Alaine C. Keebaugh

Variation in vasopressin receptor (Avpr1a) expression creates diversity in behaviors related to monogamy in prairie voles

Polymorphisms in noncoding regions of the vasopressin 1a receptor gene (Avpr1a) are associated with a variety of socioemotional characteristics in humans, chimpanzees, and voles, and may impact behavior through a site-specific variation in gene expression. The socially monogamous prairie vole offers a unique opportunity to study such neurobiological control of individual differences in complex behavior. Vasopressin 1a receptor (V1aR) signaling is necessary for the formation of the pair bond in males, and prairie voles exhibit greater V1aR binding in the reward-processing ventral pallidum than do asocial voles of the same genus. Diversity in social behavior within prairie voles has been correlated to natural variation in neuropeptide receptor expression in specific brain regions. Here we use RNA interference to examine the causal relationship between intraspecific variation in V1aR and behavioral outcomes, by approximating the degree of naturalistic variation in V1aR expression. Juvenile male prairie voles were injected with viral vectors expressing shRNA sequences targeting Avpr1a mRNA into the ventral pallidum. Down-regulation of pallidal V1aR density resulted in a significant impairment in the preference for a mated female partner and a reduction in anxiety-like behavior in adulthood. No effect on alloparenting was detected. These data demonstrate that within-species naturalistic-like variation in V1aR expression has a profound effect on individual differences in social attachment and emotionality. RNA interference may prove to be a useful technique to unite the fields of behavioral ecology and neurogenetics to perform ethologically relevant studies of the control of individual variation and offer insight into the evolutionary mechanisms leading to behavioral diversity.

Central oxytocin receptors mediate mating-induced partner preferences and enhance correlated activation across forebrain nuclei in male prairie voles.

Oxytocin (OT) is a deeply conserved nonapeptide that acts both peripherally and centrally to modulate reproductive physiology and sociosexual behavior across divergent taxa, including humans. In vertebrates, the distribution of the oxytocin receptor (OTR) in the brain is variable within and across species, and OTR signaling is critical for a variety of species-typical social and reproductive behaviors, including affiliative and pair bonding behaviors in multiple socially monogamous lineages of fishes, birds, and mammals. Early work in prairie voles suggested that the endogenous OT system modulates mating-induced partner preference formation in females but not males; however, there is significant evidence that central OTRs may modulate pair bonding behavior in both sexes. In addition, it remains unclear how transient windows of central OTR signaling during sociosexual interaction modulate neural activity to produce enduring shifts in sociobehavioral phenotypes, including the formation of selective social bonds. Here we re-examine the role of the central OT system in partner preference formation in male prairie voles using a selective OTR antagonist delivered intracranially. We then use the same antagonist to examine how central OTRs modulate behavior and immediate early gene (Fos) expression, a metric of neuronal activation, in males during brief sociosexual interaction with a female. Our results suggest that, as in females, OTR signaling is critical for partner preference formation in males and enhances correlated activation across sensory and reward processing brain areas during sociosexual interaction. These results are consistent with the hypothesis that central OTR signaling facilitates social bond formation by coordinating activity across a pair bonding neural network.

Oxytocin in the nucleus accumbens shell reverses CRFR2-evoked passive stress-coping after partner loss in monogamous male prairie voles

Loss of a partner can have severe effects on mental health. Here we explore the neural mechanisms underlying increased passive stress-coping, indicative of depressive-like behavior, following the loss of the female partner in the monogamous male prairie vole. We demonstrate that corticotropin-releasing factor receptor 2 (CRFR2) in the nucleus accumbens shell mediates social loss-induced passive coping. Further, we show that partner loss compromises the oxytocin system through multiple mechanisms. Finally, we provide evidence for an interaction of the CRFR2 and oxytocin systems in mediating the emotional consequences of partner loss. Our results suggest that chronic activation of CRFR2 and suppression of striatal oxytocin signaling following partner loss result in an aversive emotional state that may share underlying mechanisms with bereavement. We propose that the suppression of oxytocin signaling is likely adaptive during short separations to encourage reunion with the partner and may have evolved to maintain long-term partnerships. Additionally, therapeutic strategies targeting these systems should be considered for treatment of social loss-mediated depression.

RNAi knockdown of oxytocin receptor in the nucleus accumbens inhibits social attachment and parental care in monogamous female prairie voles.

