie vole prevents partner preference formation. Prairie voles also have a higher density of vasopressin receptors in the ventral pallidal area, which is the major output of the nucleus accumbens, than montane voles. Both the nucleus accumbens and ventral pallidum are key relay nuclei in the brain circuits implicated in reward, such as the mesolimbic dopamine and opioid systems. Therefore, we hypothesize that oxytocin and vasopressin may be facilitating affiliation and social attachment in monogamous species by modulating these reward pathways.

Manipulations of the AVT system shift social status and related courtship and aggressive behavior in the bluehead wrasse

Arginine vasotocin (AVT) and its mammalian homologoue arginine vasopressin (AVP) influence male sexual and aggressive behaviors in many species. We tested the effects of AVT and an AVP-V(1a) receptor antagonist on the display of alternative male tactics in a tropical coral reef fish, the bluehead wrasse Thalassoma bifasciatum. We gave AVT injections to territorial and nonterritorial males of the large and colorful phenotype (terminal phase) and an AVP-V(1a) receptor antagonist, Manning compound, to territorial males in the field. AVT increased courtship independent of status, while its effects on territoriality and aggression were dependent upon male status. In territorial males, AVT increased courtship and tended to decrease the number of chases toward initial phase individuals. In nonterritorial males, AVT increased courtship, chases toward initial phase individuals, and territorial behavior while decreasing feeding. These are all behaviors rarely seen in nonterritorial males, so AVT made these males act like territorial TP males. The AVP-V(1a) receptor antagonist had opposite effects. It decreased courtship and territorial defense, making these males act more like nonterritorial males. Manipulations of the AVT system shifted males within a single phenotype from the nonterritorial social status to the territorial social status and vice versa. Since the entire suite of behaviors related to territoriality was affected by AVT system manipulations, our results suggest that the AVT system may play a key role in motivation of behaviors related to mating.

Social behavior functions and related anatomical characteristics of vasotocin/vasopressin systems in vertebrates

The neuropeptide arginine vasotocin (AVT; non-mammals) and its mammalian homologue, arginine vasopressin (AVP) influence a variety of sex-typical and species-specific behaviors, and provide an integrational neural substrate for the dynamic modulation of those behaviors by endocrine and sensory stimuli. Although AVT/AVP behavioral functions and related anatomical features are increasingly well-known for individual species, ubiquitous species-specificity presents ever increasing challenges for identifying consistent structure-function patterns that are broadly meaningful. Towards this end, we provide a comprehensive review of the available literature on social behavior functions of AVT/AVP and related anatomical characteristics, inclusive of seasonal plasticity, sexual dimorphism, and steroid sensitivity. Based on this foundation, we then advance three major questions which are fundamental to a broad conceptualization of AVT/AVP social behavior functions: (1) Are there sufficient data to suggest that certain peptide functions or anatomical characteristics (neuron, fiber, and receptor distributions) are conserved across the vertebrate classes? (2) Are independently-evolved but similar behavior patterns (e.g. similar social structures) supported by convergent modifications of neuropeptide mechanisms, and if so, what mechanisms? (3) How does AVT/AVP influence behavior - by modulation of sensorimotor processes, motivational processes, or both? Hypotheses based upon these questions, rather than those based on individual organisms, should generate comparative data that will foster cross-class comparisons which are at present underrepresented in the available literature.

The neurobiology of attachment

It is difficult to think of any behavioural process that is more intrinsically important to us than attachment. Feeding, sleeping and locomotion are all necessary for survival, but humans are, as Baruch Spinoza famously noted, "a social animal" and it is our social attachments that we live for. Over the past decade, studies in a range of vertebrates, including humans, have begun to address the neural basis of attachment at a molecular, cellular and systems level. This review describes some of the important insights from this work.