Decreased GABAergic Biomarkers in Cerebellar, Limbic and Cortical Areas in Autism: Neuropathological and Developmental Implications

Background: Intrinsic and extrinsic GABAergic activity act as a dual inhibitory control to inhibit or disinhibit glutamatergic neurons thereby modulating the normal local and global balance of inhibitory:excitatory synaptic interactions.  In autism, this balance is hypothesized to be disturbed suggesting a developmental origin with disruption of local and/or long cerebellar and cortical circuits.  This balance is essential during neural tissue genesis including proliferation, migration and differentiation of neuronal precursors as well as synapse formation and refinement of cortical circuits. GAD65/67 are key synthesizing enzymes for GABA and reduced protein levels are associated with decreased GABA levels. Ionotropic GABA-A receptors and metabotropic GABA-B receptors play critical roles in early neuronal development and are distributed both synaptically and extrasynaptically. The levels/densites of these key GABA biomarkers are consistently reduced in cerebellar, limbic and cortical areas in postmortem examination in individuals with autism compared to age- and post-mortem interval-matched controls.

Objectives:  To provide an overview of current studies from our laboratory and others demonstrating remarkably consistent findings of decreased GAD mRNA levels, and decreased GABA-A receptors, benzodiazepine binding sites and GABA-B receptors throughout many brain areas in adult autism cases.

Methods: In situ hybridization histochemistry for GAD65/67 mRNA levels was used in the Crus II and dentate regions of the cerebellum in adult autism cases and matched controls. In vitro ligand binding techniques for GABA-A and GABA-B receptors and benzodiazepine binding sites were used in cerebellum, hippocampal formation, cingulate cortices and fusiform gyrus.

Results: Significant reductions in GAD65 mRNA levels were found in the dentate nucleus and decreases in GAD67 mRNA levels were found in Purkinje cells. Significant decreases in the density of GABA receptors and benzodiazepine binding sites were found in parts of the cerebellum, hippocampal formation, anterior and posterior cingulate cortex and in the fusiform gyrus.

Conclusions: Results from these investigations combined with reported findings from the literature suggest that in autism, specific types of GABAergic biomarkers are downregulated in brain areas that participate in many of the core neural features of the disorder.  Overall, findings suggest a dysregulation of GABAergic modulatory control of glutamatergic neurons potentially impacting neural development and in the balance of inhibitory:excitatory synaptic circuitry in the mature autism brain.

Acknowledgements:  Human tissue was obtained from the NICHD Brain and Tissue Bank for Developmental Disorders at the University of Maryland, Baltimore (Dr. R. Zielke, Director), and the Autism Tissue Program (Dr. J. Pickett, Director) and The Autism Research Foundation (Dr. M.L. Bauman, Director) via the Harvard Brain Tissue Resource Center (Dr. F. Benes, Director).  The authors are very grateful to the brain banks and to HBTRC for their generosity and professionalism in obtaining tissue for these studies.