Introduction to Autism
Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder for behavioral deficits. For more information on autism, check out this article.
To be diagnosed with ASD, an individual must show the following symptoms:
- Social interaction and communication impairments.
- Repetitive and restricted behaviors, interests, and activities.
In this article, we’ll focus on the restricted, repetitive movements in autism and we will discuss the mouse models that are being used in relevant studies.
Introduction to Restricted Repetitive Movements
Restricted, repetitive behaviors (RRBs) refer to a wide range of behaviors that involve repetition, rigidity, and invariance.[1,2] RRBs are required for the diagnosis of autism but are also present in other clinical disorders as well as in typical development.[1,2]
More specifically, RRBs include stereotyped motor movements (stereotypy refers to any behavior that is performed in excessive repetition without a clear goal, for more information on stereotypy, check out this article), repetitive handling of objects, repetitive self-harming behavior, attachments to specific objects, compulsions, rituals and routines, resistance to change/insistence on sameness, repetition of language, and narrow and restricted interests.
Moreover, RRBs can be categorized into two clusters:
- “lower-order” motor actions that are characterized by repetitive movements.
- “higher-order” behaviors that have a certain cognitive component.
Existing animal models of restricted, repetitive behavior in autism have provided us with findings regarding the genetic background (gene by environment relationships) and neuropathology of this disorder. Such findings are quite crucial for the examination of the phenotypic expression of restricted repetitive behavior in autism and the development of the appropriate treatment.
Genetics of Restricted Repetitive Behavior
Autism has a strong genetic component and it is likely that genes that control RRBs are independent of genes that control social and communication deficits. Studies have shown that repetitive behavior has been linked to mutations at disparate chromosomal loci. A large number of genes are being implicated in the expression of RRBs. Mutations of even one of these genes can lead to the expression of the behavioral phenotype.
Neuropathology of Restricted Repetitive Behavior
Repetitive behavior has been linked to regional volumetric differences. For instance, increased right caudate volumes have been found in individuals with autism with an association between right caudate volume and the insistence on sameness/resistance to change.
Moreover, restricted repetitive behavior has been linked to activation of the anterior cingulate cortex (ACC).
Animal Models of Restricted Repetitive Behavior
Animal models of restricted repetitive behavior in humans can be categorized into three classes: repetitive behavior associated with:
- targeted insults to the central nervous system (CNS),
- administration of specific pharmacological agents
- exposure to restricted environments and experience.[1,2]
1) Repetitive behavior and targeted CNS insult
Gene targeting technologies are used for the generation of mutant mouse models of various neurodevelopmental disorders. For instance:
- Mutations in the methylCpG binding protein 2 (MECP2) gene are responsible for the majority of cases of Rett syndrome, which is a developmental disorder. Mice that express truncated MeCP2 protein display repetitive forelimb movements.
- The gabrb3 homozygous knockout mouse exhibits stereotyped behavior such as intense circling and “tail-chasing”.
- The Hoxb8 homozygous mutant mouse displays compulsive grooming which leads to hair removal and self-inflicted wounds. High levels of expression of Hoxb8 have been observed in brain regions that comprise circuitry mediating obsessive compulsive disorder (OCD) symptoms in patients. This model is relevant to self-injurious behavior.
- Studies have also tested various inbred mouse strains for autistic-like traits. For example, C58/J mice exhibit nondrug-related stereotyped jumping and backward flipping behaviors. Furthermore, these mice display reduced exploratory behavior, compared to control strains. Another study showed that mice with C57BL/10 strain exhibit spontaneous repetitive vertical jumping, when this behavior is not being observed in the closely related C57BL/6 strain.
2) Drug-induced repetitive behavior
Drug-induced stereotyped behavior in animals has taught us a lot about the neurobiological basis of repetitive behavior. For instance, it has been shown that injection of the glutamate receptor ligand, NMDA into the striatum induces stereotyped behavior.
3) Repetitive behavior and environmental restriction
Housing restrictions in animals (e.g., zoo, farm, laboratory) might cause abnormal repetitive behaviors. For instance, it has been shown that deer mice (Peromyscus maniculatus) display repetitive hindlimb jumping and backward somersaulting because of restrictions in the housing conditions (standard laboratory caging).
Repetitive Behaviors in ASD Mouse Models
ASD mouse models can be also categorized based on the repetitive behaviors that they display. Some repetitive restricted behaviors in ASD mouse models are the following:
- Marble burying
Under normal circumstances, mice scratch and brush their hair with their forelimbs for a few seconds to minutes.  However, when self-grooming is displayed at a higher rate and for a longer time, it can be considered as repetitive behavior.
