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.     

以田鼠作为动物模型来研究社会行为的进化:基因、脑与行为

田鼠属的一些近缘种间具有独特的社会行为多态性。例如Microtusochrogaster和M .pinetorum为一夫一妻制 ,而M .montanus和M .pennsylvanicus则为独居和一夫多妻制。无论是在野外还是人工饲养的条件下 ,单配制的田鼠其雌、雄成年个体一经交配即在两者之间形成长期的配偶关系并且双亲共同哺育后代。已证明神经多肽加压素 (Vasopressin)参与了田鼠单配制行为的神经调控。本篇综述了过去以及近期关于加压素调控田鼠配偶关系形成的研究结果和进展。首先 ,阐述了加压素V1a受体 (V1aR)在脑分布的种间差异 ,并以此来鉴别特定脑区在配偶关系形成中的功能 ;其次 ,探讨了运用V1aR拮抗物的药理学方法来决定究竟哪些脑区参与配偶关系的形成 ,还描述了田鼠种间V1aR基因结构和功能的不同 ,以及这些不同对V1aR在大脑的分布和行为调控潜在的作用机制 ;最后 ,讨论了最新的研究结果 ,即对一夫多妻制田鼠进行脑V1aR基因的改造 ,从而使之表现出一夫一妻制田鼠的行为。总之 ,了解复杂的社会性行为的遗传和神经机制可以加深我们对种间和种内行为分歧进化的理解     

Field tests of cis-regulatory variation at the prairie vole avpr1a locus: association with V1aR abundance but not sexual or social fidelity

The neuropeptide vasopressin and its receptor V1aR are broadly implicated in social behavior and play a central role in several key aspects of male mating tactics in voles. In the prairie vole, a microsatellite in the cis-regulatory region of the gene encoding V1aR (avpr1a) provides a potential genetic basis for individual variation in neural phenotype and behavior; recent studies found that allele length predicts V1aR expression and male social attachment in the laboratory. Here, we explore the relationship between avpr1a microsatellite length, V1aR neural phenotype, and field measures of monogamy and fitness in male prairie voles. We found significant effects of allele length on V1aR expression in structures integral to pairbond formation. These effects did not, however, translate to differences in mating tactics or reproductive success. Together, these data suggest that, while length polymorphism in the avpr1a microsatellite influences neuronal phenotype, this variation does not contribute significantly to male reproductive success and field behavior. We propose that previously reported behavioral effects may be mediated primarily by sequence variation at this locus, for which allele length is an imperfect proxy. By combining genetic, neuronal and ecological approaches, these data provide novel insights into the contribution of genotype to natural diversity in brain and behavior.     

Field tests of cis-regulatory variation at the prairie vole avpr1a locus: Association with V1aR abundance but not sexual or social fidelity

The neuropeptide vasopressin and its receptor V1aR are broadly implicated in social behavior and play a central role in several key aspects of male mating tactics in voles. In the prairie vole, a microsatellite in the cis-regulatory region of the gene encoding V1aR (avpr1a) provides a potential genetic basis for individual variation in neural phenotype and behavior; recent studies found that allele length predicts V1aR expression and male social attachment in the laboratory. Here, we explore the relationship between avpr1a microsatellite length, V1aR neural phenotype, and field measures of monogamy and fitness in male prairie voles. We found significant effects of allele length on V1aR expression in structures integral to pairbond formation. These effects did not, however, translate to differences in mating tactics or reproductive success. Together, these data suggest that, while length polymorphism in the avpr1a microsatellite influences neuronal phenotype, this variation does not contribute significantly to male reproductive success and field behavior. We propose that previously reported behavioral effects may be mediated primarily by sequence variation at this locus, for which allele length is an imperfect proxy. By combining genetic, neuronal and ecological approaches, these data provide novel insights into the contribution of genotype to natural diversity in brain and behavior.     

