A longitudinal study of neurobiological mechanisms in boys with attention-deficit hyperactivity disorder: preliminary findings

Annual research review: Infant development, autism, and ADHD--early pathways to emerging disorders

BACKGROUND

Autism spectrum disorders (ASD) and attention deficit hyperactivity disorder (ADHD) are two of the most common neurodevelopmental disorders, with a high degree of co-occurrence.

METHODS

Prospective longitudinal studies of infants who later meet criteria for ASD or ADHD offer the opportunity to determine whether the two disorders share developmental pathways.

RESULTS

Prospective studies of younger siblings of children with autism have revealed a range of infant behavioral and neural markers associated with later diagnosis of ASD. Research on infants with later ADHD is less developed, but emerging evidence reveals a number of relations between infant measures and later symptoms of inattention and hyperactivity.

CONCLUSIONS

We review this literature, highlighting points of convergence and divergence in the early pathways to ASD and ADHD.

Elucidation of neurobiology of anxiety disorders in children through pharmacological challenge tests and cortisol measurements: a systematic review

Anxiety disorders are common both in adults and children. While there have been major advances in understanding the neurobiology of anxiety disorders in adults, progress has been more limited in the elucidation of the mechanisms underlying these disorders in childhood. There is a need to delineate childhood biological models, since anxiety represents a significant clinical problem in children and is a risk factor for the subsequent development of anxiety and depression in adulthood. We conducted a review of the literature regarding pharmacological challenge tests and direct hypothalamic-pituitary-adrenal axis measurement in children with anxiety disorders, with emphasis on panic disorder and social anxiety disorder. Studies identified were contrasted with those in adult panic disorder and social anxiety disorder. Despite this broad approach few studies emerged in children, with only 22 studies meeting inclusion criteria. When contrasted with adult neurobiological models of panic disorder and social anxiety disorder, children studied showed some abnormalities which mirrored those reported in adults, such as altered baseline respiration, altered responses to CO(2) challenge tests and blunted growth hormone response to yohimbine. However, results differed from adults with panic disorder and social anxiety in some aspects of noradrenergic and serotonergic function. For endpoints studied in panic disorder children, unlike adults, displayed a lack of baseline end-tidal CO(2) abnormalities and a different hypothalamic-pituitary-adrenal pattern response under low-dose CO(2). The biology of these anxiety disorders in children may only partially mirror that of adult anxiety disorders. However, caution is required as the evidence is limited, and many studies combined patients with panic disorder and social anxiety disorder with other disorders or non-specific anxiety. Further research is required to fully understand the biology and progression of childhood anxiety disorders.

Dopamine system genes and ADHD: a review of the evidence

The search for genes influencing the development of attention-deficit/hyperactivity disorder (ADHD) has identified a number of associated genes within, or influencing, the dopamine neurotransmitter system. The focus on this system as the site of genetic susceptibility was prompted by information from animal models, particularly transgenics, as well as the mechanism of action of the psychostimulants, the primary pharmacological treatment for ADHD. Thus far, genes in the dopamine system reported as associated with ADHD, by at least one study, include the dopamine transporter, the dopamine receptors D1, D4 and D5, as well as genes encoding proteins that control the synthesis, degradation and release of dopamine. For some of these genes, replication across studies provides evidence supporting the relationship; however, for others, the data is far from conclusive and further work is needed. The quick progress in the genetic findings was initially surprising given the complexity of the phenotype and the relatively small sample sizes used in the initial studies. However, the high heritability of ADHD, as indicated by twin studies, may have contributed to the success. The genes studied so far are estimated to contribute only weakly or moderately to the risk for the development of ADHD. This may be because these genes, in fact, make only a small contribution. However, few studies have comprehensively examined the genetic information across the gene. This will lead to underestimates of risk if the polymorphism(s) tested is/are not the functional change(s) actually contributing to the genetic susceptibility and if linkage disequilibrium between tested marker(s) and causal variant(s) is weak, or if there is substantial allelic heterogeneity. While the studies thus far are very promising, virtually nothing is known on precisely how genetic variation in these genes actually contributes to risk; thus, functional studies are now required.

The interaction of psychosocial adversity and biological risk in childhood aggression

Childhood aggression has both biological and environmental underpinnings. However, the manner in which these factors interact to influence various types of aggression remains an important area of study. The current study examined the degree to which biological risk and psychosocial adversity, both alone and in combination, are associated with childhood aggression. Linear regression procedures were used to assess the extent to which biological risk status (low vs. high serotonergic responsivity, as measured by prolactin response to fenfluramine), magnitude of psychosocial risk, and the interaction of these factors predicted parent and teacher ratings of aggression and delinquency. After accounting for the independent contribution of biological and psychosocial risk, the interaction of biological and psychosocial risk was significantly associated with parent-rated aggression and marginally related to parent-rated delinquency. In contrast, no such interaction was observed for teacher-rated aggression. Findings suggest that individuals at biological risk for aggression may be particularly vulnerable to the impact of psychosocial adversity.

