2.1.4 The dopamine theory of ADHD
The original dopamine theory of ADHD suggested that ADHD was caused by a deficit in dopamine function, resulting in less dopamine, putting the brain in a so-called hypo-dopaminergic state (Levy, 1991). However, later research indicated that not all areas of the brain appeared to have a reduction in dopamine, and in fact, some areas were actually hyper-dopaminergic, meaning there is more dopamine than typically found (Castellanos, 1997). This is possible because the increases and decreases in dopamine functioning occur in different parts of the brain.
Figure 11 shows the three different dopamine pathways within the brain. One pathway runs from the substantia nigra to the dorsal striatum and is known as the nigrostriatal pathway. This pathway is a locomotor pathway. The other pathways both originate in the ventral tegmental area but the connections branch to target two different structures. The first branch of this pathway is known as the mesolimbic pathway, and this one targets the nucleus accumbens in the midbrain. The second branch projects quite widely throughout the prefrontal cortex at the front of the brain, and is known as the mesocortical pathway. These two pathways are involved in motivated behaviour, attention and response inhibition.
Drag and drop the labels onto Figure 11 below to display the main structures involved in dopamine signalling in the brain.
Research has indicated that there is a decrease in dopamine functioning in the mesocortical pathway that can have an impact on attention, including selective attention, and an increase in dopamine functioning in the nigrostriatal pathway that can result in symptoms of impulsivity and hyperactivity (Castellanos, 1997).
It is important to recognise that the dopamine theory is not without criticism. Gonon (2009) outlines concerns about the different kinds of evidence presented in favour of the dopamine theory. These are summarised in Table 4.
Table 4 Evidence for the dopamine theory of ADHD has been questioned (from Gonon, 2009)
|Type of Evidence||Evidence supporting dopamine theory||Concerns about evidence|
|Imaging||Reduced activity in areas receiving dopamine||Changes in activity are more complex and may involve multiple systems|
|Genetics||Certain variants of genes related to the dopamine transporter and dopamine receptor are found to be increased in the brains of people with ADHD||The most robust finding for DRD4 only shows it to be slightly more prevalent in individuals with ADHD than healthy people|
|Treatment||Effective treatments for ADHD act on the dopamine system||Effective treatments may also act on the noradrenalin system|
Based on some of the concerns raised about the dopamine theory of ADHD, Gonon (2009) suggests that the theory is unhelpful because it reduces research into other possible brain changes in ADHD. However, despite this criticism, dopamine is generally accepted as having a very important place in the neurobiological basis of ADHD.
Various approaches for managing ADHD are available, some of which are based on the known biological changes, including changes in dopamine signalling, while others are focused on psychosocial elements of the condition. Some of these management approaches are discussed in the next sections.