The Relative Contribution of Proximal 5′ Flanking Sequence and Microsatellite Variation on Brain Vasopressin 1a Receptor (Avpr1a) Gene Expression and Behavior

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

Polymorphism at the avpr1a locus in male prairie voles correlated with genetic but not social monogamy in field populations

Integrative studies of genetics, neurobiology and behaviour indicate that polymorphism in specific genes contributes to variation observed in some complex social behaviours. The neuropeptide arginine vasopressin plays an important role in the regulation of a variety of social behaviours, including social attachment of males to females, through its action on the vasopressin 1a receptor (V1aR). In socially monogamous prairie voles ( Microtus ochrogaster ), polymorphism in the length of microsatellite DNA within the regulatory region of the gene ( avpr1a ) encoding the V1aR predicts differences among males in neural expression of V1aRs and partner preference under laboratory conditions. However, understanding the extent to which V1aR mediates variation in prairie vole social and reproductive behaviour observed in nature requires investigating the consequences of avpr1a polymorphism and environmental influences under ecologically relevant conditions. We examined the relationship between avpr1a length polymorphism and monogamy among male prairie voles living in 0.1 ha enclosures during a time similar to their natural lifespan. We found no evidence that avpr1a genotype of males predicts variation in social monogamy measured in the field but some indices of social monogamy were affected by population density. Parentage data indicated that a male's avpr1a genotype significantly influenced the number of females with which he sired offspring and the total number of offspring sired. Total brain concentrations of V1aR mRNA were not associated with either male behaviour or avpr1a genotype. These data show that melding ecological field studies with neurogenetics can substantially augment our understanding of the effects of genes and environment on social behaviours.     

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.     

avpr1a length polymorphism is not associated with either social or genetic monogamy in free-living prairie voles

Recent discoveries of single-gene influences on social behaviour have generated a great deal of interest in the proximate mechanisms underlying the expression of complex behaviours. Length polymorphism in a microsatellite in the regulatory region of the gene encoding the vasopressin 1a receptor (avpr1a) has been associated with both inter- and intra-specific variation in socially monogamous behaviour in voles (genus Microtus) under laboratory conditions. Here, we evaluate the relationship between avpr1a length polymorphism and social associations, genetic monogamy, and reproductive success in free-living prairie vole (M. ochrogaster) populations. We found no evidence of a relationship between avpr1a microsatellite length and any of our correlates of either social or genetic monogamy in the field. Our results, especially when taken in conjunction with those of recent experimental studies in semi-natural enclosures, suggest that avpr1a polymorphism is unlikely to have been a major influence in the evolution or maintenance of social monogamy in prairie voles under natural conditions.     

Trichostatin A (TSA) facilitates formation of partner preference in male prairie voles (Microtus ochrogaster)

In the socially monogamous prairie voles (Microtus ochrogaster), the development of a social bonding is indicated by the formation of partner preference, which involves a variety of environmental and neurochemical factors and brain structures. In a most recent study in female prairie voles, we found that treatment with the histone deacetylase inhibitor trichostatin A (TSA) facilitates the formation of partner preference through up-regulation of oxytocin receptor (OTR) and vasopressin V1a receptor (V1aR) genes expression in the nucleus accumbens (NAcc). In the present study, we tested the hypothesis that TSA treatment also facilitates partner preference formation and alters OTR and V1aR genes expression in the NAcc in male prairie voles. We thus observed that central injection of TSA dose-dependently promoted the formation of partner preference in the absence of mating in male prairie voles. Interestingly, TSA treatment up-regulated OTR, but not V1aR, gene expression in the NAcc similarly as they were affected by mating — an essential process for naturally occurring partner preference. These data, together with others, not only indicate the involvement of epigenetic events but also the potential role of NAcc oxytocin in the regulation of partner preference in both male and female prairie voles.     

From endophenotypes to evolution: social attachment, sexual fidelity and the avpr1a locus

Both medical and evolutionary genetics increasingly emphasize the importance of subtle, quantitative measures of phenotype. One such 'endophenotype,' the distribution of vasopressin 1a receptor (V1aR), is a recent focus for studies of social behavior. In animal studies, the neural distribution of V1aR has been linked to both social attachment and patterns of sexual fidelity. At a genetic level, a microsatellite in the cis-regulatory region of the avpr1a locus has been linked to variation in both brain and behavior. Both sets of data become more complex as the mechanistic and evolutionary details are examined more fully. I briefly summarize recent work from animal and human studies of avpr1a and highlight parallels between comparative and clinical approaches.     

