Using a Valproic acid-induced rat model to investigate the effects of litter size on behaviouraland morphological outcomes

Abstract

Autism Spectrum disorder (ASD) constitutes a variety of neurodevelopmental disorders such as autism, Asperger’s syndrome and Rett’s syndrome, all characterised by three main features: deficits in social interaction, impaired communication and language, and the presence of repetitive and restrictive behaviours. Genetic factors, in association with environmental insults during key moments in development can result in the development of autism in early childhood. Valproic acid (VPA), an anticonvulsant drug used in the treatment of epilepsy, has been linked to an increased risk of autism when used during pregnancy. For this reason, VPA has been increasingly used to create models of autism in rodents, to understand further the behavioural characteristics, cellular and molecular underpinnings, and potential therapies for autism. An understudied area of ASD research is the variation in behavioural phenotypes exhibited by individuals with autism. Not only are males more likely to be diagnosed with ASD, but there may be many differences in the severity of ASD symptoms. This study utilised the VPA-induced autism model for analysis of the behavioural and morphological autistic-like characteristics in the offspring of valproic acid exposed dams. In addition to the repetitive and restricted behaviours, as well as cognitive, locomotor and exploratory behaviours assessed; gross anatomical characteristics were also analysed such as tail length, “the crooked tail phenotype” and relative brain mass. In addition, ELISA was used to detect and quantify whether there was residual VPA within blood sera, and what concentrations would be found in individuals of different litter sizes. Comparisons were made between neurotypical and VPA-exposed pups, as well as within the VPA litters to determine the effects of sex and litter size on autistic-like traits. Morphological findings seemed to confirm that individuals from smaller litter sizes were likely exposed to more VPA in utero, as these individuals showed the greatest tail abnormalities (significantly decreased length and the presence of tail kinks) as well as significantly larger brain mass relative to body mass. The main behavioural difference between VPA-exposed individuals and controls was seen in locomotor activity. VPA-exposed females showed greater locomotor activity than controls, however, it was those from the largest litter size that appeared to drive the trend, showing greater average speed as well as greater distance travelled in the hole board assay. In contrast, no clear pattern was seen in the performance of assays assessing cognitive, Christine Dignon MSc ( Med) Thesis 711602 7 repetitive, or exploratory behaviour of VPA-exposed individuals from different litter sizes. Similarly, no differences were seen in the reactivity to heat in the hotplate thermal assay. Importantly, ELISA did not provide reliable data on serum levels of VPA in pups. The findings of this study indicate that the autism-like behavioural phenotypes are not limited to males only and provide support for the inclusion of females in ASD studies. Furthermore, the study demonstrates the need for the inclusion of litter size as a variable in the assessment of all analyses pertaining to the features of ASD-like in rodents.