Decreased GABA receptors and benzodiazepine binding sites in the anterior cingulate cortex in autism
A. Oblak, T. T. Gibbs, G. J. Blatt. Autism Res. 2009 Aug;2(4):205-19..
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.
autistic • anterior cingulate cortex • GABA • post-mortem • ligand binding
- Input Author
The anterior cingulate cortex is heavily connected to the limbic system and prefrontal cortex. This makes it a key player in socioemotional affect, and abnormalities in this region have been linked to autism.
The GABA system has been implicated in autism. Lines of evidence: GABA receptors are important in development and autism is a developmental disorder. People with autism are more likely to get seizures (which may be caused by an underactive GABA system). Some autistic patients respond abnormally to benzodiazepines, which act on GABA receptors. Mutations in the genes for GABA receptor subunits have been linked with autism.
The authors obtained post-mortem brain slices from the anterior cingulate cortex of 7 autistic and 9 healthy patients. To measure the amount of GABA receptors, seven concentrations of tritium-muscimol was added and bound radioactivity measured to obtain a binding curve. Tritium-flunitrazepam was used to measure the amount of GABA receptors containing a benzodiazepine binding site. From the binding curves, a Bmax (indicative of the total amount of GABA receptors/benzodiazepine binding sites present) and Kd (indicative of the binding affinity) were determined and compared between groups.
The Kd was the same in all cases. This means that the binding affinity between the receptors and the ligands did not change. This suggests that the GABA receptors themselves do not differ between autistic patients and healthy controls.
The Bmax, however, did differ between autistic patients and healthy controls, for both muscimol and flunitrazepam. In all cases, the amount of GABA receptors and benzodiazepine binding sites was lower in the anterior cingulate cortex of autistic patients than in that of healthy controls.
According to the statistical tests, there was no significant effect of seizure history or anticonvulsant therapy on the Bmax for either ligand. This, however, may be due to the small number of autistic patients that had a history of seizures (n=4) or were being treated for seizures (n=3).
Any disruption in receptor levels can affect the circuitry of the brain. Therefore, a decrease in GABA receptor levels could have implications in autism.
In addition, GABA receptors play a crucial role in development (proliferation, migration, and differentiation of neurons). A disruption in this system might therefore contribute to autism. The paper below describes abnormalities in the number of tracts leading in and out of the anterior cingulate cortex.
Seizures have been shown to decrease the number of benzodiazepine binding sites in the brain, but the correlating decrease in GABA receptors in general suggests that seizures alone are not the cause of the decrease in flunitrazepam binding.
In conclusion, this study demonstrates that autistic patients have fewer GABA receptors in their anterior cingulate cortex than healthy controls. It is unclear as to what is the cause of such a decrease.
The anatomy of extended limbic pathways in Asperger syndrome: A preliminary diffusion tensor imaging tractography study
NeuroImage Volume 47, Issue 2, 15 August 2009, Pages 427-434
It has been suggested that people with autistic spectrum disorder (ASD) have altered development (and connectivity) of limbic circuits. However, direct evidence of anatomical differences specific to white matter pathways underlying social behaviour and emotions in ASD is lacking. We used Diffusion Tensor Imaging Tractography to compare, in vivo, the microstructural integrity and age-related differences in the extended limbic pathways between subjects with Asperger syndrome and healthy controls. Twenty-four males with Asperger syndrome (mean age 23 ± 12 years, age range: 9–54 years) and 42 age-matched male controls (mean age 25 ± 10 years, age range: 9–54 years) were studied. We quantified tract-specific diffusivity measurements as indirect indexes of microstructural integrity (e.g. fractional anisotropy, FA; mean diffusivity, MD) and tract volume (e.g. number of streamlines) of the main limbic tracts. The dissected limbic pathways included the inferior longitudinal fasciculus, inferior frontal occipital fasciculus, uncinate, cingulum and fornix. There were no significant between-group differences in FA and MD. However, compared to healthy controls, individuals with Asperger syndrome had a significantly higher number of streamlines in the right (p = .003) and left (p = .03) cingulum, and in the right (p = .03) and left (p = .04) inferior longitudinal fasciculus. In contrast, people with Asperger syndrome had a significantly lower number of streamlines in the right uncinate (p = .02). Within each group there were significant age-related differences in MD and number of streamlines, but not FA. However, the only significant age-related between-group difference was in mean diffusivity of the left uncinate fasciculus (Zobs = 2.05) (p = .02). Our preliminary findings suggest that people with Asperger syndrome have significant differences in the anatomy, and maturation, of some (but not all) limbic tracts.
Asperger; Limbic system; White matter; Diffusion tensor; Tractography
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ASD (which includes Asperger’s and autism) is a neurodevelopmental disorder characterized by impaired communication, difficulty in social interaction, and repetitive and stereotypic behavior (WHO, 1993). The limbic system in humans is involved in emotion regulation, attention processing, and decision making, and social behavior. Structures such as the cingulate, orbitofrontal cortex, hippocampus, and amygdale all regulate functions that ASD patients have deficits in. It has been suggested that some of the social and communication abnormalities typically found in people with ASD are secondary (Damasio and Maurer, 1978) to abnormalities in limbic structures, and perhaps also in their connectivity ([Courchesne and Pierce, 2005b]and [Wickelgren, 2005]). This study uses Diffusion Tensor Imaging and tractography to see if there are any differences in the connections in white matter tracts in the limbic system between healthy controls and Asperger’s patients.
Participants included 24 males with Asperger’s syndrome and 42 healthy controls. MRI images were acquired by multislice echo planar imaging. Whole brain volumetric measurements were acquired using Measure software.
Diffusion tensors were calculated for each voxel. From these Fractional Anisotropy (FA) maps were created which quantifies the directionality of the diffusion. Seed regions were chosen, and tracts were reconstructed by following the principle eigenvectors at each voxel. At areas where the tracts branch, new seed regions were defined at the branch so that both branches could be visualized. Number and length of streamlines (SL) were calculated for each tract. Also fractional anisotropy (FA) and mean diffusivity (MD) at regular (.5 mm) intervals along the defined tracts were extracted and the means for each tract computed
No significant difference in age, though there was a significant difference in IQ.
No statistical differences in whole brain volumes.
FA and MD values were significantly different between healthy controls and Asperger’s patients in the ILF, IFOF, and cingulated, but not after Bonferroni correction (see fig. 1 and 2).
The Asperger’s group showed higher numbers of streamlines, especially in the inferior longitudinal fasciculus and the cingulum (see Fig.3 ). Also, the Asperger’s group showed age-related differences in that their streamlines did not increase over time, whereas healthy controls did( see fig.4).
Significant differences in streamlines in inferior longitudinal fasciculus and esp. right cingulum. There do appear to be differences in microstructure and directionality of the white matter tracts in the limbic system in Asperger’s syndrome; however, these are not significant after Bonferroni correction. The Asperger’s group appear to have a higher number of streamlines but experience no growth of tracts later. Higher leptin levels were also associated with larger brain parenchymal volume, and smaller ventricular volume.
Significant differences in streamlines in inferior longitudinal fasciculus and esp. right cingulum could reflect different methods of processing of empathy and social cognition. Previous studies have showed decreased glucose metabolism in these areas; this study shows there are structural differences as well.
Anterior and posterior cortices form the “default network”, which activates when people are at rest or thinking about the future. Differences in structure in these areas could explain some of the more internal driven self-reflective thinking of ASD. FA and MD measurements, though not significant, showed higher MD and lower FA in Asperger’s patients, which suggest structural damage to axon membranes, leading to higher diffusivity and lower anisotropy.