The Basal Ganglia Has Altered Inhibitory Receptor Expression in Autism

Background:  

The basal ganglia (BG) is a collection of sub-cortical nuclei that contain mainly inhibitory GABAergic medium spiny neurons (MSN’s). The BG projects to the thalamus and has reciprocal connections with multiple cortical regions and the cerebellum. Parts of the BG are implicated in OCD, habit formation, motor, speech and language disorders. Thus, the BG is an ideal region of interest to examine the neurochemical basis of repetitive, stereotyped behavior and social communication difficulties observed in autism. This is the first study to examine changes in inhibitory circuits and receptors in specific regions of the BG in individuals with autism. Previously, Wegiel et al. 2014 reported volumetric and neuronal density changes in BG regions.

Objectives:  

Determine levels of expression of inhibitory GABA_A receptors in functional subdivisions of the BG in autism. Specifically, the dorsal striatum consisting of caudate and putamen, and the ventral striatum including the core and shell territories of nucleus accumbens (NAcc) were quantified.

Methods:  

Sixteen post-mortem cases (8 autism and 8 control) were examined in select regions of the BG in age-matched control (mean=22.6±2.9 years) and autism (mean=22.9±3.0 years) cases. Sampled regions in the dorsal striatum included projection fields from the anterior cingulate cortex (ACC) and dorsolateral pre-frontal cortex (dlPFC) in caudate and putamen. Sampled regions in the ventral striatum included projection fields from ventromedial pre-frontal cortex vmPFC and orbitofrontal cortex (OFC) to the NAcc core and shell, areas that play a critical role in reward related behaviors. Cryostat cut 20 µm sections from these regions of interest were mounted on gelatin coated glass slides and incubated with tritiated flunitrazepam ([³H]-flunitrazepam 5 nM, specific activity = 79.8 Ci/mmol, Perkin Elmer) before they were loaded into X-ray cassettes with tritium standards and apposed to tritium-sensitive film ([³H]-hyperfilm) for four weeks at room temperature. Two sections per case were used for determining total binding with the tritiated ligand and one section was used for non-specific binding with a competitive displacer (Ro-15-451, 15 nM). After exposure, the films were developed and digitized to quantify measurements of binding in femtomoles per milligram of tissue for the ligand. Analysis was performed using student’s t-test.

Results:  

Tritiated flunitrazepam binding was found to be significantly increased in the caudate region of the dorsal striatum in subjects with autism compared to controls (p=0.015). Additionally, significantly increased benzodiazepine binding site density was found in the region of caudate that receives putative projections from the ACC (p=0.03).

Conclusions:  

The results from this initial study suggest that there are significant increases in the expression of inhibitory GABA_A receptors in the striatum of subjects with autism. Normally, the limbic ACC input that specifically targets the striatal interneurons plays an important role in BG efferent projections. In autism cases, increased GABA_A receptors in the ACC recipient region of the caudate may be compensatory to an increase in excitatory input(s) and/or may represent an overexpression of receptors due to decreased GABA availability in this region. In either case, there is likely a disturbance of inhibitory/excitatory balance within BG circuitry in autism.