Mouse behavioral tasks relevant to autism: phenotypes of 10 inbred strains

Low-level neonatal thimerosal exposure: further evaluation of altered neurotoxic potential in SJL mice

Ethylmercury in thimerosal-preserved childhood vaccines has been suggested to be neurotoxic and to contribute to the etiology of neurodevelopmental disorders, including autism. Immune system function may be an important factor influencing vulnerability of the developing nervous system to thimerosal. This possibility is based in part on a report by Hornig et al. (2004, Mol. Psychiatry 9, 833-845) of neurodevelomental toxicity in SJL/J mice that develop autoantibodies when exposed to organic mercury. The present study reexamined this possibility by injecting neonatal SJL/J mice with thimerosal, with and without combined HiB and DTP vaccines. Injections modeled childhood vaccination schedules, with mice injected on postnatal days 7, 9, 11, and 15 with 14.2, 10.8, 9.2, and 5.6 mug/kg mercury from thimerosal, respectively, or vehicle. Additional groups received vaccine only or a 10 times higher thimerosal + vaccine dose. Low levels of mercury were found in blood, brain, and kidneys 24 h following the last thimerosal injection. Survival, body weight, indices of early development (negative geotaxis, righting) and hippocampal morphology were not affected. Performance was unaffected in behavioral tests selected to assess behavioral domains relevant to core deficits in neurodevelopmental disorders such as autism (i.e., social interaction, sensory gating, anxiety). In an open-field test the majority of behaviors were unaffected by thimerosal injection, although thimerosal-injected female mice showed increased time in the margin of an open field at 4 weeks of age. Considered together the present results do not indicate pervasive developmental neurotoxicity following vaccine-level thimerosal injections in SJL mice, and provide little if any support for the hypothesis that thimerosal exposure contributes to the etiology of neurodevelopmental disorders.

Mouse behavioral assays relevant to the symptoms of autism

While the cause of autism remains unknown, the high concordance between monozygotic twins supports a strong genetic component. The importance of genetic factors in autism encourages the development of mutant mouse models, to advance our understanding of biological mechanisms underlying autistic behaviors. Mouse models of human neuropsychiatric diseases are designed to optimize (i) face validity (resemblance to the human symptoms) (ii) construct validity (similarity to the underlying causes of the disease) and (iii) predictive validity (expected responses to treatments that are effective in the human disease). There is a growing need for mouse behavioral tasks with all three types of validity, to define robust phenotypes in mouse models of autism. Ideal mouse models will incorporate analogies to the three diagnostic symptoms of autism: abnormal social interactions, deficits in communication and high levels of repetitive behaviors. Social approach is tested in an automated three chambered apparatus that offers the subject a choice between spending time with another mouse, with a novel object, or remaining in an empty familiar environment. Reciprocal social interaction is scored from videotapes of interactions between pairs of unfamiliar mice. Communication is evaluated by measuring emission and responses to vocalizations and olfactory cues. Repetitive behaviors are scored for measures of grooming, jumping, or stereotyped sniffing of one location or object. Insistence on sameness is modeled by scoring a change in habit, for example, reversal of the spatial location of a reinforcer in the Morris water maze or T-maze. Associated features of autism, for example, mouse phenotypes relevant to anxiety, seizures, sleep disturbances and sensory hypersensitivity, may be useful to include in a mouse model that meets some of the core diagnostic criteria. Applications of these assays include (i) behavioral phenotyping of transgenic and knockout mice with mutations in genes relevant to autism; (ii) characterization of inbred strains of mice; (iii) evaluation of environmental toxins; (iv) comparison of behavioral phenotypes with genetic factors, such as unusual expression patterns of genes or unusual single nucleotide polymorphisms; and (v) evaluation of proposed therapeutics for the treatment of autism.

Social deficits in BTBR T+tf/J mice are unchanged by cross-fostering with C57BL/6J mothers

Inbred strains of mice are useful model systems for studying the interactions of genetic and environmental contributions during neurodevelopmental stages. We recently reported an inbred strain, BTBR T+tf/J (BTBR), which, as compared to the commonly used C57BL/6J (B6) strain, displays lower social interactions as juveniles, lower social approach in adult ages, and higher levels of repetitive self-grooming throughout developmental stages. The present study investigated whether the early postnatal maternal environment contributes substantially to the unusually low expression of social behaviors and high self-grooming in BTBR as compared to B6. Within 24h of birth, entire litters of pups were cross-fostered to either a dam of the same strain or a dam of the opposite strain. Control litters were left with their own mothers. Offspring were tested for juvenile play at postnatal day 21+/-1, for sociability at 8 weeks of age in an automated three-chambered social approach test, and for self-grooming at 9-11 weeks of age. Results indicate that deficits in play behaviors in juvenile BTBR pups were not rescued by a B6 maternal environment. Similarly, a BTBR maternal environment did not induce play deficits in B6 pups. Cross-fostering had no effect on sociability scores in adults. The high self-grooming in BTBR and low self-grooming in B6 were not affected by maternal environment. These findings favor a genetic interpretation of the unusual social behaviors and self-grooming traits of BTBR, and support the use of the BTBR inbred strain as a mouse model to study genetic mechanism of autism.

