Species differences in central oxytocin receptor gene expression: comparative analysis of promoter sequences

How the brain processes social information: searching for the social brain

Because information about gender, kin, and social status are essential for reproduction and survival, it seems likely that specialized neural mechanisms have evolved to process social information. This review describes recent studies of four aspects of social information processing: (a) perception of social signals via the vomeronasal system, (b) formation of social memory via long-term filial imprinting and short-term recognition, (c) motivation for parental behavior and pair bonding, and (d) the neural consequences of social experience. Results from these studies and some recent functional imaging studies in human subjects begin to define the circuitry of a "social brain." Such neurodevelopmental disorders as autism and schizophrenia are characterized by abnormal social cognition and corresponding deficits in social behavior; thus social neuroscience offers an important opportunity for translational research with an impact on public health.

Profound impairment in social recognition and reduction in anxiety-like behavior in vasopressin V1a receptor knockout mice

Considerable evidence suggests that arginine vasopressin (AVP) is critically involved in the regulation of many social and nonsocial behaviors, including emotionality. The existence of two AVP receptors in the brain, namely the V1a and V1b subtypes, and the lack of clear pharmacological data using selective agonists or antagonists, make it difficult to determine which receptor is responsible for the AVP-mediated effects on behavior. Here we report the behavioral effects of a null mutation in the V1a receptor (V1aR) in male mice. Male mice lacking functional V1aR (V1aRKO) exhibit markedly reduced anxiety-like behavior and a profound impairment in social recognition. V1aRKO performed normally on spatial and nonsocial olfactory learning and memory tasks. Acute central administration of AVP robustly stimulated stereotypical scratching and autogrooming in wild-type (WT), but not V1aRKO males. AVP and oxytocin (OT) mRNA and OT receptor-binding levels were similar in WT and V1aRKO mice. Given the current findings, the V1aR may provide a novel potential pharmacological target for social and affective disorders including autism, and anxiety disorders.

The prairie vole (Microtus ochrogaster): an animal model for behavioral neuroendocrine research on pair bonding

Pair bond formation has been investigated much less than many other social behaviors, perhaps in part because traditional laboratory mice and rats do not exhibit this behavior. However, pair bonding is common among monogamous animals such as the prairie vole (Microtus ochrogaster). In this review, we discuss how the prairie vole has been used as a model system to investigate the neurobiology of pair bonding. Descriptions include neuroanatomical differences between monogamous and non-monogamous voles, as well as how manipulations of vasopressin, oxytocin, dopamine, and corticosterone systems affect pair bond formation. Also summarized are potential interactions among these systems that regulate pair bonding, and the extent of sexual dimorphism in underlying mechanisms. Pair bonding in prairie voles is an excellent model system for studying central processing of social information. Understanding the mechanisms underlying this behavior may provide important insights into human disorders associated with impaired social functioning.

Is social attachment an addictive disorder?

There is a considerable literature on the neurobiology of reward, based largely on studies of addiction or substance abuse. This review considers the possibility that the neural circuits that mediate reward evolved for ethologically relevant cues, such as social attachment. Specifically, mesocorticolimbic dopamine appears important for maternal behavior in rats and pair bonding in monogamous voles. It is not yet clear that dopamine in this pathway mediates the hedonic properties of social bond formation or whether dopamine's role is more relevant to developing associative networks or assigning salience to social stimuli. The neuropeptides oxytocin (OT) and vasopressin (AVP) appear to be critical for linking social signals to the mesocorticolimbic circuit.

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.

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.

Vasopressin in the lateral septum regulates pair bond formation in male prairie voles (Microtus ochrogaster)

Male prairie voles (Microtus ochrogaster) form a pair bond with a female partner after mating, and this behavior is regulated by the neuropeptide vasopressin (AVP). The authors report that AVP in the lateral septum is important for pair bond formation. Administration of an AVP V1a receptor antagonist in the lateral septum blocked mating-induced pair bonding, whereas administration of AVP induced this behavior in the absence of mating. In addition, administration of an oxytocin (OT) receptor antagonist in the lateral septum also blocked pair bond formation induced by either mating or AVP administration, suggesting that the OT receptor blockade may have interfered with the AVP regulation of behavior. Together, these data provide evidence suggesting that AVP in the lateral septum regulates pair bond formation in male prairie voles and that this process requires access to both AVP and OT receptors.