Oxytocin modulates many aspects of social cognition and behaviors, including maternal nurturing, social recognition and bonding. Natural variation in oxytocin receptor (OXTR) density in the nucleus accumbens (NAcc) is associated with variation in alloparental behavior, and artificially enhancing OXTR expression in the NAcc enhances alloparental behavior and pair bonding in socially monogamous prairie voles. Furthermore, infusion of an OXTR antagonist into the NAcc inhibits alloparental behavior and partner preference formation. However, antagonists can promiscuously interact with other neuropeptide receptors. To directly examine the role of OXTR signaling in social bonding, we used RNA interference to selectively knockdown, but not eliminate, OXTR in the NAcc of female prairie voles and examined the impact on social behaviors. Using an adeno-associated viral vector expressing a short hairpin RNA (shRNA) targeting Oxtr mRNA, we reduced accumbal OXTR density in female prairie voles from juvenile age through adulthood. Females receiving the shRNA vector displayed a significant reduction in alloparental behavior and disrupted partner preference formation. These are the first direct demonstrations that OXTR plays a critical role in alloparental behavior and adult social attachment, and suggest that natural variation in OXTR expression in this region alone can create variation in social behavior.

Genetic Influences on Receptive Joint Attention in Chimpanzees (Pan troglodytes)

Despite their genetic similarity to humans, our understanding of the role of genes on cognitive traits in chimpanzees remains virtually unexplored. Here, we examined the relationship between genetic variation in the arginine vasopressin V1a receptor gene (AVPR1A) and social cognition in chimpanzees. Studies have shown that chimpanzees are polymorphic for a deletion in a sequence in the 5′ flanking region of the AVPR1A, DupB, which contains the variable RS3 repetitive element, which has been associated with variation in social behavior in humans. Results revealed that performance on the social cognition task was significantly heritable. Furthermore, males with one DupB+ allele performed significantly better and were more responsive to socio-communicative cues than males homozygous for the DupB- deletion. Performance on a non-social cognition task was not associated with the AVPR1A genotype. The collective findings show that AVPR1A polymorphisms are associated with individual differences in performance on a receptive joint attention task in chimpanzees.

Personality in Chimpanzees (Pan troglodytes): Exploring the Hierarchical Structure and Associations with the Vasopressin V1A Receptor Gene

One of the major contributions of recent personality psychology is the finding that traits are related to each other in an organized hierarchy. To date, however, researchers have yet to investigate this hierarchy in nonhuman primates. Such investigations are critical in confirming the cross-species nature of trait personality helping to illuminate personality as neurobiologically-based and evolutionarily-derived dimensions of primate disposition. Investigations of potential genetic polymorphisms associated with hierarchical models of personality among nonhuman primates represent a critical first step. The current study examined the hierarchical structure of chimpanzee personality as well as sex-specific associations with a polymorphism in the promoter region of the vasopressin V1a receptor gene (AVPR1A), a gene associated with dispositional traits, among 174 chimpanzees. Results confirmed a hierarchical structure of personality across species and, despite differences in early rearing experiences, suggest a sexually dimorphic role of AVPR1A polymorphisms on hierarchical personality profiles at a higher-order level.

The Evolutionary Fate of the Genes Encoding the Purine Catabolic Enzymes in Hominoids, Birds, and Reptiles

Gene loss has been proposed to play a major role in adaptive evolution, and recent studies are beginning to reveal its importance in human evolution. However, the potential consequence of a single gene-loss event upon the fates of functionally interrelated genes is poorly understood. Here, we use the purine metabolic pathway as a model system in which to explore this important question. The loss of urate oxidase (UOX) activity, a necessary step in this pathway, has occurred independently in the hominoid and bird/reptile lineages. Because the loss of UOX would have removed the functional constraint upon downstream genes in this pathway, these downstream genes are generally assumed to have subsequently deteriorated. In this study, we used a comparative genomics approach to empirically determine the fate of UOX itself and the downstream genes in five hominoids, two birds, and a reptile. Although we found that the loss of UOX likely triggered the genetic deterioration of the immediate downstream genes in the hominoids, surprisingly in the birds and reptiles, the UOX locus itself and some of the downstream genes were present in the genome and predicted to encode proteins. To account for the variable pattern of gene retention and loss after the inactivation of UOX, we hypothesize that although gene loss is a common fate for genes that have been rendered obsolete due to the upstream loss of an enzyme a metabolic pathway, it is also possible that same lack of constraint will foster the evolution of new functions or allow the optimization of preexisting alternative functions in the downstream genes, thereby resulting in gene retention. Thus, adaptive single-gene losses have the potential to influence the long-term evolutionary fate of functionally interrelated genes.

