Which neurotransmitter is primarily affected by MAOIs by their mechanism of action?

MAOIs, or monoamine oxidase inhibitors, primarily inhibit the activity of the enzyme monoamine oxidase. This enzyme is responsible for the breakdown of several neurotransmitters, including norepinephrine and serotonin. By inhibiting this enzyme, MAOIs lead to an increase in the levels of these neurotransmitters in the synaptic cleft, enhancing their availability and effects on mood and emotional regulation.

Both norepinephrine and serotonin play a significant role in mood disorders, and the increased availability of these neurotransmitters is often associated with the therapeutic effects of MAOIs in treating conditions like depression. This dual action on norepinephrine and serotonin is key to why this particular option is correct, as it highlights the comprehensive effect of MAOIs rather than focusing on just one neurotransmitter.

While dopamine is also influenced by monoamine oxidase, it is primarily norepinephrine and serotonin that are significantly impacted in the context of MAOIs and their clinical use for mood disorders. Therefore, the correct answer reflects the broader impact of MAOIs on multiple neurotransmitters that contribute to their therapeutic action.