Parent-child DRD4 genotype as a potential biomarker for oppositional, anxiety, and repetitive behaviors in children with autism spectrum disorder

Anhedonia and Hyperhedonia in Autism and Related Neurodevelopmental Disorders

Although autism spectrum disorder (ASD) is defined by impaired social communication and restricted and repetitive behaviors and interests, ASD is also characterized by impaired motivational processes. The "social motivation theory of autism" describes how social motivation disruptions in ASD in early childhood may impede the drive to engage in reciprocal social behaviors and ultimately interfere with the development of neural networks critical for social communication (Chevallier et al., Trends Cogn Sci 16:231-239, 2012b). Importantly, clinical studies and preclinical research using model organisms for ASD indicate that motivational impairments in ASD are not constrained to social rewards but are evident in response to a range of nonsocial rewards as well. Additionally, translational studies on certain genetically defined neurodevelopmental disorders associated with ASD indicate that these syndromic forms of ASD are also characterized by motivational deficits and mesolimbic dopamine impairments. In this chapter we summarize clinical and preclinical research relevant to reward processing impairments in ASD and related neurodevelopmental disorders. We also propose a nosology to describe reward processing impairments in these disorders that uses a three-axes model. In this triaxial nosology, the first axis defines the direction of the reward response (i.e., anhedonic, hyperhedonic); the second axis defines the construct of the reward process (e.g., reward liking, reward wanting); and the third axis defines the context of the reward response (e.g., social, nonsocial). A more precise nosology for describing reward processing impairments in ASD and related neurodevelopmental disorders will aid in the translation of preclinical research to clinical investigations which will ultimately help to speed up the development of interventions that target motivational systems for ASD and related neurodevelopmental disorders.

A simultaneous [11C]raclopride positron emission tomography and functional magnetic resonance imaging investigation of striatal dopamine binding in autism

The social motivation hypothesis of autism posits that autism spectrum disorder (ASD) is characterized by impaired motivation to seek out social experience early in life that interferes with the development of social functioning. This framework suggests that impaired mesolimbic dopamine function underlies compromised responses to social rewards in ASD. Although this hypothesis is supported by functional magnetic resonance imaging (fMRI) studies, no molecular imaging study has evaluated striatal dopamine functioning in response to rewards in ASD. Here, we examined striatal functioning during monetary incentive processing in ASD and controls using simultaneous positron emission tomography (PET) and fMRI. Using a bolus + infusion protocol with the D2/D3 dopamine receptor antagonist [¹¹C]raclopride, voxel-wise binding potential (BP_ND) was compared between groups (controls = 12, ASD = 10) in the striatum. Striatal clusters showing significant between-group BP_ND differences were used as seeds in whole-brain fMRI general functional connectivity analyses. Relative to controls, the ASD group demonstrated decreased phasic dopamine release to incentives in the bilateral putamen and left caudate, as well as increased functional connectivity between a PET-derived right putamen seed and the precuneus and insula. Within the ASD group, decreased phasic dopamine release in the putamen was related to poorer theory-of-mind skills. Our findings that ASD is characterized by impaired striatal phasic dopamine release to incentives provide support for the social motivation hypothesis of autism. PET-fMRI may be a suitable tool to evaluate novel ASD therapeutics targeting the striatal dopamine system.

A potential mechanistic role for neuroinflammation in reward processing impairments in autism spectrum disorder

Accumulating evidence suggests that autism spectrum disorder (ASD) may be conceptualized within a framework of reward processing impairments. The Social Motivation Theory of Autism posits that reduced motivation to interact with people and decreased pleasure derived from social interactions may derail typical social development and contribute to the emergence of core social communication deficits in ASD. Neuroinflammation may disrupt the development of mesolimbic dopaminergic systems that are critical for optimal functioning of social reward processing systems. This neuroinflammation-induced disturbance of mesolimbic dopaminergic functioning has been substantiated using maternal immune activation rodent models whose offspring show aberrant dopaminergic corticostriatal function, as well as behavioral characteristics of ASD model systems. Preclinical findings are in turn supported by clinical evidence of increased mesolimbic neuroinflammatory responses in individuals with ASD. This review summarizes evidence for reward processing deficits and neuroinflammatory impairments in ASD and examines how immune inflammatory dysregulation may impair the development of dopaminergic mesolimbic circuitry in ASD. Finally, future research directions examining neuroinflammatory effects on reward processing in ASD are proposed.

