Serotonin Transporter Knockout Mice and Maternal Stress: A Potential Animal Model of Autism

This study examined whether an interaction between the serotonin transporter genotype and maternal stress results in a mouse with characteristics of autism. Serotonin transporter knockout heterozygous dams and wild-type dams were stressed during different gestational periods. Their offspring then underwent a series of behavioral tasks to test sociability, anxiety levels, and level of cerebellar functioning. We predicted that mice that experienced prenatal stress and had a deficit in the serotonin transporter would produce the anxiety, cerebellar and social interaction deficiencies seen in autism. In contrast, we hypothesized that without the presence of prenatal stress, there is no difference in the baselines of the control groups and the serotonin transporter knockout mice. Our results did not support our hypothesis, suggesting that the serotonin transporter knockout mice and prenatal stress paradigm is not a valid animal model of autism. However, we did find effects on sociability, anxiety, and locomotor functioning, indicating an interaction of serotonin transporter knockout mice and maternal stress. Mouse Model of Autism 3 Serotonin Knockout Mice and Maternal Stress: A Potential Animal Model of Autism Autism spectrum disorder (ASD) is a developmental disorder that is classified by impairments in social interactions and communication, as well as repetitive and restrictive behaviors (Cook, 1997; Crawley, 2004; DiCicco-Bloom, 2006; Tuchman, 2003). The etiology of ASD is largely unknown, but several factors have been suggested. By creating an animal model of autism, these factors can further be investigated. One suggested factor contributing to the development of autism is the serotonin transporter (5-HTT). The function of 5-HTT is to uptake extracellular serotonin (5-HT) and transport 5-HT back into the cell (Murphy, 2004). Polymorphisms of the 5-HTT gene can alter 5-HTTs functioning, including uptake, binding ability, and expression (Lesch, 1996; Murphy, 2004). An insertion/deletion within the promoter region of the 5HTT gene results in either the long or short variant polymorphism, respectively (Lesch, 1996; Murphy, 2004). It has been discovered that possessing at least one allele of the short variant of the 5-HTT gene, SLC6A4, is associated with numerous clinical conditions, including depression and anxiety (Capsi, 2003; Lesch, 1996). Harari et al (2002) discovered that carriers of the short allele show greater amygdala activation to fearful stimuli. This suggests the possibility that carriers of the short allele have greater reactivity to stressful stimuli. Similar to the human literature, 5-HTT knockout (KO) mice exhibit anxiety-like behaviors (Holmes, 2003). These 5-HTT KO mice also display increased levels of extracellular 5-HT, decreased intracellular concentrations of 5-HT, and reduced transporter binding availability and uptake activity in comparison to wild-type and Mouse Model of Autism 4 heterozygous 5-HTT mice (Bengel, 1998; Kim, 2005; Montanez, 2003). While the heterozygous 5-HTT KO mice have similar 5-HT levels as wild-type mice, their uptake activity and transporter binding availability is reduced roughly 50% in comparison to wild-type mice (Bengel, 1998; Montanez, 2003). This reduction of the heterozygotes functioning is similar to that seen in human studies investigating the short allele of the 5HTT gene (Lesch, 1996). Therefore, in order to best model the human 5-HTT function, heterozygous 5HTT KO mice should be used. Another suggested factor contributing to the development of ASD is prenatal stress (Beversdorf, 2005; Kinney, 2007). In a study conducted by Beversdorf et al. (2005), it was discovered that mothers of children with autism had a significant increase in stress during prenatal weeks 21-32 in comparison to normally functioning children, as well as children with Down syndrome. It was also discovered that a peak at 25-28 weeks gestation was observed for the autistic group (Beversdorf, 2005). In investigating the effects that naturally occurring stressors have on prenatal development, Louisiana parishes were researched for their differing prevalence rates of autism. The prevalence rates increased in a storm intensity-dependent manner, especially when the hurricanes and/or tropical storms were experienced during the late 2 or early 3 trimester (Kinney, 2007). Both of these studies implicate prenatal stress as a possible risk factor in the development of ASD. Acute stress produces increased levels of circulating glucocorticoids and 5-HT, which has the ability to alter fetal development (Gaspar, 2003; Leonard, 2006). In contrast, chronic stress has been shown to decrease 5-HT levels, which can be seen by measuring peripheral 5-HT blood platelet levels (Bianchi, 2002). Cote et al. (2007) Mouse Model of Autism 5 found that maternal peripheral 5-HT is crucial in the neuronal development of mice. Thus, stress-related changes in 5-HT levels may have the ability to significantly alter development of mice offspring. While numerous studies have investigated the independent effects of prenatal stress and of increased circulating 5-HT levels on the etiology of autism, few have investigated the interactions between these factors. In addition, little is known about the effects caused by variation in the timing of prenatal stress on both neuronal development and behavioral implications. Therefore, the present study will conduct a 2x3 experiment, measuring genotype and maternal stress. The genotype conditions will consist of the 5HTT KO mice and of wild-type mice that serve as a genetic control. The stress conditions are: no stress (control), early prenatal stress (gestational days 8-10), and late prenatal stress (gestational days 16-18). We hypothesize that without the presence of prenatal stress, there will be no difference in the baselines of the control groups and the 5-HTT KO heterozygous mice. In contrast, we hope to establish that the presence of prenatal stress and a deficit in 5-HTT expression, as seen in the 5-HTT KO mice, produces the anxiety, exploration, and social interaction abnormalities seen in autism.