Effects of avpr1a length polymorphism on male social behavior and reproduction in semi‐natural populations of prairie voles (Microtus ochrogaster)

Intraspecific variation in sociosexual behavior has typically been investigated in the context of its relationship with environmental factors, but neurogenetic factors can also influence sociosexual behavior. In laboratory studies of prairie voles (Microtus ochrogaster), length polymorphism of microsatellite DNA within the gene (avpr1a) encoding the vasopressin 1a receptor is correlated with variation in male sociosexual behavior. However, field studies of prairie voles have found the relationship between male avpr1a microsatellite allele length and sociosexual behavior to be more ambiguous, possibly because most males had alleles of intermediate length. We tested the hypothesis that avpr1a microsatellite allele length mediates male sociosexual behavior in field settings by releasing voles into field enclosures where every male possessed two avpr1a microsatellite alleles at least one standard error longer or shorter than the mean length in their population of origin. Voles from an Illinois and Kansas population were examined separately as social monogamy appears more prevalent in the Illinois population. Illinois males with long avpr1a microsatellite alleles had smaller home ranges and overlapped a greater proportion of the home range of the female that they overlapped the most. Kansas males showed the opposite pattern. Illinois, but not Kansas, males with long avpr1a microsatellite alleles sired offspring with more females and sired more litters. Our results support the hypothesis that genetic variation associated with the avpr1a gene plays a role in mediating male prairie vole sociosexual behavior in nature. However, the relationship between specific male behaviors and male avpr1a microsatellite allele length sometimes differed significantly between Kansas and Illinois voles, suggesting relationships between specific male sociosexual behaviors and polymorphism associated with the avpr1a locus are complex, possibly involving specific nucleotide sequences or other population‐specific genetic differences.     

Complex selection on a regulator of social cognition: Evidence of balancing selection,regulatory interactions and population differentiation in the prairie vole Avpr1a locus

Adaptive variation in social behaviour depends upon standing genetic variation, but we know little about how evolutionary forces shape genetic diversity relevant to brain and behaviour. In prairie voles (Microtus ochrogaster), variants at the Avpr1a locus predict expression of the vasopressin 1a receptor in the retrosplenial cortex (RSC), a brain region that mediates spatial and contextual memory; cortical V1aR abundance in turn predicts diversity in space use and sexual fidelity in the field. To examine the potential contributions of adaptive and neutral forces to variation at the Avpr1a locus, we explore sequence diversity at the Avpr1a locus and throughout the genome in two populations of wild prairie voles. First, we refine results demonstrating balancing selection at the locus by comparing the frequency spectrum of variants at the locus to a random sample of the genome. Next, we find that the four single nucleotide polymorphisms that predict high V1aR expression in the RSC are in stronger linkage disequilibrium than expected by chance despite high recombination among intervening variants, suggesting that epistatic selection maintains their association. Analysis of population structure and a haplotype network for two populations revealed that this excessive LD was unlikely to be due to admixture alone. Furthermore, the two populations differed considerably in the region shown to be a regulator of V1aR expression despite the extremely low levels of genomewide genetic differentiation. Together, our data suggest that complex selection on Avpr1a locus favours specific combinations of regulatory polymorphisms, maintains the resulting alleles at population‐specific frequencies, and may contribute to unique patterns of spatial cognition and sexual fidelity among populations.     