For instance, BTBR T + tf/J (BTBR) mice (an inbred strain without corpus callosum) are used as a model of idiopathic autism and exhibit increased repetitive self-grooming. This excessive self-grooming can be treated by inhibiting mGluR5 activity. Moreover, Contactin-associated protein-like 2 (Cntnap2) −/− mice also display abnormal grooming, which can be treated with risperidone, an antagonist to the dopamine D2 receptor (D2R). For more information on self-grooming, check out this article
C57BL/6 mouse is a normal control mouse that is frequently used in behavioral research and jumps using its hind limbs. In contrast with C57BL/6 mice, the C58/J mice are characterized by excessive jumping. Also, Shank2 −/− mice exhibit repetitive jumping and scrabbling behaviors.
4) Marble burying:
Marble burying is characterized by burying marbles scattered on the bedding into the bedding. This behavior can be associated with anxiety to a novel context and exploration. Some mice with increased marble burying also exhibit increased locomotor activity. Such mouse models are: Shank1 −/− mice, Ephrin-A −/− and Ephrin-A3 −/− mice, and C58/J mice. On the other hand, mice with decreased marble-burying also exhibit decreased locomotor activity. Such mouse models are BTBR mice, Eif4ebp2 −/− mice, and FMR1 −/− mice.
Resistance to Change/Insistence on Sameness
Cognitive flexibility, or resistance to change, can be assessed in animals using tasks that differ in complexity. For instance, the performance of deer mice was examined in a procedural learning task, in which mice had to learn to turn down the right or left arm of a T-maze for reinforcement (for more information on the T-maze, check out this article).
After the acquisition, the reinforced arm would be reversed. It was found that high levels of stereotypy in deer mice are associated with deficits in reversal learning in the T-maze. Also, environmental enrichment is linked to better reversal learning and decreased stereotyped motor behavior.
Neurocircuitry of Restricted Repetitive Behavior
A) Cortico-basal ganglia circuitry in repetitive behavior
It is also quite important to gain further knowledge regarding the neurocircuitry of restricted repetitive behavior. Pathways that link areas of the cortex and basal ganglia are considered to be involved in the expression of repetitive behaviors. The striatum is a key component of this circuitry.
The SAPAP3 knockout mouse has been used to point out the importance of cortico-basal ganglia circuitry in repetitive behavior. SAPAP3 is a postsynaptic scaffolding protein, which is highly expressed in the striatum and plays a major role in regulating glutamatergic cortico-striatal synapses. The SAPAP3 knockout mice display excessive grooming that might induce lesions to the head, neck, and snout.
B) Environmental enrichment and repetitive behavior
Studies have examined the role of cortical-basal ganglia circuitry in mediating stereotypy in deer mice by blocking corticostriatal glutamatergic projections or nigrostriatal dopaminergic projections using pharmacological agents. Intrastriatal administration of either the NMDA receptor antagonist MK-801 or the D1 dopamine receptor antagonist SCH23390 leads to attenuation of stereotypy.
C) Direct and indirect pathways in repetitive behavior
Repetitive behavior in deer mice is associated with an imbalance in the activity of the direct and indirect pathways of the basal ganglia, indicating overactivity of the direct pathway. A study measured the concentrations of the striatal neuropeptides dynorphin and enkephalin in dorsolateral striatum deer mice that exhibited both high and low stereotypy (housed in standard cages).
Decreased leu-enkephalin content and increased [dynorphin]/[enkephalin] content ratios have been found in the high-stereotypy mice relative to low-stereotypy mice. Also, a significant negative correlation between striatal enkephalin content (indirect pathway) and frequency of stereotypy has been found in deer mice. On the other hand, a significant positive correlation between the dynorphin/enkephalin content (direct pathway) and frequency of stereotypy has been found in these mice.
The development of repetitive behavior in individuals with autism involves long-term, striatal plasticity, which depends on experience. Studies of habit learning or habit formation examine such neuroadaptations. Molecular mechanisms involved in such experience-dependent neuroadaptations involve transcription factors that can cause changes in gene expression.
For instance, in mouse models of L-DOPA-induced dyskinesias, activation of ERK, which is the extracellular-regulated kinases that mediate downstream transcription, is restricted to the direct pathway neurons. Compulsive wheel running leads to the activation of the transcription factor ∆FosB in striatal direct pathway neurons. Transgenic animals that selectively overexpress ∆FosB in striatal neurons (direct pathway) display compulsive wheel running, whereas wheel running is inhibited in animals that overexpress the gene in enkephalin-containing neurons (indirect pathway).
The use of animal models is quite important to understand the pathophysiology of restricted repetitive behavior and thus, develop the appropriate treatments. Greater use of animal models should be used to gain further knowledge in the different domains that can lead to the expression of restricted repetitive behavior.
Future research should focus on specific brain regions that are associated with restricted repetitive behaviors in ASD mouse models.
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