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.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

Toward a Neurobiology of Attachment

Although much has been written about the psychological aspects of attachment, still relatively little is known about how the brain organizes this complex behavior or how attachment alters brain organization. Recent research has suggested that neuropeptides may be important as neural modulators for several complex behaviors. This review examines the research on one specific neuropeptide, oxytocin, to describe approaches to the neurobiology of attachment at several levels, including molecular, cellular, and whole animal studies. A key to understanding oxytocin's role in behavior is a careful analysis of the pattern of oxytocin receptors in the brain. The available data support the hypothesis that oxytocin is critical for maternal behavior and pair-bond formation in select nonhuman animals. Humans have oxytocin and brain oxytocin receptors, but the role of this neuropeptide system in human attachment remains highly speculative.

Hypothalamic vasopressin gene expression increases in both males and females postpartum in a biparental rodent

In previous studies, the closely related neuropeptide hormones oxytocin and vasopressin have been implicated in the central mediation of parental behaviour. Several studies in rats and sheep have demonstrated a role for oxytocin in the initiation of maternal behaviour. Recently, a few studies in a biparental species, the prairie vole (Microxytocinus ochrogaster) have suggested that vasopressin is important for paternal care. The present study investigated this latter possibility by measuring changes in vasopressin and oxytocin hypothalamic gene expression 1 day and 6 days following parturition in prairie voles which show paternal care and in montane voles (M. montanus) which lack paternal care. In prairie voles, vasopressin gene expression increased in both males and females postpartum, relative to sexually naive controls. In the non-paternal montane vole, no change in vasopressin gene expression was observed in either sex. In contrast to this species difference in vasopressin gene expression, hypothalamic oxytocin gene expression increased in both prairie and montane vole females, but not in males of either species. To augment measures of gene expression, we assessed vasopressin (V1a) and oxytocin receptor binding in both species. Although forebrain vasopressin V1a receptor binding was not altered following parturition in either species, oxytocin receptor binding increased in the ventromedial nucleus of the hypothalamus in females, but not males, in both prairie and montane voles. In summary, vasopressin gene expression increases in both males and females postpartum in a biparental species and oxytocin gene expression and receptor binding increase selectively in females. These results are consistent with earlier reports of a role for vasopressin in paternal care and for oxytocin in maternal behaviour.

The role of sex steroids in the oxytocin hormone system

The sex steroids and the peptide hormone oxytocin are both ancient modulators of the reproductive system of most metazoan species responsible for tissue differentiation and acute events respectively. In vivo experimentation implies estrogenic control of both the oxytocin (OT) gene and that for its receptor (OTR). Yet neither gene promoter appears able to bind classic estrogen-dependent nuclear receptors (ER) in vitro. The literature is confused by some transfected cell culture experiments which suggest that the human and rat OT gene promoter can be regulated by both ER alpha and ER beta through a major hormone response element at -160 bp upstream of the transcription start site. These findings depended, however, upon the presence of a high molar excess of the nuclear estrogen receptor. The current consensus suggests that the sex steroids are acting indirectly on both the OT and OTR genes, possibly involving intermediate transcription factors or cofactors. They may also act upon the OTR at the cell membrane, though more study is needed before the few current observations can be generalized. Due to the OT system being so ancient and fundamental to all aspects of reproduction, it is likely that the mechanisms by which the sex steroids influence this system are going to be of general importance to many other basic aspects of reproductive control.

