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Review
. 2016 Feb 9:10:27.
doi: 10.3389/fnins.2016.00027. eCollection 2016.

Striatal Circuits as a Common Node for Autism Pathophysiology

Affiliations
Review

Striatal Circuits as a Common Node for Autism Pathophysiology

Marc V Fuccillo. Front Neurosci. .

Abstract

Autism spectrum disorders (ASD) are characterized by two seemingly unrelated symptom domains-deficits in social interactions and restrictive, repetitive patterns of behavioral output. Whether the diverse nature of ASD symptomatology represents distributed dysfunction of brain networks or abnormalities within specific neural circuits is unclear. Striatal dysfunction is postulated to underlie the repetitive motor behaviors seen in ASD, and neurological and brain-imaging studies have supported this assumption. However, as our appreciation of striatal function expands to include regulation of behavioral flexibility, motivational state, goal-directed learning, and attention, we consider whether alterations in striatal physiology are a central node mediating a range of autism-associated behaviors, including social and cognitive deficits that are hallmarks of the disease. This review investigates multiple genetic mouse models of ASD to explore whether abnormalities in striatal circuits constitute a common pathophysiological mechanism in the development of autism-related behaviors. Despite the heterogeneity of genetic insult investigated, numerous genetic ASD models display alterations in the structure and function of striatal circuits, as well as abnormal behaviors including repetitive grooming, stereotypic motor routines, deficits in social interaction and decision-making. Comparative analysis in rodents provides a unique opportunity to leverage growing genetic association data to reveal canonical neural circuits whose dysfunction directly contributes to discrete aspects of ASD symptomatology. The description of such circuits could provide both organizing principles for understanding the complex genetic etiology of ASD as well as novel treatment routes. Furthermore, this focus on striatal mechanisms of behavioral regulation may also prove useful for exploring the pathogenesis of other neuropsychiatric diseases, which display overlapping behavioral deficits with ASD.

Keywords: autism spectrum disorders; circuit; dorsal striatum; mouse models; nucleus accumbens (NAcc); synaptic transmission.

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Figures

Figure 1
Figure 1
Simplified schematic of the input and local connectivity of the mammalian basal ganglia. (A) Diagram of known excitatory inputs to distinct striatal sub-regions including dorsomedial striatum (DMS), dorsolateral striatum (DLS), and nucleus accumbens (NAc). All striatal sub-regions are under dopaminergic and serotonergic neuromodulatory control (orange). (B) Diagram of major dorsal striatal cell types and their downstream connectivity within basal ganglia circuits. In contrast to striatal inputs, nearly all cell types within the basal ganglia are inhibitory. The striatum is comprised of D1R+ (red) and D2R+ (green) medium spiny neurons (MSNs), as well as a smaller population of local circuit interneurons (tan). The direct pathway projection of D1R+ MSNs is diagrammed in red and the indirect pathway projection of D2R+ MSNs is drawn in green. (C) Diagram of the major cell types and connectivity of the nucleus accumbens core. Note that in contrast to the strict divergence of D1R+ and D2R+ MSNs in the dorsal striatum, D1R+ MSNs of the nucleus accumbens project both to the ventral tegmental area and the ventral pallidum.
Figure 2
Figure 2
Striatal dysfunction and major autism spectrum disorder symptom domains. Schematic illustrating hypothetical connections between abnormalities of striatal function and the varied clinical phenotypes observed in ASDs.
Figure 3
Figure 3
Mouse models of ASD-associated genes display abnormalities in striatal structure and function. (A) FMR1 KO mice display abnormalities in the regulation of endocannabinoid signaling within the striatum, with decreased 2-arachidonylglycerol (2-AG) production in the NAc leading to altered plasticity of excitatory inputs (left) and increased 2-AG production in the dorsal striatum enhancing depression of inhibitory transmission onto dorsal striatal MSNs (right). (B) 16p11.2 heterozygotes exhibit an increase in excitatory synaptic transmission onto D2R+ MSNs of the NAc (left) while both the dorsal and ventral striatum have larger overall numbers of D2R+ MSNs (right). (C) Loss of Met receptor function within ventral telencephalic progenitors leads to an increase in the number of parvalbumin and somatostatin-positive striatal interneurons at the expense of cortical interneuron populations (left). Met receptor KO mice display enhanced connectivity of superficial layer cortical neurons that synapse on corticostriatal projection neurons (right). (D) Shank3 KO mice exhibit gross abnormalities in synaptic structure, alterations in the protein architecture of synapses and a decrease in general excitatory synaptic strength within the dorsal striatum. (E, left) Cntnap2 KO mice exhibit deficits in migration of interneuron progenitors such that striatal interneuron populations are decreased. Cntnap4 KO mice demonstrate enhanced release of dopamine specifically within the nucleus accumbens (right).
Figure 4
Figure 4
Circuit-specific analysis of ASD-related behaviors points toward an underlying striatal deficit. (A) Targeted removal of oxytocin receptor within the dorsal raphe demonstrates a key role for this molecule in formation of social conditioned place preference, a measure of social reward. Oxytocin receptor functions on dorsal raphe terminals within the nucleus accumbens to regulate the release of serotonin, which can hetero-synaptically modulate excitatory transmission onto MSNs through presynaptic Htr1b receptors. (B) Removal of Neuroligin-3, a synaptic adhesion molecule associated with ASD, from D1R+ MSNs of the nucleus accumbens is sufficient to drive the enhanced formation of repetitive motor routines, as assayed by learning on the accelerating rotarod (KOs display an increased and earlier stereotyped pattern of foot placements compared with WTs). In addition, Neuroligin-3 KO mice have a cell type- specific deficit in inhibitory transmission onto accumbens D1R+ MSNs, which presumably leads to an increase in output from this circuit.

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