Decreased GABAA receptors and benzodiazepine binding sites in the anterior cingulate cortex in autism

Abstract

The anterior cingulate cortex (ACC; BA 24) via its extensive limbic and high order association cortical connectivity to prefrontal cortex is a key part of an important circuitry participating in executive function, affect, and socio-emotional behavior. Multiple lines of evidence, including genetic and imaging studies, suggest that the ACC and gamma-amino-butyric acid (GABA) system may be affected in autism. The benzodiazepine binding site on the GABA(A) receptor complex is an important target for pharmacotherapy and has important clinical implications. The present multiple-concentration ligand-binding study utilized (3)H-muscimol and (3)H-flunitrazepam to determine the number (B(max)), binding affinity (K(d)), and distribution of GABA(A) receptors and benzodiazepine binding sites, respectively, in the ACC in adult autistic and control cases. Compared to controls, the autistic group had significant decreases in the mean density of GABA(A) receptors in the supragranular (46.8%) and infragranular (20.2%) layers of the ACC and in the density of benzodiazepine binding sites in the supragranular (28.9%) and infragranular (16.4%) lamina [corrected]. These findings suggest that in the autistic group this downregulation of both benzodiazepine sites and GABA(A) receptors in the ACC may be the result of increased GABA innervation and/or release disturbing the delicate excitation/inhibition balance of principal neurons as well as their output to key limbic cortical targets. Such disturbances likely underlie the core alterations in socio-emotional behaviors in autism.

Figure 1

A Nissl stained section from a control case (4103) demonstrates the cytoarchitecture of the human anterior cingulate cortex (Area 24a–c). The box illustrates our sampling scheme dividing the cortex into supragranular (I–III) and infragranular (V–VI) layers.

Figure 2

Summary of mean K_d values for GABA_A (2a) and BZD (2b) receptor binding in autistic and control cases. There was no significant difference between autistic and control cases in either ligand or layers. Geometric means for K_d are reported here.

Figure 3

Sample binding curves from two cases demonstrating specific binding in the supragranular layers from each concentration of tritiated ligand. Data from [³H]muscimol is in the upper panel and data from [³H]flunitrazepam is in the bottom panel.

Figure 4

Examples of pseudocolored images of [³H]-muscimol binding (15 nM) in autistic (4a) and control (4b) cases. Solid black arrows demonstrate the location of sampling in the supragranular layers; arrows with dashed lines indicate sampling in the infragranular layers. In the [³H]muscimol binding experiments, the number of GABA_A receptors in the supragranular (p=0.02) and infragranular layers (p=0.04) was significantly (**) decreased (4c).

Figure 5

Examples of individual [³H]muscimol binding curves. Specific binding of [³H]-muscimol to the supragranular (a) and infragranular (b) layers of the anterior cingulate cortex in seven autistic and nine control subjects. Smooth curves indicate fits to the hyperbolic binding equation.

Figure 6

Examples of autistic (6a) and control (6b) pseudocolored images of [³H]-Flunitrazepam (15nM) binding in the anterior cingulate cortex. Solid black arrows demonstrate the location of sampling in the supragranular layers; arrows with dashed lines indicate sampling in the infragranular layers. There was a significant decrease (**) in benzodiazepine binding sites in the supragranular layers (p=0.002) and a trend (*) in the infragranular layers (p=0.06) in autism (6c).

Figure 7

Examples of individual [³H]-flunitrazepam binding curves. Specific binding of [³H]-flunitrazepam to the supragranular (7a) and infragranular (7b) layers of the anterior cingulate cortex in six autistic and nine control subjects. Smooth curves indicate fits to the hyperbolic binding equation.