Investigating cortical oscillations, coherence and seizure susceptibility in a mouse model of autism
Document TypeMasters Research thesis
Access StatusOpen Access
© 2018 Andrew Argyropoulos
Autism spectrum disorder (ASD) is a pervasive neurodevelopmental disorder diagnosed by difficulties in social communication, repetitive behaviour and/or restricted interests. A high proportion of ASD patients also experience seizures and abnormal brain activity as recorded via electroencephalography, however the underlying biological mechanisms of increased seizure susceptibility in ASD are unknown. Neuroligin-3 (NL3) is a neuronal adhesion protein involved in regulating synaptic structure and function. A rare point mutation at position 451 of the Neuroligin-3 amino acid sequence converts an arginine to a cysteine residue is associated with ASD and reduces NL3 protein levels by 90%. This NL3 R451C mutation was identified in two Swedish brothers with ASD, one of whom was diagnosed with comorbid epilepsy. The mutation is replicated in NL3R451C mice that exhibit reduced preference for social interactions, increased repetitive behaviours, and increased performance in memory and some motor testing. NL3R451C mice also display region-specific differences in inhibitory and excitatory neurotransmission in brain slices increased dendritic complexity, and reduced number of PV-inhibitory interneurons, all of which could contribute to changes in seizure susceptibility and oscillatory activity. Several animal models of ASD show spontaneous seizures and increased susceptibility to experimentally induced seizures, however whether NL3R451C mice have altered seizure susceptibility is unknown. It is well established that brain neuronal activity generates electrical rhythms that underlie cognitive phenomena in humans and may be altered in ASD. Gamma oscillations in the 30-100Hz range are frequently increased with demanding cognitive load and are thought to orchestrate spatially disparate cortical regions. Several clinical studies have pointed to alterations in gamma oscillations and in the coherence of oscillatory activity measured between two disparate cortical regions in ASD patients. To determine whether gamma oscillations are altered in NL3R451C mice, the glutamatergic NMDA receptor antagonist ketamine (20mg/kg; a potent inducer of gamma oscillations) was administered to adult male and female NL3R451C and WT mice and oscillatory activity recorded via electrocorticography (EcOG). PV-interneurons have been experimentally isolated to reveal they are the major drivers of gamma rhythms following ketamine administration. Baseline oscillatory activity from 6 and 10 week old male mice was examined as well as interhemispheric coherence in male and female NL3 mutants and wild type littermates. Gamma oscillatory power in male wild type and NL3R451C mice was comparable following administration of saline and was enhanced to a similar degree following ketamine (20mg/kg) administration. Non-gamma oscillations were also unchanged by ketamine administration. Interhemispheric coherence, however, was significantly higher in 6- week old male NL3R451C compared to control mice for both beta and gamma oscillation ranges. In 10-week old male mutants, there was a significant effect detected for theta, alpha and beta oscillations. Because female mice carry two copies of the X-linked NL3 gene it was hypothesised that any oscillatory activity effects may be exacerbated by the R451C mutation. Gamma band oscillations following administration of ketamine (20 mg/kg) were statistically not significant when WT, heterozygous and homozygous females were compared. Additionally, no statistically significant differences were found for interhemispheric coherence in females. To gain further understanding of the role of the NL3 R451C mutation in epilepsy, seizure susceptibility was investigated in NL3R451C male mice using pentalenetetrazole (PTZ) at both low (20 and 30 mg/kg) and high (50mg/kg) doses. Administration of high dose PTZ is associated with behavioural changes culminating to convulsive seizures. The Racine behavioural scale was used to quantify behavioural seizure severity over time in response to high dose PTZ. In mice, low dose PTZ results spike-wave discharges (SWDs) and a loss of responsiveness resembling absence seizures in patients. Electroencephalography (EcOG) recordings of neuronal activity at the cortical surface was utilised to detect SWDs following administration of low dose PTZ over a 30 minute period. NL3R451C mice showed a strong trend for shorter SWD duration although EcOG analyses of SWD frequency and duration showed similar susceptibility in both mutants and WT littermates to low dose PTZ-induced seizures. Following administration of high-dose PTZ (50mg/kg), NL3R451C mutants were slower to progress to severe seizure scores and overall progressed to milder seizures on the racine scale over the 30-minute testing period. The results of the present study show reduced seizure susceptibility and increased coherence in male NL3R451C mice. These changes might reflect underlying structural and neurochemical alterations present in NL3R451C mice. The reported increase in cortical inhibitory neurotransmission in NL3R451C mice may underlie the resistance to PTZ-induced seizures identified in this study. Similarities in gamma oscillations may indicate similar functioning of NMDA receptors and the PV-interneurons that are strongly implicated in the generation of gamma rhythms. Higher interhemispheric coherence in male NL3R451C mice could stem from differences in cognitive, behavioural and cortical changes present in this model. Further research is needed to clarify relationships between behaviour and oscillatory activity in NL3R451C mice.
Keywordsautism; ASD; electroencephalography; EEG; coherence; gamma; oscillations; seizures; neuroligin-3; ketamine
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