A sequence-based variation map of 8.27 million SNPs in inbred mouse strains

A dense map of genetic variation in the laboratory mouse genome will provide insights into the evolutionary history of the species and lead to an improved understanding of the relationship between inter-strain genotypic and phenotypic differences. Here we resequence the genomes of four wild-derived and eleven classical strains. We identify 8.27 million high-quality single nucleotide polymorphisms (SNPs) densely distributed across the genome, and determine the locations of the high (divergent subspecies ancestry) and low (common subspecies ancestry) SNP-rate intervals for every pairwise combination of classical strains. Using these data, we generate a genome-wide haplotype map containing 40,898 segments, each with an average of three distinct ancestral haplotypes. For the haplotypes in the classical strains that are unequivocally assigned ancestry, the genetic contributions of the Mus musculus subspecies--M. m. domesticus, M. m. musculus, M. m. castaneus and the hybrid M. m. molossinus--are 68%, 6%, 3% and 10%, respectively; the remaining 13% of haplotypes are of unknown ancestral origin. The considerable regional redundancy of the SNP data will facilitate imputation of the majority of these genotypes in less-densely typed classical inbred strains to provide a complete view of variation in additional strains.

A seizure-prone phenotype is associated with altered free-running rhythm in Pten mutant mice

Conditional deletion of Pten (phosphatase and tensin homolog on chromosome ten) in differentiated cortical and hippocampal neurons in the mouse results in seizures, macrocephaly, social interaction deficits and anxiety, reminiscent of human autism spectrum disorder. Here we extended our previous examination of these mice using electroencephalogram/electromyogram (EEG/EMG) monitoring and found age-related increases in spontaneous seizures, which were correlated with cellular dispersion in the hippocampal dentate gyrus. Increased spontaneous locomotor activity in the open field on the first and the second day of a 3-day continuous study suggested heightened anxiety in Pten mutant mice. In contrast, the mutants exhibited decreased wheel running activity, which may reflect reduced adaptability to a novel environment. Synchronization to the light-dark cycle was normal, but for up to 28 days under constant darkness, the Pten mutants maintained a significantly lengthened and remarkably constant free-running period of almost exactly 24 h. This result implies the involvement of Pten in the maintenance of circadian rhythms, which we interpret as being due to an effect on the phosphatidylinositol 3-kinase (PI3K) signaling cascade.

Social approach behaviors in oxytocin knockout mice: comparison of two independent lines tested in different laboratory environments

Oxytocin mediates social affiliation behaviors and social memory in rodents. It has been suggested that disruptions in oxytocin contribute to the deficits in reciprocal social interactions that characterize autism. The present experiments employed a new social approach task for mice which is designed to detect low levels of sociability, representing the first diagnostic criterion for autism. Two lines of oxytocin knockout mice were tested, the National Institute of Mental Health line in Bethesda, and the Baylor/Emory line at the University of North Carolina in Chapel Hill. Similar methods were used for each line to evaluate tendencies to spend time with a stranger mouse versus with an inanimate novel object with no social valence. Adult C57BL/6J males were tested identically, as controls to confirm the robustness of the methods used in the social task. Comprehensive phenotyping of general health, neurological reflexes, olfactory and other sensory abilities, and motor functions was employed to assess both lines. No genotype differences were detected in any of the control measures for either line. Normal sociability, measured as time spent with a novel stranger mouse as compared to time spent with a novel object, was seen in both the NIMH and the Baylor/Emory lines of oxytocin null mutants, heterozygotes, and wild-type littermate controls. Normal preference for social novelty, measured as time spent with a second novel stranger as compared to time spent with a more familiar mouse, was seen in both the NIMH and the Baylor/Emory lines of oxytocin null mutants, heterozygotes, and wild-type littermate controls, with minor exceptions. Similar behavioral results from two independent targeted gene mutations, generated with different targeting vectors, bred on different genetic backgrounds, and tested in different laboratory environments, corroborates the negative findings on sociability in oxytocin mutant mice. Intact tendencies to spend time with another mouse versus with a novel object, in both lines of oxytocin knockouts, supports an interpretation that oxytocin plays a highly specific role in social memory, but is not essential for general spontaneous social approach in mice.

