GABA Receptor Binding Density in the Striatum of Individuals with Autism: Novel Findings for Consideration When Designing Human Clinical Autism Studies with Inhibitory Modulators

Background:

The basal ganglia (BG) is a collection of sub-cortical nuclei that contain inhibitory GABAergic spiny projection neurons (SPN’s). The BG projects to the thalamus and has reciprocal connections with multiple cortical regions and the cerebellum. The BG nuclei 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. Prior clinical trials using GABA receptor modulators as treatment in autism have yielded mixed results due to poor behavioral endpoint design. Therefore, the current investigation specifically examines the role of GABA_A receptor changes compared to GABA_B in select regions of the BG in postmortem cases from individuals with autism compared to typically developing controls. Previously, Wegiel et al. (2014) reported volumetric and neuronal density changes in specific BG regions in postmortem autism cases, and Oblak et al. (2011) found a decrease in GABA_B receptor binding in the cingulate cortex, a source of diffuse limbic input to the dorsal striatum.

Objectives:

To determine the binding density of inhibitory GABA_A receptors compared to GABA_B in the BG, specifically, the dorsal and the ventral striatum in autism cases versus controls.

Methods:

Forty-one post-mortem cases were examined in select regions of the BG in age- and PMI-matched cases (n=20 control and n= 21 autism). Sampled regions in the dorsal striatum included caudate and putamen and sampled regions in the ventral striatum included the NAcc core and shell. Cryostat cut 20 µm sections from these regions were incubated with either [³H]-Flunitrazepam 5 nM (GABA_A) or [³H]-CGP 54626 3 nM (GABA_B) then loaded into X-ray cassettes with tritium standards and apposed to tritium-sensitive film for twelve weeks and fourteen weeks respectively. 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. After exposure, the films were developed and digitized on a MCID platform to quantify measurements of binding in femtomoles per milligram of tissue for the ligand. Analysis was performed using student’s t-test.

Results:

Significantly high GABA_A expression revealed by [³H]-Flunitrazepam binding was observed in dorsal striatum (p=0.0024) and ventral striatum (p=0.0103) in autism. In contrast, GABA_B expression revealed by [³H]-CGP 54626 binding was found to be unchanged in the dorsal striatum as well as the ventral striatum in subjects with autism compared to controls (p>0.05).

Conclusions:

Since GABA_A receptors are present mainly post-synaptically, increased GABA_A receptors with unchanged GABA_B receptors implies postsynaptic alteration, which adds evidence to excitatory/inhibitory imbalance in select regions of the autism brain. Increased expression of GABA_A receptors is a possible compensation to increased excitatory cortical inputs. Understanding the consequences of GABA receptor alterations in the BG is important for dissecting BG circuitry and its effects on cortical and BG related behaviors. Future human studies using inhibitory modulators should consider changes to GABA receptor targets in the striatum when designing experiments and planning end point outcome measures for clinical trials.