Relationship between absolute and relative ratios of glutamate, glutamine and GABA and severity of autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental pathology characterized by an impairment in social interaction, communication difficulties, and repetitive behaviors. Glutamate signaling abnormalities are thought to be considered as major etiological mechanisms leading to ASD. The search for amino-acidic catabolytes related to glutamate in patients with different levels of ASD might help current research to clarify the mechanisms underlying glutamate signaling and its disorders, particularly in relation to ASD. In the present study, plasma levels of the amino acids and their derivatives glutamate, glutamine, and γ-aminobutyric acid (GABA), associated with their relative ratios, were evaluated using an enzyme-linked immunosorbent assay (ELISA) technique in 40 male children with ASD and in 38 age- and gender-matched neurotypical health controls. The Social Responsiveness Scale (SRS) was used to evaluate social cognition, and the Childhood Autism Rating Scale (CARS) was used to assess subjects’ behaviors. Children with ASD exhibited a significant elevation of plasma GABA and glutamate/glutamine ratio, as well as significantly lower levels of plasma glutamine and glutamate/GABA ratios compared to controls. No significant correlation was found between glutamate levels and the severity of autism, measured by CARS and SRS. In receiver operating characteristic (ROC) curve analysis, the area under the curve for GABA compared to other parameters was close to one, indicating its potential use as a biomarker. Glutamine appeared as the best predictive prognostic markers in the present study. The results of the present study indicate a disturbed balance between GABAergic and glutamatergic neurotransmission in ASD. The study also indicates that an increased plasma level of GABA can be potentially used as an early diagnostic biomarker for ASD.