Monoamine oxidase A gene polymorphism predicts adolescent outcome of attention-deficit/hyperactivity disorder

ADHD is generally deemed to be a highly heritable disorder with mean heritability of 0.75. The enzyme monoamine oxidase (MAO), which has both A and B types, has long been considered a candidate pathological substrate for ADHD, and more recently, the genes for both MAO enzymes have been examined as mediators of the illness. Previous studies indicated that 30-50% of children with ADHD will experience symptoms that persist into adolescence and will have more significant impairment in social and neuropsychological functioning compared to those whose symptoms have remitted. Genes may also influence these characteristics of the disorder, and in this context MAO genes may also be candidates for moderating the presentation of ADHD. The current study examined the association between adolescent outcome of ADHD and MAO gene polymorphisms, including the 941T > G polymorphism in exon 8 (rs1799835) and 1460C > T polymorphism in exon 14 (rs1137070) of the MAOA gene, and the A > G polymorphism in intron13 (rs1799836), C > T polymorphism in the 3'UTR (rs1040399), and 2327T > C polymorphism in exon15 of the MAOB gene. Significant associations were observed between the MAOA gene polymorphisms and ADHD remission. Due to the small sample size and the possibility of phenotypic and etiologic heterogeneity of ADHD outcomes across ethnic or geographic groups, these results must be replicated before they can be generalized to other populations.

Contribution of 5-HT2A receptor gene -1438A>G polymorphism to outcome of attention-deficit/hyperactivity disorder in adolescents

Attention-deficit/hyperactivity disorder (ADHD) typically emerges before 7 years of age and may persist into adolescence or adulthood. The adolescent outcome can be classified into four types, including non-remission, syndromatic remission, symptomatic remission, and functional remission. Genetic factors are believed to contribute to symptom stability and change across development, so adolescent outcome may be a sub-phenotype for molecular genetic studies of ADHD. Serotonin system genes are prime candidates for this sub-phenotype, since the development of this neurotransmitter system parallels the course of ADHD. The current study examined the association between adolescent outcome in ADHD and serotonin system genes, including the -1438A>G polymorphism of the serotonin 2A receptor gene (HTR2A) and the -759C>T polymorphism of the serotonin 2C receptor gene (HTR2C). The -1438A>G polymorphism was found to be related to remission in ADHD, especially functional remission (P = 0.029). Due to potential phenotypic and etiologic heterogeneity in ADHD, the results of this study must be replicated in additional samples before they can be generalized to other populations.

Elevated childhood serotonergic function protects against adolescent aggression in disruptive boys

OBJECTIVE

This longitudinal study examined whether responsiveness of the neurotransmitter serotonin (5-HT) in childhood predicts adolescent aggression.

METHOD

Boys (N = 33) with disruptive behavior disorders who received assessments of central 5-HT function via the prolactin response to fenfluramine between 1990 and 1994 when they were 7 to 11 years old were re-evaluated clinically on average 6.7 years later.

RESULTS

After accounting for baseline aggression, early 5-HT function accounted for a significant proportion of variance in adolescent aggression. This prospective relationship of childhood 5-HT function with adolescent aggression (r = -0.71) and antisocial behavior (r = -0.59) was found primarily in adolescents who were aggressive during childhood. Irrespective of childhood aggression, no child with high 5-HT function was particularly aggressive at follow-up.

CONCLUSIONS

Low childhood 5-HT function appears important, but not sufficient, for the emergence of adolescent aggression. However, early high 5-HT function may protect against adolescent violence and aggression.

Familial correlates of central serotonin function in children with disruptive behavior disorders

Previous findings suggest a relationship between childhood aggression, parental history of aggression and central serotonin (5-HT) function. The present study extended these findings by examining the impact of childhood aggression and central 5-HT function on the incidence of psychopathology in first- and second-degree relatives of pre-pubertal children with disruptive behavior disorders. Family history of psychopathology was obtained for 58 aggressive and 44 non-aggressive clinically referred children who were further sub-divided based on central 5-HT function. Central 5-HT function was assessed by measuring the prolactin response to a 1 mg/kg oral dose of d,l-fenfluramine. Aggressive children with low-prolactin responses to fenfluramine had a significantly greater incidence of first- and second-degree relatives with aggressive and antisocial characteristics compared to both non-aggressive children and aggressive children with high-prolactin responses. No group differences were found in the frequency of relatives with symptoms of cognitive impairment or inattention and hyperactivity. These data suggest that there are both familial and non-familial forms of aggression in children, and that only the familial type is associated with reduced 5-HT function.

