This study investigates the brain response to a single acute dose of Arbaclofen, the R-enantiomer of the GABA-B agonist Baclofen, compared to a single dose of placebo in healthy men with and without autism spectrum disorder.

Previous research suggests that GABAergic drug compounds could shift brain excitation and inhibition (E-I) in the healthy brain and in neurodevelopmental psychiatric conditions, such as autism spectrum disorder (ASD) - where this balance is disrupted. A study by Ajram et al. (2017) has shown an E-I shifted towards more GABA in individuals with ASD, and not in controls, after a single dose of the anti-glutamatergic and pro-GABAergic drug Riluzole. Moreover, brain connectivity patterns in ASD patients where shifted towards the ones observed in the control group. However, it was unclear whether this changes could be driven by GABA receptors, thus more specific probes may help to clarify the mechanism underlying the E-I coordination in ASD. Therefore, this study will use neuroimaging and electrophysiology to investigate the brain E-I coordination in ASD compared to control participants when the system is responding to a single dose of the specific GABA-B (STX209) receptor agonist. 50 adult individuals with ASD and 50 neurotypical adults (25 males and 25 females per group) will be invited to participate. Each participant will receive a single dose of the drug (15mg or 30mg Arbaclofen) or matched placebo). Brain activity and neurochemistry will be investigated using magnetic resonance imaging. Further data will be collected through questionnaires, behavioural tasks, blood samples, and sensory tasks using electroencephalography and retinal imaging.

Inclusion Criteria:

• ASD participants must pass diagnostic threshold for ASD on the Autism Diagnostic Interview-Revised (if and informant is available)
• ASD participants must be currently symptomatic on Autism Diagnostic Observation Schedule (ADOS)
• Age 18-60 years
• Can give informed consent
• medication free in the month preceding participation; but regular medication (used in a stable dose over the two months previous to participation) with drug which does not affect glutamate or GABA directly may be permitted
• IQ>70

Exclusion Criteria:

• IQ<70
• history of psychosis, co-morbid major mental illness, significant physical illness (heart disease, high blood pressure, seizures) habitual substance misuse (including alcohol)
• ASD caused by a known genetic syndrome e.g. Fragile X or 22q11 deletion syndrome,
• past/present treatment for epilepsy
• Change of medication dose/start of a new pharmacological therapy in the month prior to participation

Participants and investigators were blinded to the drug condition