The genomes of the South American opossum (Monodelphis domestica) and platypus (Ornithorhynchus anatinus) encode a more complete purine catabolic pathway than placental mammals

The end product of purine catabolism varies amongst vertebrates and is a consequence of independent gene inactivation events that have truncated the purine catabolic pathway. Mammals have traditionally been grouped into two classes based on their end product of purine catabolism: most mammals, whose end product is allantoin due to an ancient loss of allantoinase (ALLN), and the hominoids, whose end product is uric acid due to recent inactivations of urate oxidase (UOX). However little is known about purine catabolism in marsupials and monotremes. Here we report the results of a comparative genomics study designed to characterize the purine catabolic pathway in a marsupial, the South American opossum (Monodelphis domestica), and a monotreme, the platypus (Ornithorhynchus anatinus). We found that both genomes encode a more complete set of genes for purine catabolism than do eutherians and conclude that a near complete purine catabolic pathway was present in the common ancestor of all mammals, and that the loss of ALLN is specific to placental mammals. Our results therefore provide a revised history for gene loss in the purine catabolic pathway and suggest that marsupials and monotremes represent a third class of mammals with respect to their end products of purine catabolism.

Identification of Variables Contributing to Superovulation Efficiency for Production of Transgenic Prairie Voles (Microtus ochrogaster)

BackgroundThe prairie vole (Microtus ochrogaster) is an emerging animal model for biomedical research because of its rich sociobehavioral repertoire. Recently, lentiviral transgenic technology has been used to introduce the gene encoding the green fluorescent protein (GFP) into the prairie vole germline. However, the efficiency of transgenesis in this species is limited by the inability to reliably produce large numbers of fertilized embryos. Here we examined several factors that may contribute to variability in superovulation success including, age and parentage of the female, and latency to mating after being placed with the male.MethodsFemales produced from 5 genetically distinct breeder lines were treated with 100 IU of pregnant mare serum gonadotrophin (PMSG) and immediately housed with a male separated by a perforated Plexiglas divider. Ovulation was induced 72 hr later with 30 IU of human chorionic gonadotropin (hCG) and 2 hrs later mating was allowed.ResultsSuperovulation was most efficient in young females. For example, females aged 6-11 weeks produced more embryos (14 +/- 1.4 embryos) as compared to females aged 12-20 weeks (4 +/- 1.6 embryos). Females aged 4-5 weeks did not produce embryos. Further, females that mated within 15 min of male exposure produced significantly more embryos than those that did not. Interestingly, there was a significant effect of parentage. For example, 12 out of 12 females from one breeder pair superovulated (defined as producing 5 or more embryos), while only 2 out of 10 females for other lines superovulated.ConclusionsThe results of this work suggest that age and genetic background of the female are the most important factors contributing to superovulation success and that latency to mating is a good predictor of the number of embryos to be recovered. Surprisingly we found that cohabitation with the male prior to mating is not necessary for the recovery of embryos but is necessary to recover oocytes. This information will dramatically reduce the number of females required to generate embryos for transgenesis in this species.

PRTFDC1 Is a Genetic Modifier of HPRT-Deficiency in the Mouse

Lesch-Nyhan disease (LND) is a severe X-linked neurological disorder caused by a deficiency of hypoxanthine phosphoribosyltransferase (HPRT). In contrast, HPRT-deficiency in the mouse does not result in the profound phenotypes such as self-injurious behavior observed in humans, and the genetic basis for this phenotypic disparity between HPRT-deficient humans and mice is unknown. To test the hypothesis that HPRT deficiency is modified by the presence/absence of phosphoribosyltransferase domain containing 1 (PRTFDC1), a paralog of HPRT that is a functional gene in humans but an inactivated pseudogene in mice, we created transgenic mice that express human PRTFDC1 in wild-type and HPRT-deficient backgrounds. Male mice expressing PRTFDC1 on either genetic background were viable and fertile. However, the presence of PRTFDC1 in the HPRT-deficient, but not wild-type mice, increased aggression as well as sensitivity to a specific amphetamine-induced stereotypy, both of which are reminiscent of the increased aggressive and self-injurious behavior exhibited by patients with LND. These results demonstrate that PRTFDC1 is a genetic modifier of HPRT-deficiency in the mouse and could therefore have important implications for unraveling the molecular etiology of LND.