Altered neural connectivity in excitatory and inhibitory cortical circuits in autism

Abstract

Converging evidence from diverse studies suggests that atypical brain connectivity in autism affects in distinct ways short- and long-range cortical pathways, disrupting neural communication and the balance of excitation and inhibition. This hypothesis is based mostly on functional non-invasive studies that show atypical synchronization and connectivity patterns between cortical areas in children and adults with autism. Indirect methods to study the course and integrity of major brain pathways at low resolution show changes in fractional anisotropy (FA) or diffusivity of the white matter in autism. Findings in post-mortem brains of adults with autism provide evidence of changes in the fine structure of axons below prefrontal cortices, which communicate over short- or long-range pathways with other cortices and subcortical structures. Here we focus on evidence of cellular and axon features that likely underlie the changes in short- and long-range communication in autism. We review recent findings of changes in the shape, thickness, and volume of brain areas, cytoarchitecture, neuronal morphology, cellular elements, and structural and neurochemical features of individual axons in the white matter, where pathology is evident even in gross images. We relate cellular and molecular features to imaging and genetic studies that highlight a variety of polymorphisms and epigenetic factors that primarily affect neurite growth and synapse formation and function in autism. We report preliminary findings of changes in autism in the ratio of distinct types of inhibitory neurons in prefrontal cortex, known to shape network dynamics and the balance of excitation and inhibition. Finally we present a model that synthesizes diverse findings by relating them to developmental events, with a goal to identify common processes that perturb development in autism and affect neural communication, reflected in altered patterns of attention, social interactions, and language.

Keywords: GAP-43; anterior cingulate cortex; myelinated axons; parvalbumin-positive interneurons; prefrontal cortex (PFC); ratio of excitation and inhibition; short-range and long-distance pathways; white matter.

Figure 1

High resolution segmentation of the white matter. (A) Coronal view of a representative ACC (A32) tissue slab. Dotted lines indicate gross (macroscopic) distinction of superficial (SWM) and deep (DWM) white matter, based on subsequent microscopic analysis. (B,C) Fluorescent photomicrographs of coronal sections from A32 white matter after labeling of axons with a neurofilament protein antibody (NFP-200; green). Light microscopic segmentation of superficial (B) and deep (C) white matter is based on the distinct orientation of axons at different depths from the gray matter. Axons in the superficial white matter travel mainly perpendicular to the surface of the cortex (B, axons appear mainly as thin lines), whereas in the deep white matter most axons travel parallel to the cortical surface (C, axons appear mainly as green dots). (D,E) EM photomicrographs show mostly elongated axon profiles in the superficial white matter (D) and mostly circular axon profiles in the deep white matter (E). Adapted from Zikopoulos and Barbas (2010).

Figure 2

Changes in myelinated axons below prefrontal cortices in adults with ASD. (A) In the superficial white matter (SWM) below ACC (area 32) the relative density of small (thin) axons (±SEM) is increased in the autistic cases, and more axons branch and express GAP-43. These data suggest increased local connectivity of ACC in ASD. In contrast, in the deep white matter (DWM) below ACC the relative density of large axons is reduced in ASD, suggesting weakening of long-range connectivity. Thinning of the myelin in axons of all sizes just below OFC (area 11) suggests weak local connections. (B,C) Laminar and overall neuronal density below ACC, OFC, and LPFC is similar in adults with ASD and controls and is not correlated with the changes in axons below PFC. (D) EM photomicrograph of axons in the superficial white matter below ACC of an adult with ASD. (E) Collapsed image (z-projection) from a three-dimensional confocal stack shows myelinated axons branching, labeled with NFP-200 (green). A branching axon is pseudo colored with orange/yellow hue for visualization (yellow arrowheads point to branches). (F) Image from a three-dimensional confocal stack with double immunofluorescence shows GAP-43 (red) in axons labeled with NFP-200 (green). Some myelinated axons contain GAP-43 in their axolemma, which is transported to axon terminals and branching points. Colocalization of the two antibodies is rendered white. (G) EM photomicrographs show differences in myelin thickness in OFC between control and autistic adults, apparent in all axon size groups.

Figure 3

Map of human frontal areas. (A) Lateral view of the human brain shows the dorsolateral prefrontal area 9 and its relationship with other frontal areas. Dotted lines indicate the coronal level used for analysis. (B) One centimeter thick slab of frontal cortex shows the region sampled in the dorsolateral prefrontal cortex (red dotted-line square). See Appendix for abbreviations.

Figure 4

There is a decrease in the ratio of parvalbumin (PV) to calbindin (CB) inhibitory neurons in area 9 of the human brain in autism. (A) Fluorescent photomicrograph shows the preferential laminar distribution of CB (red) in the superficial layers and PV (green) in the middle-deep layers of the human dorsolateral prefrontal cortex. (B,C) High magnification photographs of CB and PV neurons in the human dorsolateral prefrontal cortex (indicated by blue arrows). (D) Preliminary results show lower density of PV neurons in autistic cases (cells/mm³ ± standard deviation). (E,F) Low magnification photographs of PV neurons in the dorsolateral prefrontal cortex (indicated by blue arrows) of control and ASD adults.

Figure 5

Relationship of axonal features to developmental events. Changes in axons and inhibitory neurotransmission affect network dynamics in ASD. ACC exhibits local overconnectivity in ASD, which combined with changes in the ratio of inhibitory neurons in LPFC can tip the balance of excitation and inhibition. OFC exhibits weak local connectivity in ASD due to thinning of the myelin, which may affect conduction velocity. Overall, prefrontal areas exhibit weakening in their long-distance connections. This connectivity pattern is supported by structural and functional data. Black lines indicate typical connectivity and purple lines indicate connectivity in ASD. The thickness of the line indicates the strength of a connection.