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Objective: There is considerable evidence supporting a genetic component in the etiology of attention-deficit/hyperactivity disorder (ADHD). Because stimulant medications act primarily on the dopaminergic system, dopamine system genes are prime candidates for genetic susceptibility factors for ADHD. Previous studies by several groups have observed a significant association of ADHD and an allele with 7 copies of the 48 base pair repeat in the third exon of the dopamine D4 receptor., Method: The authors sought to replicate these previous findings by collecting an independent sample of families from Toronto, Ontario, Canada, and confirming this finding in an expanded sample of ADHD families collected from Irvine, California. Using the transmission disequilibrium test (TDT), the authors tested for biased transmission of the 7-repeat allele at the exon III polymorphism of the dopamine D4 receptor locus in these samples of ADHD subjects., Results: Biased transmission of the 7-repeat allele from parents to ADHD probands and their affected siblings was observed in the 2 new samples of families collected in Toronto and Irvine (TDT chi2 = 2.711, 1 df, one-sided p value = .050) and for these samples combined with the 52 families previously reported from Irvine (TDT chi2 = 6.426, 1 df, one-sided p value = .006)., Conclusions: The results of this study further support the possibility of a role of the dopamine D4 receptor locus in ADHD.

Methylphenidate is the most common treatment for attention deficit hyperactivity disorder (ADHD) and has been shown to improve attention and behaviour. However, the precise nature of methylphenidate on specific aspects of attention at different dose levels remains unclear. We studied methylphenidate effects in ADHD from a neurophysiological perspective, recording event-related potentials (ERPs) during attention task performance in normal controls and children with ADHD under different dose conditions. Twenty children with ADHD and 20 age matched controls were assessed with a continuous performance task requiring subjects to identify repeating alphabetic characters. ERPs and behavioural measures were recorded and analyzed for trials where a correct response was made. The ADHD group was assessed off drug (baseline) and on placebo, low (0.28 mg/kg) and high (0.56 mg/kg) dose levels of methylphenidate. The results showed that the ADHD group at baseline was more impulsive and inattentive than controls and had shorter P2 and N2 latencies and longer P3 latencies. Low dose methylphenidate was associated with reduced impulsivity (fewer false alarms) and decreased P3 latencies, whereas the higher dose level was associated with reduced impulsivity and less inattention (more hits), as well as increased P2 and N2 latencies and decreased P3 latencies. Amplitudes were unaffected and there were no adverse effects of the higher dose for any of the children. These results suggest differential dosage effects and a dissociation between dose levels and aspects of processing.

Although the role of genetics in personality has been studied extensively at a phenomenological level, only lately has the investigation of specific genes been performed. Recent reports suggest that DNA variants of the dopamine D4 receptor gene (DRD4) are associated with the personality trait of novelty seeking; however, others fail to replicate this finding. Such conflicting results suggest either a weak effect, an association only in certain populations, or a false-positive resulting from population stratification. We provide a critical analysis of genetic studies of DRD4 variants with novelty seeking, alcoholism, drug abuse, and attention deficit hyperactivity disorder. Evidence for the role of DRD4 in novelty seeking is inconclusive, with a number of methodological concerns. Use of more conservative statistical criteria for significance, employing gene haplotypes, as well as linkage disequilibrium studies, are recommended. The molecular biology of the D4 gene is also reviewed.

Previously in this journal, we reported an association of the dopamine D4 receptor gene (DRD4) and attention deficit hyperactivity disorder (ADHD). In a population-association (case-control) study of 39 children with a refined phenotype of ADHD and 39 ethnically matched controls, we observed an increased percentage of the 7 repeat allele (29% vs 12%) and the 7+ genotype (49% vs 21%) in the ADHD group compared to the control group. In a replication and an extension of our initial study, we recruited another sample of ADHD subjects and found percentages of the 7 repeat allele (28%) and the 7+ genotype (48%) consistent with our previous findings. We used a family-based approach to evaluate a predicted association of DRD4 and ADHD based on a test of allele transmission focused on the 7 repeat allele. We identified 52 families based on the diagnosis of the refined phenotype of ADHD in the proband and the availability of DNA from both biological parents as well as the proband. Haplotype relative risk (HRR) analysis was performed to test our a priori hypothesis and produced significant results (chi-square = 4.65, P < 0.035). This provides additional evidence that the DRD4 gene is associated with a refined phenotype of ADHD.

Event-related potentials (ERPs) were compared among three groups, each with 13 subjects: (1) ADHD non-responders to methylphenidate treatment; (2) ADHD responders to methylphenidate treatment; and (3) normal control children. Response to methylphenidate was determined through extensive psychoeducational and cognitive assessments during a 4-week double-blind medication assessment. ERPs were recorded each week from 13 active electrodes during a visual feature detection task and a semantic classification task. Significant group effects were found for N2 and P3b latencies due to longer latencies for the ADHD children. Off medication, there were no differences between responders and non-responders. However, on methylphenidate non-responders had significantly longer P3b latencies than responders. Cognitive testing also revealed differential performance on medication between non-responders and responders on the paired-associate learning (PAL) task. Thus, both cognitive and ERP measures were found to differentiate ADHD non-responders and responders to methylphenidate treatment.