Oxytocin, vasopressin, and the neuroendocrine basis of pair bond formation

Several lines of evidence support a role for oxytocin and vasopressin in complex social behaviors, including parental care, sex behavior, and aggression. Recent studies in a monogamous mammal, the prairie vole, suggest an additional role for both peptides in the formation of pair bonds. Central administration of oxytocin facilitates and administration of an oxytocin antagonist inhibits partner preference formation in female prairie voles. Conversely, vasopressin facilitates and a V1a receptor antagonist inhibits pair bonding in males. A potential cellular basis for these effects is the species-specific pattern of expression of oxytocin and V1a receptor in reward pathways of the prairie vole brain. At a molecular level, comparative sequencing of the oxytocin and V1a receptors reveals species differences in the promoter sequences that may guide regional expression in the brain. Transgenic mice created with the 5' flanking region of the prairie vole oxytocin receptor gene demonstrate that sequencing in this region influence the pattern of expression within the brain. The unique promoter sequences of the prairie vole OTR and V1a receptor genes and the resulting species-specific pattern of regional expression provide a potential molecular mechanism for the evolution of pair bonding behaviors and a cellular basis for monogamy.

Oxytocin, vasopressin, and autism: is there a connection?

Autism is a poorly understood developmental disorder characterized by social impairment, communication deficits, and compulsive behavior. The authors review evidence from animal studies demonstrating that the nonapeptides, oxytocin and vasopressin, have unique effects on the normal expression of species-typical social behavior, communication, and rituals. Based on this evidence, they hypothesize that an abnormality in oxytocin or vasopressin neurotransmission may account for several features of autism. As autism appears to be a genetic disorder, mutations in the various peptide, peptide receptor, or lineage-specific developmental genes could lead to altered oxytocin or vasopressin neurotransmission. Many of these genes have been cloned and sequenced, and several polymorphisms have been identified. Recent gene targeting studies that alter expression of either the peptides or their receptors in the rodent brain partially support the autism hypothesis. While previous experience suggests caution in hypothesizing a cause or suggesting a treatment for autism, the available preclinical evidence with oxytocin and vasopressin recommends the need for clinical studies using gene scanning, pharmacological and neurobiological approaches.

Neuroendocrine perspectives on social attachment and love

The purpose of this paper is to review existing behavioral and neuroendocrine perspectives on social attachment and love. Both love and social attachments function to facilitate reproduction, provide a sense of safety, and reduce anxiety or stress. Because social attachment is an essential component of love, understanding attachment formation is an important step toward identifying the neurobiological substrates of love. Studies of pair bonding in monogamous rodents, such as prairie voles, and maternal attachment in precocial ungulates offer the most accessible animal models for the study of mechanisms underlying selective social attachments and the propensity to develop social bonds. Parental behavior and sexual behavior, even in the absence of selective social behaviors, are associated with the concept of love; the analysis of reproductive behaviors, which is far more extensive than our understanding of social attachment, also suggests neuroendocrine substrates for love. A review of these literatures reveals a recurrent association between high levels of activity in the hypothalamic pituitary adrenal (HPA) axis and the subsequent expression of social behaviors and attachments. Positive social behaviors, including social bonds, may reduce HPA axis activity, while in some cases negative social interactions can have the opposite effect. Central neuropeptides, and especially oxytocin and vasopressin have been implicated both in social bonding and in the central control of the HPA axis. In prairie voles, which show clear evidence of pair bonds, oxytocin is capable of increasing positive social behaviors and both oxytocin and social interactions reduce activity in the HPA axis. Social interactions and attachment involve endocrine systems capable of decreasing HPA reactivity and modulating the autonomic nervous system, perhaps accounting for health benefits that are attributed to loving relationships.

Serotonin and neuropeptides in affiliative behaviors

The neuropharmacological study of serotonin and behavior has followed two fundamentally different strategies. One approach has used behavior as a dependent variable for assaying drug effects. To characterize serotonergic drugs, most studies have used relatively simple behaviors, such as locomotor activity, startle, exploration, operant responses, and sleep. A second approach has focused on behavior, with drugs used as tools to elucidate the physiological role of serotonin. These studies have increasingly focused on behaviors of ethological importance, including aggression, sexual behavior, and other forms of social interaction. Here we review studies using this approach to focus on one particular kind of social interaction: affiliation.