Rates of adherence to pharmacological treatment among children and adolescents with attention deficit hyperactivity disorder

Pharmacological intervention, mainly with psychostimulants, alone or with psychotherapy or behavioural modification, was found to be effective in increasing sustained attention span, improving concentration, reducing hyperactive behaviour and improving areas of academic deficits in children and adolescents with the diagnosis of attention deficit-hyperactivity disorder (ADHD). Despite their proven efficacy, noncompliance of the children and adolescents to the prescribed medication presents serious problems to patients and health care providers alike.

OBJECTIVE

To investigate the rate of adherence to prescribed medication in a clinically referred sample of children and adolescents diagnosed as having ADHD. In addition, the stability of reports of adherence over a 3 month period was explored.

METHOD

Fifty-one children and adolescents (males: n = 42; females: n = 9) between the age of 7 years and 16.6 years diagnosed with ADHD and their parents were administered a children behaviour checklist, a teacher report form scale and a compliance with treatment opinion and attitude scale.

RESULTS

There were very high reports of adherence by children to prescribed medications for ADHD with rates of compliance greater than 70%. Correlation between the children and adolescents' reports and the parents' reports revealed high agreement both at the end of week 1 and at the end of the study (week 12). There were also findings of stability of adherence reports over a 3 month period.

CONCLUSION

The results of this study documented high rates of adherence to medication prescribed for symptoms of attention deficit hyperactivity disorder in a sample of children and adolescents. Their reports of adherence were well correlated with parents' reports. Several factors were found to be related to the high level of adherence.

Relationship between central serotonergic function and aggression in prepubertal boys: effect of age and attention-deficit/hyperactivity disorder

Data indicate that diminished central serotonergic (5-HT) function is related to aggression in adults, but discrepant findings in children suggest that age or the presence of attention-deficit/hyperactivity disorder (ADHD) may influence this relationship. This study examined whether age or ADHD affects the association between 5-HT and aggression in 7-11-year old clinically-referred boys. Forty-six boys were divided into non-aggressive ADHD, aggressive ADHD, and aggressive non-ADHD groups based on responses to interviews and ratings of behavior. Central 5-HT function was assessed by measuring the prolactin response to a 1-mg/kg oral dose of D,L-fenfluramine. There was no significant difference in the prolactin response across the three groups of boys. Furthermore, when examined dimensionally, prolactin response was largely unrelated to ratings of aggression, even after controlling for ADHD. Finally, age was not associated with prolactin response, and had no effect on the relationship between prolactin response and aggression. This study provides further evidence that there is no clear relationship between central 5-HT function and aggression in disruptive boys. Moreover, these data do not confirm the hypothesis that age or the presence of ADHD influence the relationship between 5-HT and childhood aggression.

Serotonin and aggression in children

Research consistently indicates that in animals and adults, reduced central serotonergic (5-HT) function is associated with increased aggression. This relationship has been elucidated via cerebrospinal fluid monoamine metabolite levels, hormonal responses to pharmacologic challenge using serotonergic probes, platelet receptor binding studies, and, more recently, through molecular genetic approaches. In contrast, studies examining the relationship of 5-HT to aggression in children have been characterized by inconsistent findings. The literature examining the relationship between central 5-HT function and aggression is reviewed. Several hypotheses that might account for the discrepancies in the child literature are examined.

Serotonergic-1a activity and contingent negative variation

While cholinergic, dopaminergic, noradrenergic, and gabaergic effects on contingent negative variation (CNV) have been largely described, little is known about serotonergic influence. Therefore, the relationship between CNV and serotonergic activity as reflected by prolactin (PRL) response to flesinoxan, a 5-HT(1A) full agonist, has been investigated in 28 healthy volunteers. To investigate the clinical implications of the relationship between CNV and serotonergic-1a activity, a group of 43 depressed patients was included in the study. Results among healthy volunteers showed a significant negative relationship between PRL response to flesinoxan and CNV amplitude at Fz, but no relationship for the other electrodes (Cz and Pz). In depressed patients, the relationships were not significant. Overall, this study does not support serotonergic effects on CNV. However, this information is indirect (correlations) and is limited to 5-HT(1A) activity.