Multiple Interacting Neurotransmitters |
| Dopamine is a precursor in the synthesis of norepinephrine. Tyrosine hydroxylase activity, which is the rate limiting enzyme in dopamine synthesis, can be inhibited by norepinephrine levels. [Click HERE if you would like to review this biochemical pathway.] Therefore, it is possible that medications affecting one neurotransmitter will indirectly affect levels of another neurotransmitter (18-21).
The effectiveness of the d and l isomers of amphetamine in the treatment of AD/HD has been compared (17). These isomers have identical effects on the noradrenergic function but differing effects on dopamanergic function. Both improved AD/HD symptoms but improvements of inattention varied between the two groups. This would suggest that both norepinephrine and dopamine are involved in the etiology of AD/HD but attention was primarily mediated via dopamine pathways. This has been supported by trials with clonidine in children with AD/HD. Clonidine is an alpha-2 noradrenergic agonist and does not directly affect dopamine pathways. It is effective in treating AD/HD symptoms. However, clonidine primarily improves the hyperaroused/aggressive behaviors and is not as effective as other agents for improving inattention. This would support the hypothesis that specific behavioral components of AD/HD are mediated by different neurotransmitter pathways. |