Functional Roles of Norepinephrine and Dopamine in ADHD

Introduction

While norepinephrine (NE) and dopamine (DA) are certainly not the only neurotransmitters involved in ADHD, there is considerable evidence that these neurotransmitters play essential roles in attention and thinking. It may be an artifact to attempt to attribute unique functions to each neurotransmitter since they collaborate in facilitating many cognitive and affective functions. Both agents contribute to maintaining alertness, increasing focus, and sustaining thought, effort, and motivation. NE and DA are structurally very similar, differing only in the presence of a hydroxyl group; DA is a precursor to NE synthesis in the brain. However, distinctions in their sources of origin and their projections in the brain and differences in the behavioral effect of selective alternations suggest that these neurotransmitters have discrete complementary roles in the brain. Although these neurotransmitters affect related components of attention, they activate distinct receptors including specific subtypes of NE and DA, usually identified as D1, D2, and D3 receptors, etc.^[1]

Arising from the locus coeruleus (LC), a small area in the basal brain located near the pons, are the cell bodies from which NE is generated. From this region, they project widely and diffusely in the brain. The broad dispersion of NE itself suggests a broad role for NE as a "neuromodulator," a term contributed by Floyd Bloom, the pioneer of functional research on NE. The concept of a neuromodulator suggests a more generalized impact of NE on tonic and phasic arousal than might be mediated by a more discretely localized neurotransmitter.^[2]