The V1a vasopressin receptor is necessary and sufficient for normal social recognition: a gene replacement study

Vasopressin modulates many social and nonsocial behaviors, including emotionality. We have previously reported that male mice with a null mutation in the V1a receptor (V1aR) exhibit a profound impairment in social recognition and changes in anxiety-like behavior. Using site-specific injections of a V1aR-specific antagonist, we demonstrate that the lateral septum, but not the medial amygdala, is critical for social recognition. Reexpressing V1aR in the lateral septum of V1aR knockout mice (V1aRKO) using a viral vector resulted in a complete rescue of social recognition. Furthermore, overexpression of the V1aR in the lateral septum of wild-type (wt) mice resulted in a potentiation of social recognition behavior and a mild increase in anxiety-related behavior. These results demonstrate that the V1aR in the lateral septum plays a critical role in the neural processing of social stimuli required for complex social behavior.     

小型田鼠:研究单配制进化与调控的模型

哺乳动物的单配制通常被认为是社会性单配制,它不是单纯地由性行为来决定,而是由诸多因素,包括长期的pair bond、夫妻双方共同抚育后代、免近亲交配以及雌雄两性相似等来决定的。在这篇综述中,我们论述了如何以啮齿类田鼠属(Microtus)为模型,通过比较研究来帮助我们理解社会性单配制的进化以及其神经调控机制。对田鼠属的研究不仅证实了单配制起源于艰苦的生存条件的假说,而且还证实了雌性性选择可能有利于维持单配制。不仅如此,哺乳动物单配制的进化还需要雄性的prosocial行为的不断强化。例如,亲近行为可以促进pairbond的形成并强化雄性对后代的哺育行为,而这种强化则来源于神经多肽催产素(OT)和加压素(AVP)与类固醇类激素的相互作用。催产素和加压素调控pairbond和双亲哺育行为的表达,而单配制和多配制田鼠的催产素和加压素受体在脑内的分布有显的不同。比较研究揭示了小型田鼠单配制的调控机制,而种内差异和行为上的可塑性则有助于我们进一步理解这种机制。比如,在某些条件下,多配制的草原田鼠(Microtus pennsylvanicu)的雄性个体具有哺育后代的行为。尽管草原田鼠的加压素Vla受体在脑内的分布与其他多配制的田鼠相似,但是如果脑室注射加压素,仍可以诱发其哺育后代的行为。同样是单配制的橙腹田鼠(Microtus ochrogaster),生活在：Illnois的显示出高水平的prosocial行为,而生活在Kansas的则表现出较低水平的社会性行为。即使两个种群的催产素或加压素Vla受体在脑内的分布相同,它们的雌激素受体表达水平显不同,这在雄性个体表现尤其明显。与Kansas的雄性个体相比,在终纹床核(bed rucleus of the stria tenninalis)和杏仁核中区(medial amygdala)这两个调控亲近行为和攻击行为的脑区,Illinois的雄性个体的α雌激素受体的水平要低得多。这些研究表明对雌激素的低敏感程度有利于高水平地表达prosocial行为并降低特定类型的攻击行为。     

Genetic variation in the developmental regulation of cortical avpr1a among prairie voles

Early experiences can have enduring impacts on brain and behavior, but the strength of these effects can be influenced by genetic variation. In principle, polymorphic CpGs (polyCpGs) may contribute to gene‐by‐environment interactions (G × E) by altering DNA methylation. In this study, we investigate the influence of polyCpGs on the development of vasopressin receptor 1a abundance in the retrosplenial cortex (RSC‐V1aR) of prairie voles (Microtus ochrogaster). Two alternative alleles (‘HI’/‘LO’) predict RSC avpr1a expression, V1aR abundance and sexual fidelity in adulthood; these alleles differ in the frequency of CpG sites and in methylation at a putative intron enhancer. We hypothesized that the elevated CpG abundance in the LO allele would make homozygous LO/LO voles more sensitive to developmental perturbations. We found that genotype differences in RSC‐V1aR abundance emerged early in ontogeny and were accompanied by differences in methylation of the putative enhancer. As predicted, postnatal treatment with an oxytocin receptor antagonist (OTA) reduced RSC‐V1aR abundance in LO/LO adults but not their HI/HI siblings. Similarly, methylation inhibition by zebularine increased RSC‐V1aR in LO/LO adults, but not in HI/HI siblings. These data show a gene‐by‐environment interaction in RSC‐V1aR. Surprisingly, however, neither OTA nor zebularine altered adult methylation of the intronic enhancer, suggesting that differences in sensitivity could not be explained by CpG density at the enhancer alone. Methylated DNA immunoprecipiation‐sequencing showed additional differentially methylated regions between HI/HI and LO/LO voles. Future research should examine the role of these regions and other regulatory elements in the ontogeny of RSC‐V1aR and its developmentally induced changes.     

