Which neurotransmitter is primarily affected in depression?

The correct choice is serotonin. This neurotransmitter plays a crucial role in regulating mood, emotion, and behavior. In the context of depression, research has shown that decreased levels of serotonin can lead to feelings of sadness, hopelessness, and a lack of interest or pleasure in activities. This understanding has led to the development of many antidepressant medications that primarily function by increasing serotonin levels in the brain, aiming to alleviate the symptoms of depression.

While norepinephrine and dopamine also contribute to mood regulation and can be involved in depression, the primary focus in many cases of depressive disorders has been on serotonin. Acetylcholine, although significant in other functions such as attention and learning, is not directly implicated as the primary neurotransmitter in the pathology of depression.