Post-Mortem Analysis of Amygdala Neuron Morphology in ASD

Background:  

The amygdala is a medial temporal lobe structure posited to function as a danger, salience and value detector. It evaluates the emotional significance of environmental stimuli and coordinates appropriate responses. This has highly significant implications for the development of social interactions and thus makes it an ideal target for study in terms of autism spectrum disorders (ASD). Previous MRI and post-mortem studies from our lab have determined that the amygdala is larger in children diagnosed with ASD compared to typically developing (TD) controls, a difference that resolves by adulthood (Schumann et al., 2004; Schumann & Amaral, 2006).  

Objectives:  

The neurobiological underpinnings responsible for the differences in amygdala volume remain unknown. Our laboratory has been exploring two conceivable hypotheses: 1) Are there a greater number of neurons or 2) are the neurons physically larger in the amygdala of young children with ASD? This latter hypothesis is addressed here in a study of lateral nucleus neuron morphology across a range of ages from 4-46 years.

Methods:  

Tissue from 17 TD and 16 ASD cases was assembled from a combination of samples from the NIH NeuroBioBank (previously NICHD brain and tissue bank) and the brain repository at UC Davis MIND Institute. Blocks of amygdala tissue approximately 1.5x1.5x0.5cm were excised from the temporal lobe and neurons were visualized using a modified Golgi-Kopsch staining technique. After dehydration and embedding in parlodion, 150µm sections were cut on a sliding microtome. 10 lateral nucleus neurons per case were selected (fully impregnated, central within the slice and free from obscurities) to be traced using Neurolucida software (MBF Biosciences). Measures of dendrite morphology such as total dendritic length, segment count and spine density were analyzed using Neurolucida Explorer (MBF Biosciences) and statistical analyses conducted in SPSS v22 (IBM).  

Results:  

In an early analysis of a small subset (6 TD and 5 ASD cases) of the full case list, we have found a trend for a linear decrease in spine density across age (F_(1,9)=2.98, P=0.11), however this did not differ between the ASD and the TD group. Similarly with this small subset, no significant group differences were found in total dendritic length or spine density.

Conclusions:  

This is the largest post-mortem analysis comparing neuron morphology in ASD vs. typically developing controls and the first study of its kind in the amygdala. The findings help to resolve our understanding of why the amygdala grows larger in children with autism spectrum disorders.