Modeling Higher-Order Repetitive Behavior in the C58 Mouse Strain

Background: The diagnostic domain of restricted, repetitive behavior (RRBs) represents a heterogeneous set of behaviors but has been shown to cluster into two subtypes: lower-order repetitive body movements (e.g., hand flapping) and higher-order repetitive behavior, such as circumscribed interests and insistence on sameness. Most work with mouse models relevant to the repetitive behavior in ASD have focused on lower-order repetitive behaviors with less attention to modeling higher-order repetitive behaviors. The C58 inbred mouse strain exhibits a robust lower-order repetitive behavior phenotype involving high levels of hindlimb jumping and backward somersaulting (Muelhman et al., 2012; Ryan et al., 2010). This model has yet to be systematically characterized for the presence of higher-order repetitive behaviors. Understanding the pathophysiology of the full range of repetitive behaviors is critical to identifying therapeutic drug strategies for individuals with ASD.

Objectives: The purpose of this study was to extend characterization of the C58 animal model to include higher-order repetitive behavior by examining resistance to change or inflexibility using reversal learning of a positional discrimination as well as extinction of this conditioned behavior. C58 mice were compared to control (C57BL/6) mice on these measures using an appetitive operant nose-poke task.

Methods: C58 (n=11) and C57BL/6 (n=8) mice of both sexes were tested for lower-order motor stereotypy at 8 weeks of age. At 9 weeks of age, all mice were food deprived across 3-4 days to approximately 85% of their free feeding weight and behavioral testing sessions were conducted daily. Each session was 70 minutes, which included a 10 minute habituation session in the operant chamber with the house light off prior to each 60 minute operant session. During the operant testing, mice were reinforced on a fixed ratio schedule (FR1) to either the right or left side of the operant chamber. Once a mouse met acquisition criterion of 85% correct for 4 consecutive days, reversal learning was tested by switching the nose poke side associated with reinforcement. Once the reversal learning criterion was reached (85% correct, 4 consecutive days), the mouse entered the extinction phase and nose pokes to either side of the chamber did not result in reinforcement.

Results: Our findings demonstrated that C58 mice had greater difficulty in switching during reversal learning and exhibited higher rates of perseverative responding during extinction compared to C57BL/6J mice. Additionally, frequency of hind limb motor stereotypy was directly correlated with perseverative errors.

Conclusions: Overall, these findings suggest that C58 mice demonstrate cognitive inflexibility relative to C57/BL6 mice and that the two forms of RRB may have overlapping neurocircuitry. Futhermore, these data show the viability of the C58 stain as a model for both higher-order and lower-order repetitive behavior. Extending animal model work to include higher order repetitive behavior is critical to identifying the underlying novel potential therapeutic targets that can be used in the development of pharmacotherapies for ASD.