Pharmacological modulation of Excitation-Inhibition balance in Autism Spectrum Disorders : a translational approach

There are currently no effective pharmacologic treatments for the core symptoms of Autism Spectrum Disorder (ASD); and candidate treatments are often assumed to act in a similar way in people with and without ASD. This may not be the case, as there is recent evidence that the excitatory (E) glutamate and inhibitory (I) GABA systems, which are crucial to brain development and function, are altered in ASD. Abnormalities of E-I balance at rest in ASD are described in the literature, however findings often contradict and mostly stem from cross-sectional studies. To date, no prior studies of ASD have examined the 'responsivity' of the E-I system to pharmacologic challenge. No single technique can fully capture E-I dynamics in the living brain, however E-I balance can be measured at a number of 'levels', using different species-specific modalities. Hence, in the present thesis, I aimed to explore the effect of pharmacologically modulating E-I balance at the i) intracellular, ii) functional, and iii) extracellular level, in both humans, and an animal model of ASD (the Neurexin1a knock out rat), after a pharmacological challenge with the E-I acting drug, riluzole. To first capture E-I dynamics at the intracellular level, I investigated riluzole-evoked changes in bulk tissue levels of E-I neurotransmitters in the medial prefrontal cortex (mPFC) and basal ganglia (BG)- regions known to be important in ASD- using Magnetic Resonance Spectroscopy and ex vivo tissue analyses in humans and rats, respectively. In humans, I compared the change in 'Inhibitory Index' - the GABA fraction within the pool of glutamate plus GABA metabolites - post riluzole challenge in adult men with (n = 17) and without (n = 20) ASD. Despite comparable baseline measures of GABA and Glx (glutamate + glutamine) in the mPFC, I found the response of the Inhibitory Index to riluzole to be diametrically opposite in men with and without ASD. In contrast, in the BG, I reported a significant group difference in baseline Glx, in which the ASD cohort had lower Glx levels compared to controls, yet both groups responded in the same direction to riluzole challenge, with an increase in the Inhibitory Index. These findings were partially replicated in the Neurexin1a knock out rat- with lower subcortical glutamate also reported in an equivalent basal ganglia region (the caudate putamen), compared to wild type controls. However, no differences in E-I responsivity were observed in the rodent in either strain or brain region. Given the reported group differences in E-I responsivity profile of the human mPFC, and that E-I imbalances are likely to affect the whole brain, I next explored the responsivity of functional connectivity of the mPFC with the rest of the brain, using resting-state functional Magnetic Resonance Imaging (rs-fMRI). In the ASD group, functional connectivity was abnormal at baseline, but restored to control levels after riluzole administration. Conversely, riluzole had no effect on the functional connectivity in the control group, thus highlighting again a group difference in responsivity. Finally, to examine whether extracellular changes in E-I dynamics were driving differences in functional connectivity, I explored neurotransmitter efflux into the extracellular space in the living rat brain, using in vivo microdialysis. However, riluzole had no measurable effect on glutamate nor GABA efflux in either wild type, or Neurexin1a knock out rats. Overall, I have, for the first time, identified that E-I balance at both the intracellular and functional level may be shifted in adult men with ASD, and that the brain in ASD is pharmacologically different to that of controls. This has implications for future drug discovery in ASD and the results of this thesis could be translated forwards to enhance future clinical trial design. Further to this, I found the Neurexin1a knock out rat to have a comparable baseline biology to the human condition, which may support its use in future studies of back-translate these findings. Such work will be necessary to further understand E-I pharmacology in ASD, and further validate E-I balance as a tractable therapeutic treatment target for this increasingly prevalent condition.