An overview of ADHD-related content on Genes to Cognition Online.

Attention Deficit/Hyperactivity Disorder (ADHD) is most often diagnosed in children and has three subtypes – inattentive, hyperactive, and combined. While all children have trouble paying attention or being over-active, ADHD is a chronic condition that is functionally impairing. The Dana review article Background to ADHD provides a comprehensive summary of the disorder. In ADHD as a Genetic Disorder, Professor Judith Rapoport explains that while the disorder has clearly heritable causes, the search for candidate genes has been complex. Genes involved in the transporting and receiving dopamine have been associated with ADHD and dopamine, along with serotonin and norepinephrine are associated biochemicals. It remains to be discovered how brain cells are affected in ADHD, and research into the functions of neurons, synapses, and glial cells will doubtless shed some light on this issue. In Delayed Brain Development, Professor Philip Shaw describes how the brain is slower to mature in ADHD. Specific associated brain areas include the amygdala, hippocampus, and frontal cortex. Lead toxins, prenatal factors, and diet are some environmental factors that may contribute to ADHD. GENES In ADHD as a Genetic Disorder, Professor Judith Rapoport discusses research highlighting ADHD as a highly heritable disorder. A number of candidate genes have been identified, including Use the chromosome Map of Disorders and Processes to explore some of the candidate genes associated with ADHD the dopamine transporter gene (DAT1) and the dopamine D4 receptor (DRD4). New techniques including whole genome scanning are providing increasingly powerful measures for determining the genetic cause of this and many other disorders. BIOCHEMICALS Because two of the main candidate genes associated with dopamine are involved in receiving and transporting dopamine, this biochemical is of particular interest to researchers. Dopamine fluxes in the brain and dopamine regulation in the brain is very much intimately associated with how the cortex develops, and this may underlie delayed neurodevelopment. Serotonin has also been implicated in ADHD. Researchers are examining the serotonin neurotransmitter in relation to ADHD across a number of levels from the very basic building blocks to the waste products, and how this might influence risk for developing ADHD. In a series of video clips, Professor Philip Shaw discusses the relationship between biochemicals and ADHD. Medications such as methylphenidate (Ritalin) work on dopamine receptors. CELLS In Brain Cells and ADHD, Professor Judith Rapoport explains that it has been difficult to find the cellular effects of ADHD because there are many candidate genes and so-called risk genes are never present in most of the patients. If you don’t have the kind of a specific gene, like with rare single-gene disorders, it is difficult to trace how cells might be affected. A more complete understanding of the physiology of neurons, synapses, and glial cells will doubtless take us closer to an understand of all disorders at the cellular level. BRAIN A network of brain regions has been associated with ADHD, including the amygdala, hippocampus, and frontal cortex. The cerebellum, which is involved in the coordinating voluntary movement, timing, and sequence learning is also associated. Professor Philip Shaw describes a study by his team that indicates Delayed Brain Development in children with ADHD. Their neuroimaging study of 220 children with ADHD found a delay in reaching milestones of brain development by roughly about 3 years. COGNITION The three subtypes of ADHD are inattentive, hyperactive-impulsive, and combined. Inattention refers to difficulty focusing, but an individual with ADHD may focus very well on some activities that require different forms of attention. Children with hyperactivity seem to be in constant motion and have difficulty sitting still. This symptoms tends to decrease with age. Individuals with impulsivity appear to “act before they think” and find it hard to wait for a reward. ADHD occurs three times more often in boys than in girls. ENVIRONMENT In the environment and high levels of lead in the blood are very clearly associated with ADHD. There are also, like if a mom smokes, for example. The risk of the child having ADHD is greatly increased. And that is particularly the case if the child has a certain genotype or gene makeup. There are lots of different genes which influence ADHD. One of them is of the dopamine transporter. It is called the DAT gene, and if a kid has a certain form of the DAT gene and mom smokes, their risk of having ADHD is very, very elevated indeed. So that’s a good example of how environmental risks interact with the genotype of the child. Other ones that are very well known are dietary factors. These are much more controversial, but there is certainly a good very large study from Southampton that shows that some food additives and colorants may have a very slight effect on the level of hyperactivity in all children.

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