Neuroendocrine bases of monogamy

A number of studies have implicated the neurohypophyseal peptides oxytocin and vasopressin in the central mediation of complex social behaviors, including affiliation, parental care and territorial aggression. Research on a monogamous rodent, the prairie vole (Microtus ochrogaster), suggests that these neuropeptides are also involved in the control of several behaviors associated with monogamy, including pair bonding, paternal care and mate guarding. Comparative studies using several species of vole have identified species-specific patterns of oxytocin- and vasopressin-receptor expression in the brain that appear to be associated with a monogamous versus non-monogamous social structure. Molecular studies suggest that changes in the regulation of oxytocin- and vasopressin-receptor gene expression underlie these species differences in receptor distribution and might provide a mechanism for the evolution of monogamy in voles.

Ontogeny of oxytocin and vasopressin receptor binding in the lateral septum in prairie and montane voles

Adult prairie (Microtus ochrogaster) and montane voles (M. montanus) differ in the distribution of oxytocin (OT) and vasopressin (AVP) receptor binding in the brain. The present study examined the ontogenetic pattern of these receptor bindings in the lateral septum in both species to determine whether adult differences in the receptor binding are derived from a common pattern in development. In both species, OT and AVP receptor binding in the lateral septum were detected neonatally, increased during development, and reached the adult level at weaning (third week). The progression of OT and AVP receptor differed, as OT receptor binding increased continually until weaning while AVP receptor binding did not change in the first week, increased rapidly in the second week, and was sustained thereafter. For both receptors, the binding increased more rapidly in montane than in prairie voles, resulting in species differences in receptor binding at weaning and in adulthood. Together, these data indicate that OT and AVP could affect the brain during development in a peptide- and species-specific manner in voles.

An evaluation of what the mouse knockout experiments are telling us about mammalian behaviour

The early gene knockout studies with a neurobiological focus were directed at fairly obvious target genes and added very little to our knowledge of behavioural neuroscience. On the contrary, since the behavioural consequences were often predictable, this helped confirm that the technology was working. However, a substantial number of knockouts of genes expressed in the brain have been without obvious behavioural consequences, supporting the concept of genetic canalisation and redundancy. Others have produced a behavioural deficit for which there is no obvious explanation. Many cells of different tissue types have a capacity for memory, and in the brain, cells of the hippocampus are important for spatial learning and memory. Deleting genes that are expressed in the hippocampus has received considerable attention in this behavioural context. Although the initial studies experienced problems of interpretation, considerable advances have since been made. Knockout mice are now subject to tests of different forms of learning, multicellular hippocampal recordings, and restricted gene deletion specific to cells of component regions. This multi-level approach is proving more informative. Nevertheless, there is still a need to recognise that behavioural expression is several steps removed from gene expression, and that the relationship between genes and behaviour can be reciprocal.

Gene targeting approaches to neuroendocrinology: oxytocin, maternal behavior, and affiliation

Transgenic technology affords exciting new opportunities in the field of behavioral neuroendocrinology. We have extended our research into the behavioral function of oxytocin in maternal and social behavior using two transgenic approaches: (i) targeted deletion of the oxytocin gene in mice and (ii) augmented oxytocin receptor expression in the brain. Mice genetically deficient in oxytocin can mate, give birth, and display normal maternal behavior; however, milk ejection and certain aspects of social behavior are affected. Comparative studies of oxytocin receptors have led to the observation that species differences in social organization are associated with differences in receptor distribution. Specifically, monogamous prairie voles and nonmonogamous, asocial montane voles exhibit different patterns of OT receptor expression in the brain. Transgenic mice have been created with a reporter gene driven by the prairie vole oxytocin receptor gene promoter. Analysis of the expression pattern suggests that it should be possible to manipulate receptor expression in the vole brain in order to examine the effects of receptor distribution on behavior.