Fear behaviour and its underlying physiology in the Nlgn3⁻/ʸ rat model of autism

Neurodevelopmental disorders, including autism spectrum disorder (ASD) and intellectual disabilities (IDs), are a heterogeneous range of disorders thought to affect approximately 3-5 % of the population. There are currently no effective pharmacological treatments for ASD or ID, due in part to a lack of understanding of the pathophysiology of these disorders. Single-gene mutations account for a large proportion of cases where individuals present with ASD and co-occurring moderate to severe ID; and of these, mutations in synaptic proteins have been repeatedly implicated. Mutations in the gene encoding the postsynaptic transmembrane protein neuroligin-3 (Nlgn3 ) are highly correlative with autism spectrum disorders (ASDs) and intellectual disabilities (IDs) in humans. Some of the most debilitating aspects of ASD/ID are anxiety and altered emotional responses, and although these cannot be examined directly using rodent models, we can gain insight on these aspects of the disorders by investigating behavioural tasks that involve similar brain circuitry. Previous work on Nlgn3-/y rats carried out by my colleagues examined behavioural tasks, such as fear conditioning and conditioned place avoidance, which rely on several brain areas involved in fear, anxiety and emotion. Nlgn3-/y rats show abnormal fear responses during these tasks, displaying a greater propensity to exhibit escape responses in contrast to the classic freezing behaviour seen in their wild-type littermates. In this thesis, I examine the electrophysiology of the periaqueductal grey (PAG), a midbrain area involved in the execution of flight-freeze responses. I firstly provide an electrophysiological and morphological characterisation of neuronal populations in the PAG using whole-cell patch clamp recordings in acute slices, highlighting the heterogeneity of cells in this brain area. Secondly, I identify hyperexcitability of neurons in the dorsal, but not ventral, PAG of Nlgn3-/y rats, which may promote activation of circuits eliciting flight behaviour. Furthermore, electrical stimulation of the PAG in vivo resulted in a higher proportion of Nlgn3-/y rats exhibiting flight behaviours in comparison to wild-types, suggesting the threshold for eliciting an escape response is lower in these rats. However, local field potential recordings in the PAG during fear recall following auditory fear conditioning revealed no differences between wild-type and Nlgn3-/y rats. Overall, these physiological changes in the PAG of Nlgn3-/y rats may underly the imbalance of flight-freeze behaviours they display in response to fearful stimuli. In the final chapter of this thesis, I investigate the effect of Nlgn3 loss on the physiology of two other brain areas involved in fear learning: the hippocampus and medial pre-frontal cortex. Additionally, I present a direct comparison of the electro-physiology in these brain areas with another model of ASD/ID; the Nrxn1+/- rat. Nrxn1 encodes the neurexin-1 protein, a presynaptic cell adhesion molecule that binds to neuroligin-3, mutations in which are highly correlated with ASD and ID. I find that, although both models display reduced hippocampal long-term potentiation, the majority of pathophysiologies identified in either model do not show convergence. This chapter highlights the importance of studying genetically diverse models of ASD to gain better insight into the pathophysiologies of these disorders. In summary, this thesis provides evidence that Nlgn3-/y rats display physiological deficits in brain areas involved in the 'fear circuit' that may underlie the abnormal fear responses seen in the behaviour of these animals. This work highlights novel avenues for ASD/ID research and treatment, particularly in the context of understanding the anxiety disorders and emotional behaviours often seen in individuals with autism and ID.