Using Sibling Designs to Understand Neurodevelopmental Disorders: From Genes and Environments to Prevention Programming

Neurodevelopmental disorders represent a broad class of childhood neurological conditions that have a significant bearing on the wellbeing of children, families, and communities. In this review, we draw on evidence from two common and widely studied neurodevelopmental disorders-autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD)-to demonstrate the utility of genetically informed sibling designs in uncovering the nature and pathogenesis of these conditions. Specifically, we examine how twin, recurrence risk, and infant prospective tracking studies have contributed to our understanding of genetic and environmental liabilities towards neurodevelopmental morbidity through their impact on neurocognitive processes and structural/functional neuroanatomy. It is suggested that the siblings of children with ASD and ADHD are at risk not only of clinically elevated problems in these areas, but also of subthreshold symptoms and/or subtle impairments in various neurocognitive skills and other domains of psychosocial health. Finally, we close with a discussion on the practical relevance of sibling designs and how these might be used in the service of early screening, prevention, and intervention efforts that aim to alleviate the negative downstream consequences associated with disorders of neurodevelopment.

Enhancement of gamma activity after selective activation of dopamine D4 receptors in freely moving rats and in a neurodevelopmental model of schizophrenia

Dopamine D4 receptor (D4R) mechanisms have been implicated in several psychiatric diseases, including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and autism, which are characterized by cognitive deficits. The cellular mechanisms are poorly understood but impaired neuronal synchronization within cortical networks in the gamma frequency band has been proposed to contribute to these deficits. A D4R polymorphism was recently linked to variations in gamma power in both normal and ADHD subjects, and D4R activation was shown to enhance kainate-induced gamma oscillations in brain slices in vitro. The goal of this study was to investigate the effect of D4R activation on gamma oscillations in freely moving rats during natural behavior. Field potentials were recorded in the frontal, prefrontal, parietal, and occipital cortex and hippocampus. Gamma power was assessed before and after subcutaneous injection of a D4R agonist, A-412997, in several doses between 0.3 and 10.0 mg/kg. The experiments were also repeated in a neurodevelopmental model of schizophrenia, in which rats are prenatally treated with methylazoxymethanol (MAM). We found that the D4R agonist increased gamma power in all regions at short latency and lasted for ~2 h, both in normal and MAM-treated rats. The effect was dose dependent indicated by the significant difference between the effects after 3 and 10 mg/kg in pair-wise comparison, whereas 0.3 and 1.0 mg/kg injections were ineffective. This study demonstrates the involvement of D4R in cortical gamma oscillations in vivo and identifies this receptor as potential target for pharmacological treatment of cognitive deficits.

Maternal and offspring dopamine D4 receptor genotypes interact to influence juvenile impulsivity in vervet monkeys

The merging of psychological and genetic methodologies has led to an increasing appreciation of environmental moderators of the relationships between genotype and phenotype. Here we used a nonhuman-primate model to study the moderating effect of the mother's genotype on the association of a dopamine D4 receptor (DRD4) gene polymorphism with juvenile impulsivity, assessed in a standardized social-challenge test. The results showed that juvenile carriers of the rare 5-repeat variant of the exon III 48-base-pair repeat polymorphism scored significantly higher in social impulsivity than juveniles homozygous for the common 6-repeat allele. In addition, juvenile genotype interacted with maternal genotype to influence impulsivity, with the highest rates of impulsivity found in variant offspring with variant mothers. These results highlight the importance of considering the genotype of the parents in studies of early experience and vulnerability genes for impulsivity-related traits.