Comparative distribution of central neuropeptide Y (NPY) in the prairie (Microtus ochrogaster) and meadow (M. pennsylvanicus) vole

Neuropeptide Y (NPY) has been implicated as a modulator of social behavior, often in a species-specific manner. Comparative studies of closely related vole species are particularly useful for identifying neural systems involved in social behaviors in both voles and humans. In the present study, immunohistochemistry was performed to compare NPY-like immunoreactivity (-ir) in brain tissue of the socially monogamous prairie vole and non-monogamous meadow vole. Species differences in NPY-ir were observed in a number of regions including the cortex, extended amygdala, septal area, suprachiasmatic nucleus, and intergeniculate leaf. Meadow voles had higher NPY-ir in all these regions as compared to prairie voles. No differences were observed in the striatum or hippocampus. The extended amygdala and lateral septum are regions that play a key role in regulation of monogamous behaviors such as pair bonding and paternal care. The present study suggests NPY in these regions may be an additional modulator of these species-specific social behaviors. Meadow voles had moderately higher NPY-ir in a number of hypothalamic regions, especially in the suprachiasmatic nucleus. Meadow voles also had much higher levels of NPY-ir in the intergeniculate leaflet, another key region in the regulation of circadian rhythms. Overall, species differences in NPY-ir were observed in a number of brain regions implicated in emotion, stress, circadian, and social behaviors. These findings provide additional support for a role for the NPY system in species-typical social behaviors.     

Vasopressin and oxytocin excite MCH neurons, but not other lateral hypothalamic GABA neurons

Neurons that synthesize melanin-concentrating hormone (MCH) colocalize GABA, regulate energy homeostasis, modulate water intake, and influence anxiety, stress, and social interaction. Similarly, vasopressin and oxytocin can influence the same behaviors and states, suggesting that these neuropeptides may exert part of their effect by modulating MCH neurons. Using whole cell recording in MCH-green fluorescent protein (GFP) transgenic mouse hypothalamic brain slices, we found that both vasopressin and oxytocin evoked a substantial excitatory effect. Both peptides reversibly increased spike frequency and depolarized the membrane potential in a concentration-dependent and tetrodotoxin-resistant manner, indicating a direct effect. Substitution of lithium for extracellular sodium, Na(+)/Ca(2+) exchanger blockers KB-R7943 and SN-6, and intracellular calcium chelator BAPTA, all substantially reduced the vasopressin-mediated depolarization, suggesting activation of the Na(+)/Ca(2+) exchanger. Vasopressin reduced input resistance, and the vasopressin-mediated depolarization was attenuated by SKF-96265, suggesting a second mechanism based on opening nonselective cation channels. Neither vasopressin nor oxytocin showed substantial excitatory actions on lateral hypothalamic inhibitory neurons identified in a glutamate decarboxylase 67 (GAD67)-GFP mouse. The primary vasopressin receptor was vasopressin receptor 1a (V1aR), as suggested by the excitation by V1aR agonist [Arg(8)]vasotocin, the selective V1aR agonist [Phe(2)]OVT and by the presence of V1aR mRNA in MCH cells, but not in other nearby GABA cells, as detected with single-cell RT-PCR. Oxytocin receptor mRNA was also detected in MCH neurons. Together, these data suggest that vasopressin or oxytocin exert a minimal effect on most GABA neurons in the lateral hypothalamus but exert a robust excitatory effect on presumptive GABA cells that contain MCH. Thus, some of the central actions of vasopressin and oxytocin may be mediated through MCH cells.