Dopamine Receptors and the Pharmacogenetics of Side-Effects of Stimulant Treatment for Attention-Deficit/Hyperactivity Disorder

Abstract

There is considerable interest in the pharmacogenetics of responses to stimulant treatment for attention-deficit/ hyperactivity disorder (ADHD) (Froehlich et al. 2010). By contrast, little attention has been paid to individual differences in side effects. Dopamine function in the prefrontal cortex (PFC) is a strong candidate system for understanding these (Arnsten, 2009). D1 receptors are the most abundant dopamine (DA) receptor subtype, and mediate most of the cellular effects of DA in this region (Lewis and Gonzalez-Burgos 2006). They are thought to be involved in the regulation of sustained firing of dorsolateral PFC neurons during the delay phase of delayed-response tasks that require working memory (Sawaguchi and Goldman-Rakic 1994; Seamans and Yang 2004; Lewis and Gonzalez-Burgos 2006). Sawaguchi and Goldman-Rakic (1994) showed that D1 dopamine receptors in the dorsolateral prefrontal cortex of monkeys participated in the maintenance of internalized visuospatial representations and/or in the control of eye movements governed by internal cues. They suggested that the D1 family of dopamine receptors may have a critical role in PFC-mediated working memory and cognitive deficits. Variations in D1 polymorphism rs4532 are associated with ADHD (Bobb et al. 2005) and predict side effects to medication for schizophrenia (Potkin et al. 2003; Lai et al. 2011), although in the latter studies the effects may relate to excess dopamine at the D1 receptor. In both cases, there are good reasons to suggest that D1 receptor genetics may predict side-effects from stimulant medication for ADHD. We tested the relationship of stimulant side effects to common variations in DA receptor genes using 90 Caucasian children (79.2% males) who met formal criteria for American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV) diagnosis of ADHD (American Psychiatric Association 1994); were 4–14 years of age (mean = 8.85 years; SD= 2.32); had no major neurological/physical illness; had an intelligence quotient (IQ) >70; were using stimulant medication; and were not using another psychoactive medication (full demographic and diagnostic data and medication use available from Dr. Levy). Ethics approval and fully informed consent was from the University of New South Wales. Participants gave blood (52.5%) or saliva (or both for a smaller reliability check sample) via Oragene saliva collection kits (http://www.dnagenotek.com/). DNA extraction rates were >95% for both methods. Samples were genotyped for four dopamine receptor single nucleotide polymorphisms (SNPs): DRD1 rs4532, DRD2 rs6277, DRD3 rs6280; DRD4 rs7932167, and COMT rs4680. All were in Hardy–Weinberg equilibrium. Genotypes were determined using iPLEX Gold primer extension followed by mass spectrometry analysis on the Sequenom MassARRAY system (Sequenom, San Diego, CA) by the Australian Genome Research Facility (AGRF: http://www.agrf.org .au/). Age, weight sex, and IQ were noted, and the side effects rated as absent, mild, moderate, and severe, using a modified Barkley side-effect scale (Barkley et al. 1990). Medication usage was as follows: Primary medication Ritalin (immediate release [IR]) n = 43, dosage mean = 22.03 SD = 11.7; Ritalin (modified release) n = 14, dosage mean = 26.43 SD = 6.33; Concerta n = 12, dosage mean = 29.25 SD = 7.79; dexamphetamine n = 19, dosage mean = 8.37 SD = 4.47; dexamphetamine (long acting [Dex LA]) n = 2, dosage mean = 10 SD = 0; secondary medication Ritalin (IR) n = 5, dosage mean = 14.0 SD = 10.25; Catapres n = 9, dosage mean = 25.05 SD = 10.83. Secondary medications were distributed as follows: 4 of the 43 children receiving primary Ritalin IR were also receiving Catapres at night; 6 of the 14 children receiving Ritalin (modified release) were receiving a secondary medication, that is, 3 were also receiving Catapres at night, and 3 were receiving an additional Ritalin IR during the day. Two of the 19 children receiving dexamphetamine were also receiving Ritalin IR during the day; none of those on Dex LA were receiving an secondary medication; 2 of the 12 on Concerta were receiving Catapres at night. Medication doses were standardized to a standard Ritalin (IR) dose of 10 mg by the following formula: Dexamphetamine 5mg =Ritalin IR 10mg; Ritalin (modified release) 10mg =Ritalin IR 5mg b.i.d.; Concerta 18mg =Ritalin IR 5mg t.i.d. We reduced the 16 side effects items to factors using principal components analysis. Both parallel analysis and Velicer’s minimum average partial (MAP) test indicated that there were three nonrandom factors; therefore, we generated a three factor solution using Varimax rotation with Kaiser normalization. Factor loadings are available from Dr. Levy. The three factors, nausea, withdrawal