Affiliative behavior, ultrasonic communication and social reward are influenced by genetic variation in adolescent mice

Social approach is crucial for establishing relationships among individuals. In rodents, social approach has been studied primarily within the context of behavioral phenomena related to sexual reproduction, such as mating, territory defense and parental care. However, many forms of social interaction occur before the onset of reproductive maturity, which suggests that some processes underlying social approach among juvenile animals are probably distinct from those in adults. We conducted a longitudinal study of social investigation (SI) in mice from two inbred strains to assess the extent to which genetic factors influence the motivation for young mice to approach one another. Early-adolescent C57BL/6J (B6) mice, tested 4–6 days after weaning, investigated former cage mates to a greater degree than BALB/cJ (BALB) mice, irrespective of the sex composition within an interacting pair. This strain difference was not due to variation in maternal care, the phenotypic characteristics of stimulus mice or sensitivity to the length of isolation prior to testing, nor was it attributable to a general difference in appetitive motivation. Ultrasonic vocalization (USV) production was positively correlated with the SI responses of mice from both strains. Interestingly, several USV characteristics segregated with the genetic background of young mice, including a higher average frequency and shorter duration for the USVs emitted by B6 mice. An assessment of conditioned place preference responses indicated that there was a strain-dependent difference in the rewarding nature of social contact. As adolescent mice aged, SI responses gradually became less sensitive to genetic background and more responsive to the particular sex of individuals within an interacting pair. We have thus identified a specific, genetic influence on the motivation of early-adolescent mice to approach one another. Consistent with classical theories of motivation, which propose a functional relationship between behavioral approach and reward, our findings indicate that reward is a proximal mechanism through which genetic factors affect social motivation during early adolescence.

What's wrong with my mouse model?

Stress plays a key role in pathogenesis of anxiety and depression. Animal models of these disorders are widely used in behavioral neuroscience to explore stress-evoked brain abnormalities, screen anxiolytic/antidepressant drugs and establish behavioral phenotypes of gene-targeted or transgenic animals. Here we discuss the current situation with these experimental models, and critically evaluate the state of the art in this field. Noting a deficit of fresh ideas and especially new paradigms for animal anxiety and depression models, we review existing challenges and outline important directions for further research in this field.

Social reward among juvenile mice

Mammalian social relationships, such as mother–offspring attachments and pair bonds, can directly affect reproductive output. However, conspecifics approach one another in a comparatively broad range of contexts, so conceivably there are motivations for social congregation other than those underlying reproduction, parental care or territoriality. Here, we show that reward mediated by social contact is a fundamental aspect of juvenile mouse sociality. Employing a novel social conditioned place preference (SCPP) procedure, we demonstrate that social proximity is rewarding for juvenile mice from three inbred strains (A/J, C57BL/6J and DBA/2J), while mice from a fourth strain (BALB/cJ) are much less responsive to social contact. Importantly, this strain-dependent difference was not related to phenotypic variability in exploratory behavior or contextual learning nor influenced by the genetic background associated with maternal care or social conditioning. Furthermore, the SCPP phenotype was expressed early in development (postnatal day 25) and did not require a specific sex composition within the conditioning group. Finally, SCPP responses resulted from an interaction between two specifiable processes: one component of the interaction facilitated approach toward environments that were associated with social salience, whereas a second component mediated avoidance of environmental cues that predicted social isolation. We have thus identified a genetically prescribed process that can attribute value onto conditions predicting a general form of social contact. To our knowledge, this is the first definitive evidence to show that genetic variation can influence a form of social valuation not directly related to a reproductive behavior.

Assessing autism-like behavior in mice: variations in social interactions among inbred strains

Autism is a pervasive developmental disorder, with characteristics including impairments in reciprocal social interaction, impaired communication, and repetitive/stereotyped behaviors. Despite decades of research, the etiology of autism remains elusive. Thus, it is important that we pursue all avenues, in attempting to understand this complicated disorder. One such avenue is the development of animal models. While autism may be uniquely human, there are behavioral characteristics of the disorder that can be established in animal models. Evidence supports a genetic component for this disorder, and over the past few decades the mouse has been a highly valuable tool for the elucidation of pathways involved in many human disorders (e.g., Huntington's disease). As a first step toward establishing a mouse model of autism, we studied same-sex social behavior in a number of inbred mouse strains. In Study 1, we examined intra-strain social behavior of male pairs after one mouse had 15 min prior exposure to the testing chamber. In Study 2, we evaluated intra-strain and inter-strain social behavior when both mice were naive to the testing chamber. The amount and type of social behavior seen differed between these studies, but overall there were general inbred strain differences in social behavior. Some strains were highly social, e.g., FVB/NJ, while others displayed low levels of social behavior (e.g., A/J, BTBR T+tf/J). These strains may be useful in future genetic studies to determine specific genes involved in mouse social behavior, the findings of which should in turn help us to determine some of the genes involved in human social behavior and